10.1 Freshwater Biogeochemistry (Lakes and Rivers)
3 min read•july 25, 2024
Freshwater ecosystems are dynamic hubs of biogeochemical activity. , driven by carbon, nitrogen, and phosphorus, forms the backbone of these systems. These processes fuel and , shaping the intricate food webs that thrive in lakes and rivers.
Physical factors like and play crucial roles in nutrient distribution. Meanwhile, human activities such as and 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
- 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
- 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
- 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
- involves CO2 fixation and organic matter breakdown
- 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 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
- in lakes forms epilimnion, metalimnion, and hypolimnion
- 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 lead to oxygen depletion in bottom waters ()
- 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
- 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)
- 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
Key Terms to Review (20)
Acid Rain: Acid rain refers to any form of precipitation that is unusually acidic, containing elevated levels of hydrogen ions (low pH). This phenomenon is primarily caused by the emissions of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), which can react with water vapor in the atmosphere to form sulfuric and nitric acids, leading to significant ecological impacts.
Algal Blooms: Algal blooms are rapid increases in the population of algae in aquatic systems, often resulting in vibrant green, red, or brown water. These blooms can lead to significant ecological impacts, such as oxygen depletion and the production of harmful toxins, affecting aquatic life and water quality. The primary drivers of algal blooms often include nutrient pollution from phosphorus and nitrogen, particularly from agricultural runoff and wastewater.
Biomagnification: Biomagnification is the process by which the concentration of toxic substances increases in organisms at each successive level of the food chain. This phenomenon occurs because certain pollutants, such as heavy metals or persistent organic pollutants, are not easily broken down or eliminated by organisms, leading to higher concentrations as these substances move up through trophic levels. It highlights the interconnectedness of ecosystems and the potential dangers of environmental contaminants.
Carbon cycle: The carbon cycle is the series of processes through which carbon atoms circulate in the Earth's systems, including the atmosphere, biosphere, hydrosphere, and geosphere. This cycle plays a crucial role in regulating Earth’s climate, supporting life, and maintaining ecological balance by involving various reservoirs and fluxes of carbon across different spheres.
Chemical Stratification: Chemical stratification refers to the layering of chemical properties and concentrations in a body of water, often seen in lakes and rivers due to variations in temperature, density, and dissolved substances. This phenomenon influences the distribution of nutrients and pollutants, as well as the biological communities that thrive at different depths. Understanding chemical stratification is crucial for studying freshwater ecosystems and their biogeochemical cycles.
Decomposition: Decomposition is the biological and chemical process by which organic matter is broken down into simpler organic and inorganic materials, releasing nutrients back into the environment. This process plays a crucial role in nutrient cycling, influencing soil health, carbon storage, and ecosystem productivity.
Detritus: Detritus refers to the decomposing organic matter, such as dead plants and animals, along with the associated microbial communities and inorganic materials. This material plays a vital role in nutrient cycling and energy flow within ecosystems, contributing to both soil fertility and the dynamics of aquatic environments. By breaking down detritus, microorganisms release nutrients back into the ecosystem, which can be taken up by plants and other organisms, forming an essential part of the food web.
Eutrophic lakes: Eutrophic lakes are freshwater bodies characterized by high nutrient levels, particularly phosphorus and nitrogen, leading to excessive growth of algae and aquatic plants. This phenomenon can result in low oxygen levels in the water, especially during decomposition, and is often a response to agricultural runoff and other human activities that increase nutrient inputs.
Eutrophication: Eutrophication is the process by which water bodies become enriched with nutrients, often leading to excessive growth of algae and other aquatic plants. This phenomenon can disrupt ecosystems, contribute to oxygen depletion, and harm aquatic life, making it a critical concern in the study of biogeochemistry.
Hypoxia: Hypoxia refers to a condition in which there is a deficiency of oxygen in the water, often leading to detrimental effects on aquatic life. This phenomenon can occur due to various human activities that disrupt nutrient cycles, particularly the nitrogen and phosphorus cycles, leading to an overgrowth of algae and subsequent oxygen depletion in bodies of water. Understanding hypoxia is essential in addressing issues related to aquatic ecosystems and their health.
L. S. Redfield: L. S. Redfield was an influential oceanographer and biogeochemist known for formulating the Redfield Ratio, which describes the consistent elemental composition of marine phytoplankton. This ratio highlights the stoichiometric balance of carbon, nitrogen, and phosphorus in oceanic ecosystems, connecting nutrient dynamics to primary production in freshwater systems like lakes and rivers.
M.A. Palmer: M.A. Palmer is a notable figure in the field of biogeochemistry, particularly recognized for his work on freshwater ecosystems. His research has focused on the biogeochemical processes occurring in lakes and rivers, which include nutrient cycling, organic matter dynamics, and the influence of human activities on aquatic systems. Understanding Palmer's contributions is essential for grasping how nutrient inputs and ecological interactions affect freshwater environments.
Nitrogen cycle: The nitrogen cycle is the biogeochemical process through which nitrogen is converted between its various chemical forms, enabling it to be used by living organisms. This cycle involves several key processes including nitrogen fixation, nitrification, denitrification, and ammonification, connecting various Earth's spheres and influencing ecosystem dynamics.
Nutrient Cycling: Nutrient cycling refers to the movement and exchange of organic and inorganic matter back into the production of living matter. This process is vital as it connects various biological, geological, and chemical components of the Earth, ensuring that essential nutrients like carbon, nitrogen, and phosphorus are continuously recycled within ecosystems.
Oligotrophic lakes: Oligotrophic lakes are freshwater bodies characterized by low nutrient concentrations, especially nitrogen and phosphorus, which leads to low biological productivity. These lakes often have clear waters and support fewer algae and aquatic plants, creating a unique ecosystem that thrives in low-nutrient conditions. The clarity of oligotrophic lakes is usually a result of the limited availability of nutrients that would otherwise promote algal blooms.
Phosphorus Cycle: The phosphorus cycle is the biogeochemical process through which phosphorus moves through the lithosphere, hydrosphere, and biosphere. This cycle is vital for living organisms as phosphorus is a key component of DNA, RNA, and ATP, playing a critical role in energy transfer and genetic information.
Primary Production: Primary production refers to the process by which autotrophs, such as plants and phytoplankton, convert inorganic carbon (primarily CO₂) into organic compounds through photosynthesis or chemosynthesis. This process is crucial because it forms the foundation of the food web and affects nutrient cycling, energy flow, and the overall health of ecosystems.
Stratification: Stratification refers to the layering of different physical and chemical properties within a water body, such as lakes and rivers, often influenced by factors like temperature, density, and nutrient concentrations. This layering can significantly impact biological processes, including the distribution of organisms, nutrient cycling, and overall ecosystem health. Understanding stratification is essential for studying the dynamics of freshwater ecosystems and their responses to environmental changes.
Thermal stratification: Thermal stratification is the process in which water in a lake or other body of water forms distinct layers based on temperature differences, with warmer water on the surface and cooler water below. This layering affects the distribution of nutrients, oxygen levels, and overall ecosystem dynamics in freshwater environments, influencing both biological and chemical processes within lakes and rivers.
Water residence time: Water residence time refers to the average length of time that water spends in a specific reservoir, such as lakes or rivers, before being replaced or leaving that system. This concept is essential in understanding the dynamics of freshwater ecosystems, including how nutrients and pollutants are cycled, how organisms interact with their environment, and how water quality can change over time. The residence time can influence biogeochemical processes, affecting productivity and ecosystem health.