SCOPE 13 - The Global Carbon Cycle

ABSTRACT

The major input of organic carbon in the ocean appears to be primary productivity in the surface waters. Associated heterotrophic activity is mainly responsible for the recycling of labile organic compounds produced by primary producers. The resulting efficiency of recycling in the upper layer of the ocean is probably >90%. The remaining 10% is distributed into deep-sea pools of refractory dissolved and particulate organic carbon (DOC and POC). These pools are probably in a quasi-steady state as result of losses of POC to the sediment and deep-sea biological oxidation of POC and DOC in the water column and at the sedimentwater interface. In addition, the pools appear to be partially interconvertible through as yet poorly understood processes. Difficulties in clearly defining these processes have arisen, at least in part, as a result of analytical uncertainties in the determinations of DOC and POC.

The chemical compositions of DOC and POC are discussed and various interactions, such as intermolecular condensation and the formation of metal ion complexes, are examined. Cycling of DOC and POC is presented in terms of reservoirs of labile and refractory substances and estimates of the various fluxes are given. Furthermore, a discussion of the major existing problems in the areas of sources, cycling, and nature of DOC and POC in the ocean is presented.

11.1 INTRODUCTION

The purpose of this study is to summarize dissolved and particulate organic carbon (abbreviated as DOC and POC) cycles in the sea and to point out areas of ignorance and reported conflict in this field. Other detailed surveys with somewhat different emphases can be found elsewhere (e.g.: Menzel, 1974; Williams, 1975; Degens and Mopper, 1976; Morris and Eglinton, 1977).

11.2 INPUTS

The conventional definition of POC and DOC, based on filtration with a 0.45 µm filter, is used in this report. As a consequence of this definition, as much as 15% of DOC may be colloidal material (Sharp, 1973a). Table 11.1 and Figure 11.1 show the dimensions of the inputs and reservoirs of DOC and POC in the sea. It is emphasized that the values of DOC adopted in this report are obtained by wet chemical (Menzel and Vaccaro, 1964; Böhme, 1975) and/or photo-oxidation methods (Armstrong et al., 1966; Ehrhardt, 1969; Collins and Williams, 1977), and are generally lower by a factor of 2 to 3 than those of high-temperature wet or dry combustion techniques (Skopintsev and Timofeyeva, 1962; Sharp, 1973b; Gordon and Sutcliffe, 1973). The values for chemical/photo-oxidation are probably low as a result of incomplete oxidation of refractory DOC, while the high values for high-temperature combustion are often related to contamination problems. The combustion methods, after contamination problems are eliminated, yield values which are probably closer to reality than the chemical/photo-oxidation methods; however, the technique is not yet suitable for routine analysis. Furthermore, none of these methods has the necessary precision needed to discern slight natural fluctuations of DOC in the water column. Figure 11.2 clearly shows the effect of methodology on the resultant DOC profile obtained in an oceanic water column. From the above, it can be concluded that to the present day, the magnitude of one of the largest reservoirs of organic carbon on the earth (Figure 11.1) is known only within a factor of 2 to 3. Critical discussions of the various methods for determining DOC are presented by Wangersky (1975) and Collins and Williams (1977).

Table 11.1 Inputs, reservoirs and losses of organic material in the sea (modified from Williams, 1975)

Figure 11. 1 (a) Distribution of organic carbon pools in the ocean. (b) Comparison of oceanic and land organic carbon pools. (After Cauwet, 1978. Reproduced by permission of Gauthier-Villars Editeur, Paris.)

