Long-term changes of the zooplankton variability in a turbid environment: The Gironde estuary (France)

Abstract

Zooplanktonic variability at the maximum turbidity zone was investigated for the Gironde estuary for 18 years. This ecosystem is characterized by a high concentration of suspended matter and five dominant zooplankton species: the copepods Eurytemora affinis, Acartia bifilosa, A. tonsa and the mysids Neomysis integer and Mesopodopsis slabberi. Four major sources of variation in density were analyzed: (a) spatial variation (vertical and longitudinal variability) and (b) temporal variation (seasonal and annual variability) in the oligo-mesohaline area of the estuary. Temporal variability was related to environmental parameters: temperature, salinity, suspended matter concentration and active chlorophyll in the water column. Seasonal and annual variability were identified as significant sources of variation which explains the fluctuations of all zooplankton densities. No long-term trend was observed for zooplankton densities except for A. tonsa, a species recently introduced into the oligo-mesohaline area of the estuary. Temperature and salinity were important factors explaining the interannual variability of E. affinis as well as of both mysids. Suspended matter concentration controlled the long-term trends of E. affinis and A. tonsa. Annual variability of A. bifilosa and both mysid species was also explained by the proportion of active chlorophyll. Seasonal variability, calculated as the between-month variation, represented the dominant type of variability for all species. Seasonal variations of copepods were essentially explained by salinity. N. integer was significantly correlated with temperature and copepod densities, whereas M. slabberi was correlated with temperature and salinity. Zooplankton also showed significant spatial variation. Mysids demonstrated significant differences between surface and bottom densities whereas vertical variability of copepod densities did not represent a significant source of variation. In terms of longitudinal variability, density of E. affinis was similar along the salinity gradient, whereas Acartia spp. and mysids occurred at significantly greater densities at the downstream station. The high turbidity, which characterizes the Gironde estuary, seems to be a determining factor that acts directly on copepod temporal variability and indirectly on mysid long-term fluctuations by limiting the nutritional quality of the environment.