A mass balance model for the Gulf of Cadiz: implications for the Marine Strategy Framework Directive Descriptor 4 (Food webs)

Abstract

The Marine Strategy Framework Directive (MSFD) aims to achieve Good Environmental Status (GES) of the European waters by 2020. In a legislative framework, this Directive promotes the ecosystem approach to the management of human activities having an impact on the marine environment, integrating the concepts of environmental protection and sustainable use. In order to achieve GES, the Directive lists 11 Descriptors associated to specific criteria and indicators. Of those, Descriptor 4 (D4) addresses the food web structure and energy transfer between different components of the marine ecosystems. For the further development of candidate D4 indicators, the use of models provides essential information to understand dynamical aspects of the marine food webs and therefore helps to implement the MSDF in European Seas. In this regard, Ecopath with Ecosim (EwE) is the most applied mass balance modelling tool worldwide since it typically includes all trophic levels from primary producers and detritus to apex predators and humans. In the context of D4, some indicators arising from the EwE models have been used in the initial assessment of the environmental status (e.g. biomass of the functional groups) or otherwise are currently being discussed as candidate indicators (e.g. ecological network analysis indicators). Keeping the above information in mind, this work presents the main indicators and outputs coming from the EwE model applied in the Gulf of Cadiz in 2009. In a brief overview of the model, a total of 43 functional groups integrated into five trophic levels were identified. Most groups showed high values of ecotrophic efficiencies, indicating that the production of each group was extensively used in terms of predation or exploitation by fisheries within the ecosystem. In addition, this ecosystem was characterized by the dominance of the pelagic fraction, a strong benthic-pelagic coupling and a significant role of detritus within the system. The main flows were originated from the groups at the base of the food web transferring the energy to the upper trophic level groups. Flows to detritus were mainly originated from trophic level II and the most efficient trophic transfer occurred from trophic level III to IV. Concerning the indicators related to the food web complexity, the relatively low omnivory index of the system suggested that the Gulf of Cadiz food web is rather linear. This result indicated that in general consumers feed on a single trophic level. The main keystone species identified were cephalopods, sparids, rose shrimp, dolphins and macrozooplankton, all presenting important overall effects on the remaining functional groups. Finally, additional information coming from the network analysis and the ecosystem indicators defined the Gulf of Cadiz as a relatively immature and a notably stressed ecosystem, partly explained by the intensive human exploitation since ancient times. In conclusion, the application of this mass balance model contributes greatly to the knowledge of the marine food web structure and functioning and may therefore be useful for monitoring progress toward the GES under the MSDF.