The DOC and POC imputs to the sea are usually divided into two categories: allochthonous (or external), such as from land and atmosphere, and autochthonous (or internal), such as from in situ photosynthesis. The results of stable carbon isotope analyses (Sackett and Thompson, 1963; Hunt, 1968; Williams and Gordon, 1970; Nissenbaum and Kaplan, 1972; Eadie and Jeffrey, 1973; Gearing et al., 1977) and direct chemical analyses of specific terrigenous `tag' compounds, such as glucose/ribose (Mopper, 1973; Mopper et al., 1977) lignin-related material (Pocklington and MacGregor, 1973; Gardner and Menzel, 1974) and humic substances (Ishiwatari, 1973, 1974; Stuermer and Harvey, 1974; Stuermer, 1975) indicate that, at the present time, autochthonous sources are dominant in the sea. Only nearshore environments appear to be influenced by land input, as shown in Figure 11.3. Thus, even though the organic content of rivers can be quite high, the quantitative significance of this source in the sea is low. The controlling factor appears to be organic flocculation in estuaries (Sieburth and Jensen, 1968; Kranck, 1973; Brown, 1975, 1977). As a result of this process, it is difficult to estimate the river input to the sea.

Figure 11.2 A comparison of dissolved or total organic carbon concentrations reported at various stations and dates in the Northwest Atlantic Ocean by authors using different analytical methods. Symbols designate the average concentration within successive layers: 0100m, 100500m, 5001000m, 10002000m, 20003000m, and deeper than 3000 m. Open squares, Duursma (1965) wet oxidation; open circles, Menzel (1967) wet oxidation; open triangles, Sharp (1973a) wet oxidation; solid squares, Sharp(1973b) wet high temperature combustion; solid triangles, Gordon and Sutcliffe(1973) dry combustion; solid circle,Skopintsev et al. (1966) dry combustion. (After Gordon and Sutcliffe, 1973. Reproduced by permission of Elsevier Scientific Publishing Co.)

Atmospheric sources, such as dry fallout and rainfall, are insignificant sources for particulate organic matter (POM) and dissolved organic matter (DOM) when compared to primary productivity (Duce and Duursma, 1977).

It is generally agreed that phytoplankton is the major source of DOC and POC in the oceans. In fact, their spatial concentrations usually follow those of plankton densities in the sea. Figure 10.3 (Chapter 10, this volume) shows the distribution of primary productivity in the sea.

11.3 NATURE AND INTERACTIONS OF DOC AND POC 

11.3.1 DOC

DOC is derived. from plankton both directly, through exudates (Hellebust, 1974), and indirectly, through lysis and partial mineralization of dead cells. These DOC compounds are rather labile and are eliminated rapidly by heterotrophic activity (dominantly microbial) in the surface layer of the sea (Ramsay, 1974; Bell et al., 1974; Pomeroy, 1974; Berman, 1975; Azam, 1976; Williams and Yentsch, 1976; Lee and Bada, 1977). A small fraction, probably less than 10% (Ogura, 1975) is apparently converted into high-molecular-weight complexes, which are then incorporated into a vast pool of biologically refractory DOC (Menzel, 1974).

Deep-water biotic and abiotic oxidation processes, in combination with. intermolecular condensation reactions (Stuermer, 1975; Hunter and Liss, 1977), and the formation of metal ion complexes (Williams, 1969; Siegel, 1971; Rashid and Leonard, 1973; Gonye and Carpenter, 1974; Signer, 1974; Batley and Florence, 1976; Perdue et al., 1976; Picard and Felbeck, 1976), will have the effect of further eliminating the more labile constituents, thereby increasing the degree of inertness (Barber, 1968; Ogura, 1970a) and overall molecular weight of POC and DOC. An increase in molecular weight of DOC with depth is, in fact, observed (Maurer, 1971; Ogura, 1974). Degens (unpublished data) determined that the bulk of DOC in the euphotic zone has a molecular weight <5000, with a maximum at 1500. In contrast, most of the DOC at a depth of 5000 m has a molecular weight >10 000. A concurrent increase in the degree of metal complexation with depth is also expected, although this parameter has not been studied.

Figure 11.3 13C values of organic matter of surface sediments from the continental shelf of the northeastern United States (after Hunt, 1968). Values of 30 to 24 indicate terrigenous sources, while values 24 to 18 indicate marine plankton sources. (Degens and Mopper, 1976. Reproduced with permission from Chemical Oceanography, Vol. 6 (2nd edn.), eds. J. P. Riley and R. Chester. Copyright by Academic Press Inc. (London) Ltd.)

