Marine macrophyte (seagrass and macroalgae) communities have been reported to be generally net autotrophic with an excess of organic production relative to their respiration. This excess of organic matter can be stored in the sediment or exported to adjacent ecosystems. Carbon burial in marine macrophyte communities represents less than 6 % of the net community production, so a large percentage of the excess of organic matter produced must be exported to adjacent waters in dissolved and particulate form. In this thesis, I examined the net dissolved organic carbon (DOC) production by macrophyte communities and its contribution to the DOC export from these ecosystems to adjacent waters. The results obtained showed that, in general (more than 80% of the estimates), marine macrophyte communities acted a net source of DOC to the ecosystems, as expected. Globally the net DOC release by marine macrophyte communities was estimated at 0.23 PgC y-1, which can contribute to support the metabolism of adjacent waters.

Due to the contribution of the marine macrophyte communities, along with other vegetated habitats (e.g. mangrove forests and saltmarshes) to the net DOC production in coastal waters, the coastal zone can export DOC to the open waters. Here, I studied the DOC exported from two coastal sites, one was an experimental site and the other was a natural coastal ecosystem. In both coastal systems there was a net export of DOC, supported by the macrophytes, to the adjacent waters.

There were few estimates of the DOC export from individual coastal zones to the open ocean prior to this thesis. So in order to estimate the global DOC export from the coastal to the open ocean I compiled the DOC concentration in the coastal ocean and coupled this with the water exchange between these two compartments of the ocean and the DOC concentration in the open ocean. The global DOC exported from the coastal ocean to the open ocean was estimated at 6 Pg C y-1. Thus, these results show that excess of DOC production in the coastal zone can subsidize the metabolism of the open ocean, generating a metabolic link between these two biomes.