RESEARCH ARTICLE

Phytoplankton growth and microzooplankton grazing ratesin a restricted Mediterranean lagoon (Bizerte Lagoon, Tunisia)

A. Sakka Hlaili Æ B. Grami Æ Hassine Hadj Mabrouk ÆM. Gosselin Æ D. Hamel

Received: 5 June 2006 / Accepted: 16 October 2006 / Published online: 11 November 2006� Springer-Verlag 2006

Abstract Phytoplankton growth and microzooplank-

ton grazing were investigated in the restricted Bizerte

Lagoon in 2002 and 2004. The 2002 study, carried out

at one station from January to October, showed sig-

nificant seasonal variations in phytoplankton dynamics.

High growth rates (0.9–1.04 day–1), chlorophyll a

(Chl a) concentrations (6.6–6.8 lg l–1) and carbon

biomass (392–398 lg C l–1) were recorded in summer

(July), when several chain-forming diatoms had

intensively proliferated and dominated the carbon

biomass (74%). In 2004, four stations were studied

during July, a period also characterized by the high

proliferation of several diatoms that made up 70% of

the algal carbon biomass. In 2004, growth rates (0.34–

0.45 day–1) and biomass of algae (2.9–5.4 lg Chl a l–1

and 209–260 lg C l–1) were low, which may be related

to the lower nutrient concentrations recorded in 2004.

Microzooplankton >5 lm were mainly composed of

heterotrophic dinoflagellates and ciliates. Microzoo-

plankton biomass peaked during summer (2002 320–

329, 2004 246–361 lg C l–1), in response to the en-

hanced phytoplankton biomass and production. The

grazer biomass was dominated by ciliates (71–76%) in

July 2002 and by heterotrophic dinoflagellates (52–

67%) in July 2004. Throughout the year and at dif-

ferent stations, microzooplankton grazed actively on

phytoplankton, removing 26–58% of the Chl a and 57-

84% of the primary production. In 2002, the highest

grazing impact was observed on the large algae

(>10 lm) during the period of diatom dominance.

These results have a significant implication for carbon

export to depth. Indeed, the recycling of most of the

diatom production by the microbial food web in the

upper water column would reduce the flux of material

to the seafloor. This should be considered when mod-

eling the carbon cycling in coastal environments and

under conditions of diatom dominance. During both

studies, ciliates had higher growth rates (0.5–1.5 day–1)

and a higher carbon demand (165–470 lg C l–1 day–1)

than dinoflagellates (0.1–0.5 day–1, 33–290 lg C l–

1 day–1). Moreover, when grazer biomass was domi-

nated by ciliates (in July 2002), herbivory accounted

for 71–80% of the C ingested by microzooplankton

while it accounted only for 14–23% when dinoflagel-

lates dominated the grazer biomass (in July 2004).

These results suggest that, in contrast to findings from

open coastal waters, ciliate species of the restricted

Bizerte Lagoon were more vigorous grazers of the

large algae (diatoms) than were dinoflagellates.

Introduction

Microzooplankton, including ciliates, flagellates, dino-

flagellates, sarcodines and small metazoans (Capriulo

et al. 1991), constitute a significant proportion of the

total zooplankton biomass in several aquatic ecosys-

tems (Mazunder et al. 1990). Their grazing removes a

Communicated by S.A. Poulet, Roscoff.

A. Sakka Hlaili (&) � B. Grami � H. Hadj MabroukLaboratoire de Cytologie vegetale et Phytoplanctonologie,Departement des Sciences de la Vie,Faculte des Sciences de Bizerte, 7021 Zarzouna,Bizerte, Tunisiae-mail: asma_sakka@yahoo.fr

M. Gosselin � D. HamelInstitut des sciences de la mer de rimouski (ISMER),Universite du Quebec a Rimouski, 310 allee des Ursulines,QCRimouski G5L 3A1, Canada

123

Mar Biol (2007) 151:767–783

DOI 10.1007/s00227-006-0522-y

large proportion of the phytoplankton and bacterial

production in different natural waters (McManus and

Fuhrman 1988; Murrell and Hollibaugh 1998; Verity

et al. 2002; Stelfox-Widdicombe et al. 2004) and also

plays a key role in nutrient regeneration (Caron and

Goldman 1990; Gaul et al. 1999) and dissolved organic

carbon release (Strom et al. 1997). Within the micro-

zooplankton community, protozoan organisms (het-

erotrophic flagellates and dinoflagellates as well as

ciliates) may have growth rates as high as those of

phytoplankton, which enable them to structure and

control the algal assemblage (Banse 1992; Riegman

et al. 1993). Furthermore, protozooplankton act as a

significant trophic link between small producers and

large consumers in oligotrophic regions (Sherr et al.

1986; Nielsen et al. 1993). In productive coastal waters,

where large algal cells are dominant, the mesozoo-

plankton are generally considered the most important

grazers of phytoplankton (Sommer et al. 2000). Nev-

ertheless, there is increasing evidence that ciliates and

dinoflagellates may play a more important role as

consumers of coastal phytoplankton, since their graz-

ing has been observed to account for up to 75% of the

daily algal production (McManus and Ederington-

Cantrell 1992; Strom and Strom 1996; Strom et al. 2001;

Bottjer and Morales 2005). Hence, since algal carbon

can be mostly channeled to protozooplankton in

coastal waters, the vertical export of phytoplankton

carbon via fast-sinking metazoan fecal pellets would be

reduced.

Investigations of microzooplankton grazing have

been conducted mostly in the open ocean or open

coastal areas (e.g., bays, estuaries) (Froneman and

McQuaid 1997; Strom et al. 2001; Stelfox-Widdicombe

et al. 2004). There have been only a few reports on this

subject in enclosed areas, such as coastal lagoons that

have narrow channels opening to the ocean (Sakka

et al. 2000). Some of these lagoons are highly produc-

tive and support intensive fisheries (Gamito et al.

2005). However, the role of microzooplankton grazing

in determining the fate of primary production remains

poorly documented in these ecosystems.

Bizerte Lagoon presents some of the physical and

biological characteristics of a restricted lagoon (Ga-

mito et al. 2005). Exchange with the sea takes place

through an 800 m wide channel that has a length of

7 km and an average depth of 12 m. Most studies

conducted in this lagoon have described the phyto-

plankton species composition, abundance and biomass

(Chikhaoui 2003; Sakka Hlaili et al. 2006). However,

there is no information on the growth and production

rates of phytoplankton or their control by microzoo-

plankton grazing. A recent study reported high con-

centrations of heterotrophic dinoflagellates (28–

165 · 103 cells l–1) and ciliates (360 · 103 cells l–1) in

the lagoon (Chikhaoui 2003), which exceed abun-

dances found in other Mediterranean Sea regions

(Dolan 2000; Pitta and Giannakourou 2000; Gomez

and Gorsky 2003). In the Bizerte Lagoon, the algal

species composition changes significantly among sea-

sons. Small flagellates dominate in winter while dia-

toms become more abundant in summer (Sakka Hlaili

et al. 2006). Nanoflagellates are suitable prey for mi-

crozooplankton but diatoms may also be consumed by

heterotrophic dinoflagellates, including Gyrodinium

(Hansen 1992; Strom and Strom 1996) and Protope-

ridinium (Jacobson and Anderson 1986), and ciliates

(Sime-Ngando et al. 1995). These facts suggest that

microzooplankton may constitute potential grazers of

the phytoplankton production in the lagoon through-

out the year. The aim of this study was to examine the

importance of microzooplankton grazing during dif-

ferent seasons and at different locations in Bizerte

Lagoon.

