Ocean Heat Flux, Productivity, and Carbon Cycling Dynamics

Posted on Mar 5, 2025 in Marine Sciences

Ocean Heat Flux and its Components

Heat flux (W/m2) is composed of short-wave radiation, long-wave radiation, sensible heat flux, and latent heat flux.

  • Short-wave radiation is the dominant source of heat flux, strongest in the summer, and a function of cloud cover.
  • Latent heat flux is the largest sink for surface water, proportional to the rate of evaporation, and strongest in the winter.
  • Net heat flux increases with latitude and changes with seasons.
  • h=q(w)/a, q=ρcpt

Salinity seasonality reflects the imbalance of evaporation and precipitation.

Buoyancy (b) is controlled by temperature (T) and salinity (S): b=-gρ/ρ0

Wind stress (τ-N/m2) is the net effect of wind-driven momentum and usually stronger in the winter. Monsoon wind is driven by the seasonal reversal of land-ocean temperature gradient: τ=cdρu102

Heat flux and wind stress regulate the seasonality of the oceans.

Control of Mixed Layer Depth

  • Cooling makes surface water dense, dense water sinks/mixes with deeper layer, and deepens the mixed layer.
  • Heating makes surface water light and shrinks the mixed layer.
  • The vertically integrated density increase/buoyancy decrease balances the net heat loss at the surface, and the vertically integrated density decrease/buoyancy increase balances the net heat gain at the surface.
  • Wind stress intensifies surface currents and near-surface turbulence, thus deepening the mixed layer, and the vertically integrated buoyancy is conserved.

Potential energy (PE) = ρgz

Two cases:

  1. Well-mixed with thickness h, PE = 1/2ρgh2
  2. Stratified (δρ) with equal thickness h/2, PE = 1/2ρgh2-1/4δρgh2. The well-mixed state has higher potential energy. Transition from stratified to well-mixed requires energy.

λδbh(∂h/∂t) = 2mu*3–βh

  • Potential energy gain due to deepening of mixed layer
  • Wind kinetic energy input
  • Potential energy input due to buoyancy loss

Sea surface temperature (SST) integrates the heat flux. Maximum SST lags behind the maximum heating and maximum cooling, and mixing of thermocline water cools the SST during fall and winter. Mixed layer deepening due to rapid shoaling from heating from the top during the spring and summer affects only the top thin layer. Mixed layer gradual deepening is due to mixing of subsurface water from the deeper layer.

Phytoplankton and Ocean Productivity

Phytoplankton are responsible for ~50% of photosynthesis on Earth. They harvest light, absorb CO2, and nutrients to produce organic material: 6CO2 + 6H2O → C6H12O6 + 6O2

Redfield Ratio P:N:C = 1:16:106

106CO2 + 16NO3 + H2PO4 + 12H2O + light → C106H263O110N16P + 138O2

Photosynthesis productivity is regulated by nutrients and light, as water and carbon dioxide molecules are always available in the surface ocean. High productivity occurs in the high latitudes (summer hemisphere), tropics, and coastal regions. Low productivity occurs in the subtropics and high latitudes of the winter hemisphere.

Nutrient Uptake Regimes

Two regimes of nutrient uptake (rate limiting):

  1. Low nutrient levels: Diffusion; uptake of N = 4πrK{N∞}
  2. High nutrient levels: Transporter enzyme kinetics; uptake of N = Vmax ({N∞}/Ksat + {N∞})
  • At low N, N uptake is proportional to radius (r), so smaller cell size is more advantageous.
  • At high N, N uptake is proportional to r2, so large plankton can store N internally and use it when needed.

Types of Phytoplankton

  • Pico-phytoplankton: Tiny (<1μm) single-cell photosynthesizing bacteria; prokaryotes; easily grazed and recycled; take up nutrients efficiently at very low levels.
  • Diazotrophs: Nitrogen-fixing bacteria; can use N2 as a nitrogen source.
  • Diatoms: Large (2-200 μm) photosynthetic eukaryotes that can grow quickly and form blooms; silica-based shells (frustules) make them sinkers; abundant in upwelling regions (equator, Southern Ocean).
  • Coccolithophorids: Calcium carbonate shells, CaCO3 cycling, sinkers.
  • Flagellates: Motile, mixotrophic, can grow rapidly and bloom, sometimes causing harmful algae blooms.

Phytoplankton contain light-harvesting chlorophyll pigments. Under low light, they produce more to increase capture of photons and vice versa.

Seawater is an excellent absorber of light. Penetration of light is dependent upon the particle concentration and has an exponential decay with depth: I = I0e^(z/zλ). Red light is more readily absorbed. Blue light penetrates deeper and scatters by water molecules. Typically, zλ is about 20m for blue light and only a few meters for red. The clearest water with depleted nutrients from the subtropical open ocean is deep blue, almost black.

Subpolar Productivity

Subpolar productivity features a spring/summer bloom. Upper ocean stratification regulates productivity:

  1. Spring/summer shoaling of the mixed layer
  2. Wind-induced mixing

Gross Primary Production (GPP): Total amount of photosynthesis.

Net Primary Production (NPP) = GPP – Ra (autotrophic/plant respiration).

