Ch. 11 Oxygen in Lakes

Why study oxygen?

  1. Essential for the metabolism of most organisms

    • all higher organisms require oxygen

  2. Critical role in nutrient cycling and energy flow

    • oxygen is the most crucial nutrient

Solubility of Oxygen in Water

  • Solubility of oxygen in water depends on:

    • 1) Temperature

      • warm water holds less oxygen than cold water (same with all gases → gas law)

    • 2) Pressure → increase in pressure creates greater solubility of oxygen

      • allowing more gas to be dissolved in the water

    • 3) Salinity → increase in salinity creates a decrease in solubility of oxygen

Photosynthesis and Respiration

  • Photosynthesis: CO2 + H2O → C6H12O6 + O2

    • Photosynthesis creates oxygen, which allowed oxygen in our atmosphere, which allowed for higher organisms

  • Respiration: C6H12O6 + O2 → CO2 + H2O

    • Respiration consumes oxygen

    • Most oxygen consumption comes from the decomposition of organisms

Sinks and sources of Oxygen in Lakes

  • Sources:

    • Diffusion

      • oxygen from the atmosphere is diffusing into the lake

    • Photosynthesis

      • aquatic macrophytes create oxygen in the lake using photosynthesis

    • Water inflow

      • from rivers that contain high oxygen

  • Sinks:

    • Respiration (BOD)

      • taking in oxygen and turning into CO2

    • Decomposition (BOD)

    • Photoxidation

    • Water inflow

      • from rivers that are contaminated and have less oxygen

    • Chemical oxygen demand (COD)

      • certain chemicals take in oxygen

  • BOD = biological oxygen demand

  • COD = chemical oxygen demand

  • Anoxia: absence of dissolved oxygen

    • zero oxygen

  • Hypoxia: low dissolved oxygen conditions

    • some organisms are dead before they get to zero oxygen

Lake Trophic State Classification

  • Ogliotrophic = nutrient poor, low algae production

  • Mesotrophic = medium level of nutrients, medium algae production

  • Eutrophic = high level of nutrients, high algae production

    • high algae production can lead to less light in deep water → less photosynthesis in deep water → less oxygen because of more decomposition and more respiration

Significance of Anaerobic Layer

  1. Obligate requirement of many organisms for oxygen (e.g. kills fish)

  2. Important implications for nutrient cycling

Ways to Express Dissolved Oxygen (DO)

  • 2 ways we typically express dissolved oxygen (DO) data:

    • 1) Concentration data (mg/L or ppm)

    • 2) Percent saturation (%)

Measuring Devices for Dissolved Oxygen in a Lake

  • Dissolved oxygen meter

  • Wikler titrations

    • standard method

Variations in Oxygen Profiles

  • (+) heterograde

    • bump in (+) oygen

    • created by an abundance of algae bloom at a specific depth → producing an excess amount of oxygen

  • (-) heterograde

    • (-) bump in oxygen

    • caused by respiration and decomposition

      • usually by an abundance of zooplankton

  • Example:

Changes in a Lake

  • Diurnal Changes = changes over the course of a 24h day

    • e.g. there might an abundance of photosynthesis during the day, and nothing during the night (high oxygen during the day and low oxygen at night)

Horizontal Differences in O2

  • Between basin differences

    • variations in depth, temperature stratification, and nutrient availability, leading to distinct oxygen concentrations in different areas of the lake

  • Aquatic macrophytes

    • higher plants that photosynthesize a lot creating super-stratification

  • Organic pollution from point sources (e.g. incoming river)

  • Ice cover

    • creates a barrier on the lake, decreasing oxygen coming from the atmosphere and decreases photosynthesis

    • there might be ice covering one side of the lake whereas the other side has no ice covering