Oceanic Production, Carbon Oceanic Production, Carbon Regeneration, Nutrient Cycling Regeneration, Nutrient Cycling --

Part IPart I

OCN 401 OCN 401 -- Biogeochemical SystemsBiogeochemical Systems

Reading: Reading: SchlesingerSchlesinger, Chapter 9, Chapter 9

1. Oceanic net primary production (NPP)• Global patterns• Fate of NPP

- Surface-ocean decomposition and recycling- Export to the deep sea- Autochthonous vs. allochthonous OM respiration

2. Nutrient cycling in the ocean• Internal cycles• N and P budgets• Human perturbations

OutlineOutline

Net Primary Production: Global PatternsNet Primary Production: Global Patterns

• Oceanic NPP is ~50% of total NPP on Earth- mostly as phytoplankton in surface mixed layer- seaweed accounts for only ≈ 0.1%

• NPP ranges from 130 - 420 gC/m2/yr, lowest in open ocean, highest in coastal zones

• Terrestrial forests range from 400-800 gC/m2/yr, while deserts average 80 gC/m2/yr

• O2 distribution is an indirect measure of photosynthesis:CO2 + H2O → CH2O + O2

• NPP is usually measured using O2-bottle or 14C-uptake techniques

• Controversy over magnitude of global NPP arises from discrepancies in methods for measuring NPP: estimates range over 27 - 51 x 1015 gC/yr

• O2 bottle measurements tend to exceed 14C-uptake rates because:

- Large biomass of picoplankton, only recently observed, which pass through the filters used in the 14C technique

- Picoplankton may account for up to 50% of oceanic production

- DOC produced by phytoplankton, a component of NPP, passes through filters

- Problems with contamination of samples with toxic trace elements during 14C-HCO3

- additions depress NPP

Despite disagreement on absolute magnitude of global NPP, there is consensus on the global distribution of oceanic NPP:

Chlorophyll concentration for May 2001 derived from MODIS Terra data (MOD21)

Note high productivity of coastal areas, southern ocean, and equatorial zoneseospso.gsfc.nasa.gov/ftp_docs/data_products_1.pdf

Satellite remote sensing allows quantification of chlorophyll-a, which is approximately proportional to biomass:

Fate of Net Primary Production: Decomposition Fate of Net Primary Production: Decomposition and Recycling in Surface Watersand Recycling in Surface Waters

• Most NPP (80-90%) is consumed in the surface ocean by zooplankton and free-floating bacteria (bacterioplankton)

• Bacterial decomposition >> zooplankton consumption of NPP

• Bacteria respire 30-70% of NPP

• Zooplankton are part of a trophic chain that leads to larger animals (fish, squid)

• In contrast, bacteria are consumed by bacteriovores who mineralize nutrients (converting POP and PON to PO4

3- and NO3-) and release CO2 to surface waters

• When bacteria are abundant, a large fraction of the C fixed by NPP is not passed to higher trophic levels

• Bacterial #s are depressed in cold waters, where more NPP can be passed up the food chain

Fate of Net Primary Production: Export of NPP out Fate of Net Primary Production: Export of NPP out of Surface Waters to the Deep Seaof Surface Waters to the Deep Sea

• 80-90% of NPP is degraded to inorganic compounds (PO43-, NO3

-, CO2 ) in surface waters

• The remaining 10-20% sinks below the euphotic zone to the deep ocean

• POM exported from the euphotic zone sinks at an average of ~350 m/d, so the average particle spends ~10 days in transit to the bottom

• Bacteria continue to decompose POM as it sinks, consuming O2 and producing CO2 in deep waters, but rates are limited by the cold temperatures

• Most POM (95%) is remineralized by 3000 m depth, and only small quantities reach the sediments of the deep ocean

• Comparison of rates of NPP and OM burial in deep sea sediment suggests ~98% of POM is degraded before deposition

• Burial in sediments ≈ 0.16 x 1015 gC/yr (<1% of NPP)

The global distribution of organic carbon in surface sediments, in five categories: >2.00, 1.01-2.00, 0.51-1.00, 0.25-0.50, and <0.25 % dry-weight

www.icsu-scope.org/.../cope35/chapter14.html

The distribution of organic carbon in ocean sediments is similar to that of NPP in surface waters (Fig. 9.6), except that a greater fraction of total burial (83%) occurs on the shelf:

The global distribution of phytoplankton primary production, in five categories of >500, 250-500, 150-250, 100-150, and <100 mg C m-2 day-1

www.icsu-scope.org/.../cope35/chapter14.html

The distribution of NPP in surface waters shows clear relationship to organic carbon content of underlying sediments:

The global distribution of zooplankton abundance over the upper 100 m of the water column, in four categories: >500, 201-500, 51-200, and <50 mg m-3

www.icsu-scope.org/.../scope35/chapter14.html

The distribution of zooplankton abundance correlates well with NPP in surface waters:

Fate of Net Primary Production: Autochthonous Fate of Net Primary Production: Autochthonous vs.vs. Allochthonous Allochthonous OM RespirationOM Respiration

• River-borne organic carbon is respired in the ocean

• Bulk carbon isotopic evidence shows a decline in terrestrial OM from coastal and shelf areas to the deep sea

• However, more sophisticated compound-specific isotope analysis of marine sedimentary OM indicates that mainly fine-grained, old soil OM is transported to deep waters

• The total respiration of organic carbon in the ocean exceedsauthochthonous organic carbon production, which suggests that the ocean is net heterotrophic

Nutrient Cycling in the OceanNutrient Cycling in the Ocean• NPP in the oceanic euphotic zone is generally limited by nutrient availability:

- NPP is highest where nutrients are most abundant (e.g., coastal regions)- NPP is lower in the open ocean, where nutrients levels are low

• Nutrients are stripped from surface waters by downward sinking dead organisms and fecal pellets

• The mean residence times of N, P and Si in the surface ocean << MRT of surface water

- Nutrients are non-conservative elements- Nutrient behavior is strongly controlled by biogeochemistry

• Nutrients are regenerated in the deep ocean (via microbial respiration reactions), where their concentrations are much higher than at the surface

• Pacific Ocean deep water nutrient concentrations > Atlantic Ocean deep water- Pacific deep water is older than Atlantic deep water -- has had more time to accumulate nutrients from respiration of POM- Nutrient concentrations progressively increase as NADW transits from the Atlantic to the Pacific

Dissolved Nutrient Profiles Showing Progressive Increase in Deep Water Concentrations from Atlantic to the Pacific and Indian Oceans

Nutrient Cycling in the Ocean: Internal CyclesNutrient Cycling in the Ocean: Internal CyclesThe equation for phytoplankton photosynthesis, taking N and P into account:

106CO2 + 16HNO3 - + HPO4

2- + 122H2O + 18H+ →

(CH2O)106(NH3)16(H3PO4) + 138O2 (9.5)

The overall C:N:P stoichiometric ratio in phytoplankton is, on average:

106 C : 16 N : 1 P

• Thus, C, N and P are taken up by phytoplankton on average in the atomic C:N:P ratio of 106:16:1

• This is known as the Redfield Ratio, as A.C. Redfield made this observation in 1958

• Redfield noted that the biota determine the proportions of N:P in deep waters, and the biotic demand is closely matched by the N:P ratio in upwelled waters

• Upwelling waters have an average C:N:P ratio of 800:16:1, indicating clearly that N and P (and not C) are the limiting nutrients

• Ca2+ is also taken up by biota, but they only removes 1.25% of Calcium

- Although biotic uptake is the major Ca-removal process, it affects the Ca reservoir only slightly, and Ca remains well mixed (conservative) in the ocean

• Global NPP (50 x 1015 gC/yr) requires 8.8 x 1015 gN/yr and 1.2 x 1015 gP/yr

• Rivers provide only 36 and 2 x 1012 g N and P per year• An additional 15% of required nutrients are provided by upwelling and diffusion• The balance (85-90%) of required nutrients are supplied by recycling:

• Phytoplankton maintain very low N and P levels in surface waters (Fig. 9.12)

- Much of the recycling of nutrients takes place in close proximity to the dying cell- Growing phytoplankton cells in the immediate vicinity take up N and P upon their release

- N and P are preferentially released, relative to C, during mineralization, so the C:N:P ratio of sinking particulate matter increases with depth

• Direct uptake of recycled N is in the form of NH4+, since uptake is immediate

and there is no time for nitrification prior to uptake

• In contrast, most of the N mineralized in the deep sea is nitrified, converted to NO3

-

• Nitrate also dominates input by rivers and the atmosphere

• The fraction of NPP that derives from NH4-uptake vs. that derived from NO3-uptake allows estimates of of the sources of N that support NPP:

new vs. recycled

• New inputs of N are defined as those from rivers, the deep sea, and the atmosphere, and support 10-20% of new production

• New production must balance export of nutrients from surface to deep waters in order to maintain low, steady-state nutrient concentrations in surface ocean waters

• The sources of nutrients that sustain new production globally ≈ losses of nutrients in organic debris that sinks below the thermocline to the deep sea (export)

Nitrogen and Phosphorus Budgets for the SeaNitrogen and Phosphorus Budgets for the Sea

• Redfield ratios suggest that the phytoplankton demand for N and P is closely matched by their concentrations in upwelled deep water, so that either could be the limiting nutrient (in contrast to the strong P-limitation observed in freshwaters)

• N-fixation is an important process for alleviating N-limitation, especially in the open ocean

- Requires Mo and Fe for synthesis of the N-fixing enzyme nitrogenase- Seems to require anaerobic micro-zones, found in marine snow, bundled

organisms (e.g., Trichodesmium)- Global N-fixation may account for 10-15 x 1012 g N/yr added to the oceans

• Denitrification is an important pathway for removal of fixed-N from seawater- Requires low-O2 conditions, found both in water column and in sediments- O2-minimum zones in the water column in the Pacific may result in loss of 50 - 60

x 1012 g N/yr via denitrification- Denitrification in coastal sediments may lose another 50 x 1012 g N/yr - N-losses via denitrification push oceanic production toward N-limitation

• N-inputs from dust deposition can be significant in N-limited regions, and links oceanic NPP to distant terrestrial ecosystems; pollutive atm-N from land can also have an effect on NPP down-wind from source of pollutive aerosols

• Most NPP is supported by N-remineralization in surface waters

• Only a small fraction of N is lost to the deep ocean

• MRT of available N in euphoticzone << 1 yr, while MRTof total N pool is ~10 yrs, implying ~10 cycles of N before removal

• MRTof N in deep sea is ~500 yrs

• 50% of upward N-flux is from upwelling (localized) and diffusion(more diffuse, important in open ocean)

• Many fluxes known only to a factor of 2; however modeling suggests that N fluxes are not in steady state, with a net loss of N from the oceans

NitrogenNitrogen

• Most new P enters the ocean via rivers, a small amount only from dust deposition; input N:P ratio = 40:1

• Recycling in surface waters accounts for the majority of P uptake by phytoplankton

• C:P ratio increases with depth, and with sediment burial (200:1), suggesting preferential P-mineralization

• No gaseous forms of P (in contrast to N), thus river + atmospheric inputs of P must balance burial of P

• Burial of biogenic P in open ocean ≈ river input of P

PhosphorusPhosphorus

• Burial of P in continental margin sediments can only be balanced when inputs from river-borne sediments are taken into account

• MRT of P in oceans is ~25,000 years (but there are estimates as low as 10,000 years)

• Thus, each atom of P that enters the ocean cycles ~50 times before it is removed to sediments

Human Perturbations of Marine Nutrient CyclingHuman Perturbations of Marine Nutrient Cycling• Sewage and fertilizer runoff have increased N and P input to oceans• These inputs have increased NPP in coastal areas, and perhaps ocean-wide• Greater NPP could result in greater export of POC to deep sea, potentially serving

as a sink for increasing atmospheric CO2

• However, this model suggests that increased C-uptake (0.1 x 1015 gC/yr) is small relative to the anthropogenic C-release to atmosphere (6 x 1015 gC/yr):

• Greatest human perturbations are seen in the coastal ocean- occupies only ~10% of ocean’s surface, but- accounts for ~18% of ocean productivity- contains 83% of C buried in marine sediments

• Globally averaged models mask the importance of the coastal zone

• A significant amount of POC may be transported from the shelf to the deep sea

• If global climate change alters coastal upwelling rates, significant changes in the ocean’s overall biogeochemistry should be expected

SummarySummary• Most NPP is respired by bacteria, rather than consumed by zooplankton

• Most NPP is recycled in surface waters, a small amount is exported below the thermocline, and an even smaller fraction is buried in sediments

• Oceanic N and P distributions reflect biogeochemical uptake and recycling patterns:

• Oceanic water-column nutrient profiles• N and P evolution during thermohaline circulation

• The Redfield Ratio describes the uptake ratio of C:N:P by phytoplankton, and its release during respiration

• Suggests that N and P are limiting

• Recycled N and P supports the bulk of NPP

• Human perturbations are most strongly felt in the coastal ocean, but could still cause global ocean changes in biogeochemistry