Pollution's Impact On Phytoplankton: A Delicate Balance Disrupted

Phytoplankton are the main primary producers in aquatic ecosystems, and their productivity is controlled by a number of environmental factors, many of which are influenced by human activities. Pollution is one such factor, and it can affect phytoplankton in a variety of ways.

Pollution in marine environments can arise from different sources, including the influx of domestic sewage, industrial waste, mining effluents, shipping activities, microplastics, radiation, and heat. These sources of pollution can have both direct and indirect effects on phytoplankton communities, impacting their abundance, growth strategies, dominance, and succession patterns.

For example, pollutants may accumulate in phytoplankton cells and be passed on to other trophic levels, resulting in biomagnification of certain pollutants. Additionally, pollution can alter the phytoplankton community structure and cell size, with certain species becoming dominant due to their tolerance to pollution.

The impact of pollution on phytoplankton can also be influenced by synergistic interactions with other factors such as climate change, eutrophication, and harmful algal blooms.

Understanding the effects of pollution on phytoplankton is crucial for devising suitable remediation strategies and ensuring the health of marine ecosystems.

Eutrophication

One of the most common sources of cultural eutrophication is agriculture. The use of fertilizers on fields, lawns, and golf courses can result in nutrient runoff, which can cause an increase in algae growth and blooms. These blooms can shade benthic plants, altering the overall plant community. When the algae die off, their decomposition by bacteria consumes oxygen, potentially creating anoxic conditions that can kill aerobic organisms such as fish and invertebrates.

To prevent and reverse eutrophication, it is important to minimize point source pollution from sewage and agriculture, as well as nonpoint pollution sources. Introducing bacteria and algae-inhibiting organisms, such as shellfish and seaweed, can also help reduce nitrogen pollution and control the growth of cyanobacteria, a major source of harmful algae blooms.

Ocean acidification

Phytoplankton, which are microscopic plants that form the base of the ocean food web, are among the calcifying species affected by ocean acidification. Some phytoplankton, such as coccolithophorids, experience developmental disorders due to their inability to merge with carbonates and hydrogen ions.

Studies have shown that elevated levels of carbon dioxide can have both positive and negative effects on phytoplankton. While some species may benefit from increased carbon dioxide availability for photosynthesis, others may be harmed by the resulting decrease in pH.

For example, a study by Stephanie Dutkiewicz and others found that increased ocean acidification may cause dramatic changes to phytoplankton populations. Their research, which involved compiling data from 49 papers on the growth rates of phytoplankton under lower pH levels, revealed a range of responses. Some species thrived, while others died out.

Another study, led by Jennifer Chu, found that ocean acidification may significantly alter the balance of various phytoplankton species, with some experiencing population die-offs while others migrate towards the poles as the planet warms.

Additionally, a long-term in situ mesocosm experiment by Jan Taucher and colleagues investigated the impact of ocean acidification on plankton community structure during a winter-to-summer succession. They found that elevated carbon dioxide concentrations tended to increase the biomass of copepods, a type of zooplankton, by up to 30-40%. This effect was attributed to increased primary production and phytoplankton biomass, which provided more food for the copepods.

Overall, ocean acidification is expected to have significant impacts on phytoplankton populations, with potential consequences for the marine food web and biogeochemical cycling in marine ecosystems.

Microplastics

In addition, microplastics can act as vectors for toxic chemicals, which can interfere with the development and survival of marine organisms. They can also transport invasive species over long distances.

The impact of microplastics on phytoplankton is not limited to direct ingestion. Microplastics can also alter the availability of light, which is essential for photosynthesis in phytoplankton. As microplastics accumulate in the ocean, they can block sunlight, disrupting phytoplankton's ability to produce energy through photosynthesis and leading to their decline.

The presence of microplastics in the ocean can also affect nutrient cycling. When zooplankton consume microplastics, they may exhibit reduced grazing pressure on primary producers, leading to an increase in export production and organic particle remineralisation, which consumes oxygen and returns nutrients at depth. This can have a significant impact on the oxygen levels in the ocean, exacerbating the effects of climate change-driven deoxygenation.

