Carbon as an indicator of Daphnia condition in an alpine lake

Carbon-length relationships can be used to indicate the condition of daphni ds in natural situations. We examined the Daphnia galeata population of an alpine lake (Oberer, Arosasee, Switzerland), where most individuals display diel vertical migration behaviour (DVM). Normally, migrating daphnids face a trade-off between 'predator safe areas' in the hypolimnion and 'food ric h areas' with high predation risk. However, in alpine lakes, with chlorophy ll a and POC maxima typically in deeper layers and weak temperature gradien ts, migrating species are confronted less with this trade-off. We investiga ted the seasonal changes in carbon content of D. galeata in different depth strata in relation to environmental parameters of the lake. The carbon con tent of D. galeata was high in spring, but declined considerably in summer and increased slightly in autumn. The low values indicate that D. galeata a re food limited for much of the year. The slopes of the regression lines be tween carbon content and body length varied seasonally, but were not signif icantly different among depths on a given date. In summer, D. galeata indiv iduals residing in the deep layers during the day had a significantly highe r carbon content than individuals in the surface layers. During the rest of the year, the carbon content of individuals was similar among all depth st rata. We conclude that migrating D. galeata and individuals remaining in de ep layer had better body conditions compared to non-migrating D. galeata in this alpine lake during summer, when migration amplitudes were highest.