Metal Speciation Analysis and Modeling – How do we Best Estimate the Bioavailable Form

It is well established that the availability of essential trace metals to organisms depends on their chemical speciation. It is also known now that the speciation of most essential trace metals in natural aqueous systems is governed by strong ligands of predominantly unknown identity and origin. However, equally established is that the bioavailable form of these trace metals (with the exception of iron) can be approximated by its labile species. These are usually the inorganic ligands, but also weak organic ligands. While the inorganic speciation of trace metals can be calculated reasonably accurately for seawater using thermodynamic equilibrium models, the quality of these models is becoming questionable for less standard matrices such as estuaries, hydrothermally influenced fluids, ice melts, etc. However, the accurate estimation of strong organic complexes with bioactive metals is a challenge that the both the SCOR working group 139 on ‘Organic Ligands—A Key Control on Trace Metal Biogeochemistry in the Ocean’ and SCOR working group 145 on ‘Modelling Chemical Speciation in Seawater to Meet 21st Century Needs (MARCHEMSPEC)’ has taken as their goal.

In this presentation we will give examples of improvements of analysis and evaluation procedures as a result of these joint efforts. We will furthermore present new results of Cu toxicity bioassays to different juvenile forms of marine invertebrate and algae. The application of improved methods leads to better estimates of the bioavailable Cu, as shown in correlations with biological effects observed. Directives on how we can further improve trace metal speciation modelling will be given.