Surface avoidance by freshwater zooplankton: Field evidence on the role of ultraviolet radiation

CRISTINA ALONSO,; VANINA ROCCO,; PABLO BARRIGA, JUAN; MIGUEL BATTINI,; HORACIO ZAGARESE

LIMNOLOGY AND OCEANOGRAPHY

Año: 2004 vol. 49 p. 225 - 232

0024-3590

The avoidance of surface waters by crustacean zooplankton is a common phenomenon both in marine and fresh waters.  The contemporary paradigm interprets such behavior as an antipredator strategy.  However the phenomenon has also been reported in predator-free environments, suggesting that other variables may contribute to this depth selection behavior.  In this study we investigated the possibility that ultraviolet radiation could influence the avoidance of surface waters in three inter-connected lakes differing in their transparency.  Each lake was sampled 17 or 18 times.  In the two most transparent lakes, the percentage of the population occurring in the surface layer decreased during spring and summer; but this pattern was not observed in the less transparent lake.  Such differences in the patterns of surface waters avoidance were unrelated to the abundance and vertical distribution of the dominant planktivores (early stages of galaxiid fishes), as well as to temperature and food availability.  They were  inversely related however, to the average UVR levels within the surface layer.  A UV avoidance strategy was predicted for the copepod B. gracilipes, on account of its low UV tolerance and lack of photoprotective compounds.  The present study supports this prediction and strongly suggests that surface avoidance by freshwater Patagonian zooplankton is not just a by-product benefit of an antipredator behavior, but a direct response to high UVR levels.