7-3 Freshwater Life Zones

What Are Freshwater Life Zones? Lakes, Wetlands, Rivers

Freshwater life zones occur where water with a dissolved salt concentration of less than 1% by volume accumulates on or flows through the surfaces of terrestrial biomes. Examples are standing (lentic) bodies of fresh water such as lakes, ponds, and inland wetlands.

What Life Zones Are Found in Freshwater Lakes? Life in Layers

Lakes normally consist of four distinct zones that are defined by their depth and distance from shore.

  • The top layer is the littoral zone: It consists of the shallow sunlit waters near the shore to the depth at which rooted plants stop growing, and it has a high biological diversity.

  • Next is the limnetic zone: the open, sunlit water surface layer away from the shore that extends to the depth penetrated by sunlight. As the main photosynthetic body of the lake, its producers supply the food and oxygen that support most of the lake’s consumers.

  • Next is the profundal zone: the deep, open water where it is too dark for photosynthesis.

  • Finally, at the bottom of the lake we find the benthic zone. Mostly decomposers and detritus feeders and fish that swim from one zone to the other inhabit it.

What Temperature Zones Are Found in Freshwater Lakes? Three Summer Temperature Zones

In summer, most Canadian lakes are affected by thermal stratification. Water near the surface, which is heated by the sun and mixed by the action of wind and waves, becomes a distinct layer of warm well- oxygenated water called the epilimnion.

Beneath the epilimnion, the temperature changes rapidly over a short distance; this is the thermocline. The abrupt change in temperature and relative lack of mixing serve as a barrier to the transport of nutrients, oxygen, and organisms like fish that cannot tolerate sudden temperature change.

The cold water below the thermacline is termed the hypolimnion. This layer of water often has a lower concentration of oxygen because it is isolated from the surface by the thermal stratification of the lake.

In fall, thermal stratification breaks down. The surface water, cooled by exposure to cold air, sinks to the bottom of the lake and sets up a convection that brings nutrients up from the bottom and mixes oxygen down from the surface. This period is termed fall turnover.

In spring as ice melts and the surface water warms, it actually becomes more dense; it sinks and sets up a convection mixing nutrients from the bottom and oxygen from the surface—this is spring turnover.

How Do Plant Nutrients Affect Lakes? Too Much of a Good Thing Is Not Good

Ecologists classify lakes according to their nutrient content and primary productivity:

  • Oligotrophic (poorly nourished)

  • Eutrophic (well-nourished)

  • Many lakes fall somewhere between the two extremes of nutrient enrichment and are called mesotrophic lakes.

What Are the Major Characteristics of Freshwater Streams and Rivers? A Downhill Ride

Precipitation that does not sink into the ground or evaporate is surface water. It becomes runoff when it flows into streams. The land area that delivers runoff, sediment, and dissolved substances to a stream is called a watershed, or drainage basin.

The downward flow of surface water and groundwater from mountain highlands to the sea takes place in three different aquatic life zones:

  1. Source zone: Most of these streams are not very productive because of a lack of nutrients

  2. Transition zone: The warmer water and other conditions in this zone support more producers and a variety of cool-water and warm-water fish species with slightly lower oxygen requirements.

  3. Floodplain zone: These slow-moving rivers sometimes support fairly large populations of producers such as algae and cyanobacteria and rooted aquatic plants along the shores

Streams are fairly open ecosystems that receive many of their nutrients from bordering land ecosystems. We have established farmlands and constructed dams, power plants that need cooling water, sewage treatment plants, cities, recreation areas, and shipping terminals in the watersheds along the shores of rivers and streams, especially in their transition and floodplain zones.

What Are Freshwater Inland Wetlands? Natural Sponges

Four main categories of inland wetlands are found in Canada:

  1. Marshes: The most common type of wetland, and they are dominated by cattails and other reeds, bulrushes, and pond lilies.Marshes are nutrient rich and are one of the most productive ecosystems on the planet.

  2. Swamps: Dominated by water-tolerant trees such as black spruce, black ash, cedars, silver maples, and red maples. These generally nutrient-rich and productive ecosystems provide habitat for a wide range of wild species.

  3. Fens: Typically found in the arctic and subarctic regions of Canada, fens have more nutrients and more plant diversity than bogs.

  4. Bogs: Have wet spongy ground, and there can be a floating mat of vegetation that encroaches on open water. As a result of the acidic environment, decomposition and productivity are extremely low, which accounts for the discovery of bog men

In addition to their role as habitat, inland wet- lands perform a number of important services. They absorb pollutants, improve water quality, slow soil erosion, provide flood control, and store greenhouse gases.

What Are the Impacts of Human Activities on Freshwater Systems? Using and Abusing Rivers and Wetlands

Human activities have four major impacts on freshwater systems:

  1. Dams, diversions, or canals fragment almost 60% of the world’s 237 large rivers. This alters and destroys wildlife habitats along rivers and in coastal deltas and estuaries by reducing water flow.

  2. Flood control levees and dikes built along rivers alter and destroy aquatic habitats.

  3. Cities and farmlands add pollutants and excess plant nutrients to nearby streams and rivers.

  4. Many inland wetlands have been drained or filled to grow crops or have been covered with concrete, asphalt, and buildings.