ASLO - Ocean Sciences Meeting, San Diego 1996 - Abstract

Time Between Lunch and Dinner in the Plankton: Measuring Distance
Between Organisms in Coastal Marine Zooplankton Using a Modified
Optical Plankton Counter (OPC)

W.J.S. Currie, M.R. Claereboudt, and J.C. Roff (Department of Zoology, 
University of Guelph, Guelph, ON, Canada N1G 2W1; 519-824-4120 x3635; 
wcurrie@uoguelph.ca)

The microscale distribution of planktonic communities is a significant unknown in 
marine food web dynamics. The Optical Plankton Counter (OPC) provides a new 
technology that can measure distances between successive planktonic organisms 
sized from 250 µm to 2 cm diameter. 

The OPC permits a quick unobtrusive determination of zooplankton distributions from 
the km-scale down to a cm- scale resolution. We modified a Focal Technologies OPC 
and deployed it in-situ to count and size particles around the St. Lawrence estuary 
near-coast. The hardware alteration stores a time stamp for each particle counted. 
Knowing the towing velocity allows us to calculate the distance between successive 
particles. The OPC acts as an artificial plankton feeder. The determination of food 
encounter rate for specific planktivores can be determined through an appropriate 
scaling down of the feeding response zone and velocity of the organism.

In our study, we attempt to relate physical and biological oceanographic information. 
We coupled the OPC to a CTD profiler containing an in-situ fluorometer and oxygen 
probe. Nearly 200 transects were carried out with the unit at depths from 2 to 30m. 
With this data, we can identify the various scales of variability and patterns of 
distribution for zooplankton abundance. There are other methods of continuous 
sampling which examine the spatial distribution of zooplankton, but it is our belief 
that the range of scales and the sheer volume of data collection offered by the 
modified OPC is unparalleled.