Small‐scale Spatial Distribution in Oceanic Zooplankton1

A Longhurst‐Hardy plankton recorder was used near Guadalupe Island (Baja California) to obtain information on the small‐scale horizontal patchiness of a number of zooplankton species. On one occasion, a daytime series of 11 horizontal recorder tows was taken forming a grid over an area 500 m square at 90‐m depth (total number of samples, 459). On another, a single long nighttime horizontal tow was taken at 20 m (82 samples). A method has been developed to measure spatial structure in terms of patch size and shape, number and distribution of patches in the grid area, concentration of individuals in each patch, and densities of background individuals. The patches were approximately circular with a median patch radius ranging from 13.6 to 15.6 m for the species on the grid and 38.4 to 73.1 m for the night tow. Estimates of the numbers of patches in the grid area ranged from 183 to 222. They were distributed randomly. Average densities of individuals in the patches over the grid varied from 2.6 to 5.1 times the background densities. Although the abundances of different species showed a significant degree of concordance, the relationship between the same species pair on different tows and between different species pairs was quite variable. The degree of aggregation of the less abundant species was associated with their density; the lower the density the less frequently significant departures from a random expectation were observed.