Modelling storm-eventE.colipulses from the Motueka and Sherry Rivers in the South Island, New Zealand

Abstract

Storm-induced Escherichia coli pulses in the Motueka River (2074 km 2 ) and the Sherry River (78.4 km 2 ) are modelled. The model focuses on the catchment outlets, representing key processes, including E. coli transfer to and from the river bed, with account taken of the hysteresis in, and non-linear, non-stationary, response of E. coli concentrations to river stormflows. The model fits the Motueka River observations well, but less well in the Sherry River. A greatly simplified description of headwater and riparian inputs is satisfactory at the larger catchment scale where near-field, in-channel processes dominate the response. Spatial heterogeneity in rainfall-run-off and faecal sources probably contribute to the poorer fit in the smaller catchment. Despite using a relatively small number of driving variables and parameters, the model has the potential to predict real-time E. coli input to Tasman Bay in river plumes causing shellfish and bathing beach contamination.

Motueka River plume facilitates transport of ruminant faecal contaminants into shellfish growing waters, Tasman Bay, New Zealand

“…This is largely because the AMAs lie within a well flushed embayment (Tuckey et al 2006) and are located at least 6 km away from river inputs. In addition, dry periods would be accompanied by high levels of solar radiation, which is the primary factor regulating mortality of faecal microbes (Gameson & Gould 1975;Wilkinson et al 2011). Nonetheless, the fact that faecal contaminants from the land can be tracked into mussels within the region's largest AMA during moderate flooding highlights a need to understand the mechanisms and environmental conditions that lead to a greater likelihood of faecal contamination and subsequently higher risk of pathogens within shellfish.…”

“…Nonetheless, the fact that faecal contaminants from the land can be tracked into mussels within the region's largest AMA during moderate flooding highlights a need to understand the mechanisms and environmental conditions that lead to a greater likelihood of faecal contamination and subsequently higher risk of pathogens within shellfish. Further surveys combining analyses of FIB and MST markers will also assist in determining whether successive floods result in a decline in faecal contamination in the AMAs because of flushing of the landscape and river stores of bacteria (Muirhead et al 2004;McKergow & Davies-Colley 2010;Svejkovsky et al 2010;Wilkinson et al 2011).…”

“…This marker, which targets a different region of 16s RNA gene of Bacteroidales, was detected in all mussel samples tested, providing further confirmation of ruminant animals as a source of contamination. Wilkinson et al (2011) predicted that a large portion of faecal contaminants can be transported a distance of 50 km downstream in the Motueka River. The detection of FIB of ruminant origin in the mussels offshore indicates that at least a portion of the faecal contamination is derived from the middle to upper reaches of the catchment and therefore originates at least 50 km upstream.…”

“…Accurate prediction of faecal contaminant loading for the Motueka River is possible (McKergow & Davies-Colley 2010;Wilkinson et al 2011); however, forecasting the concentration and fate of faecal contaminants in coastal waters is more difficult. Upon exiting the confines of the river, the discharged freshwater forms an expansive buoyant plume that mixes with underlying seawater and is susceptible to movement associated with currents, which will be largely influenced by wind and tides (Warrick et al 2007;Svejkovsky et al 2010).…”

“…It is likely that other types of wildlife and domestic animals also contributed to faecal contamination; however, the very high concentrations of enterococci and E. coli coupled with the strong signal of the ruminant Bacteroidales (RBac) marker in both Sherry and Motueka River samples, and current land use patterns indicate that pastoral farming (dairy, beef and sheep) is likely the main source of contamination. The Sherry River catchment accounts for only 3.8% of the total Motueka River catchment area, and (after accounting for die-off during transport) contributes an estimated 8% of the total bacteria loads measured downstream at the Woodmans Bend sampling location (Wilkinson et al 2011). Pastoral farming is also carried out in lower reaches of the catchment and it is likely that these areas contribute a larger portion of the total faecal contamination transported into Tasman Bay.…”