New Research Shows How Lakes Alter Carbon Emissions

Context

Lakes and inland water systems play a substantial and important part in the global carbon cycle, a new study reveals.

Key Highlights

• The scientists found that lake-rich stream networks emitted more carbon as CO₂ into the atmosphere than transported it downstream.
• The results highlight the importance of considering lakes and streams as part of a connected network, not as isolated ecosystems.
• The research covered:
  • 362 stream segments
  • 42 lakes
  • 32 nested Arctic stream-lake networks
  • Observations across four contrasting seasons
• The results of the study indicate that networks with more lakes hold water longer and emit more carbon to the atmosphere.

How Do Lakes Influence the Carbon Cycle?

• Longer Network Residence Time

  • Network residence time is the amount of time water spends within a connected stream and lake system.
  • For the studied networks in the Swedish Arctic, residence time varied from 42 minutes to 29 years.
  • The bulk of this variation in water residence time was due to lakes alone.
  • Lakes also retard the flow of water, giving more time for biological and chemical processing of the carbon.

• Mineralisation of Dissolved Organic Carbon

  • Dissolved Organic Carbon (DOC) is picked up from soils and ecosystems around the lake.
  • Increased water retention time allows more time for microbes to consume DOC.
  • Organic carbon is transformed into carbon dioxide (CO₂) in this process, called mineralisation.
  • The CO₂ can then be released from the surface of the lakes into the atmosphere.

• Impact on Carbon Export

  • In lake-rich networks:
  • Increased CO₂ emissions to the atmosphere.
  • Less carbon is conveyed downstream.
  • Carbon spends more time in the inland water system.
• Thus, lakes can significantly alter the balance between carbon emission and carbon export.

Why Should Lakes and Streams Be Studied Together?

• Lakes and streams have been traditionally studied separately as landscape features.
• Instead, they should be considered as part of a unified in-land water network, the research suggests.
• However, several carbon pathways should be taken into account in an integrated assessment, such as:
  • CO2 emissions to the air
  • Downstream carbon transport
  • Carbon storage
  • The transformation of DOC to CO₂.

Seasonal Impact on Carbon Emissions

• The study demonstrated that there were significant seasonal differences in carbon emissions and downstream export.
• Summer
  • Carbon emissions per downstream export were highest in summer.
  • The summer conditions are typically:
    • Higher temperatures
    • Lower stream flows
    • Drier conditions
    • Lower flows prolong the time of carbon in the network.
  • Microbial activity and DOC conversion to CO₂ also increase with increasing temperature.
• Spring and Autumn
  • The carbon emissions per unit of downstream export were relatively high in the spring and fall.
  • The more rapid the flow, the more carbon that will get carried downstream.

Significance for Climate Change

• Inland waters are gaining increasing recognition as players in the global carbon cycle.
• Climate Change affects the ability to change:
  • Temperature
  • Rainfall patterns
  • Stream flow
  • Lake levels
  • Water residence time
• Warming and a modification of the hydrological cycle in the Arctic may lead to higher processing of carbon and CO₂ emissions from inland water systems.
• Thus, lakes can become more significant in the future for the fate of carbon:
  • Stored,
  • Transported downstream, or
  • Let out in the air.

Key Terms

• Dissolved Organic Carbon (DOC)

  • Dissolved organic carbon, mostly from decaying plants, soils and other organic material, in solution in the water.

• Dissolved Inorganic Carbon (DIC)

  • Carbon present in inorganic forms such as dissolved CO₂, bicarbonate and carbonate.

• Mineralisation

  • Biological and chemical degradation of organic matter to simpler inorganic compounds, such as CO₂.

• Network Residence Time

  • The mean residence time of water in an interconnected inland water system.

Why is the Research Important?

• Underlines the under-appreciated contribution of inland waters to the global carbon budget.
• Demonstrates that lakes can act as a source or sink for carbon.
• Stresses the need for integrating ecosystem-based climate research.
• Gives insights on the possible effect of warming in the Arctic on carbon emissions.
• Proposes to model lakes and/or connected freshwater systems in climate models.

Way Forward

• Consider lakes, streams and rivers as part of the carbon cycle.
• Account for several carbon pathways for climate and ecosystem models.
• Enhance continuous monitoring of inland water carbon emissions.
• Expand understanding of the seasonal dynamics of carbon processing.
• Embed Arctic hydrology change in global climate projections.
• Increase research on role of freshwaters in global carbon budgeting.

Source: The DTE

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