Degens (1970) postulated that DOC consists predominantly of nitrogenous heteropolycondensates, which are held together by functional group condensations, urea clathrate complexes, and metal-ion coordination polyhedra. Skopintsev (1959) and Abelson and Hare (1971) postulated that the sugaramino acid condensates (melanoidins) constitute a significant fraction of inert DOC.

The chemical composition of DOC is largely unknown. Only 10 to 20% of it has been identified and found to consist of amino acids, fatty acids, carbohydrates, phenols, sterols, etc. (see Duursma, 1965; Wagner, 1969; Williams, 1975; Gagosian and Stuermer, 1977; Giger, 1977; Lee and Bada, 1977; Zsolnay, 1977; Mopper and Dawson, 1978). The analysis of the various biochemically important groups of compounds within DOC has proven to be exceedingly difficult, since these substances are present as traces in relation to inorganic salts. However, with improved analytical methods, such as the use of highly sensitive fluorescent reagents, it has been possible to inject sea-water samples directly into amino acid and sugar ion-exchange chromatographic analysers (Garrasi, 1978; Garrasi and Mopper, 1978; Mopper et al., 1978). Thus, by elimination of time-consuming and contaminating desalting and concentration steps, a detailed analysis of these compounds at sea will soon be possible using specially designed on-board analysers.

The improved analytical techniques were first tested routinely during the FLEX 76 experiment (see Chapter 1, this volume). As an illustration, we present a pattern of sugars as it develops from start to end of a spring plankton bloom (Figure 11.4).

Insights into the nature of DOC may also be gained by C/N analyses. Unfortunately several severe analytical difficulties need to be overcome (Williams, 1975) before this ratio can be of value.

11.3.2 POC

POC may also be considered as containing a labile, rapidly recycled fraction (living and dead plankton, and faecal pellets) and an older, refractory fraction. Refractory POC may be formed in part from DOC and vice versa (Parsons, 1975). The role of bacteria, bubbles, colloids, and extracellular enzymes in these transformations is not, at present, clear (Sutcliffe et al., 1963; Baylor and Sutcliffe, 1963; Riley, 1963; Riley et al., 1964; Barber, 1966; Sheldon et al., 1967; Aaronson, 1971; Sharp, 1972; Jannasch, 1973; Wheeler, 1975; Johnson, 1976).

In addition to the above-mentioned processes, POC may be formed from DOC by photochemical reactions at the airsea interface (Carlucci et al., 1969; Wheeler, 1972; Hansen, 1975; Zafiriou, 1974, 1977). However, the overall importance of these reactions to the oceanic cycling of DOC and POC still needs to be elucidated.

The average age of deep-sea DOC is approximately 3400 years (Williams et al., 1969; Arhelger et al., 1974); thus, POC formed from DOC may not only be more refractory than seston, but may also be significantly older. This refractory POC probably constitutes a major fraction of the organic input to deep-sea sediments, since the input of seston is probably considerably reduced relative to that of shallow-water sediments, as depicted in Figure 11.5. In fact, the age of organic matter in surface sediments of deep-sea and shallow-water environments has been found to be about 2000 to 3000 years and 200 to 400 years respectively (Degens, 1965). However, this age difference may also be partially attributed to differences in sedimentation rates in the two areas, as well as to homogenization of the sediments by burrowing organisms. For example, in the Argentine Basin, where the sedimentation rate is about 6 cm per 1000 years, homogenization of the upper 5 cm will yield an apparent age of about 400--600 years (assuming the organic input to be of zero age).

Figure 11.4 Shows depth and time profiles of water temperature, chlorophyll, and total dissolved sugars at a central station in the FLEX box situated in the North Sea (see Chapter 1, this volume). The time of highest sugar concentration coincides with the highest population density, represented here by the chlorophyll value. Concentration of sugars in the lower water layer remains the same as during the pre-bloom periods. This value may be taken as background concentration for this area. Ittekkot, 1978. Reproduced by permission of the author.)

