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

Abstract

Abstract. Constraining coral reef metabolism and carbon chemistry dynamics are fundamental for understanding and predicting reef vulnerability to rising coastal CO 2 concentrations and decreasing seawater pH. However, few studies exist along reefs occupying densely inhabited shorelines with known input from land-based sources of pollution. The shallow coral reefs off Kahekili, West Maui, are exposed to nutrient-enriched, low-pH submarine groundwater discharge (SGD) and are particularly vulnerable to the compounding stressors from land-based sources of pollution and lower seawater pH. To constrain the carbonate chemistry system, nutrients and carbonate chemistry were measured along the Kahekili reef flat every 4 h over a 6-day sampling period in March 2016. Abiotic process -primarily SGD fluxes -controlled the carbonate chemistry adjacent to the primary SGD vent site, with nutrient-laden freshwater decreasing pH levels and favoring undersaturated aragonite saturation ( arag ) conditions. In contrast, diurnal variability in the carbonate chemistry at other sites along the reef flat was driven by reef community metabolism. Superimposed on the diurnal signal was a transition during the second sampling period to a surplus of total alkalinity (TA) and dissolved inorganic carbon (DIC) compared to ocean endmember TA and DIC measurements. A shift from positive net community production and positive net community calcification to negative net community production and negative net community calcification was identified. This transition occurred during a period of increased SGD-driven nutrient loading, lower wave height, and reduced current speeds. This detailed study of carbon chemistry dynamics highlights the need to incorporate local effects of nearshore oceanographic processes into predictions of coral reef vulnerability and resilience.

Coral Skeleton δ15N as a Tracer of Historic Nutrient Loading to a Coral Reef in Maui, Hawaii

“…For example, nitrate concentrations within the seep discharge fluid varied from 0.45 µmol L −1 to over 70 µmol L −1 , while nitrate δ 15 N values varied from less than +40‰ to almost +75‰, all during the 6-day period, demonstrating the highly dynamic nature of this reef location. Higher nitrate concentrations are observed at the shallowest sampling sites closest to the seep 21,59 consistent with previous work at this site 52–54 . Collectively these data suggest that the two corals (LobataHead04 and LobataHead06) likely experienced somewhat different conditions both day-to-day and at any specific time.…”

“…This could explain the variability observed in δ 15 N in each coral and the differences between these corals from 1998 to 2002. The variability in coral δ 15 N values is also consistent with other δ 15 N measurements from the study area, including those in particulate organic matter (POM) (this study, see Table 1), groundwater and seawater DIN 53,54,59 , and algal tissue 35,51 , capturing how differences in exposure to nutrient-enriched effluent and post-discharge mixing can lead to differences in δ 15 N. Despite the high variability in nitrate δ 15 N, the coral skeleton provides a time-integrated signal, meaning the overall signal is robust. Importantly, the δ 15 N variability between LobataHead04 and LobataHead06 is also much smaller than the primary denitrification signal observed after 1996.…”

“…Prouty et al . 21,59 analyzed seep water and nearby seawater chemistry at this site during a 6-day intensive sampling campaign. The results of these studies revealed that bottom water carbonate chemistry, nutrient concentrations, and specifically nitrate δ 15 N were largely controlled by SGD discharge fluxes, as well as nearshore oceanographic conditions (e.g.…”

“…2016 seawater δ 15 N NO3 values (indicated by **) were previously published in Prouty et al . 59 . Values are reported as average ± standard deviation.…”

“…(2013) for detailed site description 54 ). Two of the cores collected (LobataHead04 and LobataHead06) were from within an active seep area 52–54 , whereas core LobataHead07 was collected ~150 m to the south, in an area of high coral cover and low bioerosion and background nutrient concentrations 59 . Intra-crystalline organic matter from each core was analyzed for δ 15 N variability using a newly developed analytical procedure (see methods).…”