Ultraviolet-B (UV-B) radiation has increased quickly in a short evolutionary time span due to the decrease of the ozone layer. Information about UV-avoidance strategies and their costs is, in addition to direct UV effects on organisms, an important key to understanding UV radiation as a selective ecological factor and to predicting influences of increasing UV radiation on the biosphere. My goal was to quantify factors determining UV-penetration depth and to characterize and quantify the benefit and cost of pigmentation and downward migration in the genus DAPHNIA as a model for UV-avoidance strategies in zooplankton.

Spectral UV radiation was measured in 9 lakes of different trophic states and an empirical model was developed to predict spectral attenuation coefficients based on the secchi depth and photometrical measurements. The influence of ozone, sun angle, altitude, albedo, and trophic state on the UV-penetration depth was analyzed.

Spectral UV attenuation in exuviae of melanized and transparent D. PULEX clones was measured. UV transmittance was significantly higher in the transparent clone (85%) than in weakly (19%) and strongly melanized clones (8%). The benefit of the pigmentation was quantified as the distance that a pigmented daphnia could stay higher in the water column under the protection of the melanized carapax in contrast to a transparent carapax. The distance, calculated on the basis of the above- mentioned empirical model, ranged from 0.2 m to 5.7 meters, depending on the grade of pigmentation and the trophic state of the water.

I tested UV-induced depth selection behaviour of unpigmented and pigmented DAPHNIA species in microcosms with a radiation and temperature gradient. All experiments showed a significant downward migration of UV-exposed daphnids into the colder part of the microcosms. Exposed daphnids avoided UV radiation by downward migration in spite of the anticipated cost of a colder environment. Unpigmented D. GALEATA, D. PULEX and D. PULEX OBTUSA showed a stronger UV response than D. ROSEA with carotenoids and a melanized D. PULEX clone did. The pigmentation allows the organisms to stay closer to the surface under UV radiation, and thereby reduces the cost (i.e., reduced growth rate in colder temperatures) of UV-induced downward migration.

The cost of melanin pigmentation was quantified in life table experiments, and feeding experiments with SALVELINUS SALVELINUS as a predator. Melanized DAPHNIA PULEX clones had lower intrinsic growth rates. The intermediate melanized clone showed an intermediate growth rate. In addition, predation risk was 1.1 to 2.5 times higher for melanized daphnids, depending on the size-class of the prey.