Small-scale horizontal distributions of zooplankton taxa

Abstract

Small-scale (100 to 2 400 m) horizontal distributions of major taxonomic categories (class and order) of zooplankton were measured at a depth of 90 m with an opening-closing plankton net over a 3 d period in October 1978 in the California Current. Some zooplankton categories showed evidence of diurnal vertical migration, while others had long-period temporal changes in mean abundance. Variance-to-mean ratio for large copepods and euphausiids was higher at night than during the day, while the opposite was true for chaetognaths and pteropods. Within a given category, the variance-to-mean ratio generally increased with a category's abundance. Spatial abundance variations were characterized by trends (i.e., fluctuations larger than length of the net hauls) in some taxonomic categories. No consistent differences in scales of variability were found as a function of animal size or from day to night. Correlation analysis of taxonomic counts implied that significant biological interactions occurred. The proportions of counts of taxonomic groups showed no large changes over the time-space scales sampled. However, the proportions of biomass in taxonomic groups differed from day to night due to the large variability of euphausiids. Comparisons of wet weight biomass to taxonomic counts indicated that biomass was usually less variable than taxonomic counts.