Environmental assessment of the impact of cage fish farming on water quality and phytoplankton status in Monastir Bay (eastern coast of Tunisia)

Although aquaculture development is experiencing a rapid growth on the southern Mediterranean shore, studies of its environmental impact are still limited. This study, conducted from January to December 2014, in and around a cage fish farm located in Monastir Bay (eastern coast of Tunisia), investigated the hypothesis that cage fish farming would affect the water column quality and plankton communities. Phytoplankton diversity in addition to 13 physico-chemical parameters and chlorophyll-a were all investigated at 11 experimental stations located inside and around the farm. A total of 106 phytoplankton taxa were identified at genus and/or species level, and assigned to two major (dinoflagellates and diatoms) and four minor (cyanobacteria, euglenophyceae, coccolithophorides, and dictyophyceae) groups, with a remarkable predominance of heterotrophic and mixotrophic species, in particular among dinoflagellates taxa, highlighting a top-down model of phytoplankton regulation. Across seasons, a high Shannon-Wiener diversity index (H') was recorded. Further, we have demonstrated that nutrients were the most important factors contributing to differences in species abundance, as revealed by N/P and Si/Pi ratios that were factors most affecting phytoplankton distribution. Our findings revealed that fish farming aquaculture became an additional source of nutrients in the bay. In some stations, nutrient loading has little or no trophic impact, while, at others, there is evidence that nutrients are assimilated by primary producers. Our study provides a rich information platform for conservation strategies of the bay of Monastir.