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

Nancy G Prouty,Curt D Storlazzi,Kimberly K Yates,Nathan Smiley,Olivia Cheriton,Chris Gallagher

doi:10.5194/bg-15-2467-2018

Abstract

Abstract. Constraining coral reef metabolism and carbon chemistry dynamics are fundamental for understanding and predicting reef vulnerability to rising coastal CO2 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 (Ωarag) 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.

Highlights

Coral reefs provide critical shoreline protection and important ecosystem services, such as marine habitat, and support local economies through tourism, fishing, and recreation (Hughes et al, 2003; Ferrario et al, 2014)
A rapid change was observed in the pH, DO, total alkalinity (TA), dissolved inorganic carbon (DIC), and nutrient concentrations (Fig. 2)
These results are consistent with earlier work documenting lower-pH, nutrient-enriched freshwater endmember values tightly coupled to submarine groundwater discharge (SGD) (Swarzenski et al, 2012, 2016; Glenn et al, 2013)

Summary

Introduction

Coral reefs provide critical shoreline protection and important ecosystem services, such as marine habitat, and support local economies through tourism, fishing, and recreation (Hughes et al, 2003; Ferrario et al, 2014). Coral reefs are being threatened by global climate change processes, such as increasing temperatures, sea-level rise, and ocean acidification (OA), caused by uptake of atmospheric carbon dioxide into the ocean (Orr et al, 2005). These effects are often compounded by local stressors such as over-fishing, sedimentation, land-based sources of pollution, and coastal acidification (Knowlton and Jackson, 2008) that can result from freshwater inflow, eutrophication, and/or coastal upwelling. Isolating the effects of these stressors is difficult without establishing the biological and physical controls on community calcification and production This is challenging for coral reefs adjacent to densely inhabited shorelines, where freshwater fluxes can deliver excess nutrients.

Methods

Results

Discussion

Conclusion