Assessing ecological impacts of shrimp and sewage effluent: Biological indicators with standard water quality analyses

Despite evidence linking shrimp farming to several cases of environmental d egradation, there remains a lack of ecologically meaningful information abo ut the impacts of effluent on receiving waters. The aim of this study was t o determine the biological impact of shrimp farm effluent, and to compare a nd distinguish its impacts from treated sewage effluent. Analyses included standard water quality/sediment parameters, as well as biological indicator s including tissue nitrogen (N) content, stable isotope ratio of nitrogen ( delta N-15) and amino acid composition of inhabitant seagrasses, mangroves and macroalgae. The study area consisted of two tidal creeks, one receiving effluent from a sewage treatment plant and the other from an intensive shr imp farm. The creeks discharged into the western side of Moreton Bay, a sub -tropical coastal embayment on the east coast of Australia. Characterizatio n of water quality revealed significant differences between the creeks, and with unimpacted eastern Moreton Bay. The sewage creek had higher concentra tions of dissolved nutrients (predominantly NO3-/NO2- and PO43-, compared t o NH4+ in the shrimp creek). In contrast, the shrimp creek was more turbid and had higher phytoplankton productivity. Beyond 750 m from the creek mout hs, water quality parameters were indistinguishable from eastern Moreton Ba y values. Biological indicators detected significant impacts up to 4 km bey ond the creek mouths (reference site). Elevated plant delta N-15 values ran ged from 10.4-19.6 parts per thousand at the site of sewage discharge to 2. 9-4.5 parts per thousand at the reference site. The free amino acid concent ration and composition of seagrass and macroalgae was used to distinguish b etween the uptake of sewage and shrimp derived N. Proline (seagrass) and se rine (macroalgae) were high in sewage impacted plants and glutamine (seagra ss) and alanine (macroalgae) were high in plants impacted by shrimp effluen t. The delta N-15 isotopic signatures and free amino acid composition of in habitant flora indicated that sewage N extended further from the creek mout hs than shrimp N. The combination of physical/chemical and biological indic ators used in this study was effective in distinguishing the composition an d subsequent impacts of aquaculture and sewage effluent on the receiving wa ters. (C) 2001 Academic Press.