Teaching · nutrient chemistry

Carbon and nitrogen move living carbon; iron and ligands gate where macros cannot

This page is ecology-first restoration literacy for COP visitors: how C, N, Fe, and companion nutrients shape the biological pump. It is a teaching tool — not a fertilisation prospectus, not a credit calculator, and not a field-dosing recipe.

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Numbers below are labeled Fact, Assumption, Founder report, or Unknown.

Carbon — currency of the biological pump

Dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) are the main pools visitors meet first. DIC itself is a fast equilibrium among three inorganic “pumps” that set how carbon sits at the air–sea boundary and in seawater.

  • Dissolved CO₂ pump

    Aqueous CO₂ (and H₂CO₃*) exchanges with the atmosphere. Wind, temperature, and ΔpCO₂ drive the air–sea flux. Physics owns the transfer velocity; chemistry owns the speciation that sets how much of DIC is “gas-ready.” Fact (system identity) · air–sea rate knobs stay Assumption in Flux Explorer.

  • Bicarbonate pump

    Most DIC in modern surface seawater is bicarbonate (HCO₃⁻). Biology can draw on HCO₃⁻ (and convert it) for photosynthesis; remineralisation returns carbon mainly into the DIC pool. Teaching label: Fact that HCO₃⁻ dominates DIC; pathway shares are Assumption until site-cited.

  • Carbonate pump

    Carbonate ion (CO₃²⁻) and solid CaCO₃ (calcifiers: coccolithophores, foraminifera, pteropods) form the classical carbonate / CaCO₃ pump: calcification removes alkalinity and can counterintuitively raise pCO₂ locally even while exporting particulate inorganic carbon. Fact (process identity) · magnitudes and ballasting roles are Assumption / Phase B cites.

  • Organic biological pump

    Photosynthesis draws DIC into biomass (POC/DOC). Export and remineralisation return carbon to DIC or move it below the mixed layer. More chlorophyll is not verified sequestration — export efficiency and remineralisation depth matter (Assumption in the explorer).

Together: dissolved CO₂ ↔ bicarbonate ↔ carbonate (equilibrium chemistry) plus organic and CaCO₃ export pathways. None of these teaching panels issue carbon credits.

Nitrogen — primary low-latitude limiter

Forms include nitrate (NO₃⁻), nitrite (NO₂⁻), ammonium (NH₄⁺), dissolved organic N, and N₂ gas.

Fact Moore et al. (2013) map nutrient limitation: N tends to limit productivity across much of the low-latitude surface ocean where subsurface supply is slow; Fe often limits where subsurface macronutrient supply is enhanced (Southern Ocean, eastern equatorial Pacific).

Iron — strong secondary, bioavailability first

Fact Tagliabue et al. (2017): iron regulates productivity magnitude and dynamics. Sources are diverse — dust, sediments, hydrothermal, and recycling — not dust alone.

Fact GEOTRACES-era open-ocean dissolved Fe (dFe) is typically on the order of ~0.1–1 nM; remote HNLC surfaces can sit well below 0.1 nM (Conway / GEOTRACES Intermediate Data Product syntheses).

Organic ligands usually exceed dFe and keep iron dissolved. Teaching preference: ligand∶dFe framing over mineral “more Fe” narratives.

Explorer default total Fe-binding ligands [Lt] ≈ 1.5 nM Fe-equivalents is an Assumption (typical excess-ligand order; regional medians vary).

Phosphorus, silicon, and other micros

P and Si co-shape guilds; Zn, Cu, Mn, Co, Mo are enzyme cofactors that can co-limit with Fe or N.

  • Phosphate / DOP

    Co-limiter when N∶P supply and demand diverge. Surface ranges in explorers are Assumption teaching boxes, not station means.

  • Silicic acid

    Builds diatom frustules; Si∶N shifts favour or suppress export-prone guilds. HNLC-capable defaults are teaching Assumption.

  • Other micros

    Moore et al. (2013) note vitamins and micronutrients may co-limit; spatial patterns remain unclear. Explorer v1 keeps these narrative unless a Zn slider is opened.

  • H and O (light touch)

    Water is solvent and reactant; O₂ is metabolic product/reactant and a hypoxia indicator — stop hooks live on the biology page. Air–sea exchange is owned mainly by physics.

