10.1 Freshwater Biogeochemistry (Lakes and Rivers)

Freshwater ecosystems are dynamic hubs of biogeochemical activity. Nutrient cycling, driven by carbon, nitrogen, and phosphorus, forms the backbone of these systems. These processes fuel primary production and decomposition, shaping the intricate food webs that thrive in lakes and rivers.

Physical factors like water residence time and stratification play crucial roles in nutrient distribution. Meanwhile, human activities such as eutrophication and acid rain are altering these delicate balances. Understanding these processes is key to preserving the health of our vital freshwater resources.

Biogeochemical Processes in Freshwater Ecosystems

Biogeochemical processes in freshwater

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• Nutrient cycling drives ecosystem function through element movement
  • Carbon cycle involves CO2 fixation and organic matter breakdown
    • Photosynthesis by aquatic plants and algae captures atmospheric carbon
    • Respiration by aquatic organisms releases CO2 back into water
    • Decomposition of organic matter releases nutrients and CO2
  • Nitrogen cycle crucial for protein synthesis and DNA formation
    • Nitrogen fixation by cyanobacteria converts N2 to biologically available forms
    • Nitrification oxidizes ammonia to nitrate, denitrification reduces nitrate to N2
  • Phosphorus cycle often limits primary production in freshwater
    • Adsorption and desorption of phosphates to sediments regulates availability
    • Uptake by aquatic plants and algae removes phosphorus from water column
• Primary production forms the base of aquatic food webs
  • Phytoplankton growth in open water produces oxygen and organic matter
  • Periphyton growth on surfaces provides food for grazers (snails, fish)
  • Macrophyte production in littoral zones creates habitat structure
• Decomposition recycles nutrients and organic matter
  • Microbial breakdown of organic matter releases stored nutrients
  • Release of nutrients back into the water column supports new growth
  • Formation of detritus and sediment accumulates organic matter over time

Physical factors in aquatic biogeochemistry

• Water residence time affects nutrient retention and ecosystem productivity
  • Longer residence times increase nutrient processing and primary production
  • Shorter residence times flush nutrients downstream more quickly
• Stratification creates distinct layers with different chemical properties
  • Thermal stratification in lakes forms epilimnion, metalimnion, and hypolimnion
  • Chemical stratification develops oxygen gradients and nutrient distributions
• Mixing redistributes nutrients and oxygen throughout water column
  • Seasonal turnover in temperate lakes homogenizes water chemistry
  • Wind-driven mixing in shallow lakes and rivers resuspends sediments
  • Impacts nutrient distribution and availability for primary producers

Anthropogenic impacts on freshwater biogeochemistry

• Eutrophication alters nutrient balance and ecosystem function
  • Excess nutrient input from agricultural runoff and sewage accelerates primary production
  • Increased algal blooms lead to oxygen depletion in bottom waters (hypoxia)
  • Food web structure shifts towards dominance of planktivorous fish
• Acid rain disrupts chemical balance and harms aquatic life
  • Decreased pH in water bodies affects organism physiology
  • Leaching of aluminum from soils increases toxicity to aquatic organisms
  • Changes in nutrient cycling and availability alter ecosystem productivity
• Other anthropogenic impacts modify freshwater biogeochemistry
  • Dam construction alters flow regimes and sediment transport
  • Wetland drainage reduces natural nutrient filtering and carbon storage
  • Introduction of invasive species can disrupt native biogeochemical cycles

Oligotrophic vs eutrophic lake characteristics

• Oligotrophic lakes have low nutrient concentrations and primary productivity
  • Clear water allows deep light penetration supporting diverse benthic communities
  • Oxygen-rich waters throughout water column
  • Limited algal growth results in high water clarity
  • Longer food chains support diverse fish communities (trout, whitefish)
• Eutrophic lakes contain high nutrient concentrations and primary productivity
  • Reduced water clarity due to abundant algal growth limits light penetration
  • Oxygen depletion in bottom waters from decomposition of sinking organic matter
  • Frequent algal blooms can lead to fish kills and reduced biodiversity
  • Shorter food chains dominated by planktivorous fish (carp, catfish)
• Sediment characteristics reflect trophic state
  • Oligotrophic lakes have low organic matter content in sediments
  • Eutrophic lakes accumulate high organic matter in sediments, fueling internal nutrient loading