phytoplankton-bacterioplankton interactions and carbon fluxes through microbial communities in a...

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R E S E A R C H A R T I C L E

Phytoplankton^bacterioplankton interactions and carbon£uxesthroughmicrobial communities inamicrotidal lagoon (LagoonofVenice,Northern Italy)Q1

Alessandra Pugnetti1, Paola Del Negro2, Michele Giani3, Francesco Acri1, Fabrizio Bernardi Aubry1,Franco Bianchi1, Daniela Berto3 & Annalisa Valeri2

1Istituto di Scienze Marine (ISMAR), Consiglio Nazionale delle Ricerche, Castello, Venezia, Italy; 2Dipartimento di Oceanografia Biologica (BiO), Istituto

Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy; and 3Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA),

Chioggia, Italy

Correspondence: Alessandra Pugnetti,

Istituto di Scienze Marine (ISMAR), Consiglio

Nazionale delle Ricerche, Castello 1364/A,

I-30122 Venezia, Italy. Tel.: 139 041 240 4711;

fax: 139 041 520 4126; e-mail:

[email protected]

Present address: Michele Giani,

Dipartimento di Oceanografia Biologica (BiO),

Istituto Nazionale di Oceanografia e di

Geofisica Sperimentale (OGS), Via A. Piccard

54, I-34051 Trieste, Italy.

Received 16 March 2009; revised 11 December

2009; accepted 28 December 2009.

DOI:10.1111/j.1574-6941.2010.00839.x

Editor: Riks Laanbroek

Keywords

phytoplankton productivity; bacterial

production; organic carbon; exoenzimatic

activity; Venice lagoon; carbon fluxes.

Abstract

The strength of the bacteria–phytoplankton coupling and the importance of the

microbially mediated carbon fluxes have been investigated in a microtidal lagoon

(Lagoon of Venice), with emphasis on the trophic variations, in a within-system

perspective. The large trophic heterogeneity of the three stations considered

corresponded to an elevated variability of phytoplankton biomass and production

(from 0.1 up to 300 mg C L�1 h�1), while bacteria standing stock and production

(from 2 to 8 mg C L�1 h�1) appeared, in comparison, to be much more constant.

The relationships between bacteria and the phytoplankton community could not

be related to the trophic state in a straightforward way; rather, some patterns

common to the three stations could be evidenced. In particular, the two

communities appeared to be clearly uncoupled, bacterial carbon demand (BCD)

always exceeding dissolved primary production (DPP) and, mostly, also total

primary production, independent of the station and the season considered. The

occurrence of situations in which bacterial production was larger than primary

production and the continuous prevalence of BCD over DPP implied that, quite

independent of the trophic variability, sources of organic carbon other than

phytoplankton production were necessary and available to sustain the bacterial

metabolism in the Lagoon of Venice.

Introduction

Across-ecosystem analyses of the relationships between

bacterial carbon production (BCP) or bacterial carbon de-

mand (BCD) and primary production (PP) have supported

the general conceptual model that the bacterioplankton–phy-

toplankton coupling varies as a function of ecosystem char-

acteristics. In particular, a trophic dependence of bacteria on

algae, through the photosynthetic production of dissolved

organic carbon [dissolved primary production (DPP)], would

be characteristics of aquatic ecosystems where bacteria are

carbon limited. There is a general trend towards a tight linkage

in open oceans and clear water lakes and of loose coupling in

coastal or eutrophic sites. In open seas and clear water lakes,

the general picture is one of a nutrient-controlled energy flow,

where phytoplankton particulate organic matter is the main

energy source for grazers and dissolved organic carbon (DOC)

derived from phytoplankton (e.g. direct exudation through

DPP, lyses) represents the main energy source for bacteria,

supporting all the BCD, so that the bacteria–phytoplankton

coupling is tight. In contrast, in estuaries and coastal lagoons,

the plankton system might show a net heterotrophic metabo-

lism (respiration4 gross primary production) or a seasonal

shift from net autotrophy to net heterotrophy that may be due

to turbidity and nutrient variations and due to inputs of

organic matter allochthonous to the planktonic system

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FEMS Microbiol Ecol ]] (2010) 1–12 c� 2010 Federation of European Microbiological SocietiesPublished by Blackwell Publishing Ltd. All rights reserved

F E M S E C 8 3 9 B Dispatch: 2.2.10 Journal: FEMSEC CE: Mahendra

Journal Name Manuscript No. Author Received: No. of pages: 12 TE: Deepa/MiniFEMSEC 839

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(Revilla et al., 2002; Hopkinson & Smith, 2005). As a

consequence, in transitional ecosystems, BCD may be met

not only by DPP but also by nonphytoplanktonic (leachates

from plants and exudates from macrophytes, pore water from

sediments) or by allochthonous sources (runoff, riverine

inputs) of organic carbon, leading to a loose coupling between

bacteria and phytoplankton. High bacteria biomass and BCP

may therefore occur in transitional ecosystems, where bacteria

and phytoplankton communities are often uncoupled, plank-

ton community respiration (CR) may exceed phytoplankton

photosynthesis and secondary production may be based, at

least during some seasons, on bacterial mobilization of

preformed organic matter, rather than on contemporaneous

phytoplankton photosynthesis (Gaedke & Kamjunke, 2006;

Berglund et al., 2007).

The pattern of strength variation of the bacterioplank-

ton–phytoplankton coupling appears evident from across-

ecosystem studies, but it does not necessarily hold for

smaller spatial and temporal scales. In some coastal ecosys-

tems, indeed, the reverse pattern has been observed, and a

tight coupling between DPP and BCD is recorded at the

most eutrophic sites and a loose linkage at the most

oligotrophic ones (Gonzalez-Benitez & Gattuso, 2003;

Rochelle-Newall et al., 2008).

The Lagoon of Venice (northern Adriatic Sea) is the

largest Italian lagoon and one of the largest in the Mediter-

ranean. The lagoon is intrinsically characterized by a high

heterogeneity in morphological structures and in the spatial

distribution of physical, biogeochemical and biological

parameters (Bianchi et al., 2000; Ravera, 2000; Acri et al.,

2004; Solidoro et al., 2004). The plankton communities in

this environment undergo complex spatial and temporal

dynamics. Exchanges with the sea, benthic–pelagic coupling

and river inflows appear to be important in defining the

spatial distribution of the plankton communities, even

though seasonality is the main factor leading successionQ2

(Bernardi Aubry & Acri, 2004; Bandelj et al., 2008). Phyto-

plankton community production has been traditionally

considered the main process sustaining the whole plank-

tonic food web in the lagoon (Bianchi et al., 2000; Acri et al.,

2004), while bacterial production and biomass have been

largely neglected, being studied only sporadically (Sorokin

et al., 1996, 2002). No information on the bacteria–phyto-

plankton coupling and on the carbon fluxes through the

microbial communities in Lagoon of Venice is available,

whereas long-term studies on the biomass and taxonomic

composition of phytoplankton and macrophytes are well

documented for the last 30 years (Acri et al., 2004; Sfriso

et al., 2005). The net ecosystem metabolic balance for the

whole lagoon has been calculated on the basis of the nutrient

budget, in the framework of the Land-Ocean Interactions in

the Coastal Zone activities (Giordani et al., 2005), and it

showed a state of near-balance between autotrophic and

heterotrophic processes, on a yearly base, with net auto-

trophy prevailing only in summer. A prevalent hetero-

trophic metabolism of the lagoon has been evidenced

through dissolved oxygen mass-balance models applied to

time series of high-frequency oxygen data collected in situ

(Ciavatta et al., 2008).

