Phytoplankton-pigment signatures and their relationship to spring–summer stratification in the Gulf of Gabes

Estuarine, Coastal and Shelf Science 83 (2009) 296–306

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Phytoplankton-pigment signatures and their relationship to spring–summer stratification in the Gulf of Gabes M. Bel Hassen a, *, A. Hamza a, Z. Drira a, b, A. Zouari a, F. Akrout a, S. Messaoudi a, L. Aleya c, H. Ayadi b a

ˆ, Tunisia Institut National des Sciences et Technologies de la Mer, 28 rue 2 mars 1934, 2025 Salammbo Universite´ de Sfax, Route soukra Km 3.5, BP 1171, CP 3000 Sfax, Tunisia c Laboratoire de Chrono-Environnement, Universite´ de Franche-Comte´, UMR CNRS 6249, 1 Place Leclerc, F-25030 Besançon Cedex, France b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 February 2009 Accepted 2 April 2009 Available online 15 April 2009

We investigated the phytoplankton dynamics (determined by CHEMTAX analysis of HPLC pigment data) and its relationships with nutrients and water column structure, during two oceanographic cruises in May–June and September 2006 in the Gulf of Gabes (south-eastern Mediterranean). The May–June cruise coincided with the beginning of the summer stratification, while a strong stratification occurred in September with a more than 30 m deepening of the thermocline, and a reduction of the euphotic depth. This strong stratification resulted in a shift in nitrogen sources from nitrates to ammonium as well as phosphate depletion (0.2 mM) and a decrease in silicate concentrations (<2 mM). With the exception of chlorophyll a, pigment concentrations were higher in September than in May–June samplings. The picoand nanophytoplankton were the major contributors to phytoplankton total biomass, accounting for 90% and 87% of total chlorophyll a in May–June and September, respectively. Picoplankton persisted throughout the entire survey, occupying different depth layers. Chlorophytes were present at substantial amounts (average 23% of total chlorophyll a) during May–June; however, they declined in September (average 5%). Diatoms were overall poorly represented in this study (2% of total chlorophyll a), due probably to silicate shortage. Apparently, the nutrient availability, but also the water column stability seemed to be among the major factors determining phytoplankton dynamics. Indeed, cyanobacteria were prominent in surface samples during the period of strong stratification, whereas the relative contribution of chlorophytes decreased, probably due to low phosphate availability. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: phytoplankton CHEMTAX chlorophyll b chlorophytes prochlorophytes cyanobacteria Gulf of Gabes

1. Introduction In aquatic systems, the phytoplankton community structure has been demonstrated to be governed by a combination of physical, chemical and biological environmental factors (Aleya, 1991; Sarmiento et al., 2004), and it was recognized that phytoplankton signatures might characterize specific hydrographic conditions for most of the world oceans. For example, large species (mostly diatoms) dominate in up-welling environments while picophytoplankton is a common feature of the oligotrophic stratified open ocean (Chisholm, 1992). A large set of data has been accumulating on the hydrological regime of the Mediterranean Sea, and provided evidence that the water column is well stratified in summer, mixed in winter with transition periods of stratification occurring during late spring and early winter (Be´thoux and Prieur, 1983). The study of the photosynthetic phytoplankton pigments as

* Corresponding author. . E-mail address: [email protected] (M.B. Hassen). 0272-7714/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2009.04.002

chemotaxonomical markers of phytoplankton communities over transition from mixed to stratified conditions should thus provide valuable information on the functioning of pelagic ecosystems, which, primarily, depends on the nutrient potential of waters induced by local and general hydrographic conditions (Estrada, 1991; Claustre et al., 1994; Bustillos-Guzman et al., 1995). In particular, the water mixing has been shown to induce an increase of mixed layer depth as well as changes in nutrient availability (Be´thoux and Prieur, 1983), whereas the water stratification constrained the nutrient transport from deep water masses to surface layers (Marty et al., 2002). The hydrological regime characterizing the spring-summer transition offered the best compromise between nutrient and light availability (Margalef, 1985; Reynolds, 1997) and was generally marked by the presence of large cells blooms, especially diatoms (Bustillos-Guzman et al., 1995). The change towards a well-stratified water column induced a shift in the phytoplankton size structure from large to small size algal species (Bustillos-Guzman et al., 1995; Stemmann et al., 2002). While these general features have been well described in the north Mediterranean Sea, little is known about the relationships between

M.B. Hassen et al. / Estuarine, Coastal and Shelf Science 83 (2009) 296–306

the phytoplankton dynamics, hydrographic conditions and nutrient availability in the south-eastern Mediterranean, especially in the Gulf of Gabes. In the present study, we attempted to analyse these relationships by sampling, with a high spatial resolution, the phytoplankton community (derived from the distribution of taxonomic pigments analysed by CHEMTAX), during two oceanographic cruises corresponding to transition from the spring–summer (May– June) to the summer stratified (September) conditions during 2006 in the Gulf of Gabes. This paper is aimed at providing evidence for the influence of the water stratification stage on the phytoplankton community composition and addressing the relationships between the phytoplankton community structure, the hydrographical water properties and the nutrient contents. 2. Methods 2.1. The studied area The Gulf of Gabes is located in the south Ionian Sea and occupies a wide continental shelf area (Fig. 1). The early survey conducted in this area by Brandhost (1977) reported a salinity minima mostly attributed to the presence of the Atlantic Water (AW). On a regional scale, the AW in the western Mediterranean enters the Straits of Sicily and splits into two branches. The first branch flows to the