Materials and methods

Study site

Bizerte Lagoon is located in northern Tunisia (37�8¢–37�14¢N, 9�48¢–9�56¢E; Fig. 1). It covers 150 km2 and

has a mean depth of 8 m. The lagoon is subjected to

different human impacts (urban, agricultural or

industrial) in its different sectors and it supports in-

tense fisheries and several aquaculture farms in the

northeast sector (Fig. 1). It receives a large freshwater

discharge (20 Mm3 year–1) from Ichkeul Lake through

the Tinja River (Harzallah 2002). Prevailing winds

from the northwest are the main forcing factor since

tides are negligible (Mansouri 1996). Water salinity

varies throughout the year from 32 to 40 psu, with the

higher values found off the northeast coast of the la-

goon (Dallali 2001; Harzallah 2002). The lagoon sup-

ports high chlorophyll a (Chl a) concentrations (3–

6 lg l–1; Hamdi 2002; Sakka Hlaili et al. 2006). This

high Chl a may reflect high primary production rates

due to the relatively high nutrient availability

(NO3–+ NO2

–: >1 lM; PO43–: >0.2 lM; Hamdi 2002;

Sakka Hlaili et al. 2006).

In 2002, the study was carried out at a nearshore

station of the lagoon (MC; Fig. 1) during winter (Jan-

uary and February), spring (April and May), summer

(July) and autumn (October). According to the results

of the 2002 study, the season characterized by the

highest phytoplankton and microzooplankton biomas-

768 Mar Biol (2007) 151:767–783

123

ses (i.e., summer) was chosen for the 2004 experiments,

which were conducted at four different stations (MA,

MJ, R and MB; Fig. 1). The characteristics of the study

stations are given in Table 1.

Dilution experiments

Microzooplankton grazing and phytoplankton growth

rates were estimated using the dilution method (Lan-

Fig. 1 Location of the studystations in Bizerte Lagoon,Tunisia

Table 1 Initial water conditions for the dilution experiments conducted in Bizerte Lagoon during 2002 and 2004

Station Date Waterdepth (m)

Zeu

(m)Temperature(�C)

Salinity(psu)

NO3– + NO2

–

(lM)PO4

3–

(lM)Si(OH)4

(lM)

MC 31 Jan. 02 5 3.5 12.6 38.4 0.7 0.2 ND13 Feb. 02 3.8 12.8 38.4 0.7 0.2 ND04 Apr. 02 3.8 19.0 38.5 6.0 0.7 ND04 May 02 4.0 19.4 38.5 5.5 0.6 ND08 July 02 4.0 26.6 39.2 11.0 0.6 ND16 July 02 4.0 25.8 39.5 14.0 0.7 ND28 Oct. 02 3.5 20.7 38.4 3.0 0.5 ND

MA 13 July 04 9.4 4.8 25.4 36.1 2.1 0.9 4.7MJ 17 July 04 9.2 8.0 24.7 36.2 4.9 0.3 9.1R 20 July 04 10.0 8.0 25.6 36.5 2.0 0.4 5.7MB 15 July 04 8.2 8.0 25.2 36.2 3.4 0.3 5.7

ND not determined

Zeu depth of the 10% isolume

Mar Biol (2007) 151:767–783 769

123

dry and Hassett 1982). During 2002, seven experiments

were performed at station MC between January and

October. During 2004, four experiments were con-

ducted in July at four stations. The initial water con-

ditions for each dilution experiment are given in

Table 1. The vertical attenuation coefficient for the

downward irradiance was measured with a Secchi disk

(Holmes 1970). Water temperature and salinity were

measured in situ with a microprocessor conductivity

meter (LF 196). Water was collected at the depth of

the chlorophyll maximum (2 m) using a 2.5 l plastic

water sampler PWS (Hydro-Bios) then filtered through

a 200 lm mesh screen (to remove meso- and macro-

zooplankton) and stored in polyethylene containers. A

portion of this water was filtered through a 0.2 lm

sterile capsule filter (Whatman) to remove all particles.

The filtrate was mixed with <200 lm prescreened

seawater to give five dilution factors (100, 75, 50, 25

and 5% of <200 lm prescreened water). Each dilution

mixture was then distributed into three clean 2 l

polycarbonate bottles. Initial samples were taken in

triplicate from the remaining dilution mixtures for

Chl a determination and phytoplankton and micro-

zooplankton cell counts. At the beginning of the

experiments, samples (500 ml) from the 0.2 lm filtered

seawater were frozen for later nutrient determination.

During the 2002 experiment, two dilution sets were

simultaneously performed: in the first one, bottles were

enriched with solutions of KNO3 (final concentration

of 16 lM) and NaH2PO4�2H2O (final concentration of

1 lM); in the second, neither N nor P was added.

During 2004, the dilution experiments at station R

were conducted without nutrients, with NP

(16 lM NO3– + 1 lM PO4

3–) or with NPSi [16 lM NO3– +

1 lM PO43– + 16 lM Si(OH)4]. These enrichments

provided information about the effect of nutrients on

phytoplankton growth rates.

The bottles were incubated at the sampling stations

at 2 m for 24 h. At the end of the incubation period,

final samples were removed from each bottle for Chl a

determination and phytoplankton and microzoo-

plankton cell counts.

Laboratory analyses

Nutrient concentrations were determined by spectro-

photometric methods (NO3– + NO3

–: Wood et al. 1967;

PO43–: Murphy and Riley 1962; Si(OH)4: Mullin and

Riley 1955). Water samples (500 or 1,000 ml) for

determination of Chl a and pheopigments were filtered

on 10 and 0.2 lm polycarbonate membranes. Filtration

was carried out under low vacuum pressure. Pigment

concentrations were estimated using the spectropho-

tometric method of Lorenzen (1967) and following the

procedure given by Parsons et al. (1984) after 24 h

extractions in 90% acetone at –5�C in the dark. Chl a

in the <10 lm size fraction was estimated from the

difference between total (0.2 lm) and >10 lm con-

centrations.

