Uptake of groundwater nitrogen by a near-shore coral reef community on Bermuda

Zoe C. Sims, Anne L. Cohen, Victoria H. Luu, Xingchen T. Wang, Daniel M. Sigman

Research output: Contribution to journalArticlepeer-review

Abstract

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 15N = 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.

Original languageEnglish (US)
Pages (from-to)215-228
Number of pages14
JournalCoral Reefs
Volume39
Issue number1
DOIs
StatePublished - Feb 1 2020

All Science Journal Classification (ASJC) codes

  • Aquatic Science

Keywords

  • Bermuda
  • Coral calcification
  • Macrobioerosion
  • Nutrient enrichment
  • Submarine groundwater discharge

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