The effect of ultraviolet radiation on the vertical distribution and mortality of estuarine zooplankton

Abstract

The effect of UV-B radiation on the vertical distribution of three calanoid copepod species (Tortanus dextrilobatus, Acartiura spp. and Acanthacartia spp.) and three larval stages of Pacific herring, Clupea pallasi (1-, 7-, and 14-day-old larvae) was investigated. A series of 2 m high columnar tanks equipped with infra-red light and video-microscopy was used to investigate the vertical distribution of zooplankton in the presence and absence of UV-B radiation. In the presence of UV-B radiation, T. dextrilobatus and 1-day-old C. pallasi resided about 50 cm deeper than in the absence of UV-B, while Acartiura spp. and Acanthacartia spp. showed no (or only minimal) change in vertical distribution. Mortality experiments were also conducted outdoors in which each copepod species was exposed to full or reduced natural radiation levels. Only T. dextrilobatus showed an increase in mortality when exposed to full radiation. Our results showed that T. dextrilobatus and 1-day-old C. pallasi larvae were sensitive to UV radiation (UVR), and to reduce or eliminate UV-induced stress, they avoided the surface of the water column when UV-B radiation was present. Copepod species were chosen to span a range of pigmentation: T. dextrilobatus (heavily pigmented), Acartiura spp. (moderately pigmented) and Acanthacartia spp. (not pigmented). The pigmentation did not appear to play a role in UVR tolerance of the copepods, but may be a factor determining UV tolerance of C.pallasi.