higher animals; produces food and oxygen that supports most of a lake’s
consumers.
■ PROFUNDAL ZONE—deep, no-light regions, too dark for photosynthesis,
low oxygen levels, inhabited by fish adapted to cool, dark waters.
■ BENTHIC ZONE—bottom of lake, organisms can tolerate cool temperatures
and low oxygen levels.Figure 4.9 Lake zonationTypes of Lakes
Lakes are often classified according to their production of organic matter. The
three general categories include: 1. OLIGOTROPHIC (YOUNG LAKE)—deep, cold,
small surface area relative to depth, nutrient-poor, phytoplankton are sparse, not
very productive, don’t contain much life, waters often very clear, and sediments
are low in decomposable organic matter. Characteristics include: ■ Steep
shorelines going down to the water’s edge ■ Primarily conifer trees (pines)
along the shore ■ Deep and drop off quickly
■ Bottom is mostly rocky
■ Water is very clear
■ Very few aquatic weeds