Distribution, frequency and intensity of key phytoplanktonic harmful species are impacted by environmental conditions. These later are expected to change. Recently in the Brest bay, Alexandrium minutum was responsible for several harmful algal blooms (HABs) associated with toxin production causing paralytic shellfish poisoning (PSP). Beside, Lepidodinium chlorophorum is a dinoflagellate which locally causes green waters and hypoxia along the Bay of Biscay. Previous studies revealed that L. chlorophorum success was related to the failure of Karenia mikimotoi, by competitive exclusion. It follows that phytoplankton composition has to be taken into account as environmental conditions. Herein, the study aims to assess the combined effect of changes in habitat conditions and community structure on the occurrence of HAB species, on a spatial-temporal scale. For the investigation we use the revisited Hutchinson’s niche concept, the subniche theory to understand: 1) how do the harmful species and the community respond to environmental variation? 2) How do the biotic interaction constraint the harmful species subniche? And 3) what species can be found to be potential competitors or indicator for HAB? The combined dataset from the French National Phytoplankton and Phycotoxin Monitoring Network (REPHY), the Daoulex project and satellite imagery was used. A total of 44 stations, from Dunkerque down to the Bay of Arcachon, over the period of 1998 to 2017 and with 50 taxonomic unit. The Outlying Mean Index (OMI) analysis was used to observe the species community distribution along the environmental gradients. The Within Outlying Mean indexes (WitOMI) were calculated under environmental conditions promoting high (H) and low (L) abundance bloom. In addition, the Indicating Species Analysis (ISA) was performed to expose species assemblages characterizing the phytoplankton community in each habitat condition.
36 taxa had significant niche and were mostly distributed along nutrient and salinity gradients. The two species of interest Lepidodinium chlorophorum (Lepi) and Alexandrium minutum (Alex) seemed to have similar affinity for summer-like environmental condition. Alex has a larger niche due to greater affinity to the estuarine-like conditions. The subniche of the two species had a similar response to the environmental variation; they both had a greater environmental restrain when they occurred in high abundance. The difference in environmental condition between H and L appeared to be link local hydrodynamic which increases or lift the limitation of potential resources. On the other hand, the biological constraint exerted the subniche of the harmful were greater for Alex in H condition and higher for Lepi in L condition. The species assemblages resulting from the ISA analysis confirm the previous findings by showing a greater number of indicating taxa under H condition for Alex (3 against 1 for H and L respectively) and in L condition for Lepi (6 against 3 for L and H condition) . The potential use of these taxa as indicator for HAB or as suitable competitor for limiting their occurrence will be discussed. The exceptional high occurrence of Alex seemed to be the combination of high environmental and biotic pressure. The occurrence of Lepi appeared to be more controlled by environmental conditions than by the community.