Modelling nitrogen cycle in a small intertidal estuary: respective influence of environmental factors and cultivated oysters

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The Baie des Veys, located on the French coast of the English Channel is an open intertidal estuary (37 km²) with an important oyster farming activity. The main cultivated species is the Pacific oyster, Crassostrea gigas, with a standing stock of ca. 10 500 tons. This ecosystem is influenced by four rivers which drain an important catchment basin (3 500 km²). Previous results showed that phytoplankton production is mainly influenced by terrestrial nitrogen inputs rather than oceanic nitrogen inputs. In this study, nitrogen dynamics were evaluated by developing an ecosystem box model taking into account all major nitrogen sources. This model simulates the Baie des Veys nutrient-phytoplankton-oyster food web by coupling a primary production model and an oyster ecophysiological model based on the Dynamic Energy Budget theory. The model is split in two compartments, the water column and the sediment. It was validated using in situ measurements of chlorophyll a, nutrients and biometric data on oyster growth. This coupled model allowed us to study the Baie des Veys nitrogen cycle using two different approaches. First, simulations with average environmental conditions were achieved with and without oyster stock in order to highlight the influence of cultivated oysters (filtration, excretion, biodeposition) on nitrogen dynamics. Second, different scenarios with very contrasted environmental conditions (freshwater and nutrient inputs, meteorology) were tested in order to estimate the influence of the environment on the Baie des Veys nitrogen cycle. Finally, the model results were compared with available data from other cultivated ecosystems using relevant indicators.