Samples for the identification and enumeration of

diatoms and autotrophic flagellates were preserved

with acid Lugol’s solution (3% final concentration) and

with formaldehyde (1% final concentration), respec-

tively. Microzooplankton samples were fixed in acid

Lugol’s solution at a final concentration of 10%. The

abundance of cells >5 lm was determined under an

inverted microscope (100· objective) (Utermohl 1931;

Lund et al. 1958) on settled volumes of 100 ml. At least

200 cells in each sample were counted. Unfortunately,

the smaller phototrophs and heterotrophic ultraflagel-

lates were not quantified because of a technical prob-

lem with epifluorescence microscopy. Lugol’s is

recognized to be a good fixative for ciliates since it

preserves a great number of them (Leakey et al. 1994;

Stoecker et al. 1995). However, the preservation in acid

Lugol’s is problematic for dinoflagellates because it

does not allow discrimination between the heterotro-

phic and autotrophic forms. Since the studies of Larsen

and Sournia (1991) and Stoecker (1999) have shown

that many, if not most, dinoflagellates are phago-

trophic, all dinoflagellates (including species of

Gymnodinium, Gyrodinium, Protoperidinium, Toro-

dinium and Dinophysis) and ciliates (identified as

aloricates and tintinnids) were considered as grazers in

our results. Other microzooplankton, such as copepod

nauplii and choanoflagellates (Monosiga marina), were

extremely rare in the 100 ml samples and were there-

fore excluded from our estimates.

During each experiment, the dimensions of at least

50 cells of each algal and protozoan taxa were mea-

sured using a calibrated ocular micrometer (Table 2).

Biovolumes were determined by applying standard

geometric formulae to each species, as proposed by

Hillebrand et al. (1999). For diatoms, the vacuole

volume was subtracted from the cell volume (Hille-

brand et al. 1999), assuming a plasma layer thickness of

1 lm, and the C content was obtained using a con-

version factor of 0.11 pg C lm–3 (Strathmann 1967).

The biovolumes were converted to cell carbon using a

factor of 0.22 pg C lm–3 for autotrophic flagellates

(Booth et al. 1988), 0.14 pg C lm–3 for dinoflagellates

(Lessard 1991) and 0.19 pg C lm–3 for ciliates (Putt

and Stoecker 1989). The carbon biomasses of phyto-

plankton and microzooplankton were estimated by

multiplying their cell carbon by their abundances. The

conversion factors used in our study can be not very

770 Mar Biol (2007) 151:767–783

123

accurate (Menden-Deuer and Lessard 2000), but they

still be usually applied to estimate the carbon biomass

of planktonic organisms from the microscopy size

measurements.

Calculation

Microzooplankton herbivory

In the dilution protocol, phytoplankton are assumed to

grow exponentially with time (Landry and Hassett

1982):

Nt=Neðk�gÞt0 ð1Þ

The apparent growth rate of phytoplankton (r, day–1)

is calculated as:

r=ln(Nt/N0)t�1=k� g ð2Þ

where t (day) is the incubation time, N0 and Nt are the

initial and final Chl a concentrations, respectively, k

(day–1) is the specific phytoplankton growth rate and g

(day–1) is the specific microzooplankton grazing rate.

The coefficients g and k were estimated from the

Model I linear regression of apparent growth rate (r)

against the dilution factor (Landry and Hassett 1982).

The grazing rate is the slope of the regression line and

the growth rate is the y-intercept (growth in 100%

dilution, i.e., in absence of grazers). The significance of

the regression line was examined using a t-test.

In the above calculation, we consider that dilution

affects only the density of grazers. However, micro-

zooplankton growth in undiluted water may occur

during incubation and lead to possible grazing satura-

tion, which may result in a biased estimate of the

grazing rate (Landry et al. 1995; Gallegos et al. 1996).

Recently, Dolan et al. (2000) argued that grazers must

be examined during dilution experiments to provide

information on protozoan growth and to assess possi-

Table 2 Geometric shape and size data (lm) of phytoplanktonand protozooplankton species (mean of 50 cells)

Shape A B C

DiatomsActinocyclus tenuissimus Cylinder 14 16Biddulphia sp. Cylinder 13 120Cerataulina bicornis Cylinder 20 42Chaetoceros affinis Elliptic prism 7 18 20Chaetoceros debilis Elliptic prism 12 12 10Chaetoceros laciniosus Elliptic prism 26 16 10Chaetoceros similis Elliptic prism 7 12 7Chaetoceros simplex Elliptic prism 10 10 14Chaetoceros tenuissimus Elliptic prism 5 15 7Cyclotella sp. Sphere 12Cylindrotheca closterium Cylinder 1 58Dactyliosolen fragillissimus Cylinder 9 50Delphineis spp. Elliptic prism 11 23 10Entomoneis pellucida Cylinder 14 38Guinardia delicatula Cylinder 16 75Guinardia striata Cylinder 13 120Gyrosigma sp. Double cone 18 134Lennoxia faveolata Half-elliptic

prism2 25 2

Leptocylindrus danicus Cylinder 8 26Leptocylindrus minimus Cylinder 2 20Navicula delicatula Elliptic prism 24 45 5Navicula directa Cylinder 6 28Navicula sp. Elliptic prism 10 190 5Nitzschia fontifuga Cylinder 3 19Nitzschia lineola Cylinder 2 84Nitzschia sigma Cylinder 2 30Nitzschia sp. Cylinder 2 7Paralia sulcata Cylinder 10 4Pseudo-nitzschia delicatissima Cylinder 2 61Pseudo-nitzschia

pseudodelicatissimaCylinder 2 78

Pseudo-nitzschia sp. Cylinder 2 69Rhizosolenia imbricata Cylinder 5 76Rhizosolenia spp. Cylinder 7 62Skeletonema costatum Cylinder 8 5Thalassiosira spp. Cylinder 10 4

FlagellatesChlamydomonas coccoides Sphere 5Chlamydomonas reginea Prolate spheroid 5 13Chlamydomonas sp. Prolate spheroid 5 16Dunaliella spp. Prolate spheroid 7 8Hillea spp. Prolate spheroid 5 8Mamiella spp. Sphere 5Nephroselmis spp. Sphere 7Pachysphaera sp. Sphere 5Plagioselmis spp. Cone + half

sphere5 8

Pyramimonas spp. Cone 4 7Rhodomonas spp. Cylinder 7 12Tetraselmis spp. Prolate spheroid 3 6Unidentified flagellates Sphere 5

DinoflagellatesDinophysis spp. Ellipsoid 32 42 32Gymnodinium spp. Ellipsoid 7 15 7Gyrodinium spp. Ellipsoid 6 12 6Protoperidinium spp. Double cone 26 34Torodinium spp. Prolate spheroid 20 64

CiliatesCodonellopsis sp. Cylinder 15 30

Table 2 continued

Shape A B C

Helicostomella spp. Cone 20 190Lohmanniela oviformis Ellipsoid 9 14 7Parafavella spp. Cone 15 30Strombidium spp. Cone 14 30Tintinnus spp. Cylinder 7 55Tintinnopsis spp. Cylinder 20 50Undella clevei Cylinder 12 25Uronema marinum Cylinder 8 16

A transapical axe (or diameter), B apical axe (or length), Cprevalvaire axe (or cross-section)

Mar Biol (2007) 151:767–783 771

123

ble artifacts in grazing rate estimates. During our

experiments, the net growth rate of microzooplankton

(R; day–1) was calculated as:

