3.3 Phosphorus cycle

The phosphorus cycle is a crucial component of aquatic ecosystems. It influences water quality, productivity, and overall ecosystem health. Understanding the sources, sinks, and transformations of phosphorus is essential for managing aquatic systems and preventing issues like eutrophication.

Phosphorus enters aquatic environments through weathering, decomposition, and human activities. It exists in various forms, including dissolved inorganic and organic phosphorus, as well as particulate phosphorus. Organisms uptake and assimilate phosphorus, while regeneration processes recycle it back into the water column.

Phosphorus sources and sinks

• Phosphorus enters and leaves aquatic systems through various sources and sinks, influencing the overall phosphorus budget and productivity of the ecosystem
• Understanding the major sources and sinks of phosphorus is crucial for managing water quality and preventing eutrophication in limnological systems

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• Chemical and physical weathering of phosphorus-bearing rocks (apatite) releases dissolved phosphorus into aquatic systems
• Weathering rates depend on factors such as rock type, climate, and hydrological conditions
• Glacial erosion and transport of sediments can also contribute to phosphorus inputs in some regions

Decomposition of organic matter

• Microbial decomposition of dead plant and animal material releases phosphorus back into the water column
• Breakdown of organic phosphorus compounds (phospholipids, nucleic acids) by enzymes (phosphatases) produces dissolved inorganic and organic phosphorus
• The rate of decomposition is influenced by temperature, oxygen availability, and the composition of the organic matter

Anthropogenic inputs

• Human activities can significantly increase phosphorus loads to aquatic systems through point and non-point sources
• Point sources include wastewater treatment plants, industrial discharges, and concentrated animal feeding operations (CAFOs)
• Non-point sources encompass agricultural runoff (fertilizers, manure), urban stormwater, and septic systems
• Atmospheric deposition of phosphorus from fossil fuel combustion and dust can also contribute to anthropogenic inputs

Phosphorus forms in aquatic systems

• Phosphorus exists in various forms in aquatic systems, each with different bioavailability and ecological roles
• The distribution and transformation of phosphorus forms are influenced by physical, chemical, and biological processes

Dissolved inorganic phosphorus

• Dissolved inorganic phosphorus (DIP) is the most bioavailable form for aquatic organisms, primarily existing as orthophosphate ions (PO4^3-, HPO4^2-, H2PO4^-)
• DIP concentrations are typically low in surface waters due to rapid uptake by phytoplankton and adsorption to particles
• Soluble reactive phosphorus (SRP) is a commonly measured fraction of DIP that includes both orthophosphate and some easily hydrolyzable organic phosphorus compounds

Dissolved organic phosphorus

• Dissolved organic phosphorus (DOP) consists of a diverse array of organic molecules containing phosphorus (phosphate esters, phosphonates)
• DOP is less bioavailable than DIP and requires enzymatic hydrolysis or photochemical degradation to release orthophosphate
• Some aquatic organisms (bacteria, phytoplankton) can directly utilize certain DOP compounds as a phosphorus source

Particulate phosphorus

• Particulate phosphorus (PP) includes both inorganic and organic phosphorus associated with suspended particles and sediments
• Inorganic PP is primarily composed of phosphorus adsorbed to iron, aluminum, and calcium minerals or incorporated into mineral lattices
• Organic PP encompasses phosphorus in living organisms (phytoplankton, bacteria) and detrital material
• PP can settle out of the water column and become a part of the sediment pool, potentially acting as a long-term phosphorus sink or source

Phosphorus uptake and assimilation

• Aquatic organisms acquire and incorporate phosphorus into their biomass through various uptake and assimilation processes
• The efficiency and rate of phosphorus uptake and assimilation influence primary productivity and nutrient cycling in aquatic ecosystems

Uptake by phytoplankton

• Phytoplankton are the primary producers in aquatic systems and require phosphorus for growth and metabolism
• Phosphorus uptake by phytoplankton occurs mainly through active transport of orthophosphate ions across cell membranes
• Phytoplankton can also utilize some dissolved organic phosphorus compounds through enzymatic hydrolysis at the cell surface
• Uptake rates depend on factors such as phosphorus concentration, light availability, and phytoplankton community composition

