Carbon metabolism and dissolved organic carbon (DOC) fluxes on seagrass communities: insights from plant colonization states, eutrophication and global change related factors

Abstract

Coastal vegetated communities are among the most productive ecosystems on Earth. Their role in the global carbon cycle and how they cope with global change may be more relevant than previously believed. They export large quantities of matter, both in particulate and dissolved forms to adjacent communities. Dissolved organic carbon (DOC) flux play a central role in the marine carbon cycle as an important driver of primary production for other compartments of the food web. While there has been extensive research on DOC dynamics in the open ocean, the role of coastal ecosystems in the global DOC cycle is still inadequately understood, even though these habitats tend to accumulate large amounts of DOC. Few studies have examined the DOC fluxes by marine macrophytic communities (macroalgae and seagrasses) under in situ approaches to determine their overall contribution in the whole system and their subsequent exchange with adjacent communities. Moreover, coastal vegetated communities, especially those dominated by seagrasses, are currently considered one of the most threatened ecosystems on Earth because of anthropogenic pressures, including nutrient increase and climate change. Thus, the overall objective of this Thesis was to evaluate the carbon metabolism and DOC fluxes in communities dominated by seagrasses and elucidate the effects of human–induced disturbances on the carbon dynamics of the community.

The results of this Thesis showed that macrophytic communities are highly autotrophic, with large and variable contributions of their different components of the community, and a DOC source for the plankton community. Increase in nutrients concentration triggered that the communites dominated by the macroalge Caulerpa prolifera and the seagrass Cymodocea nodosa moved from autotrophy to heterotrophy in certain seasons of the year and could increase or decrease the DOC release. The response of seagrass communities when subjected to a pH decrease was complex and showed to be species-specific. The pH decrease triggered a significant increase in gross primary production (GPP) and community respiration (R) in seagrasses, which was translated into sucrose increase in aboveground tissues rather than a higher DOC release. Water temperature was the stressor that had a higher positive effect on carbon metabolism, yielding higher seagrass productivity, growth and DOC release. A direct relationship between productivity increase and larger DOC release was found in communities dominated by seagrasses. In addition, a high correlation between DOC release and both water temperature and current velocity was found. This Thesis demonstrated that climatic change and to some extent nutrient enrichment in coastal areas may not be so detrimental than previously believed at least for temperate seagrasses, and even may benefit the productivity and resistance of some temperate species (e.g. Cymodocea nodosa) in the future.

The results of this Thesis underline the high productivity of vegetated coastal ecosystems at a local level, which support new insights in the role of the marine primary production in the ocean C sink and the role of the carbon coastal cycle in the global carbon cycle. Finally, this Thesis underscores that the role of seagrass meadows in the carbon coastal cycle will be more relevant in the near future, as higher C uptake and DOC release may occur under forecasted global change conditions.