Ecology of UV
UV introduction and basic optics
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Alpine Lake Phytoplankton and UV
Radiation
prepared by Jasmine
Saros
Alpine lakes are among the most UV transparent aquatic systems in the world. For example, attenuation coefficients at 320 nm that are as low as 0.17 m-1 and that average 0.55 m-1 have been observed in lakes of the Alps and Pyrenees (Laurion et al. 2000). Because of sparse vegetation on the catchment and low within-lake productivity, concentrations of dissolved organic material (DOM) in these lakes tend to be quite low, generally ranging from 0.2 to 1.5 mg·L-1. As a result, phytoplankton can significantly contribute to UV attenuation in these systems (Laurion et al. 2000; Sommaruga & Psenner 1997).
The chlorophyll maximum in alpine lakes is frequently observed at much greater depths than in temperate counterparts. The cause of this characteristic deep chlorophyll maximum is still unclear, and has generally received very little attention thus far. Some studies suggest a link with deeper UV penetration into these systems (Rodhe et al. 1966; Sommaruga & Psenner 1997), while other investigators suggest that phytoplankton in these oligotrophic waters are well-adapted to UV exposure (Halac et al. 1997).
Phytoplankton in alpine lakes have developed strategies to minimize damage from UV exposure. In particular, they can synthesize compounds that directly or indirectly absorb UV energy. Mycosporine-like amino acids (MAAs) are one class of these UV-absorbing compounds, with absorption maxima between 309 to 360 nm. MAAs have been found in phytoplankton and benthic cyanobacteria in an alpine lake (Sommaruga & Garcia-Pichel 1999), with MAA concentrations in phytoplankton declining with depth. Cyanobacteria can also synthesize scytonemin, which has a maximum absorption at 370 nm (Garcia-Pichel & Castenholz 1991). A third group of protective compounds employed by algae are the carotenoids, which do not directly absorb UV but are quenchers of radical oxygen species. This protective pigment is employed by the snow alga, Chlamydomonas nivalis (Bidigare et al. 1993).
References
Bidigare, R.R., et al. 1993. Evidence for a photoprotective function for secondary
carotenoids of snow algae. J. Phycol. 29: 427-434.
Garcia-Pichel, F. & R.W. Castenholz. 1991. Characterization and biological implications of scytonemin, a cyanobacterial sheath pigment. J. Phycol. 27: 395-409.
Halac, S. et al. 1997. An in situ enclosure experiment to test the solar UVB impact on plankton in a high-altitude mountain lake. I. Lack of effect on phytoplankton species composition and growth. J. Plank. Res. 19: 1671-1686.
Laurion, I., M. Ventura, J. Catalan, R. Psenner, & R. Sommaruga. 2000. Attenuation of ultraviolet radiation in mountain lakes: factors controlling the among- and within-lake variability. Limnol. Oceanogr. 45: 1274-1288.
Rodhe, W., J.E. Hobbie & R.T. Wright. 1966. Phototrophy and heterotrophy in high mountain lakes. Verh. Int. Verein. Limnol. 16: 302-313.
Sommaruga, R. & F. Garcia-Pichel. 1999. UV-absorbing mycosporine-like compounds in planktonic and benthic organisms from a high-mountain lake. Arch. Hydrobiol. 144: 255-269.
Sommaruga, R. & R. Psenner. 1997. Ultraviolet radiation in a high mountain
lake of the Austrian Alps: air and underwater measurements. Photochem. Photobiol.
65: 957-963.