In this work, we aim at evaluating the strength of the

bacteria–phytoplankton coupling and at quantifying the

importance and the significance of the microbially mediated

carbon fluxes in the Lagoon of Venice and their seasonal and

spatial variations, with an emphasis on trophic state varia-

bility. Our work focuses on the water column, which, in this

shallow environment, is the result of the continuous water–

sediment interactions. We considered PP (total, particulate

and dissolved), BCP and CR as the main indicators of the

plankton system functioning, responding to environmental

variables such as light, nutrients and organic matter avail-

ability that are commonly influenced by the catchment’s

features and disturbance due both to anthropic and to

natural changes. We wish to make a contribution to the

debate about the environmental factors that influence the

bacteria–phytoplankton linkage and the microbially

mediated C fluxes from a within-system viewpoint. In

particular, we aimed at assessing, in an area characterized

by different trophic conditions: (1) the strength of the

coupling between phytoplankton and bacterial communities

(relationships between DPP and BCD), (2) the prevalent

base of the planktonic food web (relationships between BCP

and PP) and (3) the net autotrophy vs. the net heterotrophy

nature of the planktonic system (relationships between CR

and PP).

Materials and methods

Study area

The Lagoon of Venice (Fig. 1) is a large Mediterranean

lagoon of �550 km2, located in the northern Adriatic Sea. It

is surrounded by densely inhabited shores and industrial

plants and it hosts ports, shipyards, marinas, fisheries,

aquaculture and recreational activities. The lagoon has an

average depth of �1 m and it is morphologically character-

ized by the presence of large shallow areas and by a network

of deeper (5–10 m) channels. It is connected to the Adriatic

Sea by three inlets, through which tidal currents drive water

exchanges. The tidal amplitude is�100 cm, with maxima up

to 150 cm. The lagoon can be classified as polyhaline. Twelve

main tributaries annually carry about 35 m�3 s�1 of fresh-

water in the lagoon (Zuliani et al., 2005); nitrogen and

phosphorus load are in the order of Q34000 t N year�1 and

230 t P year�1 (Collavini et al., 2005).

The Lagoon of Venice presents a high variability in most

environmental parameters and a high habitat heterogeneity.

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FEMS Microbiol Ecol ]] (2010) 1–12c� 2010 Federation of European Microbiological SocietiesPublished by Blackwell Publishing Ltd. All rights reserved

2 A. Pugnetti et al.

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Within this ecosystem, we studied three shallow water

(2–3 m deep) sites (Fig. 1) located in plankton-dominated

areas of the lagoon. In these areas, the presence of macro-

phytes is negligible and most of the pp is by the phytoplank-

ton (Acri et al., 2004; Sfriso et al., 2005).

The three stations represent different environments typi-

cal of the central and northern lagoon. St. 1 is influenced by

urban wastewaters from the town of Mestre; st. 2 is a marshy

area and it represents a typical lagoon environment; st. 3 is

close to the inlet of Lido that allows the communication of

the lagoon with the Adriatic Sea, but it is also influenced by

the inputs of freshwater channels.

Sampling

Transparency (Secchi disk), photosynthetically active radia-

tion (PAR; LiCor Li-192), temperature, salinity, dissolved

oxygen and pH (Idronaut Ocean Seven 316 multiprobe)

were measured throughout the water column. Samples for

dissolved inorganic macronutrients (Grasshoff et al., 1999),

phytoplankton and bacterial communities, respiration, par-

ticulate organic carbon (POC) and DOC were collected at

the surface water layer, representative of the whole water

column. The water column was assumed to be well mixed

because the differences in surface and bottom salinity and

temperature were minimal. Hydrological parameters, nutri-

ents and chlorophyll a (chl) were determined monthly, and

the microbial carbon parameters and fluxes seasonally

(January, April, July and October 2005). Samplings were

always performed at neap tide, in order to minimize the

effects of tidal currents.

Phytoplankton production and CR

Phytoplankton chl was determined spectrofluorometrically

according to Holm-Hansen et al. (1965). Samples for total

primary production (TPP) (14C method) were incubated

under simulated in situ conditions (natural sunlight and

maintaining in situ temperature) for 2 h around noon. Two

hundred and fifty milliliters of subsamples and a dark bottle

were inoculated with 148 kBq of NaH14CO3. TPP was

determined on duplicate subsamples (5 mL) that were

acidified and stirred for one hour, before being radioassayed.

Particulate primary production (PPP) and DPP was deter-

mined after filtrations of 10 mL of samples on 0.2 mm

polycarbonate filters at a low (o 10 mmHg) vacuum pres-

sure. The DPP was always significantly different from the

corresponding dark bottle values. In order to also take into

account the potential rapid bacterial uptake of labelled

dissolved organic carbon (DO14C) during incubations, we

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Fig. 1. Location of the sampling stations in the

Venice Lagoon.

Q13


3Phytoplankton and bacterioplankton in the Lagoon of Venice

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measured the 14C-labelled 0.2–2-mm size fraction as well, by

an additional filtration on 2-mm pore-size polycarbonate

filters. Because the 0.2–2-mm size fraction (PPpico, as the

difference between the 0.2 and the 2 mm filters) also includes

the 14C incorporated by autotrophic picoplankton, the sum

of DPP and PPpico could be taken to represent the maximum

potential carbon exudation rates.

The daily TPP was estimated considering the ratio

between the duration of the incubation (around 2 h) and

the total day length that ranged between 9 h (in winter)

and 15 h (in summer). Considering the inherent daily

variability of the phytoplankton community, related to the

tidal hydrodynamics – not assessed in the present work – the

daily TPP extrapolated from the hourly data must be

considered as a potential value.

CR was estimated as the difference in dissolved oxygen at

the beginning and after a 24-h incubation of triplicate 250-

mL water samples in the dark and at in situ temperature.

Dissolved oxygen was assayed by potentiometric Winkler

titrations (794 Basic Titrino, Metrohm). The oxygen uptake

rates were transformed into inorganic carbon production

assuming, in the absence of detailed information about

the composition of the substrate used, a respiratory quotient

of 1.

Bacterial abundance and ectoenzymaticactivities

Heterotrophic bacterial abundances were estimated on

samples fixed with 2% final concentration borate-buffered

prefiltered formalin, following a modification of the method

of Porter & Feig (1980). Subsamples (10 mL) were stained

with 40,6-diamidino-2-phenylindole (Sigma; 1 mg mL�1 final

concentration) and enumerated by an Olympus BX 60 F5

epifluorescence microscope at 1000 using a UV filter set (BP

330–385 nm). Bacterial abundance was converted into car-

bon equivalents (heterotrophic bacterial biomass, HBB)

using the conversion factor of 20 fg C per cell (Lee & Fuhr-

man, 1987).

BCP was estimated by the incorporation of 3H-leucine

(Leu-BCP) (Kirchman et al., 1985; Smith & Azam, 1992).

Triplicate 1.7-mL aliquots and two killed controls [90mL,

100% trichloracetic acid (TCA)] were amended with 20 nM

radiotracer and incubated at in situ (� 2 1C) temperature in

the dark. Incubations were stopped with 100% TCA after

1 h. The samples were processed following the microcentri-

fugation method (Smith & Azam, 1992). Radioactivity in

the samples was determined by a liquid scintillation counter

(Packard Tri-Carb 300) after the addition of 1 mL of

scintillation cocktail (Ultima Gold MV; Packard). Incor-

poration of 3H-leucine was converted into carbon produced

via bacterial protein production according to Simon &

Azam (1989), assuming a twofold isotope dilution for Leu.

