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Abstract: 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 30m 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 (0.2μM) and a decrease in silicate concentrations (<2μM). With the exception of chlorophyll a, pigment concentrations were higher in September than in May–June samplings. The pico- and 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. [Copyright &y& Elsevier]

Abstract: The spatial distribution of chlorophylls and carotenoids was recorded throughout the Gulf of Gabes (South Ionian Sea) in March 2007, and was related to patterns of the physical structure and the nutrient concentrations. Two distinct water masses were identified based on the temperature and salinity (T–S) analysis: a cool and less salty Modified Atlantic Water (MAW) and a saltier Mediterranean Mixed Water (MMW). There was no significant difference in the mean nitrogen and phosphate concentrations between MMW and MAW, although the silica values were significantly higher in MAW. The Integrated chlorophyll a mean value was about 4 mg m−2, with a maximum of 13 mg m−2 at MAW stations. Higher Chlorophyll a records in typical MAW stations were mainly due to chlorophytes, which contributed up to 58% of the pigments concentrations in the MAW and about 46% in the MMW. The contribution of chlorophytes to total Chlorophyll a was found to be relatively stable throughout the water column. The contribution of diatoms, which were twofold higher in the MMW than in the MAW, did not exceed 17% of chlorophyll a and was mainly due to subsurface maxima. The chlorophytes, pelagophytes, prymnesiophytes and cryptophytes all together accounted for more than 77% of total chlorophyll a in the MAW and about 67% in the MMW. There were statistically significant differences between MMW and MAW in the pigment contribution of cyanobacteria and pelagophytes. These two taxa accounted for 13% and 24% of chlorophyll a respectively in the MAW and MMW indicating that these differences concerned phytoplankton classes at relatively low contributions to total chlorophyll a. [Copyright &y& Elsevier]

The Gulf of Gabès in the Eastern Mediterranean Basin is one of the largest continental shelves and is a highly productive area. This is in contrast to the Eastern Mediterranean Basin as a whole which is oligotrophic to ultraoligotrophic. Over recent years, due to increased pressure from anthropological activities, there has been a decline in the biological resources in the Gulf of Gabès. This has led scientists to focus their research on the different trophic levels of the food chain and in particular on phytoplankton. Studies dedicated to phytoplankton in the Gulf of Gabès have previously relied on conventional global approaches. This work on the other hand, examines ultraphytoplankton (<10 µm) distribution at the single cell level in different water masses, during a summer cruise (2008). Three distinct water masses were identified: modified Mediterranean waters (MMW), modified Atlantic waters (MAW) and Ionian waters (IW). Despite the summer period, nutrients were abundant and non-limiting. Flow cytometry resolved five cell groups, Prochlorococcus , Synechococcus , picoeukaryotes, nanoeukaryotes and a group of unidentified cells. These cells were characterised by low fluorescence signals and scatter signals larger than that of cyanobacteria and picoeukaryotes and thus were labelled LFNano for low fluorescence nanoeukaryotes. They were found in very high abundances, whereas the other usual groups were relatively low in abundance despite the favourable growth conditions. LFNano found during a 2012 summer cruise in the same area, were also sorted by flow cytometry and observed by scanning electron microscopy. Choanoflagellates were identified, and these nanoflagellates appear to account for the low abundance in the usual ultraphytoplankton group due to their efficient feeding/grazing mechanism. However, the most abundant LFNano could not be identified. Results strongly suggest a top-down control of the ecosystem, consistent with the high productivity of the Gulf that implies a rapid transfer of matter and energy to higher trophic levels. [ABSTRACT FROM AUTHOR]