# Dietary Flexibility of Calanoid Copepods in the Sub‐Arctic Atlantic: The Role of Protistan Microzooplankton

## TL;DR

This study shows that Calanus copepods in the Arctic can switch between different food sources, including protistan microzooplankton and phytoplankton, adapting to environmental changes.

## Contribution

The study reveals the dietary flexibility of Calanus spp. by showing their reliance on protistan microzooplankton in addition to phytoplankton.

## Key findings

- Calanus spp. rely on protistan microzooplankton, accounting for 1–2 trophic steps.
- Trophic position based on glutamic acid indicates consistent herbivory in Calanus spp.
- Dietary flexibility in Calanus spp. allows adaptation to changing food availability due to climate change.

## Abstract

Zooplankton play a key role in marine food webs, transferring energy from the base of the food web to higher trophic levels. In the Arctic, warming is altering nutrient availability and primary productivity, which could alter zooplankton‐mediated transfer of energy through food webs. The Barents Sea Opening is warming rapidly, and has a strong influence on the Arctic as it is a prominent gateway for North Atlantic water advected into the polar region. Trophic position (TP) is an important metric because it identifies the location of an organism within a food web and therefore provides insight on food web functioning. Using nitrogen isotopes of amino acids in copepods, we investigated how the food web baseline and TP of the keystone Calanus species change in response to environmental gradients along the Barents Sea Opening in summer between 2010 and 2016. Spatial and interannual variation in net primary production and the North Atlantic Oscillation index both strongly influenced the nitrogen isotope baseline. We demonstrate that protistan microzooplankton play a key role in the diets of Calanus spp., accounting for 1–2 TP steps determined using alanine (TPAla) and that this varied spatially and interannually; however, the TP of Calanus spp. determined using glutamic acid (TPGlu = 2.2 ± 0.2) indicated consistent herbivorous feeding. Flexibility in the diet of Calanus spp. under differing environmental conditions suggests that Calanus spp. may be able to adapt to changing food availability created by environmental instability driven by climate change.

Zooplankton play a key role in marine Arctic food webs, transferring energy from the base of the food web to higher trophic levels. Here we show spatial and interannual variation in net primary production and the North Atlantic Oscillation index in the Barents Sea Opening, which strongly influenced the nitrogen isotope baseline. We also show that Calanus copepods rely on both protistan microzooplankton and phytoplankton. Calanus spp. reliance on the protistan food web varied spatially and temporally whilst maintaining a high degree of herbivory. Thus, protistan microzooplankton may offer an alternative food source for Calanus spp. when the availability of phytoplankton becomes limited.