Diel Vertical Migration in Deep Sea Plankton Is Finely Tuned to Latitudinal and Seasonal Day Length

Hans Van Haren,Tanya J Compton

doi:10.1371/journal.pone.0064435

Abstract

Diel vertical migration (DVM) is a ubiquitous phenomenon in marine and freshwater plankton communities. Most commonly, plankton migrate to surface waters at dusk and return to deeper waters at dawn. Up until recently, it was thought that DVM was triggered by a relative change in visible light intensity. However, evidence has shown that DVM also occurs in the deep sea where no direct and background sunlight penetrates. To identify whether such DVM is associated with latitudinal and seasonal day light variation, one and a half years of recorded acoustic data, a measure of zooplankton abundance and movement, were examined. Acoustic Doppler current profilers, moored at eight different sub-tropical latitudes in the North-Atlantic Ocean, measured in the vertical range of 500–1600 m. DVM was observed to follow day length variation with a change in season and latitude at all depths. DVM followed the rhythm of local sunrise and sunset precisely between 500 and 650 m. It continued below 650 m, where the deepest penetrable irradiance level are <10−7 times their near-surface values, but plankton shortened their time at depth by up to about 63% at 1600 m. This suggests light was no longer a cue for DVM. This trend stayed consistent both across latitudes and between the different seasons. It is hypothesized that another mechanism, rather than light, viz. a precise biochemical clock could maintain the solar diurnal and seasonal rhythms in deep sea plankton motions. In accordance with this hypothesis, the deepest plankton were consistently the first to migrate upwards.

Highlights

Diel vertical migration (DVM) in ocean zooplankton is likely to represent the largest daily migration of animals on earth, in terms of biomass [1]
The present observations show that DVM of zooplankton in the deep sea follows precise solar variations in day length, across different latitudes and matches the sun’s latitudinal influx variation
Contrary to the conventional paradigm that DVM would cease in areas where visible light does not penetrate, this study, as well as others [15,16], show that plankton continue to migrate where visible light does not penetrate

Summary

Introduction

Diel vertical migration (DVM) in ocean zooplankton is likely to represent the largest daily migration of animals on earth, in terms of biomass [1]. Plankton migrate to surface waters at dusk and return to deeper waters at dawn. This pattern of migration is considered to reflect a trade-off between to need to feed versus predator avoidance, especially in the sunlit photic zone [2,3]. Acoustics provide numerous advantages over plankton net-tows. They are non-invasive, they can detect the larger zooplankton capable of swimming out of the way of nets, they can be used for continuous monitoring [8] and they can directly measure the vertical speed of zooplankton [9]. Acoustics are limited in that they cannot provide taxonomic resolution and they have difficulty in quantifying plankton biomass with a single frequency instrument [12]

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Conclusion