Effects of nutrient enrichment on microbial biomass and class structure in groundwater-fed oligotrophic waters off west Hawai'i Island: An experimental investigation

Phytoplankton are important environmental indicators because their biomass and size structure can respond rapidly to changing conditions, including nutrient loading. Submarine groundwater discharge (SGD) is a significant contributor of natural and anthropogenic nutrients to the coastal ecosystem, with potentially adverse impacts in some areas. Our objective was to experimentally assess the sensitivity of phytoplankton and bacteria in SGD-influenced water to nutrient loading. We performed nutrient (+N, +P, +N+P) enrichment bioassays with water collected inside and outside persistent SGD plumes at Kiholo and Kaloko Bays along the arid Kona coast of Hawai'i Island, U.S.A. In situ physical / chemical conditions at Kiholo Bay reflected strong influence of SGD and the phytoplankton were dominated by pico- and nano-phytoplankton (<20 mm), which exhibited larger average cell-size in-plume. The response ratio (RR) of total phytoplankton biomass was significantly > 0 for all treatments, but the RR among the pico / nanophytoplankton fraction different, indicating that biomass responses are being driven by the larger size fraction of phytoplankton (<20 microm). For total biomass, +N+P treatments had a greater response than +N and +P. In situ physical / chemical conditions at Kaloko, reflected less influence of SGD compared to Kiholo, and both in- and out-of-plume phytoplankton showed a RR > 0 for +N and +N+P (combined) treatments, while only out-of-plume waters responded positively to +P treatments. Kaloko pico / nanophytoplankton in-plume showed a RR > 0 for +N and combined treatments. At both sites, +N and combined treatments produced significant increases in relative growth rate (Dr) for the total phytoplankton, but not the pico / nanophytoplankton. At both sites, significant drawdown of NO 3-+NO2- and PO4 3- occurred only in combined treatments applied to water collected in-plume. Despite enriched conditions in-plume, phytoplankton communities at these sites exhibited nutrient limitation through elevated biomass and changes in community composition in response to fertilization. Our findings support dual nutrient management strategies for SGD-influenced waters.