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Ocean & Coastal Management 48 (2005) 828–842

0964-5691/$ -

doi:10.1016/j

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Changes in macrophytes distribution in ahypersaline coastal lagoon associated with thedevelopment of intensively irrigated agriculture

Javier Lloret�, Arnaldo Marin, Lazaro Marin-Guirao,Josefa Velasco

Departamento de Ecologia e Hidrologia, Universidad de Murcia, 30100 Murcia, Spain

Available online 31 August 2005

Abstract

In the Mar Menor lagoon (SE Spain) recent changes in the nearby agricultural practices,

from extensive dry crop farming to intensively irrigated crops, have increased nutrient and

particulate inputs through the principal watercourse that flows into the lagoon, the Albujon

wadi. The concentration of nutrients and suspended sediments in the waters has risen,

increasing phytoplankton densities and diminishing water column transparency. As a result of

these environmental changes, the traditional main macrophyte of the Mar Menor, Cymodocea

nodosa (Ucria) Ascherson, has been replaced by the macroalga Caulerpa prolifera (Forsskal)

Lamouroux creating changes in the sediment characteristics with profound consequences to

benthic communities and also to local fishing and tourism. In the study area, seagrass

respiration exceeded photosynthesis while the macroalga presented positive rates of net

photosynthesis. C. prolifera is better adapted to these new conditions of light and nutrient

concentration and has expanded its distribution, occupying most of the bottom of the lagoon

and restricting C. nodosa to small patches in very shallow areas, where plant growth is not

limited by light and no C. prolifera can be found because of photoinhibition and emersion

stress.

r 2005 Elsevier Ltd. All rights reserved.

see front matter r 2005 Elsevier Ltd. All rights reserved.

.ocecoaman.2005.07.002

nding author. Tel.: +34968 36 4977; fax: +34 968 36 3963.

dress: jlloret@um.es (J. Lloret).

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J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842 829

1. Introduction

The decline of seagrasses worldwide has been attributed to deterioratinghabitat conditions [1,2]. Nutrient enrichment may promote phytoplanktongrowth, which increases turbidity, while the runoff of suspended sedimentsand dissolved substances further increases light attenuation in the watercolumn, especially in estuarine areas [3,4]. Decreased light availability can haveadverse effects on seagrass photosynthesis, community structure and ultimatelylong-term survival. Survival of submersed plants depends on the interactionof many biotic and abiotic factors that affect the balance between productionand respiration [5,6]. Decreased light availability constituted one of theprincipal factors that affect this balance. Increased nutrient delivery into estuarinewaters also stimulates the growth of opportunistic macroalgae, which usually havelower light requirements and constitute more competitive species under these newconditions of the environment. In this sense, seagrass loss and algal overgrowth inestuarine areas may have important implications for ecosystem production,dissolved oxygen concentration, carbon and nutrient cycling, and trophic linkagesto adjacent systems [2,7].

Due to their shallow depth and relatively weak water exchange with the open sea,lagoons are amongst the marine environments most sensitive to eutrophication.Changes in the composition and distribution of benthic flora and fauna species as aresult of eutrophication processes have been reported before in several studiescarried out in coastal lagoons and other estuarine and coastal environments [8,9].These changes may result in the loss of biological diversity of these coastalenvironments but also in profound consequences to human activities in the area,including fisheries or tourism.

The Mar Menor, a hypersaline coastal lagoon located in the southeastSpain, is one of the largest coastal lagoons of the Mediterranean coast.Several ephemeral watercourses flow into the lagoon but only after episodicrainfall events. These wadis are wide, shallow gullies, which are generallyinactive, but carry great quantities of water and sediment during floodepisodes. The torrential nature of the supplies is aggravated by the impermeablesoils and the scarce vegetation cover of the headwaters in the watershed areas. TheAlbujon wadi is the principal watercourse that flows into the Mar Menor lagoon,draining a surface of 441 km2, about one-third of the total surface of the adjacentCampo de Cartagena, one of the main horticultural areas of Europe (Fig. 1). Duringthe last 2 decades, agriculture in the watershed has changed from extensive dry cropfarming to intensively irrigated crops, using water from the Tajo-Segura riverdiversion. These changes have led to a rise in phreatic levels, since the over-exploitation of groundwater decreased. As a result, some watercourses, such as theAlbujon wadi, now maintain a regular flux, fed by ground water with high nitratelevels [10].

