Modelling the effect of Cerastoderma edule bioturbation on microphytobenthos resuspension towards the planktonic food web of estuarine ecosystem

Microphytobenthos (MPB) represents an important food source for primary consumers in estuarine ecosystems and the availability of MPB as food items results from complex physical, chemical, and biological interactions. In Baie des Veys (Lower Normandy, France), the common cockle Cerastoderma edule constitutes the major bioturbator in the ecosystem in terms of biomass. In this ecosystem, cockle bioturbation is a key process regulating the MPB erosion flux in the water column. This bivalve intensely modifies the top layer of the sediment by increasing the sediment erodibility and the fluxes of suspended chlorophyll a through the valve movements. More precisely, cockle bioturbation destabilizes the sediment surface by creating a biogenic layer that is easily eroded with tidal hydrodynamic forces. Associated MPB can then be exported to the water column to fuel higher trophic levels of the planktonic food web. The aim of this study was to develop a numerical model that reproduces the export of MPB associated to the biogenic layer erosion. Kinetics of suspended MPB, in response to increasing stress, were obtained from flume experiments in lab controlled conditions and in situ natural conditions. Following this, the suspended MPB were analyzed to respectively parameterize the model by (1) a calibration approach, and (2) an independent validation. The analysis has highlighted that the higher the biomass of cockles, the higher the MPB resuspension rates. Our model consistently reproduces the tendency encountered in laboratory analysis and with in situ natural conditions. During the validation, a small site-specific lack of adjustment was identified, but, among the macrozoobenthic community, the model can be significantly improved by considering the bioturbation activities of another ecosystem engineer, Pygospio elegans. This study thus provides reliable estimates of the daily food availability from benthic primary consumers in an estuarine system where cockles dominate the bioturbating assemblage. This model can be inserted in various model designs (0D, 1D-vertical or 3D).