Phosphorus Deposition in a Low-Phosphorus Landscape: Sources,Accuracy and Contribution to Declines in Surface Water P

Atmospheric deposition can be an important source of phosphorus (P) to watersheds in geologically low-P landscapes like the Precambrian Shield, in south-central Ontario. Recent observations in this region of declining total P (TP) concentrations in streams and lakes, as well as P limitation in vegetation, prompted this investigation into the potential contributing role of atmospheric deposition. Long-term (1980–2014) measurements of bulk TP deposition at four monitoring stations, as well as wet-only deposition at one of the sites (2012–2014) and pollen measurements in 2014, were used to assess the potential contribution of biogenic sources to TP deposition in the region. Ratios of phosphorus to potassium (P:K) in historical bulk deposition and variability in P deposition among the four sites indicated that continuously open, bulk deposition collectors are enriched in ‘local’ biogenic sources of P, particularly in the summer, fall and spring months. Furthermore, in 2014 pollen could account for as much as 30% of bulk TP deposition, and pine trees (Pinus strobus) were the most important source of pollen (>75% of collected pollen). In contrast, winter bulk deposition was the least affected by local, biogenic sources, and likely best reflects net P inputs to this region from long-range (that is, external to the watershed) sources. Catchment input–output budgets for P that use measured annual bulk deposition totals as inputs are generally strongly positive (indicating net retention/accumulation of P within watersheds), whereas annual budgets calculated using extrapolated winter deposition are in approximate balance. Balanced P budgets are more consistent with regional observations of declining surface water TP and P limitation in terrestrial vegetation. Importantly, there was no significant decline in winter TP deposition over the period of record (1980–2014), which suggests that changes in regional deposition are not responsible for declines in surface water TP. Although we recommend extrapolating winter bulk deposition measurements to approximate annual atmospheric TP input to terrestrial ecosystems in non-dusty, oligotrophic landscapes such as this, local sources of P, particularly pollen, are likely important seasonal sources of P nutrition to lakes.