Accompanying this progressive increase in inputs into the Mar Menor thedistribution of the benthic macrophytes of the lagoon has changed. Historically, themain macrophyte was the phanerogam Cymodocea nodosa (Ucria) Ascherson.

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Fig. 1. Study area: (a) General location, (b) Campo de Cartagena and the Albujon watershed (shaded),

and (c) Mar Menor lagoon and sampling stations.

J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842830

During the early 1980s, the bottom was covered by a mixed meadow of C. nodosa

and the macroalga Caulerpa prolifera (Forsskal) Lamouroux [11,12]. At the presenttime, a dense monospecific bed of C. prolifera covers most of the bottom of thelagoon, and the distribution of C. nodosa is restricted to very small patches in theshallowest areas (Fig. 2). As occurs in other Mediterranean coastal lagoons withrecent problems of nutrient enrichment, seagrasses have been progressivelysubstituted by opportunistic algae [8–10]. However, the Mar Menor lagoon differssubstantially from other areas and constitutes a particular case since no highphytoplankton densities or excessive proliferations of floating macroalgae have beenrecorded. Some authors suggested a strong top-down control mechanism of thephytoplankton by the enormous abundances of two alloctonous jellyfish species(Rhyzostoma pulmo and Cotylorhiza tuberculata) that occurred every summer [10]and this could be the reason for non-existence of algal blooms in these months sincenutrients are rapidly integrated in the trophic chain.

The substitution of C. nodosa meadows and unvegetated lagoon bottom by acontinuous and dense bed of C. prolifera have created several negative effects as aprogressive decrease of the fisheries, mainly Sparidae and Mugilidae species [13], orthe loss of bathing water quality. These changes may also pose a menace to thepopulations of several protected species as the fish Aphanius iberus and Hippocampus

ramulosus.The aim of the present study was to study the photosynthetic response of C.

nodosa and C. prolifera to the new conditions of light availability at the bottoms ofthe Mar Menor lagoon, after an increase in the nutrient inputs associated with therecent changes in agricultural practises, trying to explain the current distribution ofboth species of macrophytes in the lagoon.

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Fig. 2. Distribution of the macrophytes in the Mar Menor lagoon in (a) 1985 (adapted from Ballester [29])

and (b) the present situation (adapted from Calvın-Calvo [30]).

J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842 831

2. Methodology

2.1. Study area

The present study was carried out between September 2002 and October 2003 inthe Mar Menor coastal lagoon (135 km2, mean depth 4m, maximum 6.5m), locatedin a semi-arid region of southeast Spain, characterized by the scarcity ofprecipitation, which mainly occurs during storm events in autumn and winter. Thesalinity of the lagoon ranges between 42 and 47, due to low precipitation(o300mmyr�1) and high evaporation rates. Water temperature ranges from 10 1Cin winter to 31 1C in summer. No stratification exists in the water column. It isisolated from the Mediterranean Sea by a 22 km long sandy bar (La Manga), crossedby five very shallow channels. A transect of six sampling stations was selected inorder to characterize the different parameters of the water column adjacent to theAlbujon wadi from its mouth and following a depth gradient to Perdiguera Island,

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J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842832

located at the centre of the Mar Menor lagoon, far from the influence of thewadi (Fig. 1). The sampling stations were georeferenced using a differentialGPS: A (37143002.900N; 0151029.100W); LG1 (37142055.000N; 0151020.900W); LG2(37143000.000N; 0151000.000W); LG3 (37142045.000N; 0150030.000W); LG4(37142030.000N; 0150000.000W); LG5 (37142021.300N; 0148005.500W). The sampling wascarried out in September 2002, and January, June and October 2003, to recordseasonal variations of the physico-chemical parameters of the water column.

