Abstract
Bifrequency acoustic data, hydrological measurements and satellite data were used to study the vertical distribution of macrozooplankton in the Bay of Biscay in relation to the hydrological conditions and fish distribution during spring 2009. The most noticeable result was the observation of a ‘biocline’ during the day i.e., the interface where zooplankton biomass changes more rapidly with depth than it does in the layers above or below. The biocline separated the surface layer, almost devoid of macrozooplankton, from the macrozooplankton-rich deeper layers. It is a specific vertical feature which ties in with the classic diel vertical migration pattern. Spatiotemporal correlations between macrozooplankton and environmental variables (photic depth, thermohaline vertical structure, stratification index and chlorophyll-a) indicate that no single factor explains the macrozooplankton vertical distribution. Rather a set of factors, the respective influence of which varies from region to region depending on the habitat characteristics and the progress of the spring stratification, jointly influence the distribution. In this context, the macrozooplankton biocline is potentially a biophysical response to the search for a particular depth range where light attenuation, thermohaline vertical structure and stratification conditions together provide a suitable alternative to the need for expending energy in reaching deeper water without the risk of being eaten.
Highlights
Zooplankton play a key role in marine food webs [1] and their dynamics are closely related to the physical environment [2,3]
By taking into account both the temporal and spatial scales, we aim to explain the vertical distribution patterns exhibited by macrozooplankton with consideration of the following environmental parameters: vertical thermohaline structure and associated stratification, primary production, photic depth, and predator vertical distribution
The continuous, high resolution information provided by the acoustic method allowed us to define the biocline as the upper limit of the macroozooplankton vertical distribution and investigate its relationship with different environmental parameters and predation
Summary
Zooplankton play a key role in marine food webs [1] and their dynamics are closely related to the physical environment [2,3]. Consideration of the factors that affect the distribution and abundance of zooplankton and its role in the ecosystem is key to understanding the impact of the environment on ecosystem functioning. Zooplankton distribution varies both horizontally and vertically across a continuum of spatiotemporal scales [4,5,6], but the factors that impact on the vertical patterns are usually different from those that influence the horizontal distribution [7]. It is very important to determine the proximate environmental factors that govern the vertical distribution patterns of macrozooplankton, beyond the widespread but basic documentation describing the diel vertical migratory behaviour of mesozooplankton
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