ReviewOrganic carbon dynamics in mangrove ecosystems: A review
Introduction
Mangrove forests are known to be highly productive ecosystems with the capacity to efficiently trap suspended material from the water column. Litter from trees (leaves, propagules and twigs) and subsurface root growth provide significant inputs of organic carbon to mangrove sediments (Alongi, 1998). Litterfall is likely to represent about one third of the net primary production (Alongi et al., 2005a). A range of other sources may also provide important organic carbon inputs; including allochthonous riverine or marine material (e.g., seagrasses), autochthonous production by benthic or epiphytic micro- or macroalgae, and local water column production by phytoplankton (Bouillon et al., 2004). As a consequence, mangrove environments are sites of intense carbon processing with a potentially high impact to the global carbon budget (Borges et al., 2003, Dittmar et al., 2006, Alongi, 2007).
Mangrove-derived detritus is an important food source for decomposer food webs including many macroinvertebrates, such as sesarmid crabs (Grapsidae) that are notable in their ability to consume mangrove litter (Fratini et al., 2000, Cannicci et al., 2008). The more moderate, but in many cases considerable input of local or imported algal detritus is consumed by other animal species such as fiddler crabs (Ocypodidae) and various gastropods (Bouillon et al., 2002, Kristensen and Alongi, 2006). Irrespective of the pathways of organic matter consumption and food web structure, all organic matter that is not exported by tidal action enters the sediment where it is consumed, degraded and chemically modified. The degradation of organic matter in mangrove sediments is mediated by both aerobic and anaerobic microbial processes using a variety of electron acceptors. A fraction of mangrove detritus escapes degradation and is permanently buried within the mangrove sediments or adjacent ecosystems. While some mangrove forests largely retain detritus within their sediments (i.e. as degradation or burial), others lose a major fraction of their net primary production to adjacent coastal waters mainly through tidal forcing. Because of the regular tidal flooding and draining in many mangrove forests, the material exchange with adjacent waters can be very efficient.
In this contribution, we review and evaluate the current knowledge on organic carbon dynamics in mangrove ecosystems and its impact on other ecosystems. Fig. 1 provides an overview of the major pathways and pools of carbon associated with leaf litter and algal detritus in mangrove environments. We will first discuss the relative importance of various sources to the total ecosystem organic carbon balance and describe the chemical composition of mangrove tissues at the molecular level. Subsequently, we will discuss the function of food webs, including litter grazing invertebrates and microbial decomposers with emphasis on the behavior of organic carbon in mangrove sediments during early diagenesis, and the efficiency of permanent burial as a fate of mangrove production. Finally, we emphasize the role of outwelling and dispersal of mangrove derived organic matter that escapes decomposition for carbon dynamics in adjacent environments.
Section snippets
Mangrove ecosystem productivity
The most widely used proxy of mangrove productivity is annual litter fall, which is known to show a latitudinal gradient, being highest close to the equator (e.g., Twilley et al., 1992). Typical global average litterfall rates are in the order of ∼38 mol C m−2 year−1 (Twilley et al., 1992, Jennerjahn and Ittekkot, 2002). It must be stressed, however, that this underestimates the total net CO2 fixation by mangroves, since it does not incorporate the wood and belowground biomass production (Middleton
Chemical composition of mangrove litter
Mangrove tissues (in particular Rhizophora leaves) have been characterized in various studies that focused on: (i) the nutritional quality of mangrove-derived organic matter, (ii) the specificity of certain biomarkers to trace mangroves in paleoenvironmental reconstructions, or (iii) the organic fluxes delivered to the oceans. Numerous compound classes have been identified, including carbohydrates, amino acids, lignin-derived phenols, tannins, fatty acids, triterpenoids and n-alkanes, and up to
Mangrove foodwebs and the role of fauna in organic carbon processing
Mangrove forests are recognized as an important habitat for fauna, harboring often abundant and diverse benthic invertebrate communities (Sasekumar, 1974, Wells, 1984, Nagelkerken et al., 2008). These may further serve as important food sources for transient fauna (e.g., Sheaves and Molony, 2000) and a number of species are commercially important and are harvested for food consumption (Rönnbäck, 1999, Rönnbäck et al., 2003, Walters et al., 2008). The exact role of mangrove ecosystems in
Early stages of decomposition
Irrespective of the pathways and food web structure involved, all mangrove organic matter that is not exported by tidal action enters the sediment and is degraded or chemically modified by microorganisms. The decay of deposited mangrove litter begins with significant leaching of soluble organic substances. Newly-fallen mangrove litter loses 20–40% of the organic carbon by leaching when submerged in seawater for 10–14 days (Camilleri and Ribi, 1986, Twilley et al., 1997). The carbohydrates that
Burial and permanent storage of organic carbon in sediments
Mangrove ecosystems are able to store large amounts of organic carbon (Matsui, 1998, Fujimoto et al., 1999) and in some mangrove ecosystems, organic-rich sediments of several meters depth have been found (Twilley et al., 1992, Lallier-Verges et al., 1998). The formation of old and refractory material in mangrove sediments can be observed visually as lignified and humified (spongy) litter fragments. Accordingly, Dittmar and Lara (2001b) estimated that the average age of organic carbon in the
Outwelling and dispersal of mangrove organic matter
About four decades ago, Odum (1968) proposed a groundbreaking hypothesis in coastal ecology according to which the outwelling of litter from coastal wetlands is a major source of energy that supports much of the secondary production of estuaries and nearshore waters. Because of the regular tidal flooding and draining in most mangrove areas, the material exchange between the forests and coastal waters can be very efficient (e.g. Dittmar and Lara, 2001a). Many of the most productive mangrove
Perspectives and research directions
Over the past two decades, a large number of case studies have significantly increased our knowledge on carbon dynamics in mangrove systems and on the importance of various biogeochemical processes. We still lack, however, a complete understanding of the underlying mechanisms controlling the spatial and temporal variability of these processes as a function of changes in environmental conditions. Vegetation type, faunal composition, microbial processes and sediment structure changes along tidal
Acknowledgements
Financial support was provided by the Danish Science Research Council (Grant# 21020463), the Research Foundation Flanders (FWO-Vlaanderen) and the EU-project PUMPSEA (FP6 – INCO contract no. 510863). This is publication 4225 of the Netherlands Institute of Ecology (NIOO-KNAW).
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All authors have contributed equally to this publication.