at SciVerse ScienceDirect

Estuarine, Coastal and Shelf Science xxx (2012) 1e11

Contents lists available

Estuarine, Coastal and Shelf Science

journal homepage: www.elsevier .com/locate/ecss

Recent evolution of river discharges in the Gulf of Trieste and their potentialresponse to climate changes and anthropogenic pressure

Stefano Cozzi a,*, Claus Falconi b,1, Cinzia Comici b,2, Branko �Cermelj c,3, Nives Kovac c,3, Valentina Turk c,3,Michele Giani b,4

aCNR e I.S.MAR, Istituto di Scienze Marine, Viale Romolo Gessi 2, 34123 Trieste, ItalybOGS e Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Via A. Piccard 54, 34151 Trieste, ItalycNIB e National Institute of Biology, Marine Biology Station Piran, Fornace 41, 6330 Piran, Slovenia

a r t i c l e i n f o

Article history:Received 12 July 2011Accepted 2 March 2012Available online xxx

Keywords:runoffnutrientsfreshwatercoastal zoneeutrophicationnortheastern Adriatic

* Corresponding author. Tel.: þ39 040 305312; fax:E-mail addresses: stefano.cozzi@ts.ismar.cnr.it (S.

(C. Falconi), ccomici@ogs.trieste.it (C. Comici), cermNives.Kovac@mbss.org (N. Kovac), Valentina.Turk@minogs.it (M. Giani).

1 Tel.: þ39 040 2249742; fax: þ39 040 2249770.2 Tel.: þ39 040 2249740; fax: þ39 040 2249770.3 Tel.: þ386 5 6712900; fax: þ386 5 6712902.4 Tel.: þ39 040 2249713; fax: þ39 040 2249770.

0272-7714/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.ecss.2012.03.005

Please cite this article in press as: Cozzi, S., etchanges and anthropogenic pressure, Estuar

a b s t r a c t

Freshwater and nutrient discharges by rivers were analysed in the northeastern Adriatic continental shelffrom 1998 to 2008, in order to assess their role in the biogeochemistry of this coastal zone, as well as theirpotential future changes due to the effects of climate variability and anthropogenic pressure. River waterloads (up to 6.05 km3 yr�1) and transport of nutrients (up to 13,200 t N yr�1 for TN, 86 t P yr�1 for TP and12,400 t Si yr�1 for Si(OH)4) were high from 1998 to 2002, but they decreased by over 70% during the driestyears 2003, 2005, 2006 and 2007. The precipitation in the surrounding continental region (average of1371 mm yr�1) is the major forcing that regulates the runoff in this area, with peaks in early spring andautumn that are associated respectively to negative phases ofMediterraneanOscillation index and positivephases of West Mediterranean Oscillation index. This finding, together with the minor contribution ofsnowmelt in early spring (up to 3.2$10�5 kgm�2 s�1), indicates that the dynamics of the naturalwater cyclestill overtake in this coastal zone the effects of anthropogenic usage of continental waters.

During the last four decades, the northeastern Adriatic continental shelf has been subjected to anincreasing pressure due to a high river transport of nitrogen and, currently, only a deep phosphorusdeficiency in total (TN/TP ¼ 49e405) and inorganic (DIN/PO4 ¼ 37e418) river nutrient pools prevents itssevere eutrophication. By contrast, the decrease of river loads of nutrients from 2003 to 2007 indicatesthat recurrent water crisis might significantly lower the trophic level in this coastal zone in the future. Inthis perspective, other allochthonous sources of nutrients, like sewage loads, atmospheric deposition andbenthic fluxes might become more important for their balance, with possible implications on thestructure of this ecosystem.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

During the last decade, a wide range of impacts due to climaticchanges have been documented in Europe, with a constantlygrowing confidence, showing trends that seem to magnify thedifferences at regional scales of the availability of natural resources

þ39 040 308941.Cozzi), cfalconi@ogs.trieste.itelj@mbss.org (B. �Cermelj),bss.org (V. Turk), mgiani@

All rights reserved.

al., Recent evolution of river dine, Coastal and Shelf Scienc

and of the quality of ecosystems. In theMediterranean region, thesechanges are becoming clearly characterised by an increase ofmaximum temperatures, mainly during summer, a decrease ofannual precipitation (20% in some areas) and an increase of thefrequency of heatwaves, droughts and floods (Alcamo et al., 2007).

The coastal zones in Southern Europe subjected to a high run-off are focal marine environments for the study of the combinedeffects of sub-tropical climate changes and anthropogenicpressure, either because these ecosystems are deeply affected byatmosphere-land-sea interactions and because they provide a widerange of goods and services whose economic value depends on thequality of the environment. In most of the coasts of developedcountries in the Mediterranean basin, tourism and recreational useof bathing waters coexists with an exploitation of the marineresources based on intense fishery and aquaculture, with maritimetraffic and high concentrations of infrastructures, as well as with

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e112

high loads of pollutants originated by an expanding human pop-ulation (Brachya et al., 1994; Malone et al., 1999).

River discharge of freshwater and nutrients is a basic componentthat regulate the condition of coastal ecosystems, as it exertsa significant impact on their hydrology, biogeochemistry andproductivity. The spreading of coastal fronts has a great influence onthe stratification of thewater column and on the circulation ofwaterbodies, because of the combined effects of baroclinic gradients,topographic constrains and shallowness of these areas (Ludwiget al.,2009). Moreover, cascading of dense waters formed on the conti-nental shelves may propagate the signal of coastal waters overa much wider region of the Mediterranean (Artegiani et al., 1997).

Productivity and eutrophication problems in the coastal zonesare significantly linked to the supply of land-borne nutrients andorganic matter by the rivers and to their interaction with the localcirculation. River nutrient transport has strongly increased over thelast century, both in the continental part of Europe (Kempe et al.,1991) and along its southern coasts (Billen and Garnier, 2007),although a detailed assessment of its recent interannual fluctua-tions is still lacking for several sub-regions of the Mediterranean.During the last decades, the concern about the negative impact ofriver nutrient loads on the quality of the coastal waters has induceda more strict assessment and control of anthropogenic emissions atthe level of river drainage basins (Palmeri et al., 2005). Moreover, itis well recognised that the effects in the marine environment ofanthropogenic loads are also modulated by the climate, thusevidencing the need of an integrated approach for themanagementof these marine areas able to include all the major socioeconomicfactors and environmental forcings that act from the river basin tothe coastal zone (Brachya et al., 1994; Seitzinger et al., 2010).

The northern Adriatic continental shelf and its eastern portion,the Gulf of Trieste, is a model area for the study of the importance ofrunoff and of the supply of land-borne nutrients in a sub-tropicalcoastal marine environment. Information on long term trends ofanthropogenic loads (Olivotti et al., 1986; Degobbis et al., 2000;Turk et al., 2007) may be here compared to temporal variations oftrophic level and structure of this marine ecosystem (Malone et al.,1999; Mozeti�c et al., 2010) and to more recent changes of oceano-graphic properties (Mala�ci�c et al., 2006; Solidoro et al., 2009), inorder to better assess the mechanisms in which the variability ofriver loads affects this coastal zone.

The present study aims to fill the gap of information that stillpersists on the importance of river water and nutrient dischargesfor the biogeochemistry of the Gulf of Trieste. A comprehensiveanalysis of freshwater and nutrient loads by the rivers located in thenorthern and southern portions of the gulf has been carried out, inorder to assess their seasonal and interannual variability, theirdependence on the climatic conditions at sub-regional scale andtheir current impact on the budgets of freshwater and nutrient inthis coastal zone. Moreover, river loads were compared to otherexternal sources of freshwater and nutrients present in this costalenvironment, which may be inferred by the analysis of past studiesfocused on anthropogenic emissions and eutrophication problems.The extreme variability of the climatic conditions occurred duringthe last decade has also permitted the analysis of the potentialfuture alterations of the runoff that might be here induced by longterm trends of modification of the climate.

2. Methods

2.1. Study area

The eastern portion of northern Adriatic continental shelf isoccupied by the Gulf of Trieste, which is a shallow (<25m of depth)and semi-enclosed coastal zone approximately 20 � 25 km in size

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

(Fig. 1). Its oceanographic properties are highly variable, due toa pronounced seasonal cycles of seawater temperature (5e26 �C)and to the formation of strong salinity gradients (25e38 psu)originated by the contrasting effects of runoff and seawaterexchange at the open boundary. The occurrence of persistentcooling periods during winter causes here frequent events of densewater formation (Mala�ci�c and Petelin, 2001).

