Plankton interactions, their environmental determinants and biological/geochemical consequences in the southern Senegal cOastal LABoratory – SOLAB

The project is based on the use of an integrated multi-scale approach with a focus on the so-called synoptic scales ranging from a few days to a few weeks and on spatial scales ranging from a few kilometers to a few hundred kilometers.
SOLAB combines a wide range of observation methods (satellites, field campaigns, instrumented moorings, acoustic and optical measurements, chemical analyses, molecular biology). Based on these observations, SOLAB is designed to contribute to the evaluation and improvement of a new generation of numerical models of the coastal ocean covering physics, biogeochemistry, plankton functional biodiversity, as well as life cycle modeling of small pelagics.

The first results concern different aspects of the project: oxygen dynamics, pollutant transport, the effect of wind fluctuations on the dynamics and planktonic activity of the Senegalese coastal ocean, and the planktonic diversity observed during the autumn (November-December) which is an important transition phase for the ecosystem.

The analysis of simulations and in situ observations brings elements of understanding of deoxygenation episodes at synoptic to intra-seasonal time scales (3-30 days) that occur on the Senegalese continental shelf.

The analysis of numerical simulations also reveals the existence of circulation paths that can transport surface pollutants from the open ocean to the Senegalese coast. These paths are particularly well established between April-May and October-November. These results have important implications for the protection measures that must accompany the start of offshore oil exploitation in the next few years.

Several articles are presently in preparation. 1 article has been published and 1 book chapter is in press.

Tall, A.W. , E. Machu, V. Echevin, X. Capet, A. Pietri, K. Corréa, S.M. Sall, and A. Lazar 2021: Variability of dissolved oxygen in the bottom layer of the southern Senegalese shelf. J. Geophys. Res, 126, e2020JC016854

Machu, E., Brochier T., Capet X., et al, 2021: Marine pollution in Senegal, sources, consequences and fate, PADDLE, chapter, in press.

Several presentations of the results have been made in various venues including the 2022 Ocean Science conference (https://osm2022.secure-platform.com/a/gallery/rounds/3/details/1548).

Coastal oceans and the services they provide are socially, economically and environmentally essential, particularly for the populations that inhabit their shores. These regions are subjected to major anthropogenic stress from local origin but also as a consequence of global changes: upper ocean warming, deoxygenation, acidification, eutrophication, overfishing, and various pollutions (heavy metals, wastewaters ...). These disturbances can interact with each other, and spread/amplify in the ecosystem through modifications of key trophic interactions. To manage and anticipate the effects of these disturbances, marine environment monitoring and management tools are urgently needed. Unfortunately, in many coastal areas, scientific knowledge is insufficient to implement such tools.

This context is exacerbated in West Africa where marine resources play a vital socio-economic role, where they undergo major anthropogenic stresses and where the functioning of marine ecosystems receives only modest attention. SOLAB proposes to turn a sector of the southern Senegal shelf (SSUS) of approximately 200 km by 200 km identified as particularly favourable into a natural laboratory for the study of coastal ecosystems and trophic interactions. The overall objective of SOLAB is to improve our ability to predict and manage coastal ocean evolutions. Because of its pivotal role in marine ecosystems, plankton will be the focus of SOLAB research. The project will investigate its dynamics, diversity, structure and variability in the 3 adjacent eco-subprovinces making up the SSUS. Specifically, we seek to 1) understand how the plankton ecosystem responds to variability of its physical/biochemical drivers on scales from days to months in time, and from 1 to 100 km in space (finer physical processes will also be considered insofar as they contribute to trophic transfers); 2) clarify present-day impact of planktonic and abiotic environment variability at these scales on small pelagic fish (mainly sardinella spp which utilize the SSUC as a nursery area) and coastal shellfish; 3) take advantage of 1) and 2) to explore the possible SSUS ecosystem modifications as a result of global changes.

Our methodology relies on integrated multi-scale modelling and observations (in situ and satellite) with an emphasis on synoptic/submeso scales (from a few days to a few weeks and 1-10 km). The transitions periods between the upwelling and monsoon seasons when most of the Sardinella aurita stock migrates in or out of the SSUS will be of particular interest.

SOLAB combines a wide range of observation methods (satellites, field campaigns, instruments on long-term moorings,measurements by acoustics, optics, chemistry analyzes, metabarcoding). Based on these state-of-the-art observation activities, SOLAB is designed to contribute to the emergence of a new generation of coastal ocean numerical models of the coastal ocean (covering physics, biogeochemistry, plankton functional biodiversity) apt to model SPF life cycles with a realistic representation of their habitat and relationships to fine-scale biotic/abiotic environment.
By turning Southern Senegal into a laboratory for coastal ocean sciences SOLAB seeks to support West African societies in managing their marine environment and its future evolutions (e.g., by pursuing training activities carried out for several years by the consortium). It also more specifically seeks to contribute to the development of marine aquaculture, and provide the scientific foundations of the Senegal marine environment observatory in planning.