Fine-scale zooplankton distributions revealed with acoustics and LOPC

Session: Spatial Dynamics in the Pelagia of Large Lakes – Technological Advances and Applications

Lars Rudstam, Cornell University Bio Field Station, Dept. of Natural Resources, [email protected]
Anne Scofield, Purdue University, [email protected]
Toby Holda, Cornell Biological Field Station, [email protected]
James Watkins, Cornell University, [email protected]

Abstract

Zooplankton are inherently patchy and therefore difficult to assess with standard net tows.  Vertical tows can vary with a factor 2 between replicates taken at the same time and location.  Zooplankton distributions also show interesting vertical fine scale structures with density concentrations at certain limited depth layers.  Such fine scale layering will affect the role of zooplankton as both predators and prey, but is rarely studied or considered in the Great Lakes. We investigated these fine scale layers with hydroacoustics and laser optical plankton counters using data from Lake Ontario and Lake Michigan during day and night surveys.  Mysids dominate acoustics backscattering below the thermocline at night, making it difficult to observe distribution of other zooplankton at that time.  However, when mysids are deeper in the water column or on the bottom during the day, both methods gave similar depth distributions of the zooplankton. We investigate effects of day/night and the size of the animals present on the correlation between the two techniques.  Using a combination of optics and acoustics is likely to give us a more complete picture of the distribution of zooplankton in the Great Lakes.  Neither method can identify species however, thus net tows are still needed.