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south-eastern Mediterranean while the second flows to the south, therefore affecting the water circulation at the mouth of the Gulf of Gabes (Grancini and Michelato, 1987). We have recently shown from a study carried out during late winter in the Gulf of Gabes that the AW flows within the continental shelf area between 50 m and 100 m isobaths (Bel Hassen et al., 2009), whereas it exhibited a weak advection as summer stratification established (Bel Hassen et al., 2008). Previous observations from this area also reported a seasonal variation in AW (Be´ranger et al., 2004).

2.2. Sampling The study area encompassed the continental shelf area between 20 m and over 100 m in depth (Fig. 1). Two cruises aboard the ‘R/V Hannibal’ were conducted under a weak northeast wind during 2006 and labelled herein as (MaydJune) and (September). The first cruise (26 May–6 June) was carried out at the end of spring and the beginning of the summer stratification while the second (7–11 September) coincided with a well-established water column. The sampling locations were selected in order to better highlight the coast–offshore variability which implicitly characterized the variability across the AW, assuming that the alongshore variability was comparably unimportant. In May–June (Fig. 1) two sectors were mainly investigated. The north-east sector of the Gulf roughly between 50 m and 100 m isobaths where the AW has been previously demonstrated to flow and the coastal southern sector where the Mediterranean water has been shown particularly dominant (Bel Hassen et al., 2009). In September, the AW advection was expected to be weaker. This water mass, suspected to be the residual of enhanced spring advection, was distributed throughout a large area covering the overall zone between 50 m and 100 m isobaths during the summer-stratified period (Bel Hassen et al., 2008). Therefore, more stations were investigated in September to cover this area (Fig. 1). For each station a vertical profile of temperature and salinity was performed during daylight with a Seabird conductivity– temperature–depth (CTD) sensor (SBE 9, Seabird Electronics, USA) equipped with a 12-l Niskin rosette. For coastal well mixed stations, less than 50 m in depth, discrete samples were collected at three depths (2 m, 25 m and near bottom). For stratified offshore stations, above 50 m in depth, five depths were investigated (2 m, 10 m, 20 m, thermocline and near bottom). Sub-samples (2 l) for pigments analyses were filtered onto 47 mm-diameter glass fibre filter, Whatman GF/F. Filters for pigment analyses were immediately stored at 20  C and analysed within 1 month. Samples for nutrient analyses were preserved immediately upon collection (20  C, in the dark) and analysed within 15 days. The Secchi disk measurements were achieved during daylight. Vertical light attenuation coefficients were estimated according to Holmes (1970), and the euphotic zone depth was calculated assuming that irradiance at the bottom was 1% of surface irradiance (Cloern, 1987).

2.3. Nutrients analyses and criteria for stoichiometric and potential nutrients limitations

Fig. 1. Sampling locations and the 50 m, 100 m and 200 m isobaths in the Gulf of Gabes. Squares are sites investigated in May–June cruise and circles are sites visited in September cruise.

Nutrients analyses were performed with an automatic analyser type 3 (Bran & Luebbe) using standard methods (Tre´guer and LeCorre, 1975). The combinations of N, P and Si ratios were established to investigate potential nutrient limitations. Criteria for stoichiometric nutrient limitation, Si/N/P ¼ 16:16:1, were developed based on nutrient requirements of diatoms established by Redfield et al. (1963).

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2.4. Pigments analysis The frozen filters were cut into small pieces and chlorophyll a and carotenoids were sonicated in 100% acetone. Extracts were centrifuged to remove cellular and glass filter debris and the supernatant was filtered through a nylon membrane filter (pore size 0.45 mm). The combined extract was then analysed using reverse phase High Performance Liquid Chromatography (HPLC) according to Van Heukelem et al. (1992). Analyses were performed on a HP 1100 system equipped with an Eurospher-100 C18 (250  4.6 mm  5 mm) column and with a multi-wave length UV detector. Pigments were separated using a non-linear binary gradient using a solvent A composed of 80% methanol/20% ammonium acetate (0.5 M adjusted at pH 7.2) and a solvent B composed of 100% acetonitrile. Chlorophylls and carotenoids were detected and quantified by absorbance at 440 nm. Amounts of pigments were estimated from phytoplankton field extracts using individual calibration curves. Pigment calibration standards were purchased from DHI Water and Environment Institute (Hørsholm, Denmark). Pigment names were abbreviated as follows: chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll c3 (Chl c3), fucoxanthin (Fuco), 190 -hexanoyloxyfucoxanthin (Hex), 190 -butanoyloxyfucoxanthin (But), peridinin (Peri), alloxanthin (Allo), neoxanthin (Neo), violaxanthin (Viol), prasinoxanthin (Pras), lutein (Lut) and zeaxanthin (Zea). 2.5. Chemotaxonomic analyses of phytoplankton pigments Algal class abundances were determined from HPLC algal pigment measurements using CHEMTAX, a factor analysis programme, which estimates the contribution of each specified

