Spatial and temporal dynamics of biotic and abiotic features of temperate coastal ecosystems as revealed by a combination of ecological indicators.
Résumé
Coastal ecosystems exhibit complex spatio-temporal patterns due to their position at the interface between land and sea. This is particularly the case of temperate ecosystems where exploitation of coastal resources (fisheries and aquaculture) and intensive agricultural use of watersheds further complicate our understanding of their dynamics. The aim of the present study was to unravel the spatio-temporal dynamics of contrasted megatidal coastal ecosystems located at the same regional scale (i.e. under the same regional climate), but under different kinds of human pressure. Two kinds of ecological indicators were assessed over a period of four years at 11 locations along the coast of the Cotentin peninsula (Normandy, France). A first set of hydrobiological variables (dissolved nutrients, Chl a, temperature, salinity, etc.) was measured fortnightly in the water column. These data were analysed by principal components analysis (PCA). A second set of variables were the carbon and nitrogen stable isotope ratios of the adductor muscles of cultured Crassostrea gigas introduced every year to typify the bentho-pelagic coupling at each location. Food sources were also investigated using a mixing model with data on the isotopic composition of the food sources obtained previously. To identify which environmental variables played a significant role in determining the oyster diet, the contributions of oyster food sources were combined with environmental variables in a canonical correspondence analysis (CCA). Isotopic values of adductor muscles varied significantly between −20.12‰ and −16.79‰ for δ13C and between 8.28‰ and 11.87‰ for δ15N. The PCA distinguished two groups of coastal ecosystems that differed in their coastal hydrology, nutrient inputs, and the size of their respective watershed, irrespective of the year. In each zone, different spatial patterns in the measured variables were observed depending on the year showing that local impacts differed temporally. As revealed by CCA, food sources used by the oysters were mainly explained by salinity suggesting regional differences between ecosystems. On the west coast of the peninsula, climatic factors act in synergy with anthropogenic factors (i.e. nutrient enrichment) whereas on the east coast, climatic factors appear to be dampened by anthropogenic factors.