Figure 11.5 Some aspects of the biogeochemical cycle in the ocean. Two major organic inputs to sediment are emphasized: (1) labile particulate organic carbon or tripton, and (2) inert POC which forms from aged dissolved organic carbon (DOC). The organic input to shallow water sediments is probably dominated by inert POC and is therefore considerably older. Diagenesis of organic matter results in the release of nutrients into the environment, thereby closing the cycle (modified from Degens and Mopper, 1976. Reproduced with permission from Chemical Oceanography, Vol. 6 (2nd edn.), eds. J. P. Riley and R. Chester. Copyright by Academic Press Inc. (London) Ltd.)

Difficulties in collecting sufficiently large quantities of POM have hindered the detailed study of its chemical composition (Cauwet, 1978). POM is conventionally collected by filtering large volumes of sea-water. Handa and Yanagi (1969) employed this collection method in order to accumulate sufficient material to study the order of decay of some components of POM in the upper 50 m of the western Pacific Ocean. The following order of stability was observed: waterextractable carbohydrates > organic nitrogenous compounds > total organic matter > water-insoluble carbohydrates.

Measurements of labile phytoplankton-related substances, such as ATP, chlorophyll, and DNA, have also yielded insights into the relative composition of POC (Steele and Baird, 1961, 1965; Menzel and Ryther, 1964; Holm-Hansen et al., 1970; Sutcliffe et al., 1970). From 20 to 50% of deep-sea POM is hydrolysable (Gordon, 1970; Degens, 1970) and is composed largely of proteins and other nitrogenous compounds. Lipids have also been detected in POM hydrolysates (Agatova and Bogdanov, 1972; Pustel'nikov and Urbanovich, 1975). A hypothetical representation of marine humus is shown in Figure 11.6 and the chemical characteristics of sedimentary marine humus are given in Table 11.2.

11.4 CYCLING AND FLUXES OF DOC AND POC

Since most organic matter produced in the euphotic zone is recycled through grazing and respiration, it is difficult to calculate the flux of organic matter leaving this zone. An indirect calculation can be made if the following fluxes are estimated: (i) flux of POC to the sediment, which is subsequently permanently trapped; (ii) flux of organic matter liberated from sediment; (iii) flux of organic matter biologically oxidized at the sedimentwater interface; (iv) flux of organic matter oxidized by deep-water metabolism; and (v) flux of POC to DOC.

In the following estimates of fluxes, an average primary productivity of 100 g C/ m² yr in the sea is assumed.

Figure 11.6 A schematic representation of a marine humus. The various types of interactions which occur are described below. (1) Organic-organic condensations between protein, carbohydrate, lipid, and lignin substances (the dashed lines represent hydrogen bonds). (2) Organic-mineral interactions such as condensation and adsorption of organic compounds on to the surface of a kaolinite-type mineral; the interaction of serine with silica has been demonstrated by Hecky et al. (1973) who showed that it is enriched in the mineralized tissures of diatom cell walls. (3) Organic-metal ion interactions result in an increase in the structural order of the humic material through the formation of metal-ion coordination polyhedra; the functional groups of different organic molecules may participate in this coordination and in one of the polyhedra shown, a sulphide group (from cysteine) replaces an oxygen, and the larger ionic radius of the sulphide ion distorts the polyhedron (Saxby, 1973). The incorporation of organic compounds into metal complexes appears to protect these substances from biological degradation. (4) Mineral-metal ion-organic interactions result when oxygen atoms on the mineral surface participate in metal-ion coordination polyhedra. (5) Micelle formation; molecules which contain both hydrophobic and hydrophilic portions (e.g. fatty acids) tend to aggregate so that the hydrophilic portion interacts with the aqueous environment, and the hydrophobic portion is protected from the aqueous environment. Indirect proof of the existence of micelles in fulvic acid is given by Ogner and Schnitzer (1970). (Degens and Mopper, 1976. Reproduced with permission from Chemical Oceanography; Vol. 6 (2nd edn.), eds. J. P. Riley and R. Chester. Copyright by Academic Press Inc. (London) Ltd.)

