Biology · Ecosystems, Energy Flow & Biogeochemical Cycles
Carbon and water cycles across living and physical systems
A carbon atom can move from inorganic carbon into a producer, then into a consumer or detritus and back into inorganic forms. Water can move between air, land and surface water. Following a possible route is different from measuring every transfer or proving a small ecosystem is closed.
Begin with Organisms, Environments & Ecological Niches for ecological boundaries and scales, and Energy, Enzymes & Metabolism for chemical energy and respiration. Communities, Species Interactions & Food Webs explains why feeding relations form a web. That chapter is a reading preparation path once available, rather than a claim of an already published lesson. The budgets here introduce every symbol and require ordinary addition, subtraction and ratios. Advanced carbon-cycle measurement methods and chemical reaction mechanisms are not required.
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Use separate pool-and-transfer accounts for carbon and water, and distinguish a conceptual cycle from a closed local inventory.
Carbon moves between forms and places
Carbon occurs in organisms, detritus, dissolved substances, gases, minerals and other reservoirs. Photosynthetic carbon fixation and respiration connect selected living and inorganic pools. Feeding and mortality redistribute organic carbon. Geological and physical transfers can connect these processes to reservoirs outside an immediate ecological boundary.
The original four-pool inventory combines air/inorganic carbon, producers, consumers and detritus. It supplies photosynthetic transfer twenty, producer respiration eight, feeding six, producer litter five, consumer respiration four, consumer mortality two and detrital respiration seven. Every number counts carbon units during one interval, not energy or molecule counts.
A directed conceptual cycle shows possible transformations. A numerical inventory additionally states quantities, included pools and an interval. The supplied accounts must remain nonnegative and balanced, but the diagram does not imply every carbon atom completes a circuit before the interval ends or spends equal time in every pool.
Pool changes balance without identical stocks
Read the diagram in words
Inorganic carbon changes from one hundred to ninety-nine, producer carbon twenty to twenty-one, consumer carbon stays ten and detritus stays thirty. Total carbon stays one hundred and sixty; all seven arrows connect included pools.
| Combined inorganic carbon pool / Producer carbon / Consumer carbon / Detrital carbon | Beginning stock | Ending stock |
|---|---|---|
| Combined inorganic carbon pool | 100 | 99 |
| Producer carbon | 20 | 21 |
| Consumer carbon | 10 | 10 |
| Detrital carbon | 30 | 30 |
| Combined inorganic carbon pool / Producer carbon / Consumer carbon / Detrital carbon | Carbon fixation / Respiratory carbon return / Carbon transfer through feeding / Producer litter / Consumer mortality | Carbon units transferred during the interval |
|---|---|---|
| Combined inorganic carbon pool → Producer carbon | Carbon fixation | 20 |
| Producer carbon → Combined inorganic carbon pool | Respiratory carbon return | 8 |
| Producer carbon → Consumer carbon | Carbon transfer through feeding | 6 |
| Producer carbon → Detrital carbon | Producer litter | 5 |
| Consumer carbon → Combined inorganic carbon pool | Respiratory carbon return | 4 |
| Consumer carbon → Detrital carbon | Consumer mortality | 2 |
| Detrital carbon → Combined inorganic carbon pool | Respiratory carbon return | 7 |
| Beginning stock | Ending stock |
|---|---|
| 160 | 160 |
Complete closed original carbon-unit inventory over one interval. Air/inorganic is a combined conceptual pool, not all actual atmospheric chemistry; no empirical fluxes, molecule counts or energy units.
A water cycle also has storage
Water occupies atmospheric, surface, subsurface and biological reservoirs. Evaporation and transpiration move water into the atmosphere; precipitation, infiltration and runoff connect other parts of the system. Plants can take up water and return some to the atmosphere, while water also moves through physical pathways without first entering an organism.
The original three-pool account is a deliberately combined representation: air begins with ten water units, land/soil with thirty and surface water with twenty. Eight move from air to land as precipitation; five return as the combined evaporation/transpiration branch. Four move from land to surface water, and three from surface water to air.
