Ocean Pollution's Impact On Phytoplankton: A Complex Relationship

Ocean pollution can affect phytoplankton in a variety of ways. Phytoplankton are tiny plants that float in marine and aquatic ecosystems, and they are the foundation of the ocean ecosystem. They are responsible for producing 50-80% of the world's oxygen and are on the menu for many other organisms. However, pollution may arise from different sources, such as the influx of domestic sewage, industrial waste, and mining effluents. Shipping activities, including accidental oil spills and bioinvasion, also contribute to pollution.

Pollution affects phytoplankton communities at different levels, including abundance, growth strategies, dominance, and succession patterns. Even if no direct changes in phytoplankton communities are visible, pollutants may accumulate in phytoplankton and be passed on to other trophic levels, resulting in biomagnification of certain pollutants.

Some of the effects of pollution on phytoplankton include:

- Changes in abundance, growth, and development

- Increased resistance to antibiotics and metal salts

- Alterations in species composition and dominance

- Reduced calcification

- Impacts on the larval development of intertidal organisms

- Changes in the phytoplankton community structure and cell size

- Increased toxicity

- Disruption of the planktonic food web

The impact of oil spills on phytoplankton

Phytoplankton are the base of the food web in oceanic systems. As such, any changes to the phytoplankton community will have a ripple effect on the trophic levels above them. Oil spills in the ocean can have a substantial influence on marine ecosystems, and phytoplankton are no exception.

Oil Spills and Phytoplankton

Oil spills form oil films on the water with different thicknesses. These oil films prevent the interaction between the ocean and atmosphere, including the exchanges of gas, heat, and energy transfer. As a result, phytoplankton photosynthesis intensity changes drastically, increasing or decreasing depending on the type and concentration of oil spills.

The short-term impact of oil spills on phytoplankton could depend on the quantity and composition of the oil, while the long-term impact could be related to the biodegradation of microorganisms. Some studies have shown that in the short term, the concentrations of phytoplankton are reduced, and in the long term, an outbreak of algal blooms occurs when the Chlorophyll-a (Chl-a) concentration increases.

Phytoplankton's Response to Oil Spills

Phytoplankton can respond to the ambient light conditions by selecting their vertical position in the water column. Some phytoplankton use active motility, while others change their buoyancy by producing gas vacuoles or oil vesicles. Often, the motility of phytoplankton is directed by light and gravity. At low light intensities, cells often move to the surface using positive phototaxis and negative gravitaxis. When exposed to higher photosynthetic active radiation (PAR) and ultraviolet (UV) irradiances, the cells often switch to negative phototaxis and/or sediment to escape into deeper waters.

In the open ocean, these vertical migrations and orientation mechanisms are often overruled by the action of wind and waves, which passively transport the phytoplankton within the mixing layer. However, active movement superimposes the passive mixing, so that vertical distribution patterns of phytoplankton can be seen in the water column.

The effects of nutrient pollution on phytoplankton

Nutrient pollution can have a significant impact on phytoplankton, affecting their abundance, growth, and development. Nutrient pollution can cause eutrophication, which can lead to rapid growth and reproduction of phytoplankton. This can result in harmful algal blooms, which can have negative consequences for aquatic ecosystems, human health, and the economy.

Nutrient pollution can also alter the composition of phytoplankton communities, with certain species becoming dominant and others being suppressed. This can have cascading effects on the food web and higher trophic levels.

Additionally, nutrient pollution can affect the cell size and structure of phytoplankton, with smaller cell sizes and reduced diversity observed in polluted environments.

Nutrient pollution can also impact the accumulation and transfer of pollutants in phytoplankton, which can result in biomagnification and potential harm to other organisms and humans.

Overall, nutrient pollution can have both direct and indirect impacts on phytoplankton, with potential consequences for aquatic ecosystems and human well-being.

The effects of microplastics on phytoplankton

Microplastics are a growing concern globally due to the risks they pose to ecological communities. Phytoplankton are key ecological communities in aquatic ecosystems, providing energy to food webs and playing critical roles in ecosystem functions such as carbon cycling. Phytoplankton are not defenceless against chemical pollution; when exposed to contaminants, they may activate cellular responses to reduce their toxicity.

Microplastics can alter phytoplankton community composition. High microplastic concentrations have been shown to significantly alter the structure of phytoplankton communities, largely driven by increased abundances of cyanobacteria taxa.

Microplastics can affect phytoplankton in several ways. They can affect phytoplankton photosynthesis and growth, have toxic effects on zooplankton and affect their development and reproduction, and affect the marine biological pump and ocean carbon stock.

Zooplankton grazing of microplastics can accelerate the global loss of ocean oxygen. The reduction of grazing pressure on primary producers causes export production to increase. Consequently, organic particle remineralisation in these regions increases, which could decrease water column oxygen inventory by as much as 10% in the North Pacific and accelerate global oxygen inventory loss by an extra 0.2–0.5% relative to 1960 values by the year 2020.

