Fine-scale zooplankton distributions revealed with acoustics and optics

Zooplankton are inherently patchy and therefore difficult to assess with standard net tows. In addition, standard vertical tows cannot measure vertical structure at sub-meter scales. By combining information from a laser optical plankton counter (LOPC) and high frequency hydroacoustics (120 and 420 kHz), we found high vertical heterogeneity in zooplankton distribution in the Laurentian Great Lakes. Densities can be an order of magnitude higher in thin layers occurring within a few meters around the thermocline.  We investigated these fine scale layers using data from Lake Michigan collected during day and night surveys. The relatively large (up to 25 mm long) mysid shrimps dominated acoustics backscattering below the thermocline at night, making it difficult to observe distribution of other zooplankton with acoustics at that time.  However, mysids migrated deeper in the water column during the day, allowing us to study detailed depth distributions of copepods and cladocerans . Using a combination of optics and acoustics gave us a more complete picture of the distribution of zooplankton in the Great Lakes. We discuss the importance of this fine scale layering of zooplankton for understanding the interactions within the zooplankton community and between zooplankton and planktivorous fish. 

Primary Presenter: Lars Rudstam, Cornell University (lgr1@cornell.edu)

Authors:

Lars Rudstam, Cornell University  (lgr1@cornell.edu)

Anne Scofield, USEPA Great Lakes National Program office  (Scofield.Anne@epa.gov)

James Watkins, Cornell University  (jmw237@cornell.edu)

Hannah Blair, Cornell University  (hbb59@cornell.edu)