Resilience of the Copepod Oithona similis to Climatic Variability: Egg Production, Mortality, and Vertical Habitat Partitioning

Louise Elisabeth Cornwell,Andrew Garwood Hirst,Ceri Lewis,John T Bruun,A Atkinson,Helen S Findlay,A J Mcevoy,Timothy James Smyth,Glen Adam Tarran,Elaine S Fileman

doi:10.3389/fmars.2020.00029

Abstract

There has been an overall decline in copepod populations across the North Atlantic over the past few decades. Reasons for these declines are unclear, and several major species, including the cyclopoid copepod Oithona similis, have maintained stable populations at station L4 in the western English Channel. To identify the factors contributing to this stability, we conducted a one-year intensive study of O. similis at L4 over 2017-2018, a period of high climatic variability. For context, dominant frequency state analysis was applied to the 30-year L4 time series to derive the baseline dynamics of the Oithona spp. population. The Oithona spp. baseline demonstrated stable densities and a bimodal annual cycle. These dynamics, as well as those of reproductive output and phenological timings, were upheld in 2017-2018, indicating resilience to climatic variability. During 2017-2018, all life stages of O. similis were relatively scarce in the top 2 m of the water column, despite the presence of abundant food. Naupliar stages occurred predominantly around 10 m depth, with subsequent life stages progressively deeper. We suggest this vertical structuring may represent different trade-offs between feeding and mortality risk between ontogenetic stages. To determine the traits that contribute to population stability, we compare O. similis with the large, biomass-dominant copepod, Calanus helgolandicus. Despite having contrasting functional traits, both species have exhibited strong population stability over the time series. Our results provide evidence that mortality plays a major role in maintaining population dynamics.

Highlights

Copepod populations have undergone strong declines across the North Atlantic over the past few decades (Edwards et al, 2016; O’Brien et al, 2017)
Linear regression analysis showed that mean annual Oithona spp. density had no significant correlation with mean annual Sea surface temperature (SST) or total prey biomass (P > 0.05)
Our study shows that the numerically dominant cyclopoid species O. similis has maintained generally stable population dynamics over the L4 time series, despite high variability in both the physical and biological environment, as evidenced in our 1-year intensive study

Summary

Introduction

Copepod populations have undergone strong declines across the North Atlantic over the past few decades (Edwards et al, 2016; O’Brien et al, 2017). It is well-established that the global oceans are changing at an unprecedented rate (IPCC, 2019), the drivers behind the observed copepod declines are uncertain, and some important species have maintained stable populations. Small pelagic copepods, such as the cyclopoid Oithona similis, are some of the most numerous metazoans on Earth (Turner, 2004), and provide an important link between the microbial food web and higher trophic levels (Nielsen and Sabatini, 1996; Turner, 2004). Densities and phaenological timings of the O. similis population at this site have remained relatively stable (Castellani et al, 2016; Cornwell et al, 2018), as they have in the larger calanoid copepod, Calanus helgolandicus (Maud et al, 2015; Edwards et al, 2020)

Methods

Results

Conclusion