changed, and is mostly dominated by opportunisticspecies and gelatinous carnivores acting as top preda-tors and deadend of the food chain. Total biomass ofmesozooplankton species was decreased by a factor offive, while the abundance of opportunistic species(Noctiluca, Pleurobrachia, jelly fish Aurelia aurita, andctenophore Mnemiopsis leidyi) reached 90% of the totalzooplankton wet weight (Vinogradov et al., 1999;Kideys et al., 2000). First, Aurelia during the late 1970sand 1980s, and later Mnemiopsis during the late 1980sconsumed almost all zooplankton groups, which wereessential components in the marine food web. BecauseAurelia and Mnemiopsis had no predators in the BlackSea, their communities quickly dominated the entireecosystem, and changed its structure and, together withoverfishing, contributed to the collapse of the fishery inthe late 1980s. The situation has become so severe thatit has affected the health, well-being and standard ofliving of nearly 160 million people in the region.Economic losses exceeded $500 million per year, as esti-mated by the World Bank.

The sharp density stratification, accompanied withweak vertical circulation and mixing as well as limitedlateral influxes, inhibit ventilation of sub-pycnoclinewaters of the Black Sea from the surface. Organic mat-ter, continually sinking and decomposing, has thus ledto development of a permanent anoxia and high con-centrations of hydrogen sulfide below a depth of100–150 m during approximately the last 7000 years.The Black Sea vertical biogeochemical structure thusdiffers significantly from its typical oceanic counter-parts. The complex prey-predator interactions betweendifferent phytoplankton and zooplankton groups aretightly linked with a very efficient remineralization-

The Black Sea is a deep (about 2 km) elliptic basinwith zonal and meridional dimensions of approximate-ly 1000 km and approximately 400 km, respectively,located roughly between 28° and 42°E longitudes, 41°and 46°N latitudes (Figure 1). It has only a narrowopening to the shallow (<75 m deep) Bosphorus Strait;otherwise, it is a completely enclosed marginal sea.Until the early 1970s, the Black Sea consisted of a mosa-ic of diverse ecosystems that provided a vital habitatfor many commercial species. It supported fisheriesalmost five times richer than those of neighboringMediterranean. Then, it has been gradually trans-formed into a severely degraded marine habitat suffer-ing from the introduction of large volumes of nutrientsand contaminants from the Danube River and othersalong the northwestern coast (Mee, 1992). The state ofthe ecosystem after the mid-1970s reflected severe envi-ronmental deterioration, intense eutrophication, dra-matic decreases in biodiversity and fish stocks, espe-cially in the northwestern shelf and the Sea of Azov(Zaitsev and Mamaev, 1997).

The long-term massive fertilization of the sea bynutrients from domestic, agricultural and industrialsources has caused a substantial modification of thephytoplankton community structure and succession, aswell as an increase in the intensity, frequency andexpansion of microalgal blooms. The total phytoplank-ton biomass has increased by an order of magnitudealong the western coast, and about 3–4 times within theinterior. Individual blooms have been documented asbecoming more and more monospecific. From the foodweb point of view, the eutrophic Black Sea ecosystemhas been producing more biomass than it used to threeto four decades ago. But its taxonomic composition has

Interdisciplinary Studies Integrating The Black Sea Biogeochemistry and Circulation DynamicsTemel OguzMiddle East Technical University • Icel Turkey

Paola Malanotte-RizzoliMassachusettes Institute of Technology • Cambridge, Massachusetts USA

Hugh W. DucklowVirginia Institute of Marine Sciences, The College of William and Mary Gloucester Point, Virginia USA

James W. MurrayUniversity of Washington • Seattle, Washington USA

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This article has been published in Oceanography, Volume 15, Number 3, a quarterly journal of The Oceanography Society. Copyright 2001 byThe Oceanography Society. All rights reserved. Reproduction of any portion of this article by photocopy machine, reposting, or other means with-out prior authorization of The Oceanography Society is strictly prohibited. Send all correspondence to: info@tos.org, or 5912 LeMay Road,Rockville, MD 20851-2326, USA.

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decadal time scale) perturbed ecosystem and on bio-geochemical structures. When combined with its high-ly energetic mesoscale-dominated circulation and com-plex shelf-deep basin interactions, they make the BlackSea an excellent natural arena for testing capabilitiesand performances of interdisciplinary models. Duringthe 1990s, the major question confronted by the ripari-an countries was to identify the measures needed tostop its further deterioration. In order to prepare thescientific basis to address the environmental and socio-economic issues facing the Black Sea, some observa-tional and modeling efforts have been performed with-in the framework of several international programs. Asa part of these efforts, we have undertaken a series ofmodeling studies devoted ultimately to exploring,quantifying, and predicting circulation, ecosystem andbiogeochemical variability from the overall basin scaleto coastal/shelf domains, and over time scales extend-ing from weeks to seasons. Here, we present anoverview of our recent progress achieved on: (i) a uni-fied dynamical description of the pelagic food web andvertical biogeochemical structure via coupling of itsmajor processes and interactions between the oxic, sub-oxic and anoxic layers, and; (ii) identifying impacts ofthe circulation dynamics on biogeochemical transports.

Modeling the Food Web andBiogeochemical Pump

Constructing a mathematical framework for study-ing interdecadal transformations of the Black Seaecosystem and biogeochemistry is a challenge. It requires dealing with a complex set of interactionsbetween key autotrophic, heterotrophic and carnivore

ammonification-nitrification-denitrification chain, anda series of different types of highly complex bacterially-mediated oxidation-reduction reactions controlling thestructure and dynamics of the suboxic-anoxic transi-tion zone. The biogeochemical pump operates withinthe uppermost 100 m of the water column. The euphot-ic zone structure, covering maximally the uppermost50 m (approximately