Ocean Pollution's Impact On Freshwater Sources And Systems

Oceans are among the most valuable natural resources on Earth, governing the weather, cleaning the air, and providing food and livelihoods for millions. However, human activities have led to ocean pollution, which poses a serious threat to the health of marine ecosystems and humans alike. With 80% of marine pollution originating on land, this issue has far-reaching consequences, including negative health outcomes and the degradation of marine life and ecosystems. This paragraph introduces the topic of how ocean pollution affects freshwater, highlighting the sources and impacts of marine pollution and its potential consequences for freshwater environments and human health.

Nonpoint source pollution

The effects of nonpoint source pollution on specific waters may not always be fully assessed, but it is known to have harmful impacts on drinking water supplies, recreation, fisheries, and wildlife. For example, nonpoint source pollution can lead to eutrophication, where excess nutrients like nitrogen and phosphorus from agricultural runoff cause algal blooms, resulting in aquatic dead zones.

Atmospheric pollution

Ocean acidification is a serious consequence of atmospheric pollution. As the oceans absorb airborne carbon dioxide (CO2), chemical reactions occur that reduce seawater pH. This increased acidity has detrimental effects on many marine organisms. For example, it threatens the survival of creatures like mussels, clams, coral, and oysters, which require calcium carbonate to build their shells and skeletons. The decrease in ocean pH also contributes to coral reef bleaching and makes it more difficult for some fish to sense their prey or detect predators.

Furthermore, atmospheric pollution can lead to algal blooms, which have negative impacts on coral reefs and coastal ecosystems. These blooms occur when excess nutrients, such as nitrogen and phosphorus, are introduced into bodies of water through runoff or wastewater. Algal blooms consume oxygen and block sunlight, leading to hypoxic environments that can trigger coral bleaching events and reduce the recovery capacity of corals.

The effects of atmospheric pollution on ocean ecosystems are not limited to marine life. Pollutants can accumulate in seafood, making it harmful for human consumption. Small organisms ingest toxins, which are then passed on to larger predators, including fish that are eventually consumed by humans. This can lead to long-term health issues, cancer, and birth defects.

To address atmospheric pollution and its impact on oceans, strategies such as Carbon Capture and Storage have been proposed. This involves capturing CO2 directly at industrial or power plant sources and storing it in secure subsurface reservoirs. Additionally, measures have been enacted under international agreements to regulate the injection of carbon dioxide into sub-seabed geologic formations to mitigate climate change.

Oil spills

There are many types of oil, and the characteristics of the oil, such as viscosity, volatility, and toxicity, will influence the clean-up methods used. For example, the density of oil varies, and this can determine whether it floats or sinks in water. Oil with a density of 1.01 grams per cubic centimetre would float in saltwater but sink in freshwater.

When oil enters freshwater systems, it can have severe consequences for the organisms that live there. Oil can coat the feathers and fur of birds and marine animals, impairing their ability to insulate themselves and fly, and causing them to ingest the oil when they clean themselves. It can also suffocate fish and block light from photosynthetic plants. Oil can settle at the bottom of the water, affecting the worms, insects, and shellfish that live in the sediment. It can also be toxic to frogs, reptiles, waterfowl, and other animals that make the water their home. Oil spills can further impact vegetation, coating the leaves of grasses and plants, and preventing them from photosynthesising.

The impacts of oil spills on freshwater habitats depend on the rate of water flow and the specific characteristics of the habitat. Standing water, such as marshes or swamps with little water movement, is likely to be more severely affected than flowing water. In calm water conditions, the affected habitat may take years to restore. Oil spills in flowing water are less severe due to the natural cleaning mechanism provided by currents. However, oil can still collect along the banks of rivers, clinging to plants and grasses, which can then be ingested by animals.

While there are methods to clean up oil spills, such as using booms to contain the oil, applying chemical dispersants, skimming, and burning, there are large gaps in our knowledge about the impacts of oil spills on freshwater systems and the most effective clean-up methods.

Algal blooms

Some species of algae grow in clumps covered in a gelatinous coating, allowing cells to stick together into large surface scums in calm weather. Other algae form thick mats that float on or just below the surface along the shoreline.

While algae are typically not harmful to people, some species of algae produce toxins that can affect native aquatic organisms, livestock, pets, and even people who come into contact with them. When algal blooms block sunlight from reaching beneficial underwater plants, the ecosystem can be negatively impacted. As algae deplete the nutrient supply or move from freshwater into saltier waters, they become stressed and die. The decomposition of dying algae can reduce levels of dissolved oxygen in the water, which can be fatal for some fish species.

In addition, some algal species can cause fish kills directly, either by producing algal toxins or by clogging the gills. Dense, widespread blooms have led to fish kills and numerous reports of skin rashes, unappealing odours, and accumulations of foam and shoreline scums.

To prevent algal blooms, individuals can play a role in reducing nutrient pollution by using fertilizers wisely and only when necessary, using chemicals responsibly, and properly maintaining stormwater systems.

Hypoxic environments

Hypoxia, or low levels of dissolved oxygen, is a significant concern in both ocean and freshwater environments. It occurs when there is less than 2-3 milligrams of oxygen per liter of water and can have detrimental effects on the ecological and economic health of impacted areas.

In ocean and freshwater environments, hypoxia is often associated with the overgrowth of certain species of algae. When excess algae die, they sink to the bottom and decompose, a process that consumes oxygen, leading to oxygen depletion in the water. This, in turn, can result in the creation of "dead zones", where life cannot be sustained due to insufficient oxygen levels. These dead zones can cause die-offs of fish, shellfish, corals, and aquatic plants, impacting both the environment and industries that depend on these resources, such as fisheries.

Hypoxia occurs naturally but has become more prevalent due to human activities. Nutrient pollution, especially from agricultural runoff, fossil fuel burning, and wastewater treatment effluent, is a significant contributor to hypoxia. Climate change may also increase the occurrence of hypoxic conditions. More frequent and intense storms, coupled with warming waters, can lead to increased water column stratification, enhanced nutrient input, and reduced oxygen capacity.

The Northern Gulf of Mexico is a notable example of a hypoxic zone, with a significant dead zone forming each spring. In 2019, it covered over 6,900 square miles of the seafloor. This area is influenced by the Mississippi River, where nutrient-rich freshwater mixes with saline seawater, creating layers that prevent the mixing of oxygen-rich surface water with oxygen-poor bottom water.

Hypoxia has far-reaching consequences, affecting marine life, habitats, and human economies. It can cause physiological, developmental, growth, and reproductive abnormalities in fish and other aquatic organisms. Additionally, hypoxia disrupts essential ecosystem services like nutrient cycling and biodiversity, which are crucial for maintaining the balance and functioning of ecosystems.

Frequently asked questions