R=ln(Zt/Z0)t�1 ð3Þ

where t (day) is the incubation time and Z0 and Zt are

the initial and final abundances of microzooplankton,

respectively, in undiluted bottles. R was than used to

calculate the corrected grazing rate (g¢) derived from

the Eq. 11 of Gallegos (1989):

g’=[(Nt/N0) � eðktÞ] [(R� k)/(eðRtÞ � eðktÞ)] ð4Þ

Phytoplankton production (PChl) and consumption

(GChl) rates were calculated in terms of Chl a

(lg Chl l–1 day–1) with Eqs. 5 and 6, respectively:

PChl=k � Nm ð5Þ

GChl=g � Nm ð6Þ

where Nm is the average Chl a concentration during

the incubation (Frost 1972):

Nm=N0[eðk�gÞt � 1]/(k� g)t ð7Þ

To determine production (PC) and consumption

(GC) in terms of carbon (lg C l–1 day–1), PChl and GChl

were multiplied by the C:Chl ratio of 60, which rep-

resents the average ratio calculated from 2002 and 2004

data of Chl a and phytoplankton carbon biomass. The

percentage of production consumed is estimated as

G:P · 100, which is equal to g:k · 100. The percent-

ages of the Chl a standing stock grazed per day are

calculated as:

%Chl a consumed day�1=GChl � 100/N0 ð8Þ

Microzooplankton bacterivory

The carbon production rate of microzooplankton was

obtained by multiplying their growth rate (R) and their

carbon biomass. The production rate of micro-con-

sumers was then divided by the gross growth efficiency

of 0.30 (Straile 1997) to determine the carbon demand

(DC) needed to support the observed microzooplank-

ton growth rate. Microzooplankton bacterivory (BC)

was back calculated as DC – GC. Finally, the contri-

butions of bacterivory [BC · 100/DC] and herbivory

(GC · 100/DC) to the ingested ration of protozoa were

calculated.

Statistical analyses

Analyses were done using SPSS 8.0 statistical software

for Windows. One-way ANOVAs were used to test the

differences among dates (in 2002) or among stations

(in 2004) for Chl a, phytoplankton and protozoan

concentrations, growth rates and grazing coefficients.

The ANOVAs were followed by pair-wise multiple

comparison tests (Student–Newman–Keuls method) to

identify which groups were significantly different from

the others. When tests for normality of distribution

(test of Kolmogorov-Smirnov) and/or the homogeneity

of variance (test of Bartlett-Box) were failed, a non-

parametric test (Kruskal–Wallis one way ANOVA on

ranks) was used.

To test the effect of nutrients on algal growth rates,

paired t-tests were used to compare the values from

experiments with and without nutrient enrichment.

Paired t-tests were also used to compare estimated

rates (g and k) between algal size fractions (>10 and

<10 lm) and to compare grazing coefficients estimated

using the two methods (g and g¢). The requirement for

normality of distribution for the t-tests was respected.

Spearman correlations (rs) were used to test whether

phytoplankton estimates (Chl a, carbon biomass and

growth rate) were correlated with nutrients, tempera-

ture or microzooplankton estimates (abundance and

grazing rate).

Results

Phytoplankton community

Algal community structure during 2002

Chl a concentrations for the two size classes (<10 and

>10 lm) were significantly different among dates

(P < 0.001) during 2002. The <10 lm Chl a exhibited a

high increase during summer (5.30 lg l–1) relative to the

other seasons (0.80–2.06 lg l–1), while the >10 lm

Chl a increased progressively from January (0.35 lg l–

1) to July (1.56 lg l–1) and sharply decreased in Octo-

ber, reaching a value of 0.31 lg l–1 (Fig. 2a). The

>10 lm Chl a levels were positively correlated with

NO3– + NO2

– concentration (rs = 0.85, n = 21, P < 0.01).

The contribution of the >10 lm algae to total Chl a was

generally <20% in winter and autumn, but these cells

represented 25–33% of the total Chl a biomass during

spring and summer. Likewise, the relative contribution

of large-sized cells (like diatoms) to total phytoplankton

(>5 lm) abundance increased in April, May, July

(Fig. 3a). During spring/summer, diatoms were

772 Mar Biol (2007) 151:767–783

123

remarkably numerous (12.8–15.9 · 105 cells l–1; 171–

287 lg C l–1; Fig. 2b), contributing 50–70 and 64–74%

of the total algal cell number and carbon biomass,

respectively (Fig. 3). Indeed, several chain-forming

species (Skeletonema costatum, Leptocylindrus mini-

mus, Guinardia striata, Guinardia spp., Chaetoceros

similis, C. debilis, Dactyliosolen fragilissimus, Lenoxia

faveolata and Cerataulina bicornis) proliferated inten-

sively during these seasons (Fig. 3). The correlation

between diatom biomass and NO3– + NO2

– concentra-

tion was significant (rs = 0.80, n = 21, P < 0.01). The

winter/autumn communities of the >5 lm phytoplank-

ton were dominated by autotrophic flagellates (5.4–

7.8 · 105 cells l–1; 14.3–27.3 lg C l–1; Fig. 2b), which

contributed 83–96% and >95% of total algal cell num-

bers and carbon biomass, respectively (Fig. 3).

Algal community structure during 2004

In July 2004, the >10 lm Chl a concentrations at the

four stations were similar (0.81–1.11 lg l–1), whereas

the <10 lm Chl a levels varied significantly among

stations (P < 0.01). The values of the <10 lm Chl a at

stations MA, MJ and R (3.86–4.38 lg l–1) were higher

than those measured at station MB (2.08 lg l–1; Ta-

ble 3). For all stations, the >10 lm size class contrib-

uted 18–29% of the total Chl a. The carbon biomass of

the >5 lm phytoplankton exhibited no significant

spatial variation (P > 0.05) and varied between 209.3

and 260.0 lg C l–1 (Table 3). Diatoms (19.4–

28.9 · 105 cells l–1; 142.3–193.2 lg C l–1) were the

major component of the algal biomass, while auto-

trophic flagellates (10.0–13.2 · 105 cells l–1; 62.8–

84.1 lg C l–1) contributed less than 34% of total algal

carbon. The relative contribution of the phytoplankton

taxa at stations MA and MJ are presented in Fig. 4.

Diatoms, which made up ca. 70% of the algal cell

number and carbon biomass, were dominated by sev-

eral species of Nitzschia, Chaetoceros, Thalassiosira,

Leptocylindrus, Rhizosolenia, Cerataulina and Pseudo-

nitzschia.

Microzooplankton community

Microzooplankton community structure during 2002

The microzooplankton community during 2002 was

mainly composed of heterotrophic dinoflagellates and

ciliates (aloricates and tintinnids), whose abundances

and biomasses exhibited significant temporal variations

(P < 0.01; Fig. 5). The microzooplankton were

numerically dominated by heterotrophic dinoflagel-

lates, with abundances ranging from 77 · 103 to

193 · 103 cells l–1 (Fig. 5a). The major dinoflagellate

genera were Gymnodinium and Gyrodinium, which

were present during all seasons (Fig. 6a). During

summer, large species of Dinophysis and Protoperidi-

nium were observed in our samples, making a rela-

tively high contribution (30%) to the dinoflagellate

abundance (Fig. 6a). Because they have a higher car-

bon content (97–243 lg C l–1) and growth rate (0.5–

1.5 day–1) than heterotrophic dinoflagellates (10–

88 lg C l–1; 0.1–0.5 day–1), ciliates dominated the mi-

crozooplankton biomass (Fig. 5b). Throughout the

year, aloricates and tintinnids had similar contributions

to the ciliate assemblage, except in spring, when

aloricates made up 90% of the total ciliate number

(Fig. 6b). Aloricate ciliates were mostly dominated by

Strombidium spp. while tintinnids were represented by

species of Tintinnopsis, Codonellopsis and Parafavella.

