Coastal Groundwater Discharge as a Source of Nutrients to He‘eia Fishpond, O‘ahu, Hawai‘i

Abstract

Submarine groundwater discharge is volumetrically and chemically important to coastal zones and ecosystems. Ancient Hawaiians have known this for centuries, as Hawaiian fishponds were typically constructed not only around streams, but groundwater seepage and springs as well. To obtain a comprehensive understanding of coastal hydrology, processes, and ecosystems, is necessary to quantify SGD and SGD nutrient fluxes to coastal areas. In a Hawaiian fishpond setting, it is important to consider that SGD may be a significant source of nutrients. The chosen study site of this project was Heʻeia Fishpond, a coastal pond on the northeast coast of Oʻahu in the state of Hawaiʻi, into which Heʻeia Stream flows. To identify sources of SGD in the pond, quantify SGD, and determine nutrient fluxes from SGD, we employed tracer techniques involving measurements of the isotopes 222Rn, 223Ra, and 224Ra. Our results indicate the presence of fresh and brackish SGD, and suggest significant inputs of groundwater and groundwater derived nutrients to Heʻeia Fishpond. We found that the amount of water flux from SGD was about equal to that of Heʻeia Stream. While fresh SGD was found to bring in about the same amount of nutrients as the stream, nutrient fluxes from brackish SGD greatly outweighed those of Heʻeia Stream, suggesting that brackish SGD may be involved in the recycling of nutrients from higher trophic levels in Heʻeia Fishpond. Our results show that the contribution of nutrients to fishpond ecosystems via SGD is just as important as stream inputs and deserves more attention from the scientific community.