Outline : Carbon cycling and organic matter biogeochemistry Global carbon cycle - pools, sources, sinks and fluxes pools of organic carbon - POC, DOC - vertical & horizontal segregation, vertical fluxes Ocean productivity Biological carbon pump Preservation of organic carbon Vertical flux of POM – sediment traps Dissolved organic carbon (DOC) Concentrations & distribution Characterization of DOC pool - molecular size and reactivity Sources and fates of POM & DOM Age and long-term sinks for DOM

Operational pools of carbon in seawater POM - particulate organic matter (includes not only carbon but also H, O, N, P, S etc) DOM - dissolved organic matter (about 50% C by weight) POC - particulate organic carbon (refers only to the carbon) DOC - dissolved organic carbon PIC – Particulate inorganic carbon (CaCO3) DIC - dissolved inorganic carbon (all forms) Organic nutrient pools PON & POP (the pools of N & P that are bound in organic particles larger than the operational cut-off) DON & DOP - (the pools of N & P that are bound in organic matter that passes through the operational cut-off filter) All pools are operational! (depend on selected criteria for filtration)

0.4-0.2 µm filtration cut-off Organic particle size continuum 0.4-0.2 µm filtration cut-off

Organic carbon = Reduced carbon Includes all carbon other than CO2, HCO3-, H2CO3, CO, CO32-, and carbonate minerals Includes hydrocarbons CH4, CH3-CH3 etc & black carbon. Nearly all reduced carbon is biogenic. However, some chemical/geochemical alteration of OM takes place, petroleum and natural gas formation being notable examples. Because organic matter is mainly biogenic it typically contains not only reduced carbon but also some H, O, N, P and S etc.

Net export from surface 8-15 Global Carbon reservoirs and exchanges (Figure based on Libes; data from Table 11.1 in Emerson & Hedges) pools in 1015 gC (boxes) fluxes in 1015 gC y-1 (arrows) Atmospheric CO2 784 Terrestrial biota 600 River DIC 0.5 Exchange 90 Soil & detritus 1500 Net export from surface 8-15 Ocean DIC 38,000 Marine biota 1-2 Detrital POC 30 DOC 700 0.2 Sedimentary reservoirs are huge! Organic sediments 10,000,000 Fossil fuels 3577 Limestone & dolomite 50,000,000

Most organic carbon in the sea is dissolved or colloidal. Biomass pools are very small  Operationally-dissolved  Dissolved and Colloidal materials are operationally Dissolved

Based on Table 9.1 in Millero, 2006 Sources of organic matter to the open oceans % of total Primary production Phytoplankton 84.4 Macrophytes 6.2 Rivers 3.65 Groundwater 0.3 Atmospheric input 5.45 Rivers are a small source of organic matter to open ocean! Based on Table 9.1 in Millero, 2006

Ocean Net Primary Production in different trophic regimes Trophic zone Mixed layer Chl a (μg L-1) Net Primary Production (1015 gC y-1) % of Ocean NPP Oligotrophic < 0.1 11.0 22.7 (<100 gC m-2 y-1) Mesotrophic 0.1 -1.0 27.4 56.5 (100-300 gC m-2 y-1) Eutrophic > 1.0 9.1 18.7 (300-500 gC m-2 y-1) Macrophytes - 1.0 2.1 Total ocean production = 48.5 Total terrestrial production = 56.4 Total global production = 104.9

Global primary productivity pattern as deduced from satellite imagery Behrenfeld et al 2006. Nature 444: Considerations: Depth distribution i.e. euphotic depth Seasonal variations, esp. in polar regions Interannual variations Oceanic/oligotrophic areas– dominated by picoplankton < 2 μm Upwelling, coastal & temperate areas have larger phytoplankton (> 2 μm) as major primary producers

Behrenfeld et al 2006. Nature 444: Temporal changes in global average Chlorophyll anomaly and Net Primary Productivity (NPP) anomaly. 1997-98 was a strong El Nino year which reduced NPP. Rapid recovery ensued, with slow decline thereafter. Behrenfeld et al 2006. Nature 444:

