II. Types of Pollutants:It should be noted that most of the polluting substances occur naturally in the marine environment. Contamination, that is elevated concentrations of substances in flora or fauna, may only be labelled pollution if human-induced, because “a pollutant is a resource out of place.” Pollution, furthermore, requires substances to have a measurable adverse effect on the population of a certain species.1. Nutrients:Although it not toxic, nutrients can have severely damaging effects on the marine environment. Inputs of high levels of nitrogen and phosphorus compounds, in particular, often result in “eutrophication”. Nutrients in high concentrations, depending on the physical and chemical properties of the marine area affected, may lead to excessive growth of algae (“algae bloom”) and phytoplankton. As a consequence, oxygen concentration decreases, while concentrations of hydrogen sulphides increase. Many aquatic organisms have low resistance against hydrogen sulphides and may therefore just die off. Compounding this problem, dead algae floats on the surface and thus covers the water, making it difficult for sunlight to penetrate into the sea. Consequently, in addition to oxygen shortage, phytoplankton also lacks adequate amounts of light energy to maintain photosynthesis processes. Nutrients are mainly used as fertilisers in agriculture. Applied on fields, they drain away and are eventually carried into the sea by rivers. Therefore, estuaries and coastal areas are the prime sites in which eutrophication effects may occur due to high concentrations of nutrients. Areas where the exchange of water masses is low are equally vulnerable. Serious deterioration, for instance, has been observed in the Adriatic Sea over the last twenty years, especially in areas near the Po estuary. It carries about 100,000 tonnes/year of inorganic nitrogen and about 6,000 tonnes/year of inorganic phosphorus; total inputs from Italian sources into the northern Adriatic Sea amount to 270,000 and 24,000 tonnes/year respectively.2. Persistent Toxic Substances:The term “persistent toxic substances” (PTS) refers to a wide range of diverse substances that are mainly long-lived, noxious substances, but also less persistent substances that, because of their continuing use and dissemination, may give rise to chronic exposures over large temporal and spatial scales. While the production of some PTS has been banned, others continue to be used. Their existence in terrestrial, as well as aquatic ecosystems is thus widespread. Among substances classed as PTS, some organic compounds are particularly harmful and non-degradable. These are usually called persistent organic pollutants (POPs), referring to a group of substances that to varying extents resist photolytic, biological and chemical degradation. They are also semi-volatile, enabling longrange transport through the atmosphere. Most substances can be classified as halogenated hydrocarbons; however, metallic compounds may also have POP properties. Prominent examples include tributyl tin (TBT) and its derivatives, dibutyl tin and monobutyl tin. They are either pesticides or industrial chemicals that were once thought to possess significant societal benefits, or unintended by-products of combustion processes, such as dioxin. The growing concern that these substances evoke is reflected by the fact that after lengthy negotiations, an international convention was signed in 2001 aiming at measures to eliminate or reduce the release of POPs into the environment.
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