Total organic carbon at BATS Remember that DOC = ~98% of the TOC.

Total organic carbon at BATS Remember that DOC = ~98% of the TOC. Note the build up in DOC through the spring and summer, with subsequent export the following winter. Figure courtesy of Craig Carlson, UCSB

Labile: simple sugar monomers, amino acids)-typically nanomolar concentrations Semi-labile: amino sugars (ex. N-acetyl glucosamine)-typically ~1-10 of micromolar Refractory: largely unknown composition, rich in C relative to other nutrients (N, P); 10s of micromolar

Isolation of DOM by ultrafiltration Size selective concentration of DOM Typically solutes > 1nm are concentrated for subsequent analyses Selects for HMW fraction (about 30-35% TOC) Some salts collected also

Ultrafiltration high molecular weight DOM (HMWDOM) membrane filter >1000 D DOM fraction 30-35% TOC < 1000 D DOM fraction 65-70% TOC Photos from Dan Repeta

Final product 30-35% of total DOC

Spectral and chemical analyses of HMWDOC 13 CNMR Carbohydrate 50-70% of HMWDOC -O- -O- Acid hydrolysis followed by Monosaccharide analyses yields 7 major neutral sugars that represent 5-10% of surface water DOM Acid hydrolysis 200 150 100 50 0 R F A X Gl M Ga

NMR and carbohydrate analyses of deep sea HMWDOC surface monosaccharide distribution relative % relative % deep

Bomb 14 C Fossil fuel dilution Atmosphere Cosmogenic 14 C production Air-Sea Exchange Surface Ocean Deep Ocean Factors controlling 14 C in atmospheric and oceanic reservoirs 14 C half-life is 5730 years

History of radiocarbon in the Atmosphere and ocean Frigate shoals 14 C per mil Fiji Galapagos Prebomb value of -80 per mil

Radiocarbon in the Atlantic and Pacific Oceans Peter M. Williams and Ellen Druffel; Nature 1987, JGR 1992 DIC 14 C in surface waters of the Atlantic and Pacific has the same isotopic value. DOC is always older than DIC (by 4 kyrs in surface water) Deep ocean values of DOC are equal to a radiocarbon age of 4000-5000 yrs Either there is a source of old DOC, or DOC persists for several ocean mixing cycles

Prebomb organic matter: 14 C < -50 Post bomb organic matter production: 14 C > ~-50 and <200

Basin-scale gradients in deep ocean DOC concentrations: slow degradation and transport of organic carbon

Plankton Suspended particles Sinking particles Surface DOM Deep DOM Plankton Suspended particles Sinking particles Surface DOM Deep DOM 0 5 10 15 20 C:N ratio Stoichiometry of POM and DOM During degradation of organic material, nutrient elements are preferentially removed. Bulk pools of DOM tend to be carbon-rich, and nitrogen- and phosphorus-poor 0 100 200 300 400 500 C:P ratio

Regeneration versus assimilation of N and P Factors controlling whether nutrients are regenerated or assimilated: C:N:P ratio of the bacterial biomass C:N:P ratio of the substrate supporting growth Growth efficiency In general, the C:N:P ratio of the biomass must be greater than the C:N:P ratio of the substrate for regeneration of N or P to occur.

Major components of the biological pump: 1. Sinking particles 2. DOC Major controls on the pump efficiency: 1. Plankton production and respiration, zooplankton repackaging 2. Particulate and dissolved material stoichiometry 3. Physics (stratification and mixing)

Average surface ocean nitrate concentrations In ~1/3 of the ocean, excess nutrients are perennially available yet phytoplankton biomass is relatively low. Such regions are termed High Nitrate Low Chlorophyll (HNLC) waters

Whatever factors limit complete utilization of nutrients in HNLC regions have important consequences on the functioning of the biological pump Such processes limit new production and thus ultimately export of carbon to the deep sea.

North Atlantic Spring Bloom 47 o N

Subarctic North Pacific

What limits the accumulation of phytoplankton in large regions of the oceans? H1: Phytoplankton growth is limited by light (due to deep mixing) H2: Plankton biomass is kept low by vigorous predation H3: Nitrate uptake is inhibited by uptake of ammonium H4: Phytoplankton growth is limited by availability of specific nutrients

H1: Deep mixing results in light limited growth DEEP MIXED LAYER SHALLOW MIXED LAYER Remember the Critical Depth?

