Carbon cycle in the coastal zone: effects of global perturbations and change in the past three centuries

Chemical Geology 159 Ž1999. 283–304

Carbon cycle in the coastal zone: effects of global perturbations and change in the past three centuries Leah May B. Ver a , Fred T. Mackenzie a , Abraham Lerman a

b,)

Department of Oceanography, School of Ocean and Earth Science and Technology, UniÕersity of Hawaii, Honolulu, HI 96822, USA b Department of Geological Sciences, Northwestern UniÕersity, EÕanston, IL 60208, USA Received 2 July 1998; received in revised form 9 October 1998; accepted 15 December 1998

Abstract The coastal zone, consisting of the continental shelves to y200 m, including bays, lagoons, estuaries, and near-shore banks, is an environment that is strongly affected by two much bigger environmental reservoirs adjacent to it: the land and open ocean. In the coastal zone, as elsewhere in the Earth system, the biogeochemical cycle of carbon is coupled to, and driven by, the cycles of nitrogen and phosphorus through biological transfer processes. Human activities in the past 300 years have become an increasingly important geological factor with respect to the coastal zone through four major environmental perturbations: Ž1. C, N, and S emissions from fossil fuel burning; Ž2. changes in land-use activities resulting in gaseous C emissions, increased dissolved and particulate loads, organic matter transport, and feedbacks to biological production; Ž3. application of inorganic nitrogen- and phosphorus-containing fertilizers; and Ž4. discharges of sewage containing reactive organic C, N, and P. In addition, the mean global surface temperature of the planet has increased over this period of time by approximately 18C, perhaps also because of human activities. Starting with the year 1700 as a base for the industrial-age perturbations on land, we analyzed the consequences of these five perturbations to the carbon cycle in the coastal zone using the thirteen-reservoir, process-driven model TOTEM for the coupled C–N–P–S biogeochemical cycles. An indicator of the reliability of the model is the good agreement of its results showing the time course of increasing atmospheric CO 2 concentrations since the year 1700 with the observational results reported in the literature. During the past three centuries, there has been a significant increase in the amount of organic carbon transported from land and stored in coastal zone sediments. Of the total transported, about 65% was stored in sediments and the remaining 35% primarily recycled through exchange with the atmosphere and open ocean. The imbalance between the amounts of organic carbon produced by gross photosynthesis and remineralized has apparently increased slightly in favor of remineralization, corresponding to an increase in the degree of heterotrophy of the global coastal zone. This process, along with the release of CO 2 from the formation of CaCO 3 , counteracts the invasion of CO 2 from the atmosphere to coastal waters that is driven by the rise in atmospheric CO 2 concentrations. An analysis of a possible reduction or full collapse of the oceanic thermohaline circulation, as believed to have occurred in the past and a possibility for future centuries, indicates that the CO 2 transfer from the atmosphere to coastal waters would increase while that from the atmosphere to open ocean surface waters would decrease, if such an event took place. This is attributable to a reduced supply to the coastal zone of dissolved inorganic carbon by coastal upwelling from the deeper ocean, a process linked to the global conveyor belt of the thermohaline circulation. To date, fossil fuel CO 2 emissions to the atmosphere, changes in land-use practices, and sewage discharges have

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Corresponding author. Fax: q1-847-491-8060; e-mail: [email protected]

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 0 4 2 - X

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been the three main factors affecting the carbon cycle in the global coastal zone. The latter inputs from land have apparently produced a slight increase in the heterotrophy of the global coastal zone. However, increases in the inputs of nutrient nitrogen and phosphorus from land to the coastal zone in the future may drive its trophic state toward net production and storage Žautotrophy., thereby also increasing its potential role as a sink for atmospheric CO 2 . The direction of future change in net ecosystem production in the coastal zone strongly depends on changes in the relative magnitudes of organic carbon and nutrient N and P fluxes to the coastal zone via rivers, provided the upwelling fluxes remain constant. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Carbon cycle; Coastal zone; Global perturbations

1. Introduction Although most of the well-documented sedimentary rock record comprises terrigenous clastics and carbonates deposited in the coastal zone environment, where many observational studies have been made of modern as well as geologically older shallow-water sediments, little is known about the specific role of this region in the global carbon balance of the past, present, and future. The present-day coastal zone, including bays, estuaries, lagoons, wetlands, and shelves, comprises important sites of deposition and regeneration of organic carbon and of calcium carbonate produced in situ, including 30%– 50% of total carbonate and about 80% of total organic carbon accumulation in the ocean Že.g., Wollast, 1991, 1998; Wollast and Mackenzie, 1989; Morse and Mackenzie, 1990; Milliman, 1993; Smith and Hollibaugh, 1993.. It is a region that acts as a filter and trap of both natural and anthropogenic materials from the continents to the open ocean ŽMantoura et al., 1991., where at least 80% of the terrigenous material reaching the ocean is deposited ŽMilliman and Syvitski, 1994.. Active depositional areas in the coastal zone may have sedimentation rates as high as 30 to 60 cm per thousand years, as compared to average rates for hemipelagic and pelagic sediments of 20 cmr1000 year and 0.1–1 cmr1000 year. Coastal environments are also regions of higher biological productivity relative to that of average oceanic surface waters, making them an important reservoir in the global carbon cycle. The higher primary productivity is variably attributable to the nutrient inflows from land as well as from upwelling of deeper ocean waters along certain sections of the global coastal margin. Thus, the coastal zone with only 9% of the volume of the surface–ocean layer and 10% of the area of the

ocean is the domain of at least 10% of the total net primary production of the oceans ŽTurner and Adger, 1996. and supports more than 60% of commercial fish production ŽWorld Resources Institute, 1996.. Coastal zone environments are being heavily impacted directly and indirectly by human activities, disproportionately more than the much larger area of the open ocean. These activities include Ž1. combustion of fossil fuels and atmospheric emissions of mostly carbon and of smaller amounts of nitrogen and sulfur; Ž2. deforestation and conversion of forest land to pasture, grazing land, and urban centers, enhancing organic carbon, sediment, and nutrient inputs into the coastal zone; Ž3. application of nutrient fertilizers of nitrogen and phosphorus and pesticides to croplands and leaching of these substances into the coastal zone via river and groundwater flows, and subsequent volatilization of fertilizer and manure nitrogen to the atmosphere and rainout of this nitrogen and other substances into coastal environments; Ž4. discharge of sewage and detergents into the coastal zone; and Ž5. the global warming of the past century and its probable continuance into the 21st century, which can affect the carbon chemistry and temperature of coastal waters and possibly alter species composition and community structure and rates of organic productivity and calcification of carbonate-secreting organisms in coastal marine ecosystems. Despite the importance of the coastal zone and its substantial modification by human activities, little attention until very recently has been paid to this domain on a global scale. The neglect at this scale stems in part from the heterogeneity of the region, necessitating time consuming, regional-scale process analyses as the principal means of investigating the global coastal domain. In addition, the global coastal zone is a difficult region of the ocean to model

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ŽHofmann, 1991., because of complex physical and biogeochemical dynamics and their heterogeneity, involving fluvial and atmospheric transports that link the coastal zone to the land; gas exchange and atmospheric deposition that link it with the atmosphere; net advective transport of water, dissolved solids and particles, and coastal upwelling that connect it with the open ocean; and accumulation of sediments and organic matter that link coastal waters with sediments. Indeed most general circulation models ŽGCMs. of the coupled land surface– ocean–atmosphere system, which principally describe physical processes, have no provision for a separate coastal ocean and include it as part of the general ocean reservoir. In addition, most GCMs of the system do not consider the implications of biogeochemical processes for ocean–atmosphere exchange processes. Both of these limitations in modeling the coupled Earth system deserve attention because Ž1. coastal environments are heavily impacted today by human activities and climatic change and variability, and are presently showing strong signs of biotic impoverishment and degradation in many areas, and Ž2. biotic feedbacks to a warming of the Earth are complicated, potentially large, and involve biogeochemical processes, and thus are important in any fundamental description of the system ŽWoodwell et al., 1998.. In this paper we discuss the results of model simulations demonstrating how five major perturbations of the past three centuries of the preindustrial and current industrial age have affected the processes of carbon transport from land, and its deposition, accumulation, and in situ production in the coastal oceanic environment. In particular, we address the changing carbon balance and air–sea exchange of CO 2 in the coastal zone during this period of time. We also discuss the results of a model simulation that predicts the response of the coastal ocean to rising atmospheric CO 2 concentrations, consequent warming of the troposphere–surface system, and weakening of the thermohaline circulation of the ocean ŽManabe and Stouffer, 1994; Stocker and Schmittner, 1997.. Our analysis of the biogeochemical cycle of carbon in the global coastal zone that is coupled to, and driven by, that of nitrogen and phosphorus is based on an Earth system model ŽTOTEM or T errestrial–