Water exchange in the gulf is characterized, at its southern side,by the inflow at surface of the Eastern Adriatic Current. ModifiedLevantine Intermediate Water enters sporadically in the gulf asbottomor intermediatewaters, after flowing along the Istrian coast.The main circulation is cyclonic and the outflow of seawater mostlyoccurs along the shallow northern coast after mixing with the riverwaters (Fonda Umani et al., 2007; Solidoro et al., 2009). However,highly variable and even opposite circulations are observed, inparticular in the upper layer, due to the effect of dominant winds(Bora; ENE and Sirocco; SE) on highly stratified water columns.Residual currents in the gulf are in the range 1e3 cm s�1, but totalcurrents as high as 35 cm s�1 may be measured in the upper layer,because of the effects of tide andwind (Rajar and �Cetina,1991; Cozziet al., 2004; Mala�ci�c and Petelin, 2009).

Isonzo River (Slovene: So�ca), with a length of 136 km, a drainagebasin of 3452 km2 and a flow of 82 m3 s�1 (average for the period1998e2008), is the major source of land-borne nutrients in thiscoastal zone, in particular because of nitrate leaching from croppingareas of Venezia Giulia plain (Cantoni et al., 2003; Comici andBussani, 2007). The second tributary in the gulf, the Timavo River(Slovene: Reka) has an average flowof 27m3 s�1 and a total length of89 km, although it flows for about 40 km along its lowest partthrough a not well determined path under the Karst Plateau. Itsfreshwater load is often consistent even during periods of drought,generating a plume that mixes with that of Isonzo River and spreadsin the northern and central areas of the gulf triggering intensephytoplankton blooms, especially in late winter and autumn (Malejet al., 1995). Along the southern Slovenian Coast, the rivers Ri�zana,Bada�sevica, Drnica and Dragonja has smaller drainage basins(30e204 km2), lengths (9e28 km; Frantar, 2008) and average flows(0.25e4.0 m3 s�1). Nevertheless, their inputs concur to the increaseof freshwater content and nutrient concentration in the coastalwaters entering in thegulf at a smaller spatial scale (Turket al., 2007).

Other allochthonous sources of nutrients affect the biogeo-chemistry of the Gulf of Trieste. Urban and industrial sewagesdischarge high quantities of nutrients and organic matter and causethe diffusion of faecal coliforms and heavy metals (Cozzi et al.,2008; Mozeti�c et al., 2008; Scroccaro et al., 2010). Precipitation ispotentially a not negligible term of nutrient balance in this coastalzone, although it has been poorly studied to date (Malej et al., 1997).Benthic remineralization of organic matter and fluxes of biogenicelements from the sediments, both in oxic/hypoxic conditions, maystrongly fuel the production processes in the water column,because of the shallowness of this coastal zone (Bertuzzi et al.,1997; Faganeli and Ogrinc, 2009). As a result of the combinationbetween weak circulation and high loads of nutrients, largeportions of the northern Adriatic continental shelf (Degobbis et al.,2000) and of the Gulf of Trieste (Faganeli et al., 1985; Aleffi et al.,1992; Faganeli and Ogrinc, 2009) have been affected in the pastby dinoflagellate blooms, hypoxia and anoxia. Mucilage phenom-enon was also observed in the gulf in concomitance to the largestevents in the northern Adriatic (Fonda Umani et al., 2007; Faganeliand Ogrinc, 2009).

2.2. Compilation and analysis of data

Daily averaged flow rates (m3 s�1) of Isonzo and Timavo rivers(hereafter northern rivers) were provided by Agenzia Regionale per

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

Fig. 1. (a) Rivers in the Gulf of Trieste. Dashed line indicates the limit of the marine area that is compared to river loads. (b) Total drainage basin in the surrounding continentalregion and position of river monitoring stations.

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e11 3

la Protezione dell’Ambiente of Friuli Venezia Giulia Region (ARPA)and by ACEGAS-APS spa for the period 1998e2008. Waterdischarges referred to the monitoring stations of Pieris and Boccheof Timavo, respectively. Flow rates of the rivers located along theSlovenian Coast of the gulf were published by Slovenian Environ-mental Agency, Ministry of the Environment and Spatial Planning(ARSO). They refer to the rivers Ri�zana, Bada�sevica, Drnica andDragonja (hereafter southern rivers), which were monitored atKubed, �Salara, Pi�sine and Podka�stel stations, respectively (Fig. 1b).All the consideredmonitoring stations are close to the river mouthsand they constitute the closure of each drainage basin, so that nosubstantial additional inputs of freshwater and nutrients may beassumed to be present downstream of their section.

Data of concentration (mM) of nitrate (NO3), nitrite (NO2),ammonium (NH4), reactive phosphorus (PO4), reactive silicate(Si(OH)4), total nitrogen (TN) and total phosphorus (TP) in fresh-water at the river mouths (n ¼ 421) were provided for the period2000e2007 by monthly monitoring of ARPA, ACEGAS-APS andARSO, as well as by the Water Data Centre of European Environ-ment Agency (URL: http://www.eea.europa.eu). The concentrationof dissolved inorganic nitrogen (DIN) was calculated as the sum ofNO3, NO2 and NH4. Dissolved inorganic nutrients were determined

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

in filtered samples by standard colorimetric methods, while TN andTPwere determined in unfiltered samples by persulphate oxidationmethod followed by the colorimetric determinations of NO3 þ NO2and PO4. These methods are outlined by the national environ-mental regulations and, due to high concentrations found in riverwaters, may be considered equivalent to those largely applied inenvironmental (Clesceri et al., 1998) and oceanographic (Grasshoffet al., 1999) studies.

Annual integrated transport of river nutrients, TN and TP(F; expressed in tons per year) was estimated using the equationbased on discharge weighted means of daily transports:

F ¼"Xni¼1

ðCi$QiÞ=Xni¼1

Qi

#$QY$mA$10

�6

where Ci and Qi are nutrient concentration (mol m�3) and flow rate(m3 s�1) for each day of sampling, QY is the annual water load(m3 yr�1) and mA is the atomic mass of the considered biogenicelement. This method allows the best compensation of the biasesderiving from the different weight that concentration data assumein determining the transport, when they are collected duringperiods of freshet or drought (Kempe et al., 1991).

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e114

The impact of river discharges in the portion of the Gulf ofTrieste shown in Fig. 1a was analysed through the estimate offlushing (sFW) and turnover (sNUT) times of river water and nutri-ents. A volume of seawater equal to 8.690 km3 was calculatedapplying Kriging interpolation method (Cressie, 1990) to 0.5 km-resolution gridded bathymetry data, corresponding to a gulf surfaceof 548 km2 and an average depth of 15.86 m. The budgets ofnutrients in the gulf were afterwards calculated using medianvalues (n ¼ 1500) of their concentration in seawater, provided bythe coastal monitoring of ARPA for the period 1998e2008. Thisestimate indicated a higher content of nitrogen (343 t N of DIN,1520 t N of TN) and reactive silicate (745 t Si) in this marine areacompared to the content of phosphorus (18 t P of PO4, 165 t P of TP).

Meteorological data in the continental region surrounding thenortheastern Adriatic, which includes the mountainous portion ofIsonzo drainage basin (45.20e46.50� Lat. N, 13.20e14.20� Long. E),were obtained by NASA e Goddard Earth Sciences Data and Infor-mation Services Center (URL: http://daac.gsfc.nasa.gov/giovanni/),for the period 1998e2008. In particular, monthly averagedprecipitation was derived by Tropical Rainfall Measuring Missionproducts (TRMM; 0.25-degree resolution). Monthly averagedaccumulation (kg m�2) and melting (kg m2 s�1) of snow werederived by Monthly History Data Collections products (MERRA 2D;0.5-degree resolution).

