Aquatic systems are directly controlled by their surrounding environments and can be effective paleoenvironmental proxies if their biogeochemical processes are clearly understood. Time-series analyses of aquatic and particulate geochemistry were made between 1999 and 2001 in two very different systems on O‘ahu, Hawai‘i – Ordy Pond and Kahana Bay. Ordy Pond is a closed basin whose biogeochemistry responds rapidly to changes in the local environment. Kahana Bay is a coastal marine environment influenced by significant groundwater and surface water influx and mixing with coastal ocean water. Ordy Pond was found to be a promising tool for studying the Holocene environmental history of O‘ahu, while Kahana Bay demonstrated the applicability of natural chemical tracers to quantify submarine groundwater discharge (SGD) in Hawaiian coastal waters.

Seasonal changes in surface air temperatures were the primary control on the biochemical seasonality of Ordy Pond. When the pond was thermally stratified in the spring/summer season, oxygenated photosynthetic productivity in the epilimnion increased dramatically as indicated by profiles of dissolved O2, dissolved inorganic carbon (DIC), delta13C of DIC, and particulate inorganic and carbonate production rates. When the density gradient waned in the fall/winter season, chemically reduced hypolimnion waters mixed with the epilimnion, and surface waters became suboxic to anoxic. Organic productivity remained high (>100 mg C per sq. m per day), so it appears that the aerobic phytoplankton which dominated productivity in the spring/summer were replaced by anaerobic photosynthetic bacteria during the fall/winter. The carbonate mineral flux increased during the spring/summer due to increased dissolved carbonate mineral saturation resulting from greater total organic productivity. Modern particulate production reflected the water chemistry, and the biogeochemical processes in the modern pond were used to interpret the Holocene environmental history of the area through analysis of pond sediments. The sediments revealed three dramatic paleoenvironmental events: 1) the most recent post-glacial sea level rise around O‘ahu ~9700 years ago; 2) sea level fall ~1000 years ago following O‘ahu’s mid-Holocene 2m sea level high stand; and 3) Western human contact with O‘ahu and the introduction of plantation-scale phosphorus-fertilized agriculture.

Total submarine groundwater discharge (SGD) and the fraction of SGD consisting of terrestrial groundwater (fTGW) were measured in Kahana Bay using seepage meters and natural tracers. Lee-type seepage meters were used to measure SGD rates and collect samples of SGD directly. Radon-222, Si, Cl , and total alkalinity (TA) were used as natural tracers to calculate fTGW. Nutrient concentrations were also measured to calculate total nutrient fluxes to the bay via SGD. Ninety percent of the SGD in Kahana Bay occurs in the inner bay within 1 km of the shoreline. The average total SGD flux measured was 90 x 10^6 L per day, 16% of which was terrestrial groundwater. By comparison, the average annual surface runoff from Kahana River is 90.7 x 10^6 L per day. Estimated fluxes of total dissolved phosphorus and nitrogen by SGD to the bay were 500% and 200% greater than fluxes via surface runoff, respectively. Thus, SGD in Kahana Bay has proved to be a significant source of both fresh water and nutrient input comparable to that of the surface runoff of Kahana River.