Community metabolism and carbon budget along a gradient of seagrass(<i>Cymodocea nodosa</i>) colonization

We studied the effect of seagrass ( Cymodocea nodosa ) colonization on community metabolism and sediment conditions. The biomass of C. nodosa increased with time after seagrass colonization. The biomass increased steadily during the 6.1 yr of colonization to 9.1 mol C m −2 . Gross primary production increased from 7 to 49.3 mmol C m −2 d −1 during the first stages of the colonization and then decreased to 20 mmol C m −2 d −1 at a time when the biomass was in excess of 6 mol C m −2 . Net dissolved organic carbon (DOC) fluxes increased with time after colonization, shifting from a net uptake in patches younger than 2 yr to a net release in older patches. Community respiration (R) increased with the seagrass colonization, leading to a shift from net autotrophy in the unvegetated sediment community to net heterotrophy after C. nodosa colonization. The increase in net heterotrophy with seagrass colonization was reflected in the development of reducing conditions in the sediment. To maintain a net heterotrophic benthic metabolism and net DOC release, the C. nodosa community must receive an input of organic matter (OM) from an allochthonous source. OM inputs from sestonic material trapped by the seagrass canopy exceed 157 mmol C m −2 d −1 in developed C. nodosa communities. Thus, the seagrass community acts as an important link between the pelagic and benthic communities by trapping sestonic organic carbon.