Abstract. Using a sampling grid of 67 stations, the influence of basin-wide and subbasrascale circulation features on phytoplankton community composition and primary and new productions-was investigated m the eastern Mediterranean during winter. Taxonomic pigments were used as size class markers of phototroph groups (picophytoplankton, nanophytoplankton and microphytoplankton). Primary production rates-were computed using a light photosynthesis model that makes use of the total chlorophyll a (Tchl a) concentration profile as an input variable. New production was estimated as the product of primary production by a pigment-based proxy of thef ratio (new production/total production).

Phytoplankton blooms are an important, widespread phenomenon in open oceans, coastal waters and freshwaters, supporting food webs and essential ecosystem services. Blooms are even more important in exploited coastal waters for maintaining high resource production. However, the environmental factors driving blooms in shallow productive coastal waters are still unclear, making it difficult to assess how environmental fluctuations influence bloom phenology and productivity. To gain insights into bloom phenology, Chl a fluorescence and meteorological and hydrological parameters were monitored at high-frequency (15 min) and nutrient concentrations and phytoplankton abundance and diversity, were monitored weekly in a typical Mediterranean shallow coastal system (Thau Lagoon). This study was carried out from winter to late spring in two successive years with different climatic conditions: 2014/2015 was typical, but the winter of 2015/2016 was the warmest on record. Rising water temperature was the main driver of phytoplankton blooms. However, blooms were sometimes correlated with winds and sometimes correlated with salinity, suggesting nutrients were supplied by water transport via winds, saltier seawater intake, rain and water flow events. This finding indicates the joint role of these factors in determining the success of phytoplankton blooms. Furthermore, interannual variability showed that winter water temperature was higher in 2016 than in 2015, resulting in lower phytoplankton biomass accumulation in the following spring. Moreover, the phytoplankton abundances and diversity also changed: cyanobacteria (< 1 μm), picoeukaryotes (< 1 μm) and nanoeukaryotes (3–6 μm) increased to the detriment of larger phytoplankton such as diatoms. Water temperature is a key factor affecting phytoplankton bloom dynamics in shallow productive coastal waters and could become crucial with future global warming by modifying bloom phenology and changing phytoplankton community structure, in turn affecting the entire food web and ecosystem services.

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The responses of the plankton food web to increases in temperature and ultraviolet B radiation (UVBR, were experimentally investigated at a coastal Mediterranean site during spring. Eight moored mesocosms were used to compare natural plankton food web responses (control mesocosms) with three treatments simulating expected future local temperature and UVBR increases, as follows: (1) 3uC increase in water temperature, (2) 20% increase in incident UVBR, and (3) simultaneous 3uC increase in water temperature and 20% increase in incident UVBR. The plankton food web was resistant to elevated UVBR, having only moderate effects on plankton abundances and structure. In contrast, warming induced significant shifts in the plankton food web structure and function. Specifically, the abundance of protozooplankton (ciliates and flagellates) increased and the development time of copepods from nauplii to adults decreased. In the warm mesocosms, the emergence of copepod adult stages midway through the experiment resulted in a decrease in ciliates and consequently in an increase in heterotrophic flagellates. One unexpected result was that warming reduced the abundance of heterotrophic bacteria midway through the experiment. These results indicate a trophic-cascade effect under warming. The increase in adult copepods diminishes ciliates and in turn favors heterotrophic flagellates that consume bacteria. Warming also induced an increase in net oxygen production, indicating an increase in net primary production.

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The effects of grazing pressure and inorganic nutrient availability on the direct carbon transfer from freshly produced phytoplankton exudates to heterotrophic bacteria biomass production were studied in Mediterranean coastal waters. The short-term incorporation of ¹³C (H¹³CO₃) in phytoplankton and bacterial lipid biomarkers was measured as well as the total bacterial carbon production (BP), viral lysis and the microbial community structure under three experimental conditions: (1) High inorganic Nutrient and High Grazing (HN + HG), (2) High inorganic Nutrient and Low Grazing (HN + LG) and (3) under natural in situ conditions with Low inorganic Nutrient and High Grazing (LN + HG) during spring. Under phytoplankton bloom conditions (HN + LG), the bacterial use of freshly produced phytoplankton exudates as a source of carbon, estimated from ¹³C enrichment of bacterial lipids, contributed more than half of the total bacterial production. However, under conditions of high grazing pressure on phytoplankton with or without the addition of inorganic nutrients (HN + HG and LN + HG), the ¹³C enrichment of bacterial lipids was low compared with the high total bacterial production. BP therefore seems to depend mainly on freshly produced phytoplankton exudates during the early phase of phytoplankton bloom period. However, BP seems mainly relying on recycled carbon from viral lysis and predators under high grazing pressure.

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