Note that we found species of the large tintinnid

Helicostomella only in summer, when it contributed

33% of the ciliate abundance.

lhC

al g

µ( 1-)

0

1

2

3

4

5

6

7

8

J 13an

.

13eFb.

0A 4

p.r ya

M 40 80Ju

yl

16yluJ O 82

.tc

l C g

µ( ssamoib lagl

A1-)

0

100

200

300

400

500Autotrophic flagellates

Diatoms

Date

< 10 µm Chl a> 10 µm Chl a

a

b

Fig. 2 Temporal changes in a Chl a concentration and b algalcarbon biomass during 2002 in natural water from station MC(mean ± SD)

Mar Biol (2007) 151:767–783 773

123

The abundances of heterotrophic dinoflagellates and

ciliates were significantly correlated with the >10 lm

Chl a (rs = 0.90, n = 21, P < 0.001) but not with the

<10 lm Chl a. A significant correlation also existed

between protozoan cell numbers and diatom carbon

biomass (rs = 0.94, n = 21, P < 0.001).

Microzooplankton community structure during 2004

Microzooplankton from stations MA and MJ were

identified and enumerated. The microzooplankton

communities at both stations were dominated by het-

erotrophic dinoflagellates and ciliates, with abundances

ranged from 526 to 614 · 103 cells l–1 and biomasses

from 246 to 361 lg C l–1 (Table 3). Dinoflagellates

exhibited concentrations of 462–559 · 103 cells l–1,

biomass of 165–187 lg C l–1, and growth rate of 0.4–

0.5 day–1. Large organisms such as Protoperidinium

spp. (10–17% of the dinoflagellate abundance) were

present within this group (Fig. 7a). At both stations,

ciliates (55–66 · 103 cells l–1; 81–174 lg C l–1) had

growth rate of 0.7–0.8 day–1 and contributed 9–12%

and 33–48% of the total protozoan abundance and

biomass, respectively. Aloricate organisms, such as

Strombidium and Lohmaniellea spp. were the domi-

nant ciliates (Fig. 7b).

Phytoplankton growth

Figures 8 and 9 show the algal growth rates estimated

during the dilution experiments conducted without

nutrient enrichments. During 2002, growth rates for

total phytoplankton and for the >10 lm size fraction

varied significantly over time (P < 0.01), ranging from

0.72 to 1.04 day–1 and from 0.65 to 1.52 day–1,

respectively (Fig. 8a, c). The highest growth coeffi-

cients were recorded in July while the lowest were

estimated in January. Growth coefficients of total

Pre

cent

of

tota

l abu

ndan

ceP

rece

nt o

f to

tal b

iom

ass

0

20

40

60

80

100

31 F

eb.

31 Ja

n.

31 F

eb.

31 F

eb.

31 F

eb.

31 F

eb.

31 F

eb.

0

20

40

60

80

100

Date

Skeletonema costatumLeptocylindrus spp.Guinardia spp.Chaetoceros spp.Cerataulina bicornis

Nitzschia spp.Pseudo-nitzschia spp.Cylindrotheca closteriumOther diatomsAuto. flagellates >10 µmAuto. flagellates 5-10 µm

b

aFig. 3 Temporal changes inthe relative contribution ofalgal taxonomic groups to (a)total abundance and (b) totalC biomass of the >5 lmphytoplankton during 2002 innatural water from stationMC

Table 3 Biomasses and abundances of phytoplankton and microzooplankton in the initial water used for dilution experimentsconducted in July 2004

Station Chl a (lg l–1) Total phytoplankton Total microzooplankton

<10 lm >10 lm 105 cells l–1 lg C l–1 103 cells l–1 lg C l–1

MA 3.86 ± 0.10 0.81 ± 0.02 34.5 ± 5.0 257.2 ± 12.0 614.4 ± 18.2 361.2 ± 8.5MJ 4.38 ± 0.03 1.09 ± 0.15 40.7 ± 3.0 260.0 ± 10.0 526.2 ± 16.3 246.5 ± 7.8R 3.88 ± 0.20 1.11 ± 0.05 29.4 ± 5.1 209.3 ± 5.8 ND NDMB 2.08 ± 0.08 0.86 ± 0.05 31.2 ± 2.0 223.3 ± 3.0 ND ND

ND not determined. Mean ± SD

774 Mar Biol (2007) 151:767–783

123

phytoplankton were equivalent to production rates

of 0.8–6 lg Chl a l–1 day–1 and 48–360 lg C l–1 day–1

(Fig. 10a). The >10 lm algae accounted for 18–30%

of the total primary production during winter and

autumn while their contribution rose to 45–56%

during spring and summer. No significant difference

among dates was observed for the growth of the

<10 lm phototrophs (P > 0.05), with a mean rate of

0.75 day–1 (Fig. 8b). Except in winter, the growth

rates of the >10 lm algae were significantly higher

than those of the <10 lm phytoplankton (P < 0.01).

As for the Chl a concentration, a positive linear

relationship between growth rate and NO3– + NO2

–

levels was observed for the >10 lm algae (rs = 0.87,

n = 21, P < 0.01) but not for the <10 lm phyto-

plankton. For both phytoplankton size classes (<10

and >10 lm), the growth rates in nutrient-enriched

bottles were not significantly different (P > 0.05) from

those estimated during the experiments without any

enrichment.

During 2004, phytoplankton growth rates exhibited

significant differences among stations (P < 0.01;

Fig. 9). For total phytoplankton, the growth coeffi-

cients at stations MA, R and MB were similar (0.34–

0.37 day–1) but lower than that measured at station MJ

(0.45 day–1; Fig. 9a). Growth coefficients of total phy-

toplankton were equivalent to production rates of 1.2–

2.7 lg Chl a l–1 day–1 and 92–162 lg C l–1 day–1

(Fig. 10b). The growth rates of the <10 and >10 lm

phytoplankton ranged from 0.33 to 0.44 day–1 and from

0.35 to 0.62 day–1, respectively, with the highest rates

at station MJ (Fig. 9b, c). At most stations (MA, MJ

and R), the growth rates of the >10 lm algae were

significantly higher than those of the <10 lm phyto-

plankton (P < 0.01). Nutrient-enriched growth rates of

phytoplankton (<10 lm 0.57 day–1, >10 lm 0.52 day–1)

significantly exceeded those measured in natural water

at station R (<10 lm 0.33 day–1, >10 lm 0.41 day–1).

