Organic matter exchange and cycling in mangrove ecosystems: Recent insights from stable isotope studies

Abstract

Mangrove ecosystems are highly productive tropical coastal ecosystems which have a potentially high impact on the carbon budget of the tropical and global coastal zone. The carbon dynamics in mangrove ecosystems has been the subject of numerous studies during the past decades, but we are still far from having an integrated view of the overall ecosystem functioning in terms of organic matter processing. The application of recent analytical techniques has produced a wealth of new information but has also indicated the gaps in our knowledge on organic matter cycling in these ecosystems. This paper provides an overview of our current understanding of organic matter dynamics in mangrove ecosystems, and reviews data based on stable isotope analyses, on (i) the delineation of carbon sources in different organic matter pools, (ii) utilization patterns of organic carbon by microbial and faunal communities, and (iii) organic matter exchange between mangroves and adjacent ecosystems. Although the use of stable isotopes has a number of limitations and has not always been able to unambiguously assess source contributions, it has been invaluable in refuting some long-standing paradwigms, and has shown that source characterization is crucial in order to better estimate organic matter budgets in these dynamic ecosystems. Future studies on process rates or flux measurements should therefore ideally be combined with a variety of chemical tracers to determine the source of the organic matter considered.

Aquatic food webs in mangrove and seagrass habitats of Centla Wetland, a Biosphere Reserve in Southeastern Mexico

“…This variation may reflect residual epiphytic algae that we were unable to remove during processing, or spatial variation in δ 13 C of dissolved inorganic carbon (DIC) used as a substrate for photosynthesis. Addition of epiphytic algae (δ 13 C values for eelgrass may be expected near the connection with the Usumacinta River, and enriched values expected in the middle of dense eelgrass mats due to 13 C-enrichment of the residual DIC pool through preferential fixation of 12 C during photosynthesis (Bouillon et al, 2008). In either case, the depleted nitrogen isotope ratio of eelgrass still prevents it from being a predominant carbon source for the majority of aquatic consumer taxa collected during our survey.…”

Air–sea exchanges of CO<sub>2</sub> in the world's coastal seas

“…This phenomenon, however, may be more than counteracted by enhanced heterotrophic activity, supported by organic carbon input from rivers (Smith and Hollibaugh, 1993;Heip et al, 1995;Hedges and Keil, 1995;Hedges et al, 1997;Hansell and Carlson, 1998;Bouillon et al, 2006;Jiang et al, 2010). Additionally, direct inorganic carbon input from river water, submarine groundwater discharge and exchanges with tidal marshes and mangroves play an important role in increasing the pCO 2 of estuarine and shelf waters (Moran et al, 1991;Miller and Moran, 1997;Neal et al, 1998;Raymond and Bauer, 2001;Borges et al, , 2006Cai et al, 2003;Wang and Cai, 2004;Jahnke et al, 2005;Bouillon et al, 2008;Jiang et al, 2008aJiang et al, , 2010Chen et al, 2012).…”

Dietary habits of juveniles of the Mayan cichlid, Cichlasoma urophthalmus, in mangrove ponds of an offshore islet in Belize, Central America

“…Recent use of carbon and nitrogen SIA revealed that only a few fish species (mangrove resident species and only a few transient species) actually forage in mangroves and that most fish species sheltering in this habitat undergo trophic migrations in nearby seagrass beds (Kieckbusch et al, 2004;Bouillon et al, 2008;Nagelkerken et al, 2008;Vaslet et al, 2011). Previous studies were conducted primarily in fringing or open mangrove ecosystems, but interior mangrove habitats (such as creeks or ponds) have received less scientific scrutiny.…”

“…Due to different photosynthetic pathways and inorganic carbon sources, terrestrial and aquatic macrophytes differ in their carbon signatures. Terrestrial mangrove trees using C3 photosynthetic pathway are more depleted in 13 C (on average between -30 and -24‰) compared to submerged seagrass plants (δ 13 C > -15‰) (Fry & Sherr, 1984;Bouillon et al, 2008). Therefore δ…”