Influence of natural dissolved organic matter (DOM) on acute and chronic toxicity of the pesticides chlorothalonil, chlorpyrifos and fipronil on the meiobenthic estuarine copepod Amphiascus tenuiremis

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 (∼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.