Shrimp farming in earthen pond is one of the largest seafood growing industries among coastal mariculture, but it has faced environmental issues in recent years, e.g. overloading of supplementary feed cause deterioration of pond environments. In order to assess the impact of pond shrimp farming on the benthic environment, we investigated the changes in environmental variables and meiobenthic community structure during January to August in the earthen ponds rearing Marsupenaeus japonicus and Litopenaeus vannamei. Water and sediment samples were taken in January (before aquaculture), May (early stage) and August (late stage). We found sedimentary redox potential showed values below −150 mV in all ponds in the early and late rearing stages, indicating overall reduced and anoxic conditions. Sedimentary bulk organic matter (total nitrogen and organic carbon) accumulated along with the feeding addition during the rearing period. We found meiobenthic community structure to change over different rearing stages mainly because of an increase in nematode densities that was associated with shrimp farming activity in the studied ponds. We also investigated the functional roles of pond biota and supplementary feed by studying the benthic food web in these ponds by means of natural stable isotope ratios of carbon and nitrogen and fatty acid biomarkers in the early (May) and late (August) rearing stages. Pond biota (i.e. microalgae and meiobenthos) contributed abundantly to the diets of the two shrimp species. In the early stage, meiobenthos shared similar diets with shrimps (with both feeding on primary organic sources) and they could thus potentially compete for resources. At late rearing stages, meiobenthos represented however a functional link between primary producers and shrimps, serving as nutritional food sources for shrimp and providing them high amounts of polyunsaturated fatty acids (PUFAs). We found that the supplementary feed was not substantially consumed by the two shrimp species, which might pose an overfeeding threat to deteriorate the pond environment. |