BCD is the sum of BCP and bacterial respiration (BR),

assuming insignificant release of organic matter. Because of

methodological difficulties in correctly measuring BR, BCD

was estimated as Leu-BCP/BGE, with BGE values calculated

from the del Giorgio & Cole (1998) empirical model. The

daily rates of bacteria activity were estimated assuming the

activity rates to be constant throughout the day. Hydrolytic

ectoenzyme activities were measured using fluorogenic

substrates (Hoppe, 1993) derived from 7-amino-methyl-

coumarin (AMC) and 4-methyl-umbelliferone (MUF).

Aminopeptidase activity was assayed as the hydrolysis rate

of L-leucine-AMC. b-D-Glucosidase, a-D-glucosidase, b-D-

galactosidase, a-D-galactosidase, lipase, N-acetyl glucosami-

nidase and alkaline phosphatase were assayed using MUF

derivatives. Enzyme activities were expressed in terms of the

rate of MUF or AMC production. After evaluation of the

saturating concentration, hydrolysis was measured by in-

cubating 2.5-mL subsamples with 50 mM MUF phosphate

and 200mM leucine-AMC and other MUF substrates for 1 h

at in situ temperature in the dark. All samples were run in

triplicate with 0.2 mm filtered and boiled seawater as con-

trols. The fluorescence released by enzymatic cleavage of the

artificial substrates was measured fluorometrically at 380/

365 nm excitation and 440/455 nm emission for AMC/MUF

substrates using a Shimadzu RF 1501 fluorometer. Standard

solutions of MUF and AMC were used to perform

calibration curves. The velocity of hydrolysis (mM h�1) of b-D-

glucosidase, a-D-glucosidase, b-D-galactosidase, a-D-galactosi-

dase, lipase, N-acetyl glucosaminidase and L-leucine-amino-

peptidase was converted into the C content of the organic

component hydrolyzed from the model substrate using the

conversion factor of 72 in cases where the organic component

is glucose or a 6-C monosaccharide or a 6-C amino acid

(Hoppe, 1993) and of 216 where the organic component is

oleic acid. The sum of the enzymatic activities, expressed as

mg C L�1 h�1, represents the rate of C potentially mobilized

from the macromolecules by hydrolytic enzymes.

Organic carbon

Water samples for DOC, chromophoric dissolved organic

material (CDOM) and POC were analyzed after filtration

through precombusted (4 h, 450 1C) Whatman GF/F filters.

DOC concentrations were measured using a Shimadzu TOC

5000 Analyzer with a 1.2% Pt on silica as a catalyst at 680 1C

(Cauwet, 1994). Samples were acidified (pH = 2) with HCl

and purged with pure air for 10 min immediately before

analysis. DOC concentrations were calculated by subtracting

the system blanks (8.7� 0.9mmol L�1) and dividing by the

slope of the calibration curve (Thomas et al., 1995).

Potassium hydrogen phthalate was used as the standard.

One hundred microliters of samples were injected for each

analysis and the concentration was calculated as the average

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of three to five replicates. The reproducibility was o 3%.

POC concentrations were determined using a CHN Ele-

mental AnalyzerFisons EA1108, after acidification with HClQ4

(Nieuwenhuize et al., 1994) to remove the inorganic carbo-

nate, with high reproducibility (o 2%). Acetanilide was

used as the standard.

The UV-Vis spectra of the filtered lagoon water were

performed by a UV2 spectrophotometer ATI Unicam from

270 to 800 nm with a 5-cm quartz optical cell using Milli-Q

water as the blank, as reported by Vodacek et al. (1997). To

quantify the CDOM in each sample, we used the absorption

coefficient (aCDOM, in m�1) at 355 nm as a proxy, calcu-

lated from the sample absorbance (A) in nm�1 and path

length (L) in meters as follows:

aCDOMð355Þ ¼ 2:303ACDOMð355Þ=L

Results

Trophic characteristics and climatology

At the three selected stations, pluriannual studies for hydrol-

ogy, nutrients and plankton have been carried out (Bianchi

et al., 2003; Acri et al., 2004). These stations are reference

ones for long-term ecological research studies (LTER) in the

Lagoon of Venice, one of the LTER national and interna-

tional sites. The monthly variations of the main hydrological

and trophic parameters, averaged for the last three years

(2006–2008) and compared (median comparison, Man-

n–Whitney test) with those observed in 2005, did not show

statistically significant differences: the year 2005 therefore

represents fairly well the most recent conditions of the

Lagoon of Venice.

The main hydrological and trophic variables at the three

stations are reported, as averages for each season from the

monthly samplings, in Table 1. Temperature showed the

typical seasonal pattern, with the lowest values in winter and

the highest in summer at all the three stations. The lowest

salinity was measured at st. 1, from winter to summer, and at

st. 3 in autumn. The reversed pattern was recorded for

nutrients, which showed the highest concentrations at st. 1,

from winter to summer, and at st. 3 in autumn. At all the

stations, the dominant form of DIN was nitrate and the N/P

ratio was always much higher than 16, pointing out that

inorganic P could be a potential limiting macronutrient,

even though P was never exhausted. Chl peaked at every

station in summer; the maxima were recorded at st. 1 from

spring to autumn. St. 2 exhibited the highest salinity and the

lowest nutrient and chl concentrations.

From the seasonal samplings, it appears that the whole

water column (maximum depth: 2 m) was always euphotic

at every station, even though PAR was rapidly attenuated

and only 5–20% of the surface irradiance reached the

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Table 1. Seasonal and yearly averages (AVG) and SD of the main hydrological and trophic state parameters at the three stations

Winter Spring Summer Autumn Year

AVG SD N AVG SD N AVG SD N AVG SD N AVG SD N

St. 1

Temperature ( 1C) 7.8 2.1 3 19.3 3.7 3 25.4 2.3 3 13.0 3.2 3 16.4 7.3 12

Salinity 26.4 3.0 3 21.9 2.6 3 29.2 1.6 3 20.4 9.3 3 24.5 5.7 12

Dissolved inorganic nitrogen (mM) 51.9 51.9 3 46.4 18.1 3 34.4 18.6 3 121.8 41.6 3 63.6 42.2 12

Phosphate (mM) 0.5 0.2 3 1.3 0.8 3 1.3 0.6 3 2.1 1.5 3 1.3 1.0 12

Silicate (mM) 17.6 14.3 3 50.1 8.5 3 28.4 2.6 3 64.3 31.6 3 40.1 24.4 12

Chlorophyll a (mg L�1) 1.5 0.3 3 9.0 4.6 3 13.9 6.6 3 4.0 4.9 3 7.1 6.4 12

St. 2

Temperature ( 1C) 7.2 1.8 3 17.5 4.1 3 24.9 1.3 3 14.6 3.4 3 16.0 7.1 12

Salinity 35.4 0.9 3 34.2 1.8 3 34.8 1.3 3 32.6 2.2 3 34.2 1.8 12


Phosphate (mM) 0.1 0.0 3 0.1 0.0 3 0.1 0.0 3 0.4 0.4 3 0.1 0.2 12

Silicate (mM) 11.7 4.4 3 10.1 2.3 3 7.5 5.1 3 18.0 5.9 3 11.8 5.6 12


St. 3

Temperature ( 1C) 6.3 2.2 3 18.8 3.3 3 24.2 3.0 3 12.0 3.3 3 15.3 7.5 12

Salinity 31.5 0.6 3 29.4 3.2 3 30.5 5.3 3 16.5 6.9 3 7.5 7.5 12


Phosphate (mM) 0.11 0.03 3 0.10 0.08 3 0.3 0.3 3 2.1 1.9 3 0.7 1.2 12

Silicate (mM) 7.0 3.2 3 23.9 3.7 3 20.7 3.5 3 73.9 21.0 3 31.4 28.1 12


N, number of samples.



