Myriophyllum aquaticum (Vell.) Verd. populations in two lakes and three drainage canals in northern California were screened for nitrogen and phosphorus deficiency using emergent leaf and sten1 critical concentration criteria. Seasonal variation in productivity and concentrations of nitrogen and phosphorus in emergent tissues were examined in one of the lentic populations, Park's Lake. Analysis of emergent leaf and stem tissues from Park's Lake indicated that N and P concentrations were below the upper 95% confidence limits on the critical concentrations during much of the growing season. N:P ratios indicated that N was more limiting than P early in the growing season, and P was more limiting than N late in the growing season. The other lentic population, sampled late in the growing season, was marginally P-limited. Neither N or P limited growth in the drainage canals. Productivity, and tissue concentrations of N and P, in Park's Lake were higher in shallow (<0.5 m) than in deep areas (0.5–1.0 m). In a laboratory fertilization experiment, M. aquaticum growth was greater on deep sediments than shallow sediments. The field observations and laboratory experiments suggest that M. aquaticum productivity in deep water does not depend upon sediment nutrients. The ability of adventitious roots, formed on rhizomes in the water column, to access sediment nutrients may be an important determinant of M. aquaticum productivity.