The role of primary northern Hemisphere Teleconnectionindices (NHTi) that affect climatic conditions in the Europeancontinent was also investigated, in order to assess their significanceas proxies of the runoff in this coastal zone. Monthly averagedvalues of North Atlantic Oscillation index (NAOi), East Atlantic/WestRussia pattern (EA/WRp), East Atlantic pattern (EAp) and Scandi-navia pattern (SCAp), standardized for 1981e2010 climatology,were obtained by NOAA e National Weather Service ClimatePrediction Center (URL: http://www.cpc.ncep.noaa.gov/). Atsmaller scales, monthly averaged values of Mediterranean Oscilla-tion index (MOi; based on Algiers and Cairo reference points, CRUe

University of East Anglia, URL: http://www.cru.uea.ac.uk/cru/data/moi/) and West Mediterranean Oscillation index (WMOi; basedon Padova and Cádiz reference points, Group of Climatology e

University of Barcelona, URL: http://www.ub.edu/gc/) were alsoanalysed.

3. Results

3.1. River flows and climatic conditions

The total load of river water in the northeastern Adriaticcontinental shelf has significantly varied in the period 1998e2008(Table 1). River flows have been high from 1998 to 2002, reaching

Table 1Total runoff in the Gulf of Trieste from 1998 to 2008 and contribution of the riverslocated in its northern and southern areas.

Year (yyyy) Total runoff (km3 yr�1) N rivers (%) S rivers (%)

1998 4.530 97.3 2.71999 4.351 97.3 2.72000 6.051 97.4 2.62001 5.029 96.9 3.12002 3.439 95.2 4.82003 1.919 94.6 5.42004 3.766 95.8 4.22005 2.217 94.4 5.62006 2.365 93.1 6.92007 1.801 94.0 6.02008 4.172 96.0 4.0Median 3.766 95.8 4.2

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

a maximum values of 6.05 km3 yr�1 in 2000. The following yearshave been characterised by persistent droughts, which have causedreally scarce water loads in 2003, 2005, 2006 and a minimumvalueof 1.80 km3 yr�1 in 2007. The partition of flows between northernand southern rivers has shown that Isonzo and Timavo always exertthe most important role in this coastal zone, being their waterdischarge greater than 93.1%. However, the importance of southernrivers has increased from 2.6 to 6.9% during the driest years withrespect to those characterised by high flows.

Despite a high interannual variability, river flows showed a clearseasonal cycle in this area, being them characterised by two dryseasons in winter and summer alternated to two wet seasons inspring and autumn (Fig. 2). The highest peaks of freshwaterdischarge usually occur during October and November, althoughthese months also show the greatest interannual variability(100e2100$106 m3 month�1). Periods of drought mostly take placefrom June to September (29e517$106 m3 month�1), with a wors-ening of water scarcity during August that is exemplified by thelowest median value of runoff of the year (103$106 m3 month�1).

The dynamics of river flows is primarily linked to the precipi-tation in the surrounding continental region, both on annual andmonthly scales.With the only exception of 2004, particularly scarceannual integrated values of precipitation were observed during theperiod from 2003 to 2007, being the minimum of 1048 mm yr�1

measured in 2006 (Fig. 3a). This pronounced reduction of theprecipitation was reflected in the long crisis of annual integratedwater loads of the rivers observed during the considered decade.

On a seasonal scale, peaks of precipitation during spring andautumn occur in particular in the mountainous area of Isonzodrainage basin and in the continental sector of Slovenia. In theseseasons, values as high as 459 mm month�1, like that observed inNovember 2000, cause the increase of the runoff both of northernand southern rivers in the Gulf of Trieste. By contrast, the highestaccumulation of snow (42 kg m�2) occurs in the mountainousportion of this region (<2800 m of altitude) from December toMarch, whereas its melting (3.2$10�5 kgm�2 s�1) contributes to theincrease of river flows from February to May (Fig. 3b, c).

The analysis of the correlation between monthly values ofprimary NHTi and the anomalies of precipitation and river flows inthe Gulf of Trieste showed for NAOi an inverse relationship in theperiod 1998e2008, which indicates the well known southerly trackof storm activity and increased rainfall over Southern Europeduring the phases of low NAOi (Table 2). However, this correlationis poorly significant for most of the year. For this index, highercorrelation were found with respect to peaks of runoff only in April(r2 ¼ �0.60) and November (r2 ¼ �0.66) and with respect to lowdischarges in July (r2¼�0.59) and September (r2¼�0.50). EAp andSCAp also gave variable and poorly significant relationships withrespect to the anomalies of both precipitation and flow. Among theprimary indexes, EA/WRp showed the highest correlation with the

Fig. 2. Seasonal cycle of monthly runoff in the Gulf of Trieste.

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

Fig. 3. (a) Monthly and annual precipitation, (b) monthly averaged snow mass and (c) snow melting in the continental region surrounding the Gulf of Trieste.

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e11 5

meteorological conditions in the region of study over the year.Negative values of EA/WRp corresponded to increased river flowsin January, May, July and December. Since 2008, a phase dominatedby low values of NAOi and EA/WRp has been detected (NOAA; URL:

Table 2Coefficient of correlation (r2) among teleconnection indices and monthly anomaliesof (a) Precipitation and (b) River flows in the Gulf of Trieste, for the period1998e2008.

a) Monthly anomaly of precipitation

Month NAOi EAp EA/WRp SCAp MOi WMOi

Jan. �0.37 0.52 �0.65 0.33 �0.73 0.41Feb. �0.21 0.12 �0.30 �0.14 �0.34 0.45Mar. �0.20 �0.22 �0.29 �0.08 �0.75 0.67Apr. �0.30 0.75 �0.21 0.47 �0.27 0.70May 0.40 �0.19 0.29 �0.16 �0.35 0.23Jun. �0.35 0.34 �0.06 �0.14 0.07 0.69Jul. �0.69 �0.18 �0.55 0.21 �0.60 0.67Aug. �0.16 �0.55 �0.44 0.63 �0.36 0.40Sep. �0.36 �0.47 0.04 0.08 �0.44 0.58Oct. 0.10 �0.13 �0.29 �0.32 �0.16 0.76Nov. �0.70 0.39 �0.04 0.77 �0.51 0.71Dec. 0.03 �0.23 �0.54 0.08 �0.62 0.44

b) Monthly anomaly of river flow

Month NAOi EAp EA/WRp SCAp MOi WMOi

Jan. �0.31 0.37 �0.59 0.44 �0.70 0.28Feb. �0.33 0.22 �0.05 0.02 �0.21 0.43Mar. �0.35 �0.08 �0.04 0.03 �0.64 0.46Apr. �0.60 0.64 �0.32 �0.04 �0.21 0.87May 0.20 �0.58 0.61 �0.32 �0.71 �0.22Jun. 0.08 0.19 �0.44 �0.01 0.20 0.59Jul. �0.59 �0.20 �0.65 0.03 �0.38 0.55Aug. 0.39 �0.27 0.35 0.39 �0.33 0.41Sep. �0.50 �0.27 0.07 0.22 �0.54 0.45Oct. 0.19 0.29 �0.06 �0.07 0.41 0.92Nov. �0.66 0.32 �0.04 0.68 �0.32 0.72Dec. �0.20 �0.05 �0.60 0.21 �0.59 0.25

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

http://www.cpc.ncep.noaa.gov/), which may be connected to therecovery of higher river flows after the long water crisis of theperiod 2003e2007.

More significant correlations were obtained consideringregional indices like MOi and WMOi, because of greater proximityof the reference centres of their dipoles with the area of study(Table 2, Fig. 4cef). The presence of direct and inverse relationshipsof precipitation anomaly respectively with WMOi and MOi indicatein both cases the linkage between raining periods and persistentlow values air pressure over the central Mediterranean. Thecorrelation of these indices with river flows remains high forWMOiin case of the peaks of discharge in April, October and November(r2 ¼ 0.72e0.92), as well as during the period of low discharge inJuneeJuly (r2 ¼ 0.55e0.59).

The major role of precipitation anomaly in determining theanomaly of river flows in the gulf is confirmed by the significance oftheir linear relationship through all the seasons y ¼ 3.725$x(r2 ¼ 0.696; Fig. 4a). Snowmelt assumes positive anomalies andcontributes to the increases of river flows over the average only inthose autumns and winters where a significant accumulation ofsnow has occurred in the mountainous portion of the region(Fig. 4b).

