Trophic connectivity between offshore upwelling and the inshore food web of Banc d'Arguin (Mauritania): New insights from isotopic analysis

Abstract

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT2 ABSTRACT Banc d'Arguin (BA), Mauritania, is a nationally protected shallow gulf > 10,000 km 2 between the Sahara desert and the upwelling system off the Mauritanian coast. In the southeast, BA consists of a 500 km 2 tidal flat, the most important wintering site for shorebirds using the East Atlantic Flyway. The Mauritanian upwelling-driven phytoplankton production supports the most productive fisheries worldwide, but little is known about its trophic role in the functioning of the inshore BA food web. Using stable isotopes as trophic tracers to distinguish between upwelling-driven phytoplankton, open ocean phytoplankton, and benthic primary producers, we assessed the spatial extent to which the inshore BA food web is fuelled by upwelling-driven phytoplankton production. The δ 13 C and δ 15 N signals were characterized in dominant primary producers, benthic invertebrate taxa, and various fish species along an offshore-inshore (northwest-southeast) gradient. We also monitored the spatial and temporal extent of upwelling entering BA during 2008 with remote sensing of sea surface temperature and chlorophyll a data. The results suggest that benthic invertebrates and fishes living in the northwestern part of BA depend on the nearby upwelling phytoplankton production, but this food source does not support the intertidal benthic community in southeast BA. Furthermore, the isotopic signatures of fishes suggest weak trophic connectivity between the northern subtidal and southeastern intertidal BA. Our results support the hypothesis that the southeastern tidal flat region functions as a distinct ecosystem with a food web supported mainly by local benthic primary production, which is crucial knowledge for effective management of the pristine BA national park.

Global Systematic Review of Methodological Approaches to Analyze Coastal Shelf Food Webs

“…This is readily evident when looking at the base of the coastal shelf food webs. Coastal systems are energetically supported by a variety of sources, including local benthic or pelagic producers, as well as allochthonous organic matter which may originate either from land via freshwater run-off or from the open sea through tidal currents and upwellings (Nielsen and Navarrete, 2004;Carlier et al, 2015;Christianen et al, 2017;Meunier et al, 2017). Moreover, the proportional contribution of the different energy sources to coastal shelf food webs may also be affected by several human-induced drivers, such as coastal development, seabed disturbance, eutrophication, and changes in river flow (Tewfik et al, 2005;Al-Habsi et al, 2008;Christianen et al, 2017;Hemraj et al, 2017).…”

Pronounced Northwest African Monsoon Discharge During the Mid- to Late Holocene

“…Pollen records from West Africa presented in Hély et al (2014) indicate prolonged dry seasons and possibly associated intensified trade winds during the mid-Holocene that could have promoted upwelling along the Mauritanian coast (Talbot, 1981). However, along the inshore Banc d'Arguin corresponding to the location of the palaeoestuary, satellite derived SST and nutrient data depict a notably limited influence of upwelling today (Carlier et al, 2015). A low input of upwelled water to the essentially sheltered inner Banc d'Arguin was further supported by N isotopic signatures in fish and their potential food sources, suggesting very limited supply of upwelled nutrients to inshore species (Carlier et al, 2015).…”

“…However, along the inshore Banc d'Arguin corresponding to the location of the palaeoestuary, satellite derived SST and nutrient data depict a notably limited influence of upwelling today (Carlier et al, 2015). A low input of upwelled water to the essentially sheltered inner Banc d'Arguin was further supported by N isotopic signatures in fish and their potential food sources, suggesting very limited supply of upwelled nutrients to inshore species (Carlier et al, 2015). Considering these observations, temperature variability is likely of lesser importance to the overall δ 18 O composition of these waters in near-shore habitats, particularly for bivalves living within the palaeo-estuary.…”

Density-dependent growth of bivalves dominating the intertidal zone of Banc d’Arguin, Mauritania: importance of feeding mode, habitat and season

“…Previous studies have indicated that the food web of this intertidal area is mainly supported by local benthic primary production (i.e. seagrass and microphytobenthos), with low contributions of phytoplankton, macrophytes and epiphytes to the food-web (Wolff et al 1993b, Carlier et al 2015. Being adjacent to the Sahara desert, Banc d'Arguin does not receive freshwater inflow from rivers and precipitation is limited to occasional thunderstorms that occur at irregular intervals, sometimes several years apart (Wolff & Smit 1990, van der Geest et al 2014).…”