Drivers of surface water quality in tropical rivers: The influence of mean annual rainfall and land-use/land-cover change on fecal indicator bacteria load

Background/Question/Methods

Clean, flowing freshwater is an essential resource for sustaining island communities, providing critical habitat for unique assemblages of flora and fauna, nutrients for near-shore ecosystems, and economic, agricultural, recreational, and aesthetic uses for humans. Unfortunately, water quality on tropical islands is threatened by rapidly growing populations of humans and non-native animals (e.g., pigs, goats, cows) that contribute to significant increases in nutrients, sediment, and bacteria loads. However, few studies have quantified bacterial transport from native tropical forests over large spatial and temporal scales. Climate change is expected to alter precipitation, storm frequency, and storm intensity with resultant shifts in the amount and timing of surface water runoff delivered to streams and near-shore regions. Thus, the increased intensity and decreased frequency of storms predicted to occur in Hawai’i under current climate models will likely exert a strong control over bacterial transport in watersheds. The objective of this research was to examine the relationship between short-term rainfall and fecal indicator bacteria (FIB) transport in nine rivers across a naturally occurring mean annual rainfall (MAR) gradient in a tropical forest system on the windward coast of Hawai’i Island and then to investigate the influence of land use/land cover change (LULCC) on FIB.

Results/Conclusions

MAR had a significant effect on all three FIB concentrations (Total coliform, Enterococci, Escherichia Coli); with rivers draining higher MAR watersheds containing lower FIB. Additionally, bacteria load (FIB*flow) increased following storm events, but the rate of increase was greater in lower MAR watersheds than in higher MAR watersheds. The effect of MAR on bacterial transport suggests that changes in precipitation (more-intense storms with longer intervening dry periods) could increase the amount of FIB in streams delivered to near-shore environments during intense but intermittent rainfall events due to increased FIB in soils. As expected, LULCC resulted in increased FIB compared to native forest sites, with FIB concentrations generally greatest in lower MAR watersheds. These data indicate that anticipated changes in long-term climate patterns may also result in unexpected shifts in water quality.