Relative change in stream discharge from a tropical watershed improves predictions of fecal bacteria in near-shore environments

Statistical models poorly predict bacteria transport from tropical island watersheds to the near-shore due to inaccuracies in the characterization of rainfall, runoff and discharge regimes for tropical storms. Intense, short duration storm events on small, steeply sloped watersheds produce high rates of runoff and soil erosion, resulting in rapidly changing pulses of discharge to coastal regions that influence the physical and physiological conditions of the fate and transport of pathogens. As such, increasing rates of discharge are expected to have a different influence on sediment transport and pathogen load compared to decreasing rates of discharge. This paper uses regression modeling to examine how changing tides, runoff and the antecedent stream discharge affect the concentration of enterococci and <i>Clostridium perfringens</i>, and their response to changing environmental parameters near the mouth of Honoli‘i Stream on Hawai‘i Island over nine years. Relationships between discharge and water quality were stronger during ebb tides than flood tides and more pronounced during high flows than low flows. Including a parameter describing the relative change in discharge improved predictions of near-shore bacteria compared to models that only included discharge as it incorporates a proximate value of the energy available to transport particulates downstream. Understanding the factors that influence pathogen contamination of near-shore water bodies is critical for improving the management of watersheds and protecting public health.