We tested the hypothesis that a clear-water period, regularly observed in many meso-and eutrophic lakes, is caused by grazing herbivorous zooplankton. Such a clear-water phase occurs during mid-May in the moderately eutrophic Schiihsee and involves a rapid increase in Secchi transparency, and a drop in chlorophyll and particulate organic carbon in size fractions ~35 Nm. Maxima of zooplankton biomass and community grazing rates (170% of volume cleared per day) coincided with the greatest transparency. The algal decline was not related to nutrient depletion or climatic events. Before the clear-water phase small phytoplankton contributed up to 88% of the primary production, but the contribution of large particles was more important after the zooplankton maximum. The effects of herbivory by zooplankton were examined in a series of time-overlapping enclosure experiments. Concentrations of small (< 35 pm) particles were always higher in the bags lacking zooplankton than in the controls. A mass development of small algae occurred in the zooplankton-free bags initiated during the clear-water phase, although the presence of zooplankton stimulated the growth of large (> 35 pm) algae.

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We used a scanning spectroradiometer to conduct underwater optical surveys of 44 waterbodies during the ice-free seasons of three consecutive years in wetlands and lakes in central Saskatchewan, Canada. The waterbodies ranged widely in dissolved organic carbon (DOC) concentration (4.1-156.2 mg L Ϫ1) and conductivity (270-74,300 ohms cm Ϫ1). Although penetration of UV radiation (UV-R; 280-400 nm) in these systems was largely a function of DOC concentration, as has been reported previously, UV-R penetrated more deeply in saline waterbodies than in freshwater systems with similar DOC concentrations. Power models representing our K dUV-B or K dUV-A versus DOC relationships were described by K dUV-B ϭ 0. . UV-B radiation (280-320 nm) is not expected to penetrate deeply (typically Ͻ50 cm) in prairie lakes and wetlands because of high intrinsic DOC concentrations. However, the central plains are characteristically windy and this, coupled with the shallowness of many of these systems, suggests that biota may still be at risk from present-day and future-enhanced levels of UV-B (which may result from ozone depletion). Moreover, this risk may be exacerbated in saline systems. This could be significant, especially because saline waterbodies are often highly productive and represent important North American staging areas for shorebirds and waterfowl.Ultraviolet radiation (UV) has influenced the evolution of life on earth since it first appeared. Even in recent times, in 1 Corresponding author. AcknowledgmentsWe thank J. C. Mollison of the Instrument Technology Services for designing and constructing the parallelogram swing arm used to deploy the Optronics minicosine and submersible sphere sensors. We are grateful to H. I. Browman, Institute of Marine Research, Storebø, Norway, for supplying us with the underwater immersion correction factors used with the submersible sphere and for his sound advice on several technical aspects of the Optronics OL-754 meter. We thank R. Young and C. Rapp/C. Johnson, Optronics Laboratories Inc., Orlando, Florida, for providing us with the immersion correction factors for the minicosine sensor and for answering our technical questions, respectively. We are indebted to R. A. Bourbonniere and K. Edmondson (NWRI-Burlington) for performing DOC analyses on the 1998 samples. This research was made possible by funding through Environment Canada's National Water Research Institute to M.T.A. and R.D.R.

This report presents results on relationships between the kinetics (Vmax and Km) of β-glucosidase (GLCase) and aminopeptidase (AMPase) activity, and dissolved organic carbon (DOC) and bacterial secondary production in freshwater mesocosms of differing degrees of eutrophication. These relationships varied in different mesocosms and depended on the trophic status of water and the exudation rates of organic carbon (EOC) by phytoplankton. Close coupling of bacterial production to Vmax of GLCase activity was observed only in nutrient-enriched mesocosms. The relationship between GLCase and DOC content was also significant in enriched water. There was no correlation between the Vmax, of GLCase and DOC and bacterial production in nutrient-impoverished and control (mesotrophic) enclosures. However, the Vmax of AMPase correlated well to DOC and bacterial production in these mesocosms. AMPase activity did not correlate with DOC and bacterial production in nutrient-impoverished mesocosms. There was no relationship between bacterial biomass and enzyme activity in all studied mesocosms. Comparison of the rates of phytoplankton production of EOC and rates of the bacterial organic carbon demand (BOCD) in nutrient-impoverished mesocosms showed that EOC flux constituted, on average, 90% of BOCD. However, in nutrient-enriched mesocosms EOC contributed only, on average, 27% to the BOCD; thus, in these mesocosms, bacteria were probably organic-carbon limited. It is hypothesized that to bypass substrate limitation, bacteria produced GLCase and AMPase. These enzymes had a high specific activity and high affinity to their substrates and efficiently hydrolyzed polysaccharides and proteins, thereby supplying microorganisms with readily utilizable products of enzyme catalysis.