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Ocean–aT mosphere-Ecosystem Model. of four major interactive domains in the Earth’s surficial system: land, atmosphere, coastal zone, and open ocean ŽLerman et al., 1989; Mackenzie et al., 1993, 1995, 1998a,b; Ver et al., 1994. that have been affected by natural and anthropogenic forcings in the past three centuries, and are likely to be affected for some time in the future. TOTEM was specifically designed in part to evaluate global change in the coastal zone, as it is considered an entity separate from the open ocean surface and deep waters. 2. Definition of the coastal zone Global continental shelves, as shown on the map in Fig. 1, have a very uneven distribution around the continents and are more prominently developed in the Northern Hemisphere. Considering that the Mercator projection of the map exaggerates the extent of the continental shelves in the Arctic Ocean, the prominence of the continental shelves shows itself in Europe, East Asia, Southeast Asia, North Australia, and the eastern coasts of the Americas. The main drainage basins of the rivers are major areas of input from the continents to the coastal zone, as shown in Fig. 1, where it can also be noted that some of the bigger river outflows occur on relatively narrow or poorly developed continental shelves ŽEast coast of South America, East and West coasts of Africa.. This physiographic feature shows that the continental shelves are not merely physical links on the path between river mouths or deltas and the open ocean, but they play a broader role in the global transport and transformation of materials in the land–atmosphere–ocean system. We consider the coastal zone as the environment of continental shelves to 200 m depth, including bays, lagoons, estuaries, and near-shore banks that occupy, in various estimates, 7% to 10% of the surface area of the ocean Ž26 = 10 6 to 36 = 10 6 km2 . or approximately 9% of its surface-layer volume Ž3 = 10 6 km3 . Že.g., Lagrula, 1966; Drake and Burk, 1974.. This concept of the coastal zone includes essentially all of the continental shelves, yet it differs from some other definitions of the coastal zone that extend from some elevation on land above mean sea level to some depth on the continental shelf: for example, the international program on

286 L.M.B. Ver et al.r Chemical Geology 159 (1999) 283–304 Fig. 1. World coastal zone showing the continental shelves to y200 m depth Žshaded areas. and the major river basins on the continents. Latitudes 758S to 858N, Mercator projection. The shelf area in the Northern Hemisphere accounts for 71% of the world total. In the latitudes from 0 to 758N, the shelf area is 66% of the total.

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Land–Ocean Interactions in the Coastal Zone ŽLOICZ. defines the coastal zone as extending from q200 m on land to y200 m on the continental shelves ŽPernetta and Milliman, 1995; Smith, S.V., personal communication, 1998.. The continental shelves average 75 km in width, with a bottom slope of 1.7 mrkm, and they are generally viewed as divisible into the interior or proximal shelf, and the exterior or distal shelf ŽDrake and Burk, 1974.. The mean depth of the global continental shelf is usually taken as the depth of the break between the continental shelf and slope at approximately 200 m, although this depth varies throughout the world oceans. In the Atlantic, Emery and Uchupi Ž1984. gave the median depth of the shelf-slope break at 120 m, with the range from 80 m to 180 m. The depths of the continental shelf are near 200 m in the European section of the Atlantic, but they are close to 100 m on the African and North American coasts. Although we believe that the smaller parts of the coastal zone representing the proximal or inner shelves are the areas most sensitive to changes in inputs from land and the adjacent atmosphere, we use the coastal zone without differentiating it into inner and outer parts in our analysis of the global carbon cycle because the quality of the data does not always justify this differentiation Žsee e.g., Rabouille et al., 1997; Wollast, 1998, for a different approach..

3. Model description TOTEM is a process-driven model based on the coupling of the global biogeochemical cycles of the life-essential elements carbon, nitrogen, phosphorus, and sulfur. This unique approach to the study of the responses of the exogenic Earth system to human or natural perturbations emphasizes the dynamic interactions between the four major biologically reactive elements in an integrated land–ocean–atmosphere system ŽLerman et al., 1989; Mackenzie et al., 1993, 1995, 1998a,b; Ver et al., 1994.. The individual element cycles are linked through the biological processes of photosynthesis, autorespiration, decay, and burial. The provision for active interactions of the elements distinguishes TOTEM from most other models.

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Thirteen reservoirs are defined in the model ŽMackenzie et al., 1998a, p. 72.: the atmosphere; six terrestrial reservoirs Žliving biota, humus, inorganic soil, soilwater, shallow groundwater, and lakes.; three coastal zone reservoirs Žorganic matter, water, and sediments.; and three open ocean reservoirs Žorganic matter, surface water, and deep water.. The rivers are not defined as a reservoir because the residence time of water in rivers with respect to precipitation recharge is very short Žabout 20 days.. Element cycles are coupled at biological transfer processes through the average C:N:S:P ratios associated with oceanic and terrestrial photosynthesis ŽRedfield ratios., autorespiration, decay, and humus formation. As a first approximation, we make a simplifying assumption that these biological processes are generic and apply over many different species and environments within the terrestrial or oceanic domains, and occur with the same stoichiometric global mean elemental ratios which do not change with time. We recognize that the wide range of C:N:S:P values for terrestrial plants, phytoplankton, humus, and organic matter in marine sediments found in the published database reflects the variance of the ratios owing to differences in climatic conditions, ecosystem type, and plant physiological conditions. For example, estimates for the Redfield C:N:S:P ratio for higher land plants vary from 510:4:0.8:1 ŽDelwiche and Likens, 1977. to 882:9:0.6:1 ŽDeevy, 1973. to 2057:17:3:1 ŽLikens et al., 1981.. The C:N:S:P ratios adopted in TOTEM are 106:16:1.7:1 for oceanic plankton ŽRedfield et al., 1963., 510:4:0.8:1 for land plants Ž Delwiche and Likens, 1977 . , and 140:6.6:1.2:1 for humus ŽLikens et al., 1981.. Linear and non-linear transport and reaction kinetics equations describe all the transfer processes within the system. The time step for TOTEM calculations is usually taken as 0.015625 year Žabout 6 days. in accordance with the data on forcings which are synthesized on a daily to annual basis. The details of the flux equations, kinetic parameters, and initial conditions are given in the references at the beginning of this section. 3.1. Initial conditions We begin analysis of the behavior of the carbon cycle under anthropogenic perturbations prior to the

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year 1700, with an Earth system in an assumed quasi-steady state. This initial state is defined by the reservoir mass sizes, transport flux values for each element, and other defined constants. A major difference between TOTEM and other terrestrial carbon models and GCMs is the treatment of the observed data for atmospheric CO 2 concentrations. Whereas this dataset is used as a prescribed input function in most models of global environmental change Že.g., Bruno and Joos, 1997; Cao and Woodward, 1998; Sarmiento et al., 1998., the time course of change of atmospheric CO 2 concentration is not treated as an input function in TOTEM. Rather, the 295-year dataset for the atmospheric C reservoir, as well as that for the other elemental reservoirs in the TOTEM Earth system, are model outputs and are a result of iterative model calculations. TOTEM results for the trend in atmospheric CO 2 concentrations are then confirmed using the observed dataset ŽVer et al., 1998; see also Section 6.1.. 3.2. Forcings From the initial steady state, the system is perturbed by five forcings ŽFig. 2.: Ž1. one due to changes in a climatic variable, temperature; the other four are due to human activities, Ž2. fossil fuel burning, Ž3. land use activities, Ž4. application of N and P fertilizer to croplands, and Ž5. disposal of municipal and industrial sewage. These forcings are input functions in TOTEM. Here, we briefly discuss how these perturbations affect the partitioning of anthropogenic CO 2 as analyzed by TOTEM. 3.2.1. Temperature The mean global temperature history of the Earth for the period 1700–1995 was derived from data presented in UCARrOIES Ž1991., Nicholls et al. Ž1996., and Houghton et al. Ž1996.. For the terrestrial realm, temperature and availability of water are two of the most important climatic variables. At present, large variations in mean annual temperature Žor mean temperature of the growing season. and water balance Žas reflected in net precipitation. characterize the continental environment. In TOTEM, the response of the global ecosystem to changes in temperature is through the biologically-mediated processes of photosynthesis, plant and soil respiration,

