PHOSPHORUS AND NITROGEN EXPORT FROM FORESTED STREAM CATCHMENTS IN CENTRAL ONTARIO

The ability to predict export of algal nutrients from forested stream catchments is essential to estimate background levels of nutrients in lakes and therefore gauge the effects of anthropogenic activities on the trophic status of Ontario lakes. Data from 32 forested stream catchments in the Muskoka-Haliburton area of central Ontario collected during the 8-yr period 1976-1977 to 1983-1984 were used to develop empirical regression models of long-term average NH4+ NO3-, TON, and TP export employing mathematical transformations of geology, physiography, hydrology, and annual meteorological data. At least 6 consecutive years of sampling were necessary to avoid poor fits due to anomalous years and to produce regression models able to make reliable (R2 > 0.70) long-term average predictions of TP export. Atmospheric deposition of algal nutrients typically exceeded catchment export. Total annual phosphorus (TP) export was variable with outputs from two catchments exceeding atmospheric inputs. Total organic nitrogen (TON) export was also variable with annual export from 22 catchments exceeding atmospheric inputs, although total N (TN) export never exceeded input. Peat deposits or beaver (Castor canadensis Kuhl) ponds in most of these catchments were the likely sources of the high TP and TON export. Nitrate retention (defined as the fraction of annual deposition retained by the catchment) was high in most catchments, exceeding 0.74 in 26 of the 32 catchments. Low NO3- retention was observed primarily in those catchments with steep grades (> 5.9%). Ammonium retention was very high (> 0.87) in all catchments.