Estuarine, Coastal and Shelf Science (1998) 46, 127–142

0272–7714/98/010127+16 $25.00/0/ec970303

studies of sources and processes controlling the cycleof organic carbon in the northern Adriatic. Thesestudies are a priority because of organic matterinvolvement in two serious problems affecting theAdriatic ecosystem: hypoxic or anoxic crises in bottomwaters during the summer (Montanari et al., 1984)and sporadic massive occurrences of marine snow andmucilaginous macro-aggregates in the water column(Stachowitsch et al., 1990; Herndl, 1992).Recent findings (Moran & Hodson, 1990; Wetzel,

1992) have amended the earlier view that land-derived organic carbon formed a refractory pool(Gagosian & Lee, 1981). It is now clear that fluvialdissolved organic matter may be used directly bymarine bacteria or undergo photochemical transfor-mation that increases its bioavailability (Wetzel et al.,1995). It has been calculated that about 50% ofland-derived C can be respired annually in coastalareas on a global scale (Smith & Mackenzie, 1987).Unfortunately, the cycling of organic matter in thenorthern Adriatic sea is poorly understood and therelative importance of organic carbon sources has notbeen quantified (Najdek, 1993). Information on land-derived organic matter transported to the northernAdriatic is scarce. Pettine et al. (1985) calculated thetotal organic carbon (TOC) load of the Po as

Transport of Carbon and Nitrogen to the NorthernAdriatic Sea by the Po River

M. Pettinea, L. Patroleccoa, M. Camussob and S. Crescenziob

aWater Research Institute, C.N.R., via Reno 1, 00198 Rome, ItalybWater Research Institute, C.N.R., via della Mornera 25, 20047 Brugherio, Italy

Received 11 February 1997 and accepted in revised form 23 June 1997

Concentrations of dissolved and particulate organic carbon (DOC, POC) and particulated nitrogen (PN) were measuredin samples taken monthly for a year at two stations in the lower Po River and on four separate occasions in the mixingzone. From the results, the influence of suspended matter concentrations on POC and PN contents in the solid phase,and of flow variations on POC and PN contents in the water phase were evaluated. The behaviour of organic matter inthe mixing zone was conservative in low to medium solid transport conditions, but had a strong negative deviation withrespect to the dilution line during high solid transport, presumably due to both settling of particulate matter in theprodelta region and bacterial utilization of organic matter. Organic carbon was preferentially transported in the solidphase, in the lower Po, compared to world rivers in temperate zones. The loads of total organic carbon and nitrogen wereestimated as 25·5#104 and 15·5#104 tonnes year"1 respectively, making this river the most important contributor oforganic matter and nutrients to the Mediterranean. These high loads together with lesser inputs from other Italian riversflowing in to the northern Adriatic exert a significant impact on the productivity and trophic dynamics of this basin.

? 1998 Academic Press Limited

Keywords: organic matter; Po River; estuarine processes; carbon load

Introduction

The shallow northern part of the Adriatic sea (<14%and 3% of the total Adriatic surface area and volume,respectively) receives significant freshwater inputswhich have a marked positive impact on the produc-tivity of its inshore waters. The outflow of the PoRiver, the largest Italian river, provides over 50% ofthe freshwater input to the northern Adriatic basin.Since the Po valley is one of the most productiveagricultural areas in Italy, this river also accounts forabout 50% of the total nutrients transported into thenorthern Adriatic (Degobbis et al., 1986; Degobbis &Gilmartin, 1990). External annual contributions ofnutrients have been estimated to be of the same orderof magnitude as the regeneration rate giving similarcontributions to primary production from new andregenerated material (Degobbis & Gilmartin, 1990).This involves a high vulnerability of the northernAdriatic ecosystem to external input. Such a highvulnerability is not supported by experimental datawhich indicate limited changes in primary productionrates over the period 1971–1981 (Smodlaka &Revelante, 1983) while nutrient loads increased muchmore significantly over that period (Marchetti, 1990).This divergence highlights the need for thorough

? 1998 Academic Press Limited

128 M. Pettine et al.

20·3#104 tonnes year"1, but their estimate camefrom a limited chemical oxygen demand (COD) dataset on the lower Po, and a relationship between TOCand COD determined for the middle reaches ofthe Po.Information is available on the particulate organic C

and N (POC and PON) contents of suspended andsedimentary matter in the Gulf of Trieste, on POCand PON levels in the water column along a transectfrom the Po mouth to the Istrian coast, and also onorganic C and ä13C values in surface sediments ofdifferent regions in the Adriatic (Faganeli et al., 1988,1994; Gilmartin and Revelante, 1991; Posedel &Faganeli, 1991). However, to our knowledge nostudies have been published on dissolved organicmatter in the northern Adriatic.Integration of data on trends in water column

concentrations with data on major terrestrial inputsand in situ biological production/respiration couldimprove our understanding of the processes control-ling carbon cycling in the northern Adriatic. The inputof organic matter from the Po is particularly likely toexert a strong influence on the Adriatic in the summerwhen stratification and eddy circulation render thenorthern Adriatic semi-enclosed (Franco et al., 1982).In autumn and winter the discharge of the Po andother minor rivers (Adige, Piave, etc.) is more easilydispersed due to the prevailing cyclonic current whichdrives the plumes southward along the Italian coast.This paper presents data on the temporal variability

of particulate inorganic and organic carbon, particu-late nitrogen, dissolved organic carbon, chlorophyll aand total pigments in the lower River Po (see Appen-dix). It documents the influence of physical (flow andsuspended matter, SM) and biological (algal produc-tion) variables on the transport of carbon and nitro-gen, and describes processes which affect soluble andparticulate C and N concentrations in the mixingzone. These new data make it possible to calculatetotal net exports of carbon and nitrogen by the PoRiver and to evaluate the fluvial algal contribution tothe total transport of particulate organic matter.

Material and methods

Sampling area

Monthly water samples were collected at two stations(Pontelagoscuro and Pila, Figure 1) in the lowerstretch of the Po over the period February 1995 toMarch 1996. The Pontelagoscuro station is located290 km from the mouth, close to the city of Ferrara.Below Ferrara the Po River flows above groundlevel and does not receive further polluting inputs.

