Phosphorus dynamics in shallow subsurface waters in an uncut and cut subcatchment of a lake on the Boreal Plain

Abstract

Phosphorus dynamics in shallow subsurface waters (<2.5 m depth) were studied in harvested and unharvested subcatchments of a Boreal Plain lake. The organic soil layer was underlain by discontinuous layers of sand and clay glacial till. Total dissolved P (TDP) concentrations (6-798 µg·L-1) of discrete water samples from mineral layers (0.9-2.5 m deep) generally decreased with depth, were negatively related to Ca (rs < -0.7), and were lower in clay. When the groundwater table rose and saturated the organic layer, TDP concentrations increased in the composite (organic mineral layer) but not in the discrete (mineral layer) water samples, indicating that elevated TDP concentrations originate from the near-surface organic layer. TDP concentrations in composite samples were negatively correlated with water table depth (rs = -0.6) and were positively correlated with transmissivity (rs = 0.7) and dissolved organic C concentration (rs > 0.6). In the riparian buffer zone of the harvested subcatchment, TDP concentrations of composite samples decreased during high runoff, but these values remained higher than concentrations in the unharvested subcatchment. However, surface topography and variable depth to confining clay layers resulted in higher groundwater tables in the harvested subcatchment, especially in the cut area. Mean daily TDP export coefficients were similar between the unharvested (14 µg·m-2) and harvested (12 µg·m-2) subcatchments.