The cffccts of various environmental conditions, and of ccl1 composition, heterocyst content, and nitrogen content of algal s'amples, on fixation of N, by colonial and filamentous algae in Lake Mendota were investigated.IIctcrocyst content and temperature were significantly and positively related to acctylcnc recluction activity ( NB fixation); depth of sample collection was negatively related. Rvailablc data do not show whcthcr statistical correlations of acctylcne reduction activity with dissolved 0, and pI1 rcprcsent specific effects of Or and pJ1 on this activity, or simply reflect chemical changes caused by algal photosynthesis.During summer stratification, when surface-water content of combined inorganic nitrogcn was sevcrcly depleted, N2 fixation associated with hctcrocystous blue-green algae contributed 85% of the total Nn fixed. Bacterial N2 fixation was low compared to that of algae. Dcspitc its small overall contribution to the amlual nitrogen budget (38,000 kg of nitrogen; ca. 7% of the total input), Ns fixation is significant in maintaining blucgreen algal nuisances in surface water when non-Nn-fixing phytoplankton cannot compcte cfl&tivcly.

No abstract