engleski

The effect of precipitation on the bathing water quality at the area under anthropogenic impact

Monitoring of coastal bathing water quality is necessary to protect human health and to preserve, protect and improve the quality of the marine environment. The concentration of the indicator bacteria used to assess the quality of bathing water, Escherichia coli and intestinal enterococci, also known as Faecal Indicator Bacteria or FIB, is fluctuating during the bathing season. It is mostly affected by the type and number of the sources of faecal contamination, eventual short-term pollution events, and environmental variables such as temperature, pH, salinity, solar radiation, and precipitation. The main goal of this study is to examine whether precipitation is affecting the bathing water quality in the urban and suburban areas, which are exposed to anthropogenic impact. Additionally, we tried to examine whether an opportunistic pathogen bacteria, Staphylococcus aureus could be used as an additional parameter of microbiological pollution. To assess the effect of precipitation on the bathing water quality we monitored the concentration of E. coli and intestinal enterococci in bathing seawater. For the enumeration of E. coli, we used the ISO 9308-1:2014 method, modified by changing the temperature of incubation to enhance selectivity without negative effect to recovery, while the ISO 7899-2:2000 method was used for the enumeration of intestinal enterococci. We also measured physical-chemical parameters that can be related to microbiological contamination (water temperature, salinity, pH) by using a YSI CTD+pH probe. Additionally, we determined the concentration of S. aureus by using CHROMagar™ Staph aureus medium incubated at 37°C for 48h. Water samples were collected at 11 sites, 10 of them located in the Kaštela Bay and Split area, and one site (Stipan Jaz) located outside of the mentioned areas, which served as a control. The sampling campaigns were organized between May and October 2020, twice a month, and additionally 24h, 48h, and 96h after the precipitation. We analysed 198 water samples in total. We recorded four rain events during the bathing season, with the precipitation level varying between 4mm and 19mm. In six standard monitoring campaigns, the precipitation was absent. Bathing water quality at three sites located in the Kaštela Bay (Trogir, Kaštel Sućurac, and Vranjic) was classified as poor after the end of the sampling campaign. Trogir site was classified as poor because of the high E. coli concentrations, unlike Kaštel Sućurac and Vranjic sites where the concentration of intestinal enterococci was above the limit for poor quality. The elevated concentrations of S. aureus were recorded at all sites situated in Kaštela Bay, with the highest values in Kaštel Sućurac and Vranjic sites. E. coli and intestinal enterococci correlated significantly with S. aureus (0.52 and 0.51 respectively, p<0.001) at all sampling sites. We found a significant negative correlation with salinity for all monitored microorganisms (E. coli -0.37, intestinal enterococci -0.46, S. aureus -0.45, p<0.001). The intestinal enterococci showed a weak but significant positive correlation (0.24, p<0.001) with the temperature at all sampling sites. No statistically significant correlation was found between the monitored bacteria and precipitation. These results suggest that small levels of precipitation do not have a significant effect on the quality of bathing water in the studied sites. The moderate positive correlation with FIB and negative with salinity indicated that S. aureus is probably originating from the same sources as FIB. Additionally, the results suggest that S. aureus could be considered for use as a relevant indicator of microbiological pollution in the areas under the anthropogenic impact. The guideline values based on the relationship between S. aureus levels in bathing water and the health risk of staphylococcal infections are yet to be determined.

bathing water quality, Escherichia coli, intestinal enterococci, Staphylococcus aureus, precipitation, Kaštela Bay

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