phytoplankton pigment class to the total Chl a concentration in a water sample, and is fully described in Mackey et al. (1996). CHEMTAX software requires an initial input of a matrix of pigment: Chl a ratios for each pigment class. The pigment classes were attributed to algal taxa based on microscopic determination of the taxa present in the samples and published by our research team (Drira et al., 2009), on available data related to the sampled area (Drira et al., 2008) and on HPLC chromatograms. The pigment:Chl a ratios (Table 1) were selected from a range of values given by Mackey et al. (1996). These input ratios were already used in this same area and evidenced reliable information (Bel Hassen et al., 2008). The concentrations of chlorophyll c2 that could aid in distinguishing some prymnesiophytes, dinoflagellates and diatoms were rather low, thus not used in the CHEMTAX analyses. In addition, because diadinoxanthin rapidly converted to diatoxanthin in the light (Demers et al., 1991), it was excluded from the set of data. Divinyl chlorophylls a and b, which are diagnostic pigments for prochlorophytes (Goericke and Repeta, 1993), were not resolved from the monovinyl forms by our analytical method. The prochlorophytes were included in the CHEMTAX analysis due to the ability of the program to provide good estimates of this group, even in the absence of experimental data on the concentrations of divinyl-chlorophylls a and b (Mackey et al., 1996). Finally, nine algal categories, representing typical pigment patterns, were functionally defined by their pigment content (Table 1). The variation in pigment ratios with the light regime and hence with depth throughout the water column were investigated in a previous study conducted in this area during the summer stratified conditions (Bel Hassen et al., 2008). During this study, the data were divided into five layers, which were analysed separately by

Table 1 Input pigment:chlorophyll a ratios and output ratios for each cruises as calculated by CHEMTAX. Chlorophyll b (Chl b), chlorophyll c3 (Chl c3), fucoxanthin (Fuco), 190 -hexanoyloxyfucoxanthin (Hex), 190 -butanoyloxyfucoxanthin (But), peridinin (Peri), alloxanthin (Allo), neoxanthin (Neo), violaxanthin (Viol), prasinoxanthin (Pras), lutein (Lut), zeaxanthin (Zea). Fuco Input ratio Diatoms Dinoflagellates Prymnesiophytes Cryptophytes Pelagophytes Chlorophytes Cyanobacteria Prochlorophytes Prasinophytes Output ratio May–June Diatoms Dinoflagellates Prymnesiophytes Cryptophytes Pelagophytes Chlorophytes Cyanobacteria Prochlorophytes Prasinophytes September Diatoms Dinoflagellates Prymnesiophytes Cryptophytes Pelagophytes Chlorophytes Cyanobacteria Prochlorophytes Prasinophytes

Peri

Zea

Chl b

Allo

Hex

But

Lut

Pras

Neo

Viol

Chl c3

0.70 0.50 0.62

0.65

0.10

0.23 0.62

0.93 0.05 0.33 0.23 0.14

0.13

0.28

0.14

0.55 0.80

0.02

0.35

0.03

0.02

0.10

0.11

0.41 0.33 0.26

0.27

0.04

0.19 0.23

0.35 0.03 0.79 0.13 0.06

0.05

0.18

0.09

0.31 0.32

0.01

0.14

0.02

0.01

0.04

0.04

0.41 0.33 0.26

0.27

0.04

0.19 0.28

0.56 0.03 0.25 0.13 0.07

0.04

0.18

0.10

0.31 0.43

0.01

0.19

0.02

0.01

0.05

0.06

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CHEMTAX to allow detection of possible vertical changes in pigment:Chl a ratios. The results showed only small variations of the CHEMTAX output ratio. Moreover, in the present study, the most significant change with depth of the pigment:Chl a ratio was observed for Chl b. However, for this pigment, the difference in concentration found between the two cruises was more pronounced than that between depths on a same cruise. This is why CHEMTAX analysis was run separately for the two cruises assuming that the pigment ratios were stable with depth. Two consecutive runs of CHEMTAX were performed on each subset of data (May–June and September) using input ratios from Table 1 for the first run. The pigment ratios from the first output were then used as initial ratios for the second optimization (Wright and Van den Enden, 2000). The pigment:Chl a was allowed to vary by a factor of five.

2.6. Data analysis The maps of the hydrographical, chemical and biological parameters were performed using inverse distance interpolation method with Surfer software. The potential relationships between these variables were tested using Pearson’s correlation coefficient.

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The statistical analyses were performed with XLSTAT software for Windows.

