3.1 Carbon cycle

The carbon cycle in aquatic ecosystems is a complex interplay of processes that move and transform carbon through various pools. It involves atmospheric exchange, terrestrial inputs, and biological activities like primary production and respiration.

Understanding this cycle is crucial for limnologists as it impacts nutrient dynamics, ecosystem productivity, and greenhouse gas emissions. The balance of carbon inputs, transformations, and outputs shapes the overall health and functioning of aquatic systems.

Carbon cycle overview

• The carbon cycle in aquatic ecosystems involves the movement and transformation of carbon through various pools and processes
• Carbon enters aquatic systems through atmospheric exchange, terrestrial runoff, and primary production, and is lost through respiration, sedimentation, and outgassing
• Understanding the carbon cycle is crucial for limnologists as it influences nutrient dynamics, ecosystem productivity, and greenhouse gas emissions

Dissolved inorganic carbon

Sources of dissolved inorganic carbon

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• Atmospheric CO2 dissolution into water, which is influenced by temperature and pressure
• Weathering of carbonate and silicate rocks in the watershed, releasing bicarbonate and carbonate ions
• Respiration by aquatic organisms, which converts organic carbon to CO2
• Volcanic activity and hydrothermal vents, which can release CO2 and other gases into aquatic systems

Carbonate system equilibrium

• Dissolved inorganic carbon exists in three main forms: CO2 (aq), bicarbonate (HCO3-), and carbonate (CO32-)
• The relative proportions of these forms are determined by the pH of the water, with lower pH favoring CO2 and higher pH favoring carbonate
• The carbonate system acts as a buffer, resisting changes in pH and maintaining a stable chemical environment for aquatic organisms
• Changes in the carbonate system equilibrium can affect the availability of carbon for photosynthesis and the solubility of calcium carbonate (important for shell-forming organisms)

Dissolved organic carbon

Allochthonous vs autochthonous sources

• Allochthonous dissolved organic carbon (DOC) originates from outside the aquatic system, such as terrestrial plant and soil organic matter transported by runoff and leaching
• Autochthonous DOC is produced within the aquatic system, primarily by phytoplankton and macrophytes through exudation and cell lysis
• The relative importance of allochthonous and autochthonous DOC sources varies among aquatic systems, with allochthonous sources often dominating in small, forested streams and autochthonous sources being more important in large, productive lakes

Lability of dissolved organic carbon

• The lability of DOC refers to its susceptibility to microbial decomposition and utilization
• Highly labile DOC (e.g., simple sugars and amino acids) is rapidly consumed by heterotrophic bacteria, fueling microbial respiration and growth
• Refractory DOC (e.g., humic substances and lignin) is more resistant to microbial degradation and can persist in aquatic systems for longer periods
• The lability of DOC influences its role in aquatic food webs, with labile DOC supporting higher bacterial production and potentially transferring energy to higher trophic levels

Particulate organic carbon

Sources of particulate organic carbon

• Phytoplankton and macrophyte biomass, which can contribute to particulate organic carbon (POC) through cell death and fragmentation
• Terrestrial plant debris (e.g., leaves and wood) transported into aquatic systems by wind, runoff, and erosion
• Fecal pellets and molts produced by zooplankton and other aquatic invertebrates
• Flocculation of dissolved organic matter, which can form particulate aggregates

Sedimentation of particulate organic carbon

• POC can settle out of the water column and accumulate in sediments through the process of sedimentation
• Sedimentation rates are influenced by factors such as particle size, density, and water column turbulence
• Benthic invertebrates and microbes can consume and decompose settled POC, releasing nutrients and CO2 back into the water column
• Burial of POC in sediments represents a long-term carbon sink, potentially removing carbon from the active cycle for hundreds to thousands of years

Primary production

Factors influencing primary production

• Light availability, which is affected by water depth, turbidity, and seasonal changes in solar radiation
• Nutrient concentrations, particularly nitrogen and phosphorus, which are essential for phytoplankton growth
• Water temperature, which influences enzymatic reaction rates and can affect phytoplankton community composition
• Grazing by zooplankton and other herbivores, which can control phytoplankton biomass and influence species dominance

Measuring primary production rates

• Primary production rates can be measured using the light-dark bottle method, which compares oxygen production in clear and darkened bottles incubated in situ
• The 14C method involves adding radioactive bicarbonate to water samples and measuring the incorporation of 14C into phytoplankton biomass
• Chlorophyll a concentrations can be used as a proxy for phytoplankton biomass and potential primary production
• Advances in fluorometry and remote sensing have enabled high-resolution measurements of primary production at various spatial and temporal scales

Respiration

Bacterial respiration

• Heterotrophic bacteria play a key role in aquatic respiration, consuming organic carbon and releasing CO2
• Bacterial respiration rates are influenced by factors such as temperature, organic carbon availability, and nutrient concentrations
• In many aquatic systems, bacterial respiration can account for a significant portion of total ecosystem respiration
• The balance between bacterial respiration and primary production helps determine whether an aquatic system is a net source or sink of carbon

Zooplankton and fish respiration

• Zooplankton and fish contribute to aquatic respiration through their metabolic activities
• Respiration rates of zooplankton and fish are influenced by factors such as body size, temperature, and activity level
• Zooplankton respiration can be a significant component of total water column respiration, particularly in productive systems with high zooplankton biomass
• Fish respiration can be an important source of CO2 in aquatic systems, especially in shallow, warm waters with high fish densities

