Study of food webs

As part of the establishment of the environmental baseline for the Dieppe Le Tréport offshore wind farm, the Éolien en Mer SIG launched a Call for Projects for a study on food webs. A consortium of several laboratories coordinated by Jean-Philippe Pezy of the Joint Research Unit (UMR) 6143 Continental and Coastal Morphodynamics (M2C) won this call for projects.

Context: Food webs and their study

Food webs are the interconnected food chains of an ecosystem. The various species present form links in the web as producer or consumer species. In simplified terms, a food web can be represented as a succession of trophic levels from primary producers, to intermediate consumers, to top predators. In reality, and particularly in the marine environment, these webs can be complex and evolve according to the life stage of the species, the daily cycle, the season, etc. There is also something of a spatial continuum between closely related food webs, for example between a pelagic web (in the water column) and a benthic web (on the ocean floor).

Simplified representation of a marine food chain (Diagram: A. Michel, Roscoff Marine Biology Station)
Diagram of a marine food web obtained after modelling (Diagram: Jean-Philppe Pezy)

The study of food webs provides a better understanding of an ecosystem as a whole and the relationships between the species that make up the ecosystem. This type of study involves estimating the biomass of species at different trophic levels, the flows between these levels and how they change depending on the season. This requires specific analyses including fish stomach content analysis, stable isotope analysis and examination of the relationship between prey and top predators.

Stable isotope analyses of carbon and nitrogen are performed on all biological compartments. These stable isotopes are ecological tracers that are incorporated into the tissues of an organism directly through its diet. The carbon isotope ratio is an effective marker of consumers’ primary food sources and is thus used as a tracer of habitat and/or feeding areas. The nitrogen isotope ratio increases along trophic chains and is therefore an indicator of an organism’s trophic level and its relationships within a food chain.

Marine environment food chain illustrated by the enrichment of stable isotopes of carbon and nitrogen. The general estimation of enrichment factor (in red) between a source and its consumer is shown for each element: carbon horizontally and nitrogen vertically. (Diagram: Jean-Philippe Pezy)

In addition, for fishery resource, individual fish are used for analysis of their stomach contents. For megafauna, mainly represented by marine mammals and birds, a bibliographical study is used to define the species’ food profiles and potential prey. This may be supplemented by a study of the relationship between prey and top predators at sea. This complementary dual approach involving stable isotopes and stomach contents or food profiles makes it possible to narrow the results to these upper compartments of food webs.

Eventually, the data collected will be used to create trophic models using mathematical tools. These models simulate how food webs evolve following the application of one or more impacting factors (change in temperature, water acidity, appearance of a new substrate, etc.).

Objective of the food web study in the context of the construction of an offshore wind farm

In the context of the future implementation of the Dieppe Le Tréport offshore wind farm, knowledge of the food webs present in the area prior to construction will make it possible to assess the impact of the wind farm on the ecosystem. If species leave the area or new species become established in connection with the arrival of new substrates such as wind turbine foundations, food webs will be impacted and reorganised.

The consortium that won the SIG’s Call for Projects is composed of the following laboratories, associations and companies:

Simplified food web shown with the various organisations involved in the project (Diagram: Jean-Philippe Pezy)

The consortium will base its study on data for all biological compartments collected during Jean-Philippe Pezy’s work on his thesis as part of the study of the initial environmental status (2014-2017), and on data collected for the baseline status of the wind farm (2022-2023). These data will be supplemented by samples for specific analyses of food webs: fish stomach contents, stable isotope analyses and the study of relationships between prey and top predators.