(4) Measurement of filtered water volume

　The amount of seawater that passed through the net opening of the plankton net, that is to say the amount of seawater filtered out by the net, is called the "filtrate volume. Assuming that the plankton community was uniformly distributed in the sea, the plankton community biomass in the study area can be estimated from the "volume of filtered water. Therefore, a water filter meter (Figure 7) is used to measure the filtrate volume. The filter waterer is attached to the opening of the net.

図7  Appearance of the water filter meter (made by RIAI Co., Ltd.) and example of installation on a net 

ろ水計：water filter meter

　Figure 8 shows the structure of a water filter meter. The impeller is turned by seawater passing through the outer casing, and the rotation of the impeller is converted to rotation of the dial pointer by a reduction mechanism consisting of several gears. The amount of seawater that has passed through the mesh opening (filtration volume) is calculated from the number of rotations of the impeller.

図８　Structure of water filter meter

　The water filter meter has three or four pointers, each of which corresponds to one hundred, one thousand, ten thousand, or one hundred thousand revolutions of the impeller. In the example shown in Figure 9, "How to Read the Rotation Speed of a Water Filter Meter," the bottom pointer points between the 1000 and 2000 scales, indicating that the number of impeller revolutions is between 1000 and 2000 (the thousands digit is "1"). Similarly, if we read the middle pointer, the hundredth digit is "5," if we read the top pointer, the tens digit is "2," and if we read 1/10th of the smallest scale on the top pointer, the ones digit is "5. As a result, the number of revolutions is obtained as "1525".

図9　　How to read the rotational speed of a water filter meter

　The value of the water filter meter changes in proportion to the distance the net is towed. If the value of the filter water meter decreases despite towing the same distance, it indicates that the net has become clogged and its seawater filtration capacity has decreased. In this case, it is assumed that the seawater that cannot pass through the net flows backward, not only reducing the amount of filtered water, but also causing the plankton community that has passed through the net mouth to be expelled.