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Water column working group report
Marine Chemistry, 39 (1992) 15-25 Elsevier Science Publishers B.V., Amsterdam
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Water column working group report
INTRODUCTION
One of the central questions of this workshop has been: 'Why is oceanic organic chemistry important?' The fact that the organic material in the oceans is one of the larger reactive reservoirs of carbon on the Earth's surface is one obvious reason. Knowledge of the quantity and variety of oceanic organic components and their interactions with other systems, including food chain dynamics, ocean productivity, paleoceanography and the global carbon budget, is important for understanding global cycles of a variety of elements, not the least of which is carbon. In addition, we require detailed information of oceanic organic chemistry to determine how, and to what magnitude, anthropogenic perturbations may alter oceanic processes. For example, what role does the oceanic organic carbon play in cycling of biologically active carbon pools? How will the modification of global surface temperatures and the increase in UV radiation associated with anthropogenic trace gases affect this role? Conversely, will changes in the oceanic carbon cycle have some feedback effect on green-house warming? Organic matter in the oceanic water column is one of the most interactive organic pools on the Earth's surface because of the interfaces between the ocean and the atmosphere, sediments, and biota. With this in mind, three general questions are of central importance to improving our understanding of the role that water column processes play in oceanic chemistry. (1) What is the nature of oceanic organic matter? (2) What processes control water column organic chemistry? (3) How does water column organic chemistry in turn influence other oceanographic processes and global biogeochemical cycles? MAJOR AREAS OF CONCERN
Nature of oceanic organic matter
Recent developments in analytical techniques, in particular for dissolved organic carbon, have added a totally new dimension to this long-standing Correspondence to: J. Farrington, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
0304-4203/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
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WATER C O L U M N W O R K I N G G R O U P REPORT
question. As a result, the need to estimate total inventories of organic material in the ocean has re-emerged, including assessing the organic chemical composition of this newly detected material, as well as the previously known but still poorly characterized remaining macromolecular material. The role that macromolecular material plays in biogeochemical cycles is just beginning to be addressed by marine organic chemists. The need to pay considerable attention to accurately and precisely measuring the pool of macromolecular material in seawater and sedimentary porewaters was the topic of a separate working group at this workshop. The reader is referred to the background paper by Ishiwatari (1992) as well as the working group report which follows. Influences on the organic chemistry of the water column
Biogeochemical processes in the ocean and at its margins determine the chemical composition and behavior of organic matter in seawater. The nature of organic matter sources (i.e. autochthonous production vs. delivery of allochthonous material), the site (shallow water, deep water, sediment-water interface) and mechanism (microbial, zooplankton metabolism, abiotic) of decomposition processes, and the physical form of organic compounds (dissolved, colloidal, particulate) all are involved in controlling the organic chemistry of seawater. Two broad areas of future research have been recognized and can be categorized as (1) the transfer of organic matter across interfaces and (2) the reactivity of organic matter. Transfers of organic matter across interfaces The chemical composition and properties of organic matter in seawater result from a large number of interactive processes, many of which occur in the bulk medium. However, many important processes occur at interfaces, for example, the interface between the continents and the ocean, between particulate and dissolved pools, between the water column and the sediment, and between the atmosphere and the ocean. The nature of these sources and sinks, and the factors controlling the rates of interaction between them, are not well understood at present. Organic matter transport to and from the ocean. The bulk of the organic matter in seawater is produced in situ by marine organisms; initially by the primary producers but subsequently by the consumers and secondary producers of organic matter. Most of our knowledge of the distributions of organic compounds produced by marine organisms is severely constrained by the sampling tools available to acquire material to study and by the analytical tools available to determine molecular structures. Organic compounds biosynthesized by phytoplankton and macrozooplankton have been extensively
WATER COLUMN WORKING GROUP REPORT
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(though by no means exhaustively) inventoried. But what about the organic composition of viruses, bacteria, and microplankton? Macromolecular material (biopolymers) exuded or egested by marine organisms remains virtually uncharacterized because of difficulties in collecting it and in handling it analytically. The inventory of organic compounds consumed by organisms is likewise limited by experimental capabilities. Why is the large dissolved organic matter pool apparently resistant to consumption by marine organisms? Organic matter of continental origin is another important source of organic compounds to the ocean. Terrestrial organic material represents a significant input to coastal areas and hence to the biogeochemical processes occurring at ocean margins. Although present evidence suggests that terrestrial material is only a small fraction of the total organic matter in the ocean, its signature is detectable in sediments and in the dissolved organic matter (DOM) pool in pelagic regions. Fundamental differences in bulk chemical properties between terrestrial and marine-derived organic matter have been recognized, and there is now ample evidence, although not always at a molecular level, that organic material transferred across the continent-ocean interface is markedly different chemically, and is often unique (i.e. lignins, tannins, soil humus, xenobiotics), compared with material biosynthesized by marine organisms. It is likely that marine and terrestrial compounds behave in very different manners at the continent-ocean interface in terms of physical transport and biological behavior. The extent to which these differences are due to variable molecular structures or to the physical matrix with which the organic compounds are associated needs to be assessed. As dissolved organic material