Fig. 2. Global environmental forcings used in TOTEM. Sources of data and discussion in the text. Ža. Emissions of carbon dioxide from fossil fuel burning and land use activities; Žb. gaseous N and S emissions from anthropogenic activities; Žc. consumption of inorganic N and P agricultural fertilizer; Žd. organic C, N, and P fluxes from sewage loading into coastal zone; and Že. change in global mean temperature relative to the year 1700. The dashed lines represent projections to the year 2025, as discussed in the text.

and denitrification on land. To date, there is no observational record showing that global precipitation has changed significantly in recent centuries although its distribution may have changed and its variability increased ŽBradley et al., 1987; Diaz et al., 1989.. For example, in certain regions such as North America, there are equivocal historical data indicating an increase in precipitation during the twentieth century ŽDai et al., 1997.. In the present model, we do not address the possible effects of changes in precipitation and soilwater moisture.

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3.2.2. Fossil fuel burning The annual global CO 2 emissions from fossil fuel burning and cement production for the period 1860– 1995 are based on the estimates by Marland et al. Ž1994.. Prescribed global emissions of gaseous nitrogen oxides ŽNO x . and sulfur oxides ŽSO x . are based on the published estimates for the period 1860–1994 ŽDignon, 1992; Dignon and Hameed, 1989; Hameed and Dignon, 1992; Brown et al., 1997., and extrapolated to 1995. On the decadal to century time scale, the anthropogenic emissions of C, N, and S gases are treated as external inputs to the TOTEM atmosphere. Fossil fuel CO 2 becomes part of the well-mixed atmospheric C reservoir while NO x and SO x affect the terrestrial and oceanic reservoirs via wet and dry deposition. 3.2.3. Land–use actiÕities We use data and calculations by Kammen and Marino Ž1993. for the land-use perturbation flux for the period 1700–1850 Žpre-industrial, anthropogenic, non-fossil fuel CO 2 emissions. and estimates by Houghton Ž1991. for the more recent land use CO 2 emissions Ž1850–1990.. We made linear projections from 1990 to estimate the emissions for the period 1991–1995. Land–use activities, including deforestation, reforestation, logging, and shifting cultivation, impact the Earth system via three mechanisms. First, as a perturbation when CO 2 from land–use activities is emitted to the atmosphere. Second, as a negative feedback mechanism to rising atmospheric CO 2 concentrations. Third, as a major mechanism of material transfer from land to the coastal zone owing to soil erosion, mineral dissolution, and surface water runoff. The negative feedback to land-use activities consists of the following processes. Land-use activities change the reservoir size and residence time of terrestrial organic matter, releasing CO 2 , N, and S gases to the atmosphere and remobilizing N, P, and S to the soilwater reservoir. The increased availability of nutrients in soilwater, coupled with rising atmospheric CO 2 and warming temperatures, promotes enhanced productivity and storage of organic carbon in the terrestrial biota, thus increasing the drawdown of atmospheric CO 2 . Land–use activities affect land–coastal zone transfer processes when repeated tilling and loss of

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soil protective cover render the land more susceptible to soil degradation, particularly soil erosion ŽHoughton, 1983.. Soil erosion and water runoff contribute to the increased transport of particulate organic matter, and dissolved organic and inorganic C, N, P, and S to the coastal zone. Our analysis of the global perturbation by land– use activities is a distinctive feature of TOTEM which allows one to explore fully the effect of human activities on the coupled C, N, P, and S biogeochemical cycles. Interestingly, although the effect of land–use activities on nutrient remineralization has been mentioned in the literature ŽMelillo et al., 1993, 1996; Schimel, 1998., this process has yet to be incorporated in terrestrial–ecological models. 3.2.4. Application of nitrogen and phosphorus fertilizers The time course of change for this agricultural perturbation is based on data on global inorganic nitrogen and phosphorus fertilizer consumption for the years 1950–1995 from the United Nations FAO Žvarious years.. For data pre-dating the FAO’s collection Ž1917–1949., we used the estimates by Smil Ž1991.. In TOTEM, fertilizer application is an external input to the N and P cycles because the residence times of N and P in their respective source reservoirs of atmosphere and rock are much longer than the decadal to century time scales under consideration. Only the fraction of applied fertilizer that is not assimilated by crops is considered as a forcing in the model. With the assumption that only 45% of applied nitrogen fertilizer is converted to crop biomass, the perturbation is assumed to derive from the remaining 55%. At least 25% of the applied N fertilizer is transported to the coastal zone with surface runoff, 20% is volatilized to the atmosphere, and 10% is either stored in the soil, or leached into the continental soilwater or groundwater reservoirs ŽSmil, 1991.. More of the applied phosphorus fertilizer Žabout 70%. is assumed to remain in the soilwater reservoir after crop assimilation, most of which Žabout 50% of the total. is quickly rendered unavailable for plant uptake by precipitating out as insoluble iron, aluminum, and calcium phosphates or by occlusion with clay minerals and iron oxyhydroxides. Equal amounts are transported with surface runoff Ž10%. or leached into the soilwater Ž10%.. Unlike

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nitrogen, there is no gaseous loss of fertilizer phosphorus. 3.2.5. Sewage disposal In TOTEM, we assume that the global average sewage load is discharged untreated. Following the model treatment by Billen Ž1993. and Caraco Ž1995., we calculate the loading of municipal sewage into the coastal zone in units of moles of total C, N, and P per year using a globally averaged C, N, and P content and an average per capita rate of discharge applied to the global population. We assume that phosphate detergents originate in urban industrialized regions ŽCaraco, 1995., that very little P is removed by wastewater treatment ŽEsser and Kohlmaier, 1991., and that detergent-P is ultimately discharged into the coastal zone in the inorganic phosphate form. We calculate the global loading of detergent-P in the coastal waters of TOTEM using an average per capita consumption rate applied to the urban industrialized population.

4. Previous numerical experiments In previous papers Žreferenced above., we analyzed the partitioning of anthropogenic CO 2 in the Earth system during the past three centuries Žfrom the year 1700 to 1990.. The initial state for analysis was described by the globally averaged values of the reservoir masses and transport fluxes for the coupled cycles of C, N, P, and S. The steady state was subsequently perturbed by the five forcings described in Section 3.2. Analysis of results for the 295-year simulation confirms that TOTEM performed very well in comparison to observational data and results from other models addressing the partitioning of anthropogenic CO 2 , despite its more simplistic approach and globally aggregated reservoirs. The computed response of atmospheric CO 2 to the perturbations was confirmed by one of the major constraints on the presently accepted CO 2 budget—the combined atmospheric CO 2 data derived from ice cores ŽNeftel et al., 1985; Friedli et al., 1986; Barnola et al., 1995; Etheridge et al., 1996. and from observations at Mauna Loa ŽKeeling and Whorf, 1998. Žsee Fig. 7 and later discussion.. Observational data collected during the

decade of the 1980s also confirmed model results. For example, the 1985 rate of anthropogenic CO 2 accumulation in the atmosphere calculated using TOTEM was 3.6 Gt Cryear, consistent with direct measurements of atmospheric CO 2 accumulation Ž3.3 " 0.2 Gt Cryear for 1980–1989; Sarmiento et al., 1992.. In addition, the calculated rate of oceanic accumulation Ž1.8 Gt Cryear. for the same year also compared very well with predicted rates from ocean–atmosphere models Ž1.9 Gt Cryear for 1980– 1989; Siegenthaler and Sarmiento, 1993., and estimates of the oceanic sink as inferred from analysis of changes in oceanic and atmospheric d13 C Ž2.1 " 0.9 Gt Cryear for 1970–1990; Quay et al., 1992.. The model result for the enhanced terrestrial uptake of CO 2 Ž1.3 Gt Cryear. was also consistent with recent estimates of carbon uptake in the Northern Hemisphere forests ŽKeeling et al., 1996.. Finally, TOTEM results for the partitioning of anthropogenic CO 2 over the time course from 1700 to present compared very well with GCM model results obtained by Sarmiento et al. Ž1992. and by Bruno and Joos Ž1997., and with the global carbon cycle model ŽGLOCO. by Hudson et al. Ž1994.. Global riverine total organic carbon, nitrogen, and phosphorus fluxes to the coastal ocean, as calculated using TOTEM, generally increase by roughly a factor of two in the model simulations from 1700 to 1995. The flux increases are within the ranges of increases over the past couple of centuries as estimated by other authors using more direct observational methods and approaches other than dynamic process-based modeling Že.g., Meybeck, 1982; Wollast and Mackenzie, 1989; see later discussion.. To some extent the results above validate the performance of TOTEM and give us some confidence that we can use the modeling approach to assess global change during the past and in the future.

