Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i at Manoa

Towed Instrumentation

Optical Plankton Counter

Analytical Method

Before the return transit to Honolulu, a series of transacts are made within the 6 nautical mile radius of Station ALOHA to obtain data from a towed instrument package consisting of an Endeco Instruments 1.5 m fin, a Sea-Bird (SBE-19) CTD and a Focal Instruments Optical Plankton Counter (OPC). This is a joint project between the HOT core component and scientists from Scripps Institution of Oceanography (Mark Huntley, P.I.) which started with HOT-60 (February 1995). A WetLabs SeaStar fluorometer was added to the package for HOT-64 (July 1995). The package is deployed on a 3-conductor cable and towed at a constant depth at a speed of 8 knots. The package collects data on physical (temperature and conductivity) and biological (zooplankton biomass and size spectrum, chlorophyll a) variability in the mixed-layer of the water column. The sampling pattern typically consists of zigzags within a square extending from 22.65' N to 22.85' N, 157.9' W to 158.1' W, and a transect along the regular ship track into Honolulu. In 1996 the target towing depth was 45 m, with variations of approximately ± 5 m due to changes in ship speed and heading.


A 3-dimensional plot of particle counts at 45 m and grouped in size classes for 1996 is shown in Figure 66. A similar plot of particle volume relative to size class is shown in Figure 67. This figure shows that although most of the particles are in the smaller size range, mid-size particles dominate the spectrum on a per volume basis.