Oligotrophic lakes are deep, nutrient-poor, and low in primary productivity. C. Stream Ecosystems 1. Streams start out as freshwater springs or seeps. Riffles are shallow, turbulent stretches where water flows swiftly over sand and rock. In pools, deep water flows slowly over a smooth, sandy or muddy bottom.
Runs are fast-flowing waters with a smooth surface and a bottom of bedrock or rock and sand.
Average flow and temperature are influenced by geography, altitude, and forest shade. Volume and temperature vary with rainfall, snow melt, drought, and the seasons. Chemistry and pollution of the water depend on materials leached into, or added to, the stream. Streams erode their valleys and participate in cycles of erosion and redeposition of sediments and nutrients. The benthic province includes all the sediments and rocky formations of the ocean bottom.