In temperate regions, lakes undergo changes in density and temperature. During the spring overturn, warming and winds cause oxygen to be carried downward and nutrients to the surface. By midsummer a thermocline between the upper warmer layers and lower cooler layers prevents vertical warming. During autumn, the upper layers cool and sink causing a fall overturn. Interactions of soils, basin shape, and climate produce a continuum of trophic structure.
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.