Bioindicators of marine environment: using seaweeds as a tool for biomonitoring the quality of coastal waters. Response to a problem of seaweed beachings

To limit and prevent the degradation of the coastal ecosystems, it is necessary to develop tools for monitoring the quality of marine environment. The potential of seaweeds for coastal water quality bioindication was evaluated in the context of seaweed beachings in Grandcamp-Maisy and Courseulles-sur-Mer (Calvados). These summer season accumulations are heterogenous in quantity and quality (a mixture of Rhodophyta, Phaeophyta and Chlorophyta species) and are originated from large rocky shores highly colonized. However, the algal biodiversity of these rocky plateaus was relatively poor due to the low bathymetry and to the wide sandy beaches. The assessment of the state of algal communities with the REBENT indicator « intertidal macroalgae » revealed a “Medium” quality of coastal waters largely due to the specificity of Calvados shores. An evaluation of coastal water quality protocol more suitable named QUEROSA, was also developed. An in situ fine-scale survey of biological parameters (δ15N, nitrogen content, δ13C) of six seaweeds species was conducted on the two stranding sites in 2012 and 2013. In contrast to oceanic (Chausey archipelago) and highly anthropized (Seine estuary) reference points, a similar temporal dynamics of δ15N signatures has been highlighted at the two study sites with low δ15N in spring suggesting inputs of agricultural nitrogen and high δ15N in late summer linked to inputs of regenerated nitrogen from decomposition of seaweed beachings. The comparison of in situ δ15N of different seaweeds species coupled with an in vitro study have shown the relevance of using each algal species as bioindicator of nitrogen pollution.