The ultraviolet-B photobiology of freshwater calanoid copepods: Seasonal, population, and dietary influences

Chlorofluorocarbons are suspected to have caused a reduction in stratospheric ozone and a concomitant increase of ultraviolet B (UV-B) radiation (280-320 nm) reaching the surface (Madronich, 1993). The prospect of rising UV-B levels has spawned considerable interest in UV-B effects on organisms and ecosystems. Freshwater calanoid copepods are an excellent model for studying these effects because of their ubiquity and pivotal position in aquatic food webs.; This dissertation describes their use in acute UV-B toxicity bioassys utilizing both lamp UV-B and natural sunlight.; Freshly-collected Diaptomus minutus from the low dissolved organic carbon (DOC), high UV-B Lake Giles were consistently more tolerant of lamp and solar UV-B than were those from the moderate DOC, low UV-B Lake Lacawac. The Lake Giles population was very tolerant from late June through October, but the Lake Lacawac population was least tolerant during July and August. The Lake Giles population retained its tolerance despite late summer food limitation. Differences in lamp UV-B tolerance did not persist in cultured animals, although a culture of Lake Giles animals initiated in September was more tolerant than those initiated in December, May, or June. A September culture of Lake Lacawac animals was more tolerant than a June culture.; The vertical distribution of the Lake Giles population did not shift in response to seasonal changes in solar UV-B, but the Lake Lacawac population avoided the daytime epilimnion from May through August. Solar UV-B is strongly attenuated in Lake Lacawac, and avoidance of the epilimnion could not be attributed to solar UV-B. Variations in tolerance did not parallel seasonal variations in solar UV-B intensity in either population. Possible explanations for seasonal variations in UV-B tolerance include differential reproductive success and physiological adaptation as solar UV-B levels change.; Although some differences in UV-B tolerance might conceivably be attributable to food quality, experiments utilizing D. oregonensis showed that lamp UV-B tolerance was similar in animals fed either Cryptomonas reflexa or Pandorina morum. D. oregonensis was less tolerant of lamp UV-B than D. minutus.