The Impact of Tides on Microbial Water Quality at an Inland River Beach

Abstract

Most coastal freshwater ecosystems in the United States have semi‐tidal movements during the day. Routine monitoring of these environments is conducted once during the day when tides can be at either ebb or flood conditions, causing a variability in bacterial concentrations and misinterpretation of the illness risk associated with human activities. The occurrence and levels of enterococci (enterococci 23S rDNA [Ent23S]) and human‐ (HF183) and avian‐ (GFD) associated microbial source tracking (MST) markers were investigated using quantitative polymerase chain reaction (qPCR) along with detection of culturable enterococci and environmental parameters. Samples were collected during flood and ebb tide conditions (May–September) from a tidal river used for recreational activities. Culturable enterococci [t(420) = 2.093, p = 0.040] and Ent23S [t(420) = 2.243, p = 0.028] controlled for tide type were significantly different; higher enterococci concentrations were detected during the flood tide. Among all samples, 6% were positive for HF183, and GFD was positively correlated with Ent23S (r = 0.92, p = 0.029) and conductivity (r = 0.93, p = 0.023) during flood tide. Unlike the general assumption that ebb tide flow in a river would likely carry runoff from the land, the microbial contaminants in this case were transported from upstream via ocean water to the river during the flood tide. These results suggest that hydrology and land use patterns must be considered in sampling design when conducting future microbial water quality monitoring programs to better characterize recreational water safety in tidal rivers. Core Ideas Ocean water was carrying high enterococci into the river during flood tide. Low concentrations of HF183 occurred in 6% of the samples only during flood tide. Avian‐associated pollution was detected in 40% of the samples during both tides.

Microbial Water Quality: Monitoring and Modeling

“…Hydrological conditions can radically change microbial water quality metrics. This is well traced by the work of Aslan et al (2018) for tidal systems, where routine monitoring is conducted once during the day while tides can be at either ebb or flood conditions, causing a variability in bacterial concentrations and misinterpretation of the illness risk associated with human activities. The occurrence and levels of enterococci and human and avian molecular markers were determined in samples collected during flood‐ and ebb tide conditions (May–September) from a tidal river used for recreational activities.…”

Assessing Water Quality Dynamics in Tidally Influenced Blackwater Rivers along a Rural–Urban Gradient

“…The calculated flushing times from both studies in the region support the observations that the Urban and Inlet sites experience tidal flushing that can diminish, or remove, discrete effects of urbanization and storm events. For example, tidal fluctuations, or lack of, can significantly dilute microbiological concentrations [56,68]. A study of E. coli distribution in this region indicated spring tides flush out pollutants, whereas the neap tide increased residence time [56].…”