Where the Groundwater Meets the Sea: Ecological Impacts of Nutrient-Enriched Groundwater Discharge on Bermuda’s Near-Shore Coral Reefs

Abstract

As climate change increasingly stresses coral reefs globally, it is important to mitigate local factors, including pollution, that can push reefs past their resilience thresholds. In Bermuda, the world’s third-most densely populated nation, over 60% of residential sewage enters the groundwater through untreated cesspits. However, the impacts of sewage-enriched groundwater discharge on Bermuda’s coral reefs have been little studied. This project quantified water quality in Bermuda’s groundwater, at a coastal groundwater discharge vent, and across three North Shore reefs. Groundwater discharge is characterized by low salinity, elevated nitrate concentrations (500µM NO3–), and an elevated nitrate 15N/14N ratio (δ15N = 10.9‰) consistent with human waste. On the most heavily N-enriched site, [NO3-] averages 4µM, 10x higher than typical on Bermudian reefs without shoreline impacts, with a nitrate δ15N indicating sewage-enriched groundwater is the primary N source. Tissue δ15N in two species of benthic macroalgae collected across the three reefs is correlated with aquatic nitrate δ15N, indicating algae take up groundwater-borne N. We also quantified skeletal growth of Porites astreoides, a dominant reef-building coral on Bermuda, across the most heavily N-enriched site, finding a significant decline in calcification rates closer to the shoreline, where [NO3-] and nitrate δ15N increase. However, surveys across the three sites found that although overall coral cover is low (~5%) relative to Bermuda’s offshore reefs (~20-25%), algal and coral cover are not correlated with N enrichment. Furthermore, in a four-week herbivore exclusion experiment, algal growth and N enrichment were uncorrelated, even in the absence of herbivorous fish. Instead, algae may be phosphate-limited: phosphate is adsorbed onto Bermuda’s limestone (Simmons, 1983), and is low ([PO43-] <0.04μM) across these reefs. Further study of the aquifer’s potential phosphate saturation limits, and continued management of both PO43- inputs and herbivore populations, are critical to maintaining coral communities on these N-enriched reefs.