Survivorship of Cyclops abyssorum tatricus (Cyclopoida, Copepoda) and Boeckella gracilipes (Calanoida, Copepoda) under ambient levels of solar UVB radiation in two high-mountain lakes

We performed in situ experiments during the summer of 1995 and 1996 to asse ss the potential effect of solar ultraviolet B (UVB) radiation (290-320 nm) on the survival of Cyclops abyssorum tatricus Kozminski and Boeckella grac ilipes Daday. These species are numerically dominant within the crustacean zooplankton living in two high-mountain lakes, one located in the Austrian Alps [Gossenkollesee (GKS), 2417 m above sea level, maximum depth 9.9 m] an d another in the Chilean Andes (Laguna Negra, 2700 m above sea level, maxim um depth 320 m). The copepods were incubated in quartz tubes (11) or in qua rtz tubes wrapped with Mylar D(R) to exclude most of the UVB radiation. The organisms were exposed at 0.5 m depth for 10-72 h on cloudless days. Both lakes were very transparent to UVB and 10% of the surface radiation at the nominal wavelength of 305 nm was still present at 9.6 m in GKS and at 12.8 m in Laguna Negra. These species migrate vertically and have a maximum dayt ime distribution close to the bottom (C.abyssorum tatricus) or below 15 m d epth (B.gracilipes). Both species were red, but the carotenoid concentratio n was higher in C.abyssorum tatricus than in B.gracilipes (6.5 and 2.3 mu g mg(-1) dry weight, respectively). W-absorbing compounds with a maximum abs orption at similar to 334 nm were also detected. Cyclops abyssorum tatricus was highly resistant to UVB radiation and no significant lethal effect was observed. Boeckella gracilipes had a mortality similar to 5 times higher i n the treatment receiving full sunlight than in the Mylar treatment (3.2%) only when exposed for 70 h. The resistance of B.gracilipes was higher than that reported in the literature for the same species, suggesting the existe nce of intraspecific differences in UV sensitivity.