Coupling carbon metabolism and dissolved organic carbon fluxes in benthic and pelagic coastal communities.

Abstract

The knowledge of the production and carbon transference dynamics between adjacent communities is essential to gain a better understanding on the role of coastal areas in the global carbon cycle. Here, we assess the carbon metabolism (through dissolved oxygen –DO– concentration) and the net dissolved organic carbon (DOC) fluxes for benthic (dominated by the seagrass Cymodocea nodosa, the rhizophytic green algae Caulerpa prolifera and unvegetated sediments) and pelagic plankton communities co–habiting the same shallow coastal area in Cadiz Bay, southern Spain. Both the seagrass and macroalgae meadows were highly autotrophic, but the carbon metabolism of C. prolifera community shifted seasonally from net autotrophic to net heterotrophic. Unvegetated benthic communities were slightly heterotrophic throughout the year, while plankton community was net autotrophic. This study reveals how the different components of the ecosystem (i.e. macrophyte, sediment or plankton community) may have a seasonal variability in the contribution to the NCP in shallow coastal areas. Moreover, our results suggest that pelagic communities can be essential to maintain autotrophy of coastal system during periods of low benthic productivity. Regarding DOC, benthic communities were net DOC producers, undergoing a marked seasonality with maximum net DOC production during summer. This indicated an extra carbon assimilation not usually contemplated in productivity studies, which entails an underestimation of gross production in these communities. A strong relationship between net DOC flux and net community production (NCP) was found for the three benthic communities. The plankton community was a net DOC consumer indicating a strong coupling between the benthic and pelagic compartments in the bay. When up–scaling our results to the whole bay, the NCP estimated for the benthos was 8800 ± 3770 Tons C y−1, and the net DOC flux was 2465 ± 830 Tons C y−1, which were mainly due to vegetated communities, indicating the importance of coastal vegetated benthic ones in the carbon metabolism and DOC fluxes.