Plankton trends and community changes in a coastal sea (northern Adriatic): Bottom-up vs. top-down control in relation to environmental drivers

The shallow and land-locked northern Adriatic basin has undergone several changes at different trophic levels over the last decades, of which the most recent and basin-wide is the negative trend in concentrations of chlorophyll a over the last decade. To investigate it in detail, we performed a robust analysis of abiotic parameters and plankton community structure in the period 1989–2009 in the Gulf of Trieste. The gradual sequence of events, which encompassed hydrological and ecological systems, was identified as a regime shift in 2002/2003. Synchronised step changes in river runoff and surface salinity and decreasing concentrations of nitrate and silicate were followed by changes in chlorophyll a. The decline in phytoplankton biomass was due to the reduction of seasonal diatom blooms and the predominance of the smaller sized fraction. The changes also encompassed a higher trophic level showing significantly reduced zooplankton biomass after 2002. Our results show that despite the scientifically recognised impact of large-scale climate drivers on Adriatic plankton communities, the Gulf remains largely controlled, either through local climatic (precipitation, river runoff) or anthropogenic (eutrophication) forcing. Under the resource-replete and high biomass regime bottom-up control prevails, while in the new regime of reduced resources and biomass the switch from bottom-up to top-down control can occur seasonally.