Aquatic

The Aquatic Environment (Freshwater)

Chapter 3

I. Definition of Freshwater

• Water covers approximately 71% of the Earth's surface.

• Two main types of water on Earth:

  1. Salt water

  • Comprises 96.5% of the water in oceans (sea water with salinity of 35 PPT).

  1. Brackish water

  • Mixture of salt and freshwater (salinity between 1 to 35 PPT).

  1. Freshwater

  • Comprises less than 0.5 PPT.

    • Only 2.5% of total water is freshwater, of which 68.6% is frozen in glaciers and polar ice caps.

    • Less than 1% of freshwater is accessible for use (not all is potable).

      • Breakdown of freshwater sources:

      • 68.6% is tied up in glaciers.

      • 30.1% is groundwater.

      • 1.3% is surface freshwater, of which 0.22% is categorized as “Biological” water.

II. Distribution of Earth's Water

• Representation of global water distribution:

  • Saline Water: 96.5% (Oceans)

  • Freshwater: 2.5%

    • Groundwater: 30.1%

    • Surface water and other freshwater: 1.3%

    • Lakes: 20.1%

    • River: 0.46%

    • Swamps and marshes: 2.53%

    • Atmospheric water: 0.22%

    • Biological water: 0.22%

    • Ice and Glaciers: 68.6%

Source: Igor Shiklomanov's chapter "World fresh water resources" in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World's Fresh Water Resources.

III. Hydrological Cycle

• USGS serves as a resource for understanding the water cycle:

  • Evapotranspiration, precipitation, condensation, evaporation play critical roles in water distribution.

  • Illustrates how water moves through various states (solid, liquid, vapor) in the atmosphere and terrestrial environments.

IV. Importance of Water

1. Essential for life on Earth.

   • Facilitates most biological processes.

V. Characteristics and Physical Properties of Water

1. Natural Liquids: Very few exist apart from water; some include:

   • Mercury.

   • Liquid carbon dioxide in quartz crystals.

   • Petroleum.

2. Dipolar Moment:

   • Responsible for many unique properties of water.

   • Angle of attachment is at 104.5°; polarity causes stronger attraction between O-H than H-H, allowing water to remain as a liquid at room temperature.

3. Properties of Water:
   A. Hydrogen Bonding and Polarity: Allows water to remain liquid under normal atmospheric temperatures and pressures.
   B. Melting Point: 0 °C (32 °F)
   C. Boiling Point: 100 °C (212 °F) at sea level.
   D. Density Influence:

   • The density of water influences its behavior across ecosystems.

   • Lower density water floats on top of higher density water—it forms stable layers known as stratification.

   • The maximum density occurs at 3.98 °C, and density decreases with an increase in temperature above this point.

   • Ice Floats: Water is most dense at 4 °C; as it freezes, it becomes less dense (ice is 9% less dense than water).

     • Hydrogen bonds adjust to hold O-atoms apart forming a crystal lattice structure in ice.

4. Universal Solvent:

   • Water effectively dissolves both gases and ions.

   • Integral in weathering, dissolving nutrients from rocks, and shaping ecosystems.

   • Example: 20% of the continental land supports karst deposits from rainwater infiltrating limestone, resulting in cave systems, sinkholes, and springs.

5. Solubility Affected by Temperature:
   a. Increased temperature enhances ions' dissolution.
   b. Increased temperature reduces gas solubility.
   c. Importance for aquatic life: higher temperature increases oxygen demand but holds less oxygen.

6. Temperature-Induced Zonation in Water:
   a. Water density maximum at 3.98 °C influences stratification.
   b. Lake Temperature Layers:

   • Epilimnion: warm surface water.

   • Thermocline: rapid decline in temperature.

   • Hypolimnion: cold dense water (around 4 °C).

7. Thermal Stratification and Turnover:
   a. Tropical Lakes: thermocline persists year-round.
   b. Temperate Lakes: thermocline occurs only during summer.
   c. Autumn Turnover: Surface cools, creating uniform temperature (~4 °C)—mixes nutrients to surface layers.
   d. Spring Turnover: Ice melting leads to surface warming—mixes temperature distribution to equalize.
   e. Dimictic Lakes: Experience complete seasonal cycles (stratification twice a year).
   f. Monomictic Lakes: Stratify only in summer.

