Outwelling from arid mangrove systems is sustained by inwelling of seagrass productivity

Abstract

Mangrove forest productivity is normally sustained by nutrients from terrestrial runoff, with freshwater inputs driving the resulting outwelling of production, but arid mangroves lack this input. The movement of material between seagrass beds and mangroves was examined using the stable C and N isotopic composition of organisms, sediments and suspended matter in 3 seagrass-mangrove transects in the Arabian Gulf. The isotopic signal of suspended particulate material indicated a mixed origin that did not differ over a spring tide. Filter feeders showed significant 13C enrichment along transects from mangrove forests into seagrass beds, indicating that location within a habitat had a significant effect on isotopic composition. Similarly, 13C of both sediments and grazers increased sharply outside the mangrove forest, suggesting retention of mangrove carbon, although some outwelling was detected, the strength of which was site specific. The lack of freshwater-mediated nutrient inputs suggests any outwelling of mangrove ecosystem productivity must be balanced by inwelling, and isotopic signatures of both sediment grazers and filter feeders found within the mangrove forest confirmed the inwelling of seagrass production. Significant mangrove isotope signals in the tissue of juveniles of fishes commercially harvested offshore indicate ontogenetic movement of carbon. Additional biological movement of mangrove carbon through ontogenetic migration and 'trophic relay' is evidenced by the isotopic signature of juvenile and mature fish captured in waters exiting the mangrove forest, which indicated they fed on mangrove-sustained food webs. This study demonstrates tight coupling between arid mangroves and subtidal seagrass areas and implies that arid mangroves cannot be managed or replanted without consideration of connectivity to downstream systems such as seagrasses.