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

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 <i>Calanus</i> 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 <i>Calanus</i> spp., accounting for 1-2 TP steps determined using alanine (TP_Ala) and that this varied spatially and interannually; however, the TP of <i>Calanus</i> spp. determined using glutamic acid (TP_Glu = 2.2 ± 0.2) indicated consistent herbivorous feeding. Flexibility in the diet of <i>Calanus</i> spp. under differing environmental conditions suggests that <i>Calanus</i> spp. may be able to adapt to changing food availability created by environmental instability driven by climate change.