Source tracking and transport of microbial pollution in estuarine oyster farms

Abstract

Faecal contamination in commercial shellfish growing waters can affect seafood safety and the economy of an important coastal industry in New Zealand. Oysters are filter-feeders and can accumulate human enteric pathogens from faecally polluted growing waters and pose a risk to human health when consumed raw. Faecal pollution can enter oyster growing areas from a number of sources, including sewage discharge, septic tank seepage, boat waste discharge, and agricultural runoff. This study investigated the state and sources of faecal pollution in the Te Kapa River, a commercial oyster growing area in northern New Zealand. A sanitary survey was a useful first step for identifying possible microbial contamination sources in a watershed, and to obtain baseline data for the design of monitoring programmes to evaluate water and shellfish quality. The sanitary survey of the study site identified agricultural runoff, septic tank seepage and the Mahurangi River to be the major potential pollution sources. A temporal and spatial analysis ofmicrobial water quality in the growing area showed higher levels of faecal pollution indicators at the northern end (near the stream discharge site) and around the edges of the growing area, implicating agricultural runoff (diffuse and via the streams) as the main pollution source. Sediment was a sink and potential source of microbial pollution. The Mahurangi River was not a likely source of faecal pollution to the Te Kapa River. Environmental factors such as tide and climate (rain and wind) played an important role in the transport and fate of microbial pollution in the watershed. Tidal flow patterns appeared complex within the oyster growing area and resulted in large spatial differences in contamination potential within the oyster farm. This highlighted the necessity for careful planning of pollution monitoring sites throughout a growing area. Mass spectrometry of indicator bacteria proteins showed seabirds were an immediate source of faecal pollution. Genotyping of F+-specific RNA bacteriophages and randomly amplified polymorphic DNA (RAPD) profiles of E. coli isolated from oysters and their growing waters indicated that agricultural runoff was source of microbial contamination in this watershed most likely to be associated with pathogens. None of these techniques provided a stand-alone microbial source tracking method but each provided a useful contribution to the picture as a whole, and has application in the development of other source tracking techniques, such as pollution source-specific probes. These results show that the assessment of faecal pollution levels and identification of pollution sources in shellfish aquaculture areas in New Zealand would enhance the ability to manage these watersheds so safe seafood can be harvested for the protection of human health and the economy of an important coastal industry.