When an instrument is deployed for a long time, it is normal for organisms to start growing on it. Growth on the transducers is a form of blockage that gradually increases. The result may appear as a gradual decrease in amplitude with time, as presented in the figure below. It can still be possible to get valid velocity measurements beyond the fouling, at least for a while, before the growth becomes massive and absorbs too much of the transmitted signal. As a part of the data analysis, you have to assess the amplitude and/or correlation to determine the data quality. Data from cells closest to the instrument, should not be used if it includes the fouling (depending upon the blanking distance and fouling thickness). Then the instrument measures the velocity of the fouling and not the water. Types of fouling and how common it is depends on where in the world the deployment is. One way to prevent, or reduce, fouling is to use antifouling patches on the transducers.
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