3. Results 3.1. Hydrographic and nutrient data Depth-related trends in water temperature, density (Sigma-t) and salinity along a coast–offshore direction are shown in Fig 2. In May–June, the thermocline depth ranged between 10 m in offshore samples to more than 30 m in coast samples. In September, at the exception of the coastal stations water mixing, the water stratification was more pronounced and the thermocline established at a depth of more than 30 m. The salinity exhibited (Fig. 2) a decreasing gradient towards the offshore area, with minima (<37.5) recorded in May–June throughout the water column, at a distance ranging between 200 km and 280 km from the coast. In contrast, these minima were recorded in bottom samples in September. These minima were attributed to the AW circulation in this area, as already reported by Be´ranger et al. (2004) and extensively discussed by Bel Hassen et al. (2008, 2009).

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Fig. 2. Vertical profiles of temperature ( C), Sigma-t and salinity (P.S.U) according to a gradient of distance to the coast of May–June (left) and September (right) cruises. Distance on x axis is scaled in km from the starting point of the section: 10 E, 34 N, the ending point is located at the most distant station: 13.5 E, 35.2 N. The section width includes all the sampled stations.

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Temperature was tightly correlated with density in May–June (r ¼ 0.972, P < 0.001) and September (r ¼  0.982, P < 0.001), while salinity and density were less correlated in May–June (r ¼ 0.377, P < 0.01) and in September (r ¼  0.449, P < 0.01). Table 2 gives the mean nutrient concentrations and standard deviations and Fig 3 shows the pattern of nutrient distribution according to a coast to offshore gradient. The dissolved inorganic  þ nitrogen (DIN), representing the summation of NO 3 , NO2 and NH4 , was less concentrated in May–June than in September (not shown). Nitrate constituted the main DIN form in May–June mainly concentrated in the offshore area and with no vertical gradient, while concentrations decreased in September with the exception of a deep local patch situated at 70 m depth. Ammonium which explained the most DIN variations (r ¼ 0.973, P < 0.001) (Table 3), became more available in September, being mainly present in the coastal area. By contrast, phosphate concentrations were higher in the offshore area (average 0.43 mM) in the beginning of the water stratification (May–June); while were below 0.2 mM and were mainly distributed as small subsurface and deep local patches in September. Silicate concentrations occasionally exceeded 2 mM and were particularly present in the coastal area in May–June, whereas they represented deep patches in September. The relative proportion of available nutrients evidenced a likely silicate limitation; occurring more frequently in May–June than in September (Fig. 4). Phosphate depletion in September (Table 2), suggested that this nutrient was the most potentially limiting factor for phytoplankton growth. N:P ratios were lower in May–June than in September; however, they did not fall into the fixed criteria of potential nitrogen limitation (Fig. 4).

3.2. HPLC pigment composition Table 2 gives the mean concentrations and standard deviations of chlorophylls and carotenoids. Chlorophyll a concentrations showed the highest values during the beginning of the water stratification (1.1 mg l1), while they decreased during the wellestablished stratification (below 0.4 mg l1). The coast–offshore sections allowed identification of zones of chlorophyll maxima in both coastal and offshore areas (Fig. 5). In May–June local Deep

Table 2 The mean (plus the standard deviation) values for nutrient, chlorophyll a and major accessory pigments concentrations recorded during the two sampled periods. Chlorophyll a (Chl a), fucoxanthin (Fuco), peridinin (Per), zeaxanthin (Zea), alloxanthin (Allo), 190 -hexanoyloxyfucoxanthin (Hex), 190 -butanoyloxyfucoxanthin (But), chlorophyll b (Chl b), prasinoxanthin (Pras), lutein (Lut), neoxanthin (Neo), violaxanthin (Viol), chlorophyll c3 (Chl c3).

Si (mM) PO4 (mM) NO2 (mM) NO3 (mM) NH4 (mM) DIN (mM) Chl a (mg l1) Fuco (ng l1) Peri (ng l1) Zea (ng l1) Allo (ng l1) Chl b (ng l1) Hex (ng l1) But (ng l1) Pras (ng l1) Lut (ng l1) Neo (ng l1) Viol (ng l1) Chl c3 (ng l1)

May–June

September

0.90  1.34 0.43  0.26 0.22  0.19 1.42  0.45 0.40  0.31 2.06  0.59 0.29  0.69 6.99  19.93 6.97  19.93 11.54  12.55 4.98  4.16 76.37  307.15 7.73  16.65 3.40  7.95 6.06  9.50 2.71  7.33 5.82  10.46 7.83  15.22 12.73  21.84

0.68  0.35 0.10  0.07 0.17  0.08 0.94  0.31 2.00  1.20 3.05  1.28 0.10  0.13 23.48  82.82 42.69  88.84 46.22  41.90 10.34  10.69 109.4  278.40 58.77  236.77 28.55  64.11 20.36  62.68 4.63  11.03 70.33  193.70 9.77  14.24 26.13  48.84

