Size-Dependent Ammonium and Phosphate Uptake, and N:P Supply Ratios in an Oligotrophic Lake

One arm of oligotrophic Sproat Lake was fertilized with weekly additions of inorganic phosphorus and nitrogen (N:P = 50: 1, by atoms). There was a lag of greater than 1 mo before changes in N:P supply ratio were reflected by "relative uptake rates" (RURs) of saturating ammonium and phosphate additions. This supports previous findings that RUR is a sensitive indicator of changes in N:P supply ratio, but suggests that changes in RUR are mediated by changes in composition rather than physiological state of the plankton community. The <3 μm fraction showed a greater demand for phosphate than ammonium compared with larger cells (>3 μm), as indicated by maximum uptake rates for the two nutrients. However, maximum phosphate uptake rates for both size fractions were greater as the result of fertilization. Nonetheless, RURs for both size fractions were consistent with Synechococcus being N limited and diatoms being P limited when the lake was not fertilized. Despite the large increase in nutrient loading, dissolved phosphate turnover times decreased from 10–12 to 5–7 min, indicating increased phosphate demand relative to supply. This study emphasizes that ammonimum and phosphate kinetics data yield valuable information on the nutritional conditions experienced by phytoplankton in lakes.