is transported seaward through estuaries, varying amounts are removed via flocculation, the amount apparently depending on the river-estuarine system in question. What happens to colloidal organic matter and to particles which may eventually be formed as freshwater mixes with seawater? Organic compounds may be released from particulate matter which disaggregates or they may sorb onto newly forming particles. How do these exchanges affect the transport of organic material to the ocean? And, finally, how much organic matter produced in the ocean is transported landward, for example to be sequestered in coastal wetlands? The interface between the ocean and the atmosphere is one of the larger interfaces on earth. Eolian transport mechanisms have been shown to carry material great distances from continental sources to the open ocean; yet quantitative data on relative fluxes of aerosols and gases between the atmosphere and the ocean are few, and a comprehensive understanding of air-sea exchange is therefore limited. A number of areas are ripe for additional study. What role does the sea-surface microlayer play in controlling the composition of organic compounds transferred from the atmosphere into the underlying water column or from seawater into the atmosphere? The relative importance of biological vs. photochemical transformation processes in the microlayer
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must be quantified. Do certain components of the material transported to the ocean via the atmosphere stimulate or inhibit the primary production of new organic matter in the sea? Processes at the seafloor. Sediments accumulate at the seafloor as a result of the delivery of particulate material via either vertical or horizontal processes. The sediment-water interface is a geochemically active zone. How do the transformation reactions at the sediment-water interface vary seasonally, or with variations in the delivery or composition of organic matter from the water column (both of which may themselves vary seasonally)? We know that the flocculent material at the sediment-water interface can be resuspended by bottom currents (hence the nepheloid layer), but we have little knowledge of the importance of resuspension for introducing organic compounds from the sediments into the overlying water column. This process may be very important in continental margin regions. Pore-waters are known to contain elevated concentrations of dissolved and colloidal material compared with bottom waters, and it is probable that the composition of organic matter in porewaters is markedly different from that of bulk seawater. What is the flux and composition of organic matter moving from pore-waters into bottom waters, and how does this material influence organic geochemical processes at the seafloor? The role of the macro-, meio-, and microbenthos in early diagenetic reactions should be more fully characterized. Phase associations o f seawater organic matter. Considerable progress has been made during the past decade toward characterizing the organic composition of particulate material in the water column and in understanding its behavior and cycling. There has been less effort to characterize the composition and cycling of the dissolved organic pool. Unfortunately, however, virtually no work has been done on the material between truly dissolved and particulate, the colloidal material. We do not even have quantitative data on the inventory of colloidal material in the ocean. There no doubt is significant exchange of organic compounds between dissolved, colloidal, and particulate pools, as well as between suspended particles and rapidly sinking particles. Condensation reactions produce bio- or geoplymers and may be catalyzed by particulate material. Are the dissolved polymers transformed into particles containing the same or different macromolecular components? The physical matrix in which organic compounds are associated probably plays an important role in determing the fate of organic materials in the water column. For example, a number of studies have hypothesized that the physical packaging of marinederived and terrestrially derived organic compounds is different and results in differing apparent reactivities (or preservation) of these two organic types. However, a systematic and conclusive test of this hypothesis has yet to be conducted.
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Reactivity and preservation of organic matter The distribution of organic matter in seawater is controlled by the rates and mechanisms of transformation and decomposition. The labile material is rapidly lost from seawater, whereas the more refractory material enters the long-lived dissolved pool or, if particulate, is eventually sequestered in the sediment. The reactivity of organic compounds is related to its source, molecular structure, location in the water column, and physical packaging. At present, we have an imperfect understanding of the degradation rates and mechanisms, and even the fundamental parameters which determine reaction pathways and kinetics in nature.
Rates of organic matter cycling. Vertical profiles of dissolved and particulate organic materials suggest that most of the material biosynthesized in surface waters is degraded in the upper several hundred meters of the water column. We presume, in the absence of data to the contrary, that most of this degradation is biological. However, there is little information available on precise rates of cycling of organic compounds through the biota. We often observe the disappearance of a given substrate but not the appearance of degradation products (often we simply do not look for products, let alone make measurements of reaction kinetics under ambient conditions). What organisms are most important for various organic compounds or for the same compounds in dissolved and particulate pools? To what extent, and why, are degradation rates in deep waters different from rates in surface waters, apart from temperature and pressure effects? Even though biotic processes may dominate the water column fate of organic materials, abiotic processes certainly play a role. Clay-catalyzed chemical reactions are known to occur in sediments, but the importance of analogous reactions in the water column is unknown. Photochemistry in surface waters plays an important role in the cycling of dissolved organic matter. There is some evidence that photochemistry is important in producing macromolecular material via light-induced cross-linking reactions and also in breaking down macromolecular material into smaller substrates which are more readily handled by bacteria or which escape to the atmosphere. What are the rates of these reactions, their mechanisms, and what are the end-products? Are kinetic or thermodynamic considerations dominant? What are the rate constants which control organic-particle adsorption and organic-metal complexations, and how do these constants vary in different parts of the ocean?
