Journal of Plankton Research, Vol 21, 1823-1845, Copyright © 1999 by Oxford University Press
P Saunders, K Porter and B Taylor
In Lake Oglethorpe, Georgia, USA, herbivorous crustacean zooplankton are
abundant and dominate zooplankton biomass in winter, but are scarce
throughout most of the summer. We used a 3.5 year study of
Daphnia population dynamics to infer when food,
predators or temperature constrained growth of this population. Transitions
between winter and summer consumer assemblages are concurrent with seasonal
changes in water temperature, thermal structure
(stratification/destratification), resources
(autotrophic/heterotrophic-domained production), and predator abundance and
activity (e.g. Lepomis macrochirus and
Chaoborus punctipennis. We sampled at weekly or less
intervals from April 1992 to September 1995, and determined population
abundances for all cladoceran species. For the Daphnia
population (Daphnia ambigua + Daphnia
parvula, we measured clutch size and length for all individuals.
We used average water column temperature (where dissolved oxygen is
>1 mg l-1) to estimate egg development time
from an empirical model. Estimates of Daphnia
population birth and death rate were thus generated from abundance, egg
ratio and temperature/dissolved oxygen data. We compared observed birth
rate (bobs) with expected birth rate
(bexp>
ORIGINAL ARTICLES
Population dynamics of Daphnia spp. and implications for trophic interactions in small, monomictic lake
Institute of Ecology, University of Georgia, Athens, GA 30602, USA; Savanah River Ecology Laboratory, Drawer E, Aiken, SC 29802, USA; Present address: Ocean Sciences Centre, Memorial University of Newfoundland, St John's, NF A1C 5S7, Canada
95% CI;
predicted for food-saturated conditions at ambient temperature). For
variable (1-13 week) periods between later November and March, 1992-1995,
water temperature was the primary factor constraining
Daphnia population growth (bobs =
bexp). From about April to early November,
bobs < bexp suggested
food-limited population growth. In spring, summer and early fall
(March-October), population densities were several-fold lower than in late
fall and winter (November-February). However, all else being equal, egg
ratio and population birth rate data would have predicted that
Daphnia abundance fluctuates over equivalent ranges in
spring and fall. We interpret this discrepancy as evidence for increased
rates of extrinsic mortality during the growing season and a seasonal shift
in the relative importance of resource and predator regulation. The
duration of predator suppression of crustacean population abundance in Lake
Oglethorpe and other warm-latitude lakes (36
N-27°S) is longer (
3-7 months) than
that observed in north temperate lakes (1-2.5 months; 41-52°N).
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