Aerial and underwater carbon metabolism of a <i>Zostera noltii</i> seagrass bed in the Banc d'Arguin, Mauritania

Community respiration and primary production were measured in a dense intertidal Zostera noltii bedon the Banc d’Arguin, Mauritania (West Africa) under aerial and submerged conditions. Metabolism wasstudied in situ in dark and transparent benthic chambers. CO2 fluxes in the air were measured over a seriesof short-term incubations (3 min) using an infrared gas analyzer. Dissolved inorganic carbon fluxes werecalculated from concentration changes during one-hour underwater incubations. Air and underwaterirradiance levels were measured every minute throughout the experiments. Carbon respiration was lowerin the air (2.2mmolm−2 h−1) than underwater (5.0mmolm−2 h−1); similarly, a production-irradiancemodel fitted to the data indicated that gross maximal photosynthetic rate was markedly lower duringemergence (6.0mmolCm−2 h−1) than under water (42.7mmolCm−2 h−1). The 13C values observed inshoots indicated a decrease in atmospheric CO2 contribution, compared to dissolved inorganic carbon, inZ. noltii metabolism along a depth gradient within a single location. As the seagrass bed remains under athin layer of water at low tide at the studied site, the large difference in primary production can be mainlyattributed to photosynthesis inhibition by high pH and oxygen concentration, as well as to the negativefeedback of self-shading by seagrass leaves during emersion. The observed differences in respiration canbe explained by the oxygen deficit at night during low tide near the sediment surface, a deficit that isconsistent with the abundance of anoxia-tolerant species