Diel vertical migration (DVM) is a behaviour observed across zooplankton taxa in marine and limnetic systems worldwide. DVM influences biogeochemical cycling and carbon drawdown in oceanic systems and alters prey availability for zooplanktivorous species. DVM has been well studiedamongzooplankton,andmanyexogenousandendogenoustriggersaswellasadaptivesignificanceshavebeenhypothesized.However, second-order variability in DVM timing, the deviation of DVM times to respective dawn and dusk times throughout the year, is a less-studied phenomenon that can help identify the factors influencing migration timing as well as demonstrate the changes of DVM behaviours within and across systems. Here, we quantified seasonal trends in second-order variability of DVM timing of euphausiids at Brooks Peninsula, Clayoquot Canyon,andSaanichInletnearVancouverIsland,BritishColumbia,Canada,overmultipleyearsusingupward-facingmooredechosounders.We used generalized additive mixed models to characterize this seasonality. DVM timing relative to civil twilight times showed strong seasonalityat all locations, with euphausiids remaining near the surface longer than expected in spring and summer, and shorter than expected in winter. Euphausiids spent less time near the surface at Brooks Peninsula and Clayoquot Canyon than at Saanich Inlet throughout the year. Increased primaryproductivityinSaanichInlet,whichreducedlightpenetrationandhideuphausiidsfromvisualpredators,likelydrovethisdifference.Our findingsconfirmthatproperunderstanding ofDVMbehaviours mustaccount forseasonal variability duetocontext-specific oceanographic and ecological parameters. This is particularly pertinent when attempting to model the biogeochemical or predator–prey interactions influenced by DVMbehaviours.

Keywords:activeacoustics,biologicaloceanography,dielverticalmigration,euphausiids,predator–preyinteractions,timeseries,zooplankton.

Introduction

Euphausiidsareamajorcomponentofthezooplanktoncom- munity along the coast of British Columbia (BC) and a key trophic link between plankton and nekton (Robinson, 2000; WareandThomson,2005).Itisestimatedthatoveraquarter of euphausiid production per year off the SW coast of Van- couver Island is consumed by Pacific hakeMerluccius produc-tusand Pacific herringClupea pallasii(Robinson and Ware,1994).Thesefishpopulationsarethoughttobelimitedbythe availablebiomassofeuphausiids(WareandThomson,2005). Inresponse,euphausiidsinthisregionareknowntoperform diel vertical migration (DVM) as a mechanism to escape this predation pressure (Satoet al.,2013).

DVM has been studied for over two centuries in zooplank- ton communities globally, in both marine and freshwater ecosystems (reviewed by Bandaraet al.,2021). It has beengenerallyconsideredtobeananti-predatordefencebehaviour,ｗhereby zooplankters hide from visual predators at depth during daylight hours, then migrate to the surface to feed at night, based on the widespread observation that most zoo- planktonperformverticalmigrationsarounddawnanddusk, thetransitionsofciviltwilight(Vuorinen,1987;Pearre,2003; UrmyandBenoit-Bird,2021).