Speciation effect on bioavailability. The efficiency of biological processes in degrading organic matter can depend on the form of the organic substrate and the ability of enzyme systems to react with the organic compound, or its bioavailability. Complexation of organic matter with metals, sorption onto surfaces, or associations with colloids may 'protect' organic matter from
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enzymatic attack. There may be different degrees of bioavailability for the same organic compounds as a function of their physical form. For example, are compounds associated with particles biodegraded at significantly different rates compared with the same compounds in the dissolved pool? Why and how does adsorption of organics to surfaces and complexation with metals affect the availability of the organic compounds to organisms? These issues are important not only for fundamental understanding of the cycling of biogenic materials in the ocean but also for predicting the fate of xenobiotics which may be introduced into the marine environment either intentionally or accidentally. Consideration of the form which organic compounds take and their resulting availability to the biota is critical in the design of meaningful experiments to assess biological transformations and degradation. Effect of oceanic organic matter on other biogeochemical processes
Organic chemicals in seawater play more than merely a passive role. They actively influence major oceanographic processes and, on a larger scale, global biogeochemical cycles. Perhaps the most widely observed manifestation of this fact is the concern about acute and chronic effects on marine systems from xenobiotics. However, organic matter is also actively involved in the cycling of inorganic materials, and, on a global scale, in non-marine systems as well. Many of these processes would not occur in the absence of organic constituents, and marine organic geochemical studies provide a basis for better understanding them. Influence of organic materials & seawater on atmospheric processes As an example, we may consider the impact of seawater organic chemistry on global climate. It has long been recognized that seaspray has been a mechanism for transferring salts (e.g. sea salts) from the ocean into the atmosphere and eventually into the terrestrial environment. It is now becoming apparent that organic compounds, such as organic halides and organic sulfur compounds, are produced in seawater and delivered to the atmosphere in significant quantities. When materials such as dimethyl sulfide are oxidized in the atmosphere there is a possibility that cloud condensation nuclei form in sufficient quantities to influence global cloud patterns and the Earth's albedo. At present, it is not fully understood how feedback mechanisms operate in this ocean-atmosphere cycling of these organic and inorganic sulfur compounds. Emissions of organic halides must play a role in controlling distributions of tropospheric and stratospheric ozone (and other atmospheric oxidants). Oxidation of all ocean-derived organic matter in the atmosphere contributes to acidification of aerosols. These sea-air transfers of organic chemicals need to be quantified ultimately to allow predictions of how the ocean controls aspects of the climate and air quality of the earth.
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Impact of organic matter on chemical speciation The presence of organic matter undoubtedly plays a major role in controlling the fate of other chemicals (including pollutants) in the sea. First, we know that all chemicals in seawater solution/suspension assume some distribution of their total presence amongst a set of interconvertible dissolved species. For organic chemicals, metals, and radionuclides, one subset of these species is that resulting from associations with dissolved components of the organic pool. Thus to proceed with any efforts to estimate the fate of such substances, we must be able to calculate the equilibria (and probably the kinetics) describing these complexations. Further, to permit quantitative understandings to be applied in the future, we must recognize the chemical characteristics of seawater organic matter which dictate the intensity (or rate) of these interactions. Although such impacts of marine organic material on speciation are widely recognized, we are not yet knowledgeable enough to include them in our models to estimate the exposure of marine organisms to either nutritional or toxic compounds. A related problem involves our inability t 9 predict a priori the sorption (both equilibria and kinetics) of inorganic and organic substances to natural marine particles. As natural particles always acquire organic compounds on their surfaces, sorption involves the possibility of interactions with structural elements of surface organic layers. Such heterogeneous associations may differ from those occurring in solution only in so far as particles induce bulk electrostatic effects (i.e. electric double layers) or changes in water solvency (i.e. vicinal water). Therefore, as for solution complexation, we need to evaluate the energies of inorganic and organic sorbate interactions with structural moieties of the surface organic constituents. There is a major research problem blocking continued progress in our understanding of the cycling of specific compounds: the development of means to predict/describe how a heterogeneous mixture of organic material (dissolved and particulate) associates with specific chemicals of interest. This must include efforts to identify critical subsets of ligands or sorbent sites, to develop methods to quantify these organic moieties in seawater samples, to complete our data describing specific organic moiety-chemical interaction energies, and, finally, to clarify the reaction kinetics and/or mass transfer limitations affecting these associations. Organic compounds and marine food chains Although the primary source of organic matter in the water column is autotrophic fixation of inorganic carbon into organic matter, most of the rest of the biological community depends on organic compounds as a source of nutrition. The organic chemical composition of living and dead organic matter has a significant influence on the structure and function of the marine food web. However, the nutritional requirements of the various forms of
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marine organisms are poorly understood, and as a consequence the nutritional value of organic materials as a whole is poorly characterized. Clearly, though, the spatial and temporal distributions of organic species in the water column resulting from both primary sources (autotrophic) and secondary sources (after heterotrophic modification) will determine where marine organisms will live and how they will function. For example, regions characterized by high primary production are usually regions with important commercial fisheries. Dissolved organic chemicals are also important to marine organisms if the compounds act as chemical mediators. Secondary consumers probably sense the presence of food items at significant distances using minute quantities of dissolved compounds as chemical keys. Chemical mediators are also actively involved in reproduction and as chemical repellents to prevent colonization or ingestion. But how organisms sense these chemical signals, and what the relevant compounds actually are, is very poorly known. The large dissolved organic matter pool in seawater is an enigma, considering the efficiency of many enzyme systems for decomposing organic compounds. What has prevented the evolution of enzyme systems capable of attacking the 'refractory' material? Also, considering the enormous numbers of bacteria in the sea and their surface areas, why is dissolved organic material still so abundant, even if it is dilute? Impact of organic matter on the fate of solids in the sea The transport and dissolution/precipitation of particles in the sea is also an issue requiring consideration of the influences of organic materials. We have long realized that minerals become coated with organic matter in natural waters. This results in major changes in the surface properties of those inorganic