An Assessment of the Ecological Impacts from Low Intensity Shrimp Farm Wastewater in the Bang Pakong River Region of Thailand Using Stable Isotope Analysis

Abstract

Eutrophication of receiving water is often cited as a significant environmental impact of shrimp aquaculture. However, it is difficult to trace sporadic nutrient inputs from harvests and to separate effects of aquaculture effluent from other nutrient sources. This study attempted to trace impacts of shrimp wastewater in Thailand, where shrimp farming dominates much of the coastal landscape and economy. Stable isotopes of N and C, C:N ratios, and water quality analysis were used to evaluate the impact of shrimp farm effluent from small scale farms using semi intensive management with minimal treatment of discharged water. Farms discharge into the Chucachur Canal, a tributary to the Bang Pakong River in the Northern Gulf of Thailand. There were exceptionally high levels of nutrients in the canal, stemming both from shrimp aquaculture and from other agricultural sources. TP exceeded Thai standards for all sampling points and days; NH3 exceeded Thai standards near a pig farm source on all days, as well as in a plume of shrimp wastewater on a day when a farm was harvested. Multiple linear regression on stable isotope data indicated that insolation was the most influential variable determining δ15N and δ13C in plants in the canal. This analysis also indicated that levels of NO2 and NH3 contributed to the variation in isotope ratios measured in periphyton and hyacinth, respectively. Overall, δ15N isotope ratios indicated that although nitrogen inputs did explain some isotopic concentrations in plants, isotope ratios were driven more by physical processes like insolation and available oxygen than by inputs of particular δ15N signatures from shrimp farms. My results indicate that although it is likely that shrimp farming on the canal does impact the environment, other factors significantly influence it as well, with other anthropogenic inputs such as pig farming having a more significant and continual impact than temporal pulses of nutrients from pond harvests.