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

Abstract

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.

Carbonate system parameters of an algal-dominated reef along West Maui

“…Elevated SGD endmember nutrient concentrations are consistent with those observed from Black Point, Maunalua Bay, where effluent from proximal on-site sewage disposal is linked to excess nitrogen loads (Nelson et al, 2015;Richardson et al, 2017). As described above, an offshore gradient in nutrient concentrations was observed with enriched nutrients at the shallow sites compared to the deeper sites, consistent with a decrease in coral δ 15 N values away from the vent (Prouty et al, 2017a). Coral tissue thickness was also negatively correlated to coral tissue δ 15 N values (r = −0.66; p = 0.08), with the latter serving as a proxy for nutrient loading in algae samples along the reef flat (Dailer et al, 2010).…”

“…respectively (Prouty et al, 2017b). No diurnal pattern was observed for the nutrient data; however, there was an offshore gradient in nutrient concentrations with enriched nutrients at the shallow sites compared to the deeper sites.…”

“…According to , up to 11 m 3 d −1 of dissolved inorganic nitrogen are discharged onto the West Maui reef as the result of receiving and treating over 15 000 m 3 d −1 of sewage. Using a SGD flux rate of 87 cm d −1 at the primary seep site (Swarzenski et al, 2016), and SGD nitrate endmember concentration of 117 µmol L −1 (Prouty et al, 2017b), the nitrate flux from the primary vent site is 712 mol d −1 , clearly demonstrating excess nutrient loading. Elevated SGD endmember nutrient concentrations are consistent with those observed from Black Point, Maunalua Bay, where effluent from proximal on-site sewage disposal is linked to excess nitrogen loads (Nelson et al, 2015;Richardson et al, 2017).…”

“…Areas of discrete coral cover loss up to 100 % along the shallow coral reef at Kahekili have been observed for decades (Wiltse, 1996;Ross et al, 2012), with a history of macroalgal blooms (Smith et al, 2005). More recently, Prouty et al (2017a) found accelerated nutrient-driven bioerosion from coral cores collected along the Kahekili reef flat in response to land-based sources of nutrients. This is consistent with earlier work showing nutrification-mediated increase in plankton loads can trigger increases in filter feeders and bioeroders that endanger reef structure integrity (e.g., Fabricius et al, 2012).…”

“…The decline in coral cover along the shallow coral reef at Kahekili has been observed for decades (Wiltse, 1996;Ross et al, 2012), along with a history of macro-algal blooms (Smith et al, 2005). The shift in benthic cover from abundant corals to turf or macro-algae (primarily Ulva fasciata) and increased rates of coral bioerosion have been linked to input of nutrient-rich water via wastewater injection wells (Dailer et al, 2010Prouty et al, 2017a). Treated wastewater is injected through these wells into groundwater that flows toward the coast where it emerges on the reef through a network of small seeps and vents Swarzenski et al, 2016).…”