LAUSR

Abstract Rhizophora mangle is a mangrove forest foundation species that can create vegetated habitat, support entire animal and plant communities, and modulate ecosystem processes. Changes in freshwater subsidies over space and time may alter the performance of mangrove foundation species, presumably resulting in disproportionally large effects on energy flow. Yet the linkages between above-ground biomass and below-ground carbon reservoirs across freshwater-subsidy gradients remain poorly understood in many regions. We assessed the dynamics of litterfall production and breakdown of leaf material in a monospecific Rhizophora mangle forest thriving in a small bay in the Atrato River Delta (Southern Caribbean coast of Colombia), during a major El Nino to La Nina transition (from July 2015 to October 2016). Although parameters varied at different spatial scales within the bay, the mean annual total litter production (55 ± 15 g m−2 day−1 or 20 ± 5 Mg ha−1 yr−1) and the mean mass loss of leaf litter (1.0% and 1.7% day−1, above- and below-ground, respectively) are the highest reported for R. mangle-dominated forests worldwide. Total litter production and mass loss rates did not vary significantly over time. The lack of temporal effect was counterintuitive, because these rates typically show strong seasonal variation in mangroves elsewhere in the tropics. We concluded that mangroves in this area constitute a primary productivity hotspot (leaf fall carbon spatio-temporal range: 0.81–1.30 g C m−2 d−1). Based on a comparison of our results and data from the literature, we hypothesize that high freshwater input from the Atrato River subsidizes mangrove plants settled on the deltaic front, which sustain pervasively high litterfall rates. The high mass loss rates of leaf litter observed in this study demonstrate the importance of carbon vertical fluxes across the forest floor in fringing mangroves for carbon sinking.