In a previous lecture, we learned about the importance of water to the existence of life and it's importance in defining biomes. As we proceed in our study of animal life which originated in water and then progressed onto land, it's important to have an understanding of aquatic habitats (lakes, streams and oceans) because they have greatly affected the evolution of life forms.

Lakes

A lake is body of freshwater with three zones:

• The littoral zone extends from the shore to where rooted plants stop growing.
• The limnetic zone includes open, sunlit waters beyond the littoral to a depth where photosynthesis is no longer significant. Plankton life is abundant in the limnetic zone
• The profundal zone is the deep, open water below the depth of light penetration. Detritus sinks from the limnetic and is acted upon by decomposers.

This animation (no audio) describes lake zonation.

In temperate regions, lakes undergo changes in density and temperature. In winter, ice (less dense) forms on the surface over water that is warmer, much of it at 4 C (greatest density), and heavier. 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. Eutrophic lakes are shallow and nutrient-rich often due to agricultural and urban runoff wastes.

Streams (Audio - Not Important)

Streams start out as freshwater springs or seeps. Three kinds of habitats form along a continuum from head waters to river's end:

• 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.

Several factors affect streams:

• 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.

Oceans (Audio - Not Important)

• The open ocean consists of two vast provinces:
• The benthic province includes all the sediments and rocky formations of the ocean bottom.
• The pelagic province includes the entire volume of ocean water and is subdivided into two zones.

The neritic zone constitutes the relatively shallow water overlying the continental shelves. The oceanic zone is the water over the ocean basins. Photosynthetic activity is restricted to the surface.

Phytoplankton are the beginning of ocean food webs. Their organic remains and wastes enter the detrital webs. As much as seventy percent of the productivity may be from microscopic ultraplankton.

In hydrothermal vents which are the fissures between the earth's crustal plates, water becomes heated and laden with minerals as it escapes. Elaborate food webs are supported at these sites.

Mangroves are forests in sheltered regions along tropical coasts. Ocean waves do not reach these saltwater ecosystems, so anaerobic sediments and mud accumulate. Salt-tolerant plants with prop roots dominate.

Estuaries are partially enclosed regions where fresh and salt water meet. Estuaries are incredibly productive feeding and breeding grounds for many animals. Many marine estuaries are under assault from raw sewage, agricultural runoff, industrial wastes, and too much freshwater.

The intertidal-zone inhabitants of rocky and sandy coastlines are alternately exposed and submerged. Existence is difficult. Rocky shores have three vertically arranged zones:

• The upper littoral is submerged only during the highest possible lunar tide. It is sparsely populated.
• The mid-littoral is submerged during the regular tide and exposed at the lowest tide of the day.
• The lower littoral is exposed only during the lowest lunar tide.

Sandy and muddy shores are rather unstable stretches of loose sediments. Detrital food webs occur and invertebrates are plentiful.

This animation (Audio - Important) describes the ocean zones.

If it's daylight when you're reading this, you can take a quick trip to the Monterey Bay Aquarium and watch the Monterey Bay Cam. The height of the rocks above sea level will vary as the tides vary.

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