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bottom. According to the Kd (Table 2), PAR attenuation in the

water column was the highest at st. 1 and the lowest at st. 3.

Phytoplankton production, bacterial productionand CR

The remarkable degree of trophic variability among the

three stations was also reflected in the wide seasonal and

spatial range of chl and TPP (Table 2).

TPP increased markedly at all stations from winter to

summer, when the maxima were attained, then decreasing

in autumn, in accordance with temperature and phyto-

plankton biomass variations. Indeed, considering the whole

data set, TPP was strongly correlated with both temperature

(r = 0.82, Po 0.05, n = 12) and chl (r = 0.86, Po 0.05,

n = 12). The highest TPP was measured at st. 1 from spring

to autumn.

HBB and Leu-BCP showed a seasonal variation qualita-

tively similar to that of phytoplankton biomass, increasing

at all the stations from winter to summer and then decreas-

ing in autumn (Table 2). The highest HBB and BCP were

prevalently recorded at st. 1 (Table 2). The heterotrophic

metabolism appeared to be much less variable with respect

to the widely ranging phytoplankton production, in parti-

cular at st. 1. Leu-BCP was correlated, although weakly, with

TPP (r = 0.58, Po 0.05, n = 12).

Hourly phytoplankton production efficiency (Pb) fol-

lowed the pattern of TPP and chl at st. 1 and st. 2, while at

st. 3, it increased also from summer to autumn, when the

maximum value was attained (Table 2). The hourly BCP

efficiency (ratio between BCP and HBB, Table 2) varied

much more irregularly than Pb, without showing a clearly

identifiable pattern among stations and through seasons.

The daily values of BCP and TPP are given in Table 3. On

a daily basis, Leu-BCPd prevailed over TPPd only in winter

and summer at st. 3, in winter and spring at st. 1 and in

winter, spring and autumn at st. 2 Q5. The daily respiration

rates (CRd, Table 3) showed the same seasonal pattern as

TPP at every station, with the maximum attained at st. 1 in

summer. CRd was indeed correlated both with temperature

(r = 0.67, Po 0.05, n = 12) and with TPP (r = 0.91, Po 0.05,

n = 12), while the correlation with HBB was not significant.

Daily CR prevailed over TPPd always at st. 3, in winter and

spring at st. 2 and only in winter at st. 1.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

Table 2. Selected hydrological and biological parameters and activities at the three stations, in the four sampling periods

Winter Spring Summer Autumn

St. 1 St. 2 St. 3 St. 1 St. 2 St. 3 St. 1 St. 2 St. 3 St. 1 St. 2 St. 3

Kd (m�1) 3.8 1.5 0.1 1.7 1.2 0.8 2.5 1.6 0.6 3.5 1.3 1.4

Chlorophyll a (mg L�1) 1.3 2.1 0.3 3.8 1.2 1.6 19.9 4.9 1.8 9.7 1.0 0.9

HBB (mg C L�1) 66.5 23.0 21.8 68.5 63.2 31.9 212.3 109.3 119.1 40.8 53.4 31.5

POC (mM) 63.0 62.0 41.0 149.9 27.9 47.6 149.9 67.8 71.8 86.9 27.2 30.4

DOC (mM) 173.0 160.0 181.0 271.1 186.2 182.3 250.0 266.0 180.0 271.5 158.5 202.8

DOC/TOC (%) 73 72 82 64 87 79 63 80 71 76 85 87

aCDOM355 (nm�1) 0.3 0.8 0.6 3.9 1.0 – 3.1 1.1 0.4 3.7 1.7 0.5

TPP (mg C L�1 h�1) 0.02 1.4 0.9 15.3 5.0 11.5 300.4 71.9 16.1 92.8 5.4 12.1

DPP (mg C L�1 h�1) 0.0 0.2 0.1 1.1 1.5 0.2 3.1 0.6 0.2 4.2 0.0 0.6

PPP (mg C L�1 h�1) 0.1 1.2 0.9 14.3 3.5 11.3 297.3 71.3 15.9 88.6 5.4 11.5

PER (%) 1.0 14.3 5.6 6.9 30.0 1.7 1.0 0.8 1.1 4.5 0.2 5.0

Pb (mg C mg Chl L�1 h�1) 0.1 0.7 2.9 4.0 4.2 7.2 15.1 14.6 8.8 9.6 5.6 14.2

Leu-BCP (mg C L�1 h�1) 2.3 3.3 2.9 9.7 4.7 3.0 11.5 6.6 5.6 5.0 2.7 0.6

Leu-BCP/TPP 116.45 2.38 3.18 0.63 0.94 0.26 0.04 0.09 0.35 0.05 0.50 0.05

BCD (mg C L�1 h�1) 6.2 7.8 7.0 17.6 9.9 7.2 20.4 12.8 11.3 10.3 6.7 3.3

BCD/DPP 4 100 38.8 4 100 16.8 6.6 36.2 6.6 21.4 66.3 2.5 4 100 5.5

BCD/TPP 4 100 5.5 7.8 1.1 2.0 0.6 0.1 0.2 0.7 0.1 1.2 0.3

Leu-BCP/HBB (h�1) 0.04 0.15 0.13 0.14 0.07 0.09 0.05 0.06 0.05 0.12 0.05 0.02

CR (mg C L�1 h�1) 0.1 1.1 2.1 4.2 3.2 3.6 24.2 7.3 7.3 7.2 0.9 4.5

AMA (nM h�1) 549.7 407.1 255.0 1351.7 556.3 490.0 3155.0 796.0 475.0 753.2 181.0 231.6

APA (nM h�1) 11.3 19.6 4.0 24.6 20.0 31.8 39.7 90.9 161.6 81.8 19.2 51.4

LA (nM h�1) 1.7 90.8 25.8 24.0 15.4 24.8 12.4 8.0 2.7 840.2 878.7 55.5

a-glu (nM h�1) 2.0 7.5 3.3 4.7 4.5 6.8 9.5 7.2 5.7 15.1 2.7 2.8

b-glu (nM h�1) 3.5 1.8 0.5 5.1 3.0 6.0 11.8 15.0 18.5 22.6 4.7 3.7

a-gal (nM h�1) 1.1 2.3 1.6 3.6 3.3 7.8 7.7 6.0 0.0 116.2 6.4 7.6

b-gal (nM h�1) 1.5 3.6 0.9 1.6 1.4 2.0 7.2 2.3 1.2 19.6 2.9 5.6

NAG (nM h�1) 2.2 5.0 1.0 4.3 5.8 5.5 13.7 16.6 12.2 61.0 5.6 7.7

APA/AMA 0.021 0.048 0.016 0.018 0.036 0.065 0.013 0.114 0.340 0.109 0.106 0.222

See text for abbreviations.



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Phytoplankton exudate release and BCD

Both DPP and the percentage carbon extracellular release

(PER = DPP%TPP) exhibited a very irregular pattern,

throughout seasons and stations (Table 2). The highest

DPP were attained at st. 1 in summer and autumn. On the

contrary, the PER was the lowest at this station, while it

attained the highest values at st. 2 in spring. Considering the

whole data set, DPP was correlated with both TPP (r = 0.76,

Po 0.05, n = 12) and PPP (r = 0.74, Po 0.05, n = 12; Fig. 2).

No significant relationships were found between PER and

PPP (Fig. 2) or TPP.