5. Carbon cycle in the coastal zone In the present-day coastal zone environment, the essential elements of the carbon cycle are as shown in the schematic diagram of Fig. 3 ŽMackenzie et al., 1998b.. The inorganic carbon reservoir exchanges

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land-derived, Fo3 ., and respiration and decay or remineralization Ž FDIC .. The material balance for the organic carbon reservoir is: dCorg dt

s Ž Fo1 q Fo 2 q Fo 5 . y Ž Fo 4 q Fo 3 q FDIC . ,

Ž 1. where Corg is the mass of organic carbon in the reservoir and t is time. Gross photosynthesis Ž Fo2 . and respiration and decay Ž FDIC . are the linkages between the organic and inorganic carbon cycles. The material balance for the inorganic carbon reservoir is: dCinorg Fig. 3. Carbon cycle in the coastal zone. Conceptual model of the main reservoirs and fluxes for organic and inorganic carbon. Fluxes explained in the text. Flux Ž F . values are shown in Table 1 for the start of the model analysis in the year 1700 and the present time, computed for the year 1995. Inorganic carbon reservoir: curved arrow indicates production of CO 2 by precipitation of CaCO 3 .

CO 2 with the atmosphere Ž"Fi2 ., and receives inputs of dissolved inorganic carbon from land via rivers, surface runoff and groundwater flow Ž Fi1 ., from the CO 2 released by the precipitation of calcium carbonate, and via transport from the deeper ocean Ž Fi5 .. Outflows of inorganic carbon are via net water outflow from the coastal zone Ž Fi4 ., deposition and accumulation of calcium carbonate, formed in the coastal zone, in coastal sediments Ž Fi3 ., and use of CO 2 in photosynthesis Ž Fo2 .. The inorganic carbon cycle is linked to the organic cycle through biologically-driven reduction and oxidation processes, corresponding to primary production Ž Fo2 . and respiration and decay Ž FDIC , where DIC stands for dissolved inorganic carbon.. Analogous to the inorganic carbon cycle, there are inputs of organic carbon from land Ždissolved and particulate, Fo1 . and by transport of dissolved organic carbon from the deeper ocean, referred to as coastal upwelling Ž Fo5 .. Removal of organic carbon from the coastal zone is through net outflow to the open ocean Ž Fo4 ., accumulation in coastal sediments Žin situ produced and

dt

s Ž Fi1 " Fi 2 q Fi 5 q FDIC . y Ž Fo 2 q Fi 3 q Fi 4 . ,

Ž 2.

where Cinorg is the inorganic carbon mass in this reservoir, and the carbon dioxide flux between the atmosphere and coastal waters Ž Fi2 . can be either into the water Žq. or from the water to the atmosphere Žy.. The individual terms in Eqs. Ž1. and Ž2. show that the relative magnitudes of input from land, gross photosynthesis, respiration and decay, storage in sediments, and exchanges with the open ocean primarily determine the balance of organic carbon in the coastal zone, and the coastal zone’s potential to act as either a source or sink of atmospheric carbon based on organic metabolic processes and CaCO 3 deposition. The required nitrogen and phosphorus needed as essential nutrients in primary production in the coastal zone are imported from land, recycled from the in situ decomposition of organic matter, and transported from deeper ocean waters to the coastal zone via upwelling and vertical mixing processes. The availability of these two nutrient elements determines the rate of primary production Ž Fo2 . and hence to some extent the subsequent pathways and fate of organic carbon in the coastal zone. The effects of the major environmental forcings on land during the past three centuries, as represented by the five forcings explained in Section 3.2, will now be examined for the coastal zone.

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5.1. Net ecosystem production (NEP) and organic carbon storage The ecological concept of NEP, the net change in carbon or energy of an ecosystem over a period of time Žusually one year., relates to the balance between the rates of gross primary production and total respiration ŽSmith and Mackenzie, 1987; Smith and Hollibaugh, 1993; Woodwell, 1995.. The NEP of the coastal zone describes its net trophic state and is the difference between the rates of gross photosynthesis and ecosystem production of inorganic carbon by autotrophic and heterotrophic respiration: NEP s Fo 2 y FDIC .

Ž 3.

NEP is also a measure of the imbalance between the production Ž P . and respiration Ž R . of organic carbon within the coastal system, often denoted as P–R. A system is net heterotrophic when the amount of carbon respired and decayed in a coastal system is greater than the amount of organic carbon produced by gross photosynthesis ŽNEP - 0.. A system is net autotrophic when the amount of carbon fixed by gross photosynthesis is greater than the amount respired and decayed ŽNEP ) 0.. In a system near a steady state ŽdCorgrdt f 0., net ecosystem production can also be defined, from Eqs. Ž1. and Ž3., as NEP s Ž Fo 3 q Fo 4 . y Ž Fo1 q Fo 5 . ,

Ž 4.

which is the difference between output and input fluxes of organic carbon into the coastal zone, as shown in Fig. 3. The preceding equations confirm that in a system that is net autotrophic ŽNEP ) 0., relatively higher rates of primary production Ž Fo2 . would imply a higher storage rate of organic carbon Ž Fo3 ., which is in agreement with the observations that primary production in oceanic surface waters controls the rate of deposition of organic carbon that initially reaches the ocean floor ŽMuller and Suess, ¨ 1979; Berner and Westrich, 1985; Lee, 1992.. Another factor that also controls the trophic state ŽNEP. of the coastal zone is the export flux of organic carbon Ž Fo4 . whose magnitude is related to the rate of water movement and the transport of dissolved and particulate organic carbon to the open ocean. On a global and the geologically long time scale of Earth history, the storage of organic matter in

sediments and generation of molecular oxygen were possible because the photosynthetic production of organic matter was greater than the total autotrophic and heterotrophic respiration, i.e., global P–R ) 0. However, in an Earth system composed of several reservoirs, the relative magnitudes of photosynthetic production and remineralization of organic matter may differ in some of the individual reservoirs, such as the coastal zone or open ocean. Before we discuss our results on the carbon budgets in the coastal zone, as derived from TOTEM, we address the issues of the various estimates of net ocean–atmosphere CO 2 exchange, and of the individual terms in Eqs. Ž3. and Ž4. that control the trophic state of the coastal zone. The material balance of organic carbon in the coastal zone, which determines whether the net CO 2 flux resulting from organic metabolism is into or out of coastal waters, is difficult to assess for the preindustrial world. The global ocean was estimated by Garrels and Mackenzie Ž1972. to be a net source of CO 2 to the atmosphere in pristine time due to organic metabolism, with a calculated flux of 27 = 10 12 mol Cryear. Later estimates of net CO 2 fluxes from the ocean to the atmosphere due to processes involving organic carbon do not differ much from the early estimate in both magnitude and direction: 21 = 10 12 mol Cryear, based on the global organic carbon cycle ŽSmith and Mackenzie, 1987.; about 22 = 10 12 mol Cryear, based on organic carbon metabolism, including 3.3 = 10 12 mol Cryear from coastal waters and 18.3 = 10 12 mol Cryear from the open ocean to the atmosphere ŽWollast and Mackenzie, 1989.; and a biologically mediated flux of 23 = 10 12 mol Cryear from the global ocean, including 7 = 10 12 mol Cryear evaded from the coastal zone and 16 = 10 12 mol Cryear from the open-ocean surface waters ŽSmith and Hollibaugh, 1993.. In the most recent model of the present-day organic carbon cycle in the ocean, Wollast Ž1998. identified a proximal part of the modern coastal zone, including estuaries, marine wetlands, and bays, as net heterotrophic ŽNEP - 0., and a distal continental shelf as net autotrophic ŽNEP ) 0.. The data of Gattuso et al. Ž1998a. appear to support the conclusion of Wollast Ž1998., as do some preliminary modeling calculations of Rabouille et al. Ž1997.. The problem of the organic carbon balance in the coastal zone and open ocean is not fully resolved at