Pontelagoscuro, the closing section of the Po water-shed, is characterized by a long history of data collec-tion mostly concerning nutrients, metals, COD andBOD (IRSA, 1991). The second station, Pila, issituated on the main channel (Po di Pila) of the delta.This channel carries about 50% of the Po watervolume.Water samples were collected with a peristaltic

pump from a depth of 0·5 m (surface samples) andabout 2 m above the bottom, but this depth could varyto avoid collecting brackish water if a salt wedge waspresent. Samples were also taken in the Po river plumeat various salinities (Figure 1) in four surveys carriedout on 1 March, 21 June and 10 October 1995 and 4March 1996. The Po estuary is a non-turbid systemwhich is non-tidal for most of the year and presents alimited salt wedge only during the summer period.

Rovigo

River Po

Pontelagoscuro

Ferrara

12° E

0 25 50

km

N

45° N

Po di M

aest

raP

o de

lla

Pil

a

Pila

June 1995March

1995–1996

October 1995

Adriatic SeaPo delle TollePo di Goro e di Gnocca

F 1. Location of the sampling stations.

Analytical procedures

Particulate and dissolved matter was separated imme-diately after collection by filtering 1–2 l samplesthrough precombusted (4 h at 480 )C) and pre-weighed Whatman GF/F glass fibre filters (47 mm;0·7 ìm nominal pore size). Filtered samples werepoured into Pyrex precombusted 100 ml bottles andfrozen immediately. After filtration and washing with20 ml Milli Q water, filters were stored at "20 )C.Dissolved organic carbon (DOC) was assayed by hightemperature catalytic oxidation (HTCO) using aCarlo Erba TOC analyser (model 480). Immediatelyafter thawing, water samples were acidified to pH2–2·5 with 100 ìl ultrapure HCl and purged with N2

for 20 min to remove inorganic carbon. Carbon con-centrations were determined against potassium hydro-gen pthalate standards, correcting for the total blank(analytical system blank plus Milli Q water blank),

Transport of carbon and nitrogen by the Po River 129

which was approximately 50 ìM in our experimentalconditions. Organic carbon standards were runevery three to four samples to monitor possibleinstrument shifts. Samples, standards and blankswere measured in triplicate. The detection limit was0·5 mg l"1 (=41·7 ìM) and the standard deviationwas approximately 10%.The filters (Whatman GF/F) were dried at 60 )C

overnight, re-weighed to determine particle loadingand homogenized in an agate mortar mill. Blank filterswere processed the same way. Powdered, homo-genized samples (10–15 mg) were weighed accurately(&0·01 mg) into tin or silver cups (9#5 mm) todetermine particulate nitrogen (PN), total particulatecarbon (PC) or particulate organic carbon (POC).Samples in silver cups were acidified with 20 ìl 5 Multrapure HCl and kept at 50–60 )C for 30 min toremove inorganic carbon. Care was taken to ensurecomplete saturation of sample with HCl and to avoidsample loss. Acid treatment was repeated until effer-vescence was no longer observed (generally threetimes). PC, POC and PN were determined by hightemperature oxidation using a Carlo Erba NA 1500series 2 C/H/N/O/S analyser. Samples were run induplicate. Sulphanylamide was used to construct thecalibration curve.Carbon and nitrogen contents were expressed as

percentage of total solid and as concentration(mg l"1) of filtered water sample. Average blanklevels were 2·5&0·8 ìg C and <0·5 ìg N; detectionlimits (calculated as three times the standard deviationof the blank for carbon, and according to sensitivity ofthe instrument for nitrogen) were 2 and 0·5 ìg forcarbon and nitrogen, respectively, corresponding to0·02% C and 0·005% N for a 10 mg sample. Analyti-cal precision was &2·3 and &1·8% of the measuredvalues for total and organic carbon and &1·6% fornitrogen. Particulate inorganic carbon (PIC) wasdetermined by the difference between PC and POC.The analysis of chlorophyll a (Chl a) and phaeopig-

ments was performed spectrophotometrically using aPerkin Elmer 452 spectrophotometer following theprocedure outlined in Lorenzen (1967). The Chl aand phaeopigment concentrations were determinedby filtering 0·5–1·0 l of freshwater and 1·0–2·0 l ofseawater onto glass-fibre filters (GF/F Whatman).The filters were stored frozen, and for processing wereground with a PTFE/glass grinder and extracted with15 ml of 90% (v/v) MgCO3/saturated acetone solu-tion. The extracts were kept refrigerated for 22 h inthe dark prior to filtration through glass-fibre filters toremove particles. The optical density was read at 665and 750 nm before and after addition of 0·1 N HCl(30 ìl ml"1). The final concentrations of Chl a and

phaeopigments were calculated according to spectro-photometric equations by Lorenzen (1967). Chl a wasalso measured, on all samples, by direct insertion of afluorometric sensor into the water and noting thereadout at various depths (Sea Tech Fluorometer,Ocean Seven Idronaut). Data obtained by the twomethods were in good agreement (r=0·87, n=143,P<0·001). The concentration of sodium used as atracer in the mixing zone was measured by ICPemission spectrometry (Perkin Elmer, Plasma 40).Data sets were tested for normality by means of

the Kolmogorov–Smirnov test and in most of casesshowed a log-normal distribution. Differences be-tween surface and bottom data sets were tested bymeans of the Wilcoxon matched pairs test.

Results

PC, PIC, POC and PN concentrations

The percentage values of PC, POC and PN values inthe particulate matter of the Po River varied in theranges 3·4–21%, 0·9–18% and 0·12–3·31%, respect-ively, over the study period. The similar range ofvariation for PC and POC indicates that organicmatter had a strong influence on the variability of thedata. Particulate inorganic carbon (PIC) varied withinthe narrow range 0·5–4·0% with a mean value of1·9&0·7% (median 1·8%). Mean values and stan-dard deviations calculated from all the data were8·3&4·5%, 6·4&4·4% and 0·94&0·72% for PC,POC and PN respectively, and the correspondingmedians were 7·2%, 5·5% and 0·71% (Table 1).Surface and bottom data sets showed significantdifferences according to the Wilcoxon matched pairstest although gave close median values of 7·4–7·2%,5·9–5·4% and 0·75–0·71% for PC, POC and PN,respectively.Average particulate concentrations per unit volume

of water (mg l"1) were 4·4&4·8, 2·4&1·2 and0·35&0·18 mg l"1 for PC, POC and PN, respect-ively. The corresponding medians (2·6, 1·9, 0·31 mgl"1 for PC, POC and PN, respectively) were veryclose to the means except for PC.Seasonal variations of suspended matter (SM) and

its POC, PIC and PN contents are shown in Figure 2.POC% and PN% showed peak values at both stationson April and at the lower station (Pila) on July; thesepeaks correspond to chlorophyll peaks (see Figure 4).The lowest values of POC% and PN% were observedon the occasion of peaks in the solid transport. Thepercentage values of POC and PN appear to varyinversely with riverine suspended matter load (Figure3); this is the case for most rivers (Meybeck, 1982).

