Oxygen Cycle

For a high-level tour of how every element moves through the tank, start with Nutrient Cycling. Oxygen is the element that kills overnight — produced only while the lights are on, consumed around the clock, and the first thing to run out when respiration beats photosynthesis.

Why oxygen is the element that kills

Oxygen is the element that determines whether an ecosystem lives or dies — and it can decide in hours, not weeks. A nitrogen drain takes months to matter. A phosphorus limitation slows growth gradually. But when dissolved oxygen drops below 1–2 mg/L — a condition called hypoxia — consumers start dying within hours, and the death spiral that follows (more dead biomass means more decomposition, which consumes more oxygen, which kills more organisms) can collapse an ecosystem in a single night. Every catastrophic failure mode in the model — the sudden crash that turns a thriving jar into a cloudy graveyard — runs through oxygen.

The reason oxygen is so volatile is that its production is intermittent but its consumption is constant. Photosynthesis only runs during daylight hours; respiration and decomposition run around the clock. The ecosystem must produce enough oxygen in its ~12 hours of light to carry every organism through ~12 hours of darkness. This daily gamble — daytime surplus versus nighttime deficit — is the central tension of the oxygen cycle.

Three oxygen pools

The model tracks oxygen in three pools: dissolved oxygen in the water (which directly affects bulk-water biology), gaseous oxygen in the headspace above the water surface, and (in soil-substrate scenarios) dissolved oxygen in the sediment pore water (which gates every redox process happening below the substrate). Gas exchange continuously redistributes O₂ between water and headspace toward Henry's law equilibrium — see Dissolved Gases and Gas Exchange for the full mechanics. Pore-water oxygen is supplied by diffusion from the bulk water down through the sand cap, and (in heavily planted tanks) by radial oxygen loss leaking from rooted-plant root tissue; it is drained by the aerobic breakdown of soil organic matter and by the cryptic re-oxidation reactions at the oxic–anoxic boundary inside the substrate (reduced iron back to rust, sulfide back to sulfate, methane back to CO₂).

In a Walstad-style aged soil substrate the pore-oxygen supply-and-demand balance settles to within tens of micrograms per litre within days even when the bulk water column stays at 5–8 mg/L. This near-zero pore oxygen is the physical foundation for the spatial niche split between aerobic heterotrophs (which dominate the bulk water and the oxic surface millimetre of the detritus layer) and the anaerobic guilds — denitrifiers, sulfate reducers, methanogens — which live in the substrate pore zone and breathe nitrate, sulfate, or CO₂ instead of oxygen.

In a sealed system, total oxygen across both pools is conserved by gas exchange alone. A drop in dissolved O₂ does not necessarily mean the ecosystem is losing oxygen — it may just be moving to the headspace. To see true production and consumption, look at the total across both pools.

The day/night cycle

O₂ production happens only during daylight hours (photosynthesis), while O₂ consumption runs around the clock (respiration, decomposition, and nitrification never stop). This creates a characteristic sawtooth pattern: dissolved O₂ rises during the day and falls at night. Cool, room-temperature freshwater holds only about 8–9 mg/L of dissolved oxygen when fully saturated (Garcia & Gordon 1992), so the entire margin a tank has to survive the night is the gap between that ceiling and the ~2 mg/L where animals begin to suffocate.

For long-term ecosystem health, what matters is the integrated 24-hour balance — does daytime production outweigh the full day's consumption? If nighttime consumption consistently exceeds daytime production, dissolved O₂ trends downward over days and weeks, eventually reaching levels that stress or kill organisms. The Stability and Failure doc covers how to read these trends and what happens when the balance tips.

Oxygen production

Photosynthesis is the only source of O₂ in the model. Algae, periphyton, and macrophytes produce 1 mol of O₂ for every mol of carbon they fix from CO₂.

The PQ correction for nitrate assimilation. When producers assimilate nitrate (NO₃) instead of ammonium (NH₄), the intracellular reduction of NO₃ to NH₄ requires extra electrons from the light reactions, producing 2 extra mol of O₂ per mol of NO₃ assimilated. This photosynthetic quotient (PQ) correction means systems where producers rely heavily on nitrate produce slightly more O₂ overall.

