Climatic and Anthropogenic Impacts on Environmental Conditions and Phytoplankton Community in the Gulf of Trieste (Northern Adriatic Sea)

During the last century, human activities have exerted an increasing pressure on coastalecosystems, primarily inducing their eutrophication, with a more recent partial mitigation of thisphenomenon where improvements of environmental management practices were adopted. However,a reanalysis of the pressures on coastal zones and surrounding drainage basins is needed becauseof the alterations induced nowadays by the climate changes. A comparative analysis of long-termoceanographic and environmental data series (1986-2018) was performed, in order to highlight theeffects of anthropogenic and climatic disturbances on the phytoplankton community in the Gulf ofTrieste (GoT). After the 1980s, the decline in phytoplankton abundance was matched to increasingperiods of low runoff, an overall deficit of the precipitation and to a decrease in phosphate availabilityin the coastal waters (-0.003 micro-mol L-1 yr-1), even in the presence of large riverine inputs of nitrogenand silicates. This trend of oligotrophication was reversed in the 2010s by the beginning of a new andunexpected phase of climatic instability, which also caused changes of the composition and seasonalcycle of the phytoplankton community. Beyond the management of nutrient loads, it was shownthat climatic drivers such as seawater warming, precipitation and wind regime affect both nutrientbalance and phytoplankton community in this coastal zone.