6 - Carbon Mitigation: A Salt Marsh Ecosystem Service in Times of Change

Abstract

Salt marshes have great ecological value for the ecosystem, namely in nutrient regeneration, primary production, habitat for wildlife species, and as shoreline stabilizers. Their important role has been recently confirmed by the inclusion of these ecosystems in the Water Framework Directive. Multiple services and the value of wetlands are already well known. The major carbon sink for the planet is the oceans (38,630PgC), followed by the terrestrial zones. Considering the terrestrial sink (1400PgC), the more productive and more important zones retaining carbon are the wetlands retaining about one half to a third of the carbon (455–700PgC). Vascular plants in salt marshes are crucial to the dynamics of the estuarine ecosystem, strongly influencing the processes of retention of heavy metals, reduction of eutrophication and mitigation of carbon. Salt marsh plants (halophytes) are characterized by, among other things, being extremely productive. Their product is directly linked to the important role they play in estuaries, in terms of the value-added. The evident zonation in salt marsh vegetation is now accepted as a result of competitive advantages of superior plants to colonize particular habitats with more favorable physicochemical characteristics, leading to the drawback of less competitive species. External stresses driven by warming, like nutrient imbalances (similar to eutrophication), may lead to the success of less competitive species, through belowground competition alleviation. CO2 can be another key factor altering plant dynamics, favoring species with higher aerial biomass production and increasing competition for light. These differences in plant biomass allocation are also to be considered in terms of plant dominance as an important part of interspecific competition. With CO2 increase, the dynamics of plant distribution may be affected, as the differences in the photosynthetic metabolisms will provide some species conditions of higher production, and therefore competitive advantage. How CO2 will affect these important salt marsh areas is one of the important questions addressed in this chapter. The opposite point of view is also important to be accounted for; how can salt marshes contribute to reducing CO2? And more important from the holistic point of view; how will these changes affect the services provided to the ecosystem? Therefore, integrating the fact that these ecosystems are very productive with the generally accepted knowledge that global atmospheric CO2 is rising, it becomes important to know the present status of salt marsh systems and how their important services to the ecosystem will behave in a climate change scenario of increasing atmospheric CO2. This chapter intends to address this subject from several points of view, using a multi-disciplinary approach including microbiology, plant physiology, stable isotope discrimination, and ecological modeling.