130 M. Pettine et al.

T 1. Means, medians and distributions of surface, bottom and combined values for T, SM,DOC, TOC, PC, POC, PN and DOC/TOC ratios in water samples collected at Pontelagoscuro andPila from March 1995 to March 1996

TotalFlow (Q)(m3 s"1)

T()C)

SM(mg l"1)

DOC(mg l"1)

TOC(mg l"1)

PC(%)

Mean 1587 14·6 86·7 2·1 4·5 8·3SD 712 6·3 131·4 0·6 1·5 4·5Median 1440 15·0 32·3 2·0 4·0 7·21-ile 1100 8·6 22·4 1·7 3·4 4·83-ile 1983 20·8 60·0 2·3 5·5 10·2Minimum 680 6·4 9·0 1·3 2·3 3·4Maximum 3440 24·9 562·7 3·7 8·2 21·0

TotalPC

(mg l"1)POC(%)

POC(mg l"1)

PN(%)

PN(mg l"1)

DOC/TOC

Mean 4·4 6·4 2·4 0·94 0·35 0·5SD 4·8 4·4 1·2 0·72 0·18 0·1Median 2·6 5·5 1·9 0·71 0·31 0·51-ile 1·8 3·1 1·4 0·46 0·21 0·43-ile 4·5 8·7 3·3 1·21 0·50 0·6Minimum 1·0 0·9 0·7 0·12 0·08 0·3Maximum 22·0 18·0 5·0 3·31 0·84 0·7

Data from both surface and bottom samples werefitted to the following equations:

POC%=60·20 SM"0.67 r=0·89 S.D.&2·02 (1)

PN%=8·52 SM"0.66 r=0·87 S.D.&0·39 (2)

These significant inverse relationships suggest,following Meybeck (1982), that autochthonousparticulate organic matter in the lower Po is dilutedby land-derived particles which are poor in organicmatter.Figure 2 also shows that the data of PIC have a

different dependence on solid transport with respectto POC and PN; PIC values tend to increase linearlyas a function of suspended matter although showing ahigh scattering of values at low solid concentrations.Temporal variations of DOC, POC and PN

referred to water volume, Chl a and total pigments areshown in Figure 4. The variations of POC and PN(mg l"1) appear to reflect variations in the solidtransport, phytoplanktonic biomass and flow. Thedependence of organic matter concentration on flow(Qm3 s"1), in particular, shows two opposing effects:dilution and run-off. POC and PN values (Figure 5)decrease with increasing flow in low to mediumdischarge conditions because of prevalence of thedilution effect, increase at medium to high flows

where the run-off effect predominates and level off oreven decrease during flood periods. This distributionis somewhat different from that typical of world riverswhich show a prevalence of run-off processes with anincrease of POC with increasing flow (Meybeck,1982). As a result of these findings, the POC and PNdata were fitted to the following polynomial functionswhich include a positive term directly proportional toflow and two other terms describing the decrease ofthe dependent variable at low and very high flowconditions,

POC (mg l"1)="21·6+11776/Q+0·0135 Q"2# 10"6 Q2

r=0·92 S.D.=0·75 (3)

PN (mg l"1)="4·1+2234/Q+0·0025 Q"4#10"7 Q2

r=0·94 S.D.=0·11 (4)

The above equations are valid in the flow rangefrom about 650 to 3500 m3 s"1; out of this rangethe equations model the situation increasinglyinaccurately.With the exception of few peak values recorded

during very high solid transport conditions, PICvaried linearly with flow (Figure 6) and data werefitted to:

Transport of carbon and nitrogen by the Po River 131

PIC (mg l"1)="0·729+0·0011 Qr=0·81 S.D.=0·57 (5)

The peak values of PIC that significantly deviatefrom Equation 5 were recorded on 28 February–1 March and 9–10 October 1995 and caused bya dominant input of particulate matter from theApennines, with a higher content of carbonates thanmaterial from the Alps (see discussion).PN values showed a strong positive correlation with

POC values suggesting that organically bound nitro-gen was dominant in suspended matter from the lowerPo (Figure 7). The data were fitted to the equation

PN%=(0·15&0·01)POC(%)"(0·03&0·07)r=0·93 S.D.=0·2 (6)

which gives an average POC/PN weight ratio of6·7&0·4.

5

0

PIC

(%

)

Mar

ch

4

3

2

1

Apr

.

May

Au

g.

July

Jun

e

Sep

t.

Oct

.

Nov

.

Dec

.

Jan

.

Feb

.

Mar

ch

Mar

ch

Apr

.

May

Au

g.

July

Jun

e

Sep

t.

Oct

.

Nov

.

Dec

.

Jan

.

Feb

.

Mar

ch

4

0

PN

(%

)

3

2

1

18

0

PO

C (

%) 12

9

6

3

0.20

0.00

SM

(g

l–1)

0.12

0.08

0.04

15

Pontelagoscuro Pila

0.16 0.35 0.43 0.53

18.5

0.340.56

F 2. Temporal variations of suspended matter (SM), POC%, PN% and PIC% at the two investigated stations,Pontelagoscuro and Pila. Open bars, surface samples; shaded bars, bottom samples.

DOC concentrations

Dissolved concentrations of organic carbon were inthe range of 1·3–3·7 mg l"1 (Figure 4) and gave anaverage value of 2·1&0·6 mg l"1 and a very closemedian of 2·0 mg l"1. DOC values did not show anysignificant difference between surface and bottom

132 M. Pettine et al.

data sets according to the Wilcoxon matched pairstest (P=0·44); some differences were found on Mayand June samples. The mean values for surface andbottom samples were 2·2&0·6 and 2·1&0·6 mg l"1

respectively, while the median was 2·0 mg l"1 forboth data sets.DOC values did not show any clear seasonal behav-

iour with relatively uniform concentrations over timeat both stations as found in other fluvial systems(Aminot et al., 1990). Occasional peaks were found inhigh solid transport conditions (May and October) orfollowing algae decaying (June). The distribution ofDOC values as a function of river flow (Figure 5) isdescribed by

DOC (mg l"1)="8·4+4835/Q+0·0064 Q"10"6Q2

r=0·91 S.D.=0·56 (7)

DOC values decrease with increasing flow in low tomedium discharge conditions because of the dilutionof exogenic inputs, increase at medium to high dis-charges as a result of soil leaching by surface run-off

and decrease at very high discharges.

