Seagrass Struggles: Understanding Pollution's Impact

Seagrass meadows are vital to the health of the planet. They are flowering plants that grow in shallow marine waters all over the world, forming huge underwater meadows that provide food and shelter to a variety of species, including endangered animals such as dugongs, sea horses, and sea turtles. Seagrass also plays a crucial role in slowing global climate change by reducing acidification of seawater and acting as a carbon sink, storing about three times as much carbon as an acre of rainforest. Despite their importance, seagrass meadows are among the most threatened habitats in the world, with a football-field-sized area of seagrass disappearing every half-hour due to dredging, pollution, and disease. One of the main threats to seagrass is pollution, particularly agricultural and urban runoff, which increases nutrient levels in the water, specifically nitrogen levels, leading to rapid blooms of algae that block sunlight and kill the seagrass. Noise pollution has also been found to affect the structures within seagrass that help it detect gravity and store energy, causing it to uproot itself.

Agricultural and urban runoff

The impact of agricultural and urban runoff is evident in the decline of seagrass meadows globally. A recent study revealed that 88% of seagrass is exposed to wastewater nitrogen, leading to rapid algae blooms that deprive seagrass of sunlight. This has resulted in a concerning loss of seagrass, estimated at a football-field-sized area every half-hour.

However, there is hope in mitigating these effects. Implementing water filtration systems and improving sewage treatment plants have proven effective in reducing nitrogen levels and promoting seagrass growth. For instance, Boston's installation of a treatment facility led to an 80% decrease in nitrogen levels and a 50% increase in eelgrass. Similarly, Tampa Bay in Florida witnessed a 66% decrease in nitrogen due to sewage treatment plant upgrades.

While the implementation of such treatment facilities can be costly and challenging, individual actions can also play a role in reducing runoff pollution. Proper vehicle maintenance, responsible waste disposal, and mindful use of fertilisers and pesticides can collectively contribute to reducing the harmful effects of agricultural and urban runoff on seagrass ecosystems.

By addressing these issues, we can protect and restore seagrass meadows, recognising their importance as carbon sinks and guardians of coastal ecosystems.

Eutrophication

Cultural eutrophication involves the excessive addition of nutrients, particularly nitrogen and phosphorus, to coastal systems. This has several detrimental effects on seagrass ecosystems:

Light Reduction

Cultural eutrophication stimulates the growth of phytoplankton, epiphytes, and macroalgae, which can block sunlight, a vital resource for seagrass photosynthesis. The overgrowth of these organisms can lead to a decline in seagrass populations, as seagrass cannot survive in nutrient-rich environments due to excessive algae growth.

Toxicity

High concentrations of inorganic nitrogen and phosphorus can be toxic to seagrass. Ammonium toxicity, for example, can impair seagrass growth and survival.

Water-Column Nitrate Inhibition

Water-column nitrate enrichment can negatively impact seagrass by disrupting internal nutrient supply ratios and affecting plant metabolism.

Indirect Stressors

Cultural eutrophication can also have indirect effects on seagrass ecosystems. For example, nutrient enrichment can lead to increased sediment re-suspension, system respiration, and oxygen stress. Additionally, thick macroalgal growth can depress advective water exchange, leading to biogeochemical alterations and increased hydrogen sulfide concentrations.

• Herbivore Loss
• Altered Species Interactions

Cultural eutrophication can disrupt natural seagrass population shifts and alter species interactions. For instance, it can favor the proliferation of exotic grazers that outcompete seagrasses for space.

Carbon Sequestration Impairment

Seagrass meadows are vital carbon sinks, storing and sequestering significant amounts of carbon. However, cultural eutrophication can lead to large-scale seagrass loss, reducing their carbon storage capacity.

Noise pollution

The study found that just two hours of noise exposure damaged the organ in the seagrass responsible for detecting gravity, which could in turn affect the plant's ability to stay rooted in the soil. The number of starch grains inside the organ also decreased, which could impact the seagrass's ability to store energy and continue to grow.

