Water becomes polluted in a number of ways, but two board categories outline the different pathways of contamination: direct and indirect. Harmful fluids emitted into water habitats from factories and treatment plants are examples of direct pollution sources. Indirect pollution is released in the water via soil or rain and groundwater runoff. In 2010, a catastrophic accident led to an explosion on the oil rig Deepwater Horizon in the Gulf of Mexico. This tragedy polluted a habitat that is home to innumerable species. Aquatic ecologists show concern over the long-term effects of an oil spill of this magnitude. Deepwater Horizon is an extreme example of a direct pollution source.
As pollutants collect and the earth grows warmer, habitats change. While some organisms are at risk, others adjust to accommodate for change. Coral reefs are one underwater habitat that sustains an abundance of life. There are more than 700 species of coral that build reefs. Coral is covered in tiny polyps that feed off of light and surrounding microscopic organisms, such as phytoplankton. These polyps grow outward in search of more light and food. But the balance needed for this ecosystem to sustain itself is easily upset. Ecologists study the effects of fertilizer runoff in coral reef areas. The excess nutrients in the water system allow certain organisms to proliferate and suffocate the coral by blocking the sun. Natural events can also cause coral to die off. For example, when coral spawn, the refuse from their reproductive excretions collects. If the tide is too low to carry out the waste it will keep the sun from reaching the coral. The death of a coral reef results in the death of all the species that use it for food and shelter.
Invasive species are introduced into an ecosystem by accident or purposely. When an invasive species multiplies it can rob other organisms of nutrients and create issues for an existing ecosystem. An example is the crown-of-thorns starfish in Australia and Thailand. The starfish feeds on coral and nitrogen. When too many starfish are feeding on a reef it has the potential to kill off too much coral.
Plants can also be invasive species. The Giant Reed, or Spanish Reed, is an aquatic plant native to the Mediterranean region that causes flooding issues and crowds out native plant life. The reed was first introduced along the banks of lakes and rivers in the U.S. as a decorative piece of landscaping in the 1820s.
Aquatic ecosystems are directly affected by climate change. Longer summers with more snow run-off from melting glaciers will deplete the growth period for developing freshwater ecosystems. Fish populations, particularly migrating salmon, will suffer a blow due to the change in distribution of nutrients during the migration season. This is just one example of a change in the aquatic ecosystem because of climate change. But many species are put at risk due to longer exposure to summer heat and excessive glacial runoff.