The relationship between light and photosynthetic rate in a river community and implications for water quality modeling

Saturation of photosynthesis at naturally occurring high light intensities has widely been reported for aquatic communities. Contrary to this, using free water measurement methods, we found a relationship between light intensity and photosynthesis in a small river that was unmistakably linear on cloudless days and on days when points corresponding to times of highly variable radiation were discarded. The linear relationship obtained through the year. Simulations of nutrient‐limited growth in light‐dark bottles show that nutrient depletion effects can explain the cases of light saturation and inhibition previously reported in the literature. If light saturation and inhibition of photosynthesis are due even in part to artificially induced nutrient limitation, then the inclusion of nonlinear light terms in water quality models is inappropriate.