Biogeochemical Cycling of Nutrients, the Biological Pump, Productivity and Marine Ecology
2. Light, photosynthesis, euphotic and compensation depths
The photosynthesis-light (P–I) curve rises linearly at low irradiance with slope α (light-limited), levels off at a maximum P_max (light-saturated, limited by enzyme and nutrient supply) and may decline at very high light (photoinhibition). Light decays with depth by the Beer–Lambert law: I(z) = I₀ e^(−K_d z), with K_d the diffuse attenuation coefficient (about 0.03 to 0.05 m⁻¹ in the clearest ocean water, 0.2 m⁻¹ or more in turbid coastal water). The euphotic depth is where light falls to 1 % of the surface value: z_eu = ln(100)/K_d = 4.6/K_d. For K_d = 0.05 m⁻¹ it is 92 m; for 0.2 m⁻¹ it is 23 m.
The compensation depth is where photosynthesis just balances the plant’s own respiration, so net growth is zero; its light level is the compensation intensity I_c. Worked example. With I₀ = 400 W m⁻², I_c = 5 W m⁻² and K_d = 0.10 m⁻¹: z_c = ln(I₀/I_c)/K_d = ln 80/0.10 = 43.8 m. Over a day, cells carried through the mixed layer by turbulence sample the whole range of light. Sverdrup’s critical depth is the mixed-layer depth at which water-column production equals water-column respiration; if the mixing is deeper than this, a bloom cannot start, which is why the spring bloom of temperate seas begins when stratification shoals the mixed layer. The critical depth is greater than the compensation depth.
3. Primary and secondary production, new and regenerated production
Gross primary production (GPP) is the total carbon fixed; net primary production (NPP) = GPP − autotrophic respiration. It is measured by ¹⁴C uptake incubations, by light-and-dark bottle oxygen change, and from satellite chlorophyll with a model. Global ocean NPP is about 50 Pg C per year, roughly half the planet’s total. Biomass is dominated by tiny cells (picoplankton such as Prochlorococcus and Synechococcus) in the nutrient-poor gyres, and by large diatoms in nutrient-rich upwelling and spring-bloom waters. Secondary production is the growth of heterotrophs (zooplankton, bacteria, fish) that consume organic matter.
New production is production fuelled by nitrogen newly supplied to the euphotic zone (nitrate mixed or upwelled from below, N₂ fixation, atmospheric deposition); regenerated production uses nitrogen recycled within the zone (ammonium, urea). In steady state new production equals the organic matter exported from the zone. The f-ratio = new/(new + regenerated) ≈ nitrate uptake/(nitrate + ammonium uptake). It is high (0.5 to 0.8) in upwelling and bloom conditions and low (0.1 to 0.2) in oligotrophic gyres where recycling dominates. Worked example. Nitrate uptake 0.30 and ammonium uptake 0.90 µmol N L⁻¹ d⁻¹ give f = 0.30/1.20 = 0.25.
4. The biological pump
The biological pump is the set of processes that move carbon fixed in the surface layer to the deep ocean: phytoplankton fix CO₂, a fraction sinks as aggregates (marine snow), faecal pellets and dead cells, and most is respired at depth, returning CO₂ to the water below the surface layer, where it stays for centuries until the water returns to the surface. This lowers the surface DIC and pCO₂ and keeps the deep ocean rich in carbon and nutrients. It is one of three pumps: the soft-tissue pump (organic matter), the carbonate pump (CaCO₃ shells; forming a shell removes 2 units of alkalinity per carbon and raises surface pCO₂, opposing the soft-tissue pump) and the physical solubility pump (cold water takes up more CO₂ and sinks).
The sinking flux declines with depth as particles are consumed and remineralised, described by the Martin curve: F(z) = F₁₀₀ (z/100)^(−b), with z in metres and b about 0.86 for a typical ocean. Worked example. With F₁₀₀ = 10 mg C m⁻² d⁻¹, the flux at 1000 m is 10 × 10^(−0.86) = 1.4 mg C m⁻² d⁻¹, so only about 14 % of the carbon exported past 100 m reaches 1000 m. Deeper remineralisation means longer storage; ballast minerals (opal, CaCO₃, dust) speed sinking. The pump keeps atmospheric CO₂ lower than it would otherwise be; in HNLC areas it is weak because iron limits production.
