Assessing and predicting the influence of chromophoric dissolved organic matter on light absorption by phytoplankton in boreal lakes

Abstract

Many boreal lakes are colored brown due to strong light absorption by chromophoric dissolved organic matter (CDOM), which reduces light penetration into the water column. However, the influence of CDOM on the fraction of photosynthetically utilizable radiation (PUR) absorbed by phytoplankton from the photosynthetically active radiation (PAR) entering the lake (i.e., PUR/PAR) remains largely unknown. Here, we (1) quantified PUR/PAR values and examined the major water quality parameters determining PUR/PAR from 128 sampled boreal lakes, (2) predicted PUR/PAR values for 2250 reference boreal lakes, and (3) estimated the response of PUR/PAR to typical browning trends reported in earlier studies. The PUR/PAR values ranged from 0.4% to 17% in the sampled lakes, and a logarithmic model including CDOM and chlorophyll a (Chl a) concentration was the most parsimonious for predicting PUR/PAR values. Applying the model to the reference lakes, PUR/PAR values ranged from 0.5% to 20% (median 3%). In the model, an increase in CDOM content decreases the PUR/PAR value, but a concurrent increase in Chl a concentration with the CDOM partly compensates the negative effect of CDOM on the PUR/PAR values. Assuming that browning increases both CDOM and Chl a contents, as found for our reference lakes, our model suggests that the decrease in light absorption by phytoplankton in response to a typical degree of browning is only moderate. The moderate response of the PUR/PAR to browning may be explained by photoacclimation of phytoplankton to lowered light availability, and/or an increased loading of nutrients to lakes both leading to higher Chl a concentration.