Uptake by macrophytes

• Macrophytes (aquatic plants) obtain phosphorus from both the water column and sediments, depending on the species and environmental conditions
• Rooted macrophytes can access sediment pore water phosphorus through their root systems, while free-floating macrophytes rely on water column phosphorus
• Macrophytes can store excess phosphorus in their tissues (luxury uptake) and release it back to the water column upon senescence and decomposition

Microbial assimilation

• Bacteria and other microorganisms play a crucial role in phosphorus cycling by assimilating both inorganic and organic phosphorus compounds
• Microbial uptake of phosphorus is driven by the need for cellular growth and energy production (ATP synthesis)
• Bacteria can compete with phytoplankton for available phosphorus, especially under low-nutrient conditions
• Microbial biomass serves as a temporary phosphorus sink and can transfer phosphorus to higher trophic levels through the microbial loop

Phosphorus regeneration and release

• Phosphorus regeneration and release processes recycle phosphorus back into the water column, making it available for biological uptake
• The balance between phosphorus uptake and regeneration determines the net productivity and trophic state of aquatic ecosystems

Mineralization processes

• Mineralization is the conversion of organic phosphorus to inorganic forms through microbial decomposition and enzymatic hydrolysis
• Phosphatase enzymes (alkaline phosphatase, 5'-nucleotidase) produced by bacteria and phytoplankton break down organic phosphorus compounds
• The rate of mineralization depends on factors such as temperature, oxygen availability, and the quality of organic matter
• Mineralization can occur in both the water column and sediments, with higher rates typically observed in the sediments due to greater microbial activity

Sediment-water interactions

• Phosphorus exchange between sediments and the overlying water column plays a significant role in phosphorus cycling and internal loading
• Phosphorus can be released from sediments through desorption, dissolution of minerals (iron, calcium), and decomposition of organic matter
• The release of phosphorus from sediments is influenced by redox conditions, pH, and the presence of iron and sulfur
• Anoxic conditions in the sediments can lead to the reduction of iron oxyhydroxides and subsequent release of adsorbed phosphorus into the water column

Internal loading vs external loading

• Internal loading refers to the release of phosphorus from sediments or other in-lake sources (macrophyte senescence, fish excretion) into the water column
• External loading encompasses phosphorus inputs from the surrounding watershed, including point and non-point sources
• The relative importance of internal and external loading varies among aquatic systems and can shift over time as external loads are reduced
• Internal loading can delay the recovery of eutrophic systems even after external loads have been controlled, necessitating in-lake management strategies

Phosphorus limitation in aquatic ecosystems

• Phosphorus limitation occurs when the availability of phosphorus restricts the growth and productivity of aquatic organisms, particularly phytoplankton
• Understanding phosphorus limitation is crucial for predicting ecosystem responses to nutrient enrichment and developing management strategies

Liebig's law of the minimum

• Liebig's law states that the growth of organisms is limited by the nutrient that is in shortest supply relative to the organism's needs
• In many freshwater systems, phosphorus is often the limiting nutrient due to its low natural abundance and high biological demand
• When phosphorus is limiting, increases in phosphorus concentration can lead to rapid phytoplankton growth and potential eutrophication

N:P ratios and nutrient limitation

• The ratio of nitrogen to phosphorus (N:P) in the water column can indicate which nutrient is likely to be limiting for phytoplankton growth
• Redfield ratio (16:1 N:P) is often used as a reference point, with lower ratios suggesting potential phosphorus limitation and higher ratios indicating nitrogen limitation
• However, the optimal N:P ratio varies among phytoplankton species and can be influenced by other factors such as light and micronutrient availability

Phosphorus as a limiting nutrient

• Phosphorus limitation is common in many freshwater systems, including lakes, reservoirs, and streams
• The low solubility of phosphorus-bearing minerals and the lack of a significant gaseous phase contribute to its role as a limiting nutrient
• Phosphorus limitation can lead to shifts in phytoplankton community composition, favoring species with high phosphorus uptake affinity or the ability to utilize organic phosphorus
• Managing phosphorus inputs is often a key strategy for controlling eutrophication and maintaining water quality in phosphorus-limited systems

