Lecture 5 carbon

Freshwater Biology Overview

• Study focus: Organic Carbon in Freshwaters

Carbon Dynamics in Aquatic Systems

• Carbon Balance

  • Key processes: Photosynthesis vs. Respiration

  • Involves CO2, O2, and C

  • Is there external influence?

Sources of Carbon in Aquatic Systems

• Types of Carbon Sources:

  • Autochthonous: Produced within the aquatic system, e.g. algae, attached plants, periphyton.

  • Allochthonous: Produced outside the system, e.g. terrestrial plants.

  • Inorganic Sources: Inorganic bicarbonate (HCO3-), Atmospheric CO2.

Lake Typology Based on Carbon Balance

• Oligotrophic: Low productivity; relies primarily on autochthonous carbon.

• Mesotrophic: Intermediate productivity; mainly autochthonous.

• Eutrophic: High productivity; primarily autochthonous.

• Dystrophic: Low productivity; mainly allochthonous (often humic lakes).

Carbon in the Aquatic Medium

• Defined by size:

  • Particulate Material: > 0.2 µm

  • Soluble Components: < 0.2 µm

• Key forms:

  • POC: Particulate Organic Carbon

  • DOC: Dissolved Organic Carbon

  • Part of DOM (Dissolved Organic Matter) and POM (Particulate Organic Matter) including N & P.

  • Includes Dissolved Inorganic Carbon (DIC).

Role of Terrestrial Organic Matter

• Key Processes:

  • Surface and groundwater flow

  • Photosynthesis by benthic algae, phytoplankton, macrophytes.

  • Releases DOM through secretion and autolysis.

  • Formation of POM through flocculation and microbial metabolism.

  • Important note: Plants also respire.

Lake Metabolism Analysis

• Net Metabolism: Autotrophy vs. Heterotrophy.

  • Heterotrophic: Occurs when community respiration exceeds primary photosynthesis.

  • Indicates more organic carbon is consumed than produced by photosynthesis.

  • Sustained heterotrophy suggests organic C from outside the system is respired within it.

Seasonal Variability in Lakes

• CO2 Partial Pressures:

  • CO2 levels generally higher in lake surface than the atmosphere, indicating lakes as net sources of CO2.

  • Factors influencing seasonal dynamics: Autotrophy and heterotrophy.

Measurement Data and Variations

• Study conducted across 69 lakes (Cole et al. 1994).

• Variance in CO2 saturation impacts carbon source dynamics:

  • Net heterotrophy vs. net autotrophy

  • Measurement doesn’t consider lake sediment variability.

Deposition into Sediment Pool

• Forms of Deposition:

  • Organic matter and CaCO3.

  • Influenced by flocculation and sedimentation rates.

• Solubility varies: CaCO3 solubility increases in low temp & low pH.

Relationships with pH

• High photosynthesis leads to H+ ion uptake, increasing pH.

• In calcium-rich waters, increased precipitation of calcium carbonate can decrease pH.

Changes in Alkalinity and Chemical Reactions

• Key Reactions:

  • Photosynthesis and Respiration involve conservation of alkalinity.

  • Photosynthesis: CO2 + H2O → H+ + HCO3-

  • Respiration: Adds CO2 to the medium.

Open System Alkalinity Dynamics

• Reactions in Presence of CaCO3:

  • Respiration: Involves dissolution of CaCO3 and increased CO3²- leading to alkalinity rise.

  • Photosynthesis: Precipitates carbonates, decreasing alkalinity.

Carbon Limiting Factors

• Plant Uptake: CO2 or HCO3-.

• CO2 in water is relatively low leading to low diffusion rates (10,000 x less than air).

Plant Adaptations to Low CO2 Availability

• Mechanisms include:

  • CAM (Crassulacean Acid Metabolism): CO2 uptake at night.

  • Use of HCO3- (energy costly).

  • Heterophylly: Utilize both emergent and submerged leaves for aerial CO2 access.

Vertical Profiles of Carbon in Lakes

• Summer Profiles:

  • Oligotrophic lakes show slight CO2 increases at bottom.

  • Eutrophic lakes show lower CO2 at surface (due to phytoplankton) but high at bottom (decomposition).

Limnic Eruptions

• Rare events resulting from build-up of CO2, leading to catastrophic releases.

• Notable cases: Lake Monoun & Lake Nyos in Cameroon (1980s).

Humic Substances in Lakes

• Characteristics:

  • High molecular weight, derived from decomposition of terrestrial plants.

  • Comprise 40-60% DOM, refractory, and UV absorbent.

Impact of Humics on Lake Function

• Lakes rich in humics generally exhibit low diversity & productivity due to reduced light, acidity, and oxygen.

  • Humics can however provide organic carbon for heterotrophic bacteria, somewhat counteracting low productivity.

Measuring Water Clarity

• Secchi Disk Method:

  • Measures light penetration in lakes.

  • Critical for determining plant growth zones.

Global Role of Lakes in Carbon

• Understanding lake influence on nutrient cycles:

  • Lakes play significant role in carbon cycling globally, contradicting earlier beliefs that organics pass unchanged through freshwater.

Key Considerations

• Importance of investigating carbon dioxide's role in freshwater environments.

• Adaptations phytoplankton and macrophytes employ in low CO2 scenarios.