Zooplankton Vertical Habitat Shifts in Relation to Water Column Optical Properties

Poster Number: 
Presenter/Primary Author: 
Mark Ohman
Jean-Baptiste Romagnan

On the CCE Process Cruises in the California Current System, we used a Lagrangian sampling design to identify and track a series of discrete water parcels and entrained plankton communities through time. This is part of the "space for time" exchange that CCE scientists are using to understand the mechanisms underlying ecosystem state transitions.  On these cruises, we sampled mesozooplankton in the upper 450 m of the water column using vertically stratified MOCNESS tows, followed by ZooScan digital scanning and application of a semi-automatic machine learning algorithm to classify zooplankton images.  From these analyses, we have discovered a recurrent pattern of size-dependent diel vertical migration (DVM) in planktonic copepods of the region.  Intermediate-sized copepods consistently showed the largest amplitude DVM response, with neither the smallest nor the largest size classes exhibiting significant day-night changes in vertical distributions.  The magnitude of the peak DVM excursion varies spatially, in relation to optical properties of the ocean water column and, ultimately, to size-dependent risk due to predation by visually hunting planktivores.  These predidctable spatial changes in mesozooplankton vertical habitats can be combined with with recent evidence demonstrating a 60-year change in optical properties of the California Current (Aksnes and Ohman 2009. L&O 54:1272) to forecast altered vertical structure of the plankton community over time.