particles (e.g. surface charge reversal on iron oxides) and thereby affects their coagulation. Certain resulting effects on particle flocculation/ stabilization have been noted, but we are still unable to predict the rates of particle-particle attachment in seawater. This may be due to our inability to deal with issues such as steric hindrance deriving from the extension of organic chains from particle surfaces in flocculation phenomena. Dissolution and precipitation reactions also are affected by surface organic coatings on particles. Quantitative treatments of such dissolution kinetics as modified by organic coatings would help us understand vertical fluxes of labile solids (e.g. carbonates, silica) and associated elements (e.g. Sr, Ra) in the sea. Another major issue on which our ignorance is great concerns the factors limiting the physical detachment of particles from one another. It is possible that organic components act as surfactants and thereby encourage such disaggregation. As a result, it is clear that sedimentation of solids through the sea, and even their efficient collection into the sea-bed, is strongly influenced by the modifications of their surfaces by constituents of the organic coatings. As a result, we need to describe the character of surfaces on which these
WATER COLUMN WORKING GROUP REPORT
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coatings occur. This probably requires an understanding of the threedimensional organization of the submicron interfacial region. Application of this improved physical image to currently available physical chemical descriptions of coagulation/detachment and precipitation/dissolution is necessary to improve our ability to describe transport of particulate materials in the sea. Water column organic matter and sea-floor and sea-bed processes Organic matter in the water column is the dominant source of organic compounds to the sea-floor. It is thus an important source of nutrition to benthic organisms; it is the source of biomarkers which, if preserved in the sediment record, can provide information about paleoenvironmental conditions; and it contributes to the geological storage of organic matter. Other elements associated with sedimenting organic matter will also become part of the sedimentary record. Finally, organic compounds which are derived and modified by water column processes will influence the chemical and physical properties of interstitial fluids in both sedimentary material and hydrothermal vent areas. This provides another critical link between processes occurring in the contemporary oceanic water column and processes occurring in the seafloor. Optical properties of seawater The optical properties of seawater are strongly influenced by both dissolved and particulate organic material. Material concentrated in the sea-surface microlayer will affect the physical properties of the air-sea interface, for example, the dampening of capillary waves and thus the impact on the back-scattering of radiation, its adsorption or fluorescence at the sea surface, and its transmission into underlying surface waters. It is likely that the amount of the microlayer, and its specific molecular organic composition, will be quantitatively important in the degree of back-scattering, adsorption, transmission, etc., and in determining the spectral quality of radiation which does reach the bulk surface water. The spectrum and intensity of light penetrating into surface waters will affect the extent of photosynthesis, as photosynthesis depends on light intensity and on the available spectrum. Photochemical production and composition of parts of the organic matter pool will be dependent on the light regime as well as the organic composition, thus adding or removing material from the organic pool. In turn, the abundance and organic composition of living and dead particles will play a further role in moderating the adsorption, scattering and fluoresence of the remaining radiation. OVERVIEW
A considerable amount of information is available in a qualitative sense
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about processes in the water column. However, there are major gaps in our knowledge about rates and reaction mechanisms of specific transformations involving organic constituents and this knowledge is important for understanding the overall biogeochemistry of seawater and sediments. As many transformation reactions are mediated by biological systems and influenced by physical-chemical parameters, organic geochemists need to expand interactions across disciplines with biological, physical and inorganic geochemical colleagues. Of equal importance is the pressing need to employ novel sampling strategies and analytical techniques to characterize the composition and behavior of organic matter fractions which are outside the realm of current studies. RECOMMENDATIONS
(1) A much greater amount of quantitative information on the sources of organic matter in the ocean is required. This includes further inventorying of compounds produced by micro- and macroorganisms and the mechanisms by which organic compounds are introduced into the water column, for example, exudation from living cells vs. release from detritus. (2) The flux of organic matter between the ocean and the continents, atmosphere, and sediments needs to be guaranteed. These studies should include the determination of the extent to which material is altered during exchange, especially for the poorly characterized non-lipid material. Simultaneous sampling in the water column, across the air-sea interface, and at the sediment-water interface are needed. The importance of lateral and upward flux of particulate and dissolved organic material must be determined. (3) We must increase the use of physical-chemical properties of organic substances to quantify the behavior of these materials in the sea. This information should be applied to quantitative models comparing theoretical and field studies and predicting the behavior of other substances. The role of organic-surface interactions and organic-metal complexation on the behavior of organic substances (as well as inorganic materials) needs further attention. (4) The influence of the chemical composition of organic matter in seawater in controlling the absorption, scattering, fluorescence, and transmission of incident radiation, and the resulting light field in the ocean needs to be elucidated. Studies should also relate organic matter distributions to parameters determined by in situ and remotely based optical sensing techniques. (5) Investigations of the chemical and biological reactivity of various organic structural classes and molecular weight classes at the sea surface, in the water column, and at the sediment-water interface need to be expanded. Greater quantitative understanding is needed of the mechanisms and biologi-
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cal/abiological turnover rates under a variety of natural conditions, with different organic matter types, and by different types of organisms. Coordinated laboratory and field efforts involving investigators of different disciplines and analytical capabilities are urgently needed. (6) Continued effort is needed in relating the role of dissolved and particulate organic compounds to marine food web dynamics. What is the nutritional value of organic compounds for marine organisms? Studies on the role and function of chemical mediators in the biophysiology of marine organisms, the mechanisms by which enzyme systems function, how microorganisms attack labile organic matter and why such a large part of the dissolved organic matter pool is 'refractory' need to be included. (7) Sampling and analytical technologies, although not explicitly discussed in this report, must continue to develop, to provide new sample types for study and new analytical tools to make possible advances in characterization of oceanic organic material. Novel experimental designs are needed, both in the laboratory and in the field, to assess pressing quantitative questions about rates and mechanisms of reactions involving organic matter. REFERENCE Ishiwatari, R., 1992. Macromolecular material (humic substance) in the water column and sediments. Mar. Chem., 39: 151-166.
