Introduction: The absorption of photons in the water column of a lake can generally be attributed to four main components: algae, dissolved organic matter (DOM), non-living particulate organic matter (detritus), and the water itself. Competing with the other components for photosynthetic active radiation (PAR), algae use the absorbed photons for photosynthesis. In the first part of this thesis, the relative absorbance of PAR by the different components will be addressed. In the second part, a procedure for estimating areal primary productivity (PP) from optical measurements will be attempted. Materials and methods: 75 lakes in southern Norway and Sweden were sampled by airplane during a 4 week period (summer 2011). Absorbance coefficients of algae, DOM and detritus across the PAR spectrum were measured in the lab and combined with a spectrum of incoming solar irradiance (I0) to calculate the relative absorbance of photons constituted by the different components. Pulse Amplitude Modulated (PAM) fluorescence measurements of quantum yield of Photosystem II were used to estimate photosynthetic efficiency and combined with daily average values of PAR from July 2011, vertical profiles of scalar PAR, and the absorbance properties of the water column, to obtain maximum estimates of areal primary productivity in the lakes surveyed. Results: DOM was the dominant light absorber (mean: 44 %, range: 16 % – 64 %), followed by algae (mean: 16 %, range: 6% – 31 %), and detritus (mean: 17%, range: 5 % – 39 %). Absorbance by algae was negatively related to absorbance by DOM (P < 0.0001, R2 = 0.4). Estimates of areal PP ranged from 136 to 1597 mg C m-2 day-1, with a mean of 600 mg C m-2 day-1. The estimates were positively related to total phosphorus content, and negatively affected by total organic carbon. Estimating areal PP down to different depths indicated that the epilimnion was totally-absorbing in most lakes. Discussion: The competition for light between algae and DOM seems to be a limiting factor for areal primary productivity in the surveyed lakes. The method for estimating areal PP from optical measurements is somewhat biased, but proposes a time-efficient procedure for obtaining maximum estimates.