Carbon burial

Factors affecting carbon burial

• Sedimentation rates, which determine the amount of organic carbon delivered to the sediments
• Oxygen availability in sediments, which influences the rate of organic matter decomposition and preservation
• Sediment grain size and composition, with fine-grained, clay-rich sediments generally having higher carbon burial efficiencies than sandy sediments
• Bioturbation by benthic invertebrates, which can enhance oxygenation and decomposition of buried organic matter

Long-term carbon sequestration

• Carbon burial in aquatic sediments represents a long-term carbon sink, potentially storing carbon for hundreds to millions of years
• The efficiency of carbon sequestration depends on factors such as sedimentation rates, organic matter preservation, and the stability of the depositional environment
• Aquatic systems with high carbon burial rates (e.g., productive coastal wetlands and eutrophic lakes) can play a significant role in mitigating atmospheric CO2 levels
• However, the capacity for long-term carbon sequestration in aquatic systems is limited compared to terrestrial ecosystems, and can be affected by anthropogenic disturbances and climate change

Methane production

Methanogenesis in anoxic sediments

• Methanogenesis is the microbial production of methane (CH4) in anoxic environments, such as lake and wetland sediments
• Methanogenic archaea use organic compounds (e.g., acetate and CO2) as substrates for methane production, often in syntrophy with fermentative bacteria
• The rate of methanogenesis is influenced by factors such as temperature, organic matter supply, and the availability of alternative electron acceptors (e.g., sulfate and nitrate)
• Methane production in aquatic sediments can be a significant source of atmospheric CH4, a potent greenhouse gas

Methane oxidation and emission

• Methane produced in anoxic sediments can be oxidized by methanotrophic bacteria in the presence of oxygen, converting CH4 to CO2
• Methane oxidation can occur in the sediment-water interface or in the water column, depending on the oxygen penetration depth and mixing conditions
• The efficiency of methane oxidation determines the proportion of methane that is released to the atmosphere versus being converted to CO2
• Factors such as water depth, stratification, and ebullition (bubbling) can influence the emission of methane from aquatic systems to the atmosphere

Carbon dioxide evasion

Factors influencing CO2 evasion

• The partial pressure difference of CO2 between the water and the atmosphere, which drives the direction and magnitude of gas exchange
• Wind speed and surface turbulence, which enhance gas transfer by increasing the surface area and renewing the boundary layer
• Water temperature, which affects the solubility of CO2 and can influence the metabolic balance of the aquatic system
• Alkalinity and pH, which determine the speciation of dissolved inorganic carbon and the buffering capacity of the water

Estimating CO2 evasion rates

• Direct measurements of CO2 evasion can be made using floating chambers or eddy covariance techniques
• Indirect estimates can be obtained by measuring the partial pressure of CO2 in the water and atmosphere and applying gas transfer models
• The choice of gas transfer velocity parameterization can significantly influence the estimated CO2 evasion rates
• Scaling up CO2 evasion estimates to larger spatial and temporal scales requires accounting for variability in surface water CO2 concentrations and gas transfer velocities

Terrestrial-aquatic carbon linkages

Watershed influences on carbon inputs

• The characteristics of the surrounding watershed (e.g., land use, vegetation cover, and soil type) can strongly influence the quantity and quality of carbon inputs to aquatic systems
• Forested watersheds generally export more dissolved organic carbon to streams and lakes compared to agricultural or urban watersheds
• Watershed hydrology (e.g., runoff patterns and groundwater inputs) can affect the timing and magnitude of carbon delivery to aquatic systems
• Disturbances in the watershed (e.g., deforestation, wildfires, and urbanization) can alter the balance of carbon inputs and processing in aquatic systems

Wetland carbon cycling

• Wetlands are important transitional zones between terrestrial and aquatic ecosystems, and play a significant role in carbon cycling
• Wetland plants (e.g., emergent macrophytes and floating vegetation) can contribute to carbon fixation and storage through high primary production rates
• The anoxic conditions in wetland soils and sediments promote the accumulation and preservation of organic carbon
• Wetlands can act as both sources and sinks of carbon, depending on factors such as hydrologic connectivity, nutrient status, and disturbance regime

Anthropogenic impacts on carbon cycling

Eutrophication and carbon cycling

• Eutrophication, the excessive enrichment of aquatic systems with nutrients (primarily nitrogen and phosphorus), can significantly alter carbon cycling processes
• Increased nutrient loading can stimulate primary production, leading to higher phytoplankton biomass and potentially increased carbon fixation rates
• However, the subsequent decomposition of this excess organic matter can lead to oxygen depletion, shifts in ecosystem metabolism, and increased CO2 and CH4 emissions
• Eutrophication can also change the relative importance of different carbon pools (e.g., particulate vs. dissolved) and influence the export of carbon to downstream ecosystems

Climate change effects on carbon cycling

• Climate change is expected to have profound impacts on carbon cycling in aquatic ecosystems, through changes in temperature, precipitation, and other environmental factors
• Warmer water temperatures can increase metabolic rates, leading to higher primary production and respiration, and potentially shifting the balance between carbon fixation and release
• Changes in precipitation patterns and hydrologic regimes can alter the delivery of carbon from the watershed and the residence time of water in aquatic systems
• Rising atmospheric CO2 levels can influence the carbonate system equilibrium in aquatic systems, with potential consequences for pH, calcification, and primary production
• Climate change can also affect the distribution and phenology of aquatic organisms, with cascading effects on carbon cycling processes and ecosystem functioning