The Measurement of Phosphorus and Its Spatial and Temporal Variability in the Western Arm of Lake Superior

Abstract

Accurate and reproducible measurements of phosphorus (P) in Lake Superior are required to improve our understanding of the P cycle in this ecosystem. Several available analytical methods were tested to determine their suitability for determining low levels of total phosphorus (TP), total dissolved phosphorus (TDP), and soluble reactive phosphorus (SRP) in Lake Superior. The best results were obtained using a persulfate oxidation method to convert the TP and TDP pools to quantifiable inorganic P with subsequent colorimetric detection of SRP using flow injection analysis. The method detection limit was typically 10 nM P with an average precision of 3.7 ± 2.1% (N = 10) based on triplicates of 10 nM P standards. Samples were collected during June and September 2001 and were analyzed to determine the speciation of P and its spatial and temporal variability in the western arm of Lake Superior. Total P ranged from 20 to 200 nM with the highest concentrations and strongest spatial gradients measured in June. On average, particulate P (PP) and dissolved organic P (DOP) comprised approximately one-half of the TP and TDP pools, respectively. Based on changes in the DOP pool, we hypothesize that this pool is potentially bioavailable, and that near shore P is more reactive than the offshore P. Historically, the effect of silicic acid on colorimetric P measurements in Lake Superior has been ignored. The data presented here suggest that the historical P data reported for Lake Superior are potentially systematically high because of this silicic acid interference.