The effects of thermal pollution on phytoplankton

Thermal pollution is a pressing issue for aquatic ecosystems, and phytoplankton are particularly vulnerable to its effects. 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.

One of the most significant factors affecting phytoplankton is temperature. Phytoplankton productivity is usually limited by the availability of nutrients, and increasing temperatures can enhance stratification and decrease the depth of the upper mixing layer, reducing the upward transport of nutrients from deeper layers. This can expose phytoplankton to higher solar radiation and impair productivity.

Thermal pollution can also interact with other stressors, such as nutrient availability and solar radiation, to impact phytoplankton. For example, increasing temperatures and enhanced nutrient supply can support the occurrence of harmful algal blooms, which are often formed by dinoflagellates.

In addition, thermal pollution can affect the species composition of phytoplankton communities, favoring blooms of toxic prokaryotic and eukaryotic species. Climate change-driven ocean changes, such as ocean warming and acidification, can lead to unpredictable and nonlinear biological responses in phytoplankton, making it challenging to develop effective management strategies.

Overall, thermal pollution has significant effects on phytoplankton, altering their growth, photosynthesis, species composition, and interactions with other organisms in the ecosystem. These impacts can have cascading effects on higher trophic levels and disrupt the balance of aquatic ecosystems.

The effects of radiation on phytoplankton

Phytoplankton are the main primary producers in aquatic ecosystems. They are responsible for about half of the biomass production on Earth, and their productivity is controlled by a number of environmental factors, many of which are influenced by human activities.

Phytoplankton are not uniformly distributed across the globe. They are most abundant in the circumpolar regions, while in tropical and subtropical waters, their concentrations are 100 to 1000 times smaller. This is because UV-B levels are much higher in the tropics and subtropics than at higher latitudes. Phytoplankton are also found in large numbers in the upwelling areas along the continental shelves, where high turbidity and gelbstoff concentrations are found, accompanied by high nutrient concentrations.

Phytoplankton are not protected by an epidermal layer, and both marine and freshwater phytoplankton are affected by UV-B radiation. They are, therefore, under considerable UV-B stress even at ambient levels of radiation. Phytoplankton are also affected by UV-A and visible radiation.

Phytoplankton display a distinct vertical distribution in the water column to optimise their light input for growth and survival. They tend to move to a specific depth, which results in a typical vertical distribution pattern. This pattern is disturbed by passive mixing due to high wind and waves. Phytoplankton are also affected by the consumers that follow them, which are thus exposed to the same radiation regime.

Phytoplankton are motile, and their motility is affected by exposure to solar radiation. Both the percentage of motile cells and the swimming velocity of the still motile cells decrease dramatically when exposed to unfiltered radiation. Some phytoplankton stop moving after excessive irradiation and, since the cells are heavier than water, they sediment in the water column to a greater depth.

Phytoplankton are also affected by UV-B radiation in terms of their metabolism and development. Solar UV-B radiation affects many physiological and biochemical reactions in microorganisms. At lower doses, the radiation inhibits growth and disturbs the endogenous rhythms in many microorganisms.

Phytoplankton are also affected by UV-B radiation in terms of their pigmentation. Short-wavelength radiation bleaches the photosynthetic pigments. In contrast to higher plants, most phytoplankton do not tolerate excessive solar radiation.

Phytoplankton are also affected by UV-B radiation in terms of their proteins and pigments. Biochemical analysis of protein extracts from cells before and after UV-B exposure using SDS-PAGE shows a drastic loss of several proteins.

Phytoplankton are also affected by UV-B radiation in terms of their photosynthetic and respiratory oxygen exchange. While most instrumentation requires the transfer of biological material into the laboratory, a novel hardware device was developed that allows the determination of photosynthetic and respiratory oxygen uptake and release in organisms in their natural habitat, even using solar radiation as an actinic light source.

Phytoplankton are also affected by UV-B radiation in terms of their fluorescence. Induced chlorophyll fluorescence can be successfully used to obtain valuable information on the physiological parameters that control the photosynthetic apparatus.

Phytoplankton are also affected by UV-B radiation in terms of their DNA. The most common mechanism by which UV-B radiation affects DNA is the formation of thymine dimers, a mechanism that has also been found in animal tissues.

Phytoplankton are also affected by UV-B radiation in terms of their ability to produce mycosporine-like amino acids (MAAs), which are even passed to the next trophic level in the food web by predation.

Phytoplankton are also affected by UV-B radiation in terms of their ability to produce scytonemin, a UV-B-induced shielding substance that is incorporated into the slime sheaths of the organisms.

Any substantial depletion of the ozone layer will have detrimental effects on aquatic ecosystems. Due to the enormous size of aquatic systems, even a small loss in productivity could have substantial impacts on a global scale.

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