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Water column working group report
INTRODUCTION
One of the central questions of this workshop has been: 'Why is oceanic organic chemistry important?' The fact that the organic material in the oceans is one of the larger reactive reservoirs of carbon on the Earth's surface is one obvious reason. Knowledge of the quantity and variety of oceanic organic components and their interactions with other systems, including food chain dynamics, ocean productivity, paleoceanography and the global carbon budget, is important for understanding global cycles of a variety of elements, not the least of which is carbon. In addition, we require detailed information of oceanic organic chemistry to determine how, and to what magnitude, anthropogenic perturbations may alter oceanic processes. For example, what role does the oceanic organic carbon play in cycling of biologically active carbon pools? How will the modification of global surface temperatures and the increase in UV radiation associated with anthropogenic trace gases affect this role? Conversely, will changes in the oceanic carbon cycle have some feedback effect on green-house warming? Organic matter in the oceanic water column is one of the most interactive organic pools on the Earth's surface because of the interfaces between the ocean and the atmosphere, sediments, and biota. With this in mind, three general questions are of central importance to improving our understanding of the role that water column processes play in oceanic chemistry. (1) What is the nature of oceanic organic matter? (2) What processes control water column organic chemistry? (3) How does water column organic chemistry in turn influence other oceanographic processes and global biogeochemical cycles? MAJOR AREAS OF CONCERN
Nature of oceanic organic matter
Recent developments in analytical techniques, in particular for dissolved organic carbon, have added a totally new dimension to this long-standing Correspondence to: J. Farrington, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
0304-4203/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
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WATER C O L U M N W O R K I N G G R O U P REPORT
question. As a result, the need to estimate total inventories of organic material in the ocean has re-emerged, including assessing the organic chemical composition of this newly detected material, as well as the previously known but still poorly characterized remaining macromolecular material. The role that macromolecular material plays in biogeochemical cycles is just beginning to be addressed by marine organic chemists. The need to pay considerable attention to accurately and precisely measuring the pool of macromolecular material in seawater and sedimentary porewaters was the topic of a separate working group at this workshop. The reader is referred to the background paper by Ishiwatari (1992) as well as the working group report which follows. Influences on the organic chemistry of the water column
Biogeochemical processes in the ocean and at its margins determine the chemical composition and behavior of organic matter in seawater. The nature of organic matter sources (i.e. autochthonous production vs. delivery of allochthonous material), the site (shallow water, deep water, sediment-water interface) and mechanism (microbial, zooplankton metabolism, abiotic) of decomposition processes, and the physical form of organic compounds (dissolved, colloidal, particulate) all are involved in controlling the organic chemistry of seawater. Two broad areas of future research have been recognized and can be categorized as (1) the transfer of organic matter across interfaces and (2) the reactivity of organic matter. Transfers of organic matter across interfaces The chemical composition and properties of organic matter in seawater result from a large number of interactive processes, many of which occur in the bulk medium. However, many important processes occur at interfaces, for example, the interface between the continents and the ocean, between particulate and dissolved pools, between the water column and the sediment, and between the atmosphere and the ocean. The nature of these sources and sinks, and the factors controlling the rates of interaction between them, are not well understood at present. Organic matter transport to and from the ocean. The bulk of the organic matter in seawater is produced in situ by marine organisms; initially by the primary producers but subsequently by the consumers and secondary producers of organic matter. Most of our knowledge of the distributions of organic compounds produced by marine organisms is severely constrained by the sampling tools available to acquire material to study and by the analytical tools available to determine molecular structures. Organic compounds biosynthesized by phytoplankton and macrozooplankton have been extensively
WATER COLUMN WORKING GROUP REPORT
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(though by no means exhaustively) inventoried. But what about the organic composition of viruses, bacteria, and microplankton? Macromolecular material (biopolymers) exuded or egested by marine organisms remains virtually uncharacterized because of difficulties in collecting it and in handling it analytically. The inventory of organic compounds consumed by organisms is likewise limited by experimental capabilities. Why is the large dissolved organic matter pool apparently resistant to consumption by marine organisms? Organic matter of continental origin is another important source of organic compounds to the ocean. Terrestrial organic material represents a significant input to coastal areas and hence to the biogeochemical processes occurring at ocean margins. Although present evidence suggests that terrestrial material is only a small fraction of the total organic matter in the ocean, its signature is detectable in sediments and in the dissolved organic matter (DOM) pool in pelagic regions. Fundamental differences in bulk chemical properties between terrestrial and marine-derived organic matter have been recognized, and there is now ample evidence, although not always at a molecular level, that organic material transferred across the continent-ocean interface is markedly different chemically, and is often unique (i.e. lignins, tannins, soil humus, xenobiotics), compared with material biosynthesized by marine organisms. It is likely that marine and terrestrial compounds behave in very different manners at the continent-ocean interface in terms of physical transport and biological behavior. The extent to which these differences are due to variable molecular structures or to the physical matrix with which the organic compounds are associated needs to be assessed. As dissolved organic material is transported seaward through estuaries, varying amounts are removed via flocculation, the amount apparently depending