The rates of light limited (αB) and light saturated (PmaxB) photosynthesis per unit chlorophyll often showed significant differences between samples collected from the top and the bottom of the mixed layer in Lake Erken (59° 51′ N). The importance of these vertical variations to the photosynthetic rate of the lake as a whole was examined by systematically changing the vertical weighting given to the biomass specific photosynthetic rates associated with each depth, when calculating integral photosynthesis. These simulations, carried out over the 3 month period from July to September of 1987, suggest that vertical variations in rates of biomass specific photosynthesis could account for at most a 24 to 36 percent change in monthly rates of integral photosynthesis, this variance in integral photosynthesis was similar to that which could result from inaccurate estimates of integral biomass, but was less than the measured temporal variability in integral photosynthesis. It was not possible to identify the specific processes responsible for the vertical variations in biomass specific photosynthesis, but evidence does suggest that photoinhibition of the surface phytoplankton was partially responsible, particularly when biomass increased at the surface in the presence of blue-green algae. Simulations which increasingly redistributed the total integral biomass to the upper 1-4 m of the water column suggest that while the upward migration of blue-green algae may lead to enhanced levels of photoinhibition, the effect on integral photosynthesis may be partially offset by increased concentrations of biomass at surface light intensities.

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