Ecological studies on the zooplankton of Lake Pounui, Wairarapa

Abstract

Lake Pounui is a small, shallow, polymictic lake in the southern Wairarapa Valley (North Island, New Zealand). Seasonal cycles of abundance of phytoplankton and zooplankton in the lake are described. Concentrations of chlorophyll a were comparatively low, with a range of 0.6 mg m-3 in the winter to 4.5 mg m-3 in the autumn. The seasonal cycles of two species of Volvox are described. A distinct seasonal succession of the major zooplankton species was observed, dominance changing from Bosmina meridionalis in spring, to Ceriodaphnia dubia, to Calamoecia lucasi, to Conochilus spp. in the late autumn. Populations of Boeckella hamata, Mesocyclops leuckarti, Piona uncata exigua, and Synchaeta sp. also had notable peaks of abundance but were never numerically dominant. The effects of temperature on the development rates of Ceriodaphnia dubia and Boeckella hamata were investigated in the laboratory and temperature functions were established for each. The breeding parameters and population dynamics of the cladocerans Bosmina meridionalis and Ceriodaphnia dubia, and the calanoid copepods Calamoecia lucasi and Boeckella hamata, are described, and losses in the developmental stages of C. lucasi and B. hamata are determined. The breeding of the cladocerans was seasonal with high egg production in the spring and summer, and low in winter. Copepod breeding was aseasonal and continuous, with bursts of egg production associated with the maturation of each "generation". Age specific mortality was important in both copepod species, with per capita loss rates in the nauplii being approximately twice those recorded for similar populations in other New Zealand lakes. Changes in standing stocks of each species appeared to be controlled by variations in loss rates. The mite Piona uncata exigua is shown to be a potentially important predator on the cladoceran populations. The intrinsic rate of population change (r) was low in all four species studied (0.014 for C. lucasi to 0.048 for B. meridionalis) because changes in the birth rate were usually matched by corresponding changes in the death rate. Both cladoceran species had smaller clutch sizes and faster postembryonic development than both copepod species. However the B. meridionalis and C. lucasi populations were characterised by relatively low birth and death rates compared to the C. dubia and B. hamata populations. Intrinsic birth and death rates estimated for B. hamata were higher than have been recorded for any other New Zealand lacustrine copepod population. It is suggested that generally high mortalities for the above four major species in Lake Pounui may have prevented mechanisms of competitive exclusion that operate in other New Zealand lakes. The vertical distributions and vertical population movements of the zooplankton and Volvox spp. are described and related to resource distribution. Both species of calanoid copepods appeared to perform a type of "reverse" vertical migration.