The Biological Carbon Pump Exporting carbon below the pycnocline CO2 (g)  CO2 (aq) + H2O <=> H2CO3 <=> H+ + HCO3- <=> H+ + CO32- Air Sea Euphotic zone Photosynthesis calcification Pycnocline sinking Upwelling of high DIC, high pCO2 water POM CaCO3 Some DOM DIC & alkalinity CaCO3-rich sediment above CCD No preservation of CaCO3 below CCD CO2 Deep Sea respiration spreading Alkalinity POM CCD Ridge crest CaCO3 dissolution Carbon burial & preservation as POM and CaCO3 Non-carbonate sediment carbonates

Export Production per year Falkowski et al., 1998. Science 298:

POMflux(z) = POMflux (100)(z/100)-0.858 Flux of organic matter decreases exponentially with depth : POMflux(z) = POMflux (100)(z/100)-0.858 Where POMflux(100) is the downward flux at the base of the euphotic zone (100 m), and POMflux(z) is the flux of organic carbon at depth (z) measured with sediment traps. At 5000 meters, the flux is only 3.5% of that at the base of the euphotic zone! Very little organic matter (POM) reaches the deep ocean – and what does reach the bottom is lower quality Data for the figure of Bishop et al came from Martin et al. 1987 Vertical flux of POM is via dead phytoplankton, fecal pellets, molt shells, fragments, mucous feeding nets etc.

DOC export from surface ocean represents 8-18% of the total organic carbon export. Modeled DOC downward flux DOC/POC downward flux ratio Hansell et al., 2010

Sediment traps - particle interceptors Poison or preservative Baffle to reduce hydrodynamic effects Particle flux  Base of euphotic zone 100-200 m 500 m Capture flux decreases exponentially with depth 1000 m 3000 m

Many different designs of sediment traps have been used Time series traps - rotating cylinders within trap collect for certain period of time 1-1.5 meters Large surface area trap for oceanic sampling

Diagram of an automated time-series sediment trap used in the Arabian Sea. A baffle at top keeps out large objects that would clog the funnel. The circular tray holds collection vials. On a preprogrammed schedule (every 5 days to 1 month), the instrument seals one vial and rotates the next one into place. Scientists retrieve the samples up to a year later to analyze the collected sediment. (courtesy Oceanus magazine, WHOI) http://www.whoi.edu/instruments/gallery.do?mainid=19737&iid=10286

What results do you expect for POM captured in a sediment trap array deployed over a full oceanic depth profile? Quantity of POM? Quality of POM - C:N, specific biomolecules?, 14C-content?

Three sediment trap designs. The original funnel design (moored trap) uses a large collection area to sample marine particulates that fall to great depths. Surface waters produce enough sediment so that traps there don’t require funnels. Neutrally buoyant, drifting sediment traps catch falling material instead of letting it sweep past in the current. Drawings are not to scale. Source: http://www.whoi.edu/instruments/gallery.do?mainid=19735&iid=10286

Joaquim Goes and his team deploy simple sediment traps in the Southern Ocean

WHOI scientists Ken Buesseler and Jim Valdes with one of the neutrally buoyant sediment traps they helped design. The central cylinder controls buoyancy and houses a satellite transmitter. The other tubes collect sediment as the trap drifts in currents at a predetermined depth, then snap shut before the trap returns to the surface. (Tom Kleindinst, WHOI) http://www.whoi.edu/instruments/gallery.do?mainid=19750&iid=10286

Significance of Organic Carbon Burial Much of the present global carbon burial (preservation) is in marine environments Little organic carbon preservation in terrestrial soils except for high latitude peats. Terrestrial burial of OM has been more significant in the geological past (i.e. Carboniferous coal deposits) Significance of Organic Carbon Burial Burial and preservation of biogenic (reduced) carbon in sedimentary reservoirs removes atmospheric CO2 and allows excess O2 to remain in the atmosphere. Burial of organic matter removes some nutrient elements and trace elements. Carbon burial leads to petroleum, organic rich shales, & natural gas

Percent of primary production accumulated in the sediments The greater the overall sedimentation rate of particles, the greater the fraction of surface primary production delivered to sediments Sediment accretion rate (cm per 1000 y) 0.1 1 10 100 1000 0.001 0.01 Percent of primary production accumulated in the sediments Coastal areas – maximum of ~10% y = 0.028 x1.25 See Fig. 11.5 in Pilson for actual data graph >5000 m depth >2-5000 m depth >2000 m depth incl. Black Sea

Most burial nearshore on continental margins Burial will be a small fraction of the carbon delivered to the sediments. Most will be respired to CO2 and diffuse back to water column. Libes, Chapter 25

Reasons for high carbon burial on the continental margins: high productivity - > high POM flux to benthos high particle flux leading to faster burial rate - OM preservation tied directly to mineral surface area (see Keil et al. 94) shallow depth - less organic matter degradation on descent remineralization slower under anoxia - still a debatable issue.

