Dissolved organic matter (DOM) represents an important carbon phase in coastal environments and influences the partitioning of organic contaminants. In this study, we evaluated the role of salt-marsh sediment-derived DOM vs. DOM-free seawater on the acute and chronic toxicity of three pesticides (chlorothalonil-CHTH, chlorpyrifos-CHPY and fipronil-FIP) to the meiobenthic copepod Amphiascus tenuiremis. Acute toxicity was evaluated via standard 96-h median lethal concentration (LC50), while chronic toxicity was evaluated for 16 days using a 96-well microplate life-cycle bioassay. DOM significantly reduced (p < 0.05) acute toxicity of CHTH and CHPY to male and female copepods relative to copepods exposed in DOM-free seawater. In contrast, DOM elevated the acute toxicity of FIP to male copepods. In chronic exposures with/without DOM, CHTH and CHPY did not significantly affect copepodite-to-adult development. In these treatments, plus controls, the majority (95%) of copepodites developed into adults by 8 days. Individuals exposed to FIP in the presence of DOM showed a slower development rate than FIP individuals in DOM-free seawater. Overall, FIP exposed copepodites developed into adult copepods 4 days later than controls. CHTH, CHPY and FIP significantly reduced reproductive success by 33%, 31% and 89%, respectively. DOM, however, mitigated 30% (p=0.006) and 20% (p=0.05) of the reproductive failure attributed to CHTH and CHPY. FIP-induced reproductive failure was high (similar to 80%) in all exposures regardless of DOM presence/absence. An exponential growth model predicted that exposure to CHTH, CHPY and FIP in the absence of DOM significantly reduced (>= 38%) projected naupliar production relative to DOM-free controls. In the presence of DOM, naupliar production under CHTH and CHPY exposures was not significantly different (p > 0.05) from control projections. These results indicate that DOM generally reduced the acute and chronic toxicity of CHTH and CHPY to A. tenuiremis, while certain compound-specific pesticide (FIP): organic associations may enhance acute toxicity of FIP, particularly to male copepods. (c) 2005 Elsevier B.V. All rights reserved.
