Benthic metabolism in shallow coastal ecosystems of the Banc d'Arguin, Mauritania

Benthic primary production and respiration were investigated at 4 sites representative of the major coastal communities bordering the Sahara Desert in the Banc d'Arguin, Mauritania, Western Africa. These sites correspond to intertidal Zostera noltii beds (270 km²), intertidal bare sediments (88 km²), subtidal Cymodocea nodosa beds (374 km²), and subtidal bare sediments (311 km²). At each site, production- irradiance relationships were established in situ in November 2008 and January 2010, and used to calculate daily carbon fluxes for these communities. In intertidal areas, compared to emersion, the gross maximal photosynthetic rates for Z. noltii bed and bare sediment communities were on average 8- and 7-fold higher during immersion, respectively; community respiration rates were 3- and 18-fold higher during immersion, respectively. The Z. noltii bed was autotrophic during the 2 study periods, with a mean (±95% probability limit) daily net community production of 71.3 ± 58.6 mmol C m^-2 d ^-1. Conversely, net community production was always negative in intertidal regions and subtidal bare sediments (average -7.3 ± 46.7 mmol C m^-2 d^-1 and -47.0 ± 38.9 mmol C m^-2 d^-1, respectively); the C. nodosa bed was negative in November (-96.2 ± 85.1 mmol C m^-2 d^-1) and positive in January (33.4 ± 82.6 mmol C m^-2 d^-1). Community respiration was highest in subtidal communities, indicating active mineralization of organic matter and demonstrating that Z. noltii beds are likely to increase the biological richness of the Banc d'Arguin by exporting energy. Our results confirm the ecological importance of seagrass beds in the net coastal carbon fluxes and justify their protection.