Population resilience of the copepod Oithona similis to climatic variability: egg production, mortality, and vertical habitat partitioning

The past few decades have seen a marked decline in copepod populations across the North Atlantic. Despite this, the pelagic copepod Oithona similis has maintained stable population densities in the English Channel. Using 30 years of time series data, high-resolution sampling and predictive modelling, we investigated this apparent resilience in the O. similis population at the Western Channel Observatory station L4. Considering its status as a major ecosystem component on a global scale, O. similis is an important species upon which to study the impact of climatic variability. We therefore assessed how population density and reproductive output respond to environmental variation across seasonal and inter-annual time scales. To determine how contrasting functional traits are reflected in seasonal dynamics, O. similis population density and reproductive output was compared with that of the copepod Calanus helgolandicus. Seasonality of population density and reproductive output was strongly decoupled in both species, indicating that optimum conditions for these parameters occur at different times of the year. Following this, O. similis population dynamics were studied over 2018, a year characterised by an exceptionally cold spring and subsequent rapid rate of warming. We explored how Oithona spp. population dynamics during this year of high climatic variability diverged from the baseline seasonal dynamic derived from the L4 time series. The baseline seasonal dynamic was maintained in 2018, evidence for the resilience of this species to climatic variability. Depth-resolved sampling revealed clear ontogenetic differences in the O. similis vertical profile, with depth distribution increasing with successive life stages. Avoidance of the top few metres of the water column was observed across all life stages. High mortality rates in early life stages coincided with the autumn population decline. Interestingly, this decline occurred at a time of high prey carbon biomass and the persistence of water column stratification. This presentation discusses the multiple mechanisms that combine to drive the seasonal dynamics and vertical distribution of this globally important species.