MEPS 755:45-61 (2025)  -  DOI: https://doi.org/10.3354/meps14796

ABSTRACT: Rapid environmental changes in high-latitude marine ecosystems have led to warmer ocean temperatures and shifts in zooplankton abundance, community composition, and life history. Variations in zooplankton abundance and phenology (inferred changes in egg production timing and differences in developmental rates that result in changes to observed life stage composition) are tightly coupled to climate processes. Furthermore, zooplankton represent a critical trophic foundation for commercially valued fish communities. To better understand how zooplankton phenology has responded to recent climate perturbations, we used generalized additive models to examine changes in life stage composition for 2 copepod species groups in the Eastern Bering Sea—Calanus spp. (C. glacialis and C. marshallae) and Neocalanus spp. (N. plumchrus and N. flemingeri)—across a 20 yr time series. We found that annual shifts in Calanus spp. and Neocalanus spp. phenology are mediated by temperature and spring southwesterly wind frequency. Specifically, warmer temperatures and reduced spring southwest wind frequency were associated with younger copepod life stages, suggesting an earlier spring phenology. We also tested if an extended zooplankton growing season has enabled the production of a second Calanus spp. generation. We found evidence of earlier spring Calanus spp. reproduction and observed high proportions of younger life stages in the fall, suggesting a second generation in some warm years. Collectively, these results suggest fundamental climate-linked changes to copepod phenology and reproduction, which impact the population dynamics of copepods and could consequently disrupt elements of the Bering Sea trophic structure that rely upon these energetically dense species.

KEY WORDS: Copepod · Phenology · Reproduction · Bering Sea