Coastal waters can have much higher DOC Dissolved organic carbon - the largest pool of organic matter in seawater Measured by converting DOC into CO2 via: Wet-chemical oxidation High temperature catalytic combustion UV-oxidation Sealed tube combustion DOC concentrations are 70-100 µM in surface waters of the open ocean, and 35-50 µ M at depth. Coastal waters can have much higher DOC

Surface ocean (30 m) DOC concentrations Dots are measured values, background color field is modeled Hansell et al., 2010

Deep ocean (3000 m) DOC concentrations decrease along ocean conveyor (meridional overturning circulation) Dots are measured values, background color field is modeled NADW starts with about 46 µM DOC The semi-labile fraction of DOC degrades during the long transit from North Atlantic to the Pacific. What is left (~34 M) is ultra-refractory since it survived the ~1000 y trip through the deep ocean. This DOC is present as background DOC in surface waters and has an average age of ~6000 years. Hansell et al., 2010

DOC Concentration (µM) B C D DOC Concentration (µM) 1000 2000 3000 4000 Depth (m) 10 20 30 40 50 60 70 Labile DOC; Small pool; τ = hours to days Semi labile DOC; larger pool (25-30 µM) in sfc; τ = weeks to months Open ocean surface DOC concentration is about 70 µM. It is about 44 µM in the deep Sargasso and about 34 µM in the deep Pacific. Ultra-refractory DOC; τ = >6000 y Refractory DOC; τ = ~1000 years After Benner, 2002

The average 14C age of deep DOC is 6000 years|!

DOC is generally conservative with salinity in estuaries DOC (µM) Salinity 36 400 75 Freshwater end-member Implies terrestrial DOC delivery to ocean – but most is lost on shelf (see next slide) In fact, some modification of riverine DOC takes place in estuaries, but conservative pattern still observed Seawater end-member ~80-100 µM

DOC concentration decreases away from shore Much of the DOC delivery to ocean is lost on the shelf, close to shore

Constituents of DOM High molecular weight >5000 Da (includes colloids) proteins polysaccharides (mucus, structural polymers) nucleic acids some humic substances Medium Molecular weight 500-5000 Da humic substances (refractory) oligopeptides, oligonucleotides lipids pigments Low molecular weight < 500 Da monomers (sugars, amino acids, fatty acids) osmolytes (DMSP, betaines, polyols) toxins, pheromones and other specialty chemicals See Chapter 22 in Libes for structures of organic compounds Moderate lability Mixed lability – some very refractory High lability

Shift

Examples of some polysaccharides that might be part of a semi-labile, high molecular weight pool of DOM. Pectin contains O-methoxy groups Chitin is an amino sugar, i.e. it contains N

Depolymerization - Polymer hydrolysis Conversion of high molecular weight DOM or POM into low molecular weight DOM Carried out primarily by bacteria but really a consortium of microbes. Proteins -> free amino acids & peptides by proteases Polysaccharides to monosaccharides by glucosidases, chitinases, cellulases Peptides to amino acids by peptidases RNA or DNA to nucleotides by nucleases

Origin of labile DOM in seawater Exudates - Amino acids, sugars, some high molecular weight labile polysaccharides - rapidly consumed Death or lysis of cells - rapid uptake by bacteria Sloppy feeding - leaking of phytoplankton cell contents Digestion - Digestor theory. Jumars, Penry et al. Zooplankton maximize their organic matter assimilation by maximizing throughput not by being highly efficient. This results in considerable release of DOC from fecal pellets and zooplankton.

Marine Snow. Agglomerated organic matter - amorphous aggregates Enriched with bacteria and protozoans possible low oxygen conditions elevated nutrients Still understudied. Some species of phytoplankton release mucilage i.e. Phaeocycstis sp. TEP - Transparent ExoPolymer. Is a form of marine snow Marine Snow or aggregates caused by surface phenomenon. Enrichment of OM at surfaces of bubbles, waves convergence zones. You can make snow in the lab by rotating filtered water samples in bottle. Snow, and DOC make, sea foam.

