Marine Biology

HARDY'S CONTINUOUS PLANKTON RECORDER

It was first used by the scientists of the Edinburg Oceanographic Laboratory, UK for the collection of the zooplankton in the North Sea and North Atlantic. By this instrument, not only rapid sampling are possible but also the collection of data on the distribution of plankton over large distance within a short time.

It is a torpedo shaped plankton sampler and consists of various parts such as water tunnel, two rolls of net silk (27 meshes/cm; mesh width, 300 µm) of about 15 cm width, gear mechanism, propeller, stabilizing fin, horizontal stabilizing fin, diving plane, vibration damper, storage spool, formalin tank, etc. When the instrument is towed from a vessel, the propeller is turned by the passage of the water. Following this, the gears and the adjoining spools containing net silk strips are also simultaneously activated. When water with plankton flows through the tunnel, one of the rolls of silk from the lower spool runs up through the water tunnel in the same way as a film, in camera and filters the plankton which streams through the small squarish mouth (2cm²) of the instrument. Before this net strip containing filtered plankton reaches the spool immersed in the formalin tank, another roll of silk from the upper spool immersed in the formalin tank, another roll of silk from the upper spool which acts as a covering net strip covers the collecting strip and keeps the plankton on it. Then both the strips are wound up on the storage spool which is located in a container filled with formalin solution to preserve the plankton in the strips. The water without plankton is drained through the other end of the sampler. When the storage spool is full after the operation, the same is removed and sent to the laboratory for analysis. As this instrument needs no attention while operation, it can be used by even untrained personnel.

The rate of the movement of the net strip is so regulated that its 1 cm advance is equivalent to one nautical mile of the trawling stretch. A loaded recorder has a provision to operate it continuously for about 500 nautical miles. Owing to the greater mesh width of the net strips, only zooplankton is collected through this sampler. However, if finer monofilament nylon net strips are employed and collections are contemplated for shorter distances maintaining slow speed then this instrument may also have a brighter scope for microphytoplankton collections in our seas in the near future.