Experiments on filtration in plankton nets

Abstract

Experiments were carried out in wind tunnels, in a test tank, and at sea, to investigate the interaction between the water column and a towed plankton net. Flow patterns about stationary nets held in a stream of air were observed and photographed. The velocity and turbulence of the stream near the net, and the drag upon the net, were measured. Experimental tows were made in the laboratory and in the field to test the effect of various factors on "filtration efficiency". This interaction between the discharge and the flow around the net leads to enhanced filtration. The effect is accentuated by a flared mouth which increases the velocity differential. The effect is reduced by encasing the net, which separates the interacting streams and probably causes the stream from the net surface to lose momentum. Filtration efficiency is independent of towing velocity, except for a gradual decline at velocities less than 1-2 kt. Filtration efficiency is also independent of filtering area in nets of usual length, but declines sharply in nets shorter than twice their mouth diameter. There is a zone of turbulence and reduced velocity in the centre of a net with bridles. At approximately one-quarter of a radius from the ring, the stream is flowing at mean mouth velocity. This seems to be the most suitable site for a flowmeter.

Commercial Exploitation of Zooplankton in the Norwegian Sea

“…Gjøsund and Enerhaug (2010) compare the theoretical model with flume tank measurements with finemeshed net cones, and demonstrate among other things how the filtration efficiency for typical plankton nets increases with increasing towing velocity, and decreases with decreasing velocity (Figure 3). This is contrary to common belief, as noted also by Tranter and Heron (1967); there is a widespread and persistent, but incorrect perception that filtration efficiency generally decreases as towing velocity increases. In plankton sampling the towing velocity is therefore often recommended to be low, and it is also assumed that a low towing velocity reduces clogging (Sournia, 1978).…”

“…In plankton sampling the towing velocity is therefore often recommended to be low, and it is also assumed that a low towing velocity reduces clogging (Sournia, 1978). However, Tranter and Heron (1967) found that so-called flared samplers clogged more readily in field experiments than unflared samplers. A flared sampler implies reduced velocity inside the net and towards and through the net wall, hence hydrodynamically it is equivalent to an unflared sampler with lower filtration efficiency.…”

“…A general recommendation is that R should be greater than 3 to have high initial (i.e. before any clogging occurs) filtration efficiency (Tranter and Heron, 1967), and greater than 6 to have an additional buffer against clogging (Harris et al, 2000). However, this ratio involves only the porosity and taper angle of the net; it does not account for the Reynolds number effect described above.…”

Design and wind tunnel testing of a size sampling in-situ net system (SSISNET)

“…A factor closely related to this area ratio is the net filtration ratio (FR) and is given by FR -f (8) where B is the net porosity.…”

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“…Studies noting underestimation of zooplankton from mesh sizes > 35 µm often found relatively small error rates (i.e., estimates within the same order of magnitude) or did not use sample sizes adequate to provide a reasonable estimate of effect size (Likens and Gilbert 1970;Bottrell et al 1976;Ejsmont-Karabin 1978). Furthermore, reductions in mesh size have a negative effect on filtration rate (Tranter and Heron 1967;Likens and Gilbert 1970;Orcutt and Pace 1984). A researcher wishing to sample crustacean zooplankton and rotifers would likely have to use different methods for each group, which doubles both field and laboratory effort.…”