Despite the major role of Isonzo River in determining the runoffin this coastal zone, freshwater loads by the other minor rivers mayalso have important effects at local scale, in particular because ofthe distinct timing that often characterise the occurrence of theperiods of high and low discharge (Fig. 5). Isonzo River has oftenshown high monthly flows from 1998 to 2002, being these yearscharacterised by a high annual runoff. The other rivers have mostlyfollowed the dynamics of Isonzo during this period, with peaks ofdischarge in late winter and autumn. By contrast, high flowsseldom occurred for Isonzo River during the dry period 2003e2007,but they repetitively occurred in the minor rivers determiningfrequent peaks of local runoff. Opposite to the dynamics of the

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

Fig. 4. Monthly river flow anomaly vs (a) precipitation and (b) snowmelt anomalies. (c, d) Precipitation and (e, f) river flow anomalies vs MOi and WMOi.

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e116

freshets, drought periods were rare for Isonzo River from 1998 to2002, but they have frequently affected the other minor rivers. Lowflows were instead frequent both for Isonzo and for minor riversduring the dry period 2003e2007. This behaviour indicated thatthe smallest rivers of the Gulf of Trieste can be affected by repetitivefreshets (droughts) during dry (wet) years differently from rivers,like Isonzo, that have a greater drainage basin.

3.2. River nutrient loads

The current concentrations of DIN, PO4 and Si(OH)4 in riverwaters are approximately 50, 10 and 30 times higher that inseawater in the Gulf of Trieste, whereas those of TN and TP arerespectively 20 and 2 times higher (Table 3). Several minor rivershave concentration levels significantly higher with respect to thoseof Isonzo River, indicating that they have a greater importance fornutrient budgets in the gulf with respect to freshwater budgets.Riverine transport of nutrients is characterised by a large excess ofNO3, which constitute 60% of TN, whereas the load of phosphorus isconstituted for 70% by PO4 and for 30% by dissolved and particulate

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

organic phosphorus. The discharge of N-nutrients in most of theserivers strongly exceeds either that of phosphorus (TN/TP¼ 49e405,DIN/PO4 ¼ 37e418) and that of silicon (Si/DIN ¼ 0.5e1.0), both ininorganic and organic fractions.

In the period 2000e2007, the total transport of rivers in thiscoastal zone has been in the range 3900e13,200 t N yr�1 for TN,33e86 t P yr�1 for TP and 3400e12,400 t Si yr�1 for Si(OH)4 (Fig. 6).The clear decreasing trend and the particularly low values estimatedin 2003, 2005, 2006 and 2007 were strictly related to the reductionof annual integrated water load of rivers. This feature indicated thata reduction of 60e70% of the supply of riverine nutrients wasexperienced by this marine ecosystem during the water crisis of2003e2007 with respect to the beginning of the decade.

The partitioning of the transport of nutrients between northernand southern rivers also showed significant interannual differences(Table 4). In the period 2000e2007, the contribution of TN(95.3e87.3%) and Si(OH)4 (95.4e89.5%) from northern rivers hasdecreased with a trend consistent with that of the freshwaterbudgets. During this phase, the contribution of southern rivershas reached respectively 12.7 and 10.5%, indicating their greater

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

Fig. 5. Timing of freshets (monthly loads > 3� quartile) and droughts (monthly loads < 1� quartile) in the rivers of the Gulf of Trieste in comparison to the total annual load(H ¼ high, M ¼ medium, L ¼ low).

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e11 7

importance for the supply of nutrients in the gulf during periods ofwater scarcity. Differently from the other nutrients, thecontribution of TP by northern (76.1e96.3%) and southern(3.7e23.9%) rivers was characterised by large interannualoscillations scarcely linked to the quantity of freshwater. Thisbehaviour was probably due to the fact that phosphorus is nota major biogenic element in river waters like nitrogen and silicon,which originate by the leaching from soils caused by the meteoricwaters.

4. Discussion

4.1. Interannual variability of runoff

During the last decade, the Southern Europe have been affectedby intense summer heatwaves that have caused recurrent periodsof drought and large wildfires, with an estimated reduction of 30%of gross primary production of terrestrial ecosystems over thecontinent during the major crisis of 2003 (Alcamo et al., 2007). Therunoff of Italian rivers in the northern Adriatic continental shelf hasstrongly decreased from 2003 to 2007 (38%) with respect to theaverage value of the period 1995e2000 (z60 km3 yr�1; Cozzi andGiani, 2011). In the previous period 1971e2000, Adriatic rivers ofSlovenia and the mountainous area of Isonzo drainage basin havebeen subjected to a long tern reduction of annual runoff (6%), asa result of a rather constant precipitation and an increasedevapotranspiration of soils (11%) caused by the increase of airtemperature (Frantar, 2007). A significant trend of reduction offreshwater discharge (20%) was also detected in the whole Medi-terranean basin between 1960 and 2000 (Ludwig et al., 2009).

Our study showed that the annual runoff in the Gulf of Triestehas been characterised by a similar crisis after 2002 that has lead toa minimum value of 1.80 km3 yr�1 in 2007, corresponding toa decrease by 64% of freshwater load compared to the average of1998e2001 (Table 1). Despite a higher contribution to the total loadof minor rivers located along the southern coast during these driestyears (2.7e6.9%), this finding indicates a great exposure of thiscoastal zone to the dynamics of water cycle in the mountainous

Table 3Median value of concentration and molar ratios of nutrients in river water.

River NO3 (mM) NH4 (mM) NO2 (mM) DIN (mM) PO4 (mM) Si(OH)4 (

Isonzo 87.1 0.3 2.9 90.2 0.3 72.1Timavo 112.9 0.4 2.8 116.1 0.5 64.9Ri�zana 128.7 2.5 0.6 131.8 0.4 65.3Bada�sevica 221.6 6.9 1.2 229.6 1.0 132.8Drnica 127.2 5.5 1.0 133.7 3.4 135.3Dragonja 188.6 2.6 0.3 191.5 1.2 135.9

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

portion of Isonzo drainage basin, which is the major reservoir ofcontinental waters for the gulf.

Several important effects may be expected in this marineenvironment in case of a similar decrease of the runoff. A minordevelopment of coastal fronts and the decrease of the intensity ofhaloclines might weaken the overall cyclonic circulation, whichoriginates here by the combination of topographic constrains andbaroclinic gradients (Mala�ci�c and Petelin, 2009). During the period1991e2003, increases of temperature (þ0.2 �C yr�1) and salinity(þ0.3 psu yr�1) in the surface waters were observed in the gulf,particularly during summer (Mala�ci�c et al., 2006). Once thesetrends weremainly the result of the extreme conditions occurred in2003, they suggest that the future evolution of the hydrology of thegulf will be substantially modulated by the combination betweenannual oscillations of runoff in the surrounding continental regionand long term climatic changes, which are in turn dominated by theoverall atmospheric warming.

The presence of haloclines that segregates low-salinity upperwaters enriched of riverine nutrients favours diatom-dominatedphytoplankton blooms in late winter and autumn (Malej et al.,1995; Cantoni et al., 2003). During summer, a persistent retentionof low-salinity waters regulates the sedimentation of the biomassand it may lead to hypoxic conditions in the deeper waters, inparticular when the deeper pycnocline reduce the volume of theunderlying bottom layer (Faganeli and Ogrinc, 2009). For thesereasons, changes of the dynamics of haloclines might directly affectstanding stocks and structure of plankton communities in this area(Mozeti�c et al., 2012).

Finally, the northern sandy coast of the gulf shows amorphologythat is significantly modulated by the pattern of sedimentation ofthe solid transport carried by Isonzo River. An annual averagedsolid transport of 150 g m�3, was estimated in the past for IsonzoRiver, with peaks up to 1000 g m�3 during the most extremefreshets (Mosetti, 1983). Currently, Isonzo mouth may be consid-ered a micro tidal, low-energy and fine-grained deltaic system. Itspattern of sediment deposition follows the marked stratification ofthe water column in the estuary, during the prevailing periods ofmedium-low flows, whereas a turbulent mixing and a larger

mM) TN (mM) TP (mM) TN/TP (molar) DIN/PO4 (molar) Si/DIN (molar)

154 0.4 405 336 0.8123 0.6 199 418 0.5203 1.2 173 245 0.5515 2.6 171 240 0.5300 5.1 49 37 1.0292 1.6 139 141 0.8

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

Fig. 6. Total river transport in the Gulf of Trieste of (a) nitrogen, (b) phosphorus and (c)reactive silicon.

