Diel variation of the zooplankton and its environment at neritic stations in the Inland Sea of Japan and the north-west Gulf of Mexico

Abstract

Diel variations in the zooplankton and its environment were investigated at two, contrasting neritic stations. The first (BG-1), in the Inland Sea of Japan, was mixed and eutrophic, while the second (GM-1), in the north-west Gulf of Mexico, was stratified and oligotrophic. Intensive studies were conducted at each station in late summer for 2–3 days. Dissolved nutrients and the particulate matter were evenly distributed in time and space at BG-1, but were variable, and often maximal at depth in a nepheloid layer, at GM-1. For each station, ∼20 categories of zooplankton were enumerated in samples collected with a plankton pump and retained on ∼100 μm mesh filters. In general, the zooplankton at BG-1 exhibited little diel variation in abundance and distribution. By contrast, most types of zooplankton at GM-1 performed diel vertical migrations, though primarily within the lower half of the water column between the thermocline and nepheloid layer. Significantly, similar taxa and stages did not always behave similarly in these two, differing environments, nor did the zooplankton at GM-1 tend to aggregate at the depths of maximal particle abundance or primary productivity. We suggest that studies of diel variation of the distribution and abundance of the zooplankton often require more intense sampling, in time and space, in environments which are stratified rather than mixed.