2.2. Freshwater inputs estimation

The freshwater discharge of the Albujon wadi was estimated from measurementsof depth and current velocity in a channel cross-section made in the station locatedat the mouth of the Albujon wadi (A). Sampling was carried out seven times in thestudy period to record annual variations, and values were averaged to estimate theannual discharge of the wadi. Annual inputs of nutrients and suspended sedimentswere estimated by extrapolating the concentration data and discharge calculationsmade in each season for a whole year.

2.3. Water sampling

At each sampling point measurements of water temperature, salinity, pH anddissolved oxygen were made in situ using a multi-parametric recorder (WTW,MultiLine P4).

Photosynthetic active radiation (PAR) was measured at different depths in thewater column using a photometer LI-COR 1000. Light extinction coefficient (k) ofthe water column was obtained from photometer measurements, adjusting data to anexponential curve [14]:

Iz ¼ Io � e�kz,

where Iz is the irradiance at z depth, Io the irradiance at the water surface and k thelight extinction coefficient.

Water samples were taken at 1m depth with a Niskin bottle and kept in the darkat 4 1C for transportation to the laboratory for later analyses of the nutrients andchlorophyll contents. Samples were filtered through pre-ashed and pre-weighed GF/F glass-fibre filters. The filters were oven-dried at 60 1C to constant dry weight, toestimate the suspended sediments, and then ashed for 5 h at 450 1C to estimate theparticulate organic matter (POM) content. Chlorophyll a concentrations weredetermined by spectrophotometry, following extraction in 90% acetone. Ammoniumwas converted to ammonia by adding 10M NaOH solution and measured with anammonia selective electrode connected to a pH/mV meter. The rest of the nutrientswere determined according to standard methods: nitrate by the cadmium reductionmethod, nitrites by sulfanylic acid colorimetry, and phosphate by ascorbic acidcolorimetry [15].

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J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842 833

2.4. Photosynthesis– respiration calculations

Gross photosynthesis of C. prolifera and C. nodosa, during an annual period, wascalculated following a Michaelis–Menten kinetic, and respiration rates of bothspecies of macrophytes were obtained from lineal adjustments versus watertemperature, considered as the best approximations to these calculations byTerrados (1991) in the Mar Menor lagoon [16]. Gross photosynthesis of themacrophyte follows the expression

GPh ¼ PhmaxðIb � R=aÞ=½ðPhmax=aÞ þ ðIb � R=aÞ,

where GPh is the gross photosynthesis rate, Phmax the maximum photosynthesis, Ib

the bottom irradiance, R the respiration rate, and a the photosynthesis efficiency.Values of a, and lineal adjustments for R and Phmax versus water temperature in

the different seasons were also calculated according to Terrados (1991) for bothspecies in the Mar Menor lagoon [16].

Sinusoidal irradiance curves were plotted at 15min intervals by considering themean maximum daily irradiances and the mean photoperiod for each seasonobtained from data of the closest meteorological station. These data were comparedwith the field measurements finding a strong correspondence between them. Theirradiance at the bottom was then calculated from the water column light extinctioncurves, previously obtained for each sampling station and season.

Gross photosynthesis was calculated for each time interval (15min) and integratedto obtain daily gross photosynthesis. Respiration was calculated from the linealadjustment, considering the seasonal mean temperatures of the water column andintegrating for a whole day. Both variables were subtracted, obtaining values of dailynet photosynthesis in every season.

Light compensation depth (Zc), defined as the depth in which net photosynthesisreaches 0, was also calculated from these models.

2.5. Data analysis

To estimate the degree of association between the results obtained in themeasurements of the different parameters, data were standardized by subtracting themean and dividing by the standard deviation before running a Pearson correlationanalysis (computer application Statistica v4.5r).