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e118

transport of fine material over the external delta bar crest prevailsduring short peaks of discharge (Covelli et al., 2004). These char-acteristics indicate that delta and shape of the coast in this areamight easily change as a consequence of a persistent alteration ofriver outflows.

4.2. Seasonal cycle of runoff and climate forcings

The region surrounding the gulf of Trieste is an area rich ofcontinental waters and characterised by a high annual precipitation(average of 1371 mm yr�1 in 1998e2008). However, periods ofextremely low precipitation may easily occur in winter(6 mm month�1), as well as spring and summer (11 mm month�1;Fig. 3). Because of the scarce extension of drainage basins, shortterm oscillations of precipitation cause here pronounced riverpulses and droughts, which lead to an extreme variability of thetotal freshwater load in the gulf (29e2100$103 m3 month�1). Theimportance of the precipitation is confirmed by its direct rela-tionship with the total runoff in the gulf over all the seasons(Fig. 4a). Snowmelt in FebruaryeMarch may significantlycontribute to the river flow during the years characterised by a highaccumulation of snow mass, like in 2000, 2004 and 2006 (Fig. 3).However, the recent years have been frequently characterised bya scarce presence of snow in the Eastern Alps and a further decrease

Table 4Contributions of northern and southern rivers to the total transport of TN, TP andSi(OH)4 in the gulf.

Year TN (%) TP (%) Si(OH)4 (%)

N rivers S rivers N rivers S rivers N rivers S rivers

2000 95.3 4.7 96.3 3.7 95.4 4.62001 94.3 5.7 84.8 15.2 94.0 6.02002 94.4 5.6 88.5 11.5 91.4 8.62003 93.0 7.0 84.9 15.1 93.1 6.92004 88.4 11.6 92.7 7.3 95.3 4.72005 90.8 9.2 89.6 10.4 94.1 5.92006 87.3 12.7 76.1 23.9 89.5 10.52007 91.4 8.6 81.9 18.1 91.8 8.2

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

of its contribution has to be expected as a result of the climatewarming in the region (Alcamo et al., 2007). Our study has alsoshown the predominance of natural forcings to determine thecurrent seasonal cycle of runoff in the Gulf of Trieste. This is not anobvious characteristics of this area due to the numerous regulationsystems of the drainage network that have been built during thelast century. For example, the dams of Doblar, Plave and Solkanwere built in the drainage basin of Isonzo River to exploit itshydropower potential in 1939, 1940 and 1984, respectively. On theother hand, it should be further considered that the northernAdriatic is one of the few sites in the Mediterranean where densewaters are formed (Artegiani et al., 1997; Mala�ci�c and Petelin,2001). Strong cooling periods during autumn and winter, due toa persistent Bora wind, coupled with a scarce runoff on the conti-nental shelf may increase the southward fluxes of dense watersthrough the Adriatic propagating their biogeochemical signal towider regions (Boldrin et al., 2009).

The analysis of extremes indicate that, during the year with highflows, peaks of discharge of all rivers are basically in phase, whereasperiods of water scarcity occur only in the minor rivers. During theyears characterised by low flows, the period of water scarcity arecommon to all rivers, but the minor ones can discharge highquantities of water even in the absence of a similar behaviour ofIsonzo (Fig. 5). These differences make more complex the assess-ment of the overall freshwater advection in the area.

NAOi and several other primary teleconnection indices havebeen applied in the northern hemisphere of the Earth, as a tool forthe prediction of the prominent mode of low-frequency oscillationsof large scale climate patterns and for the implementation ofclimate models. These indices have been used to analyse theoccurrence of extreme meteorological events in Europe and in theMediterranean basin, such as heavy precipitation and heatwaves(Scaife et al., 2008; Izaguirre et al., 2010). However, the linkagebetween primary NHTi and a significant remaining fraction of thevariability of regional meteorological conditions is still not wellidentified, in particular concerning the patterns of precipitation inseveral areas of the Mediterranean. This feature has suggested theneed to define other regional teleconnection indices more suitableto catch these climatic trends at smaller scales (Brunetti et al.,2002; Vicente-Serrano et al., 2009).

It was already shown that, in the case of the northern Adriaticregion, the significance of NAOi as an index of the precipitation ismainly restricted to the winter period. However, this is a poorlyraining season, in which an increased occurrence of anticyclonessince the 1980’s should further induce less frequent wet days andwider ranges of daily temperatures in the future (Brunetti et al.,2002). The analysis of the interannual trends of regional indexeslike WMOi and MOi has demonstrated their better consistency toexplain the decreasing precipitation trends in northern Italy(Piervitali et al., 1999). Similarly, WMOi has showed in the presentstudy the highest overall correlation with the anomalies ofprecipitation (r2 up to 0.76) and river flows (r2 up to 0.92) in theGulf of Trieste for the period 1998e2008. In themiddle Adriatic, thefluctuation of salinity in the intermediate layer due to an enhancedinflowof Levantine IntermediateWater was recognised to be linkedto atmospheric pressure fields characterised by higher values overthe mid-northern Atlantic and lower values over the Mediterra-nean, which were more consistent with the trend of MOi ratherthan of NAOi (Grbec et al., 2003). All these findings suggest thatoceanographic properties and hydrological conditions in the Adri-atic Sea are better correlated to these regional teleconnectionindices rather than to NAOi. Therefore, their significance should bebetter deepened in future ecological studies addressed to theassessment of climatic effects on the evolution of this marineecosystem.

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e11 9

On the whole, model forecast and projection of NHTi has sug-gested in the area of study a future increase of evapotranspirationdue to air warming, an overall decrease of precipitation, in partic-ular during summer, as well as a reduction of snow cover over theAlps (Alcamo et al., 2007; Frantar, 2007). All these processes mightcause significant modifications of the seasonal cycle of freshwaterand a long term reduction of its availability in the regionsurrounding the Gulf of Trieste.

4.3. River transport of nutrients and eutrophication

During the period 2000e2007, river transport in the north-eastern Adriatic has reached a maximum of 13,200 t N yr�1 for TN,86 t P yr�1 for TP and 12,400 t Si yr�1 for Si(OH)4, but a decrease of60e70% of these values was experienced by this marine ecosystemduring the water crisis in 2003e2007 (Fig. 6). Our analysis showedthat the variability of river nutrient loads is mainly linked to therunoff that, in turn, varies in dependence on the precipitation overthe region surrounding the gulf. This finding indicates that theconcomitant presence of point and diffuse sources of nutrientslimit the temporal oscillations of their concentration in riverwaters, whereas the significant differences of concentrationobserved among the rivers has to be mostly ascribed to differentlevels of anthropogenic pressure acting in each drainage basin(Table 3). The important effect of precipitation regime on thetransport of river nutrients further indicates that ongoing climatechanges might still overtake the effects of human activities indetermining the future level of nutrient supply in this coastalzone.

A higher contribution of the minor rivers to the total transportwas observed during the driest years, with an increase two timeshigher for TP (3.7e23.9%) than for TN (4.7e12.7%) and Si(OH)4(4.6e10.5%). These data indicate that recurrent freshwatershortages might reduce the river supply of nitrogen and siliconmore than that of phosphorus, making the ratios of land-bornenutrients discharged in the marine environment less unbalancedwith respect to the years characterised by high flows.

Long term trends of river transport may be also inferred for thiscoastal zone through the analysis of published data. A total trans-port of 2920 t N yr�1 for TN and 548 t P yr�1 for TP was estimated inthe Gulf of Trieste by Olivotti et al. (1986), of which the largestcontribution being attributed to sewage systems (60% for N and 80%for P). Since the end of the 1970’s, average concentration of DIN inIsonzo River waters has strongly increased (60.0e90.2 mM N),because of the increase of NO3 concentration. The concentrationof Si(OH)4 (35.2e72.1 mM Si) has also increased, whereas the levelof PO4 has been basically stable (0.41e0.30 mM P; Bregant andCatalano, 1978). During the same three decades, the concentra-tion of DIN in the waters of Ri�zana (31.6e131.8 mMN) and Dragonja(55.0e191.5 mM N) rivers has showed even greater increases,always in the presence of constant PO4 emissions (Faganeli andTu�snik, 1983; Turk et al., 2007). The stability of PO4 transportover the last three decades is a common feature for the northernAdriatic rivers, which has mainly been the result of the reduction ofphosphate in the detergents introduced by Italian regulation in1986 (Cozzi and Giani, 2011). However, this process has caused anoverall progressive unbalance of N and P loads, with increases ofDIN/PO4 ratios from 69-149 to 141-336 over the period also in theGulf of Trieste.

