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Toxicity to aquatic life

How does the aquatic ecosystem react on pollutants?

Introduction

Types of freshwater pollution

Acids & alkalis

Anions

Detergents

Gases

Heat

Metals

Nutrients

Organic pollution

Pathogens

Definitions

The harmful effects that chemicals have upon individual organisms depend on many different factors. Not only the difference between the freshwater species, but also the form in which pollutants occur, and if the pollutant shows up in lentic or lotic systems. To measure the toxicity there has to get done some toxicity tests. Then there is clarity what the dose is of a chemical that a type of specie will die, which will be expressed in a LC50 or LD50.

What can be the toxicity of the different pollutants on aquatic ecosystems?

The effects of pollutants on the whole organism are considered under three main headings, namely neurophysiological, behavioural and reproductive effects. These effects can often be inter-related: neurological changes can affect behaviour; changes in behaviour can affect reproduction and so on.
A compound doesn't always exert an effect on a target organism or a community. It always depends on the concentration of that compound and the time of exposure to it. These effects eventually can be either acute or chronic. Acute toxicity occurs rapidly, are clearly defined, often fatal and rarely reversible. Chronic effects develop after long exposure to low doses or long after exposure and may ultimately cause death.
A poison is lethal when it causes death, or sufficient to cause it, by direct action. And it is sub lethal when the poison is below the level that directly causes death. Then it results in the regression of the physiological or behavioural processes of the organism, and its overall fitness is reduced. Only in the case of radioactive pollution, it is likely that it will cause irreversible effects at the ecosystem.

The effects of pollution on freshwater species are registered in the loss of some species, with maybe some profits for some of them. There normally is a reduction in diversity but not necessarily numbers of individual species, and a change in the balance of such processes as predation, competition and materials cycling. Because of the complexity of pollution, the effects of take-up in the aquatic life are also depended on the pollutants characteristic feature. If two or more poisons are present together in an effluent they may exert a combined effect to an organism, which can be additive, antagonistic or synergistic.

An example of an additive interaction is the combined toxicity of zinc and cadmium to fish. Calcium in antagonistic to lead, zinc and aluminium. Copper is more than additive with chlorine, zinc, cadmium and mercury, while its decreases the toxicity of cyanide. The toxicity to the mayfly Baetis rhodani of phenol and ammonia at low concentrations is additive, but at higher concentrations the effect is more than additive.

In the following overview, you can see what the fate of wastes in the aquatic environment is and how their community responses.


Fig.1

Toxicity testing
Aquatic systems reflects perturbations in the environment. So fish and invertebrates can often be used to indicate the health of an aquatic system because chemicals can accumulate in invertebrates from the water and sediment and in fish from water, sediment, and the food chain.
The monitoring of these effects is extremely important to regulate and remediate pollution. To test the toxicity, they can apply biomarkers to detect low-level pollution in aquatic systems. First there has to be developed the right biomarkers, and then they can be applied in the Daphnia magna (fig2.) to detect pollution in contaminated groundwater with an aim to develop measures of toxic hazard and risk.

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