Fifty years of ecological changes: Regime shifts and drivers in a coastal Mediterranean lagoon during oligotrophication.

Thau lagoon is a large Mediterranean coastal lagoons and it supports traditional shellfish farming activities. It has been subject to eutrophication leading to major anoxic events associated with massive mortalities of shellfish stocks. Since the 1970s, improvements have been made to wastewater treatment systems, which have gradually led to oligotrophication of the lagoon. The aim of our study was to determine how the decrease in nutrient inputs resulted in major ecological changes in Thau lagoon, by analysing five decades of time-series (1970-2018) of observations on pelagic and benthic autotrophic communities. We were able to identify two periods during the oligotrophication process. Period 1 (1970-1992) was considered a eutrophic period, characterised by the shift from seagrass dominance to dominance of red macroalgae. Period 2 (1993-2018), characterised by improved eutrophication status, was further divided into three: a transition phase (1993-2003) during which the water column continued to recover but the benthic community lagged behind in recovery and in partial resilience; a regime shift (2003-2006), after which the water column became oligotrophic and seagrass began to recover (2007-2018). Considering anoxia crises as indicators of ecosystem resilience and resistance, we used a generalised linear model to analyse meteorological and environmental data with the aim of identifying the triggers of summer anoxia over the study period. Among the meteorological variables studied, air temperature had the strongest positive effect, followed by the period and wind intensity (both negative effects) and by rainfall in July (positive effect). The risk of triggering anoxia was lower in period 2, evidence for the increasing resistance of the ecosystem to climatic stress throughout the oligotrophication process. At the ecosystem scale and in the long term perspective, the ecological gains related to oligotrophication are especially important in the context of climate change, with more frequent and severe heat waves predicted.

Dataset on marine ecosystem services supplied by coral reefs, sandy beaches and coastal lagoons in different eutrophication states.

This data article provides indicators of Ecosystem Service (ES) supply for coral reefs, sandy beaches and coastal lagoons in different ecological states regarding eutrophication. 14 ES are considered: food through fisheries; material; molecules; coastal protection; nutrient regulation; pathogen regulation; climate regulation; support of recreational and leisure activities; contribution to a pleasant landscape; contribution to culture and territorial identity; emblematic biodiversity; habitat; trophic networks; recruitment. For each ecosystem 3 to 4 eutrophication states are described. Indicators of ES supply are filled on the basis of a literature review supplemented with expert-knowledge. A semi-quantification of the indicator value is finally provided. Tendencies and trade-offs between ES are analyzed in How does eutrophication impact bundles of ecosystem services in multiple coastal habitats using state-and-transition models [1].

Spatial patterns in coastal lagoons related to the hydrodynamics of seawater intrusion.

Marine intrusion was simulated in a choked and in a restricted coastal lagoon by using a 3D-hydrodynamic model. To study the spatiotemporal progression of seawater intrusion and its mixing efficiency with lagoon waters we define Marine Mixed Volume (VMM) as a new hydrodynamic indicator. Spatial patterns in both lagoons were described by studying the time series and maps of VMM taking into account the meteorological conditions encountered during a water year. The patterns comprised well-mixed zones (WMZ) and physical barrier zones (PBZ) that act as hydrodynamic boundaries. The choked Bages-Sigean lagoon comprises four sub-basins: a PBZ at the inlet, and two WMZ's separated by another PBZ corresponding to a constriction zone. The volumes of the PBZ were 2.1 and 5.4 millions m3 with characteristic mixing timescale of 68 and 84days, respectively. The WMZ were 12.3 and 43.3 millions m3 with characteristics mixing timescale of 70 and 39days, respectively.