Spatial and temporal variability in organic carbon sources and storage in an oceanic mangrove system adjacent to shrimp aquaculture

Abstract:

Areas dedicated to shrimp aquaculture have increased dramatically over the last 50 years. In the coupled shrimp farm–mangrove system of Khung Krabaen Bay (KBB), Thailand, effective management requires understanding the fate of aquaculture organic matter (OM) in the coastal environment and the impact of aquaculture effluent on the C storage potential of mangroves. Our study had two broad aims: (1) to use carbon stable isotope ratios (δ¹³C) in primary producers and pools of particulate and sediment OM (POM, SOM) from the KKB mangrove and marine ecosystem to determine how shrimp aquaculture OM contributes to the coastal environment; and (2) to examine how mangrove organic C (OC) storage changes spatially along the land-to-ocean ecotone and whether ~30 years of shrimp aquaculture adjacent to the mangrove have altered the long-term OM composition and C storage in mangrove sediments. We found consistently depleted mangrove SOM δ¹³C signatures (–29.4‰ to –28.2‰) indicating that mangrove leaf litter is the primary source of OM to mangrove sediments, and there is little evidence that marine and shrimp pond OM contributes to the mangrove habitat. Stable SOM δ¹³C signatures in sediment cores over the last ~30-50 years indicate that the dominant source of OM to the mangrove forest C pool has not changed. In contrast, relatively low δ¹³C values for marine SOM (–25.7‰ to –23.9‰) overlap with the δ¹³C of shrimp feed (–25.3‰) and we found an increase in the contribution of shrimp aquaculture feed and mangrove vegetation OM to the marine ecosystem over the past ~20 years. Across the mangrove ecotone, we captured low C storage (27.0-63.0 Mg C/ha for core depths of 24-35 cm) in the mangrove fringing land, intermediate C storage (29.1-108.9 Mg C/ha for core depths of 20-35 cm) in the interior mangrove, and periods of high C pulses to the ocean fringe mangrove which resulted in greater C storage (162.4 Mg C/ha for 50 cm core length). These high C pulses to mangroves adjacent to the ocean may indicate that flooding of the bay with aquaculture effluent causes irregularity in OC sedimentation, but overall increases OC storage. Presently, it appears that OM released in shrimp aquaculture is having a larger impact on the marine environment and the immediately adjacent ocean-fringing mangroves.