These trends indicate that this coastal zone has been subjectedto a growing anthropogenic pressure due to overloads of nitrogenand reactive silicon and that only a deep phosphorus deficiencycurrently prevents its severe eutrophication. By contrast, theoccurrence of recent periods of low runoff has temporarilydecreased the load of nutrients to values similar to those estimated

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

in the past. This strong reduction had important consequences onthe trophic level of this marine environment. Recently, a reductionof the concentration of PO4 and NH4 has been reported in coastalareas of the northern Adriatic during the last 30 years (Solidoroet al., 2009), while a decrease of chlorophyll a concentration wasobserved both in the northern Adriatic and in the Gulf of Triesteafter 2000 (Mozeti�c et al., 2010). In the ecosystem of the Gulf ofTrieste, the last decade has also been characterised by regime shiftsleading to a greater prevalence throughout the year of smaller sizedphytoplankton and to a decrease of the largest seasonal diatomblooms, with a possible increase of the importance of regeneratedproduction (Mozeti�c et al., 2012). These changes indicated the basicrole of bottom-up control of nutrient availability on primaryproducers, as well as that the state of this coastal zone mightdefinitively change from mesotrophic to oligotrophic conditions incase of prevailing future periods of low runoff.

Besides the long term changes of river transport, the currentweight of the other sources of continental nutrients should bebetter estimated as, similarly to other coastal zones in industri-alised countries, they could increase their importance in case ofa permanent reduction of river flows.

Several points of discharge of sewages exist in the gulf, throughdirect emissions along the coastline and through underwaterpipelines that release the wastewaters in the bottom layer of itscentral area. The major Italian sewage disposal plant collectsdomestic wastewaters and runoff of the city of Trieste (z220,000Equivalent Inhabitants) and discharges them, after a primarytreatment, through a pipeline of 7.5 km in length. Its water flow(z1.39 m3 s�1) corresponds to z2% of river runoff, but the meanloads of DIN (1.79 t N d�1) and PO4 (0.13 t P d�1) may constituterespectively 12% and 31% of the transport by Isonzo River (Cozziet al., 2008). Along the Slovenian coast, sewage plants collectdomestic wastewaters and runoff of the cities of Piran, Izola andKoper (z80,000 Equivalent Inhabitants) and discharge on theaverage 0.25 m3 s�1 of wastewater and 0.74 t N d�1 and 0.10 t P d�1

of TN and TP, respectively (Turk et al., 2007). A comprehensiveestimate of the load of nutrients carried by sewage systems is stilllacking for this coastal zone. However, the available studies suggestthat these anthropogenic inputs are not negligible for the balanceof phosphorus and that they might become important even fornitrogen in the case of reduced river flows. Moreover, sewagedischarges have a different dynamics with respect to the advectionof river waters, as they mainly impact the bottom waters and arecharacterised by high concentrations of NH4 and urea instead ofNO3. For this reason, a different nutrient cycling induced by theirpreponderance in the area might have implications even on theplankton community structure.

Little information exists on the role of atmospheric depositionof nutrients in Adriatic coastal zones, although their concentra-tions in rainwater is comparable to those of the other continentalwaters because of the large anthropogenic emissions fromurbanised and industrialised areas (Faganeli and Tu�snik, 1983;Malej et al., 1997). During summer 1993, average concentrations of116.80 mM N, 3.82 mM P and 2.97 mM Si were measured in rain-water for TN, TP and Si(OH)4 in the southern zone of the Gulf ofTrieste (Malej et al., 1997). If these data would be applied to thetotal considered area, they would correspond to mean inputs of1.16 t N d�1, 0.08 t P d�1 and 0.07 t Si d�1 respectively. Bycomparing these values with daily river transports during recentsummers characterised by similar flows (2000e2002), it may beinferred that atmospheric deposition may be negligible for TN(z8% of river load) and Si(OH)4 (z0.5%), but not for TP (z60%).Moreover, it should be noticed that the atmospheric deposition isan episodic process that may strongly increase during the rainyperiods in early spring and autumn.

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e1110

Finally, the exchange of nutrients between sediments andbottomwaters is another important component of their balance inthe coastal zones, as high productivity and shallowness of thesemarine environments tend to increase the fraction of primaryproduction that settles on the seafloor (15e20%) and that issusceptible to be remineralised (Faganeli and Ogrinc, 2009).Despite benthic fluxes of nutrients highly vary in this continentalshelf area, depending on sediment characteristics and oxygenlevels, a general behaviour was already inferred. The most impor-tant fluxes of nutrients from the sediments to the water column areconstituted by NH4 (annual average of 0.8 � 0.7 mmol N m�2 d�1)and Si(OH)4 (2.59 � 2.3 mmol N m�2 d�1), both in oxic and anoxicconditions. The flux of NO3 is lower (0.17 � 0.73 mmol N m�2 d�1)and it may be reversed under anoxia. The release of PO4 isabout one order of magnitude lower of those of N-nutrients(0.029 � 0.05 mmol P m�2 d�1; Bertuzzi et al., 1997). If thesedata, that refer to a site in the centre of the gulf, would be extendedto its total area, annual averaged fluxes of 7 t N d�1, 0.5 t P d�1

and 40 t Si d�1 would be obtained for DIN, PO4 and Si(OH)4,respectively.

On the whole, the comparison between data of runoff presentedin this study and available past studies has suggested that ifa progressive reduction of nutrient transport by rivers will takeplace in the northeastern Adriatic during the next decades,nitrogen and silicon overloads could be substantially reduced,whereas phosphorus balance could be less affected by this alter-ation due to a greater importance of the other non-riverine externalsources of this biogenic element.

4.4. Flushing times and nutrient budgets in the gulf

The impact of rivers on this coastal zone mainly depends on thecirculation, which is an essential component for the flushing ofstanding stocks of biomass, ventilation of deeper waters andremoval of pollutants, through the renewal of upper and deeperwaters (Malej et al., 1995; Cozzi et al., 2008; Faganeli and Ogrinc,2009). The volume of freshwater discharged by rivers is negli-gible when compared to the total volume of the Gulf of Trieste,being the values of sFWz20months in the years of high regime andz50months in those of low regime (Table 5). However, it should beconsidered that the advection of river waters mostly affects anupper layer of 5 m of depth (z30% of the total volume of the gulf),which can accumulate freshwater until to reach contents as high as5% in its northern area. For this reason, flushing times of surfacelow-salinity waters may be significantly lower (1e23 d), in partic-ular when an active circulation is induced by strong winds (Cantoniet al., 2003).

By contrast, the transport of river nutrients emerges as a basiccomponent of the biogeochemistry of this coastal zone. Riversupply of DIN and Si(OH)4might replace thewhole budgets of thesenutrients in the gulf in 0.5e0.9 months, during the years with highrunoff and 1.0e2.6 months during driest years. The transport of TNis also high (sTN ¼ 1.4e4.6 months), in spite of the large fraction of

Table 5Flushing time of river water (sFW) and turnover time of nutrients (sNUT) calculatedwith respect to total volume and nutrient budgets in the Gulf of Trieste.