3. Results

3.1. Freshwater discharge estimations

Freshwater discharge estimations in each season corresponded to 181.63 l s�1 inspring, 78.10 l s�1 in summer, 2687.03 l s�1 in autumn and 1148.78 l s�1 in winter. Ascan be seen, the highest values corresponded to the rainy season, which occurredprincipally in autumn and winter. Nutrients and particulate inputs to the lagoon

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Table 1

Freshwater run-off and nutrient and particulate inputs from the Albujon wadi

Parameter Spring Summer Autumn Winter

Discharge (l s�1) 181.63 78.10 2687.03 1148.78

NO3 (kg day�1) 958.18 78.62 15 331.68 7307.71

NO2 (kg day�1) 21.60 5.18 105.41 50.98

NH4 (kg day�1) 19.87 34.56 659.23 157.25

PO4 (kg day�1) 19.01 15.55 24.19 1675.30

Suspended sediments (kg day�1) 87.88 1.88 410 109.00 1985.90

POM (kg day�1) 122.13 102.58 2460.73 2667.89

J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842834

from the Albujon wadi reached their highest values in autumn and remained high inwinter, decreasing the rest of the year (Table 1).

Our calculations were extrapolated for a whole year, estimating a total dischargeof 20Hm3 yr�1 of freshwater. The annual inputs from the Albujon wadi to thelagoon were also calculated, and corresponded to 5373 ton yr�1 of total suspendedsediments, 1568 ton yr�1 of dissolved inorganic nitrogen (DIN) and 125 ton yr�1 ofsoluble reactive phosphorus (SRP).

3.2. Water column analysis

Values of chlorophyll a, suspended sediments, k, DIN and SRP followed adecreasing gradient with the distance from the mouth of the wadi, emphasizing therole of the Albujon wadi as a source of nutrients (principally nitrate) and suspendedmaterial to the Mar Menor lagoon (Table 2). The highest values of k correspondedto the station closest to the mouth of the Albujon wadi (LG1). This station alsoshowed the highest variation of this parameter along the year, mainly in autumnwhen peak values were reached. Values of k maintained relatively constant along thestudy period in the stations located far from the influence of the wadi (Fig. 3).

Significant Pearson negative correlation was found between values of k anddistance from the mouth (r ¼ �0:42, po0:05). This parameter also showed positivecorrelations with total suspended solids (r ¼ 0:77, po0:001), nitrites (r ¼ 0:77,po0:001) and chlorophyll a (r ¼ 0:55, po0:01) concentrations.

According to the guidelines of the European Environment Agency, and followingthe criteria for assessment of nutrients levels in Mediterranean waters, the nutrientconcentration in the stations located in the centre of the lagoon and far from theinfluence of the wadi corresponded to oligotrophic waters except in winter when itreached values corresponding to moderate eutrophic waters (around 1.5 mmol l�1 ofnitrate). However, the stations located close to the influence of the wadi presentedhigher concentrations of nitrate most of the year and could be considered aseutrophic waters [17].

ARTIC

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Table 2

Values of the physico-chemical parameters measured in the lagoon

Parameter Spring Summer Autumn Winter

LG1 LG2 LG3 LG4 LG5 LG1 LG2 LG3 LG4 LG5 LG1 LG2 LG3 LG4 LG5 LG1 LG2 LG3 LG4 LG5

Temperature (1C) 28.3 26.9 26.6 26.3 25.8 31.2 30.8 31.2 30.5 29.9 18.9 18.7 19.0 18.6 18.7 10.8 10.2 9.8 10.0 10.4

Salinity 45.5 45.7 45.7 46.0 46.2 45.9 46.2 46.0 46.2 46.1 33.6 44.6 44.7 45.0 45.3 41.9 40.8 41.7 41.7 41.6

pH 8.60 8.63 8.60 8.60 8.52 8.52 8.60 8.63 8.60 8.60 8.56 8.51 8.55 8.50 8.52 8.61 8.66 8.60 8.60 8.50

Dissolved oxygen (mg l�1) 11.30 9.50 9.63 9.60 8.03 8.32 7.65 8.29 7.22 8.95 9.04 8.64 8.98 9.38 9.27 9.95 9.66 9.99 9.90 9.75

