Rainfall and Streamflow Effects on Estuarine Staphylococcus aureus and Fecal Indicator Bacteria Concentrations

Abstract

Pathogenic bacteria in nearshore waters are a public health threat, and many have watershed sources. Hence, understanding direct and indirect causes of bacterial loading can improve awareness and watershed management. Rainfall-driven runoff influences river discharge, affecting pathogen transport to the ocean. This study assessed pathogen loading to nearshore waters under varying weather conditions within Hilo Bay, Hawaii, from 2014 to 2017. Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and fecal indicator bacteria (FIB) were quantified in the bay, rivers, and potential watershed sources using culturebased methods. Relationships between their concentrations with rainfall, river discharge, and water quality data were examined. Staphylococcus aureus, MRSA, and FIB were present within Hilo Bay and its rivers, as well as road runoff, sewage, and soils; MRSA was less prevalent. Staphylococcus aureus and FIB concentrations increased with rainfall and river discharge. Turbidity and salinity were the best water quality parameters for predicting bacteria concentrations, with positive and negative relationships, respectively. Our results suggest that more intense storms, especially after longer dry periods between events, will increase S. aureus and FIB loads to nearshore waters, as storms comprise >80% of annual river loads. Our models can be used to assess recreational water users’ health risks and predict future water quality conditions with changing rainfall patterns. Rainfall and Streamflow Effects on Estuarine Staphylococcus aureus and Fecal Indicator Bacteria Concentrations Louise M. Economy, Tracy N. Wiegner,* Ayron M. Strauch, Jonathan D. Awaya, and Tyler Gerken Pathogenic bacteria in nearshore waters are a public health threat, with many originating from vertebrate fecal waste (Rochelle-Newall et al., 2015). One such bacterium is Staphylococcus aureus. Originally, it was only documented within healthcare settings, causing skin rashes, painful abscesses, necrotizing fasciitis, and scaled-skin syndrome (Chambers, 2001), but with the rise in community-acquired S. aureus infections, it is now a recognized environmental human health threat (Zetola et al., 2005). Recreational water use is thought to be a pathogen exposure route (Charoenca and Fujioka, 1993; Zetola et al., 2005), as S. aureus and one of its antibiotic resistant strains, methicillin-resistant S. aureus (MRSA), have been found in nearshore waters (Goodwin and Pobuda, 2009; Esiobu et al., 2013). An epidemiological study in Florida found that bathers reported skin infections six times more than nonbathers (Fleisher et al., 2010), and in Hawaii, they were four times more likely to develop S. aureus infections (Charoenca and Fujioka, 1995). Hawaii State has two times more MRSA infections than the national average (Chaiwongkarjohn et al., 2011), and the number has increased 40% since 2000, with the highest incidences on Hawaii Island among those 18 and under (Pellegrin, personal communication, 2017). These findings highlight health concerns with recreation water use, especially as MRSA is listed as a high-priority health threat on the World Health Organization’s antibiotic-resistant pathogen list (WHO, 2017). Therefore, it is imperative that S. aureus watershed sources and factors controlling their export are identified. The presence of S. aureus in nearshore waters has largely been attributed to human skin shedding during swimming (Elmir et al., 2007). However, recent studies found S. aureus and MRSA within watersheds, including in rivers (Viau et al., 2011), urban road runoff (Selvakumar and Borst, 2006), swine farm soils (Schulz et al., 2012), and on pets and wildlife (Faires et al., 2009; Porrero et al., 2013). Staphylococcus aureus and MRSA are also found in sewage (Rosenberg Goldstein et al., 2012), which can seep into groundwater in areas with onsite sewage disposal systems (Whittier and El-Kadi, 2014). Hence, factors controlling S. aureus and MRSA transport to nearshore waters will impact human health. Abbreviations: AIC, Akaike information criterion; CFU, colony-forming units; FIB, fecal indicator bacteria; MRSA, methicillin-resistant Staphylococcus aureus; MPN, most probably number; NARS, Natural Area Reserve System. L.M. Economy, Tropical Conservation Biology and Environmental Science Graduate Program, Univ. of Hawaii at Hilo, 200 W. Kawili St., Hilo, HI 96720; T.N. Wiegner, Marine Science Dep., Univ. of Hawaii at Hilo, 200 W. Kawili St., Hilo, HI 96720; A.M. Strauch, Dep. of Natural Resources and Environmental Management, Univ. of Hawaii at Mānoa, Honolulu, HI 96822; J.D. Awaya, Biology Dep., Univ. of Hawaii at Hilo, 200 W. Kawili St., Hilo, HI 96720; T. Gerken, Geography and Environmental Science Dep., Univ. of Hawaii at Hilo, 200 W. Kawili St., Hilo, HI 96720. Assigned to Associate Editor Lisa Durso. © 2019 The Author(s). Re-use requires permission from the publisher.