Year: 2000 2001 2002 2003 2004 2005 2006 2007

sFW (month) 17.2 20.7 30.3 54.3 27.7 47.0 44.1 57.9sDIN (month) 0.5 0.7 0.7 1.0 0.7 1.3 1.3 1.5sTN (month) 1.4 1.7 2.5 3.6 2.4 4.5 3.5 4.6sPO4 (month) 3.0 5.3 5.7 9.0 6.0 6.6 9.4 10.8sTP (month) 23.0 26.9 37.2 60.2 35.7 40.0 45.0 58.3sSi(OH)4 (month) 0.7 0.9 1.3 2.4 1.2 2.1 2.0 2.6

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

dissolved and particulate organic nitrogen that is of marine originin seawater. River transport of phosphorus is significantly lowerboth for inorganic (sPO4 ¼ 3.0e10.8 months) and total(sTP ¼ 23.0e60.2 months) pools, as expected in an area that iscurrently under conditions of strong phosphorus deficiency. Thesedata confirm that, although the rivers are still the major allochth-onous source of phosphorus, its availability for productionprocesses strongly depends on the contributions of other conti-nental and atmospheric inputs of anthropogenic origin, as well asby its recycling in the pelagic and benthic compartments. Finally,the pronounced oscillations of turnover times of river nutrientsobserved in the period 2000e2007 indicated that the runoff ispotentially able to cause substantial interannual changes of thebiogeochemical characteristics of the northeastern Adriatic conti-nental shelf, including structure and productivity of this marineecosystem. This variability should be further studied, in order tobetter evaluate if this recent water crisis may be basically consid-ered a temporary ecosystemic oscillation or a phase consistent witha longer trend.

5. Conclusions

River loads constitute the major allochthonous source of fresh-water and nutrients in the Gulf of Trieste, at a level that is currentlyable to deeply modulate hydrology, biogeochemistry and produc-tivity of this coastal zone. During the last decades, river transporthas been characterised by a constant increase of nitrogen andsilicon emissions of anthropogenic origin, which has not causeda more severe eutrophication of this coastal ecosystem mostly dueto a reduced emission of phosphorus. However, water crisis inSouthern Europe originated by recent multi-year climatic oscilla-tions has shown the potential capacity of climate changes toreverse the common condition of growing eutrophication of thecoastal zones in industrialised countries. The strong reduction offreshwater loads and transport of nutrients by the rivers observedduring the period 2003e2007 has induced temporary changes ofthermo-haline conditions as well as a reduction of the standingstocks of biomass that might anticipate a future permanent oligo-trophication of this area.

The great exposure of river flows to the regime of precipitationin the continental region surrounding the gulf, which will be easilysubjected to a future overall reduction and to a tropicalisation of itsseasonal cycle, appears to be one of the more important mecha-nisms through which the climate changes may affect the trophy ofthis coastal marine ecosystem. For these reasons, teleconnectionindices more suitable to catch the meteorological variability ata regional scale and more detailed climate models should beimplemented, in order to test the implications that these envi-ronmental changes might have not only on hydrological andchemical characteristics of this coastal environment, but also onmarine biota and ecosystem structure. Moreover, the possibledecrease of river flows in this area will potentially increase theimpact of other non-riverine anthropogenic emissions. This findingpoints out the need for a better assessment of the possible socio-economic costs and of the challenges to the sustainability of thehuman activities that might derive in this area from the reductionof freshwater resources.

Acknowledgements

The authors wish to thank ARPA of Friuli Venezia Giulia Region,ACEGAS-APS s.p.a., ARSO and European Environment Agency forthe data of flow rates and nutrients in river waters. Particularthanks are due to dr. Sergio Predonzani (ARPA) for the statistics ofnutrients in seawater. The analysis of meteorological data in the

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005

S. Cozzi et al. / Estuarine, Coastal and Shelf Science xxx (2012) 1e11 11

area of study was produced with the Giovanni online data system,developed and maintained by the NASA GES DISC.

References

Alcamo, J., Moreno, J.M., Nováky, B., Bindi, M., Corobov, R., Devoy, R.J.N.,Giannakopoulos, C., Martin, E., Olesen, J.E., Shvidenko, A., 2007. Europe. In:Parry, M.L., Canziani, O.F., Palutikof, J.P., van der Linden, P.J., Hanson, C.E. (Eds.),Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution ofWorking Group II to the Fourth Assessment Report of the IntergovernmentalPanel on Climate Change. Cambridge University Press, Cambridge, UK,pp. 541e580.

Aleffi, F., Orel, G., Del Piero, D., Vio, E., 1992. Oxygen conditions in the gulf of trieste(High Adriatic). In: Vollenweider, R.A., Marchetti, R., Viviani, R. (Eds.), MarineCoastal Eutrophication. Elsevier, pp. 431e440.

Artegiani, A., Bregant, D., Paschini, E., Pinardi, N., Raicich, F., Russo, A., 1997. TheAdriatic sea general circulation. Part I: air-sea interactions and water massstructure. Journal of Physical Oceanography 27, 1492e1514.

Bertuzzi, A., Faganeli, J., Welker, C., Brambati, A., 1997. Benthic fluxes of dissolvedinorganic carbon, nutrients and oxygen in the Gulf of Trieste (Northern Adri-atic). Water, Air and Soil Pollution 99, 305e314.

Billen, G., Garnier, J., 2007. River basin nutrient delivery to the coastal sea: assessingits potential to sustain new production of non-siliceous algae. Marine Chem-istry 106, 148e160.

Boldrin, A., Carniel, S., Giani, M., Marini, M., Bernardi Aubry, F., Campanelli, A.,Grilli, F., Russo, A., 2009. Effects of bora wind on physical and biogeochemicalproperties of stratified waters in the northern Adriatic. Journal of GeophysicalResearch 114, C08S92. doi:10.1029/2008JC004837.

Brachya, V., Juhasz, F., Pavasovic, A., Trumbic, I., 1994. Guidelines for IntegratedManagement of Coastal and Marine Areas with Special Reference to theMediterranean Basin. Mediterranean Action Plan - United Nations EnvironmentProgramme, Split, Croatia, p. 80.

Bregant, D., Catalano, G., 1978. Condizioni chimiche e fisiche alla foce del fiumeIsonzo. CNR - Istituto Talassografico di Trieste, pubbl. n. 545, p. 12.

Brunetti, M., Maugeri, M., Nanni, T., 2002. Atmospheric circulation and precipi-tation in Italy for the last 50 years. International Journal of Climatology 22,1455e1471.

Cantoni, C., Cozzi, S., Pecchiar, I., Cabrini, M., Mozeti�c, P., Catalano, G., FondaUmani, S., 2003. Short-term variability of primary production and inorganicnitrogen uptake related to the environmental conditions in a shallow coastalarea (Gulf of Trieste, N Adriatic Sea). Oceanologica Acta 26, 565e575.

Clesceri, L.S., Greenberg, A.E., Eaton, A.D., 1998. Standard Methods for the Exami-nation of Water and Wastewater, twentieth ed. APHA (American Public HealthAssociation), AWWA (American Water Works Association), WPCF (WaterEnvironment Federation Publication), United Book Press, Inc, Baltimore,Maryland, ISBN 0-87553-235-7.

Comici, C., Bussani, A., 2007. Analysis of the River Isonzo discharge (1998e2005).Bollettino di Geofisica Teorica ed Applicata 48, 435e454.

Covelli, S., Piani, R., Faganeli, J., Brambati, A., 2004. Circulation and suspendedmatter distribution in a microtidal deltaic system: the Isonzo river mouth(northern Adriatic Sea). Special Issue. Journal of Coastal Research 41, 130e140.

Cozzi, S., Adami, G., Barbieri, P., Cantoni, C., Catalano, G., Crisciani, F., Fiorotto, V.,Olivo, P., Purini, R., Raicich, F., Reisenhofer, E., 2004. Matching monitoringand modelling in the gulf of trieste (Italy). Marine Pollution Bulletin 48,587e592.

Cozzi, S., Giani, M., 2011. River water and nutrient discharges in the NorthernAdriatic Sea: current importance and long term changes. Continental ShelfResearch 31, 1881e1893.

Cozzi, S., Reisenhofer, E., Di Monte, L., Cantoni, C., Adami, G., 2008. Effect of envi-ronmental forcing on the fate of nutrients, dissolved organic matter and heavymetals released by a coastal wastewater pipeline. Chemistry and Ecology 24,87e107.

Cressie, N.A.C., 1990. The origins of Kriging. Mathematical Geology 22, 239e252.Degobbis, D., Precali, R., Ivan�ci�c, I., Smodlaka, N., Fuks, D., Kvedere, S., 2000. Long-

term changes in the northern Adriatic ecosystem related to anthropogeniceutrophication. International Journal of Environment and Pollution 13,495e533.

Faganeli, J., Av�cin, A., Fanuko, N., Malej, A., Turk, V., Tu�snik, P., Vri�ser, B., Vukovi�c, A.,1985. Bottom layer anoxia in the central part of the gulf of Trieste in the latesummer of 1983. Marine Pollution Bulletin 16, 75e78.

