. We present an overview of the plankton studies conducted during the last 25 years in the epipelagic offshore waters of the Mediterranean Sea. This quasi-enclosed sea is characterized by a rich and complex physical dynamics with distinctive traits, especially in regard to the thermohaline circulation. Recent investigations have basically confirmed the long-recognised oligotrophic nature of this sea, which increases along both the west-east and the north-south directions. Nutrient availability is low, especially for phosphorous (N:P up to 60), though this limitation may be buffered by inputs from highly populated coasts and from the atmosphere. Phytoplankton biomass, as chl a, generally displays low values (less than 0.2 μg chl a l−1) over large areas, with a modest late winter increase. A large bloom (up to 3 μg l−1) is observed throughout the late winter and spring exclusively in the NW area. Relatively high biomass values are recorded in fronts and cyclonic gyres. A deep chlorophyll maximum is a permanent feature for the whole basin, except during the late winter mixing. It is found at increasingly greater depths ranging from 30 m in the Alboran Sea to 120 m in the easternmost Levantine basin. Primary production reveals a west-east decreasing trend and ranges between 59 and 150 g C m−2 y−1 (in situ measurements). Overall, the basin is largely dominated by small autotrophs, microheterotrophs and egg-carrying copepod species. The microorganisms (phytoplankton, viruses, bacteria, flagellates and ciliates) and zooplankton components reveal a considerable diversity and variability over spatial and temporal scales, although the latter is poorly studied. Examples are the wide diversity of dinoflagellates and coccolithophores, the multifarious role of diatoms or picoeukaryotes, and the distinct seasonal or spatial patterns of the species-rich copepod genera or families which dominate the basin. Major dissimilarities between western and eastern basins have been highlighted in species composition of phytoplankton and mesozooplankton, but also in the heterotrophic microbial components and in their relationships. Superimposed to these longitudinal differences, a pronounced biological heterogeneity is also observed in areas hosting deep convection, fronts, cyclonic and anti-cyclonic gyres or eddies. In such areas, the intermittent nutrient enrichment promotes a switching between a small-sized microbial community and diatom-dominated populations. A classical food web readily substitutes the microbial food web in these cases. These switches, likely occurring within a continuum of trophic pathways, may greatly increase the flux towards higher trophic levels, in spite of the apparent heterotrophy. Basically, the microbial system seems to be both bottom-up and top-down controlled. A "multivorous web" is shown by the great variety of feeding modes and preferences and by the significant and simultaneous grazing impact on phytoplankton and ciliates by mesozooplankton.

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Plankton in the open Mediterranean Sea: a review

Why is the Eastern Mediterranean phosphorus limited

. Analysis of 20-year time-series of the vertically averaged salinity and nutrient data in the Southern Adriatic shows that the two parameters are subject to strong decadal variability. In addition, it is documented that nutrient and salinity variations are out of phase. Nutrients in the Ionian and in the Adriatic vary in parallel except that generally the nutrient content in the Adriatic is lower than in the Ionian, a fact that has been attributed to primary producer consumption following the winter convective mixing. As shown earlier, North Ionian Gyre (NIG) changes its circulation sense on a decadal scale due to the Bimodal Oscillating System, i.e. the feedback mechanism between the Adriatic and Ionian. Cyclonic circulation causes a downwelling of the nitracline along the borders of the NIG and a decrease in the nutrient content of the water flowing into the Adriatic across the Otranto Strait, and vice versa. In addition, the highly oligotrophic central area of the Ionian shows annual blooms only during cyclonic NIG circulation. Inversion of the sense of the NIG results in the advection of Modified Atlantic Water or of the Levantine/Eastern Mediterranean waters in the Adriatic. Here, we show that the presence of allochtonous organisms from Atlantic/Western Mediterranean and Eastern Mediterranean/temperate zone in the Adriatic are concurrent with the anticyclonic and cyclonic circulations of the NIG, respectively. On the basis of the results presented, a revision of the theory of Adriatic ingressions formulated in the early 1950s is proposed.

