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

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