Regulatory role of sesarmid crabs in nutrient dynamics and implications for the productivity of mangroves

Abstract

The ecological role of brachyuran crabs in mangroves has been extensively studied, but their role as mediators of biogeochemical processes of essential nutrients remains unclear. We hypothesized that speciose sesarmid crabs in tropical mangroves can improve nutrient transfer rate (NTR) and enhance primary productivity by reducing nutrient recycling bottlenecks. Using stable isotope labels and tidal mesocosms simulating the mangrove environment (with and without crabs), we traced the transfer of carbon (C) and nitrogen (N) across the system (from mangrove leaf litter to mangrove seedlings, the microphytobenthos (MPB), and sediment). We compared NTR between the two treatments by measuring the 13C and 15N enrichment levels (as a proxy for NTR) of mangrove seedlings, MPB, and sediment. The significantly higher enrichment level of sediment, MPB and seedlings (for N) in mesocosms with crabs indicated higher NTR driven by these animals. Such increased NTR also led to higher primary productivity, supporting our hypothesis. A consumer-driven nutrient dynamics (CND) framework was established to provide a comprehensive understanding of the role of sesarmid crabs in mediating biogeochemical processes. It is clear that the reciprocal nutrient flow between MPB and sesarmid crabs, and the biogeochemical hotspots created in surface sediment underpin this CND effect. A system picture emerges whereby nutrient flow is strongly regulated by sesarmid crabs promoting the productivity of mangroves by (1) enhancing nutrient remineralization; (2) retaining nutrients; (3) creating biogeochemical hotspots; and (4) improving overall nutrient transfer rate.