The open ocean is a vital player in Earth's biogeochemistry. Phytoplankton drive primary production , converting inorganic carbon to organic matter. Nutrient limitation , especially by nitrogen , phosphorus, and iron , controls productivity. The biological pump transports organic matter from surface to deep waters.
Ocean circulation shapes nutrient distribution . Upwelling brings nutrient-rich deep waters to the surface, while deep water formation ventilates the ocean interior. Climate change impacts ocean biogeochemistry through acidification, deoxygenation, and shifts in primary production. The ocean acts as a crucial carbon sink, absorbing CO2 through solubility and biological pumps.
Open Ocean Biogeochemical Processes
Biogeochemical processes in open oceans
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Primary production
Photosynthesis by phytoplankton converts inorganic carbon to organic matter
Net primary production measures carbon fixed minus respiration
Gross primary production accounts for total carbon fixed without subtracting respiration
Phytoplankton responsible for ~50% of global primary production (diatoms, coccolithophores)
Nutrient limitation
Liebig's Law of the Minimum states growth limited by scarcest resource
Major limiting nutrients nitrogen, phosphorus, iron control productivity
Redfield ratio C : N : P = 106 : 16 : 1 C:N:P = 106:16:1 C : N : P = 106 : 16 : 1 reflects elemental composition of marine organic matter
Iron limitation common in high-nutrient, low-chlorophyll (HNLC) regions (Southern Ocean)
Biological pump
Vertical transport of organic matter from surface to deep ocean
Sinking particles and fecal pellets carry carbon downward
Remineralization in water column releases nutrients and CO2
Export production measures organic matter escaping surface layer
Carbon sequestration occurs when organic matter reaches seafloor
Ocean circulation and nutrient distribution
Upwelling
Wind-driven coastal upwelling brings nutrient-rich deep waters to surface
Equatorial upwelling driven by trade winds and Ekman transport
Upwelling regions highly productive (Peruvian coast, Benguela Current)
Deep water formation
Thermohaline circulation driven by temperature and salinity differences
North Atlantic Deep Water (NADW) forms in Nordic and Labrador Seas
Antarctic Bottom Water (AABW) forms in Weddell and Ross Seas
Deep water formation ventilates ocean interior and transports nutrients
Nutrient distribution
Surface waters depleted in nutrients due to biological uptake
Deep waters enriched in nutrients from remineralization
Nutrient spiraling describes repeated uptake and release along circulation path
Vertical profiles show low surface concentrations, increasing with depth
Dissolved gases
Oxygen minimum zones form where respiration exceeds ventilation
Carbon dioxide solubility pump driven by temperature-dependent solubility
Methane hydrates found in cold, high-pressure environments (continental slopes)
Gas seeps release methane from seafloor (hydrothermal vents, cold seeps)
Climate Change Impacts and Ocean-Climate Interactions
Climate change impacts on ocean biogeochemistry
Ocean acidification
Atmospheric CO2 dissolves in seawater, lowering pH
Carbonate system equilibrium: C O 2 + H 2 O ↔ H 2 C O 3 ↔ H C O 3 − + H + ↔ C O 3 2 − + 2 H + CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+ \leftrightarrow CO_3^{2-} + 2H^+ C O 2 + H 2 O ↔ H 2 C O 3 ↔ H C O 3 − + H + ↔ C O 3 2 − + 2 H +
Impacts calcifying organisms by reducing carbonate ion availability
Changes carbonate saturation state, affecting shell formation (corals, pteropods)
Ocean deoxygenation
Warmer waters hold less dissolved oxygen
Enhanced stratification reduces vertical mixing and ventilation
Oxygen minimum zones expand, threatening marine ecosystems
Alters biogeochemical cycles, particularly nitrogen and phosphorus
Changes in primary production
Shifts phytoplankton community structure (smaller species favored)
Alters nutrient availability and limitation patterns
Impacts biological pump efficiency and carbon export
Regional variations expected (increases in polar regions, decreases in tropics)
Ocean's role as global carbon sink
Ocean carbon sink
Solubility pump dissolves CO2 in cold, dense waters
Biological pump exports organic carbon to deep ocean
Ocean absorbs ~25% of anthropogenic CO2 emissions annually
Carbon cycle feedbacks
Changes in ocean circulation affect CO2 uptake and storage
Alterations in marine ecosystems impact carbon cycling
Potential release of methane hydrates amplifies warming
Ocean-atmosphere interactions
Air-sea gas exchange driven by partial pressure differences
El Niño-Southern Oscillation (ENSO) influences CO2 fluxes
Long-term changes in ocean heat content affect CO2 solubility
Future projections
Ocean carbon sink may saturate, reducing uptake capacity
Tipping points in ocean-climate system (thermohaline circulation shutdown)
Uncertainties in climate models and biogeochemical feedbacks complicate predictions