Lecture 12 - Aquatic chemistry and nutrient cycle

• chemicals in a aquatic ecosystem are either dissolved or particulate
• particles can be further divided into those that settle and those that will not
• ions largely originate from terrestrial environments through weathering and erosion
• aquatic ecosystems in more easily weathered geologic formations have more ions
• solubility of ions depends on temperature and pH and concentration of the ions
• concentration of some ions depends on interactions with biota
• Hydrogen ions

  • expressed as pH (more equals lower)
  • determines the acidity of water
• Alkalinity/Acidity

  • capacity of water to react with acids and bases
  • linked to hardness -> concentration of magnesium and calcium ions
• Turbidity

  • amount of light absorption by water
  • influenced by suspended particles and presence of dissolved organic molecules
• Total suspended solids

  • amount of suspended particles in the water (organic and inorganic)
• Redox Potential

  • relative availability of electrons for chemical reactions
  • determined general chemical environment of water
  • determines which chemical reactions occur
  • critical for biogeochemical cycling (oxidized or reduced)
  • Primarily determined by concentration of dissolved O2 gas
  • the more the oxygen the stronger the oxidizing opportunities
• oxygen is found in water as dissolved oxygen gas
• DO = dissolved gas
• the amount of DO found in water is a function of metabolic activity, diffusion, temperature, proximity to atmosphere (O2 source)
• DO is expressed as concentration and % saturation
• Saturation Concentration -> equilibrium concentration between water and atmosphere -> dependent on altitude and temperature
• decreased O2 and pressure at high altitude cause low DO
• due to metabolic activity DO is often sub-saturated or supersaturated
• consumption of DO is associated with aerobic respiration and addition of DO associated with photosynthesis. when consumption outweighs addition can lead to anoxic or anaerobic conditions
• when addition outweighs consumption supersaturation occurs
• photosynthesis-irradiance relationships -> describe effects of light on photosynthesis
• What are the three factors that effect photosynthesis?

  temperature, nutrients, water velocity
• how does temperature effect photosynthesis?

  rate of photosynthesis increases with each 10 degree increase
• how do nutrients effect photosynthesis?

  stimulate photosynthetic rates
• how does water velocity effect photosynthesis?

  enhances availability of CO2 to cells, removes inhibitory chemicals
• Respiration

  • primary route of O2 consumption
  • increases with biomass
  • quality and abundance of organic carbon main control
  • controlled by temp
  • lack of DO can be an issue
• Controls on distribution of DO in the Environment

  • varies with depth in lake
  • DO is depleted in hypolimnion over summer months
  • varies from day to night
  • balance between respiration and photosynthesis
  • depends on reaeration and biomass