BCD (Tables 2 and 3) increased at every station from

winter to summer, then decreasing in autumn. The highest

values, from spring to autumn, were recorded at st. 1. On a

daily basis, DPPQ6 (Table 3) represented only a small fraction

(o 5%) of BCD and it was therefore never sufficient to

completely sustain the BCD. Also, the maximum potential

daily DPP (DPP1PPpico) was never sufficient to meet BCD

on a daily basis. Taking into account the TPP, as a non-

contemporaneous DOC pool potentially available for bac-

teria, BCD was met by TPP only in summer and autumn at

st. 1. No significant correlations were found between BCD

and DPP or TPP (Fig. 3) and between BCP and DPP (Fig. 3).

Enzymatic activities and potential organiccarbon mobilization

Among the assayed extracellular enzymatic activities (Table

2), aminopeptidase activity (AMA) showed the highest

values. The maxima were always recorded at st. 1 and in

summer at every station. The alkaline phosphatase activity

(APA) was quite low and it showed the same similar pattern

as for AMA only at st. 2 and 3, while it attained the

maximum in autumn at st. 1, appearing decoupled from

the seasonal pattern of phytoplankton and bacteria biomass

here (Table 2). The APA/AMA ratio, proposed as a proxy of

phosphorus deficiency in microbial assemblages (Sala et al.,

2001), was always o 1, indicating good phosphorus avail-

ability in the lagoon waters. The specific activity of APA and

AMA (APA and AMA/HBB and APA/chl, Table 2) showed a

very irregular pattern among both seasons and stations.

The lipase- and the enzymatic-degrading polysaccharides

activities varied irregularly, without showing a clearly

identifiable pattern among stations and through seasons

(Table 2).

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

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53

Table 3. Daily metabolic rates at the three stations, in the four seasons

Winter Spring Summer Autumn

St. 1 St. 2 St. 3 St. 1 St. 2 St. 3 St. 1 St. 2 St. 3 St. 1 St. 2 St. 3

TPP (mg C L�1 day�1) 0.1 6.6 4.3 107.5 35.4 80.4 2295 552.4 123.5 539.2 31.2 71

DPP (mg C L�1 day�1) 0 0.9 0.2 7.4 10.6 1.4 23.7 4.6 1.3 24.4 0.1 3.5

Leu-BCP (mg C L�1 day�1) 55.9 79.9 68.7 232.8 112.8 72.0 276.0 158.4 134.4 119.2 64.3 13.2

BCD (mg C L�1 day�1) 148.8 186.2 168.8 422.1 237.1 173.9 488.7 307.5 270.5 247.1 161.9 78.7

Cmob (mg C L�1 day�1) 976.6 1209.1 587.0 2493.0 1072.3 1023.9 5602.4 1498.3 899.9 6062.3 4906.7 735.4

CR (mg C L�1 day�1) 2.4 26.9 50.8 100.3 75.6 87.3 581.7 175.2 175.9 173.1 22.2 108.0

BCP/TPP 4100 12.1 16.0 2.2 3.2 0.9 0.1 0.3 1.1 0.2 2.1 0.2

BCD/TPP 4100 28.2 39.3 3.9 6.7 2.2 0.2 0.6 2.2 0.5 5.2 1.1

BCD/DPP 4100 4100 4 100 57.2 22.3 4 100 20.6 66.7 4 100 10.1 4 100 22.3

CR/TPP 24.0 4.1 11.8 0.9 2.1 1.1 0.3 0.3 1.4 0.3 0.7 1.5

DOC/Cmob (days) 2.1 1.6 3.7 1.3 2.1 2.1 0.5 2.1 2.4 0.5 0.4 3.3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Log particulated primary production(µg C L–1 h–1)

Log

diss

olve

d pr

imar

ypr

oduc

tion

(µg

C L

–1 h

–1)

St. 1 St. 2 St. 3

0

5

10

15

20

25

30

35

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0 50 100 150 200 250 300 350Particulated primary production

(µg C L–1 h–1)

Per

cent

age

extr

acel

lula

rre

leas

e (%

)

St. 1 St. 2 St. 3

(a)

(b)

Fig. 2. (a) Log–log plot between hourly dissolved primary production

and particulated primary production. (b) Plot of photosynthetic extra-

cellular release vs. particulate primary production.



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The potential bacterial carbon mobilization (Cmob, Table

3), calculated from bacterial enzymatic activity, increased

from winter to summer at every station. The highest values

were measured at st. 1. Daily BCD always appeared to be

satisfied by Cmob daily rates, at every station and season

(Table 3).

Plotting all the data together, Cmob was correlated with

Leu-BCP (r = 0.64, Po 0.05, n = 12), TPP (r = 0.76, n = 12,

Po 0.005) and DPP (r = 0.88, Po 0.001, n = 12). The

correlation of Cmob with Leu-BCP and with BCD suggests

that the carbon potentially mobilized by enzymatic activity

could be used for the bacterial carbon requirements. On the

other hand, the relationships between Cmob and DPP con-

firm the trophic link between heterotrophic bacteria and

algae mediated through DPP, usually composed of easy-

to-degrade molecules (Amon et al., 2001). Cmob was also

positively correlated with TPP even if TPP needs a variable

lag phase before it is available for bacterial uptake.

Dissolved and POC

The main part of the total organic carbon in the lagoon

waters was in the dissolved form (64–87%) at each station

(Table 1). The highest POC concentrations were measured

at st. 1, where the relative incidence on TOC was lower

(64–76%). POC concentrations were generally the highest in

spring–summer, while DOC did not show a clearly identifi-

able pattern among stations and through seasons.

The magnitude of chromophoric dissolved organic car-

bon (aCDOM355) varied on average from 0.3 to 3.9 nm�1,

with minimum values in winter (mean: 0.6� 0.3 nm�1).

The highest values were generally recorded at st. 1 and the

minima at st. 3. aCDOM355 was inversely correlated

with salinity (r = 0.73, n = 11, Po 0.05) and directly with

chl (r = 0.65, n = 11, Po 0.05,) and DPP (r = 0.74, n = 11,

Po 0.01), indicating that possibly both riverine inputs of

organic matter and phytoplankton exudation could contri-

bute to the formation of CDOM. The amount of nonab-

sorbing DOC concentration was extrapolated from the

positive correlation (r = 0.74, n = 11, Po 0.01, DOC-

25.1� aCDOM3551170) between aCDOM and DOC. The

CDOM, on average, contributed to 19% of the mean DOC

concentration.

The significant (r = 0.86, n = 11, Po 0.001) linear rela-

tionship between aCDOM355 and Cmob and of aCDOM355

and DPP (r = 0.74, n = 11, Po 0.01) suggests an important

role of the bacteria in the transformation and elaboration of

CDOM.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

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33

34

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38

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40

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42

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46

47

48

49

50

51

52

53

0.0

0.5

1.0

1.5

2.0

2.5

3.0(a) (b)

Log total primary production(µg C L–1 h–1)

Log

bact

eria

l car

bon

prod

uctio

n (µ

g C

L–1

h–1

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Log dissolved primary production(µg C L–1 h–1)

Log

bact

eria

l car

bon

prod

uctio

n (µ

g C

L–1

h–1

)0.0

0.5

1.0

1.5

2.0

2.5

3.0(c)

Log total primary production(µg C L–1 h–1)

Log

bact

eria

l car

bon

dem

and

(µg

C L

–1 h

–1)

St. 1 St. 2 St. 3 St. 1 St. 2 St. 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0(d)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0Log dissolved primary production

(µg C L–1 h–1)

Log

bact

eria

l car

bon

dem

and

(µg

C L

–1 h

–1)

St. 1 St. 2 St. 3St. 1 St. 2 St. 3

Fig. 3. Log–log plots between: (a) bacterial carbon production and total primary production, (b) bacterial carbon production and dissolved primary

production, (c) bacterial carbon demand and total primary production, (d) bacterial carbon demand and dissolved primary production.



