Ecological Studies on New Zealand Lacustrine Zooplankton with Special Reference to Boeckella propinqua Sars (Copepoda: Calanoida)

Abstract

A study was made of the zooplankton of a small, warm monomictic, closed, alkaline lake. This was located on Mayor Island, an offshore island of the North Island of New Zealand. Morphometric, thermometric, and chemical aspects of this lake are discussed. The vertical distribution of the zooplankton was investigated by the vertical hauling of a closing net and by the horizontal towing of a Clarke Bumpus sampler. The vertical distribution of zooplankton biomass and of the different planktonic crustacean species at different times of the year is discussed. When thermal stratification was well developed, all crustacean and rotatorian species had a very sharply defined lower limit between the upper limit of the metalimnion and the thermocline; minimal population densities existed some distance above this lower limit. A reversed diurnal vertical migration is reported for Daphnia carinata and the mature males of Boeckella propinqua. The following investigations and findings refer to Boeckella propinqua: the vertical distribution of four population components (CI-V, C VI male, male, C VI female, female , and C VI female, female, with eggs) at different times of the year is discussed; departure of sex ratio from 1 : 1 is reported; cyclic seasonal changes, which correlated inversely with temperature during part of the year, occurred in mean length and mean number of eggs in sacs; mean egg number also correlated with mean body length; considerable simultaneous interpopulational variation in mean egg number was found; length measurements of adults from November surface collections showed a bimodal distribution, and at the same time vertical variation occurred in the nature of size-frequency distributions-in both sexes the upper mode was progressively diminished with increasing depth and finally eliminated; in these November collections the percentage of lower modal females with eggs was much smaller than that for upper modal ones; when surface collections showed unimodal size distributions there appeared to be no vertical variation in size-frequency distributions; variation in the lobulation of the last metasomal segment of females is mentioned; the occurrence of a cestode cysticercoid stage in male and female stages CV and VI, and parasitic castration of C VI male, male, is reported. General observations are made on the Rotatoria and the vertical distribution of Filinia longiseta is dealt with. Comparisons are made between the zooplankton of a number of mainland lakes and that of the Mayor Island lake.