5. Zooplankton, the food web, upwelling and HNLC regions
Zooplankton are drifting animals: microzooplankton (ciliates, flagellates) graze small phytoplankton, and mesozooplankton (copepods dominate) graze larger cells and are food for fish larvae and planktivorous fish. Alongside the classical chain (diatom to copepod to fish to predator) runs the microbial loop: bacteria use dissolved organic carbon released by cells and are grazed by flagellates, recycling nutrients in the surface layer. The transfer of energy between trophic levels is inefficient, usually quoted as about 10 % (range roughly 10 to 20 %); the rest is respired or lost to detritus. Worked example. With 15 % transfer efficiency and three transfers (producers to the fourth trophic level), production of 2000 units at the base gives 2000 × 0.15³ = 6.75 units at the top. Long chains of many levels therefore support small top-predator biomass, and short chains (diatom to anchovy) support large fisheries.
Upwelling regions bring nutrient-rich water into the sunlit zone and are the most productive: the eastern-boundary currents (Humboldt off Peru and Chile, California, Canary, Benguela), the equatorial divergence, and the Arabian Sea, where the southwest monsoon winds drive coastal and open-ocean upwelling in summer. They cover a small fraction of the ocean but supply a large share of the fish catch. HNLC regions (high nitrate, low chlorophyll: the subarctic North Pacific, the eastern equatorial Pacific and the Southern Ocean) have unused nitrate because iron is too scarce; this is the basis of Martin’s iron hypothesis, supported by deliberate iron-addition experiments that triggered blooms.
6. Marine ecology: ecosystems, coral reefs and oxygen minimum zones
The ocean is divided by depth and distance: the neritic (shelf) and oceanic (open) provinces; the sunlit epipelagic (to about 200 m), the mesopelagic (to about 1000 m) and the dark bathypelagic below; and the benthic (sea-floor) realm. Coastal systems (estuaries, mangroves, salt marshes, seagrass beds) have high production per unit area, are nurseries for fish and shelter the coast. The open ocean has low production per unit area but covers most of the planet, so its total is large; the oligotrophic gyres are microbial, with high f-ratio only where nutrients are supplied. Deep-sea hydrothermal vents run on chemosynthesis, not light.
Coral reefs are built by corals that host symbiotic algae (zooxanthellae) in their tissue, which supply much of the coral’s energy and help it lay down aragonite skeletons. They thrive in clear, shallow, warm (roughly 23 to 29 °C), nutrient-poor water with high light and are highly productive and biodiverse. Bleaching occurs when prolonged warming of a degree or so above the usual summer maximum drives the algae out, and acidification lowers Ω and weakens calcification. India’s main reef areas are the Gulf of Mannar, the Gulf of Kachchh, Lakshadweep and the Andaman and Nicobar Islands. Oxygen minimum zones (OMZs) form at mid-depth where respiration outpaces ventilation, strongest under productive upwelling in the eastern tropical Pacific and in the northern Indian Ocean (Arabian Sea, and to a lesser extent the Bay of Bengal). Oxygen is so low that denitrification removes fixed nitrogen, and hypoxia (commonly taken near 2 mL L⁻¹) restricts most animals. Warming and stratification are expected to expand low-oxygen waters.
Key takeaways
- Redfield C : N : P = 106 : 16 : 1 sets how much nitrate is used per carbon fixed; limiting nutrients are N in most waters and Fe in HNLC regions.
- Nitrate and phosphate are depleted at the surface and peak near 1000 m; silicate keeps rising deeper.
- Light decays as I₀e^(−K_d z); the euphotic depth is ln(100)/K_d and the compensation depth is where light equals I_c.
- The f-ratio is new/(new + regenerated), high in upwelling and blooms and low in oligotrophic gyres; new production equals export in steady state.
- The biological pump exports carbon; flux falls as F₁₀₀(z/100)^(−0.86), and the carbonate pump opposes the soft-tissue pump.
- Trophic transfer is about 10 to 20 %; corals host symbiotic algae, and OMZs drive denitrification.
Practice questions (16)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The Redfield ratio C : N : P of marine plankton and deep-water nutrients is, by atoms, about
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Answer: B — 106 : 16 : 1
The classic ratio is 106 carbon to 16 nitrogen to 1 phosphorus. The other orders place the numbers on the wrong elements.Why do silicate concentrations keep increasing with depth below the nitrate maximum?
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Answer: C — Diatom opal dissolves more slowly than organic matter decays
Opal dissolves more slowly than organic matter is remineralised, so the silicate release is deeper than that of nitrate and phosphate. Silicate is taken up by diatoms, so it is not unused.The f-ratio of a plankton community is 0.7. This indicates that
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Answer: D — most production uses new nitrogen, as in upwelling or a bloom
f = new/(new + regenerated), so f = 0.7 means 70 % is nitrate-based new production, typical of upwelling or blooms. A low f, 0.1 to 0.2, is the oligotrophic gyre case.The euphotic depth is conventionally defined as the depth at which the light is
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Answer: A — 1 % of its surface value
The convention is 1 % of surface irradiance, z_eu = 4.6/K_d. Light is never exactly zero at that depth, and 50 % occurs within a few metres of the surface in many waters.Which statement about HNLC regions is correct?
