Nutrient Pollution

Brian E. Lapointe, Ph.D.
Director, Marine Nutrient Dynamics Program
Division of Marine Science
Harbor Branch Oceanographic Institution, Inc.
5600 US 1 North, Ft. Pierce, FL 34946 USA



The Republic of Trinidad and Tobago, located in the southeast corner of the Caribbean Sea off the coast of Venezuela, is influenced seasonally by floodwaters of the Orinoco River. Because of the direct influence of the Orinoco plume, Trinidad has little coral reef development compared to its more offshore sister island of Tobago. Discharges from the Orinoco River influence Tobago’s coral reefs during the wet season with lower salinity and higher turbidity floodwaters, which reduce light availability needed for coral growth. This chronic, seasonal stress has long affected Tobago’s coral reefs but has not prevented the development of massive and biologicaly diverse coral reef formations, such as those found at Buccoo Reef and Culloden Reef.

Over the past two decades there has been increasing concern among scientists, resource managers, and the public alike regarding the ecological impacts of localized runoff and nutrient pollution on Tobago’s coral reefs. Increased turbidity and sedimentation from deforestation is well known to stress corals and can be fatal in some situations. Enrichment of coastal waters with nitrogen(N) and phosphorus (P) from deforestation, agricultural and urban runoff, and sewage, although more subtle, has become the largest pollution problem facing the vital coastal waters of the wider Caribbean region. Nutrient pollution is the common thread that links an array of environmental problems that include eutrophication, harmful algal blooms, “dead zones”, fish kills, loss of seagrasses and coral reefs, and even some marine mammal and seabird deaths. Because coral reefs have adapted over hundreds of millions of years to clear, clean water with low concentrations of N and P, the impacts of nutrient pollution on coral reefs can be particularly severe.

To address the status and extent of nutrient pollution on Tobago’s fringing reefs, a seasonal (“wet” versus “dry”) study of water quality and benthic biota was undertaken at a variety of Tobago’s fringing coral reefs in 2001.

The results showed:

  • Nutrient over-enrichment of Tobago’s fringing coral reefs, especially Buccoo Reef, from local nutrient sources has triggered ecological changes that have decreased living coral cover and biological diversity.
  • At Buccoo Reef, reduced coral cover correlated significantly with increased cover of macroalgae and the zoanthid Palythoa both of which are indicators of nutrient enrichment on Caribbean coral reefs.
  • Recent encroachment of the seagrasses Thalassia testudinum (turtle grass) and Halodule wrightii (Cuban shoalweed) into the sandy sediments of Nylon Pool are symptomatic of nutrient enrichment.
  • Coral diseases, including white band disease, yellow band disease, and black band disease, occurred at a number of reef sites around Tobago. The dominant coral at Culloden Reef — Montastrea annularis — was impacted by an outbreak of yellow band disease that has apparently developed in just the past few years following increased deforestation and development of its watershed.
  • Concentrations of dissolved inorganic nitrogen (DIN) increased significantly on reef sites around Tobago from the dry season to the wet season. The increased DIN concentrations resulted in significant decreases in concentrations of soluble reactive phosphorus (SRP). Low DIN:SRP ratios (< 15:1) year-around indicate N-limitation of algal growth in Tobago’s coastal waters.
  • High values (> 3 o/oo) of 15N/14N in macroalgae from the Buccoo Reef Complex and other fringing reefs off southwestern Tobago occurred during both wet and dry seasons and were indicative of land-based sewage N pollution from the upland watershed.
  • The 15N/14N values of macroalgae from Black Jack Hole off Little Tobago Island increased from relatively low values (< 3 o/oo) in the dry season to high values (> 5.0 o/oo) in the wet season, indicating increased dispersion of sewage N during periods of peak runoff.
  • Levels of phytoplankton biomass, measured as chlorophyll a, increased from the dry season to the wet season at reef sites around Tobago that had relatively low impacts of sewage enrichment (e.g. Black Jack Hole and Kelliston Drain off Speyside). Reefs that were chronically impacted by sewage pollution showed relatively little effect of the wet season runoff on chlorophyll a.
  • Some of the highest coral cover of the study occurred at Black Jack Hole off Little Tobago Island, the site with the lowest annual mean concentrations of DIN, chlorophyll a, and 15N/14N in macroalgae.

These results support the hypothesis that recent increases in local nutrient pollution, especially from sewage, have pushed Tobago’s coral reefs over the threshold indicative of eutrophication on Caribbean coral reefs. To restore and protect its vitally important coral reefs, Tobago should work to reverse nutrient pollution and sedimentation wherever possible. Meeting this goal will require an array of strategies and approaches tailored to specific reefs and upland developments. For some reefs such as the Buccoo Reef Complex and Mt. Irvine, diversion of human sewage and animal wastes may be sufficient to reverse eutrophication and restore reef health. For most fringing reefs, however, the solutions will be more complex and may involve incentives to reduce deforestation, sewage pollution, urbanization, fertilizer use, and agricultural activities.