Effects of ultraviolet-B radiation on marine zooplankton communities in a shallow estuarine system

Responses of natural zooplankton communities to enhanced input of UV-B radiation were investigated in outdoor mesocosms during the summer and the winter-spring bloom period of 1994. Two 4 week long experiments were conducted in stratified water columns in the summer of 1994. The levels of UV-B radiation were 50% higher than the ambient level of UV-B radiation measured at 313 nm. The winter-spring experiment lasted for 10 weeks from late-January to early April during the phytoplankton bloom period. The mesocosm water was well mixed. Treatment mesocosms received 100% higher levels of UV-B (measured at 313 nm) radiation than the control mesocosms.;Zooplankton communities in the mesocosm were not significantly affected by enhanced levels of UV-B radiation either during the summer or during the winter-spring experiment. A few cases of reduced abundances in UV-B enhanced treatments compared to controls were found. In summer experiment 1, nauplii of the copepods Acartia hudsonica and Centropages hamatus were significantly reduced in the UV treatment. Adult A. hudsonica in UV-B enhanced mesocosms were lower in the abundance in some parts of the winter-spring experiment. Other compartments of the food web in the mesocosms did not show significant treatment effects. Thus the effects detected are thought to be related with direct damages from UV-B radiation rather than indirect effects through trophic relations. Short-term survival tests on the nauplii suggested that there was a significant increase of mortality in the surface water (0.1 m) due to high levels of UV-B radiation. The abundances of other zooplankton were not reduced in high UV-B mesocosms.;Rapid attenuation of UV-B radiation in the water column and complex biological interactions could have resulted in reduced impacts of UV-B radiation in our mesocosms compared to short-term laboratory studies. An avoidance of the surface water was observed in the copepodites of A. hudsonica during the summer experiment. Comparison of the survival of A. hudsonica nauplii showed different susceptibility to UV-B radiation depending on the UV-B levels that the populations had been exposed to previously in the winter-spring experiment, suggesting the possibility of adaptation to increasing levels of UV-B radiation.