600

18

SM (mg l–1)

PO

C (

%)

3

15

12

9

6

200 4000 600

4

SM (mg l–1)

PN

(%

)

3

2

1

200 4000

F 3. Behaviour of the percentage contents of carbon and nitrogen in the solid phase (POC% and PN%) as a functionof suspended matter concentration. , surface samples; , deep samples, POC%, y=60·20x"0·67, r2=0·80; PN%,y=8·52x"0·66, r2=0·75.

Phytoplankton biomass and algal POC

Temporal variations of Chl a and total pigmentsmeasured at Pontelagoscuro and Pila over the studyperiod are reported in Figure 4. From this figure it isevident that the highest concentrations of Chl a andphaeopigments occurred in April and July 1995,

which are typical blooming periods. Total pigmentconcentrations varied from 2·2 to 180 ìg l"1 consid-ering both surface and bottom layers, while chloro-phyll a values ranged from 0·2 to 91·5 ì gl"1. Medianvalues (ìg l"1) of both Chl a (7·9 with 1·8 and 15·8for the 1- and 3-ile) and total pigments (18·0 with 3·9and 24·4 for the 1- and 3-ile) differed from thecorresponding means (15·0&20·6 and 26·6&35·6,respectively). No differences were found for the sur-face and bottom data sets for both Chl a (P=0·61) andtotal pigments (P=0·10) according to the Wilcoxonmatched pairs test although the significance of the testwas lower for total pigments.High chlorophyll concentrations were mostly found

at low discharges since low flow conditions tend tofavour the development of phytoplanktonic commu-nities as found for other rivers (Relexans et al., 1988;Descy et al., 1987).The chlorophyll data were converted to algal POC

by using the factors 20 and 30 for total pigments andChl a, respectively. The choice of these factors wasbased on the lowest values of the ratio POC/totalpigments and POC/Chl a, observed during the mostproductive periods (Figure 8). These conversionfactors are similar to those used for other rivers. APOC/Chl a ratio of 25 was used by Admiraal et al.(1992) in the lower river Rhine, while a value of 30,based on the lower limit of POC/total pigment ratios,was used in the Loire River (Relexans et al., 1988); afactor of 35 was used in the Loire and other Frenchrivers (Vire, Oise, Garonne) for calculating algal POCfrom chlorophyll a based on the SCOR-Unesco

Transport of carbon and nitrogen by the Po River 133

150

0

Ch

loro

phyl

l a (

µg l

–1)

Mar

ch

120

90

60

30

Apr

.

May

Au

g.

July

Jun

e

Sep

t.

Oct

.

Nov

.

Dec

.

Jan

.

Feb

.

Mar

ch

Mar

ch

Apr

.

May

Au

g.

July

Jun

e

Sep

t.

Oct

.

Nov

.

Dec

.

Jan

.

Feb

.

Mar

ch

1.0

0.0

PN

(m

g l–1

)

0.8

0.6

0.4

0.2

6

0

PO

C (

mg

l–1)

4

3

2

1

4

0

DO

C (

mg

l–1) 3

2

1

5

Pontelagoscuro Pila

F 4. Temporal variations of DOC, POC and PN referred to water volume, Chl a and total pigments at the twoinvestigated stations, Pontelagoscuro and Pila. Open bars, surface samples; shaded bars, bottom samples; – – –, surface totalpigments; - - -, bottom total pigments.

equations (Relexans & Etcheber, 1982; Dessery et al.,1984).The estimates of algal POC from Chl a were slightly

lower than those from total pigments. Data (mg l"1)were fitted to (r=0·92; SD=0·25)

Algal POCChl a=(0·80&0·05)#

Algal POCtotal pigment+(0·03&0·05) (8)

Algal POC calculated from Chl a data which repre-sent fresh phytoplanktonic biomass ranged from 0·01

to 2·75 mg l"1 with a mean value of 0·45&0·62 mgl"1 compared to the mean value for total POC of2·39&1·20 mg l"1. These algal POC values indicatea contribution to total carbon transport that is in therange 0·2–100%. The highest contributions wererecorded in April (67–83%), June (28–35%) and July(66–103%) on surface waters; in the same periods,bottom waters showed contributions of 45%, 22–32%and 75–93%, respectively. The Wilcoxon matchedpairs test gave no difference for surface and bottomdata sets of algal POC estimated from Chl a (P=0·65).

134 M. Pettine et al.

4000

5

Q (m3 s–1)

DO

C (

mg

l–1)

1000

4

3

2

1

2000 30000

4000

1.0

Q (m3 s–1)

PN

(m

g l–1

)

1000

0.8

0.6

0.4

0.2

2000 30000

4000

6

Q (m3 s–1)

PO

C (

mg

l–1)

1000

4

3

2

1

2000 30000

5

F 5. Behaviour of particulate organic carbon andnitrogen in the water phase (POC and PN as mg l"1) anddissolved organic carbon as a function of flow. , surfacesamples; deep samples.

3500

18

Q (m3 s–1)

PIC

(m

g l–1

)

1000

3

15

12

9

6

500 1500 2000 2500 30000

F 6. Distribution of particulate inorganic carbon(PIC, mg l"1) as a function of flow (values represented byopen circles were not considered in the regression analysis,see text). , Surface samples; , deep samples.y=0·001x"0·73, r2=0·66.

18

3.5

0POC %

PN

%

10

1.0

3.0

2.5

2.0

1.5

0.5

2 4 6 8 12 14 16

F 7. Relationship between particulate organic carbonand particulate nitrogen (all samples). y=0·15x"0·03,r2=0·87.

Annual loads of organic carbon

Mass fluxes of particulate inorganic and organiccarbon, particulate nitrogen and dissolved organic

carbon to the northern Adriatic basin were calculatedby four different methods. They were based on:(a) the mean of products of the instantaneous concen-trations and the mean daily discharge for the samplingday; (b) the product of the arithmetic means of theconcentrations and discharges; (c) the product ofdischarge-weighted concentrations and the meanannual discharge; and (d) the sum of the products ofdaily discharge and concentration extended to a wholeyear by converting mean daily flow to concentrationsby means of Equations 3, 4, 5 and 7:

where Ci is the concentration on the ith day in g m"3,

Qi is the average daily discharge in m3 s"1, n is the

number of measurements and 31·536 is a factor toextrapolate loads in g s"1 to tonnes year"1;

with the same notations as for (a);

where for Ci, Qi and n are as explained in (a), Qy isthe mean annual discharge (m3 year"1) and 10"6

converts g m"3 to tonnes year"1;

where Cti is the theoretical concentration (g m"3)calculated by the concentration/flow relationships,

Transport of carbon and nitrogen by the Po River 135

T 2. Annual loads of POC, DOC, PN and PIC discharged by the Po River into the Adriatic,according to different calculation methods (for details see text)

MethodPOC

(tonnes year"1)DOC

(tonnes year"1)PN

(tonnes year"1)PIC

(tonnes year"1)

aa 13·4#104 11·2#104 19·4#103 14·6#104

ab 14·8#104 12·0#104 20·3#103 16·2#104

ba 12·0#104 10·6#104 17·5#103 10·2#104

ca 13·4#104 12·1#104 19·4#103 14·6#104

cb 14·7#104 11·9#104 20·0#103 16·0#104

da 14·9#104 12·5#104 17·2#103 7·5#104

aCalculation performed by using data from Pontelagoscuro and Pila.bCalculation performed by using only data from Pontelagoscuro.

