The Vertical Distribution of Microzooplankton and Some Ecological Observations

Abstract

Major taxonomic groups of the micro-zooplankton populations were identified and their numerical abundance determined inseveral depth intervals over the upper 100 or 200m from coastal (Sta. I), continental shelf (Sta. II and III), and oceanic (Sta. IV) sites in the north-eastern Pacific Ocean. Micro-zooplankton is defined as all animal plankters which passed 202 μ mesh filter cloth. The samples were obtained with a submersible pump system and deck-mounted plankton collecting unit which separated the material into three size fractions. In addition to micro-zooplankton abundance, chlorophyll a, phaeopigments, and seston dry weight levels were determined on each sample.

Total micro-zooplankton numbers averaged 730, 740, 970, and 270 × 10³/m³ over the euphotic zone at Stations I–IV, respectively, and 250, 140, 88, and 140 × 10³/m³ below thecompensation depth at the respective sites. Organisms passing through 35 μ mesh cloth( −35 sample) dominated the populations throughout the water column sampled at each site. Averaged from the depth intervals sampled, the numbers of organisms retained by 35 μ cloth were 25, 12, 7, and 8% of the total micro-zooplankton at Stations I–IV, respectively, while at all sites animals retained by 103 μ cloth (+ 103 sample) were only 1–2%. Protozoans were numerically dominant in the −35 and +35 samples with metazoans relatively most abundant in +103 samples.

A decrease in the average numbers of micro-zooplankton/m³ in the euphotic zone was observed between neritic and oceanic sites. However, the greatest number of microzooplankton per unit chlorophyll a was found at the open ocean site and the lowest number at the coastal station. The possible ecological significance of this distribution is discussed.