Influence of dissolved organic carbon on the speciation, bioavailability and toxicity of metals to aquatic biota in soft water lakes

Abstract

Cu and Cd are extremely toxic to aquatic biota in low alkalinity water. In these calcium poor (<5 mgL-1), moderately acidic (pH <7.0) waters, the role of organic complexation is important in determining the speciation and bioavailable forms of these metals. The current paradigm of metal-organism interaction, the Free Ion Activity Model, states that the biological response of an organism to metal exposure is a function of the free aqueous metal ion activity and not the total metal concentration. Hence, metal toxicity should be proportional to the free metal ion activity (a measure of the reactive metal ion concentration). The usefulness of this model for predicting metal bioavailability in nature lake waters with natural dissolved organic carbon (DOC) is unclear.

The effect of DOC on the acute toxicity of Cu and Cd to Hyalella azteca and Pimephales promelas was studied in water from St. Mary's Lake, a soft water Precambrian Shield lake. DOC was a significant modifier of Cu toxicity but not of Cd toxicity. Mean 96-h Cu LC50sranged from 6.5 to 53.8 ug-1 and mean 96-h Cd LC50s ranged from 2.7 to 23.9 ug-1. H.azteca had similar sensitivity as P. promelas to Cu but was more than an order of magnitude more sensitive to Cd exposure than P. promelas.

The chronic toxicity of Cu and Cd to larval fathead minnows (P. promelas) was examined in water from two low alkalinity lakes with DOC concentrations of 2.3 to 6.7 mg-L-1 respectively. Both metals were toxic at low ppb (ug-L-1) levels. Metal speciation was determined by dialysis and with Cu and Cd ion specific electrodes. Differences in the observed toxic effects of Cu exposure between the two lakes when expressed as total Cu, were minimized when expressed as the dialysis fraction. When effects from the Cu exposure were expressed as a function of the free Cu ion concentration, the toxicity was roughly similar between the two lakes. The opposite pattern was observed for Cd toxicity. Differences in the observed toxic effects between the two lakes were minimized when expressed as total Cd. When expressed as the dialysis fraction, the effects of Cd exposure were more pronounced in Dickie Lake than in Halls Lake. Estimates of free Cd ion activity also suggest that the effects of Cd exposure were more pronounced in Dickie Lake than in Halls Lake.

These results indicate that while Cu toxicity appears to be a function of the free Cu ion concentration, Cd toxicity may not be a function of the free Cd ion concentration. More precise free Cd ion concentration determinations in natural waters are required in order to conclude that the free Cd ion concentration was not proportional to the observed toxicity. However, it is clear that the observed Cd toxicity was not due to the free Cd ion concentration only. Measured ligand-bound Cd complexes are clearly bioavailable to larval fathead minnow, as indicated by similar toxicity observed in the fish as a function of total Cd concentration, even though the amount of ligand-bound Cd is different in the two lakes.

These results suggest that the FIAM may not be applicable to all divalent metals. With natural DOC, the results validated the model for Cu (a metal that forms strong covalent bonds with organic ligands) but were ambiguous for Cd (a metal that forms weak electrostatic bonds with organic ligands). Calculated site-specific conditional stability constants (Log K') for the lake water DOC were compared to published fathead minnow gill Log K's for both metals. Both fish gills and lake water DOC had similar mean stability constants for Cu (between 7.4 - 8). However, fish gills had stability constants for Cd 2-3 orders of magnitude higher than lake water DOC (8.6 verses between 5.3 - 6). The enhanced toxicity of Cd in the presence of DOC appears to be due to a kinetically controlled dissociation reaction between Cd bound to organic ligands in the vicinity of the fish gill and not to direct toxicity of Cd-DOC complexes.