Organic Carbon Cycling

Forms of Organic Matter (OM)

Dissolved Organic Matter (DOM): The component of organic matter which is dissolved in water.
Particulate Organic Matter (POM): The component of organic matter that exists in particulate form.
Total Organic Matter (OM): Encompasses both DOM and POM.
Total Organic Carbon (TOC): Refers to the total amount of carbon in organic compounds, comprising dissolved organic carbon (DOC) and particulate organic carbon (POC).
Other Elements in OM: Includes nitrogen (N), phosphorus (P), oxygen (O), sulfur (S), and hydrogen (H).

Visual Representation of Organic Matter

Figure 28-1: Illustrates the forms of organic matter in aquatic ecosystems, showing the relationships among DOM and POM. the distinction between TOC (both DOC and POC) and the presence of other elements within these components.
Figure 28-2: Depicts the molecular weights of different organic matter found in aquatic ecosystems, including humic acids (HAC), fulvic acids (FA), hydrocarbons (HC), carbohydrates (CHO), fatty acids (FA), and amino acids (AA). Sizes of various organic matter components are provided, indicating their classification based on size from larger organisms (like zooplankton) down to dissolved components.

Dissolved Organic Carbon (DOC) Concentration Ranges

Figure 28-3: Shows the range of DOC concentrations within tributaries of the Moise River in Quebec, Canada. Ranges from 2 to 50 mg C L-1. A comparison of average DOC and water color across lakes in northern Michigan is also provided, highlighting DOC and color measurements.

DOC and POC

Median Organic Carbon Content in Aquatic Ecosystems

Table 23-1: Displays median values of total organic carbon (TOC), dissolved organic carbon (DOC), particulate organic carbon (POC), and the DOC:POC ratios across different habitats:
- Open Ocean: 0.5 mg L⁻¹ (DOC: 0.45 mg L⁻¹, POC: 0.05 mg L⁻¹, Ratio: 9:1)
- Ground Water: 0.7 mg L⁻¹ (DOC: 0.65 mg L⁻¹, POC: 0.05 mg L⁻¹, Ratio: 13:1)
- Precipitation: 1.1 mg L⁻¹ (DOC: 1.0 mg L⁻¹, POC: 0.1 mg L⁻¹, Ratio: 10:1)
- Oligotrophic Lakes: 2.2 mg L⁻¹ (DOC: 2.0 mg L⁻¹, POC: 0.2 mg L⁻¹, Ratio: 10:1)
- Rivers: 7.0 mg L⁻¹ (DOC: 5.0 mg L⁻¹, POC: 2.0 mg L⁻¹, Ratio: 3:1)
- Eutrophic Lakes: 12.0 mg L⁻¹ (DOC: 10.3 mg L⁻¹, POC: 1.7 mg L⁻¹, Ratio: 6:1)
- Wetlands-Marshes: 17.0 mg L⁻¹ (DOC: 15.3 mg L⁻¹, POC: 1.7 mg L⁻¹, Ratio: 9:1)
- Bog Water: 33.0 mg L⁻¹ (DOC: 30.3 mg L⁻¹, POC: 2.7 mg L⁻¹, Ratio: 11:1)
- World Average (Surface Waters): 6.5 mg L⁻¹ (DOC: 0.6 mg L⁻¹, Ratio: 10:1)

DOC Concentration Across Various Ecosystems

Figure 28-10: Shows boxplots representing dissolved organic carbon (DOC) concentrations across a continuum from wetlands to the ocean, based on data from over 12,000 unique measurements. It indicates significant variability in DOC levels across different environments.
Figure 28-11: Demonstrates the concentrations of particulate organic carbon (POC) at over 1100 gauge stations along rivers in the United States, indicating the spatial dynamics of POC distribution.

Model of Carbon Movement in Ecosystems

Figure 23-2: Simplified compartment model illustrating organic matter transport within a stream ecosystem, detailing the processes of leaching, metabolism, respiration, and the role of various organisms. Identifies key pathways of organic matter movement, from upstream inputs through to downstream ecosystems.

