Final answer:
Ice forms an insulating layer on the surface of lakes, protecting dormant lake grass and sustaining the lake's ecosystem by preventing the complete freezing of the lake, allowing for light penetration and oxygen turnover.
Step-by-step explanation:
The impact of ice on a population of lake grass, specifically Carex lacustris, that goes dormant in a frozen lake during winter, can be understood by recognizing the protective role ice plays. Ice, being less dense than liquid water, forms on the surface of lakes, creating an insulatory layer that preserves the aquatic ecosystems beneath.
This insulating barrier prevents the entire lake from freezing solid, which would be lethal to the plant life within it. The dormant lake grass is thereby protected from the expansion of ice crystals that would rupture cell membranes, causing irreversible damage.