Carbon

Organic Carbon

carbon from or in organisms

Inorganic Carbon

CO₂ (carbonic acid), carbonate (HCO₃^-), and bicarbonate (CO₃^2-)

Photosynthesis

CO₂ + H₂O (sunlight + chlorophyll) → C₆H₁₂O₆ + O₂

inorganic C → organic C

Respiration

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

are freshwaters or oceans more important in the global C cycle (while recognizing that there is extreme uncertainty)?

freshwaters

sources of organic C in lakes

1. Input from streams (allochthonous)

2. Production by phytoplankton (autochthonous)

sources of inorganic C in lakes

1. Atmosphere (CO2) diffusion

2. Respiration by consumers

Organic C processes in lakes

Respiration of organic C

Inorganic C processes in lakes

Primary production

Organic C sinks in lakes

1. Sedimentation and burial

2. Export (transport) to downstream

Inorganic C sinks in lakes

1. Photosynthesis (taking up CO₂)

2. Export (transport) to downstream

3. Atmosphere (release of CO₂)

Phytoplankton Production >> Respiration

net sink (ex: Duck Pond)

Phytoplankton Production << Respiration

net source (ex: Pandapas)

are most freshwater sources or sinks?

sources (emit CO₂ to atmosphere)

Methane (CH₄)

25-34x more potent of a greenhouse gas than CO2

How is methane produced?

via respiration under anoxic conditions:

1. Anoxic sediments

2. Anoxic bottom waters

Two main methane release processes to atmosphere:

1. Diffusion

2. Ebullition (bubbles)

Methane accumulates in anoxic hypolimnion during summer stratified period and is released to atmosphere during ____ ____________

fall turnover

Where are lakes that are substantial sources of methane to the atmosphere?

In the Arctic

do freshwaters or oceans bury more C?

freshwaters

do freshwaters or terrestrial ecosystems emit more greenhouse gases?

freshwaters

From ice cores, we can see our current high CO2 levels are ___________

extraordinary

Carbon dioxide (CO₂) — inorganic carbon

very soluble and reacts with water to form carbonic acid

carbon dioxide - carbonic acid - bicarbonate ion process

C reacts with water to make carbonic acid: CO₂ + H₂O <-> H₂CO₃

Carbonic acid dissociates: H₂CO₃ <-> H⁺ + HCO₃^-

Bicarbonate dissociates: HCO₃^- <-> H⁺ + HCO₃^-2

Dissolved inorganic carbon (DIC)

= sum of carbon dioxide (carbonic acid) + bicarbonate + carbonate

carbonate equilibrium is formed by…

chemical conversions between carbon dioxide, carbonic acid, and bicarbonate

bicarbonate equilibrium is coupled to ___

pH

as the proportion of _____________ ions change, the pH will respond

inorganic C

Weathering of carbonate rocks (e.g., limestone)

The presence of carbonate and bicarbonate buffers the solution against pH change

Limestone buffers acid rain by increasing water’s ___________ (acid buffering capacity)

alkalinity

If carbonate or bicarbonate are not present, pH can…

change rapidly

Very high photosynthesis can…

deplete CO₂ and cause CaCO₃ to precipitate (encrust marl on aquatic plants/algae)

marl

calcareous deposits

lake whiting

water suddenly becomes chalky white due to the precipitation of calcium carbonate (CaCO₃) particles

what is required for lake whiting to happen?

BOTH high background levels of calcium carbonate + rapid drawdown of CO₂

Why is Lake Superior’s pH going up?

Warming temp increases algae (chlorophyll), it takes up more CO₂, and pH increases because there’s not enough calcium carbonate to buffer pH change. Using up CO₂ leaves just HCO₃ and CO₃ in solution, causing higher pH

The C cycle in freshwaters is expected to change with _________ change

climate

If alkalinity is high…

algal blooms can cause marl deposits

If there is low alkalinity…

algal blooms can increase pH