Suspended sediments (mgDW l�1) 4.53 2.93 2.80 2.80 2.53 17.67 16.67 17.67 11.40 14.40 42.28 9.15 15.60 7.70 7.24 12.20 15.07 15.00 13.33 11.60

POM (mgAFDWl�1) 2.93 1.73 1.67 1.47 1.67 5.93 6.40 6.60 5.00 6.00 6.13 3.03 3.61 2.79 2.78 4.87 5.47 5.53 5.13 5.27

Light extinction coefficient (k) 0.44 0.25 0.30 0.25 0.18 0.63 0.46 0.49 0.30 0.29 1.60 0.44 0.39 0.43 0.44 0.53 0.34 0.33 0.25 0.13

Chlorophyll a (mgm�3) 0.95 0.96 0.78 0.64 0.49 13.67 2.90 3.95 4.26 3.27 6.60 3.02 2.50 1.72 1.56 1.53 1.50 1.26 1.47 0.81

NO3 (mmol l�1) 1.67 2.03 2.21 0.93 0.23 3.22 4.94 0.73 0.25 0.03 19.76 27.01 8.27 7.74 1.34 6.75 55.08 13.16 6.03 2.80

NO2 (mmol l�1) 0.09 0.10 0.12 0.14 0.02 0.17 0.16 0.06 0.11 0.10 3.69 0.23 0.22 0.09 0.02 0.10 0.81 0.23 0.10 0.06

NH4 (mmol l�1) BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL 0.56 BDL 0.19 BDL BDL BDL BDL BDL BDL BDL

PO4 (mmol l�1) BDL BDL BDL BDL BDL BDL BDL BDL BDL BDL 0.53 0.39 BDL 0.05 BDL 0.16 0.13 BDL BDL BDL

BDL: below detection limit.

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Fig. 3. Values of light extinction coefficient vs distance to the mouth of the Albujon wadi.

J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842836

3.3. Photosynthesis– respiration calculations

Maximum values of net photosynthesis were obtained in summer and minimumvalues in winter for both species of macrophytes. Spring and autumn presentedsimilar intermediate values.

C. nodosa presented negative values of net photosynthesis in spring (except LG2station), autumn and winter. Summer values of this parameter were positive andsimilar to those obtained for C. prolifera in this season. C. prolifera showed positivevalues of net photosynthesis most of the year except in winter (Fig. 4).

The values of light compensation depth (Zc) obtained for C. nodosa were alwaysbelow those obtained for C. prolifera and, except in summer, above the depth of thesite considered (Table 3).

4. Discussion

The recent change in agricultural use to intensively irrigated crops in the Campode Cartagena, which is drained by the Albujon wadi, has resulted in a continuousflow of this watercourse, thus loosing its episodic behaviour, and greater quantities

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Fig. 4. Values of net photosynthesis of (a) Cymodocea nodosa and (b) Caulerpa prolifera along a year.

J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842 837

of nutrients and suspended material being introduced into the lagoon. This dischargehas promoted an increase in the nutrient concentrations in the Mar Menor lagoon,creating a change in the trophic status of the waters. The rise in the inputs ofnutrients into European seas has been identified as one of the major menaces toEuropean coastal environments since eutrophication phenomena, especially in semi-enclosed water bodies, such as coastal lagoons, or the Baltic and Adriatic Seas,create severe problems to these environments and may greatly affect fisheries andalso human health and recreational uses of marine coastal zones [17].

