Uptake of groundwater nitrogen by a near-shore coral reef community on Bermuda
Nutrient enrichment can slow growth, enhance bioerosion rates, and intensify algal competition for reef-building corals. In areas of high human population density and/or limited waste management, submarine groundwater discharge can transfer anthropogenic nutrients from polluted groundwater to coastal reefs. In this case study, we investigate the impact of submarine groundwater discharge on a near-shore reef in Bermuda, where over 60% of sewage generated by the island’s 64,000 residents enters the groundwater through untreated cesspits. Temperature, salinity, pH, and alkalinity were monitored at a groundwater discharge vent, three locations across the adjacent coral reef (0–30 m from shore), and a comparison patch reef site 2 km from shore. Groundwater discharge was characterized by low salinity, low aragonite saturation state (Ωar), high alkalinity, elevated nitrate + nitrite (NO3 – + NO2 –; hereafter, “NO3 –”) concentrations (> 400 µM), and an elevated 15N/14N ratio of NO3 – (δ15N = 10.9 ± 0.02‰ vs. air, mean ± SD). Rainfall and tidal cycles strongly impacted groundwater discharge, with maximum discharge during low tide. NO3 – concentrations on the near-shore reef averaged 4 µM, ten times higher than that found at the control site 2 km away, and elevated NO3 – δ15N at the near-shore reef indicated sewage-contaminated groundwater as a significant nitrogen source. Tissue δ15N of Porites astreoides, a dominant reef-building coral, was elevated by 3‰ on the near-shore reef compared to the control site, indicating that corals across the near-shore reef were assimilating groundwater-derived nitrogen. In addition, coral skeletal density and calcification rates across the near-shore reef were inversely correlated with NO3 – concentration and δ15N, indicating a negative coral health response to groundwater-borne nutrient inputs. P. astreoides bioerosion rates, in contrast, did not show an effect from the groundwater input. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.