. 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

Plankton primary production of the World Ocean.

Several Dinophysis species produce diarrhoetic toxins (okadaic acid and dinophysistoxins) and pectenotoxins, and cause gastointestinal illness, Diarrhetic Shellfish Poisoning (DSP), even at low cell densities (<103 cells·L−1). They are the main threat, in terms of days of harvesting bans, to aquaculture in Northern Japan, Chile, and Europe. Toxicity and toxin profiles are very variable, more between strains than species. The distribution of DSP events mirrors that of shellfish production areas that have implemented toxin regulations, otherwise misinterpreted as bacterial or viral contamination. Field observations and laboratory experiments have shown that most of the toxins produced by Dinophysis are released into the medium, raising questions about the ecological role of extracelular toxins and their potential uptake by shellfish. Shellfish contamination results from a complex balance between food selection, adsorption, species-specific enzymatic transformations, and allometric processes. Highest risk areas are those combining Dinophysis strains with high cell content of okadaates, aquaculture with predominance of mytilids (good accumulators of toxins), and consumers who frequently include mussels in their diet. Regions including pectenotoxins in their regulated phycotoxins will suffer from much longer harvesting bans and from disloyal competition with production areas where these toxins have been deregulated.

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Dinophysis Toxins: Causative Organisms, Distribution and Fate in Shellfish

The Mediterranean Sea has been strongly influenced by human activities for millennia. Although the environmental history of its surrounding terrestrial ecosystems has received considerable study, historical changes in its marine realm are less known. We used a multidisciplinary approach combining paleontological, archeological, historical, fisheries, and ecological data to reconstruct past changes in marine populations, habitats, and water quality in the Adriatic Sea. Then, we constructed binary food webs for different historical periods to analyze possible changes in food-web structure and functioning over time. Our results indicate that human activities have influenced marine resource abundance since at least Roman times and accelerated in the nineteenth and twentieth centuries. Today, 98% of traditional marine resources are depleted to less than 50% of former abundance, with large (>1 m) predators and consumers being most affected. With 37% of investigated species rare and 11% extirpated, diversity has shifted towards smaller, lower trophic-level species, further aggravated by more than 40 species invasions. Species providing habitat and filter functions have been reduced by 75%, contributing to the degradation of water quality and increased eutrophication. Increased exploitation and functional extinctions have altered and simplified food-web structure over time, especially by changing the proportions of top predators, intermediate consumers, and basal species. Moreover, simulations of species losses indicate that today’s ecosystems may be less robust to species extinctions than in the past. Our results illustrate the long-term and far-reaching consequences human activities can have on marine food webs and ecosystems.

Historical Changes in Marine Resources, Food-web Structure and Ecosystem Functioning in the Adriatic Sea, Mediterranean

An ecological model of the Northern and Central Adriatic Sea: Analysis of ecosystem structure and fishing impacts

Long-term changes of basic biological and chemical parameters at two stations in the middle Adriatic

Seasonal variations in abundance and carbon biomass of ciliated protozoa and micrometazoa were studied from May 1998 to November 1999 in the eutrophicated area of Kastela Bay (Middle Adriatic Sea). Ciliates showed peaks in spring and autumn, primarily due to changes in the abundance and biomass of tintinnines, which participated in total ciliate abundance and biomass with 40.48 and 60.02%, respectively. The highest tintinnine density was 4,278 ind. l−1, while their average biomass varied from 0.611 to 26.557 μgC l−1 . Maximal average density and biomass of non-loricates were 1,430 ind. l−1 and 3.925 μgC l−1, respectively. The micrometazoa community was dominated by copepod nauplii, especially during the summer and autumn. The copepod biomass ranged between 3.47 and 26.75 μgC l−1 . High abundance and biomass values of the investigated zooplankton groups point to an important role of these organisms in the secondary production in the Bay, indicating that they may be (1) a crucial factor in controlling the populations of nano-/pico-phytoplankton and heterotrophic nanoflagellates, and (2) a significant prey for larger micrometazoans.

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Temporal variability in abundance and biomass of ciliates and copepods in the eutrophicated part of Kaštela Bay (Middle Adriatic Sea)

The first data substantiating the occurrence of diarrhetic shellfish poisoning (DSP) in the central region of the Adriatic Sea (Kastela Bay) in the summer of 1995 is presented. Toxicity was investigated using bio-assay and HPLC-FD analysis. Analysis of samples displaying toxicity, in addition to small concentrations of okadaic acid (OA), suggested the presence of an unknown derivatized compound at high concentration. A long term survey of the phytoplankton community has been conducted. The origin of mussel toxicity was traced to Dinophysis sacculus, a suspected toxic species.

A survey of shellfish toxicity in the central Adriatic Sea

Deep or subsurface chlorophyll- a maximum (DCM) was studied at one station in the middle Adriatic from December 1996 to June 1998. Chlorophyll- a concentration, abundance, volume carbon concentration, size-fraction of phytoplankton and phytoplankton community structure were determined. In addition, physical and chemical factors as well as nutrients were determined. The DCM occurs during both the vertical mixing and stratification period in the middle Adriatic Sea. It is most frequent between 50 and 75 m. It is located below the pycnocline and it is associated with the nutricline. Phytoplankton size-fraction and community structure vary seasonally. The DCM is most pronounced during spring phytoplankton blooms with diatom dominance. Procaryotic picoplankton Synechococcus sp. was abundant in DCM during summer stratification. The DCM represents both a biomass maximum and a phytoplankton adaptation to low irradiance.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0025315402005131

Deep chlorophyll- a maximum at one station in the middle Adriatic Sea

Seasonal and vertical distribution of tintinnids, non-loricate ciliates and micrometazoa were studied in Kastela Bay (central Adriatic Sea) throughout 1995. The species composition of tintinnids and copepods were studied as well. This is the first estimation of non-loricate ciliate biomass in the coastal area of the central Adriatic. Non-loricate ciliates were quantitatively the best represented ciliated protozoa, whereas nauplii were the most numerous micrometazoan organisms. Temperature affected the distribution of most micrometazoan components of the zooplankton and that of non-loricate ciliates. The temperature-dependent presence of individual size categories of non-loricate ciliates was also established. Apart from the interaction between microzooplankton groups, the influence of biotic factors, such as phytoplankton, bacteria, non-pigmented nanoflagellates (NNF) and mesozooplankton, was also discussed. The abundance of ciliates was controlled by both food supply (phytoplankton and NNF) and micrometazoan grazing. The results point to very complex trophic relationships within the planktonic community, suggesting that microzooplankton could be an important link between the microbial food web and higher trophic levels.

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Živana Ninčević

Papers

Long-term changes of basic biological and chemical parameters at two stations in the middle Adriatic

Temporal variability in abundance and biomass of ciliates and copepods in the eutrophicated part of Kaštela Bay (Middle Adriatic Sea)

A survey of shellfish toxicity in the central Adriatic Sea

Deep chlorophyll- a maximum at one station in the middle Adriatic Sea

Seasonal and vertical distribution of the ciliated protozoa and micrometazoa in Kastela Bay (central Adriatic)