Sediments of deep canadian shield lakes: observations of gross structure and biological significance.

Sediments of deep Canadian shield lakes have a firm mud-water interface and an intricately structured, oxygenated surface. Surface relief is not uniform, but is broken by small ridges and upright chironomid tubes. The sedimentary material behaves like a weak jelly and becomes flocculent only when violently disturbed. Sculpins were observed to rest on and, when started, to hide in the oxygenated layers. Sequestering of nutrients in the bottom sediments is enhanced by the structuring of the substrate surface below 10 meters, and may inhibit nutrient recycling at overturn.