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present Žsee Kempe, 1995, for review.. In TOTEM, we accepted the estimate of Smith and Hollibaugh Ž1993. for the long-term preindustrial organic carbon balance of the coastal zone, y7 = 10 12 mol Cryear, recognizing the fact that this estimate may be subject to further refinements and that the ultimate outcome will significantly influence our estimates of organic carbon fluxes for the initial steady state condition of the coastal zone prior to 1700. Note that this initial value of NEP is the difference between the two very large fluxes of gross primary production and total ecosystem autotrophic and heterotrophic respiration ŽTable 1, Fo2 y FDIC .. Import of organic and inorganic carbon from the deeper ocean Ž Fo5 , Fi5 . are global averages in our model, and these include several processes that deliver water and nutrients to the coastal zone. The processes include, for example, Ž1. divergence of water masses and upwelling at eastern boundary currents of the ocean, Ž2. cold ring formation and upwelling of deep water onto shelves, Ž3. formation of oceanic fronts inducing vertical turbulence and leading to mixing of the water column, and Ž4. seasonally induced vertical turbulence sufficient to mix the water column at the western boundaries of the ocean Že.g., Summerhayes et al., 1995; Wollast, 1998.. In this paper, these various physical processes are not distinguished and are collectively referred to as coastal upwelling. Thus, at the boundary of the coastal zone with the open ocean, upwelling and onwelling provide fluxes of water and nutrients from the open ocean that is represented in TOTEM by

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only two reservoirs, surface ocean and deep-ocean water ŽMackenzie et al., 1998a, p. 72.. It is important to reiterate that because of the global nature of TOTEM, the supply of dissolved inorganic and organic carbon and nutrients to the coastal zone are global average values, consistent with our use of the term coastal upwelling in this paper. The main carbon fluxes shown in Fig. 3 are presented in Table 1 for the initial conditions prior to the year 1700 and for the year 1995 Žfinal conditions.. For the period 1700 to 1995, the five environmental perturbations, described in an earlier section and used in the model, produce the results for the carbon cycle discussed here. The organic and inorganic carbon fluxes ŽTable 1. generally show significant increases for inputs from land to the coastal zone Ž Fo1 , Fi1 ., accumulation in coastal sediments Ž Fo3 , Fi3 ., gross photosynthesis Ž Fo2 ., and remineralization of organic matter Ž FDIC .. Since the year 1700, the heterotrophy of the coastal zone, as shown in Fig. 4a, has increased slightly from NEP s y7 = 10 12 to y10 = 10 12 mol Cryear ŽTable 1 and Eq. Ž4... Concomitantly, there were significant increases in the organic carbon flux from land ŽFig. 4b. and sedimentary storage of organic carbon in coastal marine sediments ŽFig. 4c.. These increases are in agreement with the conclusions of Ludwig et al. Ž1996., and Smith and Hollibaugh Ž1993. who observed that ecosystem production of the coastal zone, particularly the proximal areas, can be controlled by the delivery of terrestrial organic matter via the rivers. Although the riverine

Table 1 Carbon fluxes in the coastal zone Flux a

Organic carbon

Year 1700

Year 1995

Inorganic carbon

Input from land Gross photosynthesis CO 2 air–sea exchange Storage in sediments Exchange to open ocean Coastal upwelling Respiration and decay

Žq. Fo1 Žq. Fo2

34 600

53 748

9 27 9 607

19 32 9 756

Žq. Žy. Ž". Žy. Žy. Žq. Žq.

Žy. Žy. Žq. Žy.

Fo3 Fo4 Fo5 FDIC

Fi1b Fo2 Fi2 Fi3 Fi4 Fi5 FDIC

Year 1700

Year 1995

32 600 y17 6c 468 456 607

41 748 y10 6.4 483 457 756

The year 1700 marks the initial year of anthropogenic and climatic perturbations for the current industrial age, as used in TOTEM analysis. Flux Ž F . notation as in Fig. 3; Žq. is input to the coastal zone, Žy. is output; flux units are 10 12 mol Cryear. a The fluxes are rounded to the nearest whole number; thus inputs do not exactly balance outputs. b Dissolved inorganic carbon flux, does not include detrital calcium carbonate. c Long-term calcium carbonate accumulation flux ŽMorse and Mackenzie, 1990..

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Fig. 4. Changes during the past 295 years, computed from TOTEM. Ža. Net ecosystem production ŽNEP, or the difference between gross production and remineralization of organic carbon, P – R .. Žb. Transport of organic carbon from land to the coastal zone Ž Fo1 ., and Žc. accumulation rate of organic carbon in coastal sediments Ž Fo3 ., both showing increases since preindustrial time.

input of inorganic nutrients, N and P, has also increased significantly along with the input of organic carbon, the in situ new incremental production of organic matter induced by the added nutrients has been insufficient to reverse the coastal zone from its net heterotrophic Žexcess of remineralization. to net autotrophic Žexcess of photosynthetic production. state. Obviously the TOTEM calculated change in NEP is small Žabout 43%. relative to the uncertainties in the much bigger fluxes, and one could conclude that global coastal zone NEP since the year 1700 has changed little, if at all, despite the large additions of organic matter, N, and P from human activities to this region. Regardless of the assumed initial magnitude of NEP in the global coastal zone in the year 1700, the trends in NEP discussed above are robust. Sensitivity analysis using TOTEM with different initial values of NEP in the year 1700 shows that the calculated overall trend in NEP toward increasing heterotrophy for the past 300 years is maintained. However, if one starts with a coastal ocean that is strongly autotrophic ŽNEP ) 0., then the system never crosses over into an heterotrophic state. On the contrary, for any initial condition of net heterotrophy for the model simulation of 300 years, the coastal zone

progressively becomes slightly more heterotrophic during this period. One value of a model of the nature of TOTEM is that it points to processes and fluxes that are not well known or constrained on a global scale. The importance of the nutrient N and P inputs from land, and their role in the coupled C–N–P cycles, are demonstrated by the following consideration. Human land-use activities increase the rates of recycling of the soil humus. When humic material of an average molar C:N:P ratio of 140:10:1 is mineralized, the remobilized N and P can ideally support growth of land plants of an average molar C:N:P ratio of 510:4:1. On land, about 370 mol Žs 510 y 140. of the required C can thus be sequestered directly from the atmosphere, and about 6 mol of N can be added to the continental soil–water reservoir. Part of this N is lost to the atmosphere through denitrification, and part is transported to the coastal zone by rivers. As a whole, the mass of N released through land-use activities dominates other anthropogenic sources of N on land, such as agricultural fertilizer and atmospheric deposition. The fractions of N and P released by land-use activities which reach the coastal zone become available there for primary production of an average molar C:N:P ratio of 106:16:1. The supply of organic carbon by upwelling to the coastal zone Ž Fo5 . amounts to 17% to 27% of the organic carbon flux from land Ž Fo1 .. However, upwelling is more significant in terms of the import of nutrient N and P from the deep ocean, potentially supporting an amount of new production equivalent to 10%–30% of total production in the coastal zone. In addition, it also should be kept in mind that upwelling supplies sufficient dissolved inorganic carbon to satisfy the Redfield-ratio requirements of the upwelling nitrogen and phosphorus in coastal primary production. Further changes in land-use practices or in ocean circulation patterns in the future are likely to affect the nutrient N and P supply to the coastal zone by fluvial flow or coastal upwelling, thereby affecting the coastal carbon cycle and carbon balance through the C–N–P coupling. The cumulative amount of additional organic carbon transferred from land to the coastal zone in the past 300 years, according to our model analysis, was 1255 = 10 12 mol C. This is the incremental mass

