Abstract
The outwelling paradigm argues that mangrove and saltmarsh wetlands export much excess production to downstream marine systems. However, outwelling is difficult to quantify and currently 40–50% of fixed carbon is unaccounted for. Some carbon is thought outwelled through mobile fauna, including fish, which visit and feed on mangrove produce during tidal inundation or early life stages before moving offshore, yet this pathway for carbon outwelling has never been quantified. We studied faunal carbon outwelling in three arid mangroves, where sharp isotopic gradients across the boundary between mangroves and down-stream systems permitted spatial differentiation of source of carbon in animal tissue. Stable isotope analysis (C, N, S) revealed 22–56% of the tissue of tidally migrating fauna was mangrove derived. Estimated consumption rates showed that 1.4% (38 kg C ha−1 yr−1) of annual mangrove litter production was directly consumed by migratory fauna, with <1% potentially exported. We predict that the amount of faunally-outwelled carbon is likely to be highly correlated with biomass of migratory fauna. While this may vary globally, the measured migratory fauna biomass in these arid mangroves was within the range of observations for mangroves across diverse biogeographic ranges and environmental settings. Hence, this study provides a generalized prediction of the relatively weak contribution of faunal migration to carbon outwelling from mangroves and the current proposition, that the unaccounted-for 40–50% of mangrove C is exported as dissolved inorganic carbon, remains plausible.
Highlights
The unquantified role of migratory fauna in the fate of carbon, means that mangrove carbon budgets are unresolved
The study indicates that only small amounts (1.4%) of mangrove leaf litter produced is consumed and only 0.8% outwelled by fauna via two modes: through the excretion and respiration of the ingested mangrove matter by migratory fauna during low water periods when the mangroves are dry; and through the mortality of migratory fauna
We suggest arid mangroves are ideal sites for studying the faunal movement of carbon due to the lack of rainfall driven outwash of mangrove carbon that increases the certainty that the mangrove carbon in the tissues of mobile fauna originated from feeding within the mangrove forest
Summary
The unquantified role of migratory fauna in the fate of carbon, means that mangrove carbon budgets are unresolved. Mangrove forests achieve a steady state once the forest reaches maximum biomass at around 20–30 years through a constant process of mortality and renewal (Lugo, 1980) so, assuming the living biomass is not becoming more carbon dense, carbon has to be lost at a rate equal to the amount of carbon fixed as NPP This productivity is either retained within the mangrove forest, as a standing stock of live material such as wood, buried in sediments, or exported to neighbouring habitats as litter, particulate and dissolved organic carbon (POC and DOC) and dissolved inorganic carbon (DIC), or lost to the atmosphere (Bouillon et al, 2008a; Maher et al, 2013; Alongi, 2014). More recent assessments of DIC export at two sites in Australia (Maher et al, 2013; Santos et al, 2019) supported the estimates of Bouillon et al (2008a), Alongi (2014) suggested that only 40% of NPP was exported as DIC
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