Microzooplankton grazing

The grazing rates on total phytoplankton estimated

from the Model I linear regressions of phytoplankton

growth rate (r) against dilution levels (i.e., g) and from

Eq. 4, which accounts for grazer growth (i.e., g¢) were

not significantly different (P > 0.05; Figs. 8a and 9a),

StationMA MJ

ecnadnuba latot fo tnecreP

0

20

40

60

80

100Skeletonema costatumThalassiosira spp.Leptocylindrus spp.Rhizosolenia spp.Chaetoceros spp.Cerataulina spp.Nitzschia spp. Pseudo-nitzschia spp.Cylindrotheca closteriumOther diatoms Auto. flagellates >10 µmAuto. flagellates 5-10 µm

Fig. 4 Differences in therelative contribution of algaltaxonomic groups to totalabundance of the >5 lmphytoplankton during 2004 innatural water from stationsMA and MJ

01( ecnadnubA

3l sllec

1-)

0

50

100

150

200

250

Date

13Ja

n. eF 31

b.

0

.rpA 4 04M

ya uJ 80

yl

16Ju

yl

2O 8

ct.

l C gµ( ssa

moiB

1-)

0

50

100

150

200

250

300

350

Ciliates

Heterotrophic dinoflagellates

a

b

Fig. 5 Temporal changes in microzooplankton (a) abundanceand (b) carbon biomass during 2002 in natural water from stationMC (mean ± SD)

Mar Biol (2007) 151:767–783 775

123

Date31

.naJ 1

beF 3

.

0

.rpA 4 40

aM

y luJ 80

y61

yluJtcO 82

.

ecnadnuba fo tnecreP

0

20

40

60

80

100

0

20

40

60

80

100ecnadnuba fo tnecreP

Heterotrophic dinoflagellates

Ciliates

Gymnodinium spp.Gyrodinium spp.Torodinium spp.Protoperidinium spp.Dinophysis spp.

AloricatesTintinnids

b

aFig. 6 Temporal changes inthe relative abundance of (a)heterotrophic dinoflagellatesand (b) ciliates during 2002 innatural water from stationMC

StationMA MJ

ecnadnuba fo tne creP

0

20

40

60

80

100

0

20

40

60

80

100ecnadnuba fo tnec reP

Heterotrophic dinoflagellates

Ciliates

Gymnodinium spp.Gyrodinium spp.Torodinium spp.Protoperidinium spp.Dinophysis spp.

Aloricates

Tintinnids

a

b

Fig. 7 Differences in therelative abundance of (a)heterotrophic dinoflagellatesand (b) ciliates during 2004 innatural water from stationsMA and MJ

776 Mar Biol (2007) 151:767–783

123

indicating a non-saturated grazer feeding response.

Therefore, we used the non-biased estimated coeffi-

cients (i.e., g) for the rest of the paper.

During 2002, grazing rates on total phytoplankton

and the two size fractions varied significantly over time

(P < 0.05). The highest coefficients were estimated

Date

d( etaR

1-)

0

1

2 Total

* * ** ** ** **

0

1

2 < 10 µm phytoplankton

kg

d( etaR

1-)

* * ***

****

*

kgg'

13

.naJ31

.beF.rpA 40 04

yaM

yluJ 80 1

yluJ 6 2O 8

tc .0

1

2 > 10 µm phytoplankton

d( etaR

1-)

kg

* ****

**

** **

a

b

c

Fig. 8 Phytoplankton growth rates (k) and microzooplanktongrazing rates (obtained from Model I linear regressions ofapparent growth rate on dilution levels [g]; and from Eq. 4,which accounts for microzooplankton growth [g¢]) estimatedfrom the dilution experiments without nutrients during 2002 inBizerte Lagoon. Grazing (g) was evaluated by t-test as significantat the P < 0.05 (*) or P < 0.01 (**) level (mean ± SD)

Station

d( etaR

1-)

0

1

2 Total phytoplankton

** ** **

0

1

2 < 10 µm phytoplankton

kg

d( etaR

1-)

** ****

kgg'

**

**

MA M

J R BM

0

1

2 > 10 µm phytoplankton

d( et aR

1-)

kg

** **** **

a

b

c

Fig. 9 Phytoplankton growth rates (k) and microzooplanktongrazing rates (obtained from Model I linear regressions ofapparent growth rate on dilution levels [g]; and from Eq. 4,which accounts for microzooplankton growth [g¢]) estimatedfrom the dilution experiments without nutrients during 2004 inBizerte Lagoon. Grazing (g) was evaluated by t-test as significantat the P < 0.05 (*) or P < 0.01 (**) level (mean ± SD)

Mar Biol (2007) 151:767–783 777

123

during summer and the lowest were recorded during

winter or autumn (Fig. 8). Grazing coefficients on total

algae, ranging from 0.54 to 0.70 day–1 (Fig. 8a), cor-

responded to consumption rates of 0.5–4.1 lg Chl a l–

1 day–1 and 30–246 lg C l–1 day–1 (Fig. 10a). These

levels of grazing were equivalent to daily losses of

41.5–58.3% of the Chl a standing stock and 62–78% of

the phytoplankton production. Generally, the grazing

rates on the >10 lm cells (0.52–0.98 day–1) were sig-

nificantly higher than those on the <10 lm algae (0.43–

0.66 day–1; P < 0.0001; Fig. 8b, c). The correlation

analysis indicated that grazing rates on the >10 lm

algae exhibited a significant positive relationship with

their specific growth rates (rs = 0.86, n = 21, P < 0.01).

This was not observed for the <10 lm phytoplankton.

During 2004, grazing rates were less variable among

stations and among algal size fractions compared to

phytoplankton growth rates. Grazing by microzoo-

plankton ranged between 0.20 and 0.30 day–1 (Fig. 9).

The microzooplankton consumed 25.5–30% of the

Chl a standing stock, which was equivalent to a con-

sumption rate of 0.7–1.7 lg Chl a l–1 day–1 and 42–

102.5 lg C l–1 day–1 (Fig. 10b). The microzooplankton

grazing corresponded to a loss of 57–84% of the daily

primary production.

The contribution of herbivory and bacterivory to the

microzooplankton carbon demand is shown in Table 4.

During winter, spring and autumn 2002, bacterivory

accounted for 68–85% of the carbon ingested by pro-

tozooplankton. This corresponded to bacterial con-

sumption rates of 113–328 lg C l–1 day–1. During the

summer 2002, herbivory accounted for 71–80% of the

ingested ration of protozoa. However, in summer 2004,

herbivory accounted for only 14–23% of the carbon

demand by microzooplankton.

Discussion

During 2002, phytoplankton experienced significant

seasonal variation in their growth rates (Fig. 8), lead-

ing to changes in their biomass (Fig. 2) and production

(Fig. 10a). Throughout the year, the Chl a standing

stock (1–6.8 lg l–1) and phytoplankton production (48–

360 lg C l–1 day–1) were high due to the relatively high

growth rates (0.72–1.04 day–1) recorded during all

seasons. This may be attributed to sufficient nutrient

Station

AM

JM R B

M

0

1

2

3

Date

.naJ 13

.beF 31 04pAr.

