Seasonal variability in the relationship between body length and individual dry weight as related to food abundance and clutch size in two coexisting Daphnia species

Abstract

Dry weights of individual Daphnia galeata and D. hyalina, which coexist in Lake Con- stance, were determined weekly during a two year investigation. From each sample and each species a length — weight relationship (LWR) was calculated resulting in 54 (65) regression equations based on N = 2466 (3032) in D. galeata (D. hyalina). The constants (Ln(A)) and (B) of the LWRs showed a marked seasonal variability, ranging from Ln(A) = 1.0784 - 2.4740(1.0152 - 2.3860) and B = 2.14 — 4.20(2.11 - 4.15). The seasonal variability of the LWR was estimated with four different models. Condensing the 54 (65) equations into one seasonal cycle resulted in a model with an explained variance of r 1 = 0.873 (0.869). Three more models were established by calculating multiple regres- sions with lake stratification, average fecundity (E), and food concentration as determining variates. Differences in LWRs of the two Daphnia species were significant (p <0.001). As the most useful approximation during the period of stratification, LWRs are recommended for D. galeata: Ln(W) «= (1.5674 + 0.0287*(E)) + Ln(L)*(3.3611 + 0.0111*(E)) as a generalized LWR for an epilimnetic daphnid, and for D. hyalina: Ln(W) = (1.5593 + 0.0613'(E)) + Ln(L)*(3.2709 + 0.0017*(E)) as LWR for a daphnid which migrates diurnally between epi and hypolimnion. The explained variance of this model is r2 = 0.843 (0.826). The influence of food concentration in the epilimnion on LWRs was found to be significant on Ln(A) but not on (B).