Consequences for shorebirds of an intertidal mudflat (Brouage, NE Atlantic) food web functioning in winter

Background/Question/Methods

The migratory shorebirds of the East Atlantic flyway massively land for migratory stopover or wintering on the French Atlantic coast. The Brouage bare mudflat is one of the major stopover sites in France. The activity of Brouage mudflat at low tide is based on the development of a microalgal biofilm which feeds the benthic food-web. Any food web is characterized by a particular structure and functioning that assures the efficiency of carbon transfer and the sustainability of the ecosystem. The structure and functioning of the Brouage food web is crucial for the conservation of species landing on this area because it provides sufficient food which allows shorebirds to reach the North of Europe where they nest. The aim of this study is to describe and to understand how the Brouage food-web is organized in order to provide the sufficient nutritional needs for birds. Two food-web models were constructed based on in situ measurements which were made in February 2008 (presence of birds) and July 2008 (no birds). In order to complete the models, allometric relationships and additional data from the literature were used. The missing flow values of the food web models were estimated by the Linear Inverse Modeling Monte Carlo Markov Chain. The flow solutions obtained were used to calculate the Ecological Network Analysis (ENA) indices which estimate the emergent propertied of the functioning of a food-web. The combination of these two methods allowed the estimation of the confidence intervals for each of the ENA indices and for each models, allowing statistical comparisons.

Results/Conclusions

The total activities of the Brouage ecosystem in July and February are significantly different. This is due to a primary gross production 2.5 times higher in February. In July the lower gross primary production, probably due to extreme environmental conditions (high light and temperature), is compensated by the setting-up of a higher detritivory and bacterivory leading to a cycling of carbon that tends to be higher than in February. The specialization of the trophic links within the ecosystem does not appear to be different between the two models. The length of the trophic chain, which is shorter in February, avoids large carbon loss. It can be concluded that in February, the higher activity of the Brouage ecosystem coupled with the shorter length of the trophic length ensure the good feeding of the migratory shorebirds.