Impact of Wetland Disturbance on Phosphorus Loadings to Lakes

Description

Total phosphorus (TP) concentrations have declined in many lakes and streams across south- central Ontario, Canada over the past three decades and changes have been most pronounced in wetland-dominated catchments. In this study, long-term (1980-2007) patterns in TP concentrations in streams were assessed at four wetland-dominated catchments that drain into Dickie Lake (DE) in south-central Ontario. Two of the sub-catchments (DE5 and DE6) have particularly large wetland components (31-34 % of catchment area), and wetlands are characterised by numerous standing dead trees and many young live trees (18 – 27 year old). These two streams exhibited large peaks in TP and potassium (K) export in the early 1980s. In contrast, TP and K export from DE8 and DE10 (wetland cover 19 – 20 %) were relatively flat over the entire record (1980-2007), and field surveys indicated negligible standing dead biomass in these wetlands, and a relatively healthy, mixed-age tree community. Furthermore, K:TP ratios in the DE5 and DE6 streams were around 5 in the early 1980s; very similar to the K:P ratio found in biomass, and as stream TP levels fell through the 1980s, K:TP ratios in DE5 and DE6 stream water increased. The coincidence of high TP and K concentrations in the DE5 and DE6 streams as well as evidence of a disturbance event in their wetlands during the early 1980s suggest that the two are related. The diameter of standing dead trees and allometric equations were used to estimate the amount of TP that would have been held in readily decomposed tree tissues in the DE5 wetland. The amount of P that would have been held in the bark, twig, root and foliage compartments of just the standing dead trees at DE5 was approximately half of the amount of excess stream TP export that occurred in the 1980s. This work suggests that disturbance events that lead to wetland tree mortality may contribute to patterns in surface water TP observed in this region.
Author Keywords: Chemistry, Disurbance, Nutrients, Tree Death, Water, Wetland