Taxonomy and abundance
Sample collection
Mesozooplankton were collected using a 1 m² Multiple Opening and Closing Nets and Environment Sampling System (MOCNESS) equipped with 9 nets of mesh size 330 m m (Wiebe et al. 1976, 1985). Tows were conducted during nighttime (22:00 - 02:00 hrs, local time) at an average towing speed of 2 to 3 knots. The wire speed was adjusted to ensure the net angle remained at approximately 45°. Data collected by the MOCNESS included time, volume filtered (adjusted by the net angle) and depth.
Samples were taken at the following depth strata: 0-500, 500-400, 400-300 300-250, 250-200, 200-150, 150-100, 100-50, and 50-0 m. A fraction of each sample (~ 5%) was frozen at 80°C within minutes after the sample was collected in order to asses gut pigment contents. The rest of the sample was preserved in 5% borate-buffered formaldehyde.
Some net tows had problems caused by losing Cod-ends or ripped nets in rough sea. In the spring cruise (R9710) the surface nets were often clogged with Phaeocistes colonies. We were also unable to fractionate and recount those samples due to the high biomass of Phaeocistes colonies.
Analysis
To identify differences in taxonomic composition of zooplankton communities, samples were enumerated at least to the genus level, and, when possible, to species. For copepods, adult males, adult females and copepodite stages were enumerated.
Copepod species were identified according to the keys of Brodsky (1950), Vervoort (1951, 1955), Razouls (1994), Park (1993), and Guiglielmo & Ianora (1995).
The most abundant samples were subdivided with a Folsom splitter according to procedures described in the literature (Griffiths et al., 1984; Mc Ewen et al., 1954; Sell and Evans, 1982). Abundance of zooplankton was calculated from the total counts divided by the total volume filtered, as measured by MOCNESS (Wiebe et al., 1976).