on the river-estuarine system in question. What happens to colloidal organic matter and to particles which may eventually be formed as freshwater mixes with seawater? Organic compounds may be released from particulate matter which disaggregates or they may sorb onto newly forming particles. How do these exchanges affect the transport of organic material to the ocean? And, finally, how much organic matter produced in the ocean is transported landward, for example to be sequestered in coastal wetlands? The interface between the ocean and the atmosphere is one of the larger interfaces on earth. Eolian transport mechanisms have been shown to carry material great distances from continental sources to the open ocean; yet quantitative data on relative fluxes of aerosols and gases between the atmosphere and the ocean are few, and a comprehensive understanding of air-sea exchange is therefore limited. A number of areas are ripe for additional study. What role does the sea-surface microlayer play in controlling the composition of organic compounds transferred from the atmosphere into the underlying water column or from seawater into the atmosphere? The relative importance of biological vs. photochemical transformation processes in the microlayer
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WATER COLUMN WORKING GROUP REPORT
must be quantified. Do certain components of the material transported to the ocean via the atmosphere stimulate or inhibit the primary production of new organic matter in the sea? Processes at the seafloor. Sediments accumulate at the seafloor as a result of the delivery of particulate material via either vertical or horizontal processes. The sediment-water interface is a geochemically active zone. How do the transformation reactions at the sediment-water interface vary seasonally, or with variations in the delivery or composition of organic matter from the water column (both of which may themselves vary seasonally)? We know that the flocculent material at the sediment-water interface can be resuspended by bottom currents (hence the nepheloid layer), but we have little knowledge of the importance of resuspension for introducing organic compounds from the sediments into the overlying water column. This process may be very important in continental margin regions. Pore-waters are known to contain elevated concentrations of dissolved and colloidal material compared with bottom waters, and it is probable that the composition of organic matter in porewaters is markedly different from that of bulk seawater. What is the flux and composition of organic matter moving from pore-waters into bottom waters, and how does this material influence organic geochemical processes at the seafloor? The role of the macro-, meio-, and microbenthos in early diagenetic reactions should be more fully characterized. Phase associations o f seawater organic matter. Considerable progress has been made during the past decade toward characterizing the organic composition of particulate material in the water column and in understanding its behavior and cycling. There has been less effort to characterize the composition and cycling of the dissolved organic pool. Unfortunately, however, virtually no work has been done on the material between truly dissolved and particulate, the colloidal material. We do not even have quantitative data on the inventory of colloidal material in the ocean. There no doubt is significant exchange of organic compounds between dissolved, colloidal, and particulate pools, as well as between suspended particles and rapidly sinking particles. Condensation reactions produce bio- or geoplymers and may be catalyzed by particulate material. Are the dissolved polymers transformed into particles containing the same or different macromolecular components? The physical matrix in which organic compounds are associated probably plays an important role in determing the fate of organic materials in the water column. For example, a number of studies have hypothesized that the physical packaging of marinederived and terrestrially derived organic compounds is different and results in differing apparent reactivities (or preservation) of these two organic types. However, a systematic and conclusive test of this hypothesis has yet to be conducted.
WATER COLUMN WORKING GROUP REPORT
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Reactivity and preservation of organic matter The distribution of organic matter in seawater is controlled by the rates and mechanisms of transformation and decomposition. The labile material is rapidly lost from seawater, whereas the more refractory material enters the long-lived dissolved pool or, if particulate, is eventually sequestered in the sediment. The reactivity of organic compounds is related to its source, molecular structure, location in the water column, and physical packaging. At present, we have an imperfect understanding of the degradation rates and mechanisms, and even the fundamental parameters which determine reaction pathways and kinetics in nature.
Rates of organic matter cycling. Vertical profiles of dissolved and particulate organic materials suggest that most of the material biosynthesized in surface waters is degraded in the upper several hundred meters of the water column. We presume, in the absence of data to the contrary, that most of this degradation is biological. However, there is little information available on precise rates of cycling of organic compounds through the biota. We often observe the disappearance of a given substrate but not the appearance of degradation products (often we simply do not look for products, let alone make measurements of reaction kinetics under ambient conditions). What organisms are most important for various organic compounds or for the same compounds in dissolved and particulate pools? To what extent, and why, are degradation rates in deep waters different from rates in surface waters, apart from temperature and pressure effects? Even though biotic processes may dominate the water column fate of organic materials, abiotic processes certainly play a role. Clay-catalyzed chemical reactions are known to occur in sediments, but the importance of analogous reactions in the water column is unknown. Photochemistry in surface waters plays an important role in the cycling of dissolved organic matter. There is some evidence that photochemistry is important in producing macromolecular material via light-induced cross-linking reactions and also in breaking down macromolecular material into smaller substrates which are more readily handled by bacteria or which escape to the atmosphere. What are the rates of these reactions, their mechanisms, and what are the end-products? Are kinetic or thermodynamic considerations dominant? What are the rate constants which control organic-particle adsorption and organic-metal complexations, and how do these constants vary in different parts of the ocean?
