Learning Objectives

3.2.1 Summarize how the size of particles on the bottom of streams, as well as water velocity and clarity, change from source streams to large rivers and subsequently influence the organisms that inhabit different zones of moving waters.

3.2.2 Explain how the depth and amount of light penetration in a pond or lake influence the distribution of photosynthetic and nonphotosynthetic organisms.

Although they occupy a small portion of the terrestrial surface, freshwater streams, rivers, and lakes are a key component in the connection between terrestrial and marine ecosystems. Rivers and lakes process inputs of chemical elements from terrestrial ecosystems and transport them to the oceans. The biota of these freshwater ecosystems reflect the physical characteristics of the water, including its velocity (flowing streams and rivers vs. lakes and ponds), its temperature (including seasonal changes), how far light can penetrate it (clarity), and its chemistry (salinity, oxygen concentrations, nutrient status, and pH).

In this section, we will explore the biota and associated physical conditions found in freshwater ecosystems. In contrast to terrestrial biomes, for which only plants are used as indicators, the biological assemblages of freshwater ecosystems are characterized by both plants and animals, reflecting the greater proportional abundance of animals in aquatic ecosystems.