04M

ya80

uJyl

61uJ

yl 82

cO.t

0

1

2

3

4

5

6

7 2002l lh

C µ( eta

R1-

d 1-)

0

100

200

300

400

500

l C

µ( etaR

1-d

1-)

PChlGChl

PCGC

l lhC

µ( e taR

1-d

1-)

l C

µ( etaR

1 -d

1-)

2004

0

50

100

150

200

PCGC

PChlGChl

a

b

Fig. 10 Rates of total phytoplankton production (PChl and PC)and consumption (GChl and GC), during (a) 2002 and (b) 2004 inBizerte Lagoon (mean ± SD)

Table 4 Consumption rates of phytoplankton carbon (GC, as in Fig. 10) and bacterial carbon (BC) by microzooplankton during 2002and 2004 in the Bizerte Lagoon. The percent contributions of herbivory (GC%) and bacterivory (BC%) to the ingested ration ofmicrozooplankton are also indicated (see Materials and methods for details). For GC and BC, mean ± SD

Station Date GC (lg C l–1 day–1) BC (lg C l–1 day–1) GC% BC%

MC 31 Jan. 2002 53.1 ± 23.0 112.8 ± 15.5 32.0 68.013 Feb. 2002 62.6 ± 22.0 135.3 ± 30.6 31.6 68.404 Apr. 2002 84.6 ± 7.5 184.9 ± 18.5 31.4 68.604 May 2002 97.2 ± 14.8 328.1 ± 15.0 22.9 77.108 July 2002 229.4 ± 10.0 57.1 ± 14.8 80.1 19.916 July 2002 246.0 ± 20.0 100.0 ± 36.0 71.1 28.928 Oct. 2002 30.0 ± 5.2 165.3 ± 2.5 15.4 84.6

MA 13 July 2004 77.9 ± 4.2 487.9 ± 5.5 13.7 86.3MJ 17 July 2004 102.5 ± 8.8 341.2 ± 10.5 23.1 76.9

778 Mar Biol (2007) 151:767–783

123

availability in the lagoon during the study period (Ta-

ble 1). In support of this hypothesis, algal growth rates

were not affected by nutrient enrichments during 2002,

suggesting that nutrients were not limiting for phyto-

plankton growth. The >10 lm phytoplankton exhibited

the most pronounced seasonal variations in the growth

rate and biomass (Figs. 2 and 8c). The species com-

position of these algae also shifted among seasons.

Autotrophic flagellates dominated the >10 lm algal

assemblage in winter and autumn, but diatoms became

predominant in spring and summer (Fig. 3). During

this period, the increased water temperature may have

stimulated bacterial activity and benthic metabolism,

which in turn enhanced the mineralization and hence

contributed to the spring/summer increase of nitroge-

nous nutrients in the station MC. Increases in both of

these abiotic factors (i.e., temperature and

NO3– + NO2

–) in spring and summer (Table 1), and

presumably improved light conditions, stimulated dia-

tom growth. Indeed, the diatom growth rate was pos-

itively correlated to water temperature (rs = 0.86,

n = 21, P < 0.01) and the NO3– + NO2

– concentration

(rs = 0.95, n = 21, P < 0.001). Moreover, some diatom

species (Skeletonema costatum, Leptocylindrus mini-

mus, Guinardia striata, Guinardia spp., Chaetoceros

similis, C. debilis, Dactyliosolen fragilissimus, Lenoxia

faveolata and Cerataulina bicornis) were encountered

only during April, May, July (Fig. 3). Obviously, dia-

tom dynamics also depend on Si availability (Dugdale

and Wilkerson 1992; Carlsson and Graneli 1999). Al-

though this nutrient was not measured in the 2002

study, we presumed that it would not be limiting for

diatom growth, particularly in summer, when these

algae achieved two doublings day–1 (i.e., 1.5 day–1).

During July 2004, phytoplankton abundance and

biomass exhibited slight variations among stations

(Table 3). The communities were dominated by large

organisms such as diatoms, which exhibited a high

diversity of species (Fig. 4), as observed in July 2002.

Phytoplankton biomass (2.9–5.4 lg Chl a l–1; 209–

260 lg C l–1), production (92–162 lg C l–1 day–1), and

growth rates (0.34–0.45 day–1) in 2004 were relatively

low. The 2004 NP and NPSi additions resulted in sig-

nificant increases in the algal growth rates, indicating

that phytoplankton were nutrient limited in summer

2004.

Abundances and biomasses of ciliates and hetero-

trophic dinoflagellates, the main components of the

microzooplankton community in this study, exhibited

significant variations among seasons in 2002. During

the summer of 2002, the protozoan abundances and

biomasses peaked (Fig. 5), as a response to the en-

hanced phytoplankton biomass and production. During

this season, large dinoflagellates (Protoperidinium,

Dinophysis) were observed, but ciliates made up a

large fraction of the total microzooplankton biomass

(Fig. 5b). Water temperature, which has been reported

to affect protozoan growth (Montagnes and Lessard

1999), had little effect on the seasonal variation of

protozooplankton during 2002. There was no signifi-

cant relationship (P > 0.05) between temperature and

the concentration or growth coefficient of ciliates and

of dinoflagellates. During summer 2004, the micro-

zooplankton were also numerous (Table 3) due to the

very high concentrations of heterotrophic dinoflagel-

lates (462–559 · 103 cells l–1), which contributed 50–

67% of total protozooplankton biomass. Copepods,

which are potential predators of ciliates and hetero-

trophic dinoflagellates (Merrell and Stoecker 1998;

Suzuki et al. 1999; Jurgens et al. 1999), are the domi-

nant metazoans in the Bizerte Lagoon. Their abun-

dance is lower from June to September, which may be

due to the proliferation of the jellyfish Olindias phos-

phorica, a potential predator of copepods (Hamdi

2002). The high microzooplankton abundance that we

observed in summer 2002 and 2004 (Table 3; Fig. 5a)

may be due to the low predation by large zooplankton.

Predation by benthic suspension feeders on micro-

zooplankton is also well documented in marine habi-

tats (Le Gall et al. 1997). As our study site is relatively

shallow (Table 1), the benthic community may govern

in part the abundance and biomass of some planktonic

protozoans. Unfortunately, because of a lack of infor-

mation on the seasonal dynamics of benthic metazoans

in the lagoon, no evident relationship can be estab-

lished between the two communities.

The results from the dilution experiments high-

lighted the significance of microzooplankton grazing in

Bizerte Lagoon since it daily removed a large fraction

(57–84%) of the phytoplankton production in different

seasons (during 2002) and at different stations (during

2004). For several coastal systems, Calbet and Landry

(2004) calculated that, on average, 60% of the primary

production is grazed, but higher percentages have been

found for some near shore waters (James and Hall

1998; Murrell and Hollibaugh 1998; Bottjer and Mor-

ales 2005). The grazing coefficients estimated during

2002 (0.43–0.98 day–1) are in the range of values re-

ported from dilution experiments in other coastal

ecosystems (Burkill et al. 1987; Strom and Strom 1996;

Murrell and Hollibaugh 1998; Olson and Strom 2002).

