Analysis of phytoplankton community in the vicinity of domestic sewage outflow during stratified conditions

The response of phytoplankton community structure to anthropogenically-derived nutrient from the submarine sewage outflow was analysed in a shallow coastal area of the middle Adriatic Sea. The study was conducted during the eight-year period (2002-2010) at six stations in the Brač Channel, aiming to assess the potential change in the state of biological and environmental parameters before and after the sewage commissioning. The research was particularly focused on phytoplankton producers during the summer which is considered to be critical due to water stratification and low water dynamics. The results show an increase in phytoplankton biomass in the deep layer after the discharge activation that was significant (M-W test; U=584.5, p<0.01), but not related to any intense phytoplankton bloom or oxygen deficiency. Temporal variability of total phytoplankton abundances throughout the research period indicated a significant increase in 2005, the year following the sewage outflow activation. Pairwise Dunn's multiple comparison test emphasised the difference in the years before the sewage activation (2002, 2003) compared to 2005 (2002 vs. 2005, R=-60.92 p<0.001; 2003 vs. 2005, R=-63.92 p<0.001). This situation was soon followed by the rapid equalization in phytoplankton abundances. Considering the entire research period, there were no changes in contributions of major phytoplankton groups, but the increase in overall species diversity was recorded, supporting the view that moderate inputs may stimulate plankton taxonomic diversity in coastal ecosystems. Increased ciliate and copepod nauplii abundance was also observed but was considered to be caused by natural variability