Water Quality Issues
While not unique, the significant stressors on water quality in the Lake Simcoe watershed are related to a combination of urbanization and agricultural activities. The significant issues expanded upon in this section are; pollutions sources, habitat loss and invasive species. The current status of the lake's fish population in light of these stressors is also described.
Pollution Sources
Current Status of the Lake Simcoe Fishery
Lake Simcoe Phosphorus Water Balance
Pollution Sources
In Lake Simcoe (as in most other freshwater lakes), phosphorus is the nutrient in lowest supply. For that reason, its concentration is often believed to limit the growth of rooted plants and algae. When phosphorus concentrations are low, plant growth, including that of microscopic floating algae, is usually also low. When phosphorus is present in high concentrations, plant growth is usually abundant. When this significantly increased population of aquatic plants and algae die and decompose, dissolved oxygen is consumed and lake oxygen concentrations fall, a relationship known as eutrophication.
While phosphorus is cycled within the Lake Simcoe watershed naturally, the primary anthropogenic sources of phosphorus (and other nutrients) include; stormwater runoff, fertilizer application (agricultural and domestic), sewage effluent (from private septic systems and communal sewage treatment plants), milkhouse waste, soil erosion (from urban and rural areas), inadequate manure storage, and the drainage of marsh areas.
Studies from many systems have shown that phosphorus concentrations in lakes are directly related to the amounts of phosphorus discharged to the lakes. While this may seem obvious, it provides reassurance that if we can reduce phosphorus loadings to Lake Simcoe, concentrations of phosphorus will diminish. From the above listed sources of nutrients, it is obvious that all sectors within the watershed are responsible for its current condition and all sectors can contribute to the reduction Phosphorus inputs. For suggestions regarding what can be done to this end please see the TIPS section of the web site.
Current Status of the Lake Simcoe Fishery
In general, the patterns of change that we observe in the Lake Simcoe fishery appear to be typical of those in other lakes suffering accelerated eutrophication; a shift from coldwater species like lake trout, whitefish and herring to species preferring warmer water such as perch, bass and sunfish. We do not understand all of the reasons for these shifts, but it is clear that they relate in part to changes in nutrient cycling and in dissolved oxygen concentrations, siltation over spawning sites, shifts in available food items, and the invasion of non native species. Each species has unique requirements for spawning and of the young survival and is therefore affected differently by changes in the ecosystem.
Despite the lack of useful precedents, it is reasonable to assume that lake trout reproduction will be restored with improvements in water quality. The re establishment of whitefish may be complicated by the presence of rainbow smelt which may out compete young whitefish for food and living space. Complex interactions among species such as these make it difficult to predict what concentration of phosphorus is necessary for a self sustaining fish community, although any reductions beyond present loadings will be beneficial. Until we have restored desirable oxygen levels in deep waters, and a clean spawning habitat in shallower waters, it will be necessary to continue the rehabilitative stocking of lake trout and lake whitefish in the lake to maintain fish populations.
Phosphorus in Lake Simcoe
For each incoming water source derived from the monitoring and modelling described in the water balance section, nutrient loadings to the lake are estimated from the various pathways based on measured concentrations where possible, and estimated from known concentrations where not.
Atmospheric Deposition
Atmospheric inputs to the lake are sampled at the same locations in the watershed that precipitation inputs are quantified. Regular samples taken from bulk precipitation collectors are analyzed for total phosphorus and phosphorus loads are calculated by multiplying the concentration of bulk precipitation by recorded precipitation. The total atmospheric phosphorus load for 1998 was 40.1 tonnes.
Surface Water Inflow
Surface water inputs, either measured or modelled as descried in the water balance section, and measured total phosphorus concentrations are used to calculate sub-watershed surface water phosphorus inputs. For the purpose of quantifying the relative contributions of different sources, the phosphorus load from urban point and non-point sources that is ultimately delivered to streams was subtracted from the subwatershed input totals. The measure of surface water inputs is, therefore, representative of the phosphorus inputs in surface water in rural and agricultural areas, and is termed tributary load. The sources of nutrients in such areas include livestock, milkhouse waste and fertilizers. The total tributary phosphorus load for 1998 was 27.6 tonnes.
Urban Non-Point Source Loads
Lawn fertilizers, pet waste, and detergent from car washing represent some of the sources of nutrients in urban runoff. Selected uncontrolled urban areas (those with no water quality treatment) were monitored for total phosphorus concentrations and used to model the phosphorus loads in urban non-point sources (see LSEMS Technical Report A4). The 1998 urban runoff phosphorus load was calculated to be 21.9 tonnes.
Urban Point Source Loads
Sewage treatment plant (STP) discharge and the chemical composition of that discharge are monitored under the Municipal Utilities monitoring program. Total measured phosphorus concentrations are multiplied by recorded discharge values to yield STP phosphorus loading. The total sewage treatment plant phosphorus load for 1998 was 5.7 tonnes.
Vegetable Polder Loads
Agricultural practices in marshes within the watershed (Bradford, Keswick, Colbar and Holland) rely upon the management of water in the fields. Water is either pumped onto the fields for irrigation, or is pumped from the fields to drain excess moisture. Water pumped from marsh agricultural areas can contain significant nutrient concentrations depending upon the moisture levels at and soon after the time of fertilizer application. To evaluate phosphorus loads, discharge measurements (water pumped from fields to the Holland River) and sampled phosphorus concentrations from the Bradford marsh were extrapolated to the remaining vegetable polders. The resultant total vegetable polder phosphorus load for 1998 was 5.6 tonnes.
Relative Phosphorus Loads
The relative contributions of total phosphorus to Lake Simcoe are illustrated in this pie chart. Atmospheric deposition, as calculated, represents the most significant input. Additional precipitation stations have been added to the LSEMS monitoring network since evaluation of these data in an effort to more accurately capture the different weather patterns that exist in the watershed and that may been misrepresented using only 2 inland stations historically. Tributary sources are followed closely by urban runoff as the second and third most significant phosphorus sources, respectively. Finally, sewage treatment plants and vegetable polders complete the list of significant phosphorus sources.