The impact of microzooplankton grazing on phyto-

plankton assessed from dilution technique can be

overestimated do to the absence of copepods in the

incubated bottles. During our dilution experiments, the

abundance and the predation of copepods were not

Mar Biol (2007) 151:767–783 779

123

estimated. Nevertheless, the copepod concentrations

were low in the lagoon during summer (Hamdi 2002)

when microzooplankton had higher abundance (Fig. 5)

and the highest grazing impact (>50% of Chl a biomass

and >68% of algal production were removed). During

this season, higher grazing rates were measured for the

>10 lm phytoplankton (Fig. 8c), which were domi-

nated by several chain-forming diatoms (Fig. 3).

Moreover, microzooplankton growth was significantly

correlated to diatom (rs = 0.9, n = 21, P < 0.001) and a

strong correlation was observed between diatom

growth rate and the grazing coefficient (rs = 0.99,

n = 21, P < 0.001), indicating a tight link between mi-

cro-grazers and large preys. Therefore, although we

have no information on copepods during our experi-

ments, the microzooplankton in the lagoon seemed to

have high impact on larger faster-growing phyto-

plankton (i.e., diatoms) and hence played an important

role in controlling these algae.

The strong impact of microzooplankton on diatoms

has an important implication for carbon cycling in the

study site. Since micro-grazers removed 63–80% of the

diatom production per day, other mechanisms con-

trolling the loss of large algae, such as predation by

planktonic and benthic metazoans or vertical sinking

(Huntley et al. 1991; Alpine and Cloern 1992; Fron-

eman et al. 1996; Murrell and Hollibaugh 1998), would

be less important in the study lagoon. Hence the algal

carbon can be transferred to the higher trophic levels

via predation by copepods on ciliates and dinoflagel-

lates (Hansen et al. 1993; Kiørboe and Nielsen 1994) or

to benthic organisms as fecal material. However, the

high concentrations of microzooplankton in the la-

goon, particularly in summer, have been attributed to a

low predation by copepods, which are not abundant

during the June–September period (Hamdi 2002). This

suggests that most of the diatom production is recycled

and fuels the microbial food web while the vertical

carbon flux, which is generally important when large

algae are dominant (Longhurst 1991; Michel et al.

2002; Turner 2002), is reduced.

In general, microzooplankton consume the available

autotrophic and heterotrophic nanoplankton (Sherr

and Sherr 1994; Fonda Umani and Beran 2003), while

picobacteria are mainly used by heterotrophic and

mixotrophic nanoflagellates and small aloricate ciliates

(Simek et al. 1995; Reckermann and Veldhuis 1997;

Safi and Hall 1999; Calbet et al. 2001). However, the

microzooplankton grazing can be highly selective and

more variable, since micrograzers can shift to bacterial

diet when the usual nano-sized preys are scare, as ob-

served by Fonda Umani and Beran (2003) and Fonda

Umani et al. (2005). Our study confirms the bacteri-

vory of microzooplankton, which can in some season

account for a large fraction of their carbon demand

(e.g., in autumn; Table 4). The bacterivorous con-

sumption rates calculated in our experiments by

assuming a microzooplankton efficiency of 30% (57–

488 lg C l–1 day–1) were higher than those directly

measured by these authors (2–67 lg C l–1 day–1). This

may be due to the high microzooplankton biomass

(114–361 lg C l–1) measured in our study, which lar-

gely exceeded that found by Fonda Umani and Beran

(2003) and Fonda Umani et al. (2005) (<15 lg C l–1).

The high impact of microzooplankton upon phyto-

plankton in the Bizerte Lagoon agrees with previous

studies showing that protozooplankton are potential

consumers of coastal phytoplankton and are able to

graze a large fraction of the phytoplankton production

(McManus and Ederington-Cantrell 1992; Strom and

Strom 1996; Strom et al. 2001; Bottjer and Morales

2005). Ciliates are generally known to preferentially

graze on small prey (<20 lm) while dinoflagellates are

capable of feeding on larger cells (Bernard and Rass-

oulzadegan 1990; Hansen 1992). Several studies have

shown that dinoflagellates, including Gyrodinium and

Protoperidinium, are potential grazers of large algae

such as chain-forming diatoms and are often associated

with diatom blooms (Jacobson and Anderson 1986;

Strom and Strom 1996; Hansen and Calado 1999;

Kjaeret et al. 2000; Stelfox-Widdicombe et al. 2004). In

Bizerte Lagoon, phytoplankton, particularly large cells,

seemed to be mostly impacted by ciliates. These

organisms have higher growth rates (2002 0.5–1.5, 2004

0.7–0.8 day–1) than dinoflagellates (2002 0.1–0.5, 2004

0.4–0.5 day–1) and hence the ciliate carbon demand

(2002 165–350, 2004 200–470 lg C l–1 day–1) largely

exceeded that of dinoflagellates (2002 33–90, 2004 270–

290 lg C l–1 day–1). Protozoans can meet a part of

their carbon requirement for growth from bacterivory.

The 2002 experiments showed that bacterivory ac-

counted for 68–84.6% of the microzooplankton carbon

in winter, spring and autumn, but in summer (Table 4),

when ciliates largely dominated the microzooplankton

biomass (Fig. 5b), micro-grazers were primarily her-

bivorous since 71–80% of their ingested carbon origi-

nated from phytoplankton (Table 4). However, during

summer 2004, the dinoflagellates accounted for 50–

67% of the microzooplankton biomass, and herbivory

did not contribute significantly to the microzooplank-

ton carbon demand (14–23%). Furthermore, lower

grazing rates were recorded in July 2004 than in July

2002 (Figs. 8 and 9), although both summer algal

communities were dominated by diatoms. The fraction

of the Chl a biomass that was grazed per day in sum-

mer 2004 (26–31%) was also lower than that consumed

780 Mar Biol (2007) 151:767–783

123

during 2002 (57–58%). All these facts suggest that,

contrary to expectations, ciliates have a stronger

grazing control upon large phytoplankton than do di-

noflagellates.

In summary, our study showed the high impact of

microzooplankton grazing on phytoplankton produc-

tion throughout the year and at different stations in

Bizerte Lagoon. Grazing control was particularly pro-

nounced in summer, when the large algae (i.e., dia-

toms) contributed a high fraction of the primary

production. These results have significant implications

for carbon export. As phytoplankton production was

mostly channeled to the microzooplankton, an impor-

tant component of the microbial food web, the flux of

material to the seafloor, would be reduced. This should

be considered when modeling the carbon cycling in

coastal environments and under conditions of diatom

dominance. Our study demonstrated that the ciliate

community of the restricted Bizerte Lagoon was the

more active grazer of the large algae (diatoms) than

were the heterotrophic dinoflagellates, which is con-

trary to finding in open coastal waters.

Acknowledgments The 2002 study was carried out with thefinancial support of the Ministere de l’Enseignement Superieurde Tunisie. The 2004 study was supported by grants from theAgence Universitaire de la Francophonie (AUF) to A. S. H. andM. G. and from the Natural Sciences and Engineering ResearchCouncil (NSERC) of Canada to M. G. Experiments performedduring this study complied with the current laws of Tunisia. Wethank L. Devine for useful comments on the manuscript andanonymous reviewers for useful suggestions. This is a contribu-tion to the research programs of ISMER and Quebec-Ocean.

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