Degradation of marine ecosystem
Marine ecosystems today are under severe environmental stress due to human activity. Eutrophication can be branded as the most important factor causing degradation of a marine ecosystem today. Eutrophication can be understood as the enrichment of an ecosystem with excess chemical nutrients like nitrogen or phosphorus. Eutrophication can be considered a severe form of pollution because it promotes excessive plant growth, favoring certain species over others and forcing a change in species composition. In marine aquatic environments, enhanced growth of choking aquatic vegetation of phytoplankton disrupts the equilibrium of the ecosystem. There are serious ecological impacts of Eutrophication namely, decreased biodiversity, changes in species composition, dominance, and toxicity effects.
When a Marine ecosystem receives abnormal levels of nutrients due to human activities, there is an abnormal increase in the primary producer species such as algae in aquatic ecosystems. This is called an algal bloom. Algal blooms seriously affect the sunlight availability to bottom-dwelling organisms and cause variations in the amount of dissolved oxygen in the water. Further indiscriminate fishing activity has also led to an imbalance in the fish population. Under such eutrophic and disturbing conditions, during dark, there is severe oxygen depletion by the respiring algae and by microorganisms that feed on the increasing mass of dead algae. When dissolved oxygen levels decline to hypoxic levels, fish and other marine creatures such as shrimp, especially immobile bottom dwellers die due to suffocation. Chronic Eutrophication leads to acute anaerobic conditions promoting growth of bacteria such as Clostridium botulinum that can produce deadly toxins that can kill even marine mammals creating toxic zones called dead zones in the marine ecosystem.
Eutrophication also causes abundant availability of a normally “limiting nutrient” leading to shifts in the species composition of ecosystems. For example, an abnormal increase in nitrogen allows competitive species to invade and dominate the native inhabitant species. Algal blooms also known as “nuisance algae” or “harmful algal blooms,” and are often toxic to plants and animals. These toxic compounds find their way up the ecological food chain. An example of algal toxins finding their way into humans is the case of shellfish poisoning (Shumway 1990). Biotoxins created during algal blooms are taken up by shellfish (mussels, oysters), causing paralytic, neurotoxic, and diarrhoetic shellfish poisoning in human beings. Other marine animals act as vectors for such toxins, as in the case of ciguatera, a predator fish that accumulates the toxin and then poisons humans.
Specific sources that contribute to nutrient loading can be classified as point and nonpoint sources. Point sources are those sources where the nutrient waste travels directly from source to water as in factories that leave waste discharge pipes directly into a water body. Nonpoint sources of pollution known as ‘diffuse’ or ‘runoff’ pollution comes from ill-defined and diffuse sources. Commercial activities such as mineral extraction, disposal of Waste products such as sewage and industrial waste, fishing and tourism has damaged marine ecosystems. Some of the main pollutants affecting coastal waters are the high levels of nitrogen and phosphorous-based pollutants entering the water from inputs from agriculture and industries. Overloading coastal waters with such nutrients results in excessive phytoplankton growth and subsequent effects on the marine ecosystem. Most coastal waters are better flushed than lakes and marine phytoplankton are most often limited in their growth by a shortage of nitrogen (as nitrate or ammonium) rather than of phosphate. N-fixing cyanobacteria are uncommon in the sea. Nutrient enrichment of the sea is often associated with freshwater discharge and so often confined to waters of lower salinity. Usually, 10 mg chl m-3 is the Environmental Quality Standard (EQS) for coastal waters. Thus, if chlorophyll frequently exceeds this level in summer, a water body is declared eutrophic.
A literature review on the consequences of marine eutrophication gives an insight into the impact of eutrophication and includes reports on Red tides, Water discoloration and foaming, like the one caused by the colonial flagellate Phaeocystis pouchetii in the southern North Sea (Lancelot et al., 1987); Increased biomass, leading to extra Biological Oxygen Demand (BOD) and hence increased removal of oxygen, in enclosed waters; Shifts in species composition – from diatoms to flagellates, as the ratio of N: Si increases as in the German Bight of North Sea (Gillbricht, 1988); Toxicity – as in dinoflagellate Gyrodinium aureolum which caused fish kills in 1980 in Loch Fyne (Jones et al., 1982). Some blooms seem to be associated with potent toxins in the causative algae, while others seen to be simply because of high algal biomass. Studies have shown that some are lethal at exceedingly low cell densities. Blooms of seaweeds or macroalgae can also cause harm, in many cases as a result of the same eutrophication process that stimulate microalgal blooms. The term “harmful algal bloom” is now used by scientists throughout the world, with HAB the obligatory acronym.” (Anderson & Garrison, 1997).
The occurrence of HABs overlaps with the occurrence of eutrophic conditions. In
marine waters, the toxic Red Tide of Gyrodinium aureolum in parts of the Firth of Clyde in 1980 (Jones et al., 1982) has been associated with high biomass, and augmented by local nutrient enrichment. In contrast, paralytic shellfish poisoning, PSP, which continues to occur in shellfish
in marine waters, has been found to be caused by the dinoflagellate Alexandrium tamarense at relatively low abundance (Joint et al., 1997). Since 1999, there has been increasing public concern about Amnesic Shellfish Poisoning (ASP) on the west coast of USA resulting in closure of shellfisheries. ASP is due to a toxin called domoic acid that accumulates in shellfish, especially scallops, feeding on diatoms of the genus Pseudo-nitzschia. It has been found that this diatom has increased as a consequence of nutrients from fish farms.
Conclusion: It is important to deploy a range of optical instruments to measure plankton activity, suspended matter, and nitrate with the help of research vessels to monitor the marine ecosystem. The new research tools to monitor marine pollution include an in situ Fast Repetition Rate Fluorometer (FRRF), Pulse Amplitude Modulation Fluorometer (PAM) for measuring primary productivity and to validate Earth Observation (EO) images to provide spatial coverage of primary productivity measurements (Balls, 1994). Primary productivity can be validated by using data from FRRF and PAM, EO satellite images which have the potential to provide unrivalled spatial and temporal coverage of surface phytoplankton activity. These primary productivity data can be compared to more routinely monitored chlorophyll-a concentrations using High Performance Liquid Chromatography (HPLC) methods to identify phytoplankton pigments other than just chlorophyll (Balls, 1994). An assessment of species composition of the phytoplankton is an important measure and includes the assessment of the presence or absence of common species, observations of rare and new species and the occurrence of toxic or harmful species and nuisance algal blooms. There is an urgent need for protecting our marine ecosystems from indiscriminate human activities.
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