Attenuation of light and daily integral rates of photosynthesis attained by planktonic algae1

The daily integral rate of photosynthesis attained by planktonic algae in Lake Minnetonka, Minnesota, conforms to an equation for a rectangular hyperbola proposed by Bannister. The rate depends on the concentration of chlorophyll a in the mixed layer and also upon two kinetic parameters that are analogous to the kinetic constants in equations for reactions catalyzed by enzymes. One parameter is an upper limit, ψ, that would be attained by very dense populations, and the other is a concentration of chlorophyll, c', at which attenuation of photosynthetically active radiation (PhAR) by the algae equals background attenuation by the water. The parameters were evaluated with the same statistical procedures used to evaluate the kinetic constants for enzyme reactions. The limit, ψ, for the daily integral rate in this lake would be about 0.5 mol O2·m−2·d−1 Observed daily rates are usually <0.7 ψ, because chlorophyll in the water usually intercepts <70% of the radiant energy. Coefficients for the attenuation of PhAR by chlorophyll,, and by the water, were estimated from the linear regression of the total attenuation coefficient, ε, on chlorophyll concentration. The estimate of εc is 0.022 ± 0.005·m−1·(mg Chl·m−3)−1. Background attenuation, εw, affects integral photosynthesis in the same way as a competitive inhibitor affects enzyme reactions: it is a constituent of c′ = εw/εc which, together with chlorophyll concentration, determines the fraction of underwater PhAR intercepted by planktonic algae.