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addition to the unperturbed cumulative transport, based on a steady state flux of 34 = 10 12 mol Cryear ŽTable 1., and amounting to about 12% of the latter over 300 years. In other words, the additional cumulative mass transfer represents organic carbon that has been added to the rivers because of three centuries of human activities on land. This cumulative mass is equivalent to an average of about 4.3 = 10 12 mol Cryear added to the background river flux of organic carbon during the 300 years of model simulation. However, as can be seen from Fig. 4, much of this additional organic carbon was added to the world’s rivers during the 20th century. Of this total additional organic carbon mass delivered by rivers to the coastal zone, 820 = 10 12 mol C, or 65% of the total, were stored in coastal-margin sediments. The remaining fraction was recycled primarily through processes of respiration and decay, and exchange with the open ocean and atmosphere. The fraction of the additional organic carbon flux and cumulative mass from terrestrial sources buried in coastal-margin sediments during the past three centuries, and especially during the 20th century, qualifies as a sink of anthropogenic CO 2 . Thus the difference in the organic carbon storage flux to coastal marine sediments between 1700 and 1995 ŽTable 1. of 10 = 10 12 mol Cryear, or about 0.12 Gt Cryear, represents an enhanced flux that is a sink for anthropogenic CO 2 . This enhanced flux stems from human activities on land, with the concomitant increase of river transport of terrestrial organic matter to the coastal zone and organic C sewage discharge, and to an increase in the new production of coastal zone environments because of the accompanying enhanced river transport of nutrient nitrogen and phosphorus to the coastal zone. The calculated magnitude of the sink strength of 10 = 10 12 mol Cryear is similar to estimates made by Mackenzie Ž1981. and Sarmiento and Sundquist Ž1992. using different approaches; however, it represents only 2% of the fossil fuel flux of CO 2 to the atmosphere in 1997. Fig. 5 shows the time course of calculated dissolved inorganic nitrogen and phosphorus concentrations in world rivers during the 295 years of TOTEM simulation compared with observed concentrations from present-day pristine and polluted rivers ŽMeybeck and Ragu, 1995.. There is substantial

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Fig. 5. TOTEM computed trends of dissolved inorganic P and N by rivers into the coastal zone compared to present-day data from Meybeck and Ragu Ž1995.. Starting conditions are the long-term averages of Meybeck Ž1982.. The time trends show increases due to the interactions among the forcings and feedbacks on land. Note the abrupt increase in the TOTEM-calculated dissolved inorganic P flux in the late 1940s due to increased use of phosphorus-containing detergents.

scatter in the latter database but in general pristine streams and rivers have lower inorganic nutrient concentrations than polluted systems. This is not surprising because several authors have shown that nutrient concentrations and fluxes in river systems are closely correlated with the human population in the watershed of the river. Riverine export of both nutrients to the coastal zone correlates strongly with the population density of the watershed ŽWollast, 1981; Cole et al., 1993; Caraco, 1995.. Notice in Fig. 5 that the calculated nutrient concentrations of dissolved inorganic nitrogen and phosphorus from TOTEM in world rivers during the past 295 years of model simulation, starting with the long-term global pristine average of Meybeck Ž1982. in the year 1700, gradually increase into the field of nutrient concentrations of present-day polluted rivers and streams.

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The calculated increasing concentrations of nutrients imply increasing river fluxes because in TOTEM global river water discharge is maintained constant at 3.7 = 10 13 m3ryear Že.g., Meybeck, 1982. during the past 295 years of model simulation. It is the enhanced fluxes of nutrient nitrogen and phosphorus that are partially responsible for the increase in coastal-zone new production and the increased burial of organic matter in coastal marine sediments that qualify as a sink of anthropogenic CO 2 . However, as mentioned by Wollast Ž1991., these increased riverine nutrient fluxes may only have a small effect on coastal productivity on a global scale. Most of the increased burial flux of organic matter in the coastal zone on a global scale during the past three centuries is due to the increased river flux of total organic carbon derived from human activities on land and the subsequent burial of part of that flux in coastal marine sediments. 5.2. Carbon flows between the atmosphere and coastal zone Material balances of the global carbon cycle in preindustrial time indicate that the global ocean probably was a net source of CO 2 to the atmosphere because of deposition and accumulation of CaCO 3 in marine sediments and remineralization of organic matter ŽSmith and Mackenzie, 1987; Wollast, 1994; Wollast and Mackenzie, 1989; Smith and Hollibaugh, 1993.. In the prescribed coastal zone of TOTEM, the combined results of the remineralization of organic carbon Žflux FDIC in Fig. 3. and calcium carbonate deposition and accumulation Ž Fi3 . are addition of CO 2 to coastal waters and its evasion to the atmosphere in the year 1700 ŽFig. 6, curve b.. Since the year 1700, increases in the rate of remineralization of organic carbon Ž25%. and accumulation of carbonate in the global coastal zone Ž7%. ŽTable 1. accounted for a higher production rate of dissolved inorganic carbon in coastal waters. This would have led to an increase in the flux of CO 2 to the atmosphere, if it were not for the rise in atmospheric CO 2 concentrations because of combustion of fossil fuel and land use activities. In Fig. 6 Žcurves a and b., the results for two sensitivity runs using TOTEM of the magnitude and direction of the CO 2 flux across the air–sea interface

Fig. 6. Changes in the direction and magnitude of the computed CO 2 flux between the atmosphere and coastal waters since the year 1700. Ža. TOTEM calculations based on an initial 1700 flux of CO 2 to the atmosphere from organic metabolism of y7=10 12 mol Cryear. Žb. TOTEM calculations based on an initial 1700 flux of CO 2 to the atmosphere of y17=10 12 mol Cryear from organic carbon remineralization, plus from calcium carbonate deposition and accumulation. Positive values indicate flux into the coastal waters from the atmosphere Žq Fi2 .; negative values indicate the opposite. Fluctuations in the flux since the mid-1990s discussed in the text.

of global coastal waters are shown to provide the reader with some idea of the sensitivity of TOTEM to initial conditions. For curve Ža. the initial condition in the year 1700 is taken as the net flux of CO 2 to the atmosphere owing to organic metabolism Žy7 = 10 12 mol Cryear.; for curve Žb. the initial flux is the sum of the fluxes from organic metabolism, accumulation of in situ precipitated CaCO 3 in coastal marine sediments Žy6 = 10 12 mol Cryr., and CaCO 3 export to the continental slope Žy4 = 10 12 mol Cryr.. For curve Ža. since about the middle of the 1800s, the transfer of CO 2 from coastal surface waters to the atmosphere has continuously, but with short-term fluctuations in flux, diminished from about y7 = 10 12 mol Cryear Žy0.08 Gt Cryear. to fluctuations near zero, and to the computed reversal of the flux for the year 1995 of about q1.9 = 10 12 mol Cryear Žq0.02 Gt Cryear.. This calculation indicates that during the 20th century, the increase in dissolved inorganic carbon and, consequently, CO 2 of coastal waters because of increased heterotrophy and accumulation of in situ produced calcium carbonate was insufficient to counteract the effect of rising atmospheric CO 2 concentrations, owing to fossil fuel burning and release from land-use activities, and the air–sea exchange of CO 2 switched direction from out of the surface waters to into the waters of the coastal ocean. The increased production of dissolved