Table 11.2 Some chemical characteristics of sedimentary humic compounds (after Rashid, 1974)

11.4.1 DOC

Williams et al. (1969) employed the ¹⁴C dating method to arrive at an average age of deep-water (~2000 m) DOC of 3400 years. This age must be considered minimal, since the method used to convert DOC to CO₂, photo-oxidation, probably does not quantitatively release all highly refractory organic substances (see Section 11.2). Furthermore, contamination by ¹⁴C as a result of nuclear-bomb testing may yield misleadingly young ages. Despite these reservations, Williams (1971), on the basis of this age, calculated that about 0.5% of the mean annual primary productivity (corresponding to a flux of about 0.5 g C/m² yr) must enter the deep ocean in order to maintain the present reservoir of DOC. If a steady state is assumed for the concentration of deep-sea DOC, then the percentage of the average primary productivity leaving the euphotic cone to form deep-sea DOC (0.5%) must be compensated by an equivalent loss of this DOC, through processes such as deep-water metabolic oxidation and adsorption /condensation into POC (Riley, 1970). The latter flux is unknown; however, estimates of the former do exist. Using the DOC data of Menzel and Ryther (1970) for the North Atlantic, Craig (1971) calculated the rate of consumption of organic matter in deep water. Recalculation of Craig's results gives a consumption rate of approximately 1 to 2 g C/m² yr for a water column of 4000 m. It is important to note that Craig's calculations are based on the assumption that simple one-dimensional vertical diffusion-advection models can explain the vertical profile of the following parameters: inorganic CO₂, ¹⁴C, ¹³C, dissolved O₂, and alkalinity. On the basis of linear correlations between salinity and dissolved oxygen, Menzel and Ryther (1970) have suggested that lateral diffusion and advection are mainly responsible for the vertical profile observed for these parameters. Furthermore, the apparent biological inertness of deep-sea DOC (Rakestraw, 1947; Barber, 1968; Banoub and Williams, 1972), as well as the apparent invariance of DOC concentration with depth in the deep sea (Menzel, 1964; Menzel and Ryther, 1968, 1970; Ogura, 1970a, b; Kinney et al., 1971), have also been frequently cited as arguments against in situ deep-sea consumption of DOC. However, it should be emphasized that deep-water consumption takes place at an extremely slow rate, 4 to 12 x 10^-4 mg C/l yr, and to detect it experimentally would require a measuring period of over 200 years at present laboratory capabilities (Menzel, 1974). Furthermore, the reported depth invariance of DOC may be only a function of the measurement technique used since, as shown in Figure 11.2, the combustion methods show that DOC values either decrease slightly or go through a minimum in deep water, while the wet oxidation methods give invariant values. Thus, although the question of consumption vs. lateral mixing is still unresolved, evidence suggests that both processes affect the DOC distribution in the deep sea, but at different rates.

Comparison of Craig's calculation (1 to 2 g C/m² yr) with that of P. M. Williams (1971; 0.5 g C/m² yr) reveals that the consumption rate of deep-sea DOC is greater than the apparent supply rate by at least a factor of two to four. In order to reconcile this discrepancy, it is necessary to modify the steady-state assumption of Williams et al. (1969). As mentioned in Section 11.3.1, DOC may be viewed as consisting of two reservoirs: one that is labile and relatively young (e.g. exudates and products of cell lysis) and the other refractory and old (e.g. formed through the condensation of the former). If the extreme case is considered, in which refractory DOC consists exclusively of radioactively `dead' carbon (>60,000 years old), then young labile DOC (~200 years B.P.) must contribute 60 to 70% of the total DOC in order to yield the average <sup>14</sup> C `age' of 3400 years B.P. found by Williams et al. (1969). If, as a more realistic approach, it is assumed that the average age of the refractory DOC is in the range of 4000-6000 years B.P., then the young, labile DOC need only constitute 15-25% of the total DOC in order to yield the `age' in question. Interestingly, only 10 to 20% of DOC has been identified (see Section 11.3).

