Ontogenetic optimal temperature and salinity envelops of the copepod Eurytemora affinis in the Seine estuary (France)

Temperature and salinity are important factors shaping the habitats of estuarine ectotherms. Their respective effect varies along the life history moments of species with a complex life cycle. Estuarine species, particularly those living in the salinity gradient, are concerned by habitat changes that can reduce their fitness. Consequently, efforts to define the importance of those two environmental variables on developmental stages are required to enable forecasting estuarine species' future distributions. The present study focuses on the main component of the Seine estuary's zooplankton, i.e. the calanoid copepod Eurytemora affinis, and aims: (i) to establish the role of temperature and salinity in designing the habitat of E. affinis within the Seine estuary; and (ii) to model the habitat of three groups of E. affinis defined through the life cycle as follows: all larval instars (N1-N6), the first to fourth juvenile instars (C1-C4), and the pre-adult and adults instars (C5-Adults). For this purpose, data from intensive field studies of zooplankton sampling during 2002-2010 were used. The fine-scale data, i.e., every 10-20 min, on density and abiotic conditions (salinity, temperature) provided inputs for the computation. We established regions in salinity-temperature space where the three groups of developmental instars exhibit higher densities. The computed habitats differ between developmental groups. In general, the preferendum of salinity increases with ontogeny. The optima of temperature are rather constant between developmental stages (∼14 °C). Our model can be used to determine E. affinis functional habitat (i.e., the spatial relation with structuring factors), to carry out retrospective analysis, and to test future distributions. The present study also emphasizes the need of data from appropriate sampling strategies to conduct habitat definition.