T 3. End-member concentrations of DOC, POC and PN for the Po and northern Adriatic invarious seasonal periods

DOC(mg l"1)

POC(mg l"1)

PN(mg l"1)

POC(%)

PN(%)

March 1995River 2·90 2·37 0·39 1·68 0·28Sea 2·00 0·11 0·03 3·46 1·02

June 1995River 2·35 1·86 0·28 3·48 0·52Sea 2·90 0·54 0·09 9·92 1·93

October 1995River 2·93 4·37 0·65 1·01 0·18Sea 1·89 0·63 0·12 6·74 1·29

March 1996River 1·65 1·21 0·10 9·20 0·77Sea 1·15 0·83 0·14 9·18 1·50

and Qi is the daily discharge (m3 s"1) of each day over

the year of the study and 0·0864 is the conversionfactor to transform flow data from m3 s"1 to m3

day"1 and concentrations from g m"3 to tonnesm"3.To perform these calculations we used all the

concentration data collected at Pontelagoscuro andPila, and the flow data available for Pontelagoscuro.This approach relies on the assumption thatconcentrations remain the same in all the majordelta branches. We tested this assumption by usingsome of these methods to calculate the loads fromPontelagoscuro data only (see Table 2).Methods (a) and (b) which use the arithmetic mean

of instantaneous loads or arithmetic means of concen-trations and discharges give equal weight to each pairof variables. This may introduce a major bias in calcu-lations since concentrations measured at low dischargehave the same importance as those at peak discharge.Method (c) reduced this bias and improves the evalu-

ation of mass transport by use of discharge-weightedmeans; while method (d), based on the total dischargecurve is accurate if there is a good correlation betweenconcentration and discharge. The results obtained bythese methods are reported in Table 2.Although the differences in the results obtained by

these methods are quite limited, except for PN forwhich the regression method gave a lower estimatethan those produced by the other methods, due to thestrong underestimation of PIC by the PIC/flow rela-tionship under high solid transport conditions, webelieve that results by method (c) are more appropri-ate. Then, selected fluxes are 13·4#104, 12·1#104,14·6#104 and 19·4#103 tonnes year"1 for POC,DOC, PIC and PN, respectively. Measurements oftotal dissolved nitrogen (TDN) performed by anothergroup over the same period give a load of 135·8#103

tonnes year"1 (G. Tartari, pers. comm.). Therefore,total nitrogen discharge was estimated to be15·5#104 tonnes year"1.

136 M. Pettine et al.

100

40

Chlorophyll a (µg l–1)

PO

C/C

hlo

roph

yll a

× 1

0032

24

16

8

20 400 200

20

Total pigments (µg l–1)

PO

C/t

otal

pig

men

ts ×

100

16

12

8

50 150060 80

4

100

F 8. Behaviour of POC/Chlorophyll a (y=21·57x"1·02, r2=0·88) and POC/total pigments (y=20·15x"0·99, r2=0·85)ratios as a function of Chlorophyll a and total pigments, respectively.

10 000

4

Na mg l–1

DO

C m

g l–1

3

2

1

2000 4000 6000 80000

June 1995

12 000

4

Na mg l–1

DO

C m

g l–1

3

2

1

2000 4000 6000 80000

March 1996

10 000

F 9. Behaviour of DOC in the mixing zone as afunction of [Na+] in two surveys, both in low to mediumsolid transport conditions. March 1996, y="5E"05x+1·77, r2=0·62; June 1995, y=1E"04x+1·82, r2=0·53.

Estuarine behaviour of organic carbon

End-member concentrations for DOC, POC and PNexpressed as mg l"1 and as percentages of suspendedmatter in the estuarine surveys of March, June andOctober 1995 and March 1996, are reported in Table3. The large differences between end members of1 March 1995 and 4 March 1996 are due to thelarge difference in suspended solid concentration: on1 March 1995 this was 163 mg l"1 at Pila and on4 March 1996 it was 11 mg l"1.Concentrations of DOC in the river were generally

slightly higher than those in coastal waters except inJune 1995 when end members for the Po River andcoastal waters were 2·4 and 2·9 mg l"1, respectively.This high marine concentration was due to the highproductivity that characterizes the coastal zone in thesummer period (Zoppini et al., 1995). Concentrationsof POC and PN (mg l"1) were generally higher in theriver than in coastal waters, except for PN on 4 March1996 due to the low solid transport on that occasion.Differences were much more significant in Marchand October 1995 due to very high solid transport inthose periods. Organic carbon and nitrogen in thesolid phase (%POC and PN) were always higher incoastal waters than river waters, reflecting in theformer case a larger contribution of biomass to par-ticulate matter. The only exception was POC% on 4March 1996 since at low solid discharge the contribu-tion of organic matter to particles in the river maybecome more important: POC reached 9·2% on thisoccasion and was similar to the corresponding marineend-member.

The distribution of DOC values in Po estuarinewaters as a function of [Na+] in two different seasons(March 1996 and June 1995) with low to mediumsolid transport conditions is shown in Figure 9. Thedata indicate an approximately conservative behav-iour for DOC with respect to the dilution line, withsome scattering of data which may reflect short-term

Transport of carbon and nitrogen by the Po River 137

12 000

1.0

Na mg l–1

PN

mg

l–1

0.8

0.6

0.4

0.2

2000 4000 6000 80000 10 000

October 1995

12 000

6

Na mg l–1

PO

C m

g l–1 4

3

2

1

2000 4000 6000 80000 10 000

October 1995

12 000

4

Na mg l–1

DO

C m

g l–1

3

2

1

2000 4000 6000 80000 10 000

October 1995

5

F 10. Behaviour of DOC, POC and PN in thewater phase as a function of [Na+] in high solid transportconditions.