Photorespiration. At high O₂-to-CO₂ ratios, the enzyme RuBisCO — which normally grabs CO₂ for carbon fixation — mistakenly binds O₂ instead. Each oxygenation event wastes energy and actually consumes O₂ rather than producing it. The net O₂ production from photosynthesis accounts for this: at very high O₂/CO₂ ratios, net production can even go negative (the plant consumes more O₂ through photorespiration than it produces through normal photosynthesis). The degree of photorespiration depends on the RuBisCO specificity of each species.

This also provides a natural negative feedback: as O₂ builds up relative to CO₂, photosynthetic efficiency drops, limiting further O₂ accumulation.

Oxygen consumption

Many processes draw from the dissolved O₂ pool:

Maintenance respiration (all organisms, 24/7). Every species — algae, periphyton, macrophytes, grazers, nitrifying bacteria, and heterotrophic bacteria — respires continuously, day and night. Respiration consumes O₂ and produces CO₂ at a 1:1 ratio. The rate is proportional to biomass and limited by available O₂ via saturation kinetics (at very low O₂, respiration slows). Each species has its own oxygen affinity — bacteria can keep respiring at low O₂, while larger organisms need more. Organisms in suboptimal salinity respire harder, paying the energetic cost of osmoregulation.

Consumer SDA (specific dynamic action). When grazers digest food, the digestion itself consumes O₂. This represents about 20% of the carbon assimilated from food, consumed as an additional O₂ cost on top of maintenance respiration. Like maintenance respiration, SDA is limited by available O₂.

Nitrification. Nitrifying bacteria consume 2 mol of O₂ for every mol of NH₄ they oxidize to NO₃. This is a substantial oxygen demand — nitrification can be a major O₂ sink in systems with high ammonium loading. The nitrification rate itself requires O₂ as a substrate, so it slows under low-oxygen conditions.

Aerobic decomposition of detritus. Decomposition of dead organic matter consumes O₂, but only for the directly-mineralized fraction. Decomposition splits products two ways: most becomes DOM (for bacteria to consume later) and the rest is mineralized directly to dissolved CO₂, NH₄, and PO₄. Only that directly-mineralized fraction incurs an O₂ cost at this stage — the DOM fraction's O₂ cost is deferred until bacteria consume and respire it, avoiding double-counting. Within the direct fraction:

Suspended detritus decomposes in the water column with full access to dissolved O₂. The O₂ consumed equals the directly-mineralized carbon scaled by the aerobic fraction (which depends on O₂ availability).
Settled detritus at the bottom has reduced O₂ access. Settled decomposition draws only about 30% as much dissolved O₂ per unit of carbon mineralized as suspended decomposition; the rest of the carbon oxidation occurs via anaerobic pathways that do not draw dissolved O₂ from the water column. See Decomposition and Recycling for the full settled-decomposition mechanics.

Even under fully anoxic conditions, decomposition does not stop entirely — it continues at about 15% of the aerobic rate, representing anaerobic pathways.

Bacterial DOM processing. When heterotrophic bacteria consume dissolved organic matter, the fraction they respire (most of the carbon in fresh, labile DOM and nearly all of it in tough, refractory DOM) requires O₂. Their maintenance respiration also consumes O₂. Both are limited by O₂ availability via saturation kinetics.

How oxygen affects biology

Low oxygen has direct biological consequences:

Respiration limitation. All organisms' respiration rates are limited by O₂ concentration following saturation kinetics. At very low O₂, organisms cannot meet their maintenance energy needs, leading to energy deficits and starvation.

Consumer activity reduction. Grazer feeding activity decreases at low O₂. The rate of food intake drops according to O₂ saturation kinetics, reducing both food acquisition and the associated SDA costs.

