Influence of dissolved organic matter (DOM) source on copper speciation and toxicity to Brachionus plicatilis

Abstract

Environmental context. Organic matter dissolved in water can mitigate toxic effects of copper, which should be taken into account when estimating risks of copper pollution. The composition of this organic matter, however, can vary widely, and these variations might also need to be taken into account. This work addresses the question of organic matter quality and demonstrates that only the amount and not the source influences copper toxicity -good news for risk analysis because it simplifies predictions of the effects of copper in specific receiving waters.Abstract. The toxicity of copper in marine systems is dependent on its speciation and bioavailability. Dissolved organic matter (DOM) can complex copper, resulting in decreased bioavailability and hence decreased toxicity. The purpose of this study was to measure acute copper LC 50 values (concentration lethal to 50 % of the organisms) in natural marine waters in a sensitive organism, and identify the relationships between DOM quality and copper toxicity and speciation. Static acute copper toxicity tests (48-h LC 50 ) were performed using the euryhaline rotifer Brachionus plicatilis. Ion-selective electrode measurements of free copper were performed at the LC 50 concentrations to determine the influence of DOM source on copper speciation. LC 50 values ranged from 333 to 980 nM (21.1 to 62.3 mg L À1 ) with DOC concentrations ranging from 0.55 to 7.57 mg C L À1 . DOC was found to be protective (R 2 ¼ 0.72, P ¼ 0.016); however, the degree of protection decreased as DOC increased. This suggests salt-induced colloid formation could be occurring, resulting in a decrease of binding sites available to complex free copper. Free copper remained fairly constant between each sample site, with an average pCu of 10.14. Overall, this study is consistent with other studies that suggest free copper is the best species for predicting toxicity. Additionally, no significant correlation between DOM source and copper toxicity was observed as compared with total DOC concentration and copper toxicity, suggesting that DOM quality does not need to be taken into account for copper toxicity modelling in salt water.

Testing the Underlying Chemical Principles of the Biotic Ligand Model (BLM) to Marine Copper Systems: Measuring Copper Speciation Using Fluorescence Quenching

“…Previous research with copper and marine organisms has shown that measured free ion using ion selective electrodes is close to constant when measured at the EC 50 concentration of total dissolved copper (Cooper et al, 2014;Tait et al, 2016). This observation is consistent with BLM predictions that for a constant toxic response the free ion concentration should be constant.…”

“…Salting up was performed using a mixture individually purchased salts. The full details are given in Tait et al (2016). Rotifer (Brachionus plicatilis) 24-h median effect EC 50 values were determined for these same samples in a previous publication (Tait et al, 2016).…”

“…After measurement, the aliquot was returned to the beaker and the next volume of titrant was added. This process was repeated until the decrease in maximum intensity plateaued or until the total copper added to the sample was double the rotifer (Brachionus plicatilis) EC 50 value reported in Tait et al (2016) for the same sample.…”

“…These components were titrated against copper at fixed pH and salinity and then fit to a chemical equilibrium model to determine binding constants and capacities for the unknown ligands in the samples. To determine the number of fluorescent components in each sample, parallel factor analysis (PARAFAC) was performed (Tait et al, 2016) For each emission scan obtained during titration, linear regression was used to estimate the contributions of each fluorescent species:…”

“…In addition, this current work tests if the fluorescence-estimated speciation is consistent with the assumptions of the biotic ligand model; i.e., that constant cupric ion should be observed at total dissolved copper corresponding to measured effects concentrations for a given organism. Published rotifer toxicity data (EC 50 values) for the same samples are used for these comparisons (Tait et al, 2016).…”