12 000

1.4

Na (mg l–1)P

N (

%)

0.8

0.6

0.4

0.2

2000 4000 6000 80000 10 000

October 1995

12 000

8

Na (mg l–1)

PO

C (

%)

4321

2000 4000 6000 80000 10 000

October 1995

567

1.0

1.2

F 11. Distribution of percentage POC and PN valuesin the solid phase as a function of [Na+] in high solidtransport conditions.

negative deviations from the dilution line. On thisoccasion, the percentages of POC and PN in particu-late matter also showed a negative deviation from thedilution line (Figure 11).

Discussion

The Po River receives inputs from 22 rivers equallydistributed on the left and right banks which drainregions from the Alps to the Apennines. The averageannual discharge over the study period was 1587 m3

s"1 which is very close to the mean annual waterflow of 1510 m3 s"1 averaged from long-term dataover the period 1918–1984. The comparison of themean daily flow recorded during the sampling periodswith the corresponding long-term average monthlydischarge (Figure 12) shows some differences inthe discharge curve in spring and fall periods. Then,the year studied represents well the overall annualdischarge of the river while it is only reasonablyrepresentative of the long-term average dischargecurve.Particulate matter shows a quasi-linear relationship

with flow in the lower Po (Pettine et al., 1994),although some peaks in the solid transport may occa-sionally occur at relatively low flow. This is due tosome tributaries from the Apennines (Trebbia, Taro,Panaro, Secchia, Crostolo) which drain clayey–marlysoils making a major contribution to solid transport in

variability of riverine concentrations. The standarddeviation of our DOC measurements of about 10%compromises the interpretation of fine scale changesin concentration. The slopes of the fitted lines arenegative and positive on March and June, respectively,mainly depending on variations of organic matterconcentration in the marine end member. Particulateorganic carbon and nitrogen (as mg l"1 and %)showed similar conservative behaviour during thesame periods. Therefore, mixing dilution processesplay a dominant role, at low to medium solid trans-port, in the export of organic matter by the Po. Theestuarine behaviour of particulate organic matterappears more complex in high solid transport condi-tions (1 March and 10 October 1995). The distribu-tion of DOC, POC and PN as a function of [Na+] isshown in Figure 10 for 10 October. The behaviour ofDOC appears reasonably conservative although somevalues show a tendency to decrease at low salinity,while POC and PN (mg l"1) show significant

138 M. Pettine et al.

July

3500

0

Q m

3 s–1

Feb

.

1000

3000

2500

2000

1500

500

Jan

.

Dec

.

Nov

.

Mar

ch

Apr

il

May

Jun

e

Oct

.

Sep

t.

Au

gust

F 12. Mean daily flow over the study period (openbars) and corresponding long-term average monthly dis-charge over the period 1918–1984 (shaded area).

the Po without producing a marked increase in waterflow. These tributaries were responsible for the peaksin suspended matter and its PIC content on 28February to 1 March 1995 and 9–10 October 1996(see Figure 6).Particulate matter in the lower Po appears to be rich

in organic carbon (average POC value =6·4&4·4%)compared to the range (1·6–6%) that includes mostrivers in the world (Meybeck, 1982). The averagePOC/PN weight ratio is 7·1&2·5. Similar values forPOC/PN result from the ratio (=7·1) of the interceptsat SM=1 of Equations 1 and 2 which show a similardependence of POC and PN on suspended matter aswell as from the inverse of the slope (=6·7&0·4) ofEquation 6. Such a value for POC/PN in the lower Pois lower than the value of 8·5 (range 8–10) reported byMeybeck (1982) for world rivers and found to beinsensitive to environmental factors and suggests ahigh contribution of aquatic plant sources to theparticulate matter. This autochthonous contributionis important all year round since only 8 out of 50samples show POC/PN ratios higher than 8.The average POC value found in the lower Po is

similar to the average carbon content of suspendedmatter in the lower Rhine River (6·6&2·2%) andhigher than that in the lower Rhone (4·4&1·5%)(Santiago et al., 1994). Our average POC value gives avalue for particulate carbon export of 1·91 tonneskm"2 year"1, which is higher than the 1·2&0·38tonnes km"2 year"1 suggested by Lugo (1983) andthe 1·0 tonnes km"2 year"1 obtained from the dif-ference between TOC and DOC export rates pro-posed by Meybeck (1982), both for temperate rivers.The average value of DOC in the Po (2·1&0·6 mg

l"1) is lower than the average value of 3 mg l"1 forworld rivers in temperate environments (Meybeck,

1982), but well within the limits given by Thurman(1985) for arctic and alpine rivers. Consistent withlow DOC is the specific transport rate of dissolvedorganic matter, which at 1·65 tonnes km"2 year"1 islower than the value of 2·21&0·3 tonnes km"2

year"1 characteristic of temperate rivers (Lugo,1983).Thus, carbon seems to be preferentially transported

in the particulate phase in the Po compared to othermajor temperate world rivers. This may be explainedby a high autochthonous contribution to POC as wellas by the fact that Europe has a high weathering rate(Kempe et al., 1991). The variation of DOC/TOCratios over the study period (range 0·30–0·67) andtheir average value of 0·49&0·12 are also character-istic of highly erosive environments according toMeybeck (1982).The average TOC concentration in the lower Po

(4·5&1·5 mg l"1) and the TOC export rate (3·57tonnes km"2 year"1) are slightly lower than themedian TOC and the average export rate estimatedfor major temperate rivers (5 mg l"1 and 4 tonneskm"2 year"1, respectively; Meybeck, 1982). There-fore, the total level of organic carbon in the Po doesnot seem to be significantly altered by anthropicactivity, due to the self-purification capacity of theriver. TOC levels in some major temperate rivers(Rhine, Ems and Danube) that are polluted, havebeen reported to reach levels four to five times higherthan those in the Po (Meybeck, 1982).The average PIC value found in the lower Po

(1·9&0·7%) is about double that of major worldrivers (0·9%; Meybeck, 1982), while the average PNvalue (0·94&0·72%) falls in the upper part of therange (0·1–1·3%) that includes most rivers in theworld, as also noted above for POC.The Rhone and the Po are the two largest fresh

water inputs to the Mediterranean, after the buildingin 1970 of the imposing dike of Assuan on the Nileand the nearly complete exploitation of its waters forirrigation. Both these rivers originate from the Alpsand drain large catchment areas (Rhone 96 000 km2,and Po 70 091 km2). Population densities in thewatersheds of these rivers differ greatly (232 and 84inhabitants km"2 for the Po and Rhone, respectively).Solid transport in the Po (5·5#106 tonnes year"1) ishigher than in the Rhone (4·6#106 tonnes year"1)although the latter has a slightly higher water dis-charge rate (55 km3 year"1 vs 47 km3 year"1). Themodes of transport of organic carbon in the two riversare also quite different. TOC discharge from theRhone has been estimated at 15#104 tonnes year"1,based on recent data, one third of which is POC(Kempe et al., 1991). For the Po, POC discharge

