To have sufficient signal strength, the water must contain enough scattering materials that can reflect the transmitted signal. When the concentration of particles is low, it can be difficult to make valid velocity measurements. Vertical migration of plankton can contribute to these variations in particle density.
Zooplankton follows a movement pattern with a circadian rhythm, where they swim up in the water column at night at down during the day. This is called diurnal vertical migration. An example of how this can appear in amplitude data is given in Figure 1. Whether the lowest signal strength is during night or day depends on the orientation of the instrument. An instrument pointing up will typically have the best signal strength values during the night, and the opposite for an instrument looking down. If data needs to be discarded during the times of lowest signal strength, depends on how low it is. Assessing the signal strength is hence important in these situations. The influence of ideal vertical migration varies depending on the deployment site. Areas with a lot of other particles in the water are less dependent on zooplankton to obtain good data. Some measurement series has a combination of seasonal variation and diurnal vertical migration of zooplankton, and can as a result of that have some periods where the lower signal strength is sufficient and other times when it is not.
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