Abstract
Abstract. Diel vertical migration (DVM) is a survival strategy adopted by zooplankton that we investigated in the Corsica Channel using acoustic Doppler current profiler (ADCP) data from April 2014 to November 2016. The principal aim of the study is to characterize migration patterns and biomass temporal evolution of zooplankton along the water column. The ADCP measured vertical velocity and echo intensity in the water column range between about 70 and 390 m (the bottom depth is 443 m). During the investigated period, zooplanktonic biomass had a well-defined daily and seasonal cycle, with peaks occurring in late winter to spring (2015 and 2016) when the stratification of the water column is weaker. Zooplanktonic biomass temporal distribution in the whole water column is well correlated with biomass of primary producers, estimated with satellite data. Zooplanktonic blooming and non-blooming periods have been identified and studied separately. During the non-blooming period zooplanktonic biomass was most abundant in the upper and the deep layers, while during the blooming period the upper-layer maximum in zooplanktonic biomass disappeared and the deep layer with high zooplanktonic biomass became thicker. These two layers are likely to correspond to two different zooplanktonic communities. The evolution of zooplanktonic biomass is well correlated with chlorophyll, with phytoplankton biomass peaks preceding the upper-layer secondary production by a lag of about 3.5 weeks. Nocturnal DVM appears to be the main pattern during both periods, but reverse and twilight migration are also detected. Nocturnal DVM was more evident at mid-water than in the deep and the upper layers. DVM occurred with different intensities during blooming and non-blooming periods. One of the main outcomes is that the principal drivers for DVM are light intensity and stratification, but other factors, like the moon cycle and primary production, are also taken in consideration.
Highlights
Diel vertical migration (DVM) is one of the most important survival strategies adopted by zooplankton
The data collected by the acoustic Doppler current profiler (ADCP) are used to define the temporal and spatial variability of zooplankton DVM and zooplanktonic biomass distribution patterns during the investigated period
The small lag we found for total mean volume backscatter strength (MVBS) vs. Chl a and the zero lag for deep MVBS vs. Chl a are somewhat unexpected, but it is necessary to keep in mind that the temporal resolution of the Chl a field from the satellite is 8 d and that it is an exponentially weighted near-surface value and not an integrated value of the phytoplankton biomass within the whole euwww.ocean-sci.net/15/631/2019/
Summary
Diel vertical migration (DVM) is one of the most important survival strategies adopted by zooplankton. During migration these marine organisms can cover vertical distances of a few hundred metres. During nocturnal migration at dawn zooplankton descends and remains at depth, where the probability of being predated by a visually hunting predator is lower; at dusk zooplankton rises to the euphotic layer and stays there during night to feed on phytoplankton (Ringelberg, 2010; Zaret and Suffern, 1976). This is only one of the three most common migration patterns. The typical descent of twilight migration that occurs during night is called midnight or nocturnal sinking and is a downward
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