High prey-predator size ratios and unselective feeding in copepods: A seasonal comparison of five species with contrasting feeding modes

Abstract

There has been an upsurge of interest in trait-based approaches to zooplankton, modelling the seasonal changes in the feeding modes of zooplankton in relation to phytoplankton traits such as size or motility. We examined this link at two English Channel plankton monitoring sites south of Plymouth (L4 and E1). At L4 there was a general transition from diatoms in spring to motile microplankton in summer and autumn, but this was not mirrored in the succession of copepod feeding traits; for example the ambushing Oithona similis dominated during the spring diatom bloom. At nearby E1 we measured seasonality of food and grazers, finding strong variation between 2014 and 2015 but overall low mesozooplankton biomass (median 4.5 mg C m-3). We also made a seasonal grazing study of five copepods with contrasting feeding modes (Calanus helgolandicus, Centropages typicus, Acartia clausi, Pseudocalanus elongatus and Oithona similis), counting the larger prey items from the natural seston. All species of copepod fed on all food types and differences between their diets were only subtle; the overriding driver of diet was the composition of the prey field. Even the smaller copepods fed on copepod nauplii at significant rates, supporting previous suggestions of the importance of intra-guild predation. All copepods, including O. similis, were capable of tackling extremely long (>500 µm) diatom chains at clearance rates comparable to those on ciliates. Maximum observed prey:predator length ratios ranged from 0.12 (C. helgolandicus) up to 0.52 (O. similis). Unselective feeding behaviour and the ability to remove highly elongated cells have implications for how copepod feeding is represented in ecological and biogeochemical models.

Increasing picocyanobacteria success in shelf waters contributes to long‐term food web degradation

“…These are the more carnivorous genera that select for motile food (e.g. Centropages spp., Djeghri et al., 2018), whereas more herbivorous genera with a preference for diatoms (e.g. Oithona spp., Para‐ and Pseudocalanus spp., Djeghri et al., 2018) experienced the strongest decline.…”

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

“…For example, the autumn decline in O. similis population density in 2017-2018 would not have been predicted based on the high biomass of motile prey at this time. Indeed, the importance of food in driving O. similis population dynamics at L4 is uncertain (Castellani et al, 2016;Cornwell et al, 2018;Djeghri et al, 2018), and we propose that other factors likely override the effect of food, as discussed below.…”

Comment. What drives plankton seasonality in a stratifying shelf sea? Some competing and complementary theories

“…For the ambush feeding copepod O. similis at L4, similar decoupling between the timings of maximum egg production rate (in late summer) and copepodite abundance (in spring) has been observed (Cornwell et al ). Thus overall there is no congruence between timings of ambush or active‐feeding copepods and their suggested motile and non‐motile food sources (Djeghri et al ).…”

“…But what actually happens in the sea, with its plethora of sizes, types, and qualities of available particles? Even among the copepods there is a diversity of opinions on their feeding selectivity in natural food assemblages; a variety of methodological approaches find much less specialized feeding than that which underpins the trait‐based models (Huntley ; Hopkins ; Atkinson ; Fileman et al ; Pond and Ward ; Isari et al ; Djeghri et al ). Likewise, for protists the variable role of mixotrophy blurs distinctions of traditionally defined functional groups (Flynn et al ), requiring improved functional definitions (Mitra et al ).…”