Résumé: French Mediterranean coastal lagoons have been subject to huge inputs of urban nutrients for decades leading to the eutrophication of these vulnerable ecosystems. In response to new environmental regulations, some of the lagoons have recently been the subject of large-scale management actions targeting the waste water treatment systems located on their watersheds. While the eutrophication of coastal ecosystems is well described, recovery trajectories have only recently been studied. To assess the rapidity and the extent of the effect of the remediation actions, we analysed data from a 14-year time series resulting from the monitoring of nutrients, biomass and the abundance of phytoplankton in the water column of French Mediterranean coastal lagoons covering the whole anthropogenic eutrophication gradient. Following a 50% to 80% reduction in total phosphorus (TP) and total nitrogen (TN) urban loadings from the watershed of hypertrophic and eutrophic ecosystems, the integrative parameters chlorophyll a, TN and TP, provide evidence for a rapid response (1 to 3 years) and for an almost complete recovery, suggesting no hysteresis for the eutrophic lagoon. However, our findings also show that recovery patterns depend on the eutrophication status before remediation and may include feedback responses. The different responses revealed by our results should help stakeholders prioritise remediation actions and identify appropriate restoration goals, especially in light of the targets of the Water Framework Directive (WFD).

Résumé: Effects of sediment-released contaminants and nitrogen were assessed on phytoplankton communities sampled from Thau lagoon (France, Mediterranean Sea) and one close offshore marine station. Phytoplankton was exposed to sediment elutriate (seawater containing a mix of metals, organic chemicals, and nutrients) or to ammonium enrichment for four days using immersed microcosms exposed to natural conditions of light and temperature. Functional (production – respiration balance) and structural (taxonomy and cell densities) responses of the phytoplankton community were assessed. In the lagoon, both treatments stimulated phytoplankton growth, compare to controls. Conversely in the offshore station, the phytoplankton growth was stimulated only with the sediment elutriate addition. In offshore and lagoon stations, both treatments caused a shift in the taxonomic composition of the phytoplankton. Proliferation of potentially toxic diatoms and dinoflagellates resulted from the addition of elutriate. Correspondence analysis determined that phytoplankton from the offshore station was more sensitive to both treatments compared to the lagoon community. According to daily production and respiration balance, lagoon community metabolism remained heterotrophic (P < R) for all treatments, whereas only transient shifts to net autotrophy (P> R) were observed in the offshore community. Direct toxicity of contaminants released from sediment, if any, was therefore masked by nutrient enrichment effects, whereas indirect evidence of contaminant pressure was highlighted by changes in community composition and metabolism. (C) 2016 Elsevier B.V. All rights reserved.

Résumé: Microphytobenthos (MPB) is an important primary producer in coastal ecosystems. In oligotrophic environments, its activity may be controlled by the availability of organic or inorganic compounds but also by its migration behavior. The objective of this study was to determine, in MPB-colonized subtidal sediments, the consequences of short-term enrichments (< 24 h) of organic (alanine, glutamate, and glucose) and inorganic (ammonium, phosphate) compounds on MPB vertical migration and metabolisms, net production (NP), areal gross production (AGP), and community respiration (R). Two contrasting stations located in the southwest lagoon of New Caledonia were investigated: 1 under strong anthropogenic influence and 1 under more oceanic influence. Both stations were dominated by epipelic diatoms. Differences in net primary production were explained by diurnal variation of MPB biomass at the sediment surface, showing the importance of MPB migration in the functioning of these subtidal environments. However, a stimulation or inhibition of MPB migration did not necessarily impact the net primary production of the system; this strongly depends upon the interactions between the autotrophic and heterotrophic compartments, the latter being controlled by the environmental conditions. For the station under low anthropogenic influence, AGP and R were both significantly stimulated by alanine, glucose, and ammonium, and significantly inhibited by phosphate. The similar responses of AGP and R to enrichments suggest that autotrophs and heterotrophs were tightly coupled. Conversely, in the station under strong anthropogenic influence, AGP and R responded differently. Addition of ammonium inhibited AGP without having an impact on R, whereas addition of phosphate inhibited R whilst having no measurable effect on AGP. In this station, the coupling between autotrophs and heterotrophs was weakened, suggesting that the carbon demand of the heterotrophic compartment is probably sustained by the supplies of allochthonous organic matter rather than by exudates from the autotrophic compartment.