HOT-75 COMMEMORATIVE SCIENCE SYMPOSIUM

Particle Concentrations at the Hawaii Ocean Time-series Station ALOHA Measured with an Optical Plankton Counter

Mai Lopez and Mark Huntley

Marine Biology Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093

Concentrations of zooplankton-sized particles (300 um - 14 mm equivalent spherical diameter) at 45 m depth in the region of Station ALOHA have been measured with horizontal tows of an Optical Plankton Counter since February 1995. Data cumulated in 200-m segments, equivalent to approximately 1.3 m³, show low average particle counts in winter (min: 230/m³) and peaks in summer (max: 1900/m³). Seasonal changes were due to particles < 1.8 mm ESD which comprised 99.5% of counts and 75.3% of volume per m³. In three transacts which included contiguous day and night periods, night-time counts and volume per m³ averaged 1.5 x the daytime values. Within transacts, maximum counts/m³ were approximately 3x the minimum, while maximum particle volumes/m³ were about 12x the minimum. Zig-zag transacts within Station ALOHA show longitudinal and latitudinal gradients in particle counts. Gradient directions shifted from cruise to cruise, with no clear temporal pattern.

Chlorophyll concentrations at 45 m were determined from in situ fluorescence since August 1995. Variability increased with time scale, from hours (transect data) to days (bottle casts) to seasons. Although the absolute range of values within transacts was narrow, gradients similar to spatial scale to those for particle counts are evident. There was no systematic relationship between chlorophyll concentration and particle counts.