Pollution

What it is
‘Pollution is the introduction of contaminants into a natural environment, which causes instability, disorder, harm or discomfort to the ecosystem’ [1]. Pollution from land based sources can cause degradation in water quality, direct toxicity to marine life and create long term problems if it becomes incorporated into seafloor sediment. Polluted marine environments are less healthy, stable and resilient to additional stresses.

Kinds of pollution
There are a multitude of pollutants that affect marine ecosystems, and cause varying degrees of problems. Runoff from agriculture, aquaculture operations, golf courses and other large cultivated areas contain pesticides, fertilizers and sometimes harmful bacteria, which can poison or cause diseases in marine life and increase nutrient load in the near-shore areas. Oil and other noxious chemicals are washed off roadways during heavy rains and aerial applications of pesticides, herbicides and fertilizers blow offshore in windy conditions. In areas where the energy industry operates, there is a risk of leaks or spills from well heads, pipelines and tankers. These are examples of pollutants that are incorporated into the water mass, but another serious pollutant problem is marine debris, particularly vast quantities of non-biodegradable plastic. Noise can also be a form of pollution, and is caused by anything that creates loud or persistent acoustic signals. This may include military operations with exploding ordinance, bottom echo-sounding, or long range acoustic signals that measure ocean currents. Chronic pollution can cause long term degradation of marine systems, but a large influx of pollutants (such as an oil spill) can have larger immediate impacts and catches the public’s attention. In most cases however, the former causes more long term damage to the environment than the latter.

Why it is a problem
High nutrient concentrations stimulate growth or ‘blooms’ of planktonic algae, some of which is consumed by small plankton and feeds the local food webs. Problems caused by nutrient increase are greater in waters that have naturally low nutrient levels, particularly coral reefs. A pulse of nutrients into a coral reef ecosystem allows fast growing algae to bloom. Algae in the water column block the light that corals need to feed their internal symbiotic algal cells. If this condition persists, the corals will starve as they use the carbohydrates produced by their symbionts as an energy source. High nutrients also stimulate the growth of filamentous or bushy algae that grows on exposed hard surfaces, and out-competes coral colonies for space and light. With massive nutrient influx, the algal blooms are not incorporated into the food web, but die and sink to the seafloor where they are consumed by bacteria. Bacterial respiration uses oxygen, creating a low oxygen (hypoxic) environment. When currents or winds are strong, this low oxygen water is swept up into the larger water masses and re-oxygenated. When currents are low or stagnant, these hypoxic areas create ‘dead zones’ where nothing can live. One of the largest and most well-known dead zones is at the mouth of the Mississippi, which carries agricultural runoff from the US interior into the Gulf of Mexico. The Gulf was also the site of one of the largest oil spills in US history. The impacts of oil on marine life depends on what class of oil is released; fresh oil contains volatile compounds that are toxic to marine life and older oil products are stickier and can cause seabirds to lose insulation, marine mammals to inhale oil through their blowholes, and sticky oil products can clog delicate feeding mechanisms of a wide variety of marine organisms. In warm waters, oil is degraded relatively quickly by bacteria, but in cold climates it persists and has a long-term impact on marine life. Marine debris, primarily non-degradable plastics, is becoming a serious global problem. Many plastic products float and are concentrated by oceanic currents. The plastics do not bio-degrade but over time break down into small pieces which are consumed by a variety of marine animals including turtles and seabirds. These cannot be digested and eventually the animal dies from blocked intestinal tracts or starvation. The vast majority of fishing nets are constructed of monofilament, which again, does not bio-degrade. When nets are lost or abandoned they become ‘ghost nets’ and continue to ensnare marine life indefinitely. Noise pollution particularly affects marine mammals, which have sensitive ‘hearing’ and use acoustic signals for communication. In the US, there are regulations that prohibit acoustic activity within sight of marine mammal pods, but it is unclear how effective this is since noise can travel vast distances underwater.

What can be done to help
Better coastal management is essential to reduce the quantity of nutrients and pesticides that enter the ocean. Better farming practices, such as recycling run-off water back onto the land can greatly reduce the cost of fertilizers and contamination of the waterways. Leaving natural filters, such as wetlands or mangroves in place can remove nutrients, sediments and toxins from the water before it enters the ocean. Recycling plastic materials can help, but the amount currently recycled is minute compared with the vast quantity of plastic that is discarded. In countries that do not have the infrastructure to collect and contain waste, much of their discarded plastics are simply thrown in piles, and in coastal communities, this could end up in the ocean. Helping these countries create recycling plants and educating the people about the impacts of discarding non-degradable products may help reduce the quantity of plastic in the ocean. Moving towards only biodegradable plastics (including fishing gear) would reduce the damage caused plastic debris and ghost nets. Response to large oil spills is generally efficient in developing countries, but chronic leaks in pipes and spills in some locations can still cause problems, which could be overcome with stricter oversight and regulations.

How Marine Protected Areas can help
As with ocean acidification, MPAs cannot protect marine ecosystems from pollutants; however the presence of an MPA can be used as leverage to create better coastal management and improve the overall health of the marine environment within the MPA boundaries. A healthy ecosystem may be more resilient to stressors such as oil spills or short term pollutants.

What you can do to help
You can help by recycling plastics, purchasing food that has been produced with minimal fertilizer and pesticide input, supporting sustainable fishing practices that do not degrade marine ecosystems and supporting regulations that reduce chemical and fertilizer inputs in our waterways. You can also help by supporting efforts to create MPAs, to increase ecosystem health and resilience to external stressors such as pollution.

[1] en.wikipedia.org/wiki/Pollution