Reefs in peril
  Reefs in peril
Proximal causes of decline in health of coral reefs hot buttons for peril part of BIOLOGY OF CARIBBEAN CORAL REEFS overfishing/reef collecting on Caribbean coral reefs disease on Caribbean coral reefs SCUBA/snorkeling recreation on Caribbean coral reefs future of Caribbean coral reefs pollution on Caribbean coral reefs eutrophication of Caribbean coral reefs hot button for bleaching part of Biology of Caribbean Coral Reefs

There are several major proximal causes for reef decline. The topic of eutrophication is dealt with here, while other topics are accessible via the "hot" buttons.

NOTE a process of enrichment through input of nutrients, such as from fertiliser runoff or sewage, that leads to excessive growth of plants and bacteria and results in depletion of oxygen

subtitle button for eutrophication section of BIOLOGY OF CARIBBEAN CORAL REEFS Eutrophication

photograph of enriched area of Discovery Bay, Jamaica with overgrowths of green algae
Enrichment of nutrients by lawn and garden fertilisers, and organic sewage can lead to bacterial and algal overgrowth in coral reefs in a process known as eutrophication. Bacterial overgrowths are often a type of cyanobacteria, also known commonly as blue-green algae.

NOTE this ancient group of unicellular microorganisms is photosynthetic. Their colours actually range from reds through blue-greens to greens, browns, yellows, and blacks. Many types are toxic. They may be unicellular or filamentous, and often appear as large scummy masses that overgrow other organisms


Large growths of filamentous green alga,
possibly Chaetomorpha linum, in Discovery
Bay, Jamaica. The area is enriched by
nutrients carried in the freshwater
that percolates through the limestone
crust into the Bay


Lettuce coral Agaricia agaricites with
a growth of blue-green alga 0.6X

  photograph of ocean pollution in the form of eutrophication or enrichment from runoff waters in Coxen Hole, RoatanSmall rivulets of drainage water in the town of Coxen Hole in Roatan carry enrichment matter to the ocean in the form of garden fertilizers, decaying organic matter, and probably some household sewage. The preponderance of green alga (possibly Chaetomorpha sp.) seen here close view of filamentous green alga (Chaetomorpha sp.) growing in polluted freshwater runoff area in Coxen Hole, Roatanand in similar situations owes to the preference of many filamentous greens for growing in slightly dilute seawater.
seahorse "dive leader" cartoon for BIOLOGY OF CARIBBEAN CORAL REEFS photograph of dive boat going through a phytoplankton bloom "The dive boat is travelling through a rich growth of surface phytoplankton.  Excessive growth of phytoplankton and other algae may result from nutrient enrichment from the land in the form of runoff water carrying fertiliser residues and sewage matter"  - Grand Bahama 2002

photo composite of soap and other sewage pollution entering the reef area


Soapy sewage water entering the
sea on a Caribbean island can add
to nutrient enrichment and to
excessive growth of algae and
blue-green cyanobacteria


photograph of sewage being released off the coast of GuamSewage release into a coral-reef area can never be good, but if the vents are properly sited and the material suitably screened, filtered, and diffused, its deleterious effects can be mitigated. In fact, as shown in the accompanying photograph of sewage being released off the coast of Guam, corals may actually benefit from the nutrient subsidies being provided. Dollar 1994 Coral Reefs 13: 224.



At the time of this photograph some 18m liters of primary-
treated effluent were being discharged daily from ports
about 260m seaward from the reef line at 20m depth. A
redesigned system (2013) now discharges about 24m liters
daily from ports sited 360m from the reef at 80m depth

  photograph showing degraded reef in Little Cayman IslandAlgal overgrowths and other major ecosystem changes, such as shown on this reef, can result from localised enrichment combined with loss of herbivorous fishes, snails, and sea urchins
  Once healthy during the 1980s, a patch reef of brain corals Diploria strigosa in southern Florida was nonetheless seasonally subject to rainy-season nitrogen-loading that led to overgrowth by green algae Cladophora sp. (see Left photo below). Later, during the early 1990s, low-salinity water was regularly pumped from overflow areas in the Everglades. This caused excessive nitrogen loading, turbidity, and sedimentation, leading to overgrowth by low-salinity tolerant boring sponges, Cliona lampa, and increased mortality of Diploria (middle photo). Later, during the early 2000s, after the Everglades overflows were diverted elsewhere, Cladophora algae again became abundant (Right photo). This changed, possibly as a result of increased turbidity following Hurricane Wilma in 2006, when the boring sponges returned. Excess turbidity and nitrogen enrichment seem to be the issue. The “double whammy” effects of eutrophication from watershed drainage and tropical storms leading to cyclical overgrowths by algae and sponges, have severely affected the survival of these brain corals. Littler & Littler 2007 Coral Reefs 26: 515; photographs courtesy the authors.
photograph of brain coral Diploria strigosa in Florida with overgrowth of green alga Cladophora photograph of brain coral Diploria strigosa in Florida with overgrowth of boring sponge Cliona lampa photograph of brain coral Diploria strigosa in Florida with overgrowth of green alga Cladophora
Photograph of living brain coral Diploria strigosa in Florida with overgrowth of green alga Cladophora Photograph of partially dead brain coral Diploria strigosa in Florida with overgrowth of of sponge Cliona lampa Photograph ofdead brain coral Diploria strigosa in Florida overgrown by Cladophora vagabunda