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The main objectives of this study were to (a) study the interaction between N and P cycles in mining-affected aquatic systems and (b) to quantify release rates of sedimentary soluble reactive phosphorus (SRP) that may be related to this interaction. Sediment cores and water from Lake Bruträsket (Boliden, northern Sweden) were collected and a time series of water sampling and flow measurements was conducted in the Brubäcken stream connected to the lake. Factors affecting SRP release were studied in a sediment incubation experiment and water column experiments. Field and laboratory measurements indicated that pH and dissolved oxygen are two important factors for SRP release. At the end of the low-oxygen incubation, an SRP concentration of 56 μg L(-1) resulted in a sedimentary flux of 1.1 mg SRP m(-2) day(-1). This is ~10 times higher than the flux of 0.12 mg SRP m(-2) day(-1) obtained from depth integration of vertical SRP profiles measured in the lake, and ~100 times higher than the external flux of 0.014 mg SRP m(-2) d(-1) into the lake (based on catchment area). Field measurements indicated that oxidation of organic matter and mining-related chemicals (ammonium and thiosulphates) may result in increased internal SRP flux from the sediment. Increased P loading in the lake as a result of low-oxygen conditions could change water column total nitrogen/total phosphorus ratios from 27 to 17, consequently changing the lake from being P-limited to be co-limited by N and P. The obtained findings point to possible interaction between the cycles of nitrogen (oxygen consumption) and P (flux from sediment) that may be important for nutrient regulation in mine water recipients.

Uptake and cycling of nitrogen (N) in the littoral zone of a lake-receiving nutrient-rich mine water located in Boliden, northern Sweden, was investigated. Stable isotope tracer solutions of 15 N as NH 4? (NAM mesocosm) or NO 3 -(NOX mesocosm) were added to mesocosms enclosing plants of common reed (Phragmites australis). The 15 N abundance in various plant parts was measured at pre-defined time intervals over an experimental period of 22 days. During the course of the experiment, plant parts from the NAM mesocosms were significantly more enriched in 15 N than plant parts from the NOX mesocosms. On day 13, Dd 15 N values of the fine roots from the NAM mesocosms had reached ?8220 %, while the maximum Dd 15 N value in NOX roots was considerably lower at ?4430 %. Using 15 N values in macrophyte tissues present at the end of the experiment enabled calculations of uptake rates and % of tracer N recovered in the plant (%tracerNrecov). Maximum tracer uptake rates were higher for the NAM mesocosms (1.4 lg g -1 min -1 or 48 mg N m -2 d -1 ) compared to the NOX mesocosms (0.23 lg g -1 min -1 or 8.5 mg N m -2 d -1 ). Calculations of %tracerNrecov indicated that 1-8 and 25-44 % of added N was assimilated by plants in the NOX and NAM mesocosms, respectively. Hence, P. australis was more effective in assimilating NH 4 ? , and a larger portion of the tracer N accumulated in the roots compared to the other plant parts. Consequently, macrophyte N removal is most effective for cold-climate aquatic systems receiving mine water dominated by NH 4? . For permanent removal of N, the whole plant (including the roots) should be harvested.