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 studied among zooplankton, and many exogenous and endogenous triggers as well as adaptive significances have been hypothesized. 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, and Saanich Inlet near Vancouver Island, British Columbia, Canada, over multiple years using upward-facing moored echosounders. We used generalized additive mixed models to characterize this seasonality. DVM timing relative to civil twilight times showed strong seasonality at 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 primary productivity in Saanich Inlet, which reduced light penetration and hid euphausiids from visual predators, likely drove this difference. Our findings confirm that proper understanding of DVM behaviours must account for seasonal variability due to context-specific oceanographic and ecological parameters. This is particularly pertinent when attempting to model the biogeochemical or predator–prey interactions influenced by DVM behaviours.

Keywords: active acoustics, biological oceanography, diel vertical migration, euphausiids, predator–prey interactions, time series, zooplankton.

Introduction

Euphausiids are a major component of the zooplankton com- munity along the coast of British Columbia (BC) and a key trophic link between plankton and nekton (Robinson, 2000; Ware and Thomson, 2005). It is estimated that over a quarter 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). These fish populations are thought to be limited by the available biomass of euphausiids (Ware and Thomson, 2005). In response, euphausiids in this region are known to perform 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 beengenerally considered to be an anti-predator defence behaviour,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- plankton perform vertical migrations around dawn and dusk, the transitions of civil twilight (Vuorinen, 1987; Pearre, 2003; Urmy and Benoit-Bird, 2021). 

原创作者：上海逐海仪器设备有限公司