Sea foam generated from Phaeocystis bloom in Dutch Wadden Sea Phaeocystis globosa colony –cells embedded in mucous form spherical colony http://www.microscopy-uk.org.uk/mag/artapr01/foam.html http://www.ifremer.fr/delec-en/projets/efflores%20phyto/phaeocystis/phaeocys.htm Sea foam generated from Phaeocystis bloom in Dutch Wadden Sea

Blowing sea foam at Nags Head, North Carolina during Hurricane Sandy, October 2012 Nags Head, N.C. High winds blow sea foam into the air as a person walks across Jeanette's Pier in Nags Head, N.C., Sunday, Oct. 28, 2012 as wind and rain from Hurricane Sandy move into the area. Governors from North Carolina, where steady rains were whipped by gusting winds Saturday night, to Connecticut declared states of emergency. Delaware ordered mandatory evacuations for coastal communities by 8 p.m. Sunday. (AP Photo/Gerry Broome)

What isn’t there may be most important! Biogeochemists rule # 1 What isn’t there may be most important! Substances with low concentrations may be especially important in biogeochemical fluxes - their concentrations are low because they are desirable molecules to microbes! This axiom isn’t always true, but it often is

kloss Glycine (2 nM) production loss Concentrations of most labile, low molecular weight organic compounds are low (typically in the 1-10 nM (10-9 – 10-8 Molar) range). Compare this to total DOC concentration in surface waters of about 75 µM C. But some LMW compounds have very fast turnover. Glycine (2 nM) pseudo-steady state conc. production loss kloss Pool size Production = loss under steady state Hypothetical example of amino acid turnover k = 50 d-1 2 nM x 50 d-1 = 100 nM d-1 The flux of carbon through a particular compound is a function of: turnover (Conc. X Kloss ) and carbon content per molecule. So for this example, 100 nM glycine d-1 x 2 mol C/mol glycine = 200 nM C d-1 flux through the glycine pool. Thus, even substances with low concentrations can have high carbon fluxes if the turnover rate constant is large (fast turnover) Glycine and DMSP dissolved pools may turn over 10-50 times per day!

Carbon utilization efficiency affects trophic transfer and CO2/O2 dynamics In terms of carbon Carbon Assimilation Efficiency Microbial Growth Efficiency (MGE) Biomass Production (BP) = = BP + Respiration From the literature: MGE varies from 0.05 to 0.30 in different ocean waters (up to 0.52 in estuaries) Microbial Carbon Demand = Microbial C Production Microbial Growth Efficiency These terms are often referred to as bacterial growth efficiency (BGE) and bacterial carbon demand (BCD) (until discovery of ocean Archaea complicated things)

Oligotrophic (from some recent studies) eutrophic Oligotrophic (from some recent studies) Microbial Growth Efficiency = MGE = [Microb. Prod/(Microb. Prod + Respiration)] See also del Giorgio et al 2011. L&O 56:1-16

Turnover of higher molecular weight material is relatively slow Polysaccharide material (relatively labile) may turnover on time scales of days, and because of relatively large pool sizes (micromolar C), the mass flux can be large Turnover of humic substances and other refractory material may be very long (years) DOC in the deep sea is very refractory (14C-ages of 4000-6000 years) - this explains its nearly uniform distribution (see Bauer, Williams and Druffel et al.) Surface water DOC pool has average 14C age of ~1000 y - this DOC is composed of young (modern) carbon (14C age of +200 y) plus some of the old refractory material (14C age of ~6000 y)

How is this material ultimately removed from the ocean? If 14C-age of deep DOC is ~6000 years, then this material has survived several ocean mixing cycles. How is this material ultimately removed from the ocean? Photochemical oxidation may be the key (Mopper and Kieber et al. 1991). Photooxidation breaks down DOM into CO2 and smaller, often more labile molecules, thus returning it to biologically active pool of carbon (Kieber et al. Nature, 1989). Hansell et al. (2009) also suggest particle adsorption (scavenging) in the deep see may remove some refractory carbon

Photochemical Blast Zone - some DOM oxidized Photooxidation as a major sink for refractory DOM in the sea Photochemical Blast Zone - some DOM oxidized NADW formation. Labile DOM is utilized in relatively short time - leaving old refractory carbon to make another circuit Upwelling of refractory, old DOM Deep water transit (= 1000 y) Little alteration of old, refractory carbon This is a highly conceptualized diagram! Its not this simple!

