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Toxic materials in marine food-webs

Toxic materials Where they come from Summary Where they end up

Toxic materials Where they come from Summary Where they end up

Many different types of toxic materials enter the marine environment There are several natural sources for some toxic materials such as heavy metals and cyclic hydrocarbons. Other materials have no known natural source

Some are natural, but most are man-made

The list includes: ‘Heavy’ metals such as mercury and cadmium Pesticides such as DDT and lindane Some toxic materials fitr in more than one of these categories. For instance, the pesticide mirex has also been used as a fire-retardant Industrial compounds such as some lubricants and fire-retardants Combustion products such as dioxins

High levels of these toxic materials will kill animals However, normal levels in the environment are low For many toxic materials, it is often difficult to measure environmental loevels

At low levels, there are still effects on animals These effects include cancers and reproductive changes Effects on animals may be demonstrated at tissue concentrations of microgrammes- or even naogrammes-per kilogramme body mass Many organic toxic materials mimic natural hormones, so create disruption in the endocrine system. Some of these are oestrogen mimics, causing marked effects on male mammals

This means that they last for a long time Toxic materials are often very slow to break down in the environment and in living things This means that they last for a long time Many of these materials are designed specifically to be long-lasting, so they persist in the environment They are slow to break down in living things, and intermediate breakdown products may themselves be toxic

Toxic materials Where they come from Summary Where they end up

Few toxic materials enter directly into the marine environment Most come in from the land and via the atmosphere

Local sources include: Pesticides applied in coastal agriculture Anti-fouling treatments applied to ships Disturbance of polluted sediments by dredging Marine pollution incidents such as shipwreck Anti-fouling treatments are designed to stop plants and animals growing on ships’ hulls and other marine structures. The most notorious compounds are organo-tin materials, such as tri-butyl tin, which have caused major impacts in some areas close to shipyards where they have been applied

Transport from inland occurs by: Run-off and rivers Transport through the atmosphere Migration of animals Shipping The importance of different long-distance transport mechanisms depends on the solubility and vapour pressure

“Cold condensation’ occurs where the vapour pressure of a compound is sufficient for it to evaporate in warms air, but not in cold. This means that these materials tend to accumulate in cold environments UNEP / Frank Wania

This means that they move polewards Toxic materials that are transported in the atmosphere will tend to evaporate in warm areas and condense in cool areas “Cold condensation’ occurs where the vapour pressure of a compound is sufficient for it to evaporate in warms air, but not in cold. This means that these materials tend to accumulate in cold environments This means that they move polewards

Toxic materials Where they come from Summary Where they end up

Toxic materials enter the marine food-web largely at the bottom of the food-chain - the producers Many toxic materials condense onto particles, so that they enter the food chain via phytoplankton

These are then eaten by herbivores In turn, they are eaten by predators

These graphics indicate the build-up of toxic materials in predators These graphics indicate the build-up of toxic materials in predators. Note that the predator will eat several times its own body mass to fuel respiration and growth. In many cases, the entire toxic burden of its food will be retained Each copepod contains a small amount of toxic material from its plant food STEP

The fish stores most of the toxic material from its food STEP

The more copepods that it eats, the more toxic material is stored STEP

In turn, predators further up the food chain build up even higher levels of toxic materials The small plankton-eating fish from the last slide can be eaten by a larger predatory fish, which in turn can be eaten by a fish like a tuna or a shark Concern has been raised about heavy metal levels in marine predatory fish eaten by humans, such as swordfish, marlin and tuna

For some toxic materials, top predators in polar ecosystems show especially high levels Many well-documented studies indicate the accumulation of pesticides in polar marine mammals and birds. This is especially true for volatile compounds where ‘cold condensation’ (see slide 14) means that these compounds are gradually accumulating in polar ecosystems

Toxic materials Where they come from Summary Where they end up

You have seen that - Various toxic materials enter the marine environment Many of these come from the land or via the atmosphere

You have seen that - Toxic materials enter the food chain mainly through the producers Top predators in a food chain build up highest levels of toxic materials

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