Describing the Bay of Biscay's continental shelf food-web using an OSPAR common indicator: the Mean Trophic Level indicator.

The current development of biodiversity indicators adopted as common in the North East Atlantic (Under OSPAR Regional Sea Convention) is missing some elements necessary for their application as food-web indicators. The Mean Trophic Level (MTL) indicator is based on species biomass and their trophic level (TL). These metrics are commonly used in the food-web holistic approach of interactions in an ecosystem. Species TL, giving the position of an organism in the food web, is influenced by spatiotemporal variability and ontogeny of species. MTL is also shaped by the species biomasses and their data source (i.e. landings or survey data). The aim of the present study was to propose a methodology (1) considering a better regionalisation of the indicator (i.e. using local TL estimations and ecosystem survey data), (2) looking at the MTL sensitivity regarding ecosystem species and compartments and (3) discussing the food web approach of the indicator. Different scenarios were thus tested on MTL indicator with EVHOE survey data (i) using various data sources of TL and (ii) applying several cut-offs to focus on different compartments in the ecosystem. Species influence on each scenario was also investigated. Two species were found to be highly influencing the MTL indicator trend. Capros aper, a low TL species catching the bottom up effect in the food-web, was strongly acting on MTL when considering all species in the ecosystem. Meanwhile, Merluccius merluccius was driving the indicator when low trophic level species were cut-off. This high predator reflected more the top down effect on the ecosystem. This work concluded the need to assess the MTL indicator using three TL cut-offs (TL= 2, 3.25 and 4) to capture a holistic view of changes in the ecosystem. Furthermore, scientific surveys and local estimations of TL are crucial in order to reduce uncertainty around the MTL estimation.