Relative C:N ratios Amino acids (AA’s) < protein < lipids < carbohydrates. AA’s C:N 2-6 except for phenylalanine and tyrosine (C:N= 9) POM concentration is generally high in the upper water column and euphotic zone. Very low at depth. C:N of POM in surface ocean is generally similar to Redfield, i.e. 5-7 C:N of POM increase with depth (more labile N-containing compounds are removed in upper water column)

Molar ratios of C:N and C:P in marine plankton, DOM, and high molecular weight (HMW) DOM from the surface (<100 m) and deep (>1000 m) ocean. From Benner, 2002. Chemical composition and reactivity of marine dissolved organic matter. C:N DOM has much higher C:N and C:P than plankton (Redfield) Redfield C:P

Humic substances in the sea Complex, amorphous organic matter Gelbstoffe (colored DOM or CDOM) (contain many functional groups incl. aromatics) Humic acids - insoluble at pH < 4 Fulvic acids - soluble at all pH’s Humic acids + fulvic acids = humic substances Significant terrestrial input of humic substances to the sea via rivers, but most is destroyed on continental shelves before reaching open ocean, probably via photooxidation. Only a small fraction (~1%) of oceanic DOC is terrestrially-derived, but up to 10% of humic substances might be terrestrial (based on lignin biomarkers and 13C-content) Autocthonous humic substances - marine origin. Lack lignin moieties. Result from condensation of marine DOM - possibly via photoreactions

Soil humic acid showing amorphous structure and many functional groups Ligand bound Fe Adsorbed Al-Silicate clay No two humic molecules will be the same

Role of sediment adsorption of organic matter in the carbon cycle (after Hedges and Keil, 1999) Adsorption of organic compounds to inorganic sediment surfaces may play a role in organic carbon preservation Can be labile compounds – just not bioavailable when stuck to sediment

Organic carbon (weight percent) SA = Surface area of sediment particles OC/SA = Organic carbon per unit surface area Relatively constant amount of organic carbon per surface area Keil and Hedges, 1994. Nature 370:549

Hedges & Keil, 1995

Organic matter desorbed from sediment particles is rapidly degraded Age of the sediment layer from which Organic Matter was desorbed. This material persisted for at least 460 years but when desorbed, it degraded in days. Therefore it is labile stuff – protected by adsorption Hedges & Keil, 1995

More than monolayer equivalent Less than monolayer equivalent Percent of global organic carbon burial that occurs in different depositional environments. The largest fractions are Delta (44%) and Shelf (45%) indicating that 90% of global carbon burial occurs on ocean margins. The shading indicates where organic content is More, Less or Equivalent to monolayer absorption based on surface area of sediment particles. (after Keil & Hedges)

finish

Role of sediment adsorption of organic matter in the carbon cycle (after Hedges and Keil, 1999) Adsorption of organic compounds to inorganic sediment surfaces may play a role in organic carbon preservation Can be labile compounds – just not bioavailable when stuck to sediment

Low Mol Wt DOC High Mol Wt DOC Photo – Jeff Cornwell O2

From Davis and Benner, 2007

Reactivity of DOM vs. molecular size (after Amon and Benner, 1996) Counterintuitive? Big molecules more reactive than small? Applies to Bulk DOC – not to individual compounds Many small molecules have VERY high reactivity e.g. amino acids, DMSP

Latitudinal variation of DOC in the deep ocean Latitudinal variation of DOC in the deep ocean. The semi-labile fraction of DOC degrades during the long transit from North Atlantic to the Pacific. What is left (~34 M) is ultra-refractory since it survived the ~1000 y trip through the deep ocean. This DOC is present as background DOC in surface waters and has an average age of ~6000 years. Hansell.

Larger pool of semi-labile DOC in surface water; τ = weeks Small pool of very labile (easily degradable) DOC in surface waters; τ = hours to 1 day Larger pool of semi-labile DOC in surface water; τ = weeks Refractory DOC; τ = 1000 years Ultra-refractory DOC; τ = >6000 y Open ocean surface DOC concentration is about 70 µM. Its about 44 µM in the deep Sargasso and about 34 µM in the deep Pacific.