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On the impact of the Bimodal Oscillating System (BiOS) on the biogeochemistry and biology of the Adriatic and Ionian Seas (Eastern Mediterranean)

. This study presents a model of chlorophyll and primary production in the pelagic Mediterranean Sea. A 3-D-biogeochemical model (OPATM-BFM) was adopted to explore specific system characteristics and quantify dynamics of key biogeochemical variables over a 6 yr period, from 1999 to 2004. We show that, on a basin scale, the Mediterranean Sea is characterised by a high degree of spatial and temporal variability in terms of primary production and chlorophyll concentrations. On a spatial scale, important horizontal and vertical gradients have been observed. According to the simulations over a 6 yr period, the developed model correctly simulated the climatological features of deep chlorophyll maxima and chlorophyll west-east gradients, as well as the seasonal variability in the main offshore regions that were studied. The integrated net primary production highlights north-south gradients that differ from surface net primary production gradients and illustrates the importance of resolving spatial and temporal variations to calculate basin-wide budgets and their variability. According to the model, the western Mediterranean, in particular the Alboran Sea, can be considered mesotrophic, whereas the eastern Mediterranean is oligotrophic. During summer stratified period, notable differences between surface net primary production variability and the corresponding vertically integrated production rates have been identified, suggesting that care must be taken when inferring productivity in such systems from satellite observations alone. Finally, specific simulations that were designed to explore the role of external fluxes and light penetration were performed. The subsequent results show that the effects of atmospheric and terrestrial nutrient loads on the total integrated net primary production account for less than 5 % of the its annual value, whereas an increase of 30 % in the light extinction factor impacts primary production by approximately 10 %.

/pdf/seasonal-and-inter-annual-variability-of-plankton-l1oceo93en.pdf

Seasonal and inter-annual variability of plankton chlorophyll and primary production in the Mediterranean Sea: a modelling approach

Cold-water corals (CWC) are frequently reported from deep sites with locally accelerated currents that enhance seabed food particle supply. Moreover, zooplankton likely account for ecologically important prey items, but their contribution to CWC diet remains unquantified. We investigated the benthic food web structure of the recently discovered Santa Maria di Leuca (SML) CWC province (300 to 1100 m depth) located in the oligotrophic northern Ionian Sea. We analyzed stable isotopes (δ13C and δ15N) of the main consumers (including ubiquitous CWC species) exhibiting different feeding strategies, zooplankton, suspended particulate organic matter (POM) and sedimented organic matter (SOM). Zooplankton and POM were collected 3 m above the coral colonies in order to assess their relative contributions to CWC diet. The δ15N of the scleractinians Desmophyllum dianthus, Madrepora oculata and Lophelia pertusa (8 to 9‰) and the gorgonian Paramuricea cf. macrospina (9 to 10‰) were consistent with a diet mainly composed of zooplankton (6 to 7‰). The antipatharian Leiopathes glaberrima was more 15N-depleted (7 to 8‰) than other cnidarians, suggesting a lower contribution of zooplankton to its diet. Our δ13C data clearly indicate that the benthic food web of SML is exclusively fuelled by carbon of phytoplanktonic origin. Nevertheless, consumers feeding at the water–sediment interface were more 13C-enriched than consumers feeding above the bottom (i.e. living corals and their epifauna). This pattern suggests that carbon is assimilated via 2 trophic pathways: relatively fresh phytoplanktonic production for 13C-depleted consumers and more decayed organic matter for 13C-enriched consumers. When the δ13C values of consumers were corrected for the influence of lipids (which are significantly 13C-depleted relative to other tissue components), our conclusions remained unchanged, except in the case of L. glaberrima which could assimilate a mixture of zooplankton and resuspended decayed organic matter.

/pdf/trophic-relationships-in-a-deep-mediterranean-cold-water-1vavd4mktl.pdf

V Balboni

Papers

Particulate matter in the southern Adriatic and Ionian Sea: characterisation and downward fluxes