Understanding the fate of shrimp aquaculture effluent in a mangrove ecosystem: Aiding management for coastal conservation

Areas dedicated to shrimp aquaculture have increased dramatically over the last 50 years. Resultant land-use changes directly threaten the extent of mangroves and yield conflicts on the discharge location of aquaculture effluent. Khung Krabaen Bay (KBB), Thailand, is reforesting mangroves while increasing the efficiency of shrimp aquaculture for local farmers. In this coupled shrimp farm-mangrove system, effective management requires understanding the fate of aquaculture organic matter (OM) in the coastal environment. We examined carbon and nitrogen stable isotope ratios (delta C-13, delta N-15) 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. Here, soy-based shrimp feed resulted in low shrimp delta N-15, similar to marine POM, and thus we focus on the use of delta C-13 in tracking shrimp pond effluent in the environment. delta C-13 signatures of SOM varied significantly along a land-to-ocean gradient (-29.1 parts per thousand to -23.9 parts per thousand). We found consistently depleted mangrove SOM delta C-13 signatures (-29.4 parts per thousand to -28.2 parts per thousand) 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. In contrast, relatively low delta C-13 values for marine SOM (-25.7 parts per thousand to -23.9 parts per thousand) overlap with the delta C-13 of shrimp feed (-25.3 parts per thousand) and Bayesian mixing models indicate that shrimp aquaculture feed and mangrove vegetation contribute the greatest OM to the marine ecosystem. Compared to 20 years ago, marine SOM delta C-13 signatures are depleted by 10 parts per thousand and similar throughout KKB, indicating a homogenization of marine SOM carbon sources from 1998 to 2018. Synthesis and applications. The doubling of shrimp aquaculture in Khung Krabaen Bay (KKB) since 1998 led to increased discharge to the bay, swamping organic matter (OM) contributions from C-13-enriched seagrasses and marine plankton. Because of this increase in effluent release to KKB, the chemical impact is likely to be greater for the marine ecosystem than the mangrove and should also be a focus of conservation efforts. Continued technological improvements (e.g. closed systems, better feed efficiency) and support to local aquaculture farmers will help reduce OM discharge to coastal ecosystems and promote sustainable farming practices.