Temporal and Spatial Variability in Sediment and Phosphorus Loads to Lake Simcoe, Ontario

Abstract

Information on sediment and phosphorus loads through time and space, coupled with descriptions of trends in water quality and fish communities, provide useful reference points and goals for water management. These are needed in Lake Simcoe where the coldwater fish community and water quality have deteriorated. The sedimentation basin and thickness of post-glacial beds in Lake Simcoe were defined by sonar and, together with data on total phosphorus (P) and bulk density, used to estimate the pre-1800 sediment load of 27,300 t·yr -1 and P load of 28.2 t·yr -1. The recent sediment load of 63,900 t·yr -1 and P load of 74.6 t·yr -1 were obtained by scaling up pre-1800 rates using ratios of recent to pre-1800 mass sedimentation rates and P loads at 20 core stations. Time-variable mass sedimentation rates and P-concentration profiles facilitated the division of P load into three components—natural load, anthropogenic non-point-source load and anthropogenic point-source load. Analysis of temporal trends indicated that non-point-source P inputs peaked approximately 85–95 yr before present (BP), declined to about 30 yr BP, and increased again since then in the main basin. The dominant peak in Cook Bay occurred 35–45 yr BP after marshes at the head of the bay were drained and the lower Holland River was channelized for agricultural purposes. Point-source P was detected in cores beginning about 50–75 yr BP and increased toward the sediment-water interface. Spatial distribution of P load changed after settlement when altered limnological conditions and loss of marshes changed the P-retention coefficients (R) in bays.