An estimated 80% of marine pollution comes from land-based sources. Dirt, oil, nutrients, and chemicals enter the oceans as runoff; polluted liquid that flows from land to sea. The sources of runoff are numerous, and its impacts on coral reefs, especially those in costal areas, are profound.
In elementary school, one of my teachers presented the class with a model of our costal city. She handed a few students bottles of different colored water, and asked them to ‘water their lawns,’ ‘wash their cars,’ and ‘make it rain’ on the city. They proceeded to drench the model. Then she told us to watch where the water went. It rolled down into a small pool of clear water at the bottom of the model. We watched the pool turn to a brownish-grey hue.
The point of the model was to show us how pollutants, in the form of runoff, flow from land to sea. We couldn’t really see the ocean turn greyish-brown in real-life, so the model let us visualize where a lot of the pollution was coming from, and what it was doing to the sea water. As kids who grew up on the shore, the lesson hit home. Most of us weren’t strangers to summer time beach closures due to high levels of pollution.
Runoff poses a significant threat to coral reefs. Pollutants such as oil, fertilizers, inorganic materials, sewage, sediments, and heavy metals are washed into oceans daily. As a nonpoint source pollutant, runoff is hard to control, precisely because it enters the water in many places and because the pollutants originate from so many different sources. Both urban and rural environments contribute to the runoff problem.
Agricultural runoff poses a serious threat to coral reefs. Over-irrigation and rainstorms cause nutrients such as nitrogen and phosphorus from fertilizers to flow into the sea. Ordinary levels of nitrogen and phosphorous are essential for life on reefs, but increased levels of nutrients result in algal blooms. Algae thrive off of these nutrients, and grow at alarming rates on reefs when the water becomes over-saturated with nutrients. Eventually the algae take over, and the coral cannot compete for resources, so they die.
As the EPA points out, “When nutrient levels increase, the delicate balance that exists between corals and algae is destroyed and the algae can overgrow the corals. When this situation is prolonged, the corals are smothered and die beneath the algal carpet. This, in turn, affects the fish and other aquatic organisms using the area, leading to a decrease in animal and plant diversity and affecting use of the water for fishing and swimming.”
Katharina E. Fabricius of The Australian Institute of Marine Science has noted the impacts of nitrogen and phosphorous-rich terrestrial run-off on reefs in her paper,“Effects of terrestrial runoff on the ecology of corals and coral reefs: review and synthesis”. She writes that “considerable effort has gone into experiments studying the direct effects of elevated dissolved inorganic nitrogen (DIN, as nitrate or ammonium) and phosphate (DIP) on coral calcification, tissue growth and zooxanthellae.” She goes on to conclude that “chronically increased levels of dissolved inorganic nutrients may alter reef metabolism and reef calcification sufficiently to cause noticeable changes in coral communities.”
Australia’s Great Barrier Reef is at risk due to agricultural runoff. According to a report by students at the University of Michigan, “80% of the land adjacent to the Great Barrier Reef is farmland that supports agricultural production, intensive cropping of sugar cane, and major beef cattle grazing (GBR, 2007). These types of agriculture and cattle production pose large threats to the Great Barrier Reef close by.” Nutrients from these farm areas reach the reef, and cause damage.
Excess nutrients also reach the sea when human or animal waste is discharged as untreated wastewater into the sea or when sewer systems overflow before treatment. According to UNEP “around 60% of the wastewater discharged into the Caspian Sea is untreated, in Latin America and the Caribbean the figure is close to 80%, and in large parts of Africa and the Indo-Pacific the proportion is as high as 80-90%.” As the EPA describes, aside from adding excess nutrients to the water, this untreated sewage can bring bacteria and other pathogens to reefs that can cause coral disease and death.
Other toxic materials also impact coral reefs. Heavy metals, chemicals, and oils runoff into the ocean from urban areas and poison corals. The upset the chemical balance of water that is necessary for coral to live. Most of the oil in the world’s oceans does not come from large-scale oil spills, but rather from smaller sources such as runoff. The World Wildlife Fund states that only around 12% of the oil that enters the sea each year comes from oil spills. The US National Resources Council estimates that 36% of oil that enters the sea comes “as waste and runoff from cities and industry.”
The question that begs to be asked is what can be done to prevent damage to coral reefs by runoff? For one, the US government has enacted the “The Coastal Nonpoint Source Pollution Control Program.” The program was passed by Congress in order to achieve “coordination between state coastal zone managers and water quality experts to reduce polluted runoff in the coastal zone.”
The program, which focuses on runoff-pollution prevention and is jointly administered by NOAA and the EPA, “establishes a set of management measures for states to use in controlling polluted runoff. The measures are designed to control runoff from six main sources: forestry, agriculture, urban areas, marinas, hydromodification (shoreline and stream channel modification), and wetlands and vegetated shorelines, or riparian areas. These measures are backed by enforceable state policies and actions—state authorities that will ensure implementation of the program.” Thus the goal of the program is to give governments the tools and the power to regulate and prevent runoff. Though these laws are significant achievements, the challenge to lessen runoff is still steep, and threats to coral reefs as a result of runoff are ever-present.