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Lagoons are highly productive areas representing more than 50% of the coastline area in Languedoc-Roussillon (South of France, Mediterranean sea). These lagoons are very different in their environmental conditions, human influences, eutrophication levels and aquaculture intensity. Based on macrophyte communities associated with soft substrates, two indices of taxonomic diversity (the "average taxonomic distinctness" (Δ+) and the "variation in taxonomic distinctness" (Λ+)) were used to discriminate four of these lagoons (Thau, Salse-Leucate, Bages-Sigean and Mauguio). Bages-Sigean presented a significant higher average taxonomic distinctness (p < 0.05) and Salse-Leucate had a significant higher variation in taxonomic distinctness (p < 0.05) without considering exotic species. The index values were neither influenced by sample size nor by presence of exotic species. Thus, the "average taxonomic distinctness" was related to human activities and eutrophication level whereas the "variation in taxonomic distinctness" was more related the environmental variability, associated with the prime stressor of salinity in brackish coastal lagoons.

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A high-rate algae pond (HRAP) was tested as a second loop of water treatment in a recirculating fish rearing system to reduce water requirements and nutrient discharge levels. Three duplicated groups of sea bass (mean initial body weight 35±11 g) were reared under different system conditions (flow-through system, recirculating system and recirculating system with HRAP) for 1 year. Fish survival rate was higher in the system with HRAP, and their mean body weight was statistically higher (p<0.05) during the month of maximal climatic conditions for algae photosynthesis. After 266 days of experiment, mean fish weight was significantly higher (p<0.05) in the flow-through system than in the recirculating systems. Final fish weight was 15% higher in the flow-through system than in recirculating systems and was related to a higher daily-ingested food. The comparison of mean annual nutrient concentrations in the recirculating systems gives a statistically significant reduction of 25% of nitrogen (p<0.01) and 9% of phosphates (p<0.01) due to the HRAP. Absorption of nitrate form is responsible for nitrogen removal and is related to climatic conditions for algae growth. The phosphate precipitation at high pH (above 8) was not considered. The maximal removal rates were 0.5±0.2 g N m−2 day−1 and 0.03±0.02 g P m−2 day−1 for nitrates and phosphates, respectively, and were obtained during the optimal climatic conditions and the shortest algae harvesting frequency. These results are favourable to complete reuse of the HRAP treated water, all year long, in the recirculating rearing systems. In order to improve nitrate and phosphate removal rates, a periodic harvesting of algae is necessary and a higher water inflow in HRAP should partly make up for inorganic carbon depletion in high nitrate and phosphate effluents.

The decrease in water nutrients confirms oligotrophication in the period 1998-2016. ► The decrease in phytoplankton biomass was associated with a decrease in diatom abundance. ► The dominant taxa shifted from Skeletonema-Chaetoceros to Chaetoceros-Pseudo-nitzschia. ► The median proportion of dinoflagellates relative to diatoms increased significantly. ► The total dinoflagellate abundance did not change significantly.

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