Some metals are an essential part of the metabolism of living organisms - Iron, Calcium, Sodium and Potassium, for example. Others can be poisonous in the short term or over time - Lead and Mercury are both familiar examples.
However, the impact of these toxic metals depends first on the way in which they behave in chemical terms in the environment - whether they are available to be absorbed by the organism (bioavailable). Second, it depends on the way in which they are treated by the organism once absorbed. Some toxic metals are easily excreted by organisms; others can gradually accumulate to the point of toxic effect over time; yet others may be bound up by proteins and rendered non-toxic to be stored in relatively inert form inside the organism (bioaccumulation) so organisms can be described as tolerant of the toxic metals and may survive in polluted environments.
It has been proposed that the bioaccumulated concentrations of toxic metals in tolerant biomonitor organisms can be used as indicators of metal bioavailability - where bioaccumulated concentrations are high, bioavailability is high. This could be used to predict the ecological impact of those metals on groups of organisms that are more sensitive to metal pollution - direct measurement of low levels of metal pollution impact over time for sensitive organisms is both difficult and expensive.
Phil Rainbow, Sam Luoma, Brian Smith (Life Sciences) and colleagues addressed this proposal in the mining-affected streams of Cornwall. Mines operated over many years, now disused, have resulted in soil and stream pollution by metal-rich tailings and ore.
Their hypothesis was that metal concentrations in the caddisfly larvae Hydropsyche siltalai and Plectrocnemia conspersa, as tolerant biomonitors, indicate metal bioavailability in contaminated streams, and can be calibrated against metal-specific ecological responses of more sensitive mayflies. Bioaccumulated concentrations of Copper, Arsenic, Zinc and Lead in H. siltalai from Cornish streams were measured and related to the mayfly assemblage.
Caddisfly larva - showing its protective case made of stones and vegetation
They found that Mayflies were always sparse where bioavailabilities (measured from caddis) were high. However, mayflies were abundant and diverse where bioavailabilities of all metals were low. This was particularly evident when the combined abundance of two particular groups of mayflies (heptageniid and ephemerellid) was measured.
The results offer promise that bioaccumulated concentrations of metals in tolerant biomonitors can be used to diagnose ecological impacts on stream benthos (organisms living on the stream bed) from metal stress.
P.S. Rainbow, A.G. Hildrew, B.D. Smith, T. Geatches and S.N. Luoma (2012). Caddisflies as biomonitors identifying thresholds of toxic metal bioavailability that affect the stream benthos. Environmental Pollution 166, 196-207.
Ephemera danica - the larva of a mayfly