Trophic Transfer of Metals in Stream Ecosystems

Factors Governing Metal Concentration

• The bioaccumulation of metals in aquatic organisms is greater in streams adjacent to mining and farming activities. The diversity of species is also considerably lower in sections of the stream immediately adjacent to these activities and increases the further away from the contamination the aquatic organisms are found. Different metals dissolve in different concentrations, which immediately affect the trophic transfer of each specific metal.

First Stage Contamination

• The trophic transfer of metals begins with the uptake of metals by the larvae of aquatic insects. The accumulation of metals in the body of these larvae are related to the extent to which the sediment is contaminated in each stream. A high level of contamination in the sediment leads to the elevated accumulation of metals in the system of the larvae.

Metal Transfer

• The fry of fresh water fish predate on the larvae and adult forms of insects and are themselves eaten by larger fish and amphibians, such as frogs. These amphibians and fish are fed upon by the large apex fish predators, which in turn, are caught by such species as kingfishers and raccoons. In this way, the metal contamination finds its way up the food chain and moves from contaminated sediment to accumulation in the tissues of mammals and birds.

Contamination Indicators

• Living creatures in the trophic chain are good bioindicators of metal contamination in streams. Aquatic insects are typically used to measure the levels of contamination, as they are small, plentiful and easy to house in laboratory conditions. The amount of metal contamination unfortunately becomes more concentrated with increased trophic transfer as the prey items of each subsequent predator accumulate progressively higher amounts through their food intake. The large predators at the top of the food chain therefore exhibit the highest accumulation of metal contamination.