Eutrophication and phosphorus management

• Eutrophication is the excessive growth of algae and other aquatic plants due to nutrient enrichment, often leading to water quality degradation
• Phosphorus management is crucial for preventing and mitigating eutrophication in aquatic ecosystems

Causes and consequences of eutrophication

• Eutrophication is primarily caused by excessive inputs of phosphorus and nitrogen from anthropogenic sources (agriculture, wastewater, urban runoff)
• Consequences of eutrophication include algal blooms, reduced water clarity, oxygen depletion (hypoxia), fish kills, and loss of biodiversity
• Harmful algal blooms (HABs) can produce toxins that pose risks to human and animal health, as well as disrupt recreational activities and water supply
• Eutrophication can also lead to shifts in food web structure and alter the cycling of nutrients and carbon in aquatic ecosystems

Phosphorus control strategies

• Phosphorus control strategies aim to reduce both external and internal phosphorus loads to prevent or reverse eutrophication
• Point source controls involve improving wastewater treatment (tertiary treatment, phosphorus precipitation) and implementing discharge limits
• Non-point source controls focus on reducing phosphorus runoff from agricultural and urban areas through best management practices (BMPs)
• In-lake measures, such as phosphorus inactivation (alum treatment) and biomanipulation (fish stock management), can address internal phosphorus loading

Best management practices for phosphorus reduction

• Agricultural BMPs include nutrient management planning, conservation tillage, cover crops, and riparian buffer strips to minimize phosphorus runoff
• Urban BMPs involve stormwater management (retention ponds, constructed wetlands), low-impact development (permeable pavement, green roofs), and public education
• Watershed-scale approaches, such as total maximum daily load (TMDL) programs, set phosphorus reduction targets and coordinate efforts among stakeholders
• Integrated basin management considers the interactions between land use, water resources, and socioeconomic factors to develop holistic phosphorus control strategies

Phosphorus cycling in different aquatic systems

• The phosphorus cycle varies among different types of aquatic systems due to differences in hydrology, geomorphology, and biological communities
• Understanding the unique characteristics of phosphorus cycling in each system is essential for effective management and conservation

Lakes and reservoirs

• Phosphorus cycling in lakes and reservoirs is influenced by thermal stratification, mixing, and sediment-water interactions
• Epilimnetic phosphorus concentrations are often low during summer stratification due to uptake by phytoplankton and sedimentation
• Internal phosphorus loading from anoxic hypolimnetic sediments can be a significant source of phosphorus, especially in eutrophic systems
• Reservoirs may have higher phosphorus retention than natural lakes due to longer water residence times and greater sedimentation rates

Rivers and streams

• Phosphorus cycling in rivers and streams is characterized by downstream transport, sorption to suspended sediments, and interactions with the benthic zone
• Phosphorus retention in rivers is influenced by factors such as flow velocity, channel morphology, and the presence of riparian vegetation
• Uptake by benthic algae and microbes in the hyporheic zone can be a significant phosphorus sink in low-order streams
• Phosphorus spiraling concept describes the downstream transport and recycling of phosphorus in river networks

Wetlands and estuaries

• Wetlands and estuaries are transition zones between terrestrial and aquatic ecosystems, playing a crucial role in phosphorus transformation and retention
• Wetlands can act as phosphorus sinks through sedimentation, sorption to soil particles, and uptake by macrophytes and microbes
• Phosphorus cycling in estuaries is influenced by the mixing of freshwater and seawater, leading to the formation of phosphorus-rich turbidity maximum zones
• Tidal flushing and sediment resuspension can affect phosphorus dynamics in estuaries, while coastal wetlands (salt marshes, mangroves) can serve as phosphorus buffers

Phosphorus biogeochemistry and transformations

• Phosphorus biogeochemistry involves the complex interactions between biological, chemical, and physical processes that govern phosphorus cycling
• Understanding phosphorus transformations is crucial for predicting ecosystem responses to environmental changes and management interventions