Faganeli, J., Ogrinc, N., 2009. Oxiceanoxic transition of benthic fluxes from thecoastal marine environment (Gulf of Trieste, northern Adriatic Sea). Marine andFreshwater Research 60, 700e711.

Faganeli, J., Tu�snik, P., 1983. Carbon, nitrogen, silicon and phosphorus nutrients inthe eastern part of the Gulf of Trieste (Northern Adriatic). Acta Adriatica 24,25e41.

Fonda Umani, S., Del Negro, P., Larato, C., De Vittor, C., Cabrini, M., Celio, M.,Falconi, C., Tamberlich, F., Azam, F., 2007. Major inter-annual variations inmicrobial dynamics in the Gulf of Trieste (northern Adriatic Sea) and theirecosystem implications. Aquatic Microbial Ecology 46, 163e175.

Frantar, P., 2007. Geographical overview of water balance of Slovenia 1971-2000 bymain river basins. Acta Geographica Slovenica 47, 25e45.

Please cite this article in press as: Cozzi, S., et al., Recent evolution of river dchanges and anthropogenic pressure, Estuarine, Coastal and Shelf Scienc

Frantar, P., 2008. Water Balance of Slovenia 1971e2000. Ministry for Environmentand Spatial Planning, Environmental Agency of the Republic of Slovenia,Ljubljana, ISBN 978-961-6024-38-9, p. 119.

Grasshoff, K., Kremling, K., Ehrhardt, M., 1999. Methods of Seawater Analysis, Third,completely revised and extended ed. Wiley-VCH Verlag GmbH, Weinheim, p.600.

Grbec, B., Morovi�c, M., Zore-Armanda, M., 2003. Mediterranean oscillation and itsrelationship to salinity fluctuation in the Adriatic Sea. Acta Adriatica 44, 61e76.

Izaguirre, C., Mendez, F.J., Menendez, M., Luceno, A., Losada, I.J., 2010. Extreme waveclimate variability in southern Europe using satellite data. Journal ofGeophysical Research 115, C04009. doi:10.1029/2009JC005802.

Kempe, S., Pettine, M., Cauwet, G., 1991. Biogeochemistry of European rivers. In:Degens, E.T., Kempe, S., Richey, J.E. (Eds.), Biogeochemistry of Major WorldRivers. SCOPE, ICSU, UNEP, John Wiley & Sons Ltd, Chichester, pp. 169e211.

Ludwig, W., Dumont, E., Meybeck, M., Heussner, S., 2009. River discharges of waterand nutrients to the Mediterranean and Black Sea: major drivers for ecosystemchanges during past and future decades? Progress in Oceanography 80,199e217.

Mala�ci�c, V., Celio, M., Cermelj, B., Bussani, A., Comici, C., 2006. Interannual evolutionof seasonal thermohaline properties in the Gulf of Trieste (northern Adriatic)1991e2003. Journal of Geophysical Research 111, C08009. doi:10.1029/2005JC003267.

Mala�ci�c, V., Petelin, B., 2001. Regional studies, gulf of trieste. In: Cushman-Roisin, B.,Ga�ci�c, M., Poulain, P.M., Artegiani, A. (Eds.), Physical Oceanography of theAdriatic Sea. Past, Present and Future. Kluwer Accademic Publishers, Dordrecht,Boston, London, pp. 167e181.

Mala�ci�c, V., Petelin, B., 2009. Climatic circulation in the gulf of trieste (northernAdriatic). Journal of Geophysical Research 114, C07002. doi:10.1029/2008JC004904.

Malej, A., Mozeti�c, P., Mala�ci�c, V., Terzi�c, S., Ahel, M., 1995. Phytoplankton responsesto freshwater inputs in a small semi-enclosed gulf (Gulf of Trieste, Adriatic Sea).Marine Ecology Progress Series 120, 111e121.

Malej, A., Mozeti�c, P., Mala�ci�c, V., Turk, V., 1997. Response of summer phytoplanktonto episodic meteorological events (Gulf of Trieste, Adriatic Sea). P.S.Z.N.: MarineEcology 18, 273e288.

Malone, T.C., Malej, A., Harding Jr., L.W., Smodlaka, N., Turner, R.E., 1999. Ecosystemsat the Land-Sea margin - Drainage basin to coastal sea. Coastal and EstuarineStudies, American Geophysical Union 55, 381. Washington DC.

Mosetti, F., 1983. Sintesi sull’idrologia del Friuli-Venezia Giulia. Quaderni Dell’EnteTutela Pesca 6, 295.

Mozeti�c, P., Francé, J., Kogov�sek, T., Talaber, I., Malej, A., 2012. Plankton trends andcommunity changes in a coastal sea (northern Adriatic): bottom-up vs. top-down control in relation to environmental drivers. Estuarine, Coastal andShelf Science. doi:10.1016/j.ecss.2012.02.009.

Mozeti�c, P., Mala�ci�c, V., Turk, V., 2008. A case study of sewage discharge in theshallow coastal area of the Northern Adriatic Sea (Gulf of Trieste). MarineEcology 29, 483e494.

Mozeti�c, P., Solidoro, C., Cossarini, G., Socal, G., Precali, R., Francé, J., Bianchi, F., DeVittor, C., Smodlaka, N., Fonda Umani, S., 2010. Recent trends towards oligo-trophication of the Northern Adriatic: evidence from Chlorophyll a time series.Estuaries and Coasts 33, 362e375.

Olivotti, R., Faganeli, J., Malej, A., 1986. Impact of “organic” pollutants on coastalwaters, Gulf of Trieste. Water Science and Technology 18, 57e68.

Palmeri, L., Bendoricchio, G., Artioli, Y., 2005. Modelling nutrient emissions fromriver systems and loads to the coastal zone: Po River case study, Italy. EcologicalModelling 184, 37e53.

Piervitali, E., Conte, M., Colacino, M., 1999. Rainfall over the central-WesternMediterranean basin in the period, 1951-1995. II. Precipitation scenarios.Nuovo Cimento 22C, 649e661.

Rajar, R., �Cetina, M., 1991. Modelling of tidal and wind-induced currents anddispersion in the northern Adriatic. Acta Adriatica 32, 785e812.

Scaife, A.A., Folland, C.K., Alexander, L.V., Moberg, A., Knight, J.R., 2008. Europeanclimate extremes and the North Atlantic oscillation. Journal of Climate 21,72e83.

Scroccaro, I., Ostoich, M., Umgiesser, G., De Pascalis, F., Colugnati, L., Mattassi, G.,Vazzoler, M., Cuomo, M., 2010. Submarine wastewater discharges: dispersionmodelling in the Northern Adriatic Sea. Environmental Science and PollutionResearch 17, 844e855.

Seitzinger, S.P., Mayorga, E., Bouwman, A.F., Kroeze, C., Beusen, A.H.W., Billen, G.,Van Drecht, G., Dumont, E., Fekete, B.M., Garnier, J., Harrison, J.A., 2010. Globalriver nutrient export: a scenario analysis of past and future trends. GlobalBiogeochemical Cycles 24, GB0A08. doi:10.1029/2009GB003587.

Solidoro, C., Bastianini, M., Bandelj, V., Codermatz, R., Cossarini, G., Melaku Canu, D.,Ravagnan, E., Salon, S., Trevisani, S., 2009. Current state, scales of variability, andtrends of biogeochemical properties in the northern Adriatic Sea. Journal ofGeophysical Research 114, C07S91. doi:10.1029/2008JC004838.

Turk, V., Mozeti�c, P., Malej, A., 2007. Overview of eutrophication-related events andother irregular episodes in Slovenian sea (Gulf of Trieste, Adriatic Sea).ANNALES Series historia naturalis 17, 197e216.

Vicente-Serrano, S.M., Beguería, S., López-Moreno, J.I., El Kenawy, A.M., Angulo-Martínez, M., 2009. Daily atmospheric circulation events and extreme precip-itation risk in northeast Spain: role of the North Atlantic oscillation, theWestern Mediterranean oscillation, and the Mediterranean oscillation. Journalof Geophysical Research 114, D08106. doi:10.1029/2008JD011492.

ischarges in the Gulf of Trieste and their potential response to climatee (2012), doi:10.1016/j.ecss.2012.03.005