Hydrolyzing/fermenting cell Under anoxic conditions it takes a consortium of organisms to degrade complex organic matter Very high Mol Wt DOC Fermentative cell High Mol Wt DOC Low Mol Wt DOC Hydrolyzing/fermenting cell Respiring cell Low Mol Wt DOC

Different organic fractions degrade at different rates

Ocean productivity by province % of Ocean Prod. 81% 18% 1% These values for productivity are old and a low estimate! Other recent estimates of global ocean productivity (e.g. Martin et al. 1987) are closer to 40-50 x 1015 gC/y. The distribution percentages, however will be similar to those shown here.

Hopkinson & Vallino. Nature 433: 2005

Seasonal cycle of DOC at the BATS station in the Sargasso Sea - Carlson et al 1994 Winter mixing homogenizes upper 200m & mixes down some DOC Spring build up of DOC

Global Carbon Cycle Problem Global CO2 release is known, but net increase in atmosphere is less than predicted Where does this carbon go? Some of the carbon can be accounted for by ocean uptake (see Quay et al.), but there is a missing sink of 0.7 GT. Terrestrial biomass (i.e. trees) might be missing sink. 0.7 GT of C is only 4% of net annual primary productivity on land and 3% of ocean carbon exchange with atmosphere, therefore it is hard to discern with accuracy. Ocean exchange in particular is difficult because of spatio-temporal shifts in carbon exchange. The role of the oceans in Carbon exchange is being studied intensively!

G3 G2 G1

Carbon generally not considered limiting to primary productivity in the sea - plenty of bicarbonate or CO2 in seawater (DIC = ~2 mM). The ratio of C:N:P in surface seawater is 1000:16:1. Thus C not likely to be limiting to Primary Production. However, the form of inorganic carbon available to phytoplankton does makes a difference. Phytoplankton take up predominantly the neutral species of DIC (CO2(aq) and H2CO3) so if pCO2 is low, phytoplankton can experience carbon limitation. Some species may have “carbon concentrating mechanisms” to transport HCO3-.

Oceanic/oligotrophic areas– dominated by picoplankton < 2 μm Upwelling, coastal & temperate areas have larger phytoplankton (> 2 μm) as major primary producers Considerations: Depth distribution i.e. euphotic depth Seasonal & interannual variations -seasonal variations

Deep DOC ~5900 years old Deep DOC ~4100 years old

Fig. 2. Observed values of the total Corg rparticle surface area loading of sediment in riverine, deltaic, nondeltaic continental margin, and deep-sea environments sediments Mayer, 1994a,b; Keil et al., 1997.. Despite contributions of both terrestrial and marine Corg , the particle surface area specific Corg load of deltaic material is comparable to oligotrophic deep sea sites that are essentially entirely marine Corg , indicating major loss from deltaic sediments relative to all source material. Approximate terrestrial and marine percentages ";15% for deltaic, shelf; ";5% for deep-sea.are based on typical bulk sediment isotopic rangese.g., Showers and Angle, 1986; Emerson and Hedges, 1988; Bird et al., 1995; Keil et al., 1997.. The riverine and deep Pacific Corg loading values represent simple averages of reported data "SD indicated., deltaic and nondeltaic shelf values represent slopes of Corg vs. particle surface area regressions"SE indicated.. The asymptotic value of Corg rarea at depth in sediment is used at a given site if a depth variation below the sediment–water interface is evidentMayer, 1994a,b..

Polysaccharides & Proteins Revised Molecular Size-Reactivity Continuum Model for Marine DOC (after Amon and Benner, 1996) This modification of the figure presented in Amon and Benner, 1996, attempts to illustrate that a large fraction of the total DOC (quantity is indicated by the distance between the two curves) is high molecular weight material (>10,000 MW). The material >1,000 MW, represented by polysaccharides, is relatively labile (high reactivity) when compared with the low molecular weight material (refractory humics) near and just below 1,000 MW. Together these pools make up the bulk of the DOC concentration. On the low end of the size spectrum, most compounds are labile (amino acids etc.), but their concentrations are very low (together making only 1% of DOC) but their reactivity is VERY high. High Labile Monomers - Amino acids - DMSP - sugars Log Reactivity Low concentration Labile Polysaccharides & Proteins Refractory humic substances quantity Low 10000 MW 1000 MW 500 MW 0 MW High Low Log Molecular Size All scales are somewhat arbitrary, and should probably viewed as a log-type scale