Vulnerability of Coral Reefs to Bioerosion From Land‐Based Sources of Pollution

• Published in: Journal of geophysical research. Oceans Vol. 122; no. 12; pp. 9319 - 9331
• Main Authors: Prouty, Nancy G., Cohen, Anne, Yates, Kimberly K., Storlazzi, Curt D., Swarzenski, Peter W., White, Darla
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-12-2017
• Subjects: Acidification; aragonite saturation; Bioerosion; Calcification; Calcium; Calcium carbonate; Calcium carbonates; Carbon dioxide; Carbon dioxide atmospheric concentrations; Carbonates; Coral reef ecosystems; Coral reefs; Corals; Cores; Discharge; Ecosystems; Equivalence; Eutrophication; Geophysics; Groundwater; Groundwater discharge; Isotopes; Land pollution; Marine ecosystems; Marine pollution; Mineral nutrients; Nitrogen; Nitrogen isotopes; Nutrient sources; nutrients; Ocean acidification; Oceans; Oligotrophic waters; pH effects; Pollution; Pollution sources; Seawater; Seawater pH; Sewage; submarine groundwater; Vulnerability; wastewater
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1002/2017JC013264

Ocean acidification (OA), the gradual decline in ocean pH and [
CO32−] caused by rising levels of atmospheric CO2, poses a significant threat to coral reef ecosystems, depressing rates of calcium carbonate (CaCO3) production, and enhancing rates of bioerosion and dissolution. As ocean pH and [
CO32−] decline globally, there is increasing emphasis on managing local stressors that can exacerbate the vulnerability of coral reefs to the effects of OA. We show that sustained, nutrient rich, lower pH submarine groundwater discharging onto nearshore coral reefs off west Maui lowers the pH of seawater and exposes corals to nitrate concentrations 50 times higher than ambient. Rates of coral calcification are substantially decreased, and rates of bioerosion are orders of magnitude higher than those observed in coral cores collected in the Pacific under equivalent low pH conditions but living in oligotrophic waters. Heavier coral nitrogen isotope (δ15N) values pinpoint not only site‐specific eutrophication, but also a sewage nitrogen source enriched in 15N. Our results show that eutrophication of reef seawater by land‐based sources of pollution can magnify the effects of OA through nutrient driven‐bioerosion. These conditions could contribute to the collapse of coastal coral reef ecosystems sooner than current projections predict based only on ocean acidification.
Plain Language Summary
We show that sustained, nutrient rich, lower pH submarine groundwater discharging onto nearshore coral reefs off west Maui lowers the pH of seawater and exposes corals to nitrate concentrations 50 times higher than ambient. Rates of coral calcification are substantially decreased, and rates of bioerosion are orders of magnitude higher than those observed in coral cores collected in the Pacific. With many of Maui's coral reefs in significant decline reducing any stressors at a local scale is important to sustaining future coral reef ecosystems and planning for resiliency.
Key Points
Rates of bioerosion are greater than coral cores collected in the Pacific under equivalent low pH conditions but from oligotrophic waters
Heavier coral δ15N values pinpoint not only site‐specific eutrophication, but also a sewage nitrogen source enriched in 15N
Eutrophication of reef seawater by land‐based sources of pollution can magnify the effects of OA through nutrient driven‐bioerosion