💧Limnology Unit 3 – Freshwater Chemistry and Nutrient Cycling

Key Concepts and Definitions

• Limnology studies the biological, chemical, and physical features of freshwater ecosystems (lakes, rivers, wetlands)
• Freshwater contains dissolved ions, gases, and organic matter that influence water chemistry
  • Dissolved ions include cations (positively charged) and anions (negatively charged)
  • Dissolved gases such as oxygen, carbon dioxide, and nitrogen are essential for aquatic life
• Nutrients are chemical elements necessary for organism growth and development
  • Primary nutrients include carbon (C), nitrogen (N), and phosphorus (P)
• Nutrient cycling describes the movement and transformation of nutrients through ecosystems
• Eutrophication is the excessive growth of algae and aquatic plants due to nutrient enrichment
  • Can lead to oxygen depletion, fish kills, and reduced water quality
• Stratification occurs when water forms distinct layers based on temperature and density differences
• Turnover is the mixing of stratified layers, redistributing nutrients and oxygen throughout the water column

Chemical Properties of Freshwater

• pH measures the concentration of hydrogen ions (H+) in water, indicating acidity or alkalinity
  • Freshwater pH typically ranges from 6.5 to 8.5
  • Influenced by factors such as bedrock composition, atmospheric deposition, and biological processes
• Dissolved oxygen (DO) is crucial for aquatic life and varies with temperature, pressure, and biological activity
  • Cold water holds more oxygen than warm water
  • Photosynthesis produces oxygen, while respiration and decomposition consume it
• Alkalinity refers to the capacity of water to neutralize acids and buffer pH changes
  • Determined by the presence of carbonates, bicarbonates, and hydroxides
• Hardness measures the concentration of dissolved calcium and magnesium ions
  • Hard water has high concentrations, while soft water has low concentrations
• Specific conductance quantifies the ability of water to conduct an electrical current
  • Indicates the presence of dissolved ions and total dissolved solids (TDS)
• Water clarity is affected by suspended particles, dissolved organic matter, and algal growth
  • Secchi disk depth is used to measure water transparency

Major Ions and Water Chemistry

• Cations in freshwater include calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+)
  • Derived from weathering of rocks and minerals, atmospheric deposition, and anthropogenic sources
• Anions include bicarbonate (HCO3-), carbonate (CO32-), sulfate (SO42-), and chloride (Cl-)
  • Influence pH, alkalinity, and nutrient availability
• Ion concentrations vary based on geological setting, climate, and human activities
• Ionic composition affects the distribution and abundance of aquatic organisms
  • Some species have specific ion requirements or tolerances
• Dissolved organic matter (DOM) consists of organic compounds from terrestrial and aquatic sources
  • Plays a role in light attenuation, nutrient cycling, and microbial processes
• Redox potential (Eh) measures the tendency of a chemical species to acquire electrons and be reduced
  • Influences the speciation and mobility of nutrients and contaminants

Nutrient Cycling Overview

• Nutrient cycling involves the transfer of nutrients between abiotic and biotic components of ecosystems
• Primary productivity relies on the availability of essential nutrients like carbon, nitrogen, and phosphorus
  • Nutrients are incorporated into biomass through photosynthesis and released through decomposition
• Nutrient limitation occurs when the supply of one or more nutrients restricts biological growth
  • Liebig's Law of the Minimum states that growth is limited by the nutrient in shortest supply
• Internal cycling refers to nutrient transformations within the aquatic ecosystem
  • Includes processes like mineralization, nitrification, and denitrification
• External loading involves the input of nutrients from the surrounding landscape
  • Sources include atmospheric deposition, surface runoff, groundwater, and point sources (wastewater treatment plants)
• Nutrient retention and removal mechanisms regulate the concentration and export of nutrients
  • Sedimentation, uptake by organisms, and chemical transformations can remove nutrients from the water column
• Nutrient spiraling describes the downstream transport and recycling of nutrients in river systems

Carbon Cycle in Freshwater Systems

• Carbon enters freshwater through atmospheric CO2 dissolution, photosynthesis, and organic matter inputs
  • Dissolved inorganic carbon (DIC) includes CO2, HCO3-, and CO32-
  • Dissolved organic carbon (DOC) consists of organic compounds from terrestrial and aquatic sources
• Primary producers (algae and aquatic plants) fix inorganic carbon into organic matter through photosynthesis
• Respiration by aquatic organisms and microbial decomposition release CO2 back into the water and atmosphere
• Carbonate equilibrium regulates the distribution of inorganic carbon species based on pH and alkalinity
  • Shifts in pH can affect the availability of carbon for photosynthesis
• Methane (CH4) production occurs in anoxic sediments through methanogenesis
  • Can be a significant source of greenhouse gas emissions from freshwater systems
• Carbon burial in sediments represents a long-term sink for organic carbon
  • Influenced by sedimentation rates, oxygen availability, and microbial activity
• Dissolved organic carbon (DOC) affects light attenuation, nutrient cycling, and microbial processes
  • Colored dissolved organic matter (CDOM) can limit light penetration and primary productivity