Overall, microplastics pose a significant threat to phytoplankton and other marine organisms. Their presence in the ocean can disrupt the food web, alter nutrient cycling, and impact oxygen levels, ultimately affecting the health and productivity of aquatic ecosystems.

Heat (thermal pollution)

Thermal pollution is the rise or drop in the temperature of a natural body of water caused by human activity. Phytoplankton are microscopic photosynthetic algae that form the base of the marine food web.

Effects of heat on phytoplankton

• Phytoplankton growth and primary productivity are reduced.
• Phytoplankton biomass decreases.
• Phytoplankton diversity decreases.
• Phytoplankton distribution and abundance become more heterogeneous.
• Phytoplankton community composition changes, with cyanobacteria, diatoms, and green algae being the most affected.
• Phytoplankton phenology (the timing of bloom initiation, duration, and termination) is altered, with blooms occurring later, terminating earlier, and lasting for a shorter duration.
• Phytoplankton metabolic rates increase.
• Phytoplankton cell walls become less permeable to osmosis.
• Phytoplankton protein coagulation occurs.
• Phytoplankton enzyme metabolism is altered.

Mechanisms

• Warmer water reduces vertical mixing between the surface and deeper, nutrient-rich waters. This limits the transfer of nutrients to the surface, where phytoplankton grow, leading to reduced growth and productivity.
• Warmer water increases stratification and suppresses mixing, further inhibiting the upward transfer of nutrients.
• Warmer water increases phytoplankton metabolic rates, growth, and reproduction rates, which can lead to species overpopulation and shorter lifespans.
• Warmer water can alter the balance of microbial growth, including the rate of algae blooms, which reduce dissolved oxygen concentrations.
• Warmer water decreases dissolved oxygen levels, which can kill fish and alter food chain composition.
• Warmer water can reduce species biodiversity and foster the invasion of new thermophilic species.
• Warmer water can increase the solubility and kinetics of metals, leading to the uptake of heavy metals by aquatic organisms and potentially toxic outcomes.

Radiation

Effects of Radiation on Phytoplankton

Radioactivity in the ocean can have a direct impact on phytoplankton. Phytoplankton are the base of the marine food web and are responsible for as much photosynthesis as plants on land. Phytoplankton can take up radioactive contaminants from the seawater that surrounds them. Excessive UV-B radiation can impair photosynthesis, inhibit phytoplankton growth rates, and cause lethal DNA damage.

Indirect Effects of Radiation on Phytoplankton

Radioactivity in the ocean can also have indirect effects on phytoplankton. Phytoplankton are consumed by zooplankton, small fish, and larger animals up the food chain. Some of the contaminants end up in fecal pellets or other detrital particles that settle to the seafloor. These particles accumulate in sediments, and some radioisotopes contained within them may be remobilized back into the overlying waters through microbial and chemical processes.

Factors Affecting the Impact of Radiation on Phytoplankton

The impact of radiation on phytoplankton depends on a host of factors, including:

• The length of exposure to radiation
• The size and species of the organisms
• The radioisotopes involved
• The temperature and salinity of the water
• The amount of oxygen in the water
• The life stage of the organisms

Radioactivity can be transferred to marine organisms from contaminated sediments and food. Food may be the most important factor in the uptake of radioactivity. Consumed radioisotopes are assimilated internally through the gut, potentially a far more efficient route than if they are absorbed externally from the environment. Marine invertebrates, such as bottom-dwelling starfish and sea urchins, are particularly proficient at absorbing a wide range of ingested radioisotopes.

Biomagnification

Of particular concern for top-level consumers is the potential that radioisotopes will be concentrated as they make their way up the food chain—a process known as biomagnification. Fortunately, cesium shows only modest biomagnification in marine food chains—much less than mercury, a toxic metal, or other harmful organic compounds such as the insecticide DDT and polychlorinated biphenyls (PCBs).

Frequently asked questions