Trace metal contamination as a toxic and structuring factor impacting ultraphytoplankton communities in a multicontaminated Mediterranean coastal area

Toulon Bay is a Mediterranean coastal marine ecosystem facing important anthropogenic pressures resulting in strong and multiple chemical contamination gradients. In this study, we examined the spatial variations of both abundance and structure of ultraphytoplankton in relation with resources and potentially toxic trace metal concentrations in surface water in Toulon Bay. Sampling and on-field measurements were conducted in February 2014 and February 2015 throughout the entire bay. Strong Cd, Cu, Pb, and Zn contamination gradients in surface water were observed, with the highest levels being recorded in the most anthropized areas, while the open part of Toulon Bay presented levels close to those observed in the open sea. The abundance of Synechococcus-like cells was strongly negatively correlated to the concentrations of Cd, Cu, Pb, and Zn, whereas the abundance of picoeukaryote-like cells was positively correlated to these four trace metals. Trace metal contamination appeared to be one of the most significant contributors to ultraphytoplankton structure by statistically explaining up to 58.4% of its spatial variability. Incubation experiments were conducted to assess the specific impact of trace metal contamination on ultraphytoplankton of Toulon Bay. During around one month, ultraphytoplanktonic communities isolated from an uncontaminated site (representative from offshore communities) were exposed to an artificial cocktail of trace metals representative of the most anthropized part of Toulon Bay (in concentrations and proportions). A 90-99% decrease of total ultraphytoplankton abundance was observed within six days, followed by its recovery before the end of the experiment. This exposure to the trace metal cocktail additionally induced a shift from a Synechococcus-like dominated community, representative of the open sea, to a photosynthetic picoeukaryotes dominated one, mimicking the ultraphytoplanktonic community encountered in the most contaminated sites of the bay. Exposure to either natural waters representing the contaminated sites of the bay or elutriate of sediment resuspension did not promote a noticeable toxic effect on the total abundance of ultraphytoplankton, but affected its community structure, suggesting complex interplays.