Do zooplankton contribute to an ultraviolet clear-water phase in lakes?

Seasonal increases in the ultraviolet (UV) transparency of the surface waters of an oligotrophic lake in Pennsylvania suggest that clear-water phase (CWP) events similar to those previously observed for visible light also exist for the potentially damaging UV wavelengths. Seasonal increases in zooplankton grazers indicate that they play a role in these changes in UV that is similar to the role that zooplankton play in CWP events involving longer-wavelength visible, or photosynthetically active radiation (PAR, 400–700 nm). The potential role of zooplankton and incident UV in generating UV CWP events was investigated with a set of in situ microcosm experiments that manipulated UV and zooplankton, and followed changes in particulate and dissolved absorbance in the UV (320 nm) and PAR wavelength ranges over an 8-d period in April. Nutrients were also manipulated independently to examine the potential role of nutrient regeneration by zooplankton grazing in altering water transparency. Photobleaching by incident solar UV led to a strong and significant decrease in dissolved UV and PAR absorbance. The presence of zooplankton grazers also significantly decreased dissolved UV absorbance but increased dissolved PAR absorbance. Neither zooplankton nor UV had any significant effects on UV or PAR absorbance by particulates. In contrast, nutrient additions significantly increased dissolved absorbance in both the UV and PAR wavelength ranges, indicating that regeneration of nutrients by zooplankton offsets decreases in UV absorbance and enhances increases in PAR absorbance due to grazing. While photobleaching by UV radiation is likely to make a consistent strong contribution to UV CWP events in lakes, the net effects of zooplankton on UV transparency in a given lake will depend upon multiple factors including zooplankton density and a balance between the edibility and extent of nutrient limitation of the phytoplankton. Many temperate lakes undergo a distinct period of increased water transparency during the late spring or early summer referred to as a clear-water phase (CWP). The CWP was first described as a distinct event in the mid 1980s (Lampert et al. 1986; Sommer et al. 1986). According to the plankton ecology group (PEG) model of seasonal succession, which is based on an intercomparison of 24 different types of lakes, reservoirs, and ponds, the CWP follows the spring phytoplankton bloom and consists of a rapid decline in phytoplankton biomass accompanied by a pronounced increase in the visible transparency of the surface waters (Secchi depth). This rapid increase in visible transparency is accompanied by an increase in herbivorous zooplankton, and often a decline in nutrients as well. Both comparative analyses of multiple lakes (Sommer et al. 1986) and enclosure experiments that specifically manipulate zooplankton (Lampert et al. 1986) have implicated zooplankton grazing in generating the CWP. The PEG model identifies the CWP as lasting only a few weeks in May in mesotrophic to eutrophic lakes, but in oligotrophic lakes investigators observed that the CWP ‘‘always extended over the summer period’’ (Sommer et al. 1986). The CWP has been a useful concept in numerous studies of the regulation of planktonic processes ranging from zooplankton grazing and nutrient limitation (Lampert et al. 1986; Vyhnalek et al. 1991) to food web dynamics (Elser et al. 1995; Luecke et al. 1990), pelagic carbon metabolism (Markager et al. 1994), and the effects of major climate patterns such as the North Atlantic Oscillation (Adrian et al. 1999; Scheffer et al. 2001). In our studies of zooplankton and transparency of surface waters in oligotrophic Lake Giles in northeastern Pennsylvania, we have noted a similar seasonal increase in the visible (photosynthetically active radiation [PAR], 400– 700 nm) transparency of the water in the spring subsequent to an increase in zooplankton grazers. Consistent with the PEG model for oligotrophic lakes, this period of increased transparency continues well into the summer, with a peak that occurs sometime in July. We observed similar patterns of seasonally increasing PAR transparency in two other more productive lakes in the same region (Morris and Hargreaves 1997). We have also found pronounced seasonal increases in ultraviolet (UV) transparency that parallel increases in water transparency to PAR. We refer 1 To whom Correspondence should be addressed. Present address: Department of Zoology, Miami University, Oxford, Ohio 45056 ([email protected]). 2 Present address: Alterra, Wageningen UR, PO Box 47, 6700 AA Wageningen, The Netherlands. 3 Present address: Environmental Sciences and Engineering Department, CB#7431, The University of North Carolina, Chapel Hill, North Carolina 27599. Acknowledgments We thank Gabriella Grad Dee and David Huff for assistance in the field, Aaron Clauser for counting the zooplankton, Steve Lott and Toby Church for carrying out preliminary experiments, Mark Olson and John Bailer for advice on the statistics, Robin Wildermuth and the Blooming Grove Hunting and Fishing Club for access to Lake Giles, and Janice Poppich and the Lacawac Sanctuary for use of their facilities. Bruce Hargreaves provided the incident UV data, Don Morris provided input to the optical and DOC measurements, and Robert Moeller advised with the nutrient addition experiments and provided useful comments on the manuscript. The manuscript benefited from the comments of two anonymous reviewers and associate editor Dag Hessen. This project was supported by NSF grants DEB-9740356, DEB9973938, and DEB-IRCEB-0210972. Limnol. Oceanogr., 52(2), 2007, 662–667 E 2007, by the American Society of Limnology and Oceanography, Inc.