Other signs of possible UVR impact on the Crater Lake ecosystem include the appearance of UV-resistant organisms and the scarcity or low diversity of organisms in the surface waters of the lake. A previous study reported that phytoplankton diversity and abundance were low in the upper 20 m and only one dominant species has been identified (Nitzschia gracilis) representing about 70% of net plankton and nanoplankton in this depth range from 1985–1988 (McIntire et al., 1996). According to McIntire et al. (this issue), Nitzschia gracilis is dominant during the stratified summer period in the upper 20 m but the phytoplankton assemblage in the epilimnion also has high densities of smaller species of cyanobacteria (Aphanocapsa delicatissima and Synechocystis sp.), and lower densities of dinoflagellates Gymnodinium inversum and Peridinium inconspicuum; the chrysophytes Dinobryon sertularia and D. bavaricum were good indicators of the lower epiliminon and upper metaliminion, although their mean relative abundance was below 7% of the total cell density.
We hypothesize that the microbial community will respond to UVR stress under the stable shallow-mixing conditions in Crater Lake with the appearance of highly adapted (and thus UVR resistant) species in the upper 30 m (average Z10%,320=32 m) and moderately resistant species down to 60 m (average Z1%,320=55 m). Urbach et al. (2001) identified two bacterioplankton taxa (CL120-10 verrucomicrobiales and ACK4 actinomycetes) from Crater Lake surface waters that are likely to be resistant to UVR. The deep chlorophyll maximum occurs between 120 and 140 m (Larson et al 1996a), close to the 0.1% depth for PAR (131 m summer average for 1996–2002). The biomass maximum (based on cp660) occurs near the 1% depth for PAR (92 m summer average).
While the scarcity of zooplankton (Larson et al., 1996b) and phytoplankton (McIntire et al., 1996) at depths shallower than 40 m, where UV320 is more intense than 5–10% of incident irradiance during summer, is consistent with avoidance or poor survival because of exposure to UVR, other explanations for the phytoplankton distribution are possible. PAR irradiance is more intense than 10% of incident irradiance at depths above 44 m. The diatom Nitzschia gracilis, which frequently forms blooms near the surface (McIntire et al., 1996), is probably highly resistant to UVR. Some of our optical data are consistent with the presence of MAA photoprotective compounds in at least part of the phytoplankton community that has become adapted to conditions near the surface (ratio of Kd,320:Kd,380 in Figure 6, phytoplankton spectral absorption at 330 nm in Figure 12A, and the ratio of phytoplankton absorption at ap,330:ap,675 in Figure 12B). Eisner et al., (2003) also found changed in vivo particulate absorption spectra ratios that corresponded to changes in phytoplankton pigments (photoprotective versus photosynthetic carotenoids) that were correlated with the light intensity in the water column. Tartarotti et al., (2001) observed a correlation between MAAs in zooplankton (derived from phytoplankton) and elevation in Alpine lakes that they attributed to adaptations providing resistant to UVR exposure.
The absence of higher taxa near the surface of Crater Lake is also likely to be influenced directly or indirectly by UVR although lacking experimental evidence we cannot rule out other factors (grazing, predation, high levels of PAR). A benthic moss (Drepanocladus aduncas) has been reported in Crater Lake at depths 25–140 m with the greatest density 40–80 m (McIntire et al., 1994). Analysis of seasonal changes in morphology of this moss might reveal information about growth rate as a function of depth (Riis & Sand-Jensen, 1997) to explore the role of UVR in setting upper and lower depth limits for the population. Small zooplankton (rotifer taxa) have been reported at depths characteristic of moderate UVR but they are scarce above 40 m (Larson et al 1996b). A factor that may influence the distribution of large zooplankton grazers is predator avoidance because the intensity of PAR irradiance at noon on a typical clear summer day exceeds 5 ?mol m-2 s-1 at 100 m and 0.7 ?mol m-2 s-1 at 150 m. Laboratory experiments (summarized in Kalff, 2002) have shown that the distance at which fish can perceive large zooplankton prey becomes limiting at PAR irradiance below 0.6 ?mol m-2 s-1; more illumination is required to capture smaller prey. Small planktivorous fish and large zooplankton might be forced to stay below an optimal feeding depth in Crater Lake during the day in order to avoid their respective visual predators and then migrate to the zone of maximal phytoplankton abundance at night or twilight to feed. In Crater Lake young Kokanee salmon (taken as prey by rainbow trout) are found near the surface at night but migrate down to 100 m during the day (Buktenica & Larson, 1996). Cladoceran zooplankton migrated diurnally as well, with larger species remaining deeper during the day and night than smaller species. By avoiding visual predators these zooplankton and small fish would also avoid exposure to strong UVR. Some fish are known to use UV-A wavelengths for vision and UV-A may penetrate deeper than visible wavelengths at dawn and dusk, or whenever the irradiance penetrating the lake surface is dominated by skylight rather than direct rays from the sun (Leech & Johnsen, 2003).