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Answer: B — Nitrate is abundant but chlorophyll is low, because iron is scarce
HNLC means high nutrient (nitrate), low chlorophyll, with iron as the limiting factor in the subarctic Pacific, the equatorial Pacific and the Southern Ocean.A carbonate-shell-forming phytoplankton group blooms and a fraction of its shells sinks out of the surface layer. Compared with an organic-matter-only export, the net effect on surface-water pCO₂ is
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Answer: A — to raise it, since shell formation lowers alkalinity
Forming CaCO₃ removes 1 DIC but 2 alkalinity, and the alkalinity loss dominates, so surface pCO₂ rises: the carbonate pump opposes the soft-tissue pump. Alkalinity is lost, not gained.Oxygen minimum zones in the northern Indian Ocean are significant mainly because
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Answer: D — denitrification there removes fixed N and low oxygen limits animals
Under productive upwelling, respiration exhausts oxygen at mid-depth, and in the suboxic core nitrate is used in place of oxygen, which is denitrification. Oxygen is consumed there, not produced.Under identical sunlight, two waters have K_d = 0.04 m⁻¹ (open ocean) and 0.20 m⁻¹ (coastal). The ratio of their euphotic depths, open to coastal, is
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Answer: C — 5
z_eu = 4.6/K_d, so the ratio equals the inverse ratio of K_d: 0.20/0.04 = 5. The clear water has the deeper euphotic zone.Which of the following statements about production are correct?
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Answer: A — NPP = GPP − autotrophic respiration; B — New production is supported by nitrogen from outside the euphotic zone; C — In steady state, new production equals export production
The first three are standard. The f-ratio concerns nitrogen uptake, not zooplankton respiration.Which of the following processes remove fixed nitrogen from or add it to the ocean?
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Answer: A — Denitrification removes it, producing N₂; B — Nitrogen fixation adds it, converting N₂ to organic nitrogen; C — Nitrification converts NH₄⁺ to NO₃⁻ without changing the amount of fixed nitrogen
The first three are correct. Respiration regenerates nutrients within the ocean and does not add nitrogen from the air.Which of the following are correctly matched?
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Answer: A — Upwelling off Peru and Chile: Humboldt Current; B — Coral reef: symbiotic algae (zooxanthellae) in the coral; C — Hydrothermal vent communities: chemosynthesis
The first three are correct. Oligotrophic gyres have low f-ratios, because production is mostly regenerated.Take C : N : P = 106 : 16 : 1 by atoms. Surface nitrate is drawn down by 4.8 µmol kg⁻¹ during a bloom. The organic carbon formed is ____ µmol kg⁻¹ (one decimal place).
Numerical answer — type the value.
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Answer: 31.8
C = 4.8 × 106/16 = 508.8/16 = 31.8 µmol kg⁻¹. Using P in place of N (4.8 × 106) would be the slip.Light follows I = I₀ exp(−K_d z) with K_d = 0.05 m⁻¹. The euphotic depth, where I = 0.01 I₀, is ____ m (one decimal place).
Numerical answer — type the value.
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Answer: 92.1
0.01 = exp(−0.05 z), so z = ln(100)/0.05 = 4.605/0.05 = 92.1 m.In a surface water sample the nitrate uptake is 0.30 µmol N L⁻¹ d⁻¹ and the ammonium uptake is 0.90 µmol N L⁻¹ d⁻¹. The f-ratio is ____ (two decimal places).
Numerical answer — type the value.
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Answer: 0.25
f = 0.30/(0.30 + 0.90) = 0.25. Dividing nitrate by ammonium alone gives 0.33, which is wrong.The sinking carbon flux follows F(z) = F₁₀₀ (z/100)^(−0.86), with z in metres. If F₁₀₀ = 10 mg C m⁻² d⁻¹, the flux at 1000 m is ____ mg C m⁻² d⁻¹ (two decimal places).
Numerical answer — type the value.
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Answer: 1.38
F = 10 × (1000/100)^(−0.86) = 10 × 10^(−0.86) = 10 × 0.1380 = 1.38.Production at the base of a food chain is 2000 units and each transfer between trophic levels has an efficiency of 0.15. The production available at the fourth trophic level (three transfers) is ____ units (two decimal places).
Numerical answer — type the value.
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Answer: 6.75
Production = 2000 × 0.15³ = 2000 × 0.003375 = 6.75 units. Using four transfers would give 1.01.