Chlorophyll Maxima (DCM) were recorded at 15 m depth in the coastal area and between 30 m and 40 m depth in the offshore area, with euphotic depth deepening to more than 70 m in some locations. In September a DCM was recorded near the lower limit of the euphotic zone at 50 m in depth. Chlorophyll b was the most abundant accessory pigment during the two investigated periods (Table 2). In May–June, Chl b exhibited significant correlations respectively with lutein (r ¼ 0.40, P < 0.01) and prasinoxanthin (r ¼ 0.63, P < 0.01), whereas in September it correlated with prasinoxanthin (r ¼ 0.50, P < 0.01) and violaxanthin (r ¼ 0.61, P < 0.01). Lutein was mainly present in chlorophytes (Table 1) and the Chl b:Lut ratios were very high during both periods (Table 2), thus indicating that other Chl b-containing algae groups (prochlorophytes and/or prasinophytes) were generally present. With the exception of Chl a, all the other recorded pigments were much more concentrated in September than May–June (Table 2). The Chl b:Chl a ratio showed mean values about 0.22  0.34 in May–June and increased to 0.99  1.60 in the September cruise, Zea:Chl a shifted from 0.09  0.13 to 0.70  0.73 and Pras:Chl a from 0.03  0.04 to 0.36  0.98 over the same periods. 3.3. CHEMTAX analysis The final ratio matrices for May–June and September cruises are shown in Table 1. The relative contribution of pigment groups to Chl a is illustrated in Fig. 6. Chlorophytes, prochlorophytes and cryptophytes accounted for 65% of the total Chl a in May–June, with prochlorophytes explaining more the contribution to total chlorophyll at the DCM (Fig. 5); whereas chlorophytes and cyanobacteria contributed more to areas with minimum Chl a concentrations (Fig. 5). However, a sharp decrease in the relative contribution of chlorophytes, and to a lesser degree of prochlorophytes to Chl a occurred in September while that of cyanobacteria increased mainly at surface (16%) (Fig. 5). Dinoflagellates, pelagophytes and prymnesiophytes also increased in September representing 36% of total Chl a. Prasinophytes and cryptophytes showed similar relative contributions (exceeding 14% of Chl a) during both cruises, while diatoms contributed only 2.5% of Chl a biomass. 3.4. Nutrients and hydrographical control of phytoplankton classes The potential relationships between phytoplankton classes expressed in terms of Chl a concentrations, as determined by CHEMTAX analysis, the ambient nutrient concentrations and the hydrographical variables were tested by Pearson’s correlation coefficient (Table 3). The results showed that in May–June the DIN was negatively correlated with Chl a of five out of nine phytoplankton classes namely, cryptophytes, pelagophytes, chlorophytes, prasinophytes and prochlorophytes. Nitrate was negatively correlated with prasinophytes and prochlorophytes; whereas ammonium was positively correlated with prymnesiophytes and cyanobacteria. Phosphate showed positive correlation with dinoflagellates. In September, fewer correlations were found between phytoplankton classes, nutrient concentrations and hydrographical variables. Prasinophytes containing Chl a and pelagophytes were positively correlated with nitrites. The former group and diatoms positively correlated with salinity. 4. Discussion Dissolved inorganic nitrogen was generally high during both cruises, with nitrate exhibiting a range of concentrations similar to those reported in the western Mediterranean during either the spring-summer transition (L’Helguen et al., 2002) or in the stratified north-western Mediterranean (Lucea et al., 2003). Phosphate

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Nitrate 0 1.90 1.80

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Silicate 2.60 2.40 2.20 2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20

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Fig. 3. Vertical profiles of nitrate (mM), ammonium (mM), phosphate (mM) and silicate (mM) of May–June (left) and September (right) cruises. The section details as Fig. 2. The sign (þ) indicates the sampled depths.

concentrations were also high during the first cruise, being generally well above the values reported in the eastern Mediterranean Sea (Krom et al., 1991), but were similar to those recorded in the deep western Mediterranean water (Be´thoux et al., 1998; Lucea et al., 2003; Marty et al., 2008). We infer that phosphate terrestrial inputs into this area contributed to the increase in phosphate concentrations. Our assumption may be supported by the existence along the coast of the Gulf of Gabes of increased industrial and urban activities (Hamza-Chaffai et al., 1997). However, the highest phosphate

concentrations were mainly observed in the offshore area (Fig. 3). We suspect the AW as being probably the most supplier of phosphate to the open sea area. Our assumption may be supported, as noted earlier, by the good correlation between salinity minima and high phosphate concentrations (Table 3), which, was attributed to interference of the AW (Bel Hassen et al., 2009). AW may also induce increases of phosphate concentration indirectly via its disturbance of sediments with release upwards of nutrients, as suggested by the dominating feature of salinity in the offshore area (Fig. 2).

