Feeding and reproduction of a tropical coastal copepod across warming and copper gradients

Abstract Tropical coastal ecosystems in Southeast Asia are facing rapid warming and increasing pollution. Shallow coastal waters now frequently exceed 34 °C, potentially pushing tropical ectotherms beyond their thermal optimum while they are simultaneously exposed to copper (Cu) contamination, especially from aquaculture and shipping activities. However, how warming alters Cu toxicity in dominant tropical zooplankton remains poorly understood. We examined the effects of Cu (0, 10, 20, 30 and 40 µg L−1) and temperature (26, 29, 32 and 35 °C) across realistic gradients on the calanoid copepod Pseudodiaptomus annandalei, a dominant grazer of coastal plankton communities. Adult survival, cumulative faecal pellet production (as a proxy for energy intake), and cumulative nauplii production were quantified over seven days. No significant effects of temperature or Cu on adult survival were detected, likely reflecting variability among wild-collected individuals. In contrast, temperature was the main driver of feeding and reproductive performance, which peaked at 32 °C and declined at 35 °C. Cu exposure alone had no significant effects at 26 - 35 °C due to high variability in responses. At 32 °C, cumulative feeding and reproductive responses did not statistically differ among copper concentrations, whereas variability increased at both lower and higher temperatures. At 35 °C, Cu effects were non-linear, with nauplii production reduced at 30 µg Cu L−1 but highest at 20 µg Cu L−1, while faecal pellet production showed treatment-specific reductions, particularly in non-exposed individuals at 26 °C and in high Cu treatments at 29 and 35 °C. These findings indicate that warming can modify contaminant effects in tropical zooplankton and highlight the importance of incorporating realistic thermal regimes and natural population variability into ecological risk assessments under climate change. Competing Interest Statement The authors have declared no competing interest. Footnotes ↵a Co-first authors We have undertaken substantial revisions throughout the manuscript (approximately 50% of the text has been revised; approximately 25% of the original text has been removed and more than 30% of the new text has been added). Specifically, we have refined the formulation of all three hypotheses to improve conceptual consistency, clarified key aspects of the experimental design (including the treatment of reproductive output and feeding measurements), and revised the Discussion to better balance mechanistic interpretation with appropriate caution. We have also strengthened the biological and ecological context by more explicitly linking our findings to previous work on Pseudodiaptomus annandalei and related tropical copepods and their roles in the tropical coastal ecosystems. In addition, the Introduction has been revised to improve clarity and structure, ensuring that cited studies are directly linked to specific arguments and that conceptual statements are appropriately supported by the literature. The statistical analyses and their presentation have also been revised, including the addition of post hoc pairwise comparisons using Tukeys HSD test to identify differences among treatment combinations while controlling for multiple comparisons, as well as improved visualization of treatment effects. Temporal analyses of daily responses have also been included (Appendix S2) to further support the interpretation of interaction patterns.