Spatial and temporal patterns in total phosphorus in south-central Ontario streams: the role of wetlands and past disturbance

Abstract

Unraveling the cause(s) of declines in total phosphorus (TP) observed over the past three decades across many forested catchments in central Ontario remains a research priority. The objectives of this paper were firstly to test the assumption that weir water chemistry is representative of catchment changes at two adjacent headwater streams that have shown declines in TP and secondly use long-term measurements of weir water quality (1980–2008) to identify potential drivers of TP declines. Longitudinal measurements confirmed that weir water chemistry is generally representative of the entire stream channel, although channel-associated wetlands have a strong influence on TP. Additionally, processes operating in wetlands dictate seasonal patterns in TP, and differences in wetland cover explain the approximately twofold difference in average TP between the two catchments. Changes in wetland cover or processes, however, cannot explain declines in TP. Rather, declines in TP are most coherent with declines in stream potassium (K) and nitrate (NO3-N), which also occurred over the past 30 years. We suggest that coincident declines in TP, NO3-N, and K may be best explained by recovery from a disturbance event.

Impact of wetland disturbance on phosphorus loadings to lakes

“…Despite naturally oligotrophic conditions in the region, TP concentrations have declined in approximately ⅔ of the monitored lakes and streams in the region over the past three decades (Eimers et al 2009;Palmer et al 2011;O'Brien et al 2013), and this surprising trend is the focus of new research into causes of TP declines in surface waters. Declines in stream TP have been observed at both upland and wetland draining streams, although the largest declines have occurred in wetland-dominated catchments (Eimers et al 2009).…”

“…Although O'Brien et al (2013) did not specifically consider disturbance within wetlands, disturbance in headwater wetlands may be particularly important since wetlands in the region are known to act as sources of TP mainly because of release from organic soils and vegetation (Devito and Dillon 1993). Examples of disturbances that impact biological cycling of P in wetlands include timber harvesting, fires, floods, or insect infestations (McEachern et al 2000;Ensign and Mallin 2001), although most studies on wetland disturbance focus on the effects on nitrogen (N) export (Bernhardt et al 2003;Cleveland et al 2011).…”

Phosphorus forms and response to changes in pH in acid-sensitive soils on the Precambrian Shield

“…In addition to wetlands, riparian zones have been noted as important sources of TP export in this landscape (Carlyle and Hill 2001;O'Brien et al 2013) and seasonal changes in wetland/riparian water table height have been associated with changes in [TP] in pore and surface waters (Devito and Dillon 1993). In a recent study at Harp Lake, O'Brien et al (2013) showed that TP concentrations in streams were strongly influenced by organic soils in nearstream riparian areas and argued that seasonal variation in [TP] could be explained by processes operating in these organic-rich riparian soils.…”

“…In a recent study at Harp Lake, O'Brien et al (2013) showed that TP concentrations in streams were strongly influenced by organic soils in nearstream riparian areas and argued that seasonal variation in [TP] could be explained by processes operating in these organic-rich riparian soils. However, whilst processes occurring in organic soils could explain seasonal patterns, the long-term pattern of declining [TP] could not be attributed to changes in redox chemistry.…”

“…Points represent means of analytical triplicates 9SE. processes, landscape disturbance and declines in atmospheric deposition have been or are presently under investigation O'Brien et al 2013;Pinder et al 2014).…”