In the course of this study the highest values of nutrient and suspended materialconcentrations in the lagoon were observed in autumn, corresponding to the rainyseason and the subsequent highest input from the wadi. In addition, a physico-

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Table 3

Light compensation depth (m) of both species of macrophytes during 1 year

Species Station Distance

from the

mouth (m)

Site depth

(m)

Spring Summer Autumn Wintera

Cymodocea

nodosa

LG1 315.2 2.00 1.84 4.02 0.46 —

LG2 718.5 3.00 3.23 5.48 1.44 —

LG3 1549.0 3.50 2.69 5.16 1.84 —

LG4 2406.0 4.50 3.20 8.42 1.58 —

LG5 5149.6 5.00 4.59 8.76 2.12 —

Caulerpa

prolifera

LG1 315.2 2.00 6.45 7.22 2.89 1.72

LG2 718.5 3.00 11.31 9.86 9.00 2.69

LG3 1549.0 3.50 9.41 9.27 11.48 2.78

LG4 2406.0 4.50 11.18 15.13 9.85 3.71

LG5 5149.6 5.00 16.06 15.76 13.23 7.21

aMissing values for C. nodosa in winter indicate negative values of Zc.

J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842838

chemical gradient was observed along the transect of sampling points selected in thestudy area, reflecting the role of the Albujon wadi as the main source of nutrientsand suspended material input into the Mar Menor lagoon. The clearly definedgradients of water column transparency found in the study area resulted as aconsequence of these inputs from the wadi.

Light seems to be the principal limiting factor on the growth and distribution ofboth species of macrophyte studied in the Mar Menor lagoon, as occurs in otherestuarine and lagoonal systems [5,6,18–20]. Since nutrients were found in highconcentrations, they do not seem to be a limiting factor, allowing macrophytes togrow and expand [6,7,16,21]. As in other estuarine environments, light attenuation inthe lagoon is principally due to chlorophyll and suspended sediment concentrationsin the water column [14]. As has been reported before, freshwater inputs are animportant forcing function stimulating phytoplankton growth and decreasing watercolumn transparency [22]. The increase in the nutrient supply from the Albujon wadipromotes higher phytoplankton densities in the lagoon and has resulted in anincrease of primary production in the water column. The wadi also introducessuspended sediments, further increasing water column light attenuation, although, inour case, it mainly affected the area closest to the Albujon mouth during the rainyseason, when higher amounts of solids are introduced into the lagoon.

This situation has led to the principal macrophytes of the bottoms of the lagoonchanging from a phanerogam to an opportunistic macroalga, thus transforming thewhole benthic community present in the lagoon. Similar changes have been reportedin estuarine areas suffering eutrophication processes [7,23] but the case of the MarMenor lagoon differs from other Mediterranean areas. Before the agriculturaldevelopment of the area adjacent to the Mar Menor lagoon and the subsequentincrease in the nutrient inputs to its waters, C. nodosa was the dominant benthic

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J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842 839

macrophyte of the lagoon, but the recent changes have almost totally eliminated thisphanerogam from the zone and its growth is not possible due to a change in thewater column parameters. These changes constitute an example of sensitiveness ofestuarine environments, which may be profoundly transformed in the face of anutrient enrichment process, even in the absence of severe eutrophication events,which occurred in other Mediterranean areas, such as anoxia or algal blooms[8,9,24].

The study of the photosynthetic response of C. nodosa to light availability in thebottoms indicated the impossibility of survival in the study area since respirationrates exceeded photosynthesis most of the year (Fig. 4); any hypothetically presentplants in these bottoms should die due to the prolonged period when growth is notpossible. Net photosynthesis was higher in the summer, due to the highest irradianceof this season and the relatively high transparency of the waters, which augments thelight availability at the bottoms, thus favouring the photosynthetic response of thephanerogam. C. prolifera presents a different pattern as net photosynthesis reachespositive values most of the year except in winter, a result previously observed in theMar Menor lagoon [16]. Oxygen net consumption and the subsequent biomass lossin winter are rapidly compensated by the production occurring during the rest of theyear, allowing the alga to grow and expand. Values of net photosynthesis of bothmacrophytes versus distance to the mouth of the wadi remained relatively constantin every season because any increase in depth seemed to be compensated by adecrease in the water light extinction coefficient as distance increased along thetransect (Fig. 4).

Calculations of light compensation depth further indicated that no growth of thephanerogam is possible in this area because respiration rates were higher than grossphotosynthesis at these depths.

