Community metabolism and carbon budget along a gradient of seagrass (Cymodocea nodosa) 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 m22. Gross primary production increased from 7 to 49.3 mmol C m22 d21 during the first stages of the colonization and then decreased to 20 mmol C m22 d21 at a time when the biomass was in excess of 6 mol C m22. 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 m22 d21 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. Seagrasses form highly productive ecosystems, delivering a large carbon surplus to coastal ecosystems (Duarte and Cebrián 1996; Duarte and Chiscano 1999; Hemminga and Duarte 2000). The primary production of seagrass beds is enhanced by that of epiphytic and benthic algae, which usually bring the total primary production of seagrass meadows to values twice as high as those contributed directly by the seagrasses (Hemminga and Duarte 2000). Seagrass canopies enhance organic inputs to the sediment, both directly as inputs of seagrass tissues and as sestonic particles trapped by the seagrass canopy (e.g., Ward et al. 1984; Lemmens et al. 1996; Gacia et al. 2002). These sestonic particles are defined as particulate organic matter (POM), such as plankton and organic detritus, and also as inorganic particles suspended in seawater. Moreover, the seagrass rhizosphere releases OM and oxygen (Penhale and Wetzel 1983; Pedersen et al. 1998), thereby affecting sediment properties (e.g., Kemp et al. 1984; Hemminga 1998; Marbà and Duarte 2001). These processes stimulate bacterial activity within seagrass meadows 1 Corresponding author ([email protected]).