Nitrogen Cycle in Lakes and Rivers

• Nitrogen exists in various forms, including organic nitrogen, ammonium (NH4+), nitrite (NO2-), and nitrate (NO3-)
• Nitrogen fixation converts atmospheric N2 into biologically available forms
  • Carried out by cyanobacteria and other diazotrophic organisms
• Ammonification (mineralization) breaks down organic nitrogen into ammonium
  • Mediated by heterotrophic bacteria and fungi
• Nitrification is the oxidation of ammonium to nitrite and then to nitrate
  • Performed by nitrifying bacteria (Nitrosomonas and Nitrobacter)
  • Requires aerobic conditions and consumes oxygen
• Denitrification reduces nitrate to gaseous forms (N2 and N2O)
  • Occurs under anoxic conditions by denitrifying bacteria
  • Removes bioavailable nitrogen from the system
• Assimilation incorporates inorganic nitrogen into organic compounds by primary producers and microorganisms
• Dissimilatory nitrate reduction to ammonium (DNRA) converts nitrate to ammonium under anaerobic conditions
  • Can retain nitrogen within the system
• Nitrogen loading from anthropogenic sources (fertilizers, wastewater) can lead to eutrophication

Phosphorus Dynamics

• Phosphorus is a limiting nutrient in many freshwater systems
  • Exists in dissolved and particulate forms, both organic and inorganic
• Orthophosphate (PO43-) is the most bioavailable form of phosphorus
  • Readily assimilated by primary producers and microorganisms
• Organic phosphorus is incorporated into biomass and released through decomposition
• Adsorption and desorption processes regulate the exchange of phosphorus between water and sediments
  • Iron, aluminum, and calcium minerals can bind phosphorus in sediments
  • Redox conditions and pH influence the release of phosphorus from sediments
• Phosphorus precipitation occurs when dissolved phosphorus combines with metals to form insoluble compounds
  • Can remove phosphorus from the water column and store it in sediments
• Internal loading of phosphorus from sediments can sustain eutrophication even after external inputs are reduced
• Phosphorus retention in lakes is influenced by water residence time, sedimentation rates, and biological uptake
• Phosphorus recycling in the water column is driven by microbial processes and zooplankton excretion

Human Impacts on Freshwater Chemistry

• Eutrophication is a major consequence of human activities that increase nutrient loading
  • Agricultural runoff, sewage discharge, and urban development are common sources of nutrients
  • Leads to algal blooms, oxygen depletion, and changes in species composition
• Acidification occurs when atmospheric deposition of sulfuric and nitric acids lowers the pH of freshwater
  • Affects the solubility and toxicity of metals, and the survival of acid-sensitive organisms
• Salinization increases the concentration of dissolved salts in freshwater
  • Caused by road salt application, irrigation return flows, and saltwater intrusion
  • Alters the ionic composition and osmotic balance of aquatic organisms
• Contaminants such as heavy metals, pesticides, and pharmaceuticals can enter freshwater through various pathways
  • Accumulate in sediments and biota, causing toxicity and ecological disruption
• Hydrological modifications (dams, diversions, channelization) alter flow regimes and nutrient transport
  • Can lead to changes in water chemistry, temperature, and habitat availability
• Climate change affects freshwater chemistry through changes in temperature, precipitation, and evaporation
  • Influences stratification patterns, oxygen solubility, and nutrient cycling

Analytical Techniques and Measurements

• Water sampling techniques include grab sampling, depth-integrated sampling, and continuous monitoring
  • Proper sample collection, preservation, and storage are crucial for accurate analysis
• In-situ measurements use sensors and probes to measure parameters directly in the water body
  • Examples include temperature, pH, dissolved oxygen, and specific conductance
• Laboratory analysis involves the determination of chemical constituents in water samples
  • Spectrophotometry measures light absorption to quantify concentrations of nutrients, metals, and organic compounds
  • Ion chromatography separates and quantifies major ions based on their affinity for an ion exchange resin
• Nutrient analysis methods include colorimetry, flow injection analysis, and automated wet chemistry
  • Measure concentrations of nitrogen and phosphorus species
• Dissolved organic carbon (DOC) is typically measured using high-temperature catalytic oxidation
• Stable isotope analysis provides insights into nutrient sources, transformations, and cycling pathways
  • Isotope ratios of carbon, nitrogen, and oxygen are commonly used
• Quality assurance and quality control (QA/QC) procedures ensure the reliability and comparability of data
  • Include calibration, replication, blanks, and certified reference materials