Comments on the use of a deep tank in planktological research

Abstract

The Dalhousie University Aquatron tower tank (10 m deep, 3.66 m in diameter, with viewing ports at regular intervals down its side) can be operated in a continuous flow mode or as a closed system and has programmable illumination providing up to 10% of maximum summer illumination at the surface. Such systems have several drawbacks. Perhaps most serious is the lack of an adequate control. In the continuous flow mode the system is subject to the same type of sampling error encountered in the use of any other sampling device in a patchy environment. Temperature regulation and mixing were a problem with the tank in the static mode. As a result considerable stratification of properties occurred. Irradiance seemed to have too much directionality for normal diel migration patterns to develop in the zooplanktonic community. On the other hand, the system would seem very useful for short-term studies of behaviour and community interaction. For example, the pteropodSpiratella retroversa has been observed to regulate its buoyancy almost instantly. Typically animals remain motionless or slowly sinking (5–20 cm min⁻¹) for 1 to 5 minutes, swim up for 10 to 60s, and then return to “resting” and feeding. Mixed zooplankton populations, including such notoriously delicate animals asSagitta elegans, can also be maintained and behaviour studied. Despite a variable temperature regime, the structure of the zooplankton community remained intact for more than three months.