The combined evaporation/transpiration branch does not estimate the separate mechanisms. The land pool combines compartments that a detailed investigation might split into surface, soil and groundwater. These choices keep the first account readable; they do not deny the omitted structures or show that all precipitation follows one route.
A stable pool can have opposing transfers
Read the diagram in words
Air ten loses eight and receives five plus three, ending ten. Land thirty gains eight and loses five plus four, ending twenty-nine. Surface twenty gains four and loses three, ending twenty-one. Total remains sixty.
| Atmospheric water / Combined land and soil water / Surface water | Beginning stock | Ending stock |
|---|---|---|
| Atmospheric water | 10 | 10 |
| Combined land and soil water | 30 | 29 |
| Surface water | 20 | 21 |
| Atmospheric water / Combined land and soil water / Surface water | Precipitation / Combined evaporation and transpiration / Land-to-surface transfer / Surface-water evaporation | Water units transferred during the interval |
|---|---|---|
| Atmospheric water → Combined land and soil water | Precipitation | 8 |
| Combined land and soil water → Atmospheric water | Combined evaporation and transpiration | 5 |
| Combined land and soil water → Surface water | Land-to-surface transfer | 4 |
| Surface water → Atmospheric water | Surface-water evaporation | 3 |
| Beginning stock | Ending stock |
|---|---|
| 60 | 60 |
Closed original water-unit inventory for one interval. Land/soil and evaporation/transpiration are deliberately combined categories; no precipitation probability, global reservoir size, individual-molecule residence time or actual landscape prediction.
A cycle label does not make every local boundary closed
The assigned carbon inventory closes within four pools and the water inventory within three: there are no additional boundary crossings in either toy case. That is a supplied condition for checking conservation. A real pond or forest can receive and export both carbon and water while internal cycling continues.
In the water account, air ends with ten again, land with twenty-nine and surface water with twenty-one. An unchanged atmospheric stock coexists with eight units leaving and eight returning. Observing a steady stock would not reveal the absence of evaporation or precipitation; opposing transfers can balance.
Residence time and turnover require additional definitions of the stock and outflow rate, often with a steady-state approximation. A single transfer diagram cannot establish how long each atom or water molecule remains. Likewise, a carbon pool increase is not an estimate of climatic effect without its broader physical, temporal and boundary context.
Worked example
Compute each ending pool before combining
Carbon endings are: inorganic 100 - 20 + 8 + 4 + 7 = 99; producers 20 + 20 - 8 - 6 - 5 = 21; consumers 10 + 6 - 4 - 2 = 10; detritus 30 + 5 + 2 - 7 = 30. Both starting and ending totals are one hundred and sixty.
Water endings are: air 10 - 8 + 5 + 3 = 10; land 30 + 8 - 5 - 4 = 29; surface water 20 + 4 - 3 = 21. Their combined total remains sixty. Land loses one while surface water gains one, despite air returning to its original amount.
The totals remain constant because all arrows in each assigned inventory connect included pools. Adding an external input or export would change the combined balance; it would not prevent material from also following internal routes.
Check your understanding
Why does the producer carbon pool end at twenty-one although its production transfer is twenty?
Show reasoning
Its initial twenty receives fixation twenty but loses eight respiration, six feeding and five litter. Twenty + twenty - eight - six - five equals twenty-one.
Does unchanged atmospheric water ten mean no water moved through the air pool?
Show reasoning
No. Eight leave as precipitation while five plus three return. The opposing interval transfers balance, so stock is unchanged.
Can a river segment show water cycling while its local water account is open?
Show reasoning
Yes. Internal transfers and boundary-crossing upstream/downstream flows can occur together. A cycle describes connections; closure is a condition of the chosen inventory.
What to carry forward
Track material in specified pools and distinguish possible cycling from measured transfer amounts or local closure.
References
Resources supporting this lesson’s explanations and examples.