Partitioning processes controlling water column phosphorus concentrations in a shallow wetland

Summary 1. Phosphorus (P) concentrations in the water column of lakes and wetlands are crucial to their trophic status and ecosystem function, but quantifying the processes controlling P concentrations in the field has been a difficult task. A site-based, in-lake method is described to partition major field processes controlling P concentration in a shallow lake. 2. It involves (i) in-lake deployment of a suite of chambers that isolate in-chamber activities from atmospheric sources, groundwater input and horizontal water movement; (ii) monitoring P concentrations and relevant water properties inside and outside the isolation chambers; and (iii) calculating the contribution of each individual process by simple mathematical deduction, so as to differentiate the contributions from the different sources. 3. The method was applied at nearshore and offshore sites in a seasonal, groundwater-fed shallow lake on the Swan Coastal Plain, south-western Australia, during winter refilling. Primary (atmospheric and groundwater) and secondary processes (e.g. circulation and sediment-water interactions) were partitioned and quantified in terms of their contributions to water column P [as total P (TP; μg m−2 day−1)]. 4. Atmospheric and groundwater inputs were the two main processes contributing P loadings (1233 and 1010 μg P m−2 day−1), but their influence appeared restricted to the near-shore site. The estimated influence on TP by mixing-circulation, atmosphere and groundwater were 2.4–25 times higher near the lake margin as compared with the offshore site. The circulation and sediment-water interactions decreased water column P at the marginal site, but increased P offshore because of subsequent P release from sediment and a concurrent increase in pH. 5. Results are consistent with data reported elsewhere, and the factors that could affect the accuracy of partitioning are discussed