PHOSPHORUS SOURCES FOR PHYTOPLANKTON AND BACTERIA IN LAKE-MICHIGAN

Size-fractionation experiments on the uptake of phosphate (PO4) and recently excreted dissolved organic phosphorus (E-DOP) from phytoplankton suggest that algae and bacteria rely on different forms of phosphorus (P) in the epilimnion of P-limited Lake Michigan. Rate constants for PO4 uptake in 0-1 μm fractions generally were low relative to those measured in wholewater, suggesting that most of the uptake was by algae. Uptake of 33E-DOP in 0-1 μm fractions approximated uptake in wholewater, indicating uptake principally by bacteria. Concurrent experiments showed that (i) E-DOP and PO4 were taken up by different transport systems; (ii) bacteria have transport systems for E-DOP compounds; and (iii) cell-surface phosphatase-mediated PO4 supply to phytoplankton from E-DOP was negligible. Results suggest that pathways of PO4 and E-DOP flux in microplankton communities of P-limited large and small lakes may differ. The use of different sources of P by algae and bacteria in Lake Michigan supports the classical concept of algal-bacterial freshwater P cycling. These findings are consistent with a proposed hypothesis that, in large lakes with low allochthonous nutrient inputs, phytoplankton are P-limited and use PO4, while bacteria obtain P primarily from dissolved organic compounds and are limited by a nutrient other than P. © 1990 Oxford University Press.