Components of additive variance in life-history traits of Daphnia hyalina:seasonal differences in the response to predator signals

We attempted to separate the additive variance of life-history parameters ( size at first reproduction (SFR), proportion of total production allocated to reproduction) of Daphnia hyalina in a mesotrophic lake during different seasons into genetic components and phenotypic plasticity. Every month we r andomly isolated juveniles of 20 clones from the lake and measured their li fe-history parameters immediately after isolation, i.e., under the influenc e of all modifying factors in the lake. The measurements were repeated afte r the clones had been kept in the laboratory for at least six generations w ithout predator signals (controls) and with the addition of fish and Chaobo rus kairomones. SFR increased monotonously from July to December in Fresh f ield isolates. Laboratory controls showed always much larger SFR; but appro ached field value in December. Most of the size difference was due to mater nal effects. Reproductive allocation showed a different pattern than SFR wi th a minimum in September both in field samples and laboratory controls. Fi sh kairomone reduced SFR in July and August when fish predation is high, bu t not later in the fear. On the contrary, Chaoborus kairomone increased SFR in September and October when Chaoborus larvae are abundant in the lake, b ut not in summer. This indicates a seasonal shift in the clonal composition of D, hyalina populations towards clones adapted to the specific predation pressure. Reproductive allocation changed in response to fish kairomone in July and August, but not in response to Chaoborus kairomone. In this mesot rophic lake, fish have a stronger direct and indirect impact on Daphnia lif e history than Chaoborus larvae.