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Table 3 Pearson’s correlation coefficients of biological, chemical and hydrographical variables at all sampled stations for the two cruises. **P < 0.01,*P < 0.05. Phytoplankton biomasses are expressed in terms of chlorophyll a concentrations calculated by CHEMTAX and contributed by: diatoms (diat), dinoflagellates (dino), prymnesiophytes (prym), cryptophytes (cryp), pelagophytes (pelag), chlorophytes (chloro), cyanobacteria (cyano), prasinophytes (pras) and prochlorophytes (proc). diat May–June Si NO 2 NO 3 NHþ 4 PO4 DIN Sal. Temp. September Si NO 2 NO 3 NHþ 4 PO4 DIN Sal. 0.287** Temp.

dino

prym

cryp

pelag

chloro

cyano

pras

proc

Si

0.226*

NO 2

1.000

NO 3

NHþ 4

0.263*

0.245*

0.233*

0.260* 0.218* 0.214* 0.245*

0.322** 0.335**

1.000

1.000 0.322** 0.335** 1.000 0.511** 0.752** 0.418** 0.347** 0.469** 0.339** 0.232*

0.219* 0.294** 0.221*

0.359** 0.229*

1.000 0.222*

0.231*

1.000 1.000 0.417** 0.209*

0.317**

Concentrations in dissolved nutrients did not show clear vertical distribution trends, unlike the general feature described for the Mediterranean stratified water, with nutrients, declining at the water surface while accumulating in deep stratified waters (Estrada, 1985; Morel and Andre, 1991; Varela et al., 1994). This vertical distribution was reported as being the main factor inducing the DCM formation, the prominent feature of the Mediterranean during summer stratification (Estrada., 1985). However, during our survey, the DCM was absent in most of the sampled stations, especially during the well-established stratification of water column (Fig. 5). This discrepancy may be explained by the fact that we sampled continental shelf waters, excluding, therefore, the nutrient-rich deep waters (Minas and Minas, 1989). The shift toward strong stratified conditions was characterized by a general silicate deficiency as revealed by the sample deviations from the accepted standard molar ratios for marine diatoms between dissolved inorganic nitrogen, silicate and phosphate Si/N/P ¼ 16:16:1 (Redfield et al., 1963). In addition, the total N/P ratios far exceeded the Redfield ratio (N/P ratio >20), suggesting an overall phosphate depletion, which is consistent with reports of P limitation in the Mediterranean Sea (Jacques et al., 1973; Minas et al., 1988; Thingstad and Rassoulzadegan, 1995; Thingstad et al., 1998). The most striking finding of the present investigation was the low contribution of diatoms to Chl a concentrations (<5%), which, was also lower than that reported in the Gulf of Gabes either in previous summer (22%) (Bel Hassen et al., 2008) or during early spring mixed conditions (12%) (Bel Hassen et al., 2009). Our results were also much lower than those reported in the north-western Mediterranean Sea during not only in the stratified period (26%) (Bustillos-Guzman et al., 1995) but also during transition from spring bloom to oligotrophic conditions (30–20%) (Vidussi et al., 2000). However, recent observations in the NW Mediterranean Sea under late summer conditions reported similar relative contribution of diatoms to Chl a biomass (5%) (Marty et al., 2008). In our survey, the decline of diatoms may be explained by the low silicate concentrations falling below reported half-saturation constants for silicate incorporation by diatoms: Km ¼ 1–5 mM (Fisher et al., 1988). Thus, silicate was likely to be the limiting element for diatoms development during this study (Fig. 4). On the other hand, silicate concentrations recorded in this area over various periods of the year were generally higher than 2.5 mM (Bel Hassen unpublished data), thus higher than the concentrations observed during the

0.210* 0.257* 0.318**

DIN

Sal.

Temp.

0.233* 1.000

0.248*

PO4

0.417** 0.209* 1.000

0.377** 0.432** 0.971** 0.260*

1.000 0.252*

0.511** 0.469** 0.752** 0.418** 0.339** 0.347** 0.232* 1.000 1.000 0.437**

0.437** 1.000

0.210* 0.257* 0.318** 0.377** 0.260* 0.432** 0.971** 0.252* 1.000 0.220* 0.220* 1.000 0.511** 0.511** 1.000

