Contaminated sediment resuspension induces shifts in phytoplankton structure and function in a eutrophic Mediterranean lagoon

The combined effects of contaminants and nutrients released from sediment into the water column during resuspension-mixing events were assessed on the phytoplankton composition and productivity in the anthropized lagoon of Bizerte (Tunisia, Mediterranean Sea). During a 4-day experiment, phytoplankton was exposed in in situ immersed microcosms to sediment elutriates (untreated: El and sterilized: S-El), prepared from a sediment resuspension simulation process, and its responses were evaluated and compared with controls. Elutriate addition resulted in an enrichment of the water with contaminants (mix of metals and PAHs) and nutrients (NO\hbox{$_{\mathrm{2}}^{\mathrm{-}}$}, NH\hbox{$_{\mathrm{4}}^{\mathrm{+}}$}, PO\hbox{$_{\mathrm{4}}^{\mathrm{3-}}$}, Si(OH)4). The resulting contaminant enrichment was similar among treatments, but the nutrient enrichment was slightly higher in the S-El treatment than in the El treatment. Our results show that both elutriates strongly stimulated phytoplankton growth. They also show that the S-El-induced shifts in the phytoplankton community composition slightly enhanced the species diversity, whereas the El did not seem to have any effect on the phytoplankton community composition. In addition, our results show that the net production was stimulated by both elutriate treatments, after 24 h and 48 h of exposure to S-El and El. During the first hours, both elutriates induced a prompt increase in respiration, resulting in a temporary (2–3 days) switch of the systems from net autotrophy (P:R > 1) to net heterotrophy (P:R