In addition, noise pollution was found to have a negative impact on a symbiotic fungus inside the seagrass roots that is likely involved in boosting nutrient uptake. This type of damage could affect the seagrass's ability to sequester carbon. Overall, noise pollution has the potential to cause dire effects on marine ecosystem health, water quality, shoreline stabilization, and the climate crisis.

It is important to note that seagrass does not have ears or traditional hearing structures. However, it does have a sensory organ called an amyloplast, which is similar to the statocysts found in cephalopods and other marine animals. Amyloplasts are gravity-sensing cellular structures that help underwater plants push their roots down through seafloor sediments.

Climate change

Seagrass meadows are an important part of the marine ecosystem, providing food and shelter to many species, improving water quality, and reducing shoreline erosion. However, seagrass meadows are extremely vulnerable to climate change.

Increased water temperature

Increased water temperature will directly affect seagrass metabolism and the maintenance of a positive carbon balance, which may result in changes in seasonal and geographic patterns of species distribution.

Rising sea levels

Rising sea levels will reduce the amount of light reaching seagrass beds, thereby reducing growth rates. Some grass beds may not persist due to shoreline construction and armoring impeding their shoreward migration.

Increased salinity

Increased salinity due to decreased freshwater input and increased evaporation rates can cause physiological stress to the vegetation.

Increased water temperature and nutrient enrichment

Increased water temperature and nutrient enrichment together promote hypoxia and harmful algal blooms.

Increased precipitation

Increased precipitation will lead to increased runoff, impacting water quality by increasing nutrients, pollutants, and turbidity.

Ocean acidification

Increasing atmospheric carbon dioxide will directly elevate the amount of CO2 in coastal waters. In areas where seagrasses are carbon limited, this may increase primary production, although whether this increase will be sustained with long-term CO2 enrichment is uncertain. The impact of increases in CO2 will vary with species and environmental circumstances, but will likely include species distribution by altering the competition between seagrass species as well as between seagrass and algal populations.

Increased UV-B radiation

The effects of UV-B radiation will likely be greatest in the tropics and in southern oceans. The reaction of seagrasses to UV-B radiation may range from inhibition of photosynthetic activity, as seen for terrestrial plants and marine algae, to the increased metabolic cost of producing UV-B blocking compounds within plant tissue.

Boat damage

Boats and boating activities can cause significant damage to seagrass meadows, threatening these vital ecosystems. One of the main ways boats damage seagrass is through propeller contact and anchoring. Boat propellers can scar and destroy seagrass blades, while dropping anchors on seagrass beds can rip them apart, uprooting these important plants. In the western Mediterranean, studies suggest that beds of the endemic seagrass Posidonia have declined by 34% over the last 50 years, with trawling and anchoring being the most destructive human-caused factors.

To mitigate this damage, it is essential for boat captains to be aware of seagrass locations and take measures to avoid them. In Spain, a nonprofit organization has developed a phone app that shows all the seagrass beds around the Balearic Islands, helping boat crews navigate away from these fragile ecosystems. Responsible anchoring practices, such as avoiding seagrass beds and anchoring in sandy or muddy areas instead, can also help reduce damage to seagrass meadows.

In addition to physical damage, boats can also contribute to water pollution, which indirectly affects seagrass health. Boat maintenance and repair often involve the use of solvents, oils, paints, and cleansers, which can spill into the water or enter through runoff. Small oil spills from motors and refueling activities can introduce petroleum hydrocarbons that persist in aquatic ecosystems, harming bottom-dwelling organisms crucial to the marine food chain. Poorly maintained sanitary waste systems on boats can also lead to high bacteria levels in the water, impacting seagrass ecosystems.

To reduce pollution from boats, it is recommended that boaters use non-toxic and biodegradable cleaning products that are safe for the environment and do not harm aquatic life. Properly disposing of sewage and waste is also crucial, as improper disposal can degrade water quality and stimulate algae growth, which blocks sunlight and leads to the death of seagrass. Additionally, boaters can reduce their carbon footprint by using alternative fuels, such as biodiesel or ethanol, and incorporating renewable energy sources like solar panels.

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