Net Community Production (NCP)/Total amount of biological C fixation = NPP – Rh (heterotrophic respiration).

Observing Ocean Productivity

  1. Ship-based, in situ measurements of radioactive carbon-14 incubation, nutrient mass balance, and oxygen mass balance: C uptake = (total C in bottle)*(C-14 in particle/C-14 added) * 1.05; 24 hours (light + dark) C-14 incubation ~ NPP per day; NPP = NPP1δz1+NPP2δz2+….
  2. Satellite ocean color: Remote sensing reflectance (λ) = the ratio between reflected & incident solar radiation/SeaWiFS; date
  3. Marine particles: Ca = 10.0^(0.366 – 3.067r4s+1.930r24s+0.649r34s – 1.532r4s4); r4s = log(r443555>r490555>r510555)

Nitrogen available for phytoplankton is available as readily available depleted urea and ammonium produced by the decomposition of organic matter and nitrate (NO3) often depleted in the surface, but enriched at depths.

Recycled production of nitrogen uses urea and NH4; new production uses nitrate; new production is ~ export production and NCP.

Sinking particle = organic matter – mineral particles.

Sediment trap = rain gauge-like instrument that captures falling particles in the water.

Aggregation – physical & biological processes that transfer material to a larger particle; fragmentation – physical & biological processes that break apart particles into smaller size; e-ratio = C export/NPP.

There are more smaller particles than larger ones.

Surface water is usually depleted of nutrients by uptake; deep water is enriched with nutrients from remineralization & respiration; new deep water contains less nutrients; old deep water contains high nutrients.

When organic matter decomposes in deep water, it releases N & CO2.

Phytoplankton need trace elements such as iron; origin of Fe unknown; Fe can be released from mineral dust; continental shelf sediment hydrothermal, Fe-oxides, Fe-particle absorption may be sources; dissolved Fe is very low (1000x less than P).

Fe SW input = mass of dust deposition * fraction of total Fe in dust * soluble fraction of Fe; marine phytoplankton Fe demand 0.5% to 2% solubility; fate of mineral Fe after deposition: hydrolysis reactions, scavenging onto particles, binding with organic ligands, photochemical reactions.

Ocean Carbon Cycling and Pumps

DIC = [CO2]<1%+[HCO3-]90%+[CO32-]10%

Downward organic flux (J) is balanced by the vertical transport at steady state: the balance regulates the vertical gradient.

CO2 reacts with water to form bicarbonate and carbonate ions.

Colder SST enhances CO2 solubility; subtropic water is more alkaline due to the excess evaporation, thus enhancing buffering capacity (higher CO32-) and subject to the same [CO2] increase. Subtropical water can absorb more carbon molecules than higher latitudes. Revelle buffer factor (B) is ~10 in tropics/subtropics and ~18 in polar ocean; anthropogenic CO2 increase leads to different DIC response depending on B; B≈[HCO3-]/[CO32-]

Seawater is transported into the interior ocean; vertical circulation replaces subsurface water via ocean current and mixing.

On average, pH is ~8; pH is maintained by the balance between CO2 and alkalinity level: CO2 + CO32- +H20 ↔ 2HCO3- ; in CO2 rich ocean, the balance shifts right; [H+] = K2[HCO3-/[Co32-] = k2(2DIC-Ac)/Ac-DIC

Calcifiers can produce CaCO3, which 1 mol of CaCO3 production consumes 2 mol of alkalinity and 1 mol of DIC: Ca2+ + CO32- –> CaCO3

Production of sinking CaCO3 shifts the balance to the left, increasing CO2 in the seawater, leading to the degassing of CO2 into the atmosphere.

Carbon pumps maintain the vertical gradient of DIC.

  • Organic pump: Driven by sinking organic particles; increases thermocline/deep nutrients & carbon, also consumes oxygen.
  • Solubility pump: Driven by thermal stratification.
  • Carbonate pump: Driven by sinking CaCO3 particles; increases thermocline/deep alkalinity & carbon; decreases buffering capacity at the surface, therefore increases atmospheric CO2.
  • Winter cooling –> solubility increase –> lowers pCO2
  • Summer heating –> solubility decrease –> raises pCO2
  • Spring/summer bloom –> C export to deep ocean –> surface C decreases –> lowers pCO2
  • Winter entrainment of thermocline waters –> returning C & N into mixed layer –> C increases –> raises pCO2

Wind Stress, Ocean Gyres, and Circulation

Wind stress curl – measurement of the spin of the wind stress; vertical velocity depends on wind stress curl.

  • North Atlantic subtropical gyre: Gulf Stream & Canary Current
  • North Atlantic subpolar gyre: Labrador Current
  • South Atlantic subtropical gyre: Brazil Current & Benguela Current

Dynamic Regimes

  1. Western boundary – Gulf Stream & Brazil Current
  2. Eastern boundary – Canary & Benguela Current
  3. Interior gyre

Positive wind stress curl –> northward motion of water column (Vv)

Sverdrup circulation is equatorward everywhere in the subtropics (wind stress curl < 0)

DIC =[CO2]<1%+[HCO3-]90%+[CO32-]10%