Kinetics signatures of metal complexes in freshwaters

Metals can be toxic to biota and as nutrients they can regulate the growth of algae, affecting carbon dioxide in the atmosphere. It is established that, for many situations, the supply of metals to algae is influenced by the activity of the free metal ion. However, recent measurement and modelling studies have shown that the supply can sometimes be affected by the rate of dissociation of metal complexes in solution. There is relatively little field data for these rates. We have developed a new method for measuring these dissociation kinetics using a steady state measurement that emulates continuous uptake by biota. A kinetic signature of metal complexes in a natural water (a pictorial representation of the kinetic limitation for 9 metals) can be obtained from a single in situ deployment. This work will use model solutions to test fully and validate the method under controlled laboratory conditions. The characteristics of kinetic signatures for solutions where the complexes are dominated by either humic substances (organic matter mainly derived from soils) or algal exudates (formed within the water body) will be investigated. The theoretical interpretation of the data in terms of a distribution of rate constants will be developed in collaboration with a group in Spain. Kinetic signatures and, where possible, dissociation rate constants will be determined for 9 metals in 30 diverse freshwaters. These data will provide the first systematic survey of kinetic controls of metal supply in freshwaters. Allied to the laboratory measurements, they will be used to assess whether it is possible to use kinetic signatures to distinguish the origins of the complexes that control the supply of metals to biota.