Fishing fleets. Photo © Elle Wibisono

Land-Based Pollution

Human activities occurring far inland can have serious impacts on coral reefs. Deforestation and clearing of farmlands result in sediment erosion into rivers and eventually coastal waters. The application of excessive amounts of fertilizer and pesticides spread on crops can wash off or leach out of soils into waterways and coastal ecosystems.

sediment discharge

Massive discharge of sediment loads by a river entering the Caribbean Sea off the Meso-American coast. Photo © Malik Naumann/Marine Photobank

Overgrazing by livestock exacerbates these problems; removal of vegetation increases erosion. In addition, livestock waste adds nutrient pollution to coastal areas. Mining results in sediment runoff and either direct discharge or leaching of toxins into waterways and eventually to estuaries and coastal waters. Sediments, nutrients, and pollutants can disperse into plumes extending more than 100 km from the river mouth. ref

Solid wastes, whether randomly dumped or in designated coastal dumps or landfills, can directly kill corals or leach toxins into the inshore waters, potentially stressing corals.

Climate changes, particularly changes in precipitation patterns (e.g., heavier and more frequent precipitation in some areas) are likely to increase erosion and exacerbate sedimentation and pollution runoff into coastal waters. Increasing global populations are also likely to lead to increases in the extent and intensity of agriculture, which could further increase fertilizer use and runoff.

Marine-Based Pollution

Marine-based sources of pollution can adversely affect reef health. Commercial, recreational, and passenger vessels can threaten reefs by discharging contaminated bilge water, fuel, raw sewage and solid waste, and by spreading invasive species.

entangled fish

Discarded gill net with many fish entangled and killed in Hawaii. The net had trapped parrotfish, wrasses, goatfish, surgeonfish, coral crabs, spiny lobsters, slipper lobsters and other fishes. Photo © Frank Baensch-bluereefphoto.org/Marine Photobank

Cruise ships can contribute tremendous amounts of pollution to the marine environment. In one week, a typical cruise ship generates about 800 cubic meters of sewage, 3,700 cubic meters of greywater, half a cubic meter of hazardous waste, 8 tons of solid waste, and nearly 100 cubic meters of oily bilge water. ref While the International Convention for the Prevention of Pollution from Ships (MARPOL) provides guidelines regulating the discharge of sewage, oily bilge water, hazardous wastes, and solid waste, these regulations are met with varying degrees of compliance. ref

Marine debris from boats including plastics and abandoned fishing gear can also physically damage reefs and entangle marine species such as fish and turtles. Oil leakage from ships can cause corals to experience tissue death, change their rate of calcification, expel zooxanthellae, and lead to larval death, among other stress responses. As global demand for oil increases, the likelihood of spillage also increases. Maritime shipping and cruise tourism also continue to grow, ref leading to greater leakage of contaminated bilge water, fuel, raw sewage, and solid waste into the marine environment.

Impacts of Land- and Marine-Based Pollution

These include:

  • Deforestation and agricultural practices — can result in sediment, nutrient, and pesticide run-off into rivers and eventually coastal waters
  • Sediments — can smother and kill corals and other benthic organisms; can also reduce the ability of zooxanthellae to photosynthesize which slows coral growth
  • Excessive nutrient levels (e.g., nitrogen and phosphorous in coastal waters) — can lead to eutrophication where phytoplankton blooms block light from corals, or can stimulate algal growth that can out-compete or overgrow corals; can also lead to hypoxia, where decomposition of algae and other organisms consumes all of the oxygen in the water, leading to “dead zones” and eventually near shore ecosystem collapse
  • Oil spills — can result in coral degradation and mortality
  • Contaminated bilge water, fuel, raw sewage, and solid waste — leaked by commercial, recreational, and passenger vessels; this can threaten reefs directly and also by spreading invasive species

Land-based pollution can be addressed through a variety of land-use policies, plans and management practices. These include improved agricultural methods that can reduce erosion and runoff, increased fertilizer efficiency, preservation of coastal ecosystems (mangroves and seagrasses) that filter and trap sediments and nutrients before reaching reefs, and maintenance of vegetation along rivers to reduce nutrient and sediment run-off into waterways. Integrated coastal zone management is a tool that is adopted and applied globally to address issues of land-use impacts on coastal ecosystems.

Marine-based pollution can be addressed at local scales by: ref

  • Developing infrastructure at ports to dispose of ship-generated waste
  • Improving wastewater treatment systems on cruise ships and cargo ships
  • Routing shipping lanes away from reefs
  • Disposing of ballast water offshore to reduce the spread of invasive species in coastal waters
  • Developing effective oil spill contingency plans

Countries with coral reefs can take a significant step towards reducing marine-based pollution by enforcing national legislation that incorporates international agreements on marine pollution (e.g., the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter 1972, also called the “London Convention”).

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