Mediterranean costal lagoons have been exposed to anthropic eutrophication for decades. Thau lagoon is one of the largest among them and provides many ecosystem services including shellfish farming. Considerable efforts were made between the 1970s and the late 2000s to improve the wastewater treatment systems in the lagoon watershed. A decrease in nutrient inputs to Thau lagoon was subsequently observed, evidence for the start of a recovery process in Thau lagoon, following oligotrophication. This decrease has significant consequences for ecosystem communities, above all primary producers. In our study, we characterised and quantified long-term changes by analysing long monitoring time-series (1998–2016) of phytoplankton biomass, abundance and composition of the taxa, nutrients and climate conditions, using univariate and multivariate statistics. Our results revealed that two decades of mitigation actions in the Thau lagoon watershed have led to a significant progressive reduction in nutrient concentrations and in phytoplankton biomass (−60%), associated with a decrease in diatom abundance (−66%), particularly affecting Skeletonema spp. The changes were associated with a community shift characterised by a shift in phytoplankton taxonomic dominance from Skeletonema-Chaetoceros to Chaetoceros-Pseudo-nitzschia. The median proportion of dinoflagellates relative to diatoms increased, although the total dinoflagellate abundance did not change significantly.

We hypothesise that the shift in phytoplankton communities is the result of the mixed effects of the reduction in nutrient inputs and of climatic-related variables. Finally, we present a conceptual scheme of the main drivers of phytoplankton community structure to clarify the ecosystem functioning of a Mediterranean lagoon used for shellfish farming, under oligotrophication and climate change.

地中海沿岸泻湖已经遭受了人类富营养化的几十年。Thau泻湖是其中最大的泻湖之一，提供包括贝类养殖在内的许多生态系统服务。在1970年代至2000年代后期之间，为改善泻湖流域的废水处理系统做出了相当大的努力。随后观察到向Thau泻湖的营养物输入量减少，这是富营养化后开始在Thau泻湖中恢复过程的证据。这种减少对生态系统社区，尤其是所有初级生产者，产生了重大影响。在我们的研究中，我们使用单变量和多变量统计数据，通过分析浮游植物生物量的长期监测时间序列（1998-2016年），分类单元的丰度和组成，养分和气候条件，来表征和量化长期变化。骨骼肌属 这些变化与群落转移有关，其特征是浮游植物生物分类优势从骨骼藻-嗜油杆菌转变为Chaetoceros-假拟南芥。尽管总鞭毛丰度没有显着变化，但鞭毛藻相对于硅藻的中位数比例有所增加。

我们假设浮游植物群落的变化是营养物输入减少和气候相关变量混合影响的结果。最后，我们提出了浮游植物群落结构主要驱动力的概念性方案，以阐明在富营养化和气候变化下，用于贝类养殖的地中海泻湖的生态系统功能。