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Title: Where the Groundwater Meets the Sea: Ecological Impacts of Nutrient-Enriched Groundwater Discharge on Bermuda’s Near-Shore Coral Reefs
Authors: Sims, Zoe
Advisors: Pacala, Stephen W.
Department: Ecology and Evolutionary Biology
Certificate Program: Environmental Studies Program
Class Year: 2017
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.
Type of Material: Princeton University Senior Theses
Language: en_US
Appears in Collections:Ecology and Evolutionary Biology, 1992-2023

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