A comparison of phosphorus deficiency indicators with steady state phosphate in lakes.

A fundamental step in the management of nutrient impacted water bodies is the determination of the type and degree of nutrient limitation. However, nutrient deficiency indicators often provide inconsistent results. Recent advances in the measurement of phosphate concentrations may provide a better means to understand results from P deficiency indicators. With regards to phosphorus, deficiency indicators should predict P-limitation when phosphate concentrations are consistently low. We use this new understanding to examine the relationships between phosphate concentration and P deficiency. Patterns of steady state phosphate (ssPO(4)(3-)) concentrations and P deficiency were evaluated in 109 lakes located across Canada. Lakes encompassed a broad range in TP concentration (1.79-139.7 μg L(-1)). The relationships between ssPO(4)(3-) concentrations and simultaneously measured total P (TP), total dissolved P (TDP) and soluble reactive P (SRP) concentrations, particulate C:P and N:P ratios, alkaline phosphatase activities (APA) and phosphate turnover times (TT) were analyzed. ssPO(4)(3-) was positively correlated with TP and TDP. The ssPO(4)(3-) concentrations were 2-3 orders of magnitude lower than SRP concentrations. These two measures were only weakly correlated, suggesting that SRP is a major overestimate of PO(4)(3-). The ssPO(4)(3-) concentrations were negatively correlated with C:P and N:P ratios, and with APA, consistent with expectations. When only lakes with TT < 15 min were considered, TT was negatively correlated with TP, challenging the idea that nutrients become less limiting in more eutrophic systems. Overall, P deficiency indicators related to ssPO(4)(3-) in the expected manner. However, variability in relationships with APA and particulate stoichiometry emphasize the need for cautious interpretation of P deficiency measurements. We recommend simultaneous use of multiple techniques to confidently assess P deficiency.