Presenter: Heidi Hirsh (NOAA Atlantic Oceanographic and Meteorological Laboratory)

Description:
Understanding the drivers of nearshore biogeochemical variability is critical for evaluating how coral reefs will respond to present and future ocean acidification (OA) stress. To meaningfully predict nearshore OA impacts, we must account for the complexity of the local benthic community, as well as connectivity between offshore end member carbonate chemistry and onshore conditions. Estimation of nearshore residence time can help quantify this connectivity and determine the degree to which the benthos can influence the chemistry of the overlying water column. We present sub-island-scale estimates of residence time for three islands across the Pacific basin: Guam (Mariana Islands), Tutuila (American Samoa), and the island of Hawaiʻi (Hawaiian Islands), using km-scale Regional Ocean Modeling System output. To estimate retention time in nearshore sub-island sectors -- delineated by spatial patterns in carbonate chemistry observations -- we use modified control volume and particle trajectory based approaches. These estimates of local hydrodynamics are paired with our synthesis of observed spatial and temporal patterns in the reef carbonate system across the Pacific based on samples collected by NOAA’s National Coral Reef Monitoring Program (NCRMP) over the past decade. Together, records of benthic composition, residence time, and oceanic carbonate chemistry allow us to better describe the environmental and ecological drivers of reef-scale processes modifying local biogeochemistry. We observe increased predictive skill in models that include residence time, a testament to the validity of our residence time estimations. These results highlight the importance of local hydrodynamics as well as benthic processes in driving biogeochemical variability in coral reef environments.