Environmental forcing reshapes copepod functional diversity in the Northwest Atlantic

Zooplankton are central to pelagic ecosystem functioning, mediating energy transfer and are sensitive indicators of climate-driven change. While shifts are well documented in zooplankton taxonomy and abundance, their implications for ecosystem functioning remain less clear. Functional trait approaches provide a mechanistic framework to link community composition with ecological processes by focusing on traits that govern feeding, growth, and energy transfer. We applied a trait-based analysis to long-term copepod communities at two time series sites on the northwest Atlantic shelf collected between 1999 and 2021 (HL2 and P5) to assess how environmental variability structures functional composition and diversity. Guided by biodiversity–ecosystem functioning theory, we hypothesized that environmental change would re-organize communities along dominant trait gradients, favour smaller and more metabolically active taxa under warming, and produce region-specific responses due to contrasting hydrographic conditions. There were declines in overall copepod biomass driven by the declining contributions of large lipid-rich copepods and increasing relative importance of smaller taxa with higher metabolic rates. Functional richness and dispersion generally tracked richness-based expectations, suggesting functional redundancy despite trends in trait composition. These results demonstrate that environmental variability is reshaping copepod functioning and highlight the value of trait-based approaches for understanding ecosystem responses to climate change.