Copepod population restarts: naupliar recruitment phenology during the winter-to-spring transition

Abstract

In high-latitude marine environments, the winter-to-spring transition is characterized by order-of-magnitude increases in copepod biomass in the photic zone. However, the overwintering populations and naupliar recruitment patterns that lead to this biomass increase are poorly characterized. In this study, zooplankton, phytoplankton, and environmental parameters were monitored from early January to late March in a deep Alaskan Fjord, Resurrection Bay. Copepods were identified and enumerated using microscopy and nauplii were speciated using DNA metabarcoding. A diverse community of adult and pre-adult copepods was found overwintering below 200 m, illustrating the importance of nearshore deep-water locations as overwintering habitats. Nauplii were abundant at all depths throughout the winter and were comprised of a diverse and changing species assemblage over the study period. Thirteen species clustered into three groups based on naupliar phenology: 1) presence during the winter and absence during the spring (e.g., Neocalanus spp.), 2) absence during the winter and presence during the spring (e.g., Calanus marshallae), and 3) presence during both winter and spring (e.g., Metridia pacifica and Oithona similis). The first two groups aligned with predicted phenologies of capital- and income-breeding species, respectively, but the presence of several income-breeding species in group three was unexpected. Furthermore, a temporal mismatch between nauplii and ovigerous females in this group suggests that some copepods are overwintering as nauplii while female reproduction remains tied to phytoplankton availability. Differences in overwintering strategies between closely related species revealed temporal niche partitioning. The diversity of species and reproductive phenologies may contribute to ecosystem stability and resilience.