engleski

The base of the food web and its key players: case study of the oligotrophic southern Adriatic Sea

Picophytoplankton has been reported as the most important primary producer in the oligotrophic southern Adriatic Sea, and processes of deep winter convection as the main drivers of the phytoplankton and subsequent the whole food web dynamics. In order to test this theory, an interdisciplinary research composed of two winter cruises was conducted along the coast-open sea transect, from surface to aphotic layers of water column. To recognize key players of the base of the food web, we combined physical-chemical and bio-optical measurements with molecular and conservative microscopical signatures plankton communities. Apparent oxygen utilization (AOU) had positive values, indicating respiration, mainly from heterotrophic bacteria, as a main process in the area. The photosynthetic component of picoplankton was dominated by cyanobacteria, while eukaryotic picoplankon was 95% hetero- or mixo- trophic and 5% photoautotrophic, supporting the respiration processes within the microbial food web. Our findings suggest that chlorophyll rich waters, encountered on larger depths have to originate from the shallower depths, probably transferring by the high convection processes present during the winter in the area. Due its short duration, such convective mixing may go undetected, but its consequences are highly visible through increased phytoplankton abundance and Chl a concentration in the surface, as well as in deeper layers of the water column. Our findings support the idea the importance of deep winter convection in the southern Adriatic Sea for primary production. However, this study suggests respiration as the main process for the food web dynamic in this oligotrophic marginal sea.

food web, picoplankton, apparent oxygen utilization, bio-optics, Adriatic Sea

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