High-resolution physical and biogeochemical variability from a shallow back reef on Ofu, American Samoa: an end-member perspective
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Shallow back reefs commonly experience greater thermal and biogeochemical variability owing to a combination of coral community metabolism, environmental forcing, flow regime, and water depth. We present results from a high-resolution (sub-hourly to sub-daily) hydrodynamic and biogeochemical study, along with a coupled long-term (several months) hydrodynamic study, conducted on the back reefs of Ofu, American Samoa. During the high-resolution study, mean temperature was 29.0 °C with maximum temperatures near 32 °C. Dissolved oxygen concentrations spanned 32–178 % saturation, and pHT spanned the range from 7.80 to 8.39 with diel ranges reaching 0.58 units. Empirical cumulative distribution functions reveal that pHT was between 8.0 and 8.2 during only 30 % of the observational period, with approximately even distribution of the remaining 70 % of the time between pHT values less than 8.0 and greater than 8.2. Thermal and biogeochemical variability in the back reefs is partially controlled by tidal modulation of wave-driven flow, which isolates the back reefs at low tide and brings offshore water into the back reefs at high tide. The ratio of net community calcification to net community production was 0.15 ± 0.01, indicating that metabolism on the back reef was dominated by primary production and respiration. Similar to other back reef systems, the back reefs of Ofu are carbon sinks during the daytime. Shallow back reefs like those in Ofu may provide insights for how coral communities respond to extreme temperatures and acidification and are deserving of continued attention.
KeywordsPhysical–biological interactions Coral metabolism Diel cycle Coastal ecosystems
This manuscript benefitted greatly from the thoughtful comments of two anonymous reviewers and Heidi Hirsh. Helpful conversations with Andreas Andersson, Stephen Palumbi, and Lupita Ruiz-Jones improved this manuscript. Tim Clark, Carlo Caruso, and the rest of staff at the National Park Service of American Samoa provided logistical assistance. Steve Litvin provided able field assistance. We thank Doug Fenner for help with the ecological survey. Eric Stoutenberg and Gil Masters were kind to share wind speed data with us. Chip Young of NOAA CRED provided offshore carbon chemistry data. This study was supported by the Woods Institute for the Environment at Stanford University and a grant to RBD from the Betty and Gordon Moore Foundation. DK was partially supported by an NSF Graduate Research Fellowship. This study was conducted under Permit Number NPSA-2011-SCI-0010 issued by the National Park Service. Data from this study have been deposited at the NOAA National Centers for Environmental Information and can be obtained there.
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