JOURNAL OF PLANKTON RESEARCH | VOLUME 12 | NUMBER 4 | PAGES 743-758 | 1990
© Oxford University Press
research-article |
Phosphorus sources for phytoplankton and bacteria in Lake Michigan
US Department of Commerce, Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration 2205 Commonwealth Blvd, Ann Arbor, MI 48105 1Great Lakes Research Division, The University of Michigan Ann Arbor, Ml 48109, USA
Received on March 31, 1989; accepted on February 15, 1990 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.