8. Oxygen Dynamics:
   a. Diffusion from air enables oxygen solubility in water.
   b. Solubility decreases with increased salinity and temperature.
   c. Mixing by winds and currents is crucial.
   d. The effect of thermal stratification is oxygen stratification.

9. Specific Heat of Water:
   a. Specific heat is the heat required to raise the temperature of 1 gram of substance by 1 °C.

   • Specific heat of water = 1.0 cal/g (very high).

   • Few substances exceed this, such as liquid ammonia and hydrogen.
     b. Dictates how heat is stored and lost to bodies of water, impacting regional climates—affecting temperatures and precipitation.

10. Viscosity and Inertia:
    a. Viscosity increases with density; it affects how easy a liquid flows.

    1.  Viscosity is a fluid's resistance to flow, or how "thick" it is

       1.    Honey-high viscosity because its thick

       2. Water- low viscosity because it flows easily         

    2. Inertia is the resistance to motion change caused by size and density.
       c. Water's viscosity promotes certain adaptations among aquatic organisms (passive floating, sessile, and active swimming).

11. Cohesion and Surface Tension:
    a. Hydrogen bonding forms a cohesive surface, significant in survival for many aquatic organisms.
    b. Enables formation of a neuston layer—supports diverse organisms like topminnows.
    c. Capillary action influences moisture in sediments (considers how water interacts with sediment interstitial spaces).

12. Water Movement and Flow:
    a. Fast and slow flows lead to distinct environmental conditions depending on sediment load and water volume.
    b. Influences streamflow and stream habitat.
    c. Different flow types:

    • Brownian motion: random thermal motion seen at a molecular level.

    • Laminar flow: unidirectional and primarily near solid surfaces.

    • Turbulent flow: characterized by mixing and eddies(circulant movement of water)—significant in stream habitats.

VI. Importance of Light in Aquatic Systems

1. Sunlight reflection varies with angle, impacting aquatic ecosystems.

2. Light penetration varies with location and season; absorbed by water molecules affects temperature and photosynthesis.

3. Vertical extinction coefficient ($k$): Affects light penetration.

   • Lower $k$ → deeper light penetration, clearer water.

   • Higher $k$ → less penetration, more turbid water.

4. Types of Lakes Based on Light Absorption and Attenuation:

   • Eutrophic Lakes:

     • Characterized by high nutrient productivity, low transparency, and green appearance, low oxygennlevels are often present due to decomposition processes that consume oxygen.

   • Oligotrophic Lakes:

     • Low productivity, highly transparent, often appearing blue due to lower levels of suspended particles.

5. Light Wavelength Penetration:
   a. Red light is absorbed most quickly.
   b. Blue light has the least absorption, allowing for deeper penetration.
   c. Suspended particles further reduce light penetration.
   d. Photosynthesis in Aquatic Organisms:

   • Chlorophyll a absorbs red and blue light, reflects green.

   • Cyanobacteria adapt to utilize green light.

VII. Lotic and Lentic Ecosystems

1. Lotic Ecosystems:

   • Actively flowing water (e.g., rivers, streams).

2. Lentic Ecosystems:

   • Still or non-flowing water (e.g., lakes, ponds).

VIII. Lotic Ecosystem Characteristics

1. Streams and Catchments:

   • Catchment: Basin from which surface and groundwater flows into streams.

   • Watershed: Defined as land area that drains water into a body of water.

2. Stream Order:

   • Classifies the hierarchy of natural channels (Mississippi River as a 10th or 11th order stream).

3. Specific Landforms in Streams:

   • Pools (deep and slow), Riffles (shallow and fast) are influenced by substrate materials.

4. Riparian Zone Importance:

   • The interface between land and river, critical for maintaining ecosystem health and input of organic material.

5. River Continuum Concept (RCC):

   • Model for understanding lotic systems detailing ecological and hydrological changes downstream affecting energy input and biodiversity.