Informing coral reef conservation and management with geochemical sourcing of land-based sediment and contaminants at Olowalu, a Mission Blue Hope Spot

This talk was presented at the 15th International Coral Reefs Symposium, 03-08 July 2022 (Hybrid event), under Theme 12, Conservation and Management, Session 12E:  How can scientists and managers identify optimum catchment management actions to improve downstream condition for reefs and people?

Coral reef ecosystems are under increasing pressure from human activities and climate change, stressors that can introduce land-based sediment and contaminants to the coastal ocean and impair reef resilience. Mitigation of sediment and contaminant runoff requires knowledge about production functions, source areas, and/or transport pathways. These questions are being explored in the watersheds and reefs of Olowalu in leeward Maui, Hawaii, USA, chosen as a Mission Blue Hope Site for its high reef quality and role as a source of coral larvae for adjacent reefs, and identified as a priority restoration site after bleaching reduced live coral cover in the mid 2010s. The coral reefs off Olowalu face land-based pressures from sedimentation and contaminants produced by frequent wildfires and vehicular traffic on a major beach-front highway. Both could increase if future climate brings more weather extremes: droughts, wildfires, and floods. Population growth is also expected in Maui, and with it, increased chemical loads to the environment. This geochemical study aims to identify catchment sources of sediment accumulating on Olowalu and adjacent Ukumehame reefs. More specifically, geochemical sourcing aims to distinguish sediment originating from upper Ukumehame Gulch, which is extensively intruded by volcanic dikes, relative to basaltic sediment from lower Ukumehame and Olowalu Gulches, and from windward Kealaloloa Ridge, formed of young, more erodible alkalic lavas where many wildfires appear to originate. The study also aims to identify levels and sources of polycyclic aromatic hydrocarbons (PAHs, contaminants of concern) in watershed soil and reef sediment by using diagnostic ratios that distinguish PAHs produced by wildfires, vehicular traffic, and other human activities. Levels of anthropogenic metals in roadside soil and reef sediment also show potential ecological risk. An understanding of such land-based sediment sources, transport pathways, and contaminant sources from geochemical signatures can inform both strategic targeting of processes for catchment management and an integrated approach to coral reef ecosystem conservation, restoration, and management.