Summer internal phosphorus supplies in Shagawa Lake, Minnesota

Phosphorus budget calculations for Shagawa Lake show that significant increases in lake P during July and August 1971–1975 originated from within the lake basin. Mass increases of 2,000–2,900 kg (35–50 µ g·liter ‒1 ) were measured, corresponding to internal supply rates of 240–680 kg·wk ‒1 over a 3–9‐week interval. After anaerobi: conditions developed in late June, release from the profundal zone sediments contributed significant amounts of P to lake water. Sediment chemical properties were determined to estimate the size of the P reservoir in the profundal zone (at water depths > 6.5 m). The P content of the surficial profundal zone sediments was 0.2–0.5% (dry wt) and a significant fraction was potentially mobile (NaOH extractable). We calculate that the upper 10 cm of these sediments contain 50,000 kg of potentially mobile P, of which a small fraction is released each summer. Although external sources of phosphorus have been reduced by about 80%, the feedback of P from the sediments is supporting the productivity of Shagawa Lake and may continue to do so for many years.