Hydrobiologia 385: 23–32, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 23 Effects of ultraviolet radiation on primary productivity in a high altitude tropical lake Robert A. Kinzie III 1 , Anastazia T. Banaszak 2 & Michael P. Lesser 3 1 Hawai‘i Institute of Marine Biology, P.O. Box. 1346, K¯ ane‘ohe, HI 96744, U.S.A. 2 Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, MD 21037-0028, U.S.A. 3 Department of Zoology and Center for Marine Biology, University of New Hampshire, Durham, NH 03824, U.S.A. Received 16 January 1997; in revised form 9 May 1998; accepted 29 July 1998 Key words: Ultraviolet radiation, primary productivity, tropical lakes Abstract The aquatic photosynthetic community in Lake Waiau (elev. 3980 m, lat. 19 ◦ 48 ′ N) is exposed to intense solar ultraviolet radiation (UVR) due to the high altitude and low latitude. There is a strong negative effect of UVR on photosynthesis both in the planktonic and benthic mat communities in the lake. The mats apparently receive some protection from UVR-absorbing mycosporine-like amino acids (MAAs), while the planktonic primary producers do not appear to have such protection and exhibit no net photosynthesis at levels of UVR characteristic of the lake surface at midday. These forms may be meroplanktonic or rely on vertical mixing for protection from extended periods of exposure to UVR. Introduction General concern about the broad biological effects of increasing UV-B (290–320 nm) irradiances reach- ing the Earth’s surface has increased following the detection of biologically significant UVR effects in the Antarctic where severe reductions in stratospheric ozone have occurred (Dahlback et al., 1989, Smith et al., 1992). This heightened concern is largely due to some projections that an increase in UV-B radiation (290–320 nm) could become a significant factor in the northern hemisphere at mid-latitudes (Madronich et al., 1991; Mahlman, 1992) but especially at high alti- tudes (Blumthaler & Ambach, 1990). Impacts ranging from increases in human skin cancers and cataracts (Setlow, 1974; Taylor, 1989) to a reduction in primary production in marine (Buma et al., 1996; Caldwell, 1979; Helbling et al., 1992; Larkum & Wood, 1990; Tevini & Teramure, 1989) and freshwater (Bothwell et al., 1993, 1994; Gala & Giesy, 1991) systems have been observed (see references in recent reviews: Franklin & Forster, 1997; Häder, 1993; Häder & Worrest, 1991). There exists an extensive literature on the cellu- lar and subcellular sites of UVR damage, but there is much less known about responses at the organis- mal level and even less at the level of the community (Bothwell et al., 1993, 1994; Kelly, 1986; Lee & Lowry, 1980; Worrest et al., 1981), What informa- tion does exist about community level effects of UVR, suggests that the impacts are complex and may be non- intuitive (Bothwell et al., 1993, 1994; Newsham et al., 1997; Williamson, 1995) with the possibility of UV-A (320–400 nm) conferring some protection from the harmful effects of UV-B via photo-reactivation- mediated repair processes (Quesada et al., 1995). Tropical ecosystems have, even before any reduction in stratospheric ozone, been exposed to greater UVR fluxes than higher latitudes. This is due to a naturally thinner ozone layer over low latitudes and the more direct path of solar radiation through the atmosphere near the equator (Frederick et al., 1989; Gleason et al., 1993). Tropical aquatic organisms have been shown to be sensitive to natural levels of UVR (Jokiel, 1980) and to have protective mechanisms such as the pres- ence of UVR-absorbing compounds which can be