Transport of carbon and nitrogen by the Po River 139

ern Adriatic (18 900 km2) (Degobbis et al., 1990), it ispossible to estimate that the overall annual productionis about 170#104 tonnes year"1. Recent measure-ments in western coastal waters of the northernAdriatic under the influence of the Po (Zoppini et al.,1995) indicate the existence of a highly productivecoastal belt of about 3000 km2 where primary produc-tion amounts to 210–260 g C m"2 year"1. The totalannual phytoplankton production in this area may beabout 70#104 tonnes year"1. Therefore, the totalorganic carbon discharge by the Po amounts to about15% of the overall annual productivity in the entirenorthern Adriatic basin, and to about 36% of theannual productivity in the most productive region ofthe northern Adriatic. Under summer stratified con-ditions, the high particulate organic load in the Po hasa particularly strong influence on the narrow northernregion of the Adriatic, where fluvial suspended mattertends to be confined (Faganeli et al., 1994). Thedecomposition of organic matter transported by theriver, the respiration of planktonic material fuelled byriverine inputs of nutrients and the set up of waterstratification combine to produce recurrent hypoxia inthe bottom waters of the northern Adriatic.Measurements of bacterial production rates, at a

station in the Po River plume, based on [3H] thymi-dine incorporation (Puddu et al., 1996) gave meanintegrated values of 344 and 553 mg C m"2 day"1 inJuly and October, respectively, and 80 mg C m"2

day"1 in January. The same authors estimated thatbacteria process about 80% of the carbon synthesizedby autotrophic plankton in the more productiveperiods, while bacteria process a quantity of carbonwhich exceeds by a factor 2·6 the quantity of primaryproduction in winter. These authors considered theirvalues as conservative estimates since they were basedon a cell/thymidine conversion factor about half thatused by other workers. Therefore bacterial carbondemand (BCD) could be 1·6 and 5 times greater thanprimary production in summer and winter, respect-ively. Such considerations indicate the importance ofexternal organic matter sources, mainly by the Po, butalso by other minor Italian rivers (Adige, Brenta,Piave, Tagliamento, etc.) in the northern Adriatic.

Conclusions

The temporal variability of dissolved organic carbonand particulate carbon (total and organic) and nitro-gen have been evaluated in the lower Po River overa 1 year study. Concentrations measured at twostations, about 70 km apart, and at two depths in eachstation showed a fair homogeneity both in the watercolumn and in the longitudinal direction. Average

(13·4#104 tonnes year"1) is slightly higher thanDOC discharge (12·1#104 tonnes year"1). Thus,DOC discharge rates for these two rivers are similar;while in terms of TOC the Po export rate (25·5#104

tonnes year"1) is about 1·7 times higher than that ofthe Rhone, making the former river the largest con-tributor of organic matter to the Mediterranean. Thisorganic matter is discharged into the semi-enclosedAdriatic basin along with high quantities of nitrogenand phopshorus. The load of total nitrogen (TN) hasbeen estimated as 15·5#104 tonnes year"1, 12%of which is particulate while about 65% is due toNO3

". This TN load is about double that reportedfor the Rhone (7·4#104 tonnes year"1; El-Habr &Golterman, 1987), due to the intensive exploitationof the Po valley for agricultural uses. The NO3

"

load of about 105 tonnes year"1 is consistent withdata recorded in recent years by the Po WaterAuthority and indicates an increase in the nitrogendischarge with respect to previous estimates whichdate back to the beginning of 1980 (Kempe et al.,1991). This estimated NO3

" load is similar to theload the Loire discharges in to the Atlantic oceanwhich is of about 9#104 tonnes year"1 (Meybecket al., 1988).Due to the short residence time of waters in the Po

estuary, river borne dissolved and particulate matterpasses to the sea without significant processing ortransformation. In low to medium solid transportconditions, dissolved and particulate organic mattershow an approximately conservative behaviour as afunction of mixing, while at high solid transportparticulate organic matter shows a clear deviationfrom the dilution values. Since this deviation concernsboth POC and PN concentrations per unit volume ofwater and as percentage in the solid phase it involvesboth settling of particles and bacterial utilization oforganic matter. However, both these processes occurin the prodelta region of the coastal area and do notinfluence the net efflux of matter to the sea.Fresh phytoplankton biomass contributes an aver-

age of about 15% to the total POC exported by the Poanually. This increases to about 25% during the mostproductive period from April to August; contributionsestimated from total pigments may be expected to beabout 20% higher according to Equation 8. Similarfigures are reported for phytoplankton contributionsto the lower courses of eutrophized rivers during thesummer period: algal biomass contributed 55% ofPOC in the St. Lawrence River (Pocklington & Tan,1987), 50% in the Loire (Billen et al., 1986) and15–65% of PC in the lower Rhine (Admiraal et al.,1992).By assuming an average value of 89·6 g m"2

year"1 of carbon fixed by phytoplankton in the north-

140 M. Pettine et al.

values calculated from all the data gatheredwere 2·1&0·6, 2·4&1·2, 2·0&3·9, 4·4&4·8 and0·35&0·18 mg l"1 for DOC, POC, PIC, PC andPN, respectively. Corresponding annual discharges ofDOC, POC, PIC and PN amounted to 12·1#104,13·4#104, 14·6#104 and 19·4#103 tonnes year"1,respectively. According to these results the Po is themost important allochtonous source of organic matterfor the entire Mediterranean sea, contributing25·5#104 tonnes year"1 to the total organic carbontransport. A similar high contribution of 15·5#104

tonnes year"1 has been estimated for total nitrogeninputs.Estuarine processes in the Po River do not influence

the organic matter inputs, hence the estimated dis-charge may be considered as the net efflux of organicmatter into the sea. Dilution processes are in fact themain factors controlling dissolved and particulate con-centrations (DOC, POC, PN) in the mixing zone.Negative deviations of particulate organic carbon andnitrogen with respect to the dilution line wereobserved in high solid transport conditions owing toboth sink processes of suspended solids in the prodeltaregion and bacterial utilization of organic matter.The strong correlation between POC and PN in the

solid phase, their exponential decrease with increasingsuspended matter and, finally, the direct evaluation ofthe algal POC contribution to the total POC transportsuggest an important role of riverine autochthonousorganic matter which is more marked in the summerperiod. This fresh organic fraction, along with morerefractory organic components that dominate in win-ter and during flood events and may become availableafter photochemical transformations occurring in thecoastal area, sustain bacterial carbon demands whichlargely exceed exudated or fixed organic carbon byphytoplankton in the Adriatic ecosystem.Then, organic matter and nitrogen load discharged

by the Po, together with minor contributions by otherrivers (Adige, Brenta, Piave etc.), provide additionalenergy to the northern Adriatic system which signifi-cantly influences their productivity and trophicdynamics. The role of this major external carbonsource should be further investigated when seeking anexplanation for hypoxic conditions and massiveformation of mucilaginous aggregates that affect thissystem.