present survey (Table 2). Besides, silica showed higher concentrations in the AW than in the Mediterranean water during strong AW advection (Bel Hassen et al., 2009), suggesting that this water mass was a likely supplier of silicate in the studied area. We infer that this silicate shortage is not a permanent feature as this nutrient became depleted due to its consumption early spring by proliferating diatoms. Chlorophyll a biomass was essentially dominated by picoplankton and nanoplankton, which contributed 90% and 87% of total Chl a in May–June and September, respectively. This situation is similar to that reported by Vidussi et al. (2001) in the eastern Mediterranean Basin and by Marty et al. (2008) in the northwestern Basin, both studies reported relative contributions of about 87% for these size groups. The contribution of prokaryotes (cyanobacteria and prochlorophytes) to total Chl a averaged 26% and 28% in May–June and September respectively. This lies within the range of values found in the southern Mediterranean region by Barlow et al. (1997), where prokaryotes represented between 32% and 47% of Chl a in surface water samples; while decreasing to 17% and 28% of Chl a in the deep chlorophyll maximum. These results contrasted with the phytoplankton community of the tropical Atlantic Ocean where prokaryotes prevailed over eukaryotes (e.g. Letelier et al., 1993; McManus and Dawson, 1994; Claustre and Marty, 1995). The prochlorophyte and cyanobacteria contributions to total Chl a showed opposing trends between the two investigated periods (Fig. 6). An opposing pattern was also noticed at seasonal scale by Durand et al. (2001) at the Bermuda Atlantic timeseries study (BATS) site and by Marty et al. (2008) at day scale. These authors did not present explicit causes to explain this pattern. Our data indicated absence of temperature and salinity dependences (Table 3), since influence of light and other physical factors were not investigated. Nevertheless, the results from a study on short time scale variability of picoplankton abundance (Jacquet et al., 2002) revealed that, despite a strong gradient of temperature, cell growth and division rate of Synecochoccus and Prochlorococcus had inverse behaviour and were tightly coupled to the daily light cycle. In September, the proportion of cyanobacteria (relative to prochlorophytes) increased, probably favoured by the high DIN concentrations found in water samples (Table 2). While our observations appear to confirm those of Partensky et al. (1999) and Tarran et al. (1999), they contrasted, however, with other findings

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Fig. 4. Scatter diagrams of atomic nutrient ratios of May–June (left) and September (right). Stoichiometric (¼potential) limitation is indicated by -N, -P and -Si.

demonstrating an increasing proportion of prochlorophytes in highly oligotrophic waters (Crossbie and Furnas, 2001; Higgins et al., 2006). These two taxonomic groups occupied different depth layers (Fig. 5), with cyanobacteria being abundant near the surface, while prochlorophytes tended to concentrate below the thermocline, mainly between 20 m and 40 m, in May–June and between 30 m and 70 m, in September. A similar vertical distribution was previously reported from the northern Mediterranean (BustillosGuzman et al., 1995; Marty et al., 2002) and in the North Pacific Ocean (Campbell et al., 1994), and was attributed for cyanobacteria to their ability to be more photosynthetically competent in high light intensities (Kana and Glibert, 1987) or to light limitation

imposed by their less efficient accessory pigments (Moore et al., 1995), and for prochlorophytes to their high vertical diversity (Garczarek et al., 2007). From May–June to September, chlorophytes showed a sharp decrease in their contribution to total Chl a (Fig. 6), probably affected by phosphate shortage during strong stratification. However, previous findings in aquatic ecosystems indicate that green algae have a competitive advantage for phosphate assimilation in phosphate-depleted waters (Sommer, 1985), being even capable of outcompeting cyanobacteria for this element in ecosystems with high N/P ratios (Sommer, 1989). In addition, a previous cruise conducted in July 2005 in the Gulf of Gabes demonstrated a dominance of

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Fig. 5. Concentrations of chlorophyll a (ml1) and relative contributions (% of chlorophyll a) of chlorophytes, prochlorophytes, cyanobacteria and prasinophytes as calculated by CHEMTAX during May–June (left) and September (right) cruises. Dashed line indicates the depth of the euphotic zone.

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chlorophytes (up to 47% of autotrophic biomass) in a low P availability (Bel Hassen et al., 2008), suggesting that this nutrient might not be the only factor constraining chlorophyte development under a strong stratification. We suspect predation as being probably among the most important factors contributing to decreases in chlorophyte abundance in this study. This is supported by two observations. First, we found that chlorophytes were mainly present in the coast stations (Fig. 5) and the results from the study of the ciliate community in the Gulf of Gabes in summer showed that ciliates concentrated also along the coast (Hannachi et al., 2009). Second, we recorded increased ammonium concentrations (Table 2), suggesting that chlorophyte assemblages were under predation pressure from ciliates. Indeed, members of chlorophytes, including nanoflagellates, have been found by several authors to be commonly ingested by ciliated protozoa (Dietrich and Arndt, 2000). On the other hand, copepods have been frequently observed in the coast samples of Gulf of Gabes, averaging 80% of the total zooplankton biomass, with Oithona nana as dominant taxon (Drira et al., in press). These metazoans have been documented in other aquatic ecosystems to efficiently ingest ciliated protozoa (Hartmann et al., 1993; Atkinson, 1996). Therefore, these observations suggest likely trophic pathways between chlorophytes, ciliates and copepods typical of a microbial food web structure (Legendre and Rassoulzadegan, 1995). In September, the relative contribution of pelagophytes, prymnesiophytes and dinoflagellates increased, probably in relation to transition to oligotrophy. The distribution of pelagophytes seemed under the control of nitrogen. Our suggestion may be supported by the negative correlation found in May–June between this group and DIN concentration (Table 3), the positive relation with nitrite in September and similar relationships found in other ecosystems (Claustre et al., 1994; Barlow et al., 1997; Marty et al., 2002). The increase of the relative contribution of dinoflagellates during transition to oligotrophy contrasted with the idea that these large cells are the main contributors to high Chl a standing stocks (Claustre et al., 1994), but was in agreement with the results of Vidussi et al. (2000) who clearly showed increases in dinoflagellate assemblages over transition to enhanced oligotrophic waters. We also found a positive correlation between dinoflagellates and phosphate in May–June but not in September as the water was P-depleted, probably suggesting a continuous consumption of phosphate by growing dinoflagellates. This pattern has also been

