Ultraviolet Radiation and Bio-optics in Crater Lake, Oregon, 2005
DISCUSSION
UVR Impacts on the Crater Lake Ecosystem
The incident spectrum of sunlight (upper curves in Figure 1A) shows rapid attenuation at the shortest wavelengths because stratospheric ozone strongly absorbs UV-B wavelengths. Figure 13A also shows that summer stratospheric ozone can vary substantially (5-10%) from year to year. Thus exposure of planktonic organisms to UV-B will depend on atmospheric transparency as well as water column transparency. Because phytoplankton typically control the UVR penetration into the water column, if their growth rates or death rates are influenced by the increasing UV-B that accompanies a decline in ozone, the higher incident UV-B would create a water column that is more transparent to UVR and thus UV-B would penetrate deeper.
The observed correlation of chlorophyll-a with stratospheric ozone (Figure 13B) suggests a direct and significant impact of UV-B on the phytoplankton community. More study of this phenomenon is needed because at this point only two depth ranges (0–30 m and 40–140 m) and one ozone time period (the first 3 weeks of July and of August, averaged) have been investigated. Other controlling factors might have changed in parallel with ozone by coincidence or because the paths of the jet streams influence both ozone and weather. Rain has been suggested as a significant nutrient source for the nitrogen-depleted surface waters (McIntire et al., this issue). Of the three years since 1984 when the 0–30 m chlorophyll-a average for July–August dropped by more than 50% from the preceding year, two of those years (1992 and 2001) were also unusually dry (1992, 66% of average January–June precipitation; 2001, 60% of average).
The stronger correlation between ozone and deep chlorophyll-a compared with shallow chlorophyll-a (Figure 13B) is curious, but the inferred presence of MAAs in near-surface phytoplankton (Figures 12A, 12B), and signs of photoacclimation (Figure 11B) provide the basis for several hypotheses. It is likely that species differ in a variety of ways that would impact their ability to thrive (survive) near the surface of Crater Lake. These include: their sensitivity both to UV-B and to high levels of PAR; their ability to produce MAAs; their cell size (small cells gain less benefit from intracellular compounds that absorb UV-B); and their ability to cope with scarce nutrients. We hypothesize that phytoplankton living near the surface of Crater Lake are more resistant to UV-B even when ozone levels are high and thus respond less to declines in ozone than phytoplankton at greater depths. Any cells that are growing near their UV-B limits when ozone is high may simply sink faster than their growth rate can replace them when ozone declines. Cells with less UV-B resistance that live deeper in the water column seem more likely to respond in this way. An alternative hypothesis is that cells living near the surface have lower chlorophyll-a concentration per cell (one possible interpretation of Figure 11B), so that a similar percentage reduction in cell abundance in response to an ozone decline would involve a smaller change in chlorophyll-a in the surface waters compared to deeper in the water column.