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Considering the potential turnover rate of DOC mobiliz-

able by enzyme activities (DOC/Cmob, Table 2), they ranged

from 0.4 to 3.7 days, in autumn and winter, respectively,

with minima at st. 2 and maxima at st. 3.

Discussion

This investigation was carried out at three stations in the

Lagoon of Venice, characterized by a large heterogeneity in

salinity and nutrient concentration. The different trophic

state was revealed by substantial differences with regard to

algal biomass and production. The photosynthetic activity

of the phytoplankton, in particular, was characterized by a

very wide seasonal range (up to four orders of magnitude at

st. 1 and to one at st. 2 and 3), comparable in any case with

the few pp data available in the literature for the lagoon,

gathered during the 1980s and the 1990s (Degobbis et al.,

1986; Bianchi et al., 2000). The seasonal phytoplankton

production and biomass appear to be mainly related to that

of temperature and solar irradiance. Specific production

(Pb) was comparable with that of other transitional ecosys-

tems, characterized by an elevated nutrient availability

(Macedo et al., 2001; Montes-Hugo & Alvarez-Borrego,

2003; Azevedo et al., 2006). The bacterial community, on

the contrary, showed a quite constant metabolism (BCP

from 1.5- to 4-fold throughout the seasons) when compared

with the widely ranging pp and, apparently, bacterial

biomass and production do not respond with the same

intensity to the trophic variations.

The ratio between PP and BCP indicates whether second-

ary pelagic production is based on bacterial mobilization of

chemically bound energy or on phytoplankton photosynth-

esis, with consequences on the structure of the trophic web

(Berglund et al., 2007). In our study, the base of the

planktonic trophic appears to be largely independent from

the different trophic state, while a rather common seasonal

pattern could be recognized. A definite shift of the plankton

food web toward autotrophy (TPP/BCP � 1) occurs in

correspondence to a production efficiency (Pb) higher than

9 mg Cmg chl L�1 h�1.

BCP and TPP were significantly, although weakly, corre-

lated. The correlation between BCP and PP is often accepted

as a sufficient evidence of phytoplankton–bacterioplankton

coupling. However, the contemporaneous dependence of

bacteria on algae can only be mediated through DPP, while

TPP needs a time lag before becoming available to bacteria.

The strength of coupling between bacteria and phytoplank-

ton is therefore considered here, as proposed by Moran et al.

(2002), as the extent to which DPP meets BCD. The organic

carbon released during photosynthesis by phytoplankton is

considered one of the most important sources of labile

organic molecules for bacteria growth (e.g. Cole et al.,

1988; Baines & Pace, 1991; Norrman et al., 1995). If BCD

exceeds DPP, then bacteria must have other sources of

organic carbon for their growth. Phytoplankton exudation

produces mainly low-molecular-weight material that is

readily assimilable by bacteria and may be turned over in

o 1 day (Leboulanger et al., 1997; Amon et al., 2001). The

nonreadily assimilable OM, mostly composed by high-

molecular-weight (HMW) molecules, must be hydrolyzed

by bacteria ectoenzymes before the assimilation, giving rise

to a decoupling between bacterial and phytoplankton pro-

duction. The DPP at the three stations examined represents,

generally, a very small fraction of TPP (prevalently o 7%,

with just two peaks exceeding this value), mostly within the

lowest range reported in the literature (Baines & Pace, 1991).

The PER does not show a decreasing trend at increasing PPP,

as it is, in contrast to what is reported in other ecosystems

(Moran et al., 2002; Teira et al., 2003): indeed, it spanned

from 0% to 30%, independent of the wide range of PPP

(from 0.1 to 90 mg C L�1 h�1) and of trophic conditions.

BCD, on both an hourly and a daily basis, was, at every

station and season, always extremely greater than the actual

supply of DPP from algae, leading to the conclusion that

bacteria and phytoplankton were uncoupled. In this investi-

gation, the possibility to underestimate DPP with short-

term, end-point incubation, due to bacterial recycling of

recently produced DOC (Moran & Estrada, 2002), has been

taken into account by calculating the potential DPP, which

also includes the organic 14C in the 0.2–2 mm fraction. This

fraction will include both the 14C recycled by bacteria and

the autotrophic picoplankton production that, in this

environment, might be consistent, the APP Q7biomass repre-

senting, on average, 4 5% of the total phytoplankton

biomass in the lagoon with peaks up to 20% (Coppola

et al., 2006, 2007). Anyway, BCD was always too high

to be supported by DPP, even when considering this DPP

potential rate (DPP1PPpico).

Both the occurrence of conditions in which BCP is larger

than TPP and the continuous prevalence of BCD over DPP

imply that further sources of organic C, more than phyto-

plankton production alone, are necessary to sustain the

bacterial metabolism.

The carbon that is made available by the ectoenzymatic

mobilization always satisfies the BCD and the correlation of

Cmob with Leu-BCP suggests that bacterial metabolism was

mainly used for enzyme synthesis. On the other hand, the

relationship between Cmob and DPP and TPP confirms the

trophic link of heterotrophic bacteria on algae even if the

phytoplankton alone are unable to support BCD.

The major autochthonous, nonphytoplanktonic sources

of DOM in wetlands are leachates from plants, exudates

from benthic microalgae, or macrophytes, and pore water

from sediments and soils (Ziegler & Benner, 1999; Bertilsson

& Jones, 2003; Maie et al., 2006). Macrophytes generate

DOM either through extracellular release of photosynthate

1

2

3

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8

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or upon senescence (Bertilsson & Jones, 2003; Maie et al.,

2006; Wang et al., 2007 and references therein). In the three

shallow areas considered in this study, organic matter from

the sediments may also make an important contribution to

the organic matter in the waters (Sfriso et al., 2005), as a

consequence of both natural (e.g. wind, storm, tidal flush-

ing) and anthropogenic (clam harvesting, maritime traffic,

capital and maintenance dredging) sediment resuspension.

The resuspension was confirmed by the significant relation-

ship found from DOC concentrations in water and those at

the water–sediment interface (M. Giani, unpublished data).

The contribution of the CDOM, on average 19% of

the mean DOC concentration, probably derives from the

bacterial transformation of phytoplankton-derived DOM

and from riverine discharges into the lagoon. The highest

aCDOM values are usually observed in freshwaters and

estuaries and they decrease in coastal and offshore waters

(Blough et al., 1993). Bacterial processing of nonchromo-

phoric, algal organic matter, either particulate or dissolved,

could be responsible for the CDOM production. The

positive relationships found among aCDOM, DPP and

Cmob suggest that the bacteria could play an important role

in the formation of CDOM in the lagoon waters through the

reprocessing of phytoplankton exudates, supporting the

Rochelle-Newall & Fisher (2002) findings towards

the hypothesis that the noncolored DOM released by

phytoplankton could be transformed by bacteria in CDOM.

The relation between bacteria respiration and photo-

synthesis is expected to be different in relation to the

strength of the coupling between the two communities: the

availability of the HMW organic matter pool will allow

the bacteria to extend their activity and respiration beyond

the decline of photosynthesis. In this study, the relations

between CR, TPP and BCP are quite contradictory in this

respect. CR fell within the values generally reported for

estuarine and coastal waters (e.g. Jensen et al., 1990; Smith &

Kemp, 1995; Iriarte et al., 1996). A close coupling between

CR and TPP, but not between CR and BCP, was observed,

suggesting that phytoplankton may be responsible for a

significant fraction of CR. On the other hand, the rates of

oxygen consumption were much less variable than those of

PP: this may indicate that CR and PP do not respond with

the same intensity to the environmental variations and that

the CR is fuelled by sources other than phytoplankton

production and are also supplied more constantly through

time. In particular, in the periods in which CR exceeded PP,

stored or imported organic matter should prevalently sup-

port the planktonic system. However, in the present study,

the periods in which the system was heterotrophic

(CR4PP) were not related either to a concomitant increase

of BCP or of DOC concentrations or to BCP � PP.