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inorganic carbon in coastal waters diminished the potential of the coastal zone to act as a sink for the increasing atmospheric CO 2 , but the rise in atmospheric CO 2 concentrations was too great and the partial pressure of CO 2 in coastal waters fell below of that in the atmosphere, leading to transfer of CO 2 into the coastal zone. Curve Žb. in Fig. 6, as might be anticipated, is similar in trend and flux fluctuations to curve Ža. but, importantly, the net flux of CO 2 is always out of the ocean to the atmosphere. The magnitude of the air–ocean flux varies from about y17 = 10 12 mol Cryear Žy0.2 Gt Cryear. in 1700 to about y10 = 10 12 mol Cryear Žy0.12 Gt Cryear. in 1995 ŽTable 1.. Thus according to this analysis, the net air–sea transfer of CO 2 between the coastal zone and the atmosphere has not changed direction for the past three centuries. The similarity in fluctuations of the CO 2 flux during the 1900s for both model calculations Ža. and Žb., and possibly similar fluctuations in net ecosystem production ŽNEP, Fig. 4., may reflect reversals in the terrestrial organic carbon reservoirs Žplants and soil humus. between being net sinks or net sources of CO 2 to the atmosphere, with concomitant changes in the organic carbon and nutrient N and P flows to the coastal zone. The calculations shown in Fig. 6 clearly demonstrate that the pathway of CO 2 air–sea exchange for coastal zone waters during the past three centuries depends on the initial magnitude and direction of the CO 2 flux associated with NEP, and on the initial magnitude of calcium carbonate accumulation in the year 1700. For example, if the coastal ocean were strongly autotrophic Župtake of CO 2 . to start with, then despite the calculated slight tendency toward increased heterotrophy and calcium carbonate deposition during the past three centuries, it would have remained a strong sink for anthropogenic CO 2 , once atmospheric concentrations had risen sufficiently to overcome the flux of CO 2 out of coastal waters. In addition, because calcium carbonate deposition and accumulation in shallow-water marine sediments are strongly dependent on the rate of production of biogenic skeletons and tests of planktonic and benthonic marine organisms, any change in the carbonate chemistry andror temperature of coastal waters could affect the fluxes associated with these processes and hence air–sea exchange of CO 2 . This is

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of particular concern in terms of the future because of the potential for continuously rising temperatures and increased invasion of anthropogenic CO 2 into coastal waters. Both the warming of coastal waters and changes in their saturation state with respect to carbonate minerals can affect calcification rates of at least benthic corals and coralline algae ŽMackenzie and Agegian, 1989; Gattuso et al., 1998b., the major calcifying organisms found on coral reefs, and hence the production and accumulation of calcium carbonate in coastal marine sediments and air–sea exchange of CO 2 .

6. Global thermohaline circulation and the coastal ocean The coastal upwelling of deeper ocean waters is a source of dissolved inorganic and, to a much smaller extent, organic carbon ŽFig. 3, Table 1., as well as nutrient N and P, in the coastal zone. As mentioned in Section 5.1, the global nature of TOTEM requires that coastal upwelling be treated as a global process rather than as fluxes characteristic of only certain sections of the continental margins. One driving force behind coastal upwelling is the pattern of global oceanic circulation. Although upwelling is not uniformly distributed over the entire coastal margin of the world, its importance to the carbon cycle and biological production in certain coastal sections justifies its inclusion in our global model. Furthermore, the global oceanic thermohaline circulation, also known as ‘the conveyor belt’, is recognized as an important factor controlling the global climate and the exchange of carbon dioxide between the ocean and atmosphere. As such, we believe it important to examine the effects of changes in global thermohaline circulation on the carbon cycle and CO 2 exchange in the coastal zone. The conveyor-belt circulation of the ocean today is one of its main physical features and is responsible for the delivery of a large amount of heat to the northern Atlantic and amelioration of temperatures in Northern Europe. The geologic record of ice and sediments shows that this current has not run steadily in the past but has probably shifted from one mode of operation to another ŽBroecker, 1995, 1997.. The upper limb of the conveyor belt involves transport of

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water and heat in the upper 1500 m of the ocean to the high latitude regions of the North Atlantic where cooling, promoted by cold westerly winds from Northern Canada, takes place. The cooling increases the density of the upper ocean water to the point where it sinks and begins a journey south as the lower limb of the current. The lower limb extends all the way to the southern ocean where it merges and mixes with deep waters generated around the continent of Antarctica. The mixture of these northern and southern feed waters is then dispersed into all three major ocean basins ŽAtlantic, Indian, and Pacific.. Because this conveyor-like circulation has changed mode and reorganized in the past, on time scales possibly as short as a century or so ŽManabe and Stouffer, 1994; Stocker and Schmittner, 1997; Broecker, 1997., it might conceivably do so in the future, under the influence of global warming. Here we accept this possibility and explore the question of what could be the response of the global coastal zone to a collapse or near collapse of the conveyor-like circulation of the ocean? For consistency with our model analysis of the past three centuries, based on the five major forcings on land, we analyze the consequence of cessation of the thermohaline circulation in the ocean on a time scale of decades into the near future, to the year 2025. We emphasize that this is not a ‘prediction’ of the near future, but a model analysis of the effects of a change in the global circulation pattern on the coastal margin. 6.1. Future course of atmospheric CO2 To study the response of the coupled land surface–ocean–atmosphere system to changes in thermohaline circulation, we first performed a numerical experiment using TOTEM to estimate to the year 2025 the future course of global mean atmospheric CO 2 concentrations. The forcings on the system were those mentioned earlier in this paper ŽFig. 2., without any changes in the thermohaline circulation of the ocean. The projected forcings to the year 2025, also shown in Fig. 2, were based on the scenario of ‘business as usual’ ŽBAU. of the Intergovernmental Panel on Climate Change ŽIPCC IS92a, Houghton et al., 1996., and population and agricultural projections by agencies of the United Nations

ŽUnited Nations Population Division, 1995; United Nations FAO, various years.. The results of the IPCC BAU model for atmospheric CO 2 are shown in Fig. 7 which also gives the computed atmospheric CO 2 concentrations for the past three centuries, since 1700 to 1995, and projections to the year 2025. As mentioned previously, the agreement between our results from TOTEM and the CO 2 concentrations obtained by atmospheric measurements at Mauna Loa, HI and from ice core records of atmospheric CO 2 lends support to our analysis and the correctness of our model in its ability to simulate the functioning of the coupled C–N–P cycles within the global system. In the year 2025, the global mean atmospheric CO 2 concentration is projected to be approximately 455 ppmv. This value is about 26% greater than the observed 1995 concentration of 361 ppmv and about 25 ppmv greater than the value of 430 ppmv projected for 2025 by the IPCC ŽHoughton et al., 1996.. The rough agreement of our result with the IPCC projection is satisfactory, but the difference is important. The IPCC projection assumes that the feedbacks in the system linked to biology Žbiotic feedbacks. will not change significantly over time; for example, the IPCC scenario, based on the Bern carbon model ŽSiegenthaler and Joos, 1992; Joos et al., 1996., assumes a constant anthropogenic CO 2 fertilization factor independent of changes in ecosystem variables. The calculation based on TOTEM, however, allows for variations in the strength of the biotic feedbacks: the 6% difference in CO 2 concentrations in 2025 between the IPCC and TOTEM projections is in part due to the weakening of the terrestrial biotic sink for anthropogenic CO 2 because of the enhancement of respiration over photosynthesis caused by temperature increase and increased loss of terrestrial organic matter from changing land-use practices, both accounted for in TOTEM, but not in the IPCC projections. This finding is important because it brings to question the validity of projections of atmospheric CO 2 and, more generally, of the global carbon cycle that do not fully account for the complexity of biotic feedbacks. Next, we consider how changes in thermohaline circulation might affect the results described above. The intensity of the thermohaline circulation was estimated by Manabe and Stouffer Ž1994. for the

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Fig. 7. Rise of the atmospheric concentration of CO 2 , simulated by TOTEM for the period since 1700 to 1995, and projected for an additional 30 years to 2025. For 1700–1995, the computed curve agrees well with the reported observational data. Shown for comparison, the TOTEM-calculated CO 2 rise Žsolid line. and the IPCC Business as Usual ŽBAU. projection Žfilled diamonds; Houghton et al., 1996.. Insert: CO 2 rise for three scenarios of reduction in the oceanic thermohaline circulation by the year 2095 by 100%, 50%, and 34%, discussed in the text.

