The physical and nutritional energy values of aquatic invertebrates were studied in relation to organic carbon content.The ratio of energy content to the weight of organic carbon increased with increasing carbon. The dependence is described in regression equations which permit prediction of the cncrgy content with an accuracy of about +5% (P > 0.95 ) . Even the use of the mean conversion factor of 46 kJ g-l organic carbon yields considerably more accurate results than the use of a general conversion factor for ash-free dry weight.Accurate energy values of animals are essential for studies of animal energetics. Determination of the energy content of animals with the aid of bomb calorimeters now used requires samples relatively large in comparison with the small size of most freshwater invertebrates.Most scientists use dry weight or ash-free dry weight to estimate biomass. As the energy content of dry or ash-free dry biomass varies considerably from species to species (e.g. Cummins and Wuycheck 1971)) and within one species from season to season (e.g. Schindler et al. 1971; Wissing and Hasler 1971;Snow 1972), the use of general energy equivalents for dry or ash-free dry weight does not yield very accurate values.As noted earlier by Platt and Irwin ( 1973)) the energy equivalent for biomass expressed in terms of organic carbon should theoretically vary less than the energy equivalent for biomass expressed in terms of dry or ash-free dry weight. The use of 41.9 k J g-l C ( = 10 kcal g-l C ) as an approximate conversion factor has been recommended by Winberg (1971a). I-Iowever, new methods have made it possible to determine organic carbon rapidly and accurately even from submicrogram samples unsuitable for calorimetry.To obtain more accurate general energy equivalents for biomass expressed in terms of organic carbon, we have studied the relation between energy content and organic carbon in

We measured photochemical mineralization of dissolved organic carbon in a humic lake in situ. At a depth of 1 cm, solar radiation mineralized 19 mmol C m Ϫ3 d Ϫ1 . The rate of mineralization decreased with increasing depth with an attenuation coefficient of 23 m Ϫ1 . Consequently, most photochemical mineralization in the water column (0.99 mmol C m Ϫ2 d Ϫ1 ) took place in the top 10 cm. The rate of photochemical mineralization was also modeled as a product of three spectra: (1) scalar photon flux density, (2) the apparent quantum yield ( ), and (3) the absorption of chromophoric dissolved organic matter. We described the spectrum for apparent quantum yield as ϭ c ϫ 10 Ϫ d , where c (dimensionless) and d (nm Ϫ1 ) are positive constants. Mathematical optimization for the best fit between the measured and the modeled photochemical mineralization resulted in of 7.52 ϫ 10 Ϫ0.0122 . The based on the measurements in situ agreed with determined in a laboratory at 320, 355, and 390 nm. Using the determined , we calculated that UV-B contributed 9%, UV-A 68%, and visible light 23% to the photochemical mineralization. Half of total photochemical mineralization was due to wavelengths Ͻ360 nm. Our method for the determination of is applicable in situ, improves the prediction of photochemical reaction rates in surface waters, and offers an alternative to the determination of quantum yields at discrete wavelengths. helsinki.fi). AcknowledgmentsWe thank Anne Ojala for the global radiation measurements, Jorma Keskitalo for many kinds of assistance, Heikki Haario for advice with the Matlab software, and Martti Heikinheimo for advice in the problems of radiative transfer in the atmosphere. The quantum yields at single wavelengths were determined in the laboratory of Helge Lemmetyinen under his supervision.

Winter has traditionally been considered as an ecologically insignificant season and, together with technical difficulties, this has led winter limnology to lag behind summer limnology. Recently, rapidly expanding interest in climate warming has increased water research in winter. It has also become clear that neither winter conditions of lakes nor underice communities are as static as often supposed. Although interannual differences in water temperature are small, close to the maximum density temperature, they may have profound effect on under-ice hydrodynamics. Thus, stochastic variations in weather, particularly those preceding the time of freezing and ice melting, may have important consequences for hydrodynamics which then affect the distributions and conditions of microorganisms and probably further to higher trophic levels. Even fish distributions can be dictated by under-ice conditions and their activities as well as behavior can sometimes approach those in summer. Life in freshwater ice is one of the least studied aspects of winter limnology and recent studies suggest that a thorough evaluation is needed. Altogether there are strengthening signs that winter should be considered as an integral part in the functioning and dynamics of lakes affecting quantitative and qualitative characteristics of aquatic communities in summer. There are great prospects that more thorough understanding of the prevailing limnological conditions in winter will improve our understanding of lake ecosystems in their entirety, and there is no doubt that such an approach requires multidisciplinary and long-term studies at different spatial scales.

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