In conclusion, this study evidenced that the large within-

system trophic heterogeneity corresponded to elevated phy-

toplankton biomass and production variability, while bac-

teria standing stock and production appeared to be much

more constant. The relationships between bacteria and

phytoplankton community could not be associated with

the trophic state in a straightforward way. Rather, a common

seasonal variability could be evidenced, occurring quite

independent of the trophic state, confirming the hypothesis

of a general loose bacterioplankton–phytoplankton cou-

pling in coastal and transitional areas. The two communities

appeared, indeed, uncoupled, with BCD largely exceeding

DPP: this characteristic, which is common to other coastal

systems, in this investigation appeared to be constant and

independent of the station and the season considered. The

variations of the microbial metabolism and the shift from

autotrophy to heterotrophy seem to be mainly related to

variations in phytoplankton activity, rather than Q8on bacteria

one. However, the interplay of trophic conditions, organic

carbon availability and seasonality makes it arduous to find

a within-system clear pattern of variability related to the

spatial and trophic differences.

Acknowledgements

This research was partially supported by the CORILA

(Consortium for Coordination of Research Activities Con-

cerning the Venice Lagoon System) II Research Programme

(2004–2006). We deeply acknowledge Prof. Farooq Azam

for his valuable discussion and suggestions. We thank Loris

Dametto, Mauro Penzo and Franco Antonini for their

assistance in the field, and Elisa Ravagnan, Erica Crevatin,

Chiara Larato and Fabio Savelli for their help in the

laboratory activities.

References

Acri F, Bernardi Aubry F, Berton A, Bianchi F, Boldrin A, Camatti

E, Comaschi A, Rabitti S & Socal G (2004) Plankton

communities and nutrients in the Venice Lagoon. Comparison

between current and old data. J Marine Syst 51: 321–329.

Amon RMW, Fitznar H-P & Benner R (2001) Linkages among

the bioreactivity, chemical composition and diagenetic state of

marine dissolved organic matter. Limnol Oceanogr 46:

287–297.

Azevedo IC, Duarte PM & Bordalo AA (2006) Pelagic metabolism

of the Douro estuary (Portugal) – factors controlling primary

production. Estuar Coast Shelf S 69: 133–146.

Baines SP & Pace ML (1991) The production of dissolved organic

matter by phytoplankton and its importance to bacteria:

patterns across marine and freshwater systems. Limnol

Oceanogr 36: 1078–1090.

Bandelj V, Socal G, Park Y-S, Coppola J, Camatti E, Capuzzo E,

Milan L & Solidoro C (2008) Analysis of multitrophic

plankton assemblages in the Lagoon of Venice. Mar Ecol-Prog

Ser 368: 23–40.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53



FEMSEC 839

(BW

UK

FE

MSE

C 8

39 W

ebpd

f:=

02/0

2/20

10 0

9:53

:21

4512

72 B

ytes

12

PAG

ES

n op

erat

or=

M.C

hack

alay

il) 2

/2/2

010

9:53

:29

PM

8

7

8

7

8

7

8

7

Berglund J, Muren U, Bamstedt U & Andersson A (2007)

Efficiency of a phytoplankton-based and a bacteria-based food

web in a pelagic marine system. Limnol Oceanogr 52: 121–131.

Bernardi Aubry F & Acri F (2004) Phytoplankton distribution at

the three mouths of the Lagoon of Venice (June 2001–July

2002). J Marine Syst 51: 65–76.

Bertilsson S & Jones JB (2003) Supply of dissolved organic matter

to aquatic ecosystems: autochthonous sources. Aquatic

Ecosystems. Interactivity of Dissolved Organic Matter (Findlay

SEG & Sinsabaugh RL, eds), pp. 3–24. Elsevier Science,

Amsterdam.

Bianchi F, Acri F, Alberighi L et al. (2000) Biological variability in

the Venice lagoonQ9 . The Venice Lagoon Ecosystem. Inputs and

Interactions Between Land and Sea. Man and the Biosphere

Series, Vol. 25 (Lasserre P & Marzollo A, eds), pp. 97–125.

UNESCO and Parthenon Publishing Group.

Bianchi F, Acri F, Bernardi Aubry F, Berton A, Boldrin A, Camatti

E, Cassin D & Comaschi A (2003) Can plankton communities

be considered as bio-indicators of water quality in the Lagoon

of Venice? Mar Pollut Bull 46: 964–971.

Blough NV, Zafirou OC & Bonilla J (1993) Optical absorption

spectra of waters from the Orinoco river outflow: terrestrial

input of colored organic matter to the Caribbean. J Geophys

Res 98: 2271–2278.

Cauwet G (1994) HTCO method for dissolved organic carbon

analysis in sea water: influence of catalyst on blank estimation.

Mar Chem 47: 55–64.

Ciavatta S, Pastres R, Badetti C, Ferrari G & Beck MB (2008)

Estimation of phytoplankton production and system

respiration form data collected by a real-time monitoring

network in the Lagoon of Venice. Ecol Model 212: 28–36.

Cole J, Findlay S & Pace ML (1988) Bacterial production in fresh

and saltwater ecosystems: a cross-system overview. Mar Ecol-

Prog Ser 43: 1–10.

Collavini F, Bettiol C, Zaggia L & Zonta R (2005) Pollutants load

from the drainage basin to the Venice Lagoon (Italy). Environ

Int 31: 939–947.

Coppola J, Bernardi Aubry F, Acri A, Del Negro P & Pugnetti A

(2006) Picophytoplankton contribution to phytoplankton

community in the Venice LagoonQ10 . Scientific Research and

safeguarding of Venice, 2006. CORILA Research Programme

2004–2006. Vol. V, pp. 245–254.

Coppola J, Bernardi Aubry F, Acri F, Bianchi F & Pugnetti A

(2007) Dimensional structure of phytoplancton community in

the Venice LagoonQ11 . Scientific Research and safeguarding of

Venice, 2007. CORILA Research Programm 2004–2006. Vol. VI,

pp. 312–322.

Degobbis D, Gilmartin M & Orio AA (1986) The relation of

nutrient regeneration in the sediments of the Northern

Adriatic to eutrophication, with special reference to the

Lagoon of Venice. Sci Total Environ 56: 201–210.

del Giorgio PA & Cole JJ (1998) Bacterial growth efficiency in

natural aquatic systems. Annu Rev Ecol Syst 29: 503–541.

Gaedke U & Kamjunke N (2006) Structural and functional

properties of low- and high-diversity planktonic food webs. J

Plankton Res 28: 707–718.

Giordani G, Viaroli P, Swaney DP, Murray CN, Zaldivar JM &

Marshall Crossland JI (2005) Nutrient fluxes in the

transitional zones of the Italian coasts. LOICZ Reports and

studies No. 28, LOCZ, Texel, the Netherlands, 157pp.

Gonzalez-Benitez N & Gattuso J-P (2003) Are phytoplankton and

bacteria coupled within an aquatic oligotrophic system? Q12.

Geoph Res Abstr 5: 06854.

Grasshoff K, Cremling K & Erhardt M (1999) Methods of

Seawater Analysis. Verlag Chemie, Weinheim, Germany,

600pp.

Holm-Hansen O, Lorenzen CJ, Holmes RW & Strickland JDH

(1965) Fluorometric determination of chlorophyll. J Cons

Perm Int Explor Mer 30: 3–15.

