PARENT SESSION
Oral Session #27: Aquatic Ecology: Lake structure and function. Presiding: L.A. Hansson.
Tuesday, August 7, 2001. 8:00 AM to 11:30 AM. Hall of Ideas I.

Does Dissolved Organic Carbon (DOC) shift the balance between fish and invertebrate predators?

Wissel, Bjoern¹, Boeing, Wiebke ¹, Ramcharan, Charles¹, ¹

ABSTRACT- DOC has multiple effects on freshwater lakes, changing the vertical profiles of light, temperature, and oxygen. An increase in DOC concentration should reduce fish predation on zooplankton and invertebrate predators such as Chaoborus should become more important. Since fish prefer larger prey but Chaoborus can only ingest smaller zooplankton, an increase in DOC should cause a shift from small to large zooplankton species. To test this hypothesis, we analyzed the zooplankton composition and vertical distribution of 10 small lakes (Ontario, Canada) with various DOC concentrations. High DOC lakes were dominated by Daphnia and Holopedium, with maximum densities in the epilimnion. Clear lakes could only support large zooplankton species when a deep and oxygenated hypolimnion was available. A low light refuge (1x to 1.5 x Secchi depth) was crucial for large zooplankton, since maximum abundances were never found above this layer in clear lakes. Additionally, we performed an enclosure study evaluating the effects of DOC, Fish, and Chaoborus (2x2x2 factorial). Under Low-DOC conditions, fish eliminated large cladocerans and copepods and small species were dominant. Contrary, in High-DOC treatments, larger cladocerans and copepods were able to maintain higher abundances in treatments including fish. The zooplankton community in Control- and Chaoborus-Treatments consisted of large cladocerans and few small species. Generally, the high DOC concentration supported more and larger species in the presence of fish, confirming the importance of light and oxygen refugia.

KEY WORDS: dissolved organic carbon, invertebrate predators, food web, refuge