Acknowledgements

This study was part of the PRISMA Project, a multi-disciplinary project supported by the Italian Ministryfor Universities and Research, to investigate eutrophi-cation problems in the Adriatic sea. The authors

wish to thank F. Bacciu, IRSA-CNR Rome, andS. Valsecchi, IRSA-CNR Brugherio (Milan), fortheir assistance during field sample collection andanalyses. Thanks are also due to Dr A. Allodi of theHydrological Office of the Po for making availableflow data at Pontelagoscuro, and to D. C. Ward forhelp with the English language.

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142 M. Pettine et al.

Appendix

Surface and bottom values of water flow (Q), SM, DOC, PC, POC, PN, phaeopigments and chlorophyll a at Pontelagoscuroand Pila stations over the study period

SampleFlow (Q)(m3 s"1)

SM(mg l"1)

DOC(mg l"1)

PC(mg l"1)

POC(mg l"1)

PN(mg l"1)

Phaeopigments(ìg l"1)

Chl a(ìg l"1)

SurfacePon 28/2 1860 352·3 3·3 12·1 4·2 0·6 16·1 8·2Pila 1/3 2090 162·9 2·6 6·0 2·4 0·4 14·6 5·3Pon 10/4 750 18·5 2·2 3·9 3·3 0·4 17·1 91·5Pila 11/4 680 22·5 1·9 4·3 3·6 0·5 100·0 80·0Pon 16/5 3440 167·0 2·2 6·7 3·7 0·5 12·6 3·3Pila 15/5 3190 123·5 2·1 4·8 2·7 0·5 9·5 2·3Pon 20/6 2070 48·3 3·3 2·3 1·5 0·2 2·1 14·3Pila 21/6 1810 41·9 2·3 2·3 1·7 0·3 4·5 19·4Pon 11/7 1160 32·6 2·0 2·5 2·1 0·3 17·4 45·6Pila 11/7 1160 9·0 2·2 1·7 1·5 0·3 21·7 51·9Pon 30/8 1100 33·8 1·9 2·4 1·8 0·3 47·0 15·7Pila 30/8 1100 30·0 2·0 1·5 1·0 0·2 9·9 13·1Pon 11/9 1280 60·7 2·8 3·3 1·9 0·3 10·5 12·1Pila 13/9 1000 29·7 2·3 1·5 1·1 0·2 8·3 10·7Pon 9/10 2500 429·2 2·6 17·4 4·4 0·6 5·0 0·4Pila 10/10 2420 346·3 3·4 12·9 3·3 0·6 2·3 2·4Pon 7/11 925 22·1 1·9 2·3 1·9 0·3 4·2 14·7Pila 7/11 925 13·6 1·3 1·6 1·5 0·2 2·6 16·1Pon 5/12 1110 10·9 1·7 1·3 1·1 0·2 0·7 1·6Pila 5/12 1110 13·1 1·7 1·1 0·9 0·1 2·0 0·8Pon 23/1 1870 51·9 1·8 4·3 3·8 0·5 2·0 1·5Pila 24/1 2020 66·3 1·7 4·4 3·2 0·4 1·2 1·0Pon 19/2 1510 22·9 1·6 2·1 1·9 0·3 1·7 1·0Pila 20/2 1690 22·4 1·5 3·0 2·9 0·4 0·4 1·9Pon 4/3 1440 41·1 1·6 3·4 2·8 0·2 1·6 3·9Pila 4/3 1440 11·0 1·8 1·2 1·0 0·1 0·1 5·4

BottomPon 10/4 750 28·2 1·9 4·5 3·7 0·6 69·4 55·7Pila 11/4 680 29·4 2·0 5·2 4·0 0·7 — —Pon 16/5 3440 193·3 2·2 7·5 4·0 0·5 13·9 2·5Pila 15/5 3190 206·1 3·3 8·5 4·7 0·6 12·2 7·5Pon 20/6 2070 73·7 2·3 3·5 2·3 0·4 3·6 17·2Pila 21/6 1810 53·3 3·7 2·8 1·9 0·3 5·5 20·4Pon 11/7 1160 33·9 1·8 2·4 1·9 0·4 14·3 48·3Pila 11/7 1160 10·8 2·0 1·6 1·4 0·3 18·7 44·4Pon 30/8 1100 29·9 1·8 2·1 1·6 0·3 47·0 13·0Pila 30/8 1100 53·6 2·1 2·4 1·4 0·3 5·8 17·8Pon 11/9 1280 58·0 2·2 2·7 1·6 0·3 10·0 15·1Pila 13/9 1000 32·1 2·3 1·7 1·1 0·2 11·4 13·1Pon 9/10 2500 526·2 2·5 21·7 4·8 0·6 1·2 1·2Pila 10/10 2420 562·7 3·2 22·0 5·0 0·8 1·5 4·1Pon 7/11 925 26·7 1·4 3·0 2·7 0·4 4·6 12·6Pila 7/11 925 14·8 1·5 1·3 1·1 0·2 10·9 10·5Pon 5/12 1110 13·4 1·6 1·0 0·7 0·1 1·7 1·1Pila 5/12 1110 14·0 2·1 1·3 1·1 0·1 3·8 0·2Pon 23/1 1870 45·9 1·7 4·5 4·0 0·5 1·6 1·4Pila 24/1 2020 29·7 1·5 2·1 1·4 0·2 1·3 1·2Pon 19/2 1510 28·0 1·6 2·3 1·9 0·3 2·2 0·8Pila 20/2 1690 26·6 1·7 2·4 2·1 0·3 0·2 2·3Pon 4/3 1440 44·8 1·7 3·1 2·2 0·2 — —Pila 4/3 1440 15·3 1·5 1·7 1·4 0·1 0·5 6·1