Colored dissolved organic matter (CDOM) absorbs a substantial fraction of photosynthetically active radiation (PAR) in boreal lakes. However, few studies have systematically estimated how this light absorption influences pelagic primary productivity. In this study, 75 boreal lakes spanning wide and orthogonal gradients in dissolved organic carbon (DOC) and total phosphorus (TP) were sampled during a synoptic survey. We measured absorption spectra of phytoplankton pigments, CDOM, and non-algal particles to quantify the vertical fate of photons in the PAR region. Area-specific rates of gross primary productivity (PP A ) were estimated using a bio-optical approach based on phytoplankton in vivo light absorption and the light-dependent quantum yield of photochemistry in PSII measured by a PAM fluorometer. Subsequently, we calculated the effects of CDOM, DOC, and TP concentration on PP A . CDOM absorbed the largest fraction of PAR in the majority of lakes (mean 56.3%, range 36.9-76.2%), phytoplankton pigments captured a comparatively minor fraction (mean 6.6%, range 2.2-28.2%). PP A estimates spanned from 45 to 993 mg C m -2 day -1 (median 286 mg C m -2 day -1 ). We found contrasting effects of CDOM (negative) and TP (positive) on PP A . The use of DOC or CDOM as predictors gave very similar results and the negative effect of these variables on PP A can probably be attributed to shading. A future scenario of increased DOC, which is highly correlated with CDOM in these lakes, might impose negative effects on areal primary productivity in boreal lakes.

Boreal lakes are important net sources of greenhouse gases (GHGs). In this study we analyzed concentrations of CO 2 , CH 4 , N 2 O as well as O 2 , N 2 and argon (Ar) from the epilimnion of 75 boreal lakes covering gradients in total organic carbon (TOC), phosphorus (P) and nitrogen (N) deposition. The Ar-corrected gas saturation deficit was used as a proxy of net metabolic changes from spring overturn to mid-summer sampling (all lakes were dimictic). Emission fluxes were calculated for CO 2 , CH 4 and N 2 O based on partial pressure, water temperature and wind speed. Gas concentrations, actual and Ar-corrected, were related to lakespecific properties. TOC was the main predictor of CO 2 concentrations and fluxes, followed by total P, while total P and chlorophyll a governed CH 4 concentrations and fluxes. Nitrogen (NO 3 -or total N) were key predictors of N 2 O concentrations and fluxes, followed by total P. Altitude, area and depth were not strong predictors of CO 2 , CH 4 and N 2 O concentrations and fluxes, likely because only lakes with an area of [1 km 2 were included. CO 2 molar concentrations were negatively correlated with O 2

There is growing empirical and theoretical evidence for a positive relationship between the nitrogen (N)-to-phosphorus (P) ratio of phytoplankton and temperature. However, few have tested how the optimal supply N:P ratio; the dissolved N:P ratio at which nutrient limitation switches from one element to the other, responds to temperature. In this study, we conducted a factorial experiment crossing 12 temperature levels with 8 supply N:P ratios to determine the effect of temperature acclimation on the optimal supply N:P ratio of the microalgae Chlamydomonas reinhardtii. We found that the optimal supply N:P increased in a sigmoidal manner from 26.5 to 36.5 (atomic ratio) over a temperature gradient spanning from 10 to 188C, with the steepest change around 158C. This result is in accordance with trends observed for cellular and seston N:P ratios, and indicates that phytoplankton populations may be shifted toward N-limitation in a scenario of warmer waters.

Many boreal lakes are experiencing an increase in concentrations of terrestrially derived dissolved organic matter (DOM)-a process commonly labeled "browning." Browning affects microbial and photochemical mineralization of DOM, and causes increased light attenuation and hence reduced photosynthesis. Consequently, browning regulates lake heterotrophy and net CO 2-efflux to the atmosphere. Climate and environmental change makes ecological forecasting and global carbon cycle modeling increasingly important. A proper understanding of the magnitude and relative contribution from CO 2-generating processes for lakes ranging in dissolve organic carbon (DOC) concentrations is therefore crucial for constraining models and forecasts. Here, we aim to study the relative contribution of photomineralization to total CO 2 production in 70 Scandinavian lakes along an ecosystem gradient of DOC concentration. We combined spectral data from the lakes with regression estimates between optical parameters and wavelength specific photochemical reactivity to estimate rates of photochemical DOC mineralization. Further, we estimated total in-lake CO 2-production and efflux from lake chemical and physical data. Photochemical mineralization corresponded on average to 9% AE 1% of the total CO 2-evasion, with the highest contribution in clear lakes. The calculated relative contribution of photochemical mineralization to total in-lake CO 2-production was about 3% AE 0.2% in all lakes. Although lakes differed substantially in color, depth-integrated photomineralization estimates were similar in all lakes, regardless of DOC concentrations. DOC concentrations were positively related to CO 2-efflux and total in-lake CO 2-production but negatively related to primary production. We conclude that enhanced rates of photochemical mineralization will be a minor contributor to increased heterotrophy under increased browning.

Phytoplankton acclimates to irradiance by regulating the cellular content of light-harvesting complexes, which are nitrogen (N) rich and phosphorus (P) poor. Irradiance is thus hypothesised to influence the cellular N : P ratio and the N : P defining the threshold between N and P limitation (the 'optimal' N : P). We tested this hypothesis by first addressing the response of the optimal N : P to irradiance in a controlled experiment with Chlamydomonas reinhardtii. Then, we did a meta-analysis of experimental data on optimal and cellular N : P ratios across light gradients to test the generality of an N : P to light response within species. In both the experiment and the meta-analysis, N : P ratios decreased with irradiance, indicating that factors affecting underwater irradiance, like depth and the composition of the water, may influence the relative N : P requirement. The effect of irradiance did not differ between optimal and cellular N : P ratios, but observations of optimal N : P were on average 2.8 times higher than observations of cellular N : P.