Impact of flux, residence time and nutrient load of submarine groundwater discharge on coastal phytoplankton growth in coastal waters of Hawaii

M.S. University of Hawaii at Manoa 2011.Includes bibliographical references.Submarine groundwater discharge (SGD) is an important land-to-ocean pathway for biogeochemically significant nutrients, such as nitrate, silicate and phosphate. Introduction of new bioavailable nutrients from SGD into coastal waters can alter a coastal system's nutrient balance, and may result in increases in phytoplankton or macroalgae growth that in turn can cause other ecological shifts in biological species' composition (Valiela et al., 1990). In order to attempt a first investigation of the possible relative impact(s) of the amount, flux, and residence times of SGD and SGD-derived nutrients on coastal phytoplankton in the oligotrophic coastal waters off Hawaii, detailed studies were completed to compare the flux and residence times of SGD and SGD-derived nutrients on biological processes at three sites on the leeward-side of the Koolau Mountains along the southern shore of Oahu (the Ala Wai Canal, Black Point, and Wailupe Beach) and at one site on the leeward-side of Hualalai, on the western side of the Big Island of Hawaii (Honokohau Harbor, Hawaii). At each site, radon and short-lived radium isotopes were used to calculate the flux of SGD and SGD-derived constituents into the coastal areas and their residence times. The rate of uptake of 13C by photoautotrophs was determined using in situ incubation experiments with addition of 13C-labeled bicarbonate at most sites and was used as a relative indicator of photosynthetic activity and its response to potential input of SGD-derived nutrients. At study sites along the southern shore of Oahu, nutrient fluxes for a shoreline length of 1.18 km was 281 PO4 3-, 113,015 Si(OH)4, and 10,505 NO3-mol/d, respectively. Nutrient fluxes for Honokohau Harbor, estimated to have a 375-m wide seepage face, were 194 PO4 3-, 31,998 Si(OH)4, and 3,123 NO3-mol/d respectively. Photosynthetic carbon production rates were significantly higher for the Ala Wai Canal (1.39 μgC/L/h) than for Honokohau Harbor (0.10μgC/L/h). Using these carbon uptake rates and assuming a Redfield C:N:P ratio of 106:16:1, nitrogen and phosphorus uptake rates for both the Ala Wai Canal and Honokohau Harbor were less than 2% of the nutrients delivered by SGD. Variation in the N:P ratios observed in the groundwater and coastal waters could imply nutrient limitation. We found for the restricted estuarine-like embayments of the Ala Wai canal (water mass residence time 10-13 days) and Honokohau Harbor (water mass residence time 0.5-6 days), that primary productivity is not controlled by the availability of nutrients (nutrient supply > phytoplankton demand in both settings), but is nonetheless directly linked to each water mass' residence time. As nutrients are in excess of demand in both settings, we conclude that it is the physical longevity and residence time of the phytoplankton itself that ultimately controls each basin's ultimate level of sustained primary productivity