The distribution and behavior of tintinnids Stenosernella nucula have been measured in situ within a microlayer formed by 39 million individuals⋅liter–1 with an optical particle counting- and imaging-system. The parallel propulsions of the many animals add
up and drive strong downwelling water currents. Preliminary results for swimming-speeds, -directions and organism-distributions are presented. Probably due to gyrotaxis (Kessler, 1985, 1986) or some unknown bio/physical processes the organisms are focused into their self-generated velocity
profile. Similar phenomena have been described for very dense plankton cultures as “bioconvection” (Childress et al., 1975a, 1975b; Platt, 1961; Plesset and Winet, 1974; Plesset et al., 1975). The micropatches are 2–4 mm wide and 8–340 mm deep with organism concentrations
up to 215 million tintinnids⋅liter–1. The flows form small convection cells similar to Langmuir- or Benard-cells with distances between the patches of 8–30 mm. At the edge of the downwelling areas water velocity increases from near zero to 2–3 mm per s over
a vertical distance of less than 1 mm resulting in considerable shear. Some ecological consequences of these microturbulences and microdistributions for predator-prey relationships and particle transports in eutrophic estuaries are discussed.
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