Hypoxia mortality. Consumers and nitrifying bacteria experience increased mortality when O₂ drops below species-specific stress thresholds. Mortality increases between the stress threshold (where problems begin) and the lethal threshold (where it reaches its maximum rate). Heterotrophic bacteria are more tolerant, with lower O₂ thresholds reflecting the facultative anaerobic capability of some bacterial species. See Death and Decomposition for specific per-species thresholds.

Starvation cascade. When O₂ limits respiration, consumers cannot fully meet their metabolic needs, which increases starvation-induced mortality. This can trigger a positive feedback loop: dying organisms become detritus, which increases decomposition O₂ demand, further lowering O₂, which kills more organisms.

Photorespiration feedback. High O₂ relative to CO₂ reduces the efficiency of photosynthesis itself. When O₂ competes with CO₂ at the RuBisCO enzyme, carbon fixation and net O₂ production both decrease. In extreme cases, net O₂ production becomes negative — photosynthesis actually consumes O₂. This acts as a natural ceiling on O₂ accumulation.

Key ecological dynamics

The 24-hour balance sheet

The single most important diagnostic question for any ecosystem is: does daytime O₂ production exceed the full day's O₂ consumption? If yes, the system is sustainable. If no, dissolved O₂ trends downward over days and weeks until something dies.

This balance is harder to read than it seems. Dissolved O₂ might look healthy at 8 mg/L during the afternoon, but if it drops to 1 mg/L every night, the system is stressed. Conversely, dissolved O₂ might look low at 4 mg/L during the day, but if it only drops to 3 mg/L at night (because respiration demand is modest), the system is stable. The amplitude of the daily swing matters as much as the absolute level — large swings mean primary production and respiration are poorly matched, and the nighttime minimum is the number that determines whether organisms survive.

The starvation-decomposition spiral

The most dangerous feature of oxygen dynamics is the positive feedback loop between mortality and O₂ demand. When O₂ drops low enough to kill organisms (below ~1 mg/L for most consumers, below ~0.5 mg/L for bacteria), the dead biomass becomes detritus. Decomposition of that detritus consumes oxygen. If the additional O₂ demand from decomposing the dead pushes dissolved O₂ even lower, more organisms die, producing more detritus, consuming more oxygen. This spiral can take a system from "stressed but surviving" to "completely anoxic" in 24–48 hours.

The spiral is especially dangerous because it accelerates. Each death adds organic matter that demands oxygen to decompose. In a sealed system with no atmospheric O₂ exchange, there is no external rescue — the only O₂ source is photosynthesis, which requires living algae, which may also be dying. Breaking the spiral requires either reducing O₂ demand (by having less organic matter to decompose) or increasing O₂ supply (by having enough surviving producers to photosynthesize their way out). Systems with robust refugia that protect a core producer population from the crash are much more likely to recover.

How oxygen interacts with the other cycles

Carbon — the tightest coupling in the model. Every molecule of carbon fixed by photosynthesis releases one molecule of O₂, and every molecule of organic carbon respired consumes one — so oxygen flow and carbon flow are two views of the same process. The same daily breathing that pulls O₂ down at night drives the carbon and pH swings on the Carbon Cycle page.
Nitrogen — nitrification is one of the largest oxygen sinks in a cycling tank, burning two molecules of O₂ for every ammonium oxidized. When oxygen runs out, the nitrogen cycle hands off to the anaerobic pathways — denitrification and the rest of the diagenetic ladder — that the Nitrogen Cycle page follows.
pH — oxygen and pH move together through their shared carbon link: the nighttime respiration that drains oxygen also loads the water with CO₂ and pushes pH down, while daytime photosynthesis lifts both.
Iron and sulfur — oxygen is what keeps reduced iron and sulfide trapped in the sediment. As long as the water column stays oxic, the cryptic re-oxidation traps hold; when the water itself goes anoxic, those traps relax and reduced iron and toxic sulfide spill upward. See Iron Cycle and Sulfur Cycle.

Key references

Garcia, H.E. & Gordon, L.I. (1992). Oxygen solubility in seawater: better fitting equations. Limnology and Oceanography 37, 1307–1312.