Abiotic vs biotic processes

• Abiotic processes, such as adsorption-desorption, precipitation-dissolution, and sedimentation, influence phosphorus dynamics in aquatic systems
• Biotic processes, including uptake, assimilation, and mineralization by aquatic organisms, play a key role in phosphorus cycling
• The relative importance of abiotic and biotic processes varies among aquatic systems and can change over spatial and temporal scales
• Interactions between abiotic and biotic processes, such as the formation of metal-phosphorus complexes and the role of microbes in mineral dissolution, contribute to phosphorus transformations

Redox conditions and phosphorus dynamics

• Redox (reduction-oxidation) conditions strongly influence phosphorus speciation, solubility, and bioavailability in aquatic systems
• Under oxic conditions, phosphorus is often bound to iron, aluminum, and calcium minerals, limiting its availability to aquatic organisms
• Anoxic conditions can lead to the reductive dissolution of iron oxyhydroxides, releasing adsorbed phosphorus into the water column
• Sulfate reduction in anoxic sediments can lead to the formation of iron sulfides, further enhancing phosphorus release
• Redox-mediated phosphorus cycling is particularly important in seasonally stratified lakes and coastal systems with oxygen-depleted bottom waters

Phosphorus speciation and bioavailability

• Phosphorus speciation refers to the different chemical forms of phosphorus present in aquatic systems, each with distinct bioavailability and reactivity
• Orthophosphate is the most bioavailable form of phosphorus, readily taken up by phytoplankton and other aquatic organisms
• Organic phosphorus compounds, such as phosphate esters and phosphonates, require enzymatic hydrolysis or photochemical degradation to release bioavailable phosphorus
• Condensed phosphates (pyrophosphate, polyphosphate) are used by some microorganisms for energy storage and can be hydrolyzed to orthophosphate
• The bioavailability of particulate phosphorus depends on factors such as mineral composition, surface area, and redox conditions

Phosphorus budgets and mass balance

• Phosphorus budgets quantify the inputs, outputs, and storage of phosphorus within an aquatic system, providing insights into the overall phosphorus dynamics
• Mass balance approaches are used to assess the relative importance of different phosphorus sources and sinks and to evaluate management strategies

Inputs, outputs, and storage

• Phosphorus inputs to aquatic systems include external loading from the watershed (point and non-point sources) and atmospheric deposition
• Outputs of phosphorus occur through outflow (surface and groundwater), sedimentation, and biological uptake and removal (fish harvest, macrophyte removal)
• Phosphorus storage in aquatic systems includes water column, sediments, and biota (phytoplankton, macrophytes, fish)
• The balance between inputs, outputs, and storage determines the net phosphorus accumulation or depletion in an aquatic system over time

Retention and export of phosphorus

• Phosphorus retention refers to the amount of phosphorus that is retained within an aquatic system relative to the total input
• Retention mechanisms include sedimentation, adsorption to particles, and biological uptake and storage
• Phosphorus export occurs through outflow and biological removal, and is influenced by factors such as water residence time, flow regime, and ecosystem productivity
• The retention-export balance affects the downstream transport of phosphorus and the potential for eutrophication in receiving water bodies

Modeling phosphorus dynamics in aquatic systems

• Phosphorus models are used to simulate and predict phosphorus dynamics in aquatic systems, aiding in the understanding of ecosystem processes and the evaluation of management scenarios
• Empirical models, such as the Vollenweider model and the Dillon-Rigler model, relate phosphorus loading to in-lake phosphorus concentrations and trophic state
• Mechanistic models, such as the SWAT (Soil and Water Assessment Tool) and the WASP (Water Quality Analysis Simulation Program), incorporate detailed biogeochemical processes and spatial heterogeneity
• Coupled hydrodynamic-ecological models, like the CE-QUAL-W2 and the ELCOM-CAEDYM, integrate physical and biogeochemical processes to simulate phosphorus dynamics in complex systems
• Bayesian networks and machine learning approaches are emerging as tools for modeling phosphorus dynamics and supporting decision-making in aquatic ecosystem management