Coexistence, distribution patterns and habitat utilization of the sibling species complex Eurytemora affinis in the St Lawrence estuarine transition zone

Eurytemora affinis is the dominant copepod in the St Lawrence estuarine transition zone (ETZ). The distribution patterns of two co-occurring genetically divergent clades of E. affinis were analyzed. The Atlantic (A) and the North-Atlantic (NA) clade occurrences were spatially and temporally variable. Three and five distinctive habitats were found in June and August 2010, respectively. The habitats in the upper and the middle ETZ were predominantly defined by SPM characteristics such as particulate organic matter, chlorophyll a and protists while, in the lower ETZ, water mass properties such as nutrients, salinity and temperature were responsible for habitat differentiation. In June, the NA clade occurred throughout all habitats whereas, in August, it was mostly restricted to the middle and lower ETZ. The A clade was absent in June and was found in similar densities in the upper and middle ETZ in August. Associated with these habitats, distinct zooplankton assemblages were found, dominated numerically by at least one of the two clades, except in the most downstream habitat where Acartia spp. occurred in higher densities. Highest densities of the NA clade coincided with high food quantity and quality in the middle ETZ, whereas the A clade was equally abundant in the upper and the middle ETZ. Thus, we suggest that distribution patterns of the E. affinis clades were probably controlled by food source conditions and 'clade-specific' physiological tolerances, resulting in either spatial or temporal segregation or in coexistence of both clades in some of the habitats along the ETZ.