TROPHIC STRUCTURE AND MICROBIAL ACTIVITY IN A SPAWNING AREA OF ENGRAULIS ENCRASICOLUS

The abundance, biomass and size-structure of planktonic populations, and the microbial metabolic processes were studied in the Sicily Channel, one of the most important spawning areas in the Mediterranean for anchovy (Engraulis encrasicolus), a pelagic species of commercial interest. Results showed that prokaryotes contribute for the 83% of total carbon biomass. Microphytoplankton abundances and biomasses were dominated by autotrophic nanoflagellates and dinoflagellates (36 identified species) and contribute 11% of total biomass. The microzooplanktonic biomass showed its maximum at the surface or subsurface and its contribution was low (4%). The study highlights the prevalence of pico-sized fractions within the whole phytoplankton biomass expressed as chlorophyll content, suggesting the importance of picophytoplankton in sustaining the microbial food web in an oligotrophic area. Autotrophic production exceeded oxidation by respiration; at the same time, prokaryotic activities and eggs distribution co-varied with temperature, even if the microbial activities were quite depressed, indicating that ecosystem functioning was active at low trophic levels and confirming the general oligotrophy of the Sicily Channel. The results obtained confirmed that the area acted as a nursery for small fish and both autotrophic and heterotrophic processes supported by microorganisms were in synergy.