Epilimnetic Phosphorus Cycling: Influence of Humic Materials and Iron on Coexisting Major Mechanisms

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Abstract
Two major orthophosphate recycling mechanisms, alkaline phosphatase activity and the photodependent release of P from UV-sensitive complex P, cooccurred in epilimnetic samples from four trophically dissimilar lakes in northcentral Oklahoma. Both of these processes were strongly perturbed upon addition of low concentrations of dissolved humic material (DHM) and ferric iron, alone or in concert, to lake water samples. Planktonic orthophosphate uptake rates were also significantly altered by DHM/Fe addition, and, although the majority of P-uptake was accomplished by cells <3.0 μm in diameter, the effects of DHM/Fe were not size-selective. Additions of DHM/Fe to lake water appeared to promote elevated concentrations of orthophosphate and phosphomonoester substrates of alkaline phosphatase in lake water filtrates. Collectively, these data suggest that epilimnetic P cycling may be viewed as a continuum of processes and that the position of a given lake in the continuum is dynamic and influenced by DHM and Fe.