Sources of Organic Carbon

Watershed and Stream Contributions

Figure 23-1: Identifies potential sources of dissolved organic carbon (DOC) and hydrologic linkages in watershed contexts and floodplain/channel spatial scales, indicating the flow of organic matter from upland areas into aquatic systems.

Dynamics of DOC Movement

Figure 23-3: Conceptualizes the lateral and vertical boundaries of running water ecosystems, delineating flow pathways of dissolved organic matter and inorganic solutes from vegetative sources within stream ecosystems.

Net Ecosystem Production (NEP)

Figure 28-18: Outlines the cycling and fates of both autochthonous (produced within the ecosystem) and allochthonous (imported into the ecosystem) carbon. This diagram illustrates components such as gross primary production (GPP), net primary production (NPP), and various forms of ecosystem respiration.

Heterotrophic Respiration and DOC Flux

Figure 28-21: Demonstrates the relationship between carbon fluxes and heterotrophic respiration in a specific stream, noted over a time series that spans from May 2007 to July 2008. This graph tracks variations in flow and respiration rates over time.

C Sources in Reservoirs and Streams

Figure 23-4: Provides a generalized overview of the contributions of allochthonous organic matter in transitional aquatic systems, highlighting the productivity levels of different algal and plant forms as rivers transition into reservoirs.

Ecosystem Processes: GPP and ER

Figure 28-23 (a): Illustrates daily trends in gross primary production (GPP) and ecosystem respiration (ER), emphasizing the influence of storm events on these metrics in a temporal context spanning two years. (b) Shows the calculations of net ecosystem production (NEP), including storm discharge events.

Organic Carbon Fluxes

Figure 28-26: Illustrates dominant organic carbon fluxes within aquatic ecosystems, explaining processes such as emissions, sediment storage/burial, and organic carbon export.

Organic Carbon Spiraling in Streams

Figure 28-27: Visual representation of spiraling metrics used to evaluate organic carbon dynamics in streams, depicting interactions from upstream inputs through to downstream transport and respiration.

Changes in ER and GPP Along the River Continuum

Figure 28-28: Predictive insights into carbon transport and emission sources across the headwater to large river continuum. Highlights the contrasting dynamics of CO₂ emissions from small streams versus larger rivers based on metabolism and respiration processes.

Sources of Dissolved Organic Carbon in Lakes

Figure 23-8: Breakdown of contributions to the dissolved organic carbon (DOC) pool by various producers in lakes with increasing fertility.

Depth-Time Distribution of Organic Matter

Figures 23-7 and 23-10: Present the depth-time profiles of dissolved organic carbon (DOC) and particulate organic carbon (POC) over specified locations and timeframes, allowing for analysis of seasonal trends and organic matter distribution in lakes.

Recalcitrant and Labile Organic Carbon Sources

Figure 23-9: Categorizes common organic carbon sources based on their rates of mineralization, distinguishing between labile (LDOC) and recalcitrant (RDOC) dissolved organic carbon from different types of algae and higher vascular plants.

Contributions of Allochthonous and Autochthonous Carbon

Figure 23-13: Shows relative contributions of different sources of particulate organic carbon (POC) in lakes of varying fertility, demonstrating shifts in productivity types.

Humic Substances and Their Role

Figure 23-21: Represents the pathways of humic substances in the context of decomposition from higher plant tissues, detailing the interactions with enzymes and UV light, influencing the stability and utilization of humic compounds over time.

Sources of Recalcitrant Dissolved Organic Matter

Figure 23-28: Highlights lignocellulose as a key contributor to recalcitrant dissolved organic matter and its connection to metabolic processes across terrestrial and aquatic boundaries.

Climate Change Impacts on Organic Carbon Dynamics

Figure 23-31: Provides a longitudinal perspective on global CO₂ concentrations and temperature anomalies, linking changes in dissolved organic carbon (DOC) dynamics to broader climatic trends and anthropogenic influences.

Conclusion

Understanding organic carbon cycling is crucial in assessing ecosystem health and function, particularly concerning nutrient dynamics, carbon storage, and responses to climatic changes influencing aquatic environments.