Enterococcal numbers measured in waters of marine, lake, and river swimming sites of the Bay of Plenty, New Zealand

Abstract

Listen

Translate article

In the Bay of Plenty, New Zealand, 32 coastal beach sites, 23 lake beach sites, and 31 river swimming hole sites were sampled for enterococci as indicators of bathing suitability over 3 summers (from 1991 to 1995). The median enterococcal numbers of 5 samples collected from each site over each season were compared to the guideline medians: 35 and 33 enterococci per 100 ml for marine and fresh waters, respectively. The “5‐sample median” values generally agreed with longer‐term medians (n = 24, 28) from river and estuary sites sampled between 1990 and 1994. During the summers of 1991 and 1993, high enterococcal numbers at marine and lake swimming sites were assumed to be derived primarily from sewage effluent and farm run‐off, either directly or from freshwater inflows. Enterococcal numbers dropped significantly in these waters when effluents were either treated or removed from the waters. In 1995, all marine bathing waters complied with the guideline value of 35 enterococci per 100 ml. Evidence from long‐term monitoring of estuary sites indicated that most marine bathing sites have high‐quality bathing water with short‐term variability caused by winds, tides, and outfall discharges. In 1995, all but one lake site complied with the freshwater median guideline of 33 enterococci per 100 ml. Most upstream river sites also complied with the freshwater guideline. However, river sites in the mid reaches and down stream recorded high enterococcal numbers which increased slightly at some sites from 1992 to 1995. The most significant increase in bacterial numbers was recorded in the small river and stream swimming holes which carry low volumes of water (< 1000 1 s−1), particularly in the western area of Lake Rotorua. The main source of enterococci in river waters was most likely run‐off from dairy farms on the flood plains of the various rivers of the Bay of Plenty.