Cook Strait as a field for ecological study: Plankton
By and large the planktonic population of one water mass will differ from that of an- other either in the species present or in the composition of the population. Where several water masses are mixing the planktonic organisms collected at a point in the mixture will depend on which water masses are contributing towards the environment at that point. The relative abundance of the several populations may well be indicative of the proportions of the respective contributions and of the length of time since these were made.
The water in Cook Strait may be assumed to be a mixture of waters. The chief components probably are of subtropical and sub-antarctic origin. Water which enters the Strait from the Tasman Sea may differ from either of these. An extension of the East Cape Current, which reaches and is deflected into eastern Cook Strait at times, shows qualities indicating a tropical origin. Up-welling in and near the Straits may add yet another, different water.