RESUMEN

The link between harmful algal blooms, phytoplankton community dynamics and global environmental change is not well understood. To tackle this challenging question, a new method was used to reveal how phytoplankton communities responded to environmental change with the occurrence of an harmful algae, using the coastal waters of the eastern English Channel as a case study. The great interannual variability in the magnitude and intensity of Phaeocystis spp. blooms, along with diatoms, compared to the ongoing gradual decrease in anthropogenic nutrient concentration and rebalancing of nutrient ratios; suggests that other factors, such as competition for resources, may also play an important role. A realized niche approach was used with the Outlying Mean Index analysis and the dynamics of the species' realized subniches were estimated using the Within Outlying Mean Indexes calculations under low (L) and high (H) contrasting Phaeocystis spp. abundance. The Within Outlying Mean Indexes allows the decomposition of the realized niche into realized subniches, found within the subset of habitat conditions and constrained by a subset of a biotic factor. The two contrasting scenarios were characterized by significantly different subsets of environmental conditions and diatom species (BV-step analysis), and different seasonality in salinity, turbidity, and nutrients. The subset L environmental conditions were potentially favorable for Phaeocystis spp. but it suffered from competitive exclusion by key diatom species such as Skeletonema spp., Thalassiosira gravida, Thalassionema nitzschioides and the Pseudo-nitzchia seriata complex. Accordingly, these diatoms species occupied 81% of Phaeocystis spp.'s existing fundamental subniche. In contrast, the greater number of diatoms, correlated with the community trend, within subset H exerted a weaker biological constraint and favored Phaeocystis spp. realized subniche expansion. In conclusion, the results strongly suggest that both abiotic and biotic interactions should be considered to understand Phaeocystis spp. blooms with greater consideration of the preceeding diatoms. HABs needs must therefore be studied as part of the total phytoplankton community.

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RESUMEN

Suspended marine benthic microalgae in the water column reflect the close relationship between the benthic and pelagic components of coastal ecosystems. In this study, a 12-year phytoplankton time-series was used to investigate the contribution of benthic microalgae to the pelagic system at a site along the French-Atlantic coast. Furthermore, all taxa identified were allocated into different growth forms in order to study their seasonal patterns. The highest contribution of benthic microalgae was observed during the winter period, reaching up to 60% of the carbon biomass in the water column. The haptobenthic growth form showed the highest contribution in terms of biomass, dominant in the fall-winter period when the turbidity and the river flow were high. The epipelic growth form did not follow any seasonal pattern. The epiphytic diatom Licmophora was most commonly found during summer. As benthic microalgae were found in the water column throughout the year, the temporal variation detected in the structure of pelagic assemblages in a macrotidal ecosystem was partly derived from the differentiated contribution of several benthic growth forms.

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King scallop contamination (Pecten maximus) by domoic acid, a neurotoxin produced by some species of the diatom Pseudo-nitzschia, is highly problematic because of its lengthy retention in the bivalve tissue, leading to prolonged fishery closures. Data collected within the French Phytoplankton and Phycotoxin monitoring network (REPHY) over the 1995-2012 period were used to characterize the seasonal dynamics and the interannual variability of P.-nitzschia spp. blooms as well as the contamination of king scallop fishing grounds, in six contrasted bays distributed along the French Atlantic coast and English Channel. Monitoring revealed that these toxic events have become more frequent since the year 2000, but with varying magnitudes, frequencies and timing depending on the bay. Two bays, located in southern Brittany, exhibited both recurrent contaminations and high P.-nitzschia abundances. The Brest bay and the Seine bay were intermittently affected. The Pertuis Breton exhibited only one major toxic event related to an exceptionally intense bloom of P.-nitzschia in 2010, and the Saint Brieuc bay neither showed significant contamination nor high P.-nitzschia abundance. While high P.-nitzschia abundance appeared to be correlated to scallop toxicity, this study highlights the difficulty in linking P.-nitzschia spp. blooms to king scallop contamination through monitoring. Indeed, P.-nitzschia was determined at the genus level and data regarding species abundances and their toxicity levels are an absolute prerequisite to further assess the environmental control of ASP events. As results describe distinct P.-nitzschia bloom dynamics along the French coast, this may suggest distinct controlling factors. They also revealed that major climatic events, such as the winter storm Xynthia in 2010, can trigger toxicity in P.-nitzschia over a large spatial scale and impact king scallop fisheries all along the coast.