Spatially-explicit water quality responses to freshwater discharge in a tropical estuary, Hilo Bay, Hawai‘i

Hilo Bay, Hawai’i is an estuary of great importance to its neighboring coastal community, but is threatened by impaired water quality indicated by excessive turbidity and chlorophyll a associated with river discharges of sediments and nutrients. The Wailuku River in the western half of the bay is the primary source of freshwater discharge, hypothesized here to form a surface-water dominant half of the bay with different water quality traits than the groundwater dominant, eastern half of the bay where the spring-fed Wailoa River discharges. The water quality of both halves of the bay over different flow conditions of the Wailuku River is examined in this study using spatially explicit water quality sampling which collects hundreds of samples in either half of the bay at about every 40 meters. The dense sample shows significant differences between the two halves of the bay, with greater salinity dilution and turbidity in the surface-water dominant area. Both salinity and turbidity have a predictable relation to discharge, with salinity decreasing and turbidity increasing in higher flow conditions. Chlorophyll a , however, has a more complex relation to discharge, as chlorophyll a concentrations are greatest in high-flow conditions, but this may be confounded by the different seasons of the water quality samples. Furthermore, significantly greater chlorophyll a concentrations in the groundwater dominant half of the bay in low-flow conditions shows that discharge may be spuriously correlated to chlorophyll a , and further studies of the effects of surface-water discharge on chlorophyll a concentrations are warranted.