Dynamics and temporal evolution of toxic algal blooms from coastal Brittany sediments: First results in the Gulf of Morbihan and prospects
Résumé
Harmful algal blooms occur when toxin-producing algae grow excessively in coastal waters. In France, surveys are done by the Ifremer since 1987 through two networks, REPHY and REPHYTOX. Temporal series of toxic algae population are necessary to better assess the dynamics of change of these recurring seasonal phenomena. Unfortunately, only about 40 years of data are available which is not enough for an accurate understanding and prediction of the toxic blooms dynamics. This study aims at increasing the temporal series length by studying sediments deposited over the past decades, prior to 1987, with a focus on dinocyst assemblages through time and especially toxic cyst species, in different northern (e.g. Rance estuary, Gouessant estuary) and southern (e.g. Gulf of Morbihan, Penerf river, Bay of Vilaine) Brittany coasts which are currently experiencing toxic blooms. Before studying sediment cores (i.e., temporal dynamics), distribution maps of dinocysts will be established in the studied bays and estuaries. Here, we show the first results acquired at the scale of the Gulf of Morbihan (using 35 surface samples). The Gulf is a macrotidal estuary subdivided by two bays: an occidental one characterised by coarser grains and a direct connection to the ocean, protected by the swell thanks to few islands with North 90/North-West orientation; an oriental one dominated by finer grains, fluvial inputs and a weaker tides effect (mesotidal). Those results give us a better understanding of cyst content and conservation in nowadays Gulf’s sediments. Those new insights will be used to study the planned sediment core.
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