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