Thus, the flux calculated by Williams (1971) cannot represent the total flux of average primary productivity leaving the euphotic zone, but only that fraction ^,shunted off' to maintain the present reservoir of refractory DOC at its present residence time.

From the above estimates, the total flux of labile deep-sea DOC can be calculated. Assuming that this material is ~20% of the total DOC and has a turnover time of ~200 years, one arrives at an annual flux of ~10¹⁵ g C. This represents about 3% of the net annual primary production (Table 11.1). A summary of these different interrelationships is presented schematically in Figure 11.7. At the present state of the art, most of the fluxes in this figure can only be considered as rough approximations.

Figure 11.7 Turnover of organic matter in the open ocean (modified from Degens and Mopper, 1976. Reproduced with permission from Chemical Oceanography, Vol. 6 (2nd edn.), J. P. Riley and R. Chester. Copyright by Academic Press Inc. (London) Ltd.)

11.4.2 POC

In addition to deep-sea consumption of DOC, POC also appears to be utilized in deep water (Parsons and Strickland, 1962; Riley et al., 1965; Gordon, 1970; Agatova and Bogdanov, 1972; Eadie and Jeffrey, 1973; Pustel'nikov and Urbanovich, 1975; Krishnaswami and Sarin, 1976; Eadie et al., 1977). In fact, systematic decreases in POC concentrations as a function of depth have recently been reported (Gordon, 1977). A systematic increase in the C/N ratio of POM (particulate organic matter) from 5 to 8 in the euphotic to 12 to 15 in the deep sea (Hobson and Menzel, 1969; Gordon and Sutcliffe, 1973) indicates that proteinous material is utilized preferentially on descent. Detailed chemical analyses of POM by Handa et al. (1972) substantiate this finding. Furthermore, measurement of patchiness in deep-sea POC concentrations (Wangersky, 1976; Gordon, 1977) suggests that deep-sea metabolism occurs in patches and thus is not uniform throughout the water column.

In the past, non-utilization of POC in the deep sea was more generally accepted than today, since deep-sea profiles typically showed invariant POC concentrations (Menzel and Goering, 1966; Hobson and Menzel, 1969; Ryther et al., 1970; Banoub and Williams, 1972). However, as pointed out by Wangersky (1976) and Gordon (1977), past attempts to measure POC concentrations are open to criticism on the basis of the methods used. In fact, considerable problems are still involved in sampling deep-sea POC (Parsons, 1975; Breck, 1976), since most of the vertical flux of organic particles is represented by large, rapidly sinking particulate aggregates which are normally not captured by conventional samplers (McCave, 1975; Ichikawa and Nishizawa, 1975). For example, faecal pellets, which have been found in large quantities in sediment traps (Honjo, 1976), but rarely found in filter pads, are composed largely of readily degradable organic materials and thus may be a major source of input of utilizable POC, as well as DOC through enzymatic hydrolysis, in the deep sea. Large sediment traps, which are being placed at several depths in the ocean (Honjo, personal communication), should give valuable insights into the nature of POC, as well as its flux to the bottom.

Since the total flux of POC (inert + labile) to the sediment is still unknown, an estimate of its minimum value can be derived from the sum of the flux of organic matter, which is subsequently permanently trapped in the sediment (i.e. ~10 cm below the sediment-water interface), plus that which is biologically oxidized (mineralized) at the sediment surface. The former can be calculated for a given sediment from knowledge of the average sedimentation rate, organic carbon content, density, and water content. For Argentine Basin sediment (3500 m), an average organic carbon flux of 0.2 to 0.4 g C/m² yr was calculated by Degens and Mopper (1976, 1978). Fluxes of biological oxidation of the sediment surface have been determined by in situ measurements (Smith and Teal, 1973; Smith, 1974; Rowe et al., 1975; Smith et al., 1976). Fluxes vary from >150 g C/m² yr in shallow coastal sediments to about 0.1 g C/m² yr for sediment at a depth of 5200 m, and thus tend to correlate with depth variations in benthic biomass (Sanders and Hessler, 1969; Rowe, 1971; Rowe et al., 1974) and microbiological activity (Jannasch et al., 1971; Jannasch and Wirsen, 1973; Colwell and Kettling, 1974). Over a depth interval of 1000 to 5000 m the rate of biological consumption at the sediment-water interface may be taken as 0.5 to 3.0 g C/m² yr. Thus, when compared to the flux of organic matter permanently trapped in the sediment, it is apparent that 60 to 95% of the POC reaching deep-sea sediment is biologically consumed at the sediment-water interface. The same conclusion was reached by Eadie and Jeffrey (1973) and Eadie et al. (1977), based on ¹³C variations in POC.

