Maunalua Bay: Groundwater nutrient effects on accretion-erosion balance

RATIONALE: Coral reefs depend on an accretion-erosion balance that can be disrupted by chronic nutrient loading of coastal waters. Coastal eutrophication threatens reef accretion by shifting competitive dominance away from corals and other calcifiers toward fleshy algae and cyanobacteria and by increasing erosion rates by invertebrates. The limited mixing and nutrient-enhanced productivity and respiration mediated by reef flat biota alters the pH of the overlying water, increasing diel and tidal pH variation nearshore.  In our current Sea Grant project, we found fluctuations of 0.5-0.8 pH units nearshore compared to < 0.1 pH units offshore even in the absence of a strong nutrient gradient, and bioerosion rates were better-predicted by pH and pH variability than by nutrients.  We hypothesize that nutrient driven pH cycling has a synergistic effect with direct nutrient subsidies and may shift the accretion-erosion balance of Hawai‘i’s coastal reefs, jeopardizing coastal protection and the many other services our reefs provide.

GOALS & OBJECTIVES:  The proposed project provides an integrated picture of groundwater-derived nutrient processing across a reef flat and ultimately the impacts of this input on coral persistence in the face of shifting environments. We will 1) Assess groundwater discharge and related nutrient fluxes in Maunalua Bay.  2) Characterize the rate of physical mixing over the reef flat, thereby constraining estimates of biological nutrient uptake. 3) Combining benthic surveys with species-specific estimates of N-uptake from prior studies and C-uptake from the proposed study, to estimate species-specific contributions to nutrient uptake by the benthos. 4) Quantify the variability of critical environmental parameters across the reef, including DO, pH, nutrients, temperature, and turbidity. 5) Estimate the relative contribution of diffusion and uptake to variability of environmental parameters. 6) Accurately measure bioerosion and coral growth rates on cross-reef and along-reef gradients in groundwater influence. 7) Statistically disentangle the direct and indirect effects of nutrients on bioerosion and coral growth rates. 8) Taking advantage of a large-scale algal removal project by our community partners, directly test whether the removal of benthic biomass changes the P/R cycle dramatically enough to impact reef bioerosion and accretion rates.

METHODOLOGY:  In Maunalua Bay, alongshore transects for radon, salinity, DO, and pH with nutrient samples will capture the spatial pattern of groundwater discharge.   Cross-shore transects using radium isotopes and other parameters will allow estimates of nutrient uptake.  Field flume studies will estimate species-specific uptake of DIC.  We will measure bioerosion rates using small blocks of coral skeleton that are scanned before and after deployment using μCT and measure coral growth rate by outplanting dyed coral nubbins. 

EXPECTED OUTCOMES:  The rich combination of approaches used in this study will allow us to test whether coastal nutrient loading is changing our reefs, not just through shifts in community structure, but through changes in pH stresses across the fringing reef.  We will provide an ecosystem-scale evaluation of the effects of algal removal undertaken by Malama Maunalua and help the community to understand the relative importance of algal removal and eutrophication to the health of their reef.