AN ASSESSMENT OF MICROBIAL CONCENTRATIONS, ARSENIC ACCUMULATION, AND WATER QUALITY: VIABILITY OF SHELLFISH AND SEAWEED AQUACULTURE IN HILO BAY, HAWAIʻI ISLAND

Abstract

Hilo Bay is a heavily impacted estuarine system with cultural, economic, and ecological significance. There is growing interest in seaweed and bivalve aquaculture for commercial and restorative purposes, although water quality may constrain development. Aquaculture feasibility at the Hilo Bay Aquaculture Pilot Site (HAPS) was evaluated using microbial monitoring, physicochemical measurements (temperature, salinity, and dissolved oxygen), sediment and seaweed arsenic analyses, precipitation data, and wastewater tracer assessment.Precipitation was identified as one of several factors associated with variability in fecal indicator bacteria, but it was not a consistent or dominant driver, with relationships differing by bacterial indicator, time scale, and sampling depth. E. coli showed a significant positive relationship with 24-hour precipitation at the surface, while total coliform bacteria exhibited significant relationships across multiple precipitation windows. Enterococcus displayed depth-dependent responses, including negative relationships with longer-term precipitation in bottom waters. Non-detectable bacterial concentrations occurred across all precipitation conditions, indicating that microbial contamination was intermittent and not solely controlled by precipitation. Comparisons with Title 11 Department of Health Chapter 35 Shellfish Sanitation and National Shellfish Sanitation Program (NSSP, 2017) criteria showed more frequent exceedances during moderate and wet precipitation conditions; however, precipitation alone did not consistently predict regulatory classification. These results indicate that microbial water quality may periodically meet regulatory thresholds to allow harvest, but variability associated with precipitation events presents a constraint for consistent shellfish harvest intended for human consumption. Physicochemical parameters were not consistent drivers of microbial variability. Sediment arsenic concentrations were within previously reported ranges for Hilo Bay, reflecting legacy contamination. Arsenic concentrations in Amansia glomerata were consistent with reported values for marine macroalgae from other areas. Sucralose concentrations were detected at low levels below thresholds associated with strong wastewater signals, indicating that persistent wastewater inputs were not a dominant influence at the study site during the sampling period. Aquaculture development at the HAPS is feasible but dependent on environmental variability and regulatory constraints. Seaweed aquaculture presents the strongest opportunity, while bivalve aquaculture opportunities for human consumption may be more limited under the observed conditions. The results do not indicate barriers to use of bivalves for restorative purposes.