The Absorption of Light in Lakes: Negative Impact of Dissolved Organic Carbon on Primary Productivity

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 (PPA) 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 PPA. 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%). PPA 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 PPA. The use of DOC or CDOM as predictors gave very similar results and the negative effect of these variables on PPA 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.