Speciation effect on bioavailability. The efficiency of biological processes in degrading organic matter can depend on the form of the organic substrate and the ability of enzyme systems to react with the organic compound, or its bioavailability. Complexation of organic matter with metals, sorption onto surfaces, or associations with colloids may 'protect' organic matter from
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enzymatic attack. There may be different degrees of bioavailability for the same organic compounds as a function of their physical form. For example, are compounds associated with particles biodegraded at significantly different rates compared with the same compounds in the dissolved pool? Why and how does adsorption of organics to surfaces and complexation with metals affect the availability of the organic compounds to organisms? These issues are important not only for fundamental understanding of the cycling of biogenic materials in the ocean but also for predicting the fate of xenobiotics which may be introduced into the marine environment either intentionally or accidentally. Consideration of the form which organic compounds take and their resulting availability to the biota is critical in the design of meaningful experiments to assess biological transformations and degradation. Effect of oceanic organic matter on other biogeochemical processes
Organic chemicals in seawater play more than merely a passive role. They actively influence major oceanographic processes and, on a larger scale, global biogeochemical cycles. Perhaps the most widely observed manifestation of this fact is the concern about acute and chronic effects on marine systems from xenobiotics. However, organic matter is also actively involved in the cycling of inorganic materials, and, on a global scale, in non-marine systems as well. Many of these processes would not occur in the absence of organic constituents, and marine organic geochemical studies provide a basis for better understanding them. Influence of organic materials & seawater on atmospheric processes As an example, we may consider the impact of seawater organic chemistry on global climate. It has long been recognized that seaspray has been a mechanism for transferring salts (e.g. sea salts) from the ocean into the atmosphere and eventually into the terrestrial environment. It is now becoming apparent that organic compounds, such as organic halides and organic sulfur compounds, are produced in seawater and delivered to the atmosphere in significant quantities. When materials such as dimethyl sulfide are oxidized in the atmosphere there is a possibility that cloud condensation nuclei form in sufficient quantities to influence global cloud patterns and the Earth's albedo. At present, it is not fully understood how feedback mechanisms operate in this ocean-atmosphere cycling of these organic and inorganic sulfur compounds. Emissions of organic halides must play a role in controlling distributions of tropospheric and stratospheric ozone (and other atmospheric oxidants). Oxidation of all ocean-derived organic matter in the atmosphere contributes to acidification of aerosols. These sea-air transfers of organic chemicals need to be quantified ultimately to allow predictions of how the ocean controls aspects of the climate and air quality of the earth.
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Impact of organic matter on chemical speciation The presence of organic matter undoubtedly plays a major role in controlling the fate of other chemicals (including pollutants) in the sea. First, we know that all chemicals in seawater solution/suspension assume some distribution of their total presence amongst a set of interconvertible dissolved species. For organic chemicals, metals, and radionuclides, one subset of these species is that resulting from associations with dissolved components of the organic pool. Thus to proceed with any efforts to estimate the fate of such substances, we must be able to calculate the equilibria (and probably the kinetics) describing these complexations. Further, to permit quantitative understandings to be applied in the future, we must recognize the chemical characteristics of seawater organic matter which dictate the intensity (or rate) of these interactions. Although such impacts of marine organic material on speciation are widely recognized, we are not yet knowledgeable enough to include them in our models to estimate the exposure of marine organisms to either nutritional or toxic compounds. A related problem involves our inability t 9 predict a priori the sorption (both equilibria and kinetics) of inorganic and organic substances to natural marine particles. As natural particles always acquire organic compounds on their surfaces, sorption involves the possibility of interactions with structural elements of surface organic layers. Such heterogeneous associations may differ from those occurring in solution only in so far as particles induce bulk electrostatic effects (i.e. electric double layers) or changes in water solvency (i.e. vicinal water). Therefore, as for solution complexation, we need to evaluate the energies of inorganic and organic sorbate interactions with structural moieties of the surface organic constituents. There is a major research problem blocking continued progress in our understanding of the cycling of specific compounds: the development of means to predict/describe how a heterogeneous mixture of organic material (dissolved and particulate) associates with specific chemicals of interest. This must include efforts to identify critical subsets of ligands or sorbent sites, to develop methods to quantify these organic moieties in seawater samples, to complete our data describing specific organic moiety-chemical interaction energies, and, finally, to clarify the reaction kinetics and/or mass transfer limitations affecting these associations. Organic compounds and marine food chains Although the primary source of organic matter in the water column is autotrophic fixation of inorganic carbon into organic matter, most of the rest of the biological community depends on organic compounds as a source of nutrition. The organic chemical composition of living and dead organic matter has a significant influence on the structure and function of the marine food web. However, the nutritional requirements of the various forms of
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marine organisms are poorly understood, and as a consequence the nutritional value of organic materials as a whole is poorly characterized. Clearly, though, the spatial and temporal distributions of organic species in the water column resulting from both primary sources (autotrophic) and secondary sources (after heterotrophic modification) will determine where marine organisms will live and how they will function. For example, regions characterized by high primary production are usually regions with important commercial fisheries. Dissolved organic chemicals are also important to marine organisms if the compounds act as chemical mediators. Secondary consumers probably sense the presence of food items at significant distances using minute quantities of dissolved compounds as chemical keys. Chemical mediators are also actively involved in reproduction and as chemical repellents to prevent colonization or ingestion. But how organisms sense these chemical signals, and what the relevant compounds actually are, is very poorly known. The large dissolved organic matter pool in seawater is an enigma, considering the efficiency of many enzyme systems for decomposing organic compounds. What has prevented the evolution of enzyme systems capable of attacking the 'refractory' material? Also, considering the enormous numbers of bacteria in the sea and their surface areas, why is dissolved organic material still so abundant, even if it is dilute? Impact of organic matter on the fate of solids in the sea The transport and dissolution/precipitation of particles in the sea is also an issue requiring consideration of the influences of organic materials. We have long realized that minerals become coated with organic matter in natural waters. This results in major changes in the surface properties