Suttle, C.A. and P.J. Harrison. 1988. Ammonium and phosphate uptake rates, N:P supply ratios, and evidence for N and P limitation in some oligotrophic freshwater lakes. Limnology and Oceanography 33:186-202.

Natural assemblages of freshwater phytoplankton were grown in daily-dilution culture (0.50/d) at six N:P supply ratios (5:1, 10:1, 15:1, 25:1, 35:1, and 45:1; by atoms), to create a range of N- and P-limited conditions. After a minimum of 30 d in culture, subsamples were removed for determination of saturated phosphate and ammonium uptake rates. A relationship (y = 0.0148e exp(0.1496x)) was found between the ratio of phosphate to ammonium uptake rates (y) and N:P supply ratio (x), indicating that this index was sensitive to the N:P supply ratio under which phytoplankton are grown. Cultures grown at the highest ratio were dominated by a chroococcoid cyanobacterium (probably Synechococcus sp.), while those grown at the two lower ratios were dominated by the diatoms Nitzschia and Synedra and the chlorophyte Scenedesmus . Additional experiments were conducted on two oligotrophic lakes in an effort to determine if N and P co-occurred as limiting resources. Nutrient bioassay studies on water from one of these lakes indicated that N was in short supply among members of the phytoplankton community.