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

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 1-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 phaenological 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.

Multiple Trophic Markers Trace Dietary Carbon Sources in Barents Sea Zooplankton During Late Summer

“…With the change in primary production, i.e., the composition and availability of carbon sources, species with opportunistic feeding behavior, such as O. similis and Pseudocalanus spp. (Castellani et al, 2005;Lischka and Hagen, 2007;Cornwell et al, 2020) might further have a higher tolerance to more variable environmental conditions and an advantage over highly specialized (iceassociated) species.…”

Phenology of Oithona similis demonstrates that ecological flexibility may be a winning trait in the warming Arctic

“…similis in lower latitudes are quite different. In the western English Channel, increased numbers of these copepods are also observed in the autumn months, but a clear peak in both adult and copepodite stages occurs annually in spring, especially in March 37 , 76 , 77 . Similarly, high abundances of O .…”

Seasonal monitoring of lipid degradation processes in the western English Channel links bacterial 10S-DOX enzyme activity to free fatty acid production by phytoplankton

“…The seasonal phytoplankton community at L4 has been well documented over many years (e.g. Widdicombe et al 2010, Atkinson et al 2015, Tarran and Bruun 2015, Cornwell et al 2020.…”

“…The microzooplankton protist assemblages are dominated by ciliates and colourless dinoflagellates (defined here as heterotrophic). Ciliates typically peak at around the same time as the spring diatom bloom (Widdicombe et al 2010, Atkinson et al 2015, Cornwell et al 2020, whereas the stronger peak of dinoflagellates appears later (Atkinson et al 2015). The noncarnivorous holoplankton, which also includes copepods, starts to increase before the spring bloom and is often sustained until October (Atkinson et al 2015).…”

“…In the present work, we monitored the biotic and abiotic degradation of lipids in suspended particle material (SPM) collected in 2018 from the Western Channel Observatory (WCO, https://www.westernchannelobservatory.org.uk/) marine station L4, which is a highly seasonal temperate shelf site (Widdicombe et al 2010, Atkinson et al 2015, Cornwell et al 2020.…”