Trace element cycling in Bangladesh shrimp aquaculture systems: Implications for food and water security
Dietrich, Matthew John
Limited research has been completed thus far involving trace element cycling between environmental media in estuary environments, which is important to quantify to better understand potential impacts of pollution. In Southwest Bangladesh, aquaculture shrimp pond water composition is predominantly related to irrigation source (i.e., tidal channel water), although there are slight endogenous effects on composition as seen through shrimp pond dissolved organic carbon (DOC) being particularly sensitive to evaporation, with oxygen (O) and hydrogen (H) isotope compositions strongly positively correlated with DOC. Monsoon rains in July dilute tidal channel water more than most shrimp ponds due to restricted circulation. Shrimp ponds contained arsenic (As) and selenium (Se) concentrations above drinking water guidelines in May and July. The Se is likely anthropogenic from upstream rivers, while most As is likely geogenic in origin. Trace elements in the tidal channels that irrigate the shrimp ponds react differently with increased mixing of seawater from the Bay of Bengal (BoB). With increasing proximity to the BoB, elements such as barium (Ba) and Se decrease in dissolved concentration due to conservative mixing with seawater, while nickel (Ni) and cobalt (Co) increase in dissolved concentration by desorbing from suspended sediment due to saltwater ion exchange. Aquaculture shrimp do not contain elevated Se or As concentrations, and their elemental concentrations in general are not correlated with dissolved or sediment element concentrations. Thus, shrimp element concentrations are likely more related to biological processes and their feed, which may explain the elevated levels of toxic thallium (Tl) found in both shrimp and feed. Aquaculture shrimp and feed should be continuously monitored because of health risk from Tl. Kd(sediment/water) values are similar for shrimp ponds and 24-hr batch extract experiments, although Kd(sediment/extract) values are substantially higher for soluble cations such as strontium (Sr), sodium (Na), and magnesium (Mg), indicative of disequilibrium exchange. This disequilibrium is further supported by lack of correlations between pond sediment, pond water, and 24-hr extract element concentrations, which suggests that release of trace elements from sediment is not a major control on concentrations in the dissolved phase.