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described in coastal ponds located in the central part of the Gulf of Gabes (Abid et al., 2008). 5. Conclusions A prevalence of nanophytoplankton and picophytoplankton classes was observed which is in agreement with previous studies of the Mediterranean water. As protozoa are the main grazers of these size classes, consequently the microbial food web is likely to be the prevalent trophic pathway in the studied area. The Gulf of Gabes displayed a general oligotrophic character (e.g., low biomass) and a dominance of nanoplankton and picoplankton classes during the spring–summer stratification. However, it differed from other oligotrophic areas of the eastern Mediterranean considering the high dissolved inorganic nitrogen and phosphate concentrations. The origin of these differences, as well as the potential trophodynamics implication, remains to be evaluated. Acknowledgments This work was supported by the Tunisian funded project POEMM (LR02INSTM04), which was conducted in the National Institute of Marine Sciences and Technologies (INSTM) and Plankton and Microbiology of Aquatic Ecosystems Research Unit of the University of Sfax. The authors wish to thank the crew of the RV ‘Hannibal’ for their assistance. References Abid, O., Sellami-Kammoun, A., Ayadi, H., Drira, Z., Bouain, A., Aleya, L., 2008. Biochemical adaptation of phytoplankton to salinity and nutrient gradients in a coastal solar saltern, Tunisia. Estuarine, Coastal and Shelf Science 80, 391–400. Aleya, L., 1991. The concept of ecological succession applied to an eutrophic lake through the seasonal coupling of diversity index and several parameters. Archiv fu¨r Hydrobiologie 120, 327–343. Atkinson, A., 1996. Subantarctic copepods in an oceanic, low chlorophyll environment: ciliate predation, food selectivity and impact on prey population. Marine Ecology Progress Series 130, 85–96. Barlow, R.G., Mantoura, R.F.C., Cummings, D.G., Fileman, T.W., 1997. Pigment chemotaxonomic distributions of phytoplankton during summer in the western Mediterranean. Deep-Sea Research II 44, 833–850. Bel Hassen, M., Drira, Z., Hamza, A., Ayadi, H., Akrout, F., Issaoui, H., 2008. Summer phytoplankton pigments and community composition related to water mass properties in the Gulf of Gabes. Estuarine, Coastal and Shelf Science 77, 645–656. Bel Hassen, M., Drira, Z., Hamza, A., Ayadi, H., Akrout, F., Messaoudi, S., Issaoui, H., Aleya, L., Bouaı¨n, A., 2009. Phytoplankton dynamics related to water mass properties in the Gulf of Gabes: Ecological implications. Journal of Marine Systems 75, 216–226. Be´ranger, K., Mortier, L., Gasparini, G.P., Gervasio, L., Astraldi, M., Crepon, M., 2004. The dynamics of the Sicily Strait: a comprehensive study from observations and models. Deep-Sea Research II 51, 411–440. Be´thoux, J.P., Prieur, L., 1983. Hydrologie et circulation en Mediterrane´e Nordoccidentale. Petrole Techniques 299, 25–34. Be´thoux, J.P., Morin, P., Chaumery, C., Connan, O., Gentili, B., Ruiz-Pino, D., 1998. Nutrients in the Mediterranean Sea, mass balance and statistical analysis of concentrations with respect to environmental change. Marine Chemistry 63, 155–169. Brandhost, W., 1977. Les conditions de milieu au large de la coˆte Tunisienne. Bulletin de l’Institut National Scientifique et Technique d’Oce´anographie et de Peˆche de Salammboˆ 4, 129–220. Bustillos-Guzman, J., Claustre, H., Marty, J.C., 1995. Specific phytoplankton signatures and their relationship to hydrographic conditions in the coastal northwestern Mediterranean Sea. Marine Ecology Progress Series 124, 247–258. Campbell, L., Nolla, H.A., Vaulot, D., 1994. The importance of Prochlorococcus community structure in the central North Pacific Ocean. Limnology and Oceanography 39, 954–961. Chisholm, S.W., 1992. Phytoplankton size. In: Falkowsk, P.G., Woodhead, A.D. (Eds.), Primary Productivity and Biogeochemical Cycles in the Sea. Plenum Press, New York, pp. 213–237. Claustre, H., Kerherve, P., Marty, J.C., Prieur, L., Videau, C., Hecq, J.H., 1994. Phytoplankton dynamics associated with a geostrophic front: ecological and biogeochemical implications. Journal of Marine Research 52, 711–742. Claustre, H., Marty, J.C., 1995. Specific phytoplankton biomasses and their relation to primary production in the tropical North Atlantic. Deep-Sea Research I 42, 1475–1493.

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