cases of a doubled Ž2 = . and quadrupled Ž4 = . atmospheric CO 2 concentration. A measure of the circulation is the volume of flow Žin Sverdrups, 1 Sv s 10 6 m3rs. in the meridional direction in the North Atlantic Ocean. The present-day thermohaline circulation is equivalent to 18 Sv. In the 4 = -model scenario of Manabe and Stouffer Ž1994., by the end of 200 years, the thermohaline circulation in the North Atlantic Ocean would decrease in intensity by about 72%, equivalent to 5 Sv of flow. Consistent with their projections, we assume three values of reduction in thermohaline circulation at the end of a 100-year period beginning in 1995: a nearly full collapse Ž100% reduction., 50%, and 34% reductions by the year 2095. The computed results are shown in the insert of Fig. 7. As might be expected, the greater the reduction in the rate of thermohaline circulation, the greater the rate of increase of atmospheric CO 2 for the period 1995 to 2025. The change over these three decades is not substantial, but it represents about 5 ppmv difference between the TOTEM results for the cases of an unchanged ther-

mohaline circulation and the weakening of its intensity on a century-time scale. The reason for the greater concentration of CO 2 in the atmosphere is due to a decrease in the strength of the high-latitude oceanic sink for CO 2 , brought about by a decrease in the production and sinking rate of deep water as a consequence of the weaker meridional circulation. The ultimate cause of this change, as projected by Manabe and Stouffer Ž1994., is the warming of the surface ocean brought about by rising CO 2 concentrations and an enhanced greenhouse effect. 6.2. Partitioning of CO2 between the coastal and open ocean A weakening thermohaline circulation reduces the flux of CO 2 from the atmosphere to the open ocean as the downward transport of surface water slows. This is shown in the time course of the atmosphere– open ocean carbon flux in Fig. 8, projected to the year 2025 for the three cases of reduction Ž100%, 50% and 34%. of thermohaline circulation. The

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coastal waters by upwelling is also reduced Žcompare Fig. 3 and Table 1.. The DIC concentration in coastal waters would decrease, and even though the pH of coastal waters decreases slightly, the net result would be a lower CO 2 pressure in the water Že.g., Morse and Mackenzie, 1990.. With a continuously rising atmospheric pressure of CO 2 , its flux across the air–sea interface of the coastal ocean would continue to increase as the intensity of the thermohaline circulation decreases. Thus in contrast to the present, the coastal ocean could become a significant sink for atmospheric CO 2 if thermohaline circulation were to slow down in the future.

7. Summary and conclusions

Fig. 8. The historical and near-future time courses of the CO 2 flux between the atmosphere and surface waters of the coastal ocean Ža., and the atmosphere and surface waters of the open ocean Žb.. Results computed from TOTEM for four plausible scenarios of reduction of thermohaline circulation in 100 years, by the year 2095: 100% reduction, 50%, 34%, and no change in the thermohaline circulation.

greater the decrease in the circulation intensity, the greater the reduction in the atmosphere-to-ocean flux. For the coastal waters, however, the carbon flux from the atmosphere increases ŽFig. 8., indicating that the sink strength of the coastal zone for CO 2 increases when the thermohaline circulation weakens. This modeling result is due to the coupling between the intensity of thermohaline circulation in the open ocean and upwelling to coastal waters, which is not an unreasonable model condition for the physical behavior of mass flows in the ocean. However, one should keep in mind that if the wind field changes drastically during global warming of the planet, then it is possible that wind-driven upwelling along the western margins of continents could be modified. This possible additional forcing is not included in the model calculations. The upwelling to the coastal zone is an important source of dissolved inorganic carbon ŽDIC. for coastal waters and, when the intensity of the thermohaline circulation is reduced, the flux of DIC to

The global coastal zone is a significant modifier of the effects of anthropogenic perturbations on land that have been increasing during the past three centuries to their present importance as a global geologic agent. The carbon cycle in the coastal margin is closely linked to changes occurring on land and in the open ocean, and it is characterized by response times of decadal to century time scales. These time scales of response are indicated by the stronger rates of change in the 1900s relative to earlier times, and by the projected responses to a weakening thermohaline circulation in the open ocean. Five human forcings on the environment during the past three centuries were considered: gaseous emissions from fossil fuel burning and land-use practices; changes in the recycling, storage, and transport of organic matter on land due to changes in land use; application of chemical fertilizers containing the elements N and P, both of which are coupled to the carbon cycle; sewage discharges ultimately reaching the coastal waters; and an increase in a global climatic variable since the year 1700, mean temperature. Of these five forcings, the most pronounced effects in the coastal margin so far have been due to CO 2 emissions, changes in land use, and sewage discharges into coastal waters. The effects of the global rise in temperature by about 18C since the year 1700 and the present rates of agricultural fertilization on land were less important in comparison to the three preceding forcings. However, a continua-

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tion of the present trend of increase in coastal population and the increase in fertilizer application correlated with it may lead to a much greater significance of nutrient N and P inputs to coastal waters, resulting in higher rates of primary production and storage of organic carbon in sediments. This modeling result is in agreement with observations that regional-scale coastal environments receiving principally inorganic nutrient inputs Žsuch as N in nitrate and P in phosphate. develop toward a more carbon-storing or autotrophic state ŽSmith, 1995; Smith and Mackenzie, 1987.. Model analysis shows that the increase in atmospheric CO 2 due to emissions from land is a major factor counteracting the evasion of CO 2 from coastal waters. However, as land-use activities and sewage discharge are primarily responsible for the greater inputs of organic carbon from land to the coastal zone, these activities can be ultimately responsible for the increased production of dissolved inorganic carbon and carbon dioxide in coastal waters that further reduce the coastal zone’s ability to act as a sink for atmospheric CO 2 . An additional role of the coastal zone as a modifier of global change is in its response to the weakening of the oceanic thermohaline circulation. Results of the model analysis show that on a time scale of decades, cessation or reduction of the intensity of the thermohaline circulation could lead to an increased invasion of anthropogenic CO 2 from the atmosphere to coastal waters. This would be the result of a reduced coastal upwelling from the deeper ocean that is linked in the model to the rate of the global thermohaline circulation, and the consequently reduced input of dissolved inorganic carbon by upwelling to the coastal zone. Although the volume of the coastal zone is not big enough to control the CO 2 balance between the atmosphere and ocean, the changing direction of organic carbon production and remineralization, and carbon exchange with the atmosphere may lead to significant changes in its trophic state and the rates of storage of organic carbon in sediments. The relatively small volume of the coastal zone makes it strongly dependent on the bigger, adjacent reservoirs of C, N, and P on land and in the open ocean. Greater inputs of nutrient N and P to the coastal zone from land-use activities, inorganic fertilizer applica-

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tion, and sewage discharges are likely to increase primary production of organic matter and, consequently, increase the ratio of marine to terrigenous organic matter in coastal-margin sediments. At present, the C:N and C:P ratios in the upwelling waters to the coastal zone are high and they do not add extra N and P relative to C, to maintain additional production. In addition, an important part of the upwelling N is eventually involved in the process of denitrification and vented to the atmosphere as nitrogen gases. Because there is no major identifiable cause that might change this C:N:P relationship in coastal upwelling on the time scale of a century, the dependence of the coastal zone on changes in nutrient inputs to stimulate new production remains its strong link to the land, where any future changes in the present anthropogenic forcings can be expected to be reflected at least in the proximal coastal zone on a geologically short time scale of decades. However, if coastal upwelling water rates were to change and hence nutrient fluxes, this could strongly affect the carbon and nutrient cycles in the coastal zone.

Acknowledgements This research was supported by National Science Foundation grant EAR93-16133 to F.T. Mackenzie and A. Lerman; grants from the National Oceanic and Atmospheric Administration, Office of Global programs, to Mackenzie ŽNA37RJ0199. and Lerman ŽNA46GP0463.; and 1996 Fellowships of the Wissenschaftskolleg zu Berlin, Germany, to Lerman and Mackenzie. We are grateful to Fred Marton and Benjamin Horner-Johnson ŽNorthwestern University. for showing us how to create maps using GMT software, such as the map in Fig. 1; to Changrui Gong ŽNorthwestern University. for discussions of organic matter preservation in continental margin sediments; to Larry Atkinson ŽOld Dominion University. for references and discussions of the continental shelf physiography; to Jean-Pierre Gattuso ŽObservatoire Oceanologique Europeen, ´ ´ Monaco. for making available a paper in press; to an anonymous reviewer and to Roland Wollast ŽUniversite´ Libre de Bruxelles. for suggesting improvements to this paper and to the latter also for helpful discussions of the subject matter; and to Telu Li, Chris Measures, Brian

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Popp, Steve Smith, and Jane Tribble ŽUniversity of Hawaii. for reviewing the parts of this paper stemming from the PhD dissertation of Leah May Ver. School of Ocean and Earth Science and Technology Publication No. 4775.

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