Patterns of abundance and population dynamics of zooplankton in tropical Lake Malawi
Many tropical environments, and the dynamics of populations living in these environments, are now recognized as variable both temporally and spatially. However, tropical lakes still often are considered as relatively constant. Although seasonal patterns in physical processes in tropical lakes have been documented, the dynamics of tropical zooplankton populations remain particularly difficult to interpret, in part because they have been poorly studied.
Analyses of three years of daily weather readings from Malawi define seasonal changes in major meteorological parameters. In addition, several of these (wind velocity, evaporation, radiation and daily sunshine) can fluctuate abruptly, and often vary as much within one season as between seasons.
Seasonal and irregular climatic fluctuations influence physical processes in Lake Malawi. Seasonal cycles in mixing are induced by strong southeast trade winds that blow from May through September. Thermoclines are reestablished during the calmer and hotter months, October through March. Analyses of temperature profiles and stability of the water column show that abrupt climatic events, and particularly changes in wind velocity, disrupt seasonal cycles of mixing and stratification. Unseasonably strong winds break down shallow thermoclines and mix surface layers of the lake. Unusually calm periods result in surface warming, the formation of secondary thermoclines and an increase in stability.
Zooplankton were sampled weekly, for 2.5 years, to describe patterns of abundance. Four of the six most common crustacean species exhibit clear seasonal increases in abundance coincident with mixing in the lake and phytoplankton (predominantly diatom) blooms. In addition, each of the six major species exhibits irregular increases in population size which reflect demographic phenomena, and cannot be dismissed simply as errors caused by sampling within localized patches of plankton. Irregular fluctuations in population size confound seasonal trends, and both seasonal and annual cycles in abundance differ between years. As a result, no one year's data accurately describe zooplankton dynamics in this large tropical lake.
Demographic analyses of the two common cyclopoid species, Mesocyclops leuckarti aequatorialis and Thermocyclops neglectus, demonstrate that both seasonal and irregular fluctuations in population size are accompanied by temporal fluctuations in fecundity and survivorship. In both species, large seasonal peaks in egg production are accompanied by smaller aseasonal peaks, and fluctuations in population size are highly correlated with both seasonal and irregular peaks in the abundance of eggs (with time lags from 1-12 weeks).
Peaks in egg production are closely associated with increased abundance of females, and with increases in the proportion of females that are ovigerous. Clutch size varies irregularly in both species, but is independent of increases in the abundance of eggs or females. Survivorship estimates determined for peaks in eggs of both species suggest that survivorship fluctuates within one year as well as between years. These fluctuations are marked for Mesocyclops leuckarti aequatorialis, and have a major influence upon total fecundity and total population size.
The results of this study contradict previous impressions that tropical zooplankton
populations vary little in size and achieve a stable age structure. In Lake Malawi,
seasonal and irregular fluctuations in zooplankton abundance result from temporal
variations in the demographic processes of birth and death. These variations may be
linked with irregular climatic events, which disrupt seasonal cycles in lake processes
and may influence primary productivity and phytoplankton abundance.
NOTE: The above version of Twombly's abstract, dated 15 Dec 1982, is not her
final version but is longer and more detailed than the latter. I have
taken it from a personal copy of her dissertation. M.K. Oliver