In winter the lake is frozen over from December until May but thc dissolved oxygen content remains high.
-4 noticeable feature of the lake is t.he turbidity of the water (,,Winnipeg" is an Indian term meaning ,,muddy water".) This is mostly due to sediment brought in by rivers from the south and west, notably the Red river and the Saskatchewan. Secchi's disc readings in the lake vary from 1 metre to 1.75 metre. In this connection it may be noted that Lake Winnipeg covers the boundary between the limestones, etc. of the prairies and the Archaean rocks of the ,,Pre-Cambrian Shield" and thus shows striking differences between the character of its eastern and western shores.
Related to these geological facts are the pH values. The lake itself and its southern and western tributaries are alkaline (7.6 to 8.4). The lowest readings are from 'points near the east shore. Some of the tributaries from this side are actually on the acid side of neutral (e. g. Black river 6.8) while others are from 7.2 to 7.5.
The net result of the various physical and chemical conditions appears to be very favourable for a large development of phytoplancton. Bajkov (1930) estimates that the total amount of plancton (excluding nannoplancton) varies from 2cc. to 20cc. per 1,000 litres of water. In summer particularly, a large proportion of this consists of algae and diatoms.
This phytoplancton, either living or dead, provides one of the main direct sources of food for the insectan and other elements of the bottom fauna. The present writer has shown (1932) how the growth rate of the mayfly Hexugenia lirnbutu occulta follows closely the seasonal variation in the amount of phytoplancton in the lake. Incidentally of course the richness of the bottom fauna (speaking from a quantitative point of view) can support a large quantity of nekton. It may be mentioned in this connection that Lake Winnipeg is the main source of production of one of the largest inland fisheries industries in the world.
In considering the entomological resources of the area, the folloning types of waters can be noted briefly:
- Tributaries (and outlet). 2. Lagoons.