Sverdrup (1953) Sverdrup and the critical depth Critical depth Mixed layer

Cold temperatures and high winds often results in very deep mixing in Southern Ocean; however, the Subarctic North Pacific and Equatorial Pacific typically do not mix as deep (<120 m) as other systems that experience regular nutrient drawdown. Conclusion: although light limitation may be important in some HNLC systems, light alone is insufficient to explain lack of seasonal nitrate drawdown.

Three possible scenarios of factors limiting the accumulation of phytoplankton biomass Nutrients Grazing Grazing + nutrients Phytoplankton Biomass Phytoplankton Biomass Phytoplankton Biomass Time Time Time Control Grazers removed Nutrients (+) Grazers removed & Nutrients (+)

H2: Food web control of plankton biomass--grazers keep biomass cropped to low levels, allow nutrients to accumulate

Tightly coupled growth and grazing 0.8 LANDRY et al. (1993) GROWTH / OR GRAZING (d -1 ) 0.6 0.4 0.2 0.0-0.2 2 4 6 8 10 12 14 16 18 20 22 JUNE 1987 µ - phyto m - microzoo In the subarctic North Pacific and Eastern Equatorial Pacific, strong evidence supporting micrograzer control of algal biomass.

Remember: Production = growth rate * biomass P=µB If grazers reduce biomass, production decreases (unless growth rates increase). Thus, grazing can directly limit production.

In both the subarctic North Pacific and Equatorial Pacific, intense grazing pressure appears to restrict accumulation of phytoplankton biomass.we will revisit this later

The case for Iron Iron is essential for life: required for synthesis of chlorophyll, component of cytochromes (electron transport chain), needed for nitrate utilization (nitrate reductase), essential for N 2 fixation (nitrogenase). Iron is highly insoluble in oxygenated seawater; readily precipitates. In regions far removed from continental shelves primary Fe input occurs via atmospheric deposition and upwelling. In areas of active upwelling, demand for Fe is elevated; however, many of these regions are also far removed from terrestrial Fe sources.

Obtaining accurate measurements of Fe concentrations in the open ocean has plagued oceanographers for many years.

Various metals essential to life demonstrate nutrient like distributions in the oceans Surface depletion due to algal uptake; increasing concentrations increase through remineralization In many HNLC regions, upper ocean concentrations of Fe <0.1 nm From Morel and Price [2003]

25000 20000 15000 10000 5000 0 A little bit of Fe goes a long way Phytoplankton biomass: 106C : 16N : 1P : 0.005Fe Cellular concentrations relative to iron (moles) Phosphorus Nitrogen Carbon

There is evidence suggesting that changes in Fe supply influence atmospheric CO 2 300 [CO2] (ppmv) 280 260 240 220 200 180 CO 2 Iron 1.5 1.0 0.5 Fe (µmol/kg ice) 20 40 60 80 100 120 140 160 Age (1000 yr) Glacial-interglacial variations in CO 2 demonstrate inverse relationships to the availability of iron in seawater

Pye (1987) Dust source regions and transport routes

Dust flux overlaid on the NO 3 distribution (µm) in the upper ocean NOAA world ocean atlas, 1994 Atmospheric Fe flux (mg m -2 yr -1 ) Duce et al. 1991 10 100 1000 100 10 1 100 1000 10

Experiments done in carboys and bottles confirmed that phytoplankton growth was limited by Fe Martin et al. (1990)

THE RESULTS OF BOTTLE EXPERIMENTS MADE A BIG SPLASH BUT NOT EVERYONE WAS CONVINCED Bottle experiments demonstrated increases in Chl by the addition of iron; however, there were concerns about what might have been missed...exclusion of large grazers, sinking, mixing, etc.