of those inorganic particles (e.g. surface charge reversal on iron oxides) and thereby affects their coagulation. Certain resulting effects on particle flocculation/ stabilization have been noted, but we are still unable to predict the rates of particle-particle attachment in seawater. This may be due to our inability to deal with issues such as steric hindrance deriving from the extension of organic chains from particle surfaces in flocculation phenomena. Dissolution and precipitation reactions also are affected by surface organic coatings on particles. Quantitative treatments of such dissolution kinetics as modified by organic coatings would help us understand vertical fluxes of labile solids (e.g. carbonates, silica) and associated elements (e.g. Sr, Ra) in the sea. Another major issue on which our ignorance is great concerns the factors limiting the physical detachment of particles from one another. It is possible that organic components act as surfactants and thereby encourage such disaggregation. As a result, it is clear that sedimentation of solids through the sea, and even their efficient collection into the sea-bed, is strongly influenced by the modifications of their surfaces by constituents of the organic coatings. As a result, we need to describe the character of surfaces on which these
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coatings occur. This probably requires an understanding of the threedimensional organization of the submicron interfacial region. Application of this improved physical image to currently available physical chemical descriptions of coagulation/detachment and precipitation/dissolution is necessary to improve our ability to describe transport of particulate materials in the sea. Water column organic matter and sea-floor and sea-bed processes Organic matter in the water column is the dominant source of organic compounds to the sea-floor. It is thus an important source of nutrition to benthic organisms; it is the source of biomarkers which, if preserved in the sediment record, can provide information about paleoenvironmental conditions; and it contributes to the geological storage of organic matter. Other elements associated with sedimenting organic matter will also become part of the sedimentary record. Finally, organic compounds which are derived and modified by water column processes will influence the chemical and physical properties of interstitial fluids in both sedimentary material and hydrothermal vent areas. This provides another critical link between processes occurring in the contemporary oceanic water column and processes occurring in the seafloor. Optical properties of seawater The optical properties of seawater are strongly influenced by both dissolved and particulate organic material. Material concentrated in the sea-surface microlayer will affect the physical properties of the air-sea interface, for example, the dampening of capillary waves and thus the impact on the back-scattering of radiation, its adsorption or fluorescence at the sea surface, and its transmission into underlying surface waters. It is likely that the amount of the microlayer, and its specific molecular organic composition, will be quantitatively important in the degree of back-scattering, adsorption, transmission, etc., and in determining the spectral quality of radiation which does reach the bulk surface water. The spectrum and intensity of light penetrating into surface waters will affect the extent of photosynthesis, as photosynthesis depends on light intensity and on the available spectrum. Photochemical production and composition of parts of the organic matter pool will be dependent on the light regime as well as the organic composition, thus adding or removing material from the organic pool. In turn, the abundance and organic composition of living and dead particles will play a further role in moderating the adsorption, scattering and fluoresence of the remaining radiation. OVERVIEW
A considerable amount of information is available in a qualitative sense
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about processes in the water column. However, there are major gaps in our knowledge about rates and reaction mechanisms of specific transformations involving organic constituents and this knowledge is important for understanding the overall biogeochemistry of seawater and sediments. As many transformation reactions are mediated by biological systems and influenced by physical-chemical parameters, organic geochemists need to expand interactions across disciplines with biological, physical and inorganic geochemical colleagues. Of equal importance is the pressing need to employ novel sampling strategies and analytical techniques to characterize the composition and behavior of organic matter fractions which are outside the realm of current studies. RECOMMENDATIONS
(1) A much greater amount of quantitative information on the sources of organic matter in the ocean is required. This includes further inventorying of compounds produced by micro- and macroorganisms and the mechanisms by which organic compounds are introduced into the water column, for example, exudation from living cells vs. release from detritus. (2) The flux of organic matter between the ocean and the continents, atmosphere, and sediments needs to be guaranteed. These studies should include the determination of the extent to which material is altered during exchange, especially for the poorly characterized non-lipid material. Simultaneous sampling in the water column, across the air-sea interface, and at the sediment-water interface are needed. The importance of lateral and upward flux of particulate and dissolved organic material must be determined. (3) We must increase the use of physical-chemical properties of organic substances to quantify the behavior of these materials in the sea. This information should be applied to quantitative models comparing theoretical and field studies and predicting the behavior of other substances. The role of organic-surface interactions and organic-metal complexation on the behavior of organic substances (as well as inorganic materials) needs further attention. (4) The influence of the chemical composition of organic matter in seawater in controlling the absorption, scattering, fluorescence, and transmission of incident radiation, and the resulting light field in the ocean needs to be elucidated. Studies should also relate organic matter distributions to parameters determined by in situ and remotely based optical sensing techniques. (5) Investigations of the chemical and biological reactivity of various organic structural classes and molecular weight classes at the sea surface, in the water column, and at the sediment-water interface need to be expanded. Greater quantitative understanding is needed of the mechanisms and biologi-
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cal/abiological turnover rates under a variety of natural conditions, with different organic matter types, and by different types of organisms. Coordinated laboratory and field efforts involving investigators of different disciplines and analytical capabilities are urgently needed. (6) Continued effort is needed in relating the role of dissolved and particulate organic compounds to marine food web dynamics. What is the nutritional value of organic compounds for marine organisms? Studies on the role and function of chemical mediators in the biophysiology of marine organisms, the mechanisms by which enzyme systems function, how microorganisms attack labile organic matter and why such a large part of the dissolved organic matter pool is 'refractory' need to be included. (7) Sampling and analytical technologies, although not explicitly discussed in this report, must continue to develop, to provide new sample types for study and new analytical tools to make possible advances in characterization of oceanic organic material. Novel experimental designs are needed, both in the laboratory and in the field, to assess pressing quantitative questions about rates and mechanisms of reactions involving organic matter. REFERENCE Ishiwatari, R., 1992. Macromolecular material (humic substance) in the water column and sediments. Mar. Chem., 39: 151-166.