1. 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. 2. 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. 3. We examined carbon and nitrogen stable isotope ratios (δ13C, δ15N) 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 δ15N, similar to marine POM, and thus we focus on the use of δ13C in tracking shrimp pond effluent in the environment. 4. δ13C signatures of SOM varied significantly along a land-to-ocean gradient (–29.1 to –23.9‰). We found consistently depleted mangrove SOM δ13C signatures (–29.4 to –28.2‰) indicating 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 δ13C values for marine SOM (–25.7 to –23.9‰) overlap with the δ13C of shrimp feed (–25.3‰) and Bayesian mixing models indicate that shrimp aquaculture feed and mangrove vegetation contribute the greatest OM to the marine ecosystem. 5. Compared to 20 years ago, marine SOM δ13C signatures are depleted by ~10‰ and similar throughout KKB, indicating a homogenization of marine SOM carbon sources from 1998 to 2018. 6. Synthesis and applications. The doubling of shrimp aquaculture in KKB since 1998 led to enhanced discharge to the bay, swamping OM contributions from 13C-enriched seagrasses and marine plankton. Because of this increase in effluent release to KKB, the marine ecosystem is likely being more chemically impacted 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.06-Jan-2020

Please see the paper Hargan et al. in Journal of Applied Ecology for a description of the collection methods and more details on sample preparation. 

Dried bulk sediment subsamples from the top 1 cm of surface sediment were sieved (to remove shells, rootlets, and stones), ground, and ~30.0 mg was weighed and folded into 9x10 mm tin capsules for %N and δ¹⁵N. Acidified sediment samples (~12.0 - 20.0 mg) were weighed into separate 9 x 5 mm tin capsules for %C and δ¹³C analysis. Approximately 5.0 mg of ground mangrove and seagrass leave sample was utilized for stable isotope analysis. The ¹³C:¹²C and ¹⁵N:¹⁴N ratios were analyzed by continuous-flow isotope-ratio mass spectrometry. The spectrometer (ThemoFisher Delta V Plus stable-isotope analyzer coupled with a Flash Elemental Analyzer) was operated in dual isotope mode at The David W. and Claire B. Oxtoby Environmental Isotope Lab (Pomona College, California).

Analytical precision was determined using Acetanilide for δ¹³C (–29.53‰) and δ¹⁵N (1.18‰) (Indiana University) and %C (41.86%) and %N (16.18%) (ThermoFisher Scientific). For both δ¹³C and δ¹⁵N, the standard deviation of acetanilide on 10 replicate samples was < 0.2‰. Samples were calibrated to the standards USGS40 and USGS24 for ¹³C, and USGS40 and IAEA N-2 for ¹⁵N. Results of stable isotope analyses are reported in δ notation where δ = [(R_sample/R_standard) – 1] x 1000. R_sample are the ratios of the isotopes (i.e. ¹³C/¹²C, ¹⁵N/¹⁴N) in samples, and R_standard are the ratios of isotopes in Vienna Pee Dee Belemnite for δ¹³C and atmospheric nitrogen for δ¹⁵N.