Selleslagh, J; Lobry, J; Amara, R; Brylinski, JM; Boet, P
The ecological functioning of various French coastal
ecosystems, with special emphasis on the Canche estuary, along an anthropogenic pressure gradient
was studied using mass-balanced trophic models in order to test the relevance of functional
indices assessing the ecological status of coastal systems. A trophic model of the Canche
(eastern English Channel, France) estuarine food web was constructed using an Ecopath approach to
determine the structure and functioning of energy flows in the trophic network. The model
consisted of 15 compartments, from primary producers (trophic level TL = 1) to the top consumers
(fish, TL = 3.8). Input parameters were mainly computed from field studies conducted between 2006
and 2007. Results showed that the majority of flows came from detritus, which is an important
energy source in the ecosystem, as indicated by mixed trophic impacts. Keystone index (KSI)
revealed that amphipods, copepods, gobies and European eel were identified as key compartments in
the Canche estuarine food web, with the first two considered as important vectors for carbon
transfer from detritus to top predators. The Canche estuary trophic network has a low recycling
level (FCI = 0.8%), a low total system throughput (TST = 1364 gC. m(-2). y(-1)) and a low
ascendency (A = 1439 gC. m(-2). y(-1)), but a relatively low connectivity (CI = 0.33), high
internal relative ascendency (Ai/Ci = 35.6%) and a high omnivory index (OI = 0.04), indicating
that this estuary is immature but relatively organised and complex, with strong production but
which little is used as the estuary depends on external exchanges. In addition, the Canche
estuary, recognised as a small and low impacted estuarine ecosystem, was compared with six other
French coastal systems: the bay of Somme, the Seine estuary, the Mont Saint-Michel bay, the Loire
estuary, the Pertuis Charentais and the Gironde estuary, where Ecological Network Analyses (ENA)
have been applied using similar methods. Compared with the other six coastal systems, the Canche
estuary did not present a similar functioning to close geographically estuaries or estuaries with
similar environmental characteristics. The results of the present study showed that among
computed ENA indices, the omnivory index (OI) was positively correlated with an anthropogenic
pressure index, suggesting that such a functional index can be useful for assessing the
ecological quality of estuarine and coastal systems. (C) 2011 Elsevier Ltd. All rights reserved.