Hopkinson CSJ & Smith EM (2005) Estuarine respiration: an

overview of benthic, pelagic and whole system respiration.

Respiration in Aquatic Ecosystems (del Giorgio PA & Williams

LPJ, eds), pp. 123–147. Oxford University Press, Oxford, UK.

Hoppe HG (1993) Use of fluorogenic model substrates for

extracellular enzyme activity (EEA) measurement of bacteria.

Current Methods in Aquatic Microbial Ecology (Kemp PF, Sherr

BF, Sherr EB & Cole JJ, eds), pp. 423–431. CRC Press, Boca

Raton.

Iriarte A, de Madariaga I, Diez-Garagarza F, Revilla M & Orive E

(1996) Primary production, respiration and nitrification in a

shallow temperate estuary during summer. J Exp Mar Biol Ecol

208: 127–151.

Jensen LM, San-Jensen K, Marcher S & Hansen M (1990)

Plankton community respiration along a nutrient gradient in a

shallow Danish estuary. Mar Ecol-Prog Ser 61: 75–85.

Kirchman D, K’nees E & Hodson R (1985) Leucine incorporation

and its potential as a measure of protein synthesis by bacteria

in natural waters. Appl Environ Microb 49: 599–607.

Leboulanger C, Oriol L, Jupin H & Descolas-Gros C (1997) Diel

variability of glycolate in the eastern tropical Atlantic Ocean.

Deep-Sea Res 44: 2131–2139.

Lee S & Fuhrman JA (1987) Relationship between biovolume and

biomass of naturally delivered marine bacterioplankton. Appl

Environ Microb 53: 1298–1303.

Macedo MF, Duarte O, Mendes P & Ferreira JG (2001) Annual

variation of environmental variables, phytoplankton species

composition and photosynthetic parameters in a coastal

lagoon. J Plankton Res 23: 719–732.

Maie N, Jaffe R, Miyoshi T & Childers DL (2006) Quantitative

and qualitative aspects of dissolved organic carbon leached

from senescent plants in an oligotrophic wetland.

Biogeochemistry 78: 285–314.

Montes-Hugo MA & Alvarez-Borrego S (2003) Spatial and

temporal variation of photosynthetic parameters of

phytoplankton in a subtropical coastal lagoon. Estuar Coast

Shelf S 56: 517–525.

Moran XAG & Estrada M (2002) Phytoplankton DOC and POC

production in the Bransfield and Gerlache Straits as derived

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53



FEMSEC 839

(BW

UK

FE

MSE

C 8

39 W

ebpd

f:=

02/0

2/20

10 0

9:53

:21

4512

72 B

ytes

12

PAG

ES

n op

erat

or=

M.C

hack

alay

il) 2

/2/2

010

9:53

:29

PM

form kinetic experiments of 14C incorporation. Deep-Sea Res

Pt II 49: 769–786.

Moran XAG, Estrada M, Gasol JM & Pedros-Alio C (2002)

Dissolved primary production and the strength of

phytoplankton–bacterioplankton coupling in contrasting

marine regions. Microb Ecol 44: 217–223.

Nieuwenhuize J, Maas EM & Middelburg JJ (1994) Rapid analysis

of organic carbon and nitrogen in particulate materials. Mar

Chem 45: 217–224.

Norrman B, Zweifel UL, Opkinson CS Jr & Fry B (1995)

Production and utilization of dissolved organic carbon during

an experimental diatom bloom. Limnol Oceanogr 40: 898–907.

Porter KG & Feig YS (1980) The use of DAPI for identifying and

counting aquatic microflora. Limnol Oceanogr 25: 943–948.

Ravera O (2000) The Lagoon of Venice: the result of both natural

factors and human influente. J Limnol 59: 19–30.

Revilla M, Aristeguı A, Iriarte A, Madariaga I, Orive E, Sarobe A

& Trigueros JM (2002) Microplankton metabolism along a

trophic gradient in a shallow-temperate estuary. Estuaries 25:

6–18.

Rochelle-Newall EJ & Fisher TR (2002) Production of

chromophoric dissolved organic matter fluorescence in

marine and estuarine environments: an investigation into the

role of phytoplankton. Mar Chem 77: 7–21.

Rochelle-Newall EJ, Torreton J-P, Mari X & Pringault O (2008)

Phytoplankton–bacterioplankton coupling in a subtropical

South Pacific coral reef lagoon. Aquat Microb Ecol 50: 221–229.

Sala MM, Karner M, Arin L & Marrase C (2001) Measurement of

ectoenzyme activities as an indication of inorganic nutrient

imbalance in microbial communities. Aquat Microb Ecol 23:

301–311.

Sfriso A, Facca C, Ceoldo S & Marcomini A (2005) Recording the

occurrence of trophic level changes in the lagoon of Venice

over the ‘90s. Environ Int 31: 993–1001.

Simon M & Azam F (1989) Protein content and protein synthesis

rates of planktonic marine bacteria. Mar Ecol-Prog Ser 51: 201–213.

Smith DC & Azam F (1992) A simple, economical method for

measuring bacterial protein synthesis rates in sea water using

H3leucine. Mar Microb Food Webs 6: 107–114.

Smith EM & Kemp WM (1995) Seasonal and regional variations

in plankton community production and respiration for

Chesapeake Bay. Mar Ecol-Prog Ser 116: 217–231.

Solidoro C, Pastres R, Cossarini G & Ciavatta S (2004) Seasonal

and spatial variability of water quality parameters in the

Lagoon of Venice. J Marine Syst 51: 7–18.

Sorokin PYu, Sorokin YuI, Zakuskina OYu & Ravagnan GP

(2002) On the changing ecology of Venice lagoon.

Hydrobiologia 487: 1–18.

Sorokin YuI, Sorokin PYu, Giovanardi O & Dalla Venezia L

(1996) Study of the ecosystem of the lagoon of Venice with

emphasis on anthropogenic impact. Mar Ecol-Prog Ser 141:

247–261.

Teira E, Pazo MJ, Quevedo M, Fuentes MV, Niell FX & Fernandez

E (2003) Rates of dissolved organic carbon production and

bacterial activity in the eastern North Atlantic subtropical

Gyre during summer. Mar Ecol-Prog Ser 249: 53–67.

Thomas C, Cauwet G & Minster JF (1995) Dissolved organic

carbon in the Equatorial Atlantic Ocean. Mar Chem 49:

155–169.

Vodacek A, Blough NV, DeGrandpre MD, Peltzer ET & Nelson

RK (1997) Seasonal variation of CDOM and DOC in the

Middle Atlantic Bight: terrestrial inputs and photooxidation.

Limnol Oceanogr 42: 674–686.

Wang SR, Jin XC, Zhao HZ, Zhou XN & Wu FC (2007) Effect of

organic matter on sorption of dissolved organic and inorganic

phosphorus in lake sediment. Colloid Surface A 297: 154–162.

Ziegler S & Benner R (1999) Dissolved organic carbon cycling in a

subtropical seagrass-dominated lagoon. Mar Ecol-Prog Ser

180: 149–160.

Zuliani A, Zaggia L, Collavini F & Zonta R (2005) Freshwater

discharge from the drainage basin to the Venice Lagoon (Italy).

Environ Int 31: 929–938.

1

2

3

4

5

6

7

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9

10

11

12

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14

15

16

17

18

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20

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24

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26

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28

29

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32

33

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35

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37

38

39

40

41

42

43

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46

47

48

49

50

51

52

53



FEMSEC 839

(BW

UK

FE

MSE

C 8

39 W

ebpd

f:=

02/0

2/20

10 0

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ytes

12

PAG

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n op

erat

or=

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il) 2

/2/2

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