Redfield is important — cycling is larger

Classic Redfield C∶N∶P ≈ 106∶16∶1 (mol∶mol) — Fact as the conventional reference for bulk phytoplankton biomass stoichiometry (Redfield 1958 / reviews). It remains the right teaching anchor for how much C can sit next to available N and P (see Cpot in Flux Explorer — potential only, not export or credits).

Diet and excretion cycling is much greater than that static ratio alone. Consumers eat, metabolise, and excrete continuously. Those recycling loops keep nutrients in the sunlit ocean and help keep the biological pump running — often moving more mass on short timescales than a one-time Redfield “inventory” snapshot suggests. Assumption (order: cycling ≫ standing-stock stoichiometry for teaching) · Founder report (Ocean Flux emphasis).

Most of a consumer’s diet is not Redfield stoichiometry. Prey mix (phytoplankton guilds, detritus, microbes, other animals) rarely matches 106∶16∶1. Excreted N, P, Fe, and ligand-active organics therefore diverge from Redfield too. Assumption / Founder report until guild-cited.

Real phytoplankton communities also deviate from Redfield. Treat guild-specific deviations as Assumption unless cited. Moore et al. (2013) remain the teaching map for where N versus Fe tends to limit.

Derived teaching proxy (not a free slider): Febio ≈ min(dFe, [Lt]) — saturated-ligand ceiling (Assumption).

Algae excretions — quantify the Unknown

Phytoplankton release dissolved organic matter (exudates / excretions): ligands, DOC, DON, and other metabolites that reshape bioavailability and the microbial loop.

Unknown Global or regional mass-balance for algae excretion fluxes (DOC/DON/ligand production rates as a fraction of primary production) is not locked on this site. Published estimates span wide ranges by taxa, light, nutrient stress, and method. Phase A teaching rule: show the pathway, label magnitudes Unknown, and refuse credit maths from excretion terms.

Working placeholder for explorers (sensitivity only): excretion as ~2–40% of fixed carbon leaking to DOC depending on guild and stress — Assumption envelope, not a Fact. Prefer site- and taxa-specific cites before any public number is upgraded.

Flux Explorer — chemistry inputs

Six teaching sliders. Defaults are labeled; recommended ranges are sensitivity tools, not measurements of a named patch. Excluded: field dosing setpoints, slurry recipes, credit tCO₂e.

Symbol Label Unit Default Status
[NO₃⁻] Nitrate (surface box) µmol L⁻¹ 8 Assumption HNLC-ish ETP teaching box (Moore 2013 regime map) — not a station mean
[PO₄³⁻] Phosphate µmol L⁻¹ 0.7 Assumption Redfield-consistent with default N (~16∶1)
[Si(OH)₄] Silicic acid µmol L⁻¹ 15 Assumption diatom-capable HNLC order
dFe Dissolved Fe nmol L⁻¹ 0.15 Fact (order of magnitude) GEOTRACES open-ocean ~0.1–1 nM; default near HNLC-low
[Lt] Total Fe-binding ligands nM Fe-eq. 1.5 Assumption typical excess-ligand order
DIC Dissolved inorganic C µmol kg⁻¹ 2100 Assumption modern surface-ocean order

Try them in the Flux Explorer. Biology and physics pages supply the food-web and supply-pathway context.

Key citations

  1. Moore, C. M., et al. (2013). Processes and patterns of oceanic nutrient limitation. Nature Geoscience, 6, 701–710. https://doi.org/10.1038/ngeo1765
  2. Tagliabue, A., et al. (2017). The integral role of iron in ocean biogeochemistry. Nature, 543, 51–59. https://doi.org/10.1038/nature21058
  3. GEOTRACES Intermediate Data Product syntheses — dissolved Fe order of magnitude (e.g. Conway and related Oceanography overviews).
  4. Redfield stoichiometry — conventional C∶N∶P reference (Redfield 1958 / reviews).
  5. Ocean carbonate system / DIC speciation (CO₂(aq), HCO₃⁻, CO₃²⁻) and CaCO₃ pump — standard chemical oceanography; Phase B will add named textbooks/papers for Ω and PIC export.

Disclaimer strip

Teaching model only. Not verified carbon credits. Not commercial ocean iron fertilisation. Not a dosing playbook. See Disclaimer and Support.