Deuser (1971) calculated that 80 to 95% of POC produced in the euphotic zone of the Black Sea is recycled in the upper 200 m (just above the O₂ /H₂ S interface). Since the Black Sea is a restricted anoxic basin, these calculations cannot be applied to the oceans in general, as oxygen is present throughout and a greater efficiency of recycling is expected. For example, total DOC and POC deep-sea consumption rates of about 5 g C/m² yr (~5% of primary productivity) were calculated for the Pacific Ocean by Kroopnick and Craig (1976), on the basis of oxygen isotope fractionation of dissolved oxygen. Bacterial utilization apparently represents a sizeable fraction of this consumption, 0.2 to 2.4 g C/m² yr (Williams and Carlucci, 1976), while utilization by larger heterotrophs probably accounts for the rest (Fournier, 1966; Childress, 1968; Harding, 1974).

Table 11.3 Summary of organic fluxes in the sea

Flux	g C/m2 year	Reference
POC permanently trapped in	0.20.4	Degens and Mopper (1976)
sediment
Biological oxidation of POC in	0.53.0	Smith and Teal (1973)
sediment
Biological oxidation of POC in	2.04.0	Kroopnick and Craig (1976)
water column
Labile DOC converted to	0.40.6	Williams (1971)
refractory DOC
Deep water oxidation of	1.02.0	Craig (1971)
labile DOC
Flux of organic matter leaving	4.110.0
euphotic zone

Wong et al. (1976) constructed a simple box model to explain the distribution of particulate organic-associated iodine in the Atlantic. They concluded that only 3% of the iodine-containing POC reaches the deep sea and that less than 1% is finally incorporated into the sediment, which implies a recycling efficiency of 97% in the euphotic zone and about 70% in the deep sea (in the water column and at the sediment-water interface).

Nuclear-bomb related and naturally occurring radionuclides (Chung and Craig, 1973; Craig et al., 1973; Broecker et al., 1973; Aller and Cochran, 1976; Nozaki and Tsunogai, 1976; Thomson and Turekian, 1976) may be employed for following oceanic DOC and POC cycles; however, some difficulties of interpretation still exist (Bacon et al., 1976).

A summary of the various rates is given in Table 11.3. Assuming an average surface productivity of 100 g C/m² yr, the efficiency of recycling is about 90 to 95%.

11.5 CONCLUSIONS

There are many theories and hypotheses regarding the sources, cycling and nature of DOC and POC in the sea; however, there is insufficient substantiating data. Thus, we conclude this chapter by pointing out some of the major existing problem areas:

1. Analytical - the accurate measurement of DOC and POC dry combustion vs. wet oxidation; accurate measurement of DON (for C/N ratio); accurate measurement of ¹⁴C in DOC use of ¹⁴ C and other radionuclides as tracers of DOC and POC in marine ecosystems; extraction of metal-organic complexes determination of their nature and stability; quantitative extraction of marine `humus' nature of the so-called uncharacterized fraction changes with maturation; characterization of planktonic exudates and rates of hererotrophic utilization;

2. Sources external vs. internal; mechanisms of organic flocculation at mouths of rivers; percentage of DOC comprised of organic exudates vs. decay products;

3. Cycling biological utilization vs. nonutilization of DOC and POC in the deep sea the rates, and if utilization occurs, which organisms are responsible; transformation of DOC to POC and vice versa role of bubbles, bacteria, and extracellular enzymes; fluxes of POC leaving the euphotic zone and entering the sediment.

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