The estimations of net photosynthesis obtained in this work reflect that no growthof C. nodosa was possible in the studied area, including stations located far from theinfluence of the wadi, emphasizing the negative effects of an increase in water columnlight attenuation in the lagoon, promoted by a parallel increase of the concentrationof chlorophylls due to the recent eutrophication process. In contrast, C. prolifera

presented normal photosynthesis rates since this species found adequate lightconditions for its growth [25].

The substitution of the macrophytes in the Mar Menor lagoon was then favouredby the light limitation of the phanerogam and the recolonization of the space by aninvasive and opportunistic macroalga from the adjacent Mediterranean Sea. Thesuccess in the colonization of the bottoms of the lagoon by the macroalga is mainlydue to the diminution of the water column transparency, which not only limits C.

nodosa growth but also avoids high irradiances at the bottoms which causephotoinhibition processes or even irreversible photodamage of the alga as has beendescribed by Hader et al. (1997) [26]. No competence processes seem to have causedthis substitution and they only may appear in shallow areas where both speciescoexist, as has been reported before in other areas [7,27].

The settlement of C. prolifera have caused a progressive deterioration of thebottoms through the accumulation of organic matter and the subsequent appearance

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J. Lloret et al. / Ocean & Coastal Management 48 (2005) 828–842840

of anoxic conditions and the production of toxic acid volatile sulfides. These changesin the sediment characteristics do not allow the settlement of other flora and faunaspecies and also diminish the bathing water quality in several zones of the MarMenor lagoon (muddy bottoms, bad smell, etc.). In addition, the local fishingindustry is negatively affected by the decrease in the populations of commercial fish,as these species, mainly Sparidae and Mugilidae, prefer feeding on patches of thephanerogam or unvegetated bottoms, now covered by a dense and continuous bed ofthe macroalga. In summary, the changes of the macrophytes of the bottoms of thelagoon have not only affected the biological communities of the Mar Menor, butalso the human activities, with negative consequences to local economies.

The wide distribution of C. prolifera, and the high biomass that this represents,play an important role in the nutrient cycles of the lagoon since nutrientconcentrations in the water column may greatly diminish through their fixation bythe monospecific bed of the alga. In this sense, C. prolifera may improve watertrophic status, thus avoiding high nutrient concentrations and the subsequentproliferation of phytoplankton. C. prolifera growth does not seem to be limited bylight, as is reflected by light compensation depth calculations. However, if the lightextinction coefficient increases to values similar to those observed in autumn in thearea closest to the mouth of the wadi, most of the C. prolifera bed will find itselfbelow the light compensation depth, affecting algal survival and its capacity to fixnutrients from the water column, and, through a feed-back mechanism, moreoverdiminishing water transparency. This process could ultimately create the massivedecomposition of the macroalga and give rise to oxygen depletion phenomena in thelagoon, with catastrophic consequences for the biological communities of the MarMenor and also to the fisheries and tourism industry of the zone.

In this study we have demonstrated that the recent changes in land usage tointensive agricultural exploitation, and the subsequent increase in nutrient inputsinto the lagoon have favoured the expansion of the opportunistic macroalga C.

prolifera, which is better adapted to the new conditions of light of the environment,substituting C. nodosa from the benthos. C. nodosa growth was only possible in veryshallow zones of the lagoon where enough light for its photosynthesis reaches thebottom. The current distribution of both species of macrophytes seems to bereaching its equilibrium as the alga C. prolifera finds difficulties in colonizing veryshallow areas because of photoinhibition processes and emersion stress to which C.

nodosa is better adapted [16,26,28]. However, the continuous increase in the inputs ofnutrients to the Mar Menor could limit even more the presence of C. nodosa in thelagoon.

Acknowledgements

This work was funded by the European Regional Development Fund (ERDF),Consejerıa de Agricultura, Agua y Medio Ambiente, Region de Murcia, ProgramaSeneca 2001 (AGR/24/FS/02).

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