Bottle experiments indicated that the addition of iron shifted the phytoplankton assemblage from small cells (subject to tight grazing) to large cells (diatoms) that grow rapidly, consume nutrients, and sink. But was this due to a bottle effect? Exclusion of grazers. No iron controls +Fe

Solution: Mesoscale (100s of km) enrichment experiments to examine community level responses to iron. Iron added as acidic iron sulfate. The inert tracer SF 6 is added along with iron. Cartesian coordinate system Lagrangian system following drogues

IronEX I: 1993, Equatorial Pacific near Galapagos Islands. 443 kg of Fe into a 64 km 2 patch. Initial Fe concentrations ~0.1 nm, final target Fe concentration was 4 nm. Added 17,500 L of 0.5 M Fe solution (ph 2.0). A separate batch of 2000 L of SF6 was mixed into the iron solution. Coale et al. (1994)

IronEX I: Chl concentrations enriched in the patch and downstream of natural Fe source (Galapagos Islands). However, only weak drawdown of nitrate observed over course of experiment perhaps Fe is not the only limiting nutrient? Grazing? After day 4, the patch subducted beneath a low salinity front. Coale et al. (1998) Fe concentrations downstream of the Galapagos Islands (in the island plume) were ~1 nm.

IronEX II SF 6 Fe Chl NO 3 - pco 2 Coale et al. (1996) June 1995, Equatorial Pacific; 225 kg Fe in 72 km 2 ; Day 1 Fe concentrations ~2 nm. Fe added again on days 3 and 7 (to bring surface water concentrations to ~1 nm)

IRONEX II: Equatorial Pacific June 1995; a shift from cyanobacteria to diatoms The increase in Chl within the Fe seeded patch appear largely driven by growth of diatoms (85-fold increase in abundance). Landry et al. (2000)

12 mesoscale Fe experiments in > 10 years NO 3 mmol m -3 Boyd et al. (2007) +Fe (HNLC) High Fe +Fe (LNLC)

The resulting blooms are large enough to be viewed from space SERIES (Subarctic North Pacific) SOIREE (Southern Ocean) Thanks Dr. Jim Gower of IOS and NASA

One of the major findings from these open ocean Fe enrichment experiments was that specific components of the phytoplankton community increased in biomass following the addition of Fe. SoFeX (Southern Ocean)?? No iron Iron

Diatoms grow rapidly, then disappear? 25 20 15 fco2 (D18) 350 340 330 10 320 4e+5 5 0 310 300 290-5 280 Diatoms (cell L-1) 3e+5 2e+5 1e+5-10 -40-35 -30-25 -20-15 -10-5 0 5 SERIES (N.E. Pacific) 270 0 10 15 20 25 30 35 Days

1.0 Iron supply impacts many aspects of phytoplankton processes and ocean biogeochemistry nitrate depletion silicic acid depletion 0.8 mmol m -3 d -1 0.6 0.4 0.2 0.0 0 5 10 15 20 Days 1 ry production (mg C m -2 d -1 ) 3000 2500 2000 1500 1000 500 0 BOYD 2002 SOIREE IronEx II 0 2 4 6 8 10 12 14 16 18 Days Nitrate removal (mm m -3 ) 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 SOIREE IronEx II 0 2 4 6 8 10 12 14 Days

20 18 16 14 12 10 8 6 4 2 0 A wide range in bloom signatures Ironex-2 Soiree Eisenex Seeds SofexNorth SofexSouth Series Eifex Experiment 25 20 15 10 5 MLD versus [Chla ] 10 20 30 40 50 60 70 80 MLD (m) Ironex-1 Maximum Chl a [mg.m-3] Chl (mg m-3) De Baar et al. 2005 0-5

The HNLC condition-lessons learned from large scale manipulation experiments HNLC conditions are maintained by low Fe supply which suppresses phytoplankton growth and biomass production. Low concentrations of Fe appear to favor smaller cells (picoplankton). Growth of dominant picoplankton also suppressed by Fe supply but to a lesser extent than larger, rarer cells. Active mircrozooplankton grazing keeps picoplankton biomass low and relatively invariant, providing a highly regenerative upper ocean (rapid NH 4+ cycling).

HNLC Regions of the Ocean SUMMARY HNLC waters 30% OF OPEN OCEAN IRON SUPPLY causes the HNLC condition But some regions are also influenced by light, Grazing or silicic acid supply Biomass levels in HNLC waters are set by Grazing pressure which in turn resupplies iron Seeding a bloom and studying its development has provided important information on plankton control of biogeochemistry