Since Shuswap and Mara lakes are the source of drinking water for a large percentage of the region’s population, some residents have referred to the lakes as drinking water reservoirs due to their interest in maintaining high water quality. Yet, calling these lakes reservoirs is misinformed, as most lakes are complex ecosystems, that support a wide diversity of life forms and the biology of the lakes varies according to the season of the year and the variety of natural and man-made inputs. By and large, most of the Shuswap and Mara lakes area is classified as oligotrophic with clear, relatively nutrient-free water.
Lakes that contain higher levels of nutrients and thus higher concentrations of algae are called mesotrophic. Salmon Arm and Tappen bays are mesotrophic, due to the slower flushing rate of the water in the bays and to the increasing amount of nutrients entering the lake from the sewage treatment plant, storm water and the Salmon River, where agricultural run-off is a serious problem. Monitoring efforts are also showing an increase of nutrients in the Shuswap River.
Sediment core sampling done by the Ministry of Environment has revealed the status of the lake for the past 200 years by the annual deposits of sediment and diatoms. The core shows that Tappen Bay began to deteriorate in the 1930s and that deterioration increased in the 1970s.
In 1931, a detailed limnology study was done in Shuswap Lake that identified all the organisms present at different levels. Plankton was most abundant in the sheltered bays and the nets recovered 54 different species, including 13 types of algae. There was a similar level of shoreline invertebrate diversity that included six types of molluscs.
The number of bottom organisms varied from 128 to 449 per square yard, including worms, shrimp, mites, larvae, snails and clams. Using gill nets, the researchers found 16 different types of fish, including salmon fry, suckers, trout, whitefish, lake shiners and sculpins.
Although the majority of the lake shoreline is rocky, the study found an abundant growth of aquatic plants in the sheltered bays, which provide food, oxygen and habitat structure for other organisms. Included in the list were various forms of pondweed, quillwort, arrow grass, horsetail, and bladderwort. Interestingly, there was Eurasian water milfoil back then, which appears to prove that this plant is not a recent invasive species.
Key to the continued health of Shuswap and Mara lakes is the rapid flushing of the water, which for most of these lakes is estimated at just one to two years, with slower water movement in the bays.
Perhaps the greatest input of nutrients occurs after large salmon runs in the rivers, when many of the decomposing carcasses are washed into the lakes either in the fall or the spring.
These nutrients feed growing numbers of algae, which in turn stimulates the growth of plankton. The emerging salmon fry then feast on the plankton and the chain of life continues to circulate.
Light, temperature and wind are also key factors. The depth that light can penetrate controls the amount of photosynthesis that can occur. Shade helps cool the water, which fish require, particularly in creeks and rivers.
The temperature varies according to the time of year and the depth. During both spring and fall, there is a rapid turnover of the water column as compared to the summer when the temperature can vary from 20 degrees centigrade at the top to just 4 degrees at the bottom. Wind often circulates the water column, which lowers the temperature at the top.
While it is important that water-quality monitoring work continues and efforts are made soon to reduce the amount of nutrients entering the lake, attention should also be directed at the biodiversity of the lake. The growing number of concerns from fishers about an increase in the number of lesions and growths seen on trout raise questions about potential problems. Shuswap and Mara lakes do more than just provide drinking water for most of us. They are also a rich ecosystem that is home to countless numbers and types of organisms and we need to ensure that the delicate balance is not disturbed.
