Diffuse attenuation coefficients (&) for solar UV radiation (UVR) (305, 320, 340, 380 nm, and PAR) were measured in the mixed layer of 65 lake sites in Alaska, Colorado, and Pennsylvania and the Bariloche region of Argentina. Integrated mixed layer samples of lake water were concurrently collected, and a multivariate approach was used to model Kd with a number of optical and chemical variables.Substantial variation in transparency was observed among lakes. Attenuation depths (zlYO) for UV-B radiation ranged from several centimeters to > 10 m. In some shallow, low DOC (dissolved organic C) lakes typical of high elevation ecosystems, substantial fluxes of UVR penetrated the entire water column. In deeper lakes with low DOC concentrations, high fluxes of UVR were found in a significant proportion of the mixed layer. Much of the among-lake variation in & (87-96%) was explained by differences in DOC concentration, which strongly influenced dissolved absorbance. On average, dissolved absorbance accounted for between 33% (for PAR) and 68% (for 305 nm) of Kd measured in situ. Throughout the solar UV-A and UV-B range, Kd was best estimated with a univariate power model based solely on DOC concentration. Models are also presented that relate absorption coefficients to Kd. These models can be used with archival DOC or color data to provide approximate estimates of UV transparency of lakes.

The primary interpretive paradigm used to study lakes is their trophic status. Oligotrophic lakes have low nutrient loading and low productivity, while eutrophic lakes have high nutrients and high productivity. The strong empirical relationship between nutrient loading and productivity is a valuable tool for teaching, for research, and for management of lakes. In order to incorporate the variety of other known anthropogenic impacts on lakes, however, lake characterization needs to extend beyond the nutrient-productivity paradigm. For example, acid precipitation, heavy metal and toxic organic contaminants, increases in UV radiation, and global warming are all recognized threats to lake ecosystems. One of the key characteristics of lakes that determines how they respond to disturbances such as these is their concentration of colored dissolved organic carbon (CDOC). Here we argue that a paradigm that includes CDOC (using the absorption coefficient at 320 nm as a proxy) as well as nutrients will be useful in predicting and understanding the response of lake ecosystems to multiple stressors. We propose to resurrect the CDOC axis that was proposed by investigators earlier this century and to extend it by adding some operational definitions to permit placing some of the major lake types on the axes in a way that will help us to better understand the structure, function, and response to disturbance of lake ecosystems that are subject to natural and anthropogenic environmental changes at the local, regional, and global scales. Data from a few diverse lakes and a successional sequence in Glacier Bay, Alaska, are used to illustrate the potential utility of the 2-axis model in separating lake types.The most commonly used paradigm for studying lake ecosystems defines lakes in terms of their trophic status. Lakes with low nutrients and low organic production are considered oligotrophic, while those with high nutrient inputs and organic productivity are eutrophic (Wetzel 1983). This paradigm has been adopted by the general ecological community as well as limnologists and is in general use in introductory ecology and limnology textbooks. The trophic status paradigm is supported by a particularly strong empirical relationship between chlorophyll and phosphorus (Vollenweider 1968;Dillon and Rigler 1974;McCauley et al. 1989), by comparative studies and whole-lake experiments that have clearly demonstrated the role of nutrients and grazers in eutrophication Pace 1984; Carpenter et al. 1991), and by the widespread success of remediation of AcknowledgmentsWe thank Dan Engstrom for stimulating discussions on the role of DOC in the succession of lakes in the Glacier Bay chronosequence, Sheri Fritz, Tom Frost, and Bruce Hargreaves for providing feedback on some of these ideas in their early stages of development, Dave Krabbenhoft for discussions on Hg, and Philip Singer and James Symons for introducing us to the literature on water treatment disinfection by-products and DOC. Sheri Fritz provided the unpublished data on Brush La...

Climate warming in North America is likely to be accompanied by changes in other environmental stresses such as UV-B radiation. We apply an empirical model to available DOC (dissolved organic C) data to estimate the depths to which 1% of surface UV-B and UV-A radiation penetrate for several major regions of North America. UV attenuation depths are also estimated from DOC data collected from treatment and reference basins during the experimental acidification of Little Rock Lake, Wisconsin. In some regions of North America 25% of the lakes have 1% attenuation depths for UV-B radiation on the order of 4 m or more (western and northwestern U.S., Newfoundland). In other regions, 75% of the lakes have 1% attenuation depths for UV-B shallower than 0.5 m (Florida, upper midwestern U.S., northwestern Ontario, Quebec, and Nova Scotia). Attenuation depths for UV-A radiation are -2.5 times as deep as those for UV-B. Experimental acidification approximately doubled the estimated 1% attenuation depths for UV radiation in Little Rock Lake.The strong dependence of 1% attenuation depth on DOC below the l-2 mg liter-l DOC range suggests that UV attenuation in low DOC lakes is highly sensitive to even very small changes in DOC. We conclude that changes in climate, lake hydrology, acid deposition, and other environmental factors that alter DOC concentrations in lakes may be more important than stratospheric ozone depletion in controlling future UV environments in lakes.

Nitrite is an intermediate in the oxidation of ammonium to nitrate. An elevated ambient nitrite concentration is a potential problem for freshwater fish since nitrite is actively taken up across the gills in competition with chloride. Nitrite is a well-known toxicant for fish as well as a disrupter of multiple physiological functions including ion regulatory, respiratory, cardiovascular, endocrine and excretory processes. One critical consequence of nitrite accumulation is the oxidation of haemoglobin to methaemoglobin, compromising blood oxygen transport. Nitrite toxicity to fish varies considerably and depends on a large number of external and internal factors. Among the most important ones are water quality (e.g. pH, temperature, cation, anion and oxygen concentration), length of exposure, fish species, fish size and age, and individual fish susceptibility. Chloride concentration in water is considered one of the most important factors influencing nitrite toxicity to fish. The importance of individual factors is assessed and re-evaluated continuously.

SUMMARY. 1. Limiting nutrients for phytoplankton were studied experimentally in eight mountain lakes of central Colorado between May and November of 1984. 2. Five categories of phytoplankton limitation were identified: no limitation, N limitation, P limitation, concurrent limitation (stimulation only by simultaneous additions of N and P), and reciprocal limitation (stimulation by addition of either N or P). The phytoplankton communities of three lakes were primarily N‐limited, one was primarily phosphorus‐limited, and four showed primarily combined limitation (concurrent or reciprocal). Switching between categories of limitation was also observed within lakes. Nitrogen was the most frequently limiting nutrient; N, either alone or in combination with P, accounted for 79% of all observed instances of limitation. 3. Nine indices were tested for effectiveness in predicting phytoplankton limitation by N and P. The best indices for discriminating all limitations were ratios of dissolved inorganic N: total P (84% accuracy) and dissolved inorganic N:total dissolved P (80% accuracy). The effectiveness of these indices may be explained by the degree to which they represent N and P fractions actually available to the phytoplankton.