2. Study region
2.1 Site details
Maunalua Bay is an 8 km embayment located on the island of Oʻahu, Hawaiʻi, with three primary regions of groundwater discharge: Black Point, Kawaikui, and Wailupe (Richardson et al., 2015). Two of these groundwater-influenced coastal locations, Black Point and Wailupe, were considered for this study (Figure 1). Three sites (a, ‘high GW’; b, ‘intermediate GW’; and c, ‘low GW’ zones) that spanned a salinity gradient were monitored from each reef. Site A at Black Point was approximately 5 m offshore of the dominant groundwater submarine spring while site A at Wailupe was situated within 2 m of the dominant groundwater seep. These seeps are roughly 0.5 m in diameter and thought to be fed by preferential flow of…show more content… However, the groundwater endmembers are not uniform between the two locations as groundwater discharging at Black Point originates from an entirely separate aquifer than groundwater discharging at Wailupe. Previous studies have found marked differences in their respective nutrient compositions. (Ganguli et al., 2014, Swarzenski et al., 2014, Richardson et al., 2015).
2.2 Benthic structure
Sand, rubble, and macroalgae dominate benthic cover at sites A and B at Black Point. Seasonal blooms of green macroalgae, Bryopsis pennata, which persisted in the water column and benthos at all sites, were occurring during our Black Point sampling event. Coral cover is highest along the outer perimeter of the reef flat (i.e., site C). Benthic cover at Wailupe is dominated by macroalgae with percent cover ranging from 30 - 50% on average (Amato, 2015). Sand and rubble constitute nearly 20 - 50% of the benthos at all sites. Corals are found predominantly offshore with cover estimated as 2 - 5% at site C while zoanthid cover peaks in the nearshore sites at 2 - 12% (Amato, 2015).
3. Methods
3.1 Long-term instrument deployments
A portable NaI(TI) scintillation detector, or ‘GW Sniffer’ (Alpha Spectra, Inc.) was used to measure 222Rn in water over one hour integrated time periods using methods by Dulai et al. (2015). The GW Sniffer was encased in a steel house frame and deployed over a period of thirty days at Black Point (August 2015) and twenty five days at Wailupe (September 2015).