FLUME TANK EXPERIMENTS TO ESTIMATE THE VOLUME FILTERED BY HIGH-SPEED PLANKTON SAMPLERS AND TO ASSESS THE EFFECT OF NET CLOGGING

Two types of flowmeter were used to measure the velocity of water pass ing into Lowestoft high-speed tow nets and hence to estimate the volum e filtered. Miniature (Streamflo) flowmeters were used to estimate the average velocity through the circular opening plane by measuring the velocity at several points along the radius of the opening and taking a weighted average of these. The velocity varies along the radius of t he opening and because the Braystoke internal flowmeters, which are us ed routinely in field deployments, only measure the velocity in the ce ntral part of the opening, they do not give an accurate estimate of th e average velocity through the opening plane. This will result in bias sed estimates of volume filtered unless a correction is applied. Linea r models are used to show that the correction depends on the type of g ear and the degree of clogging of the net, but is independent of water velocity. Clogging was simulated in the experiments using impervious patches of material, and can be estimated in the field from the ratio of internal:external flowmeter readings. Corrections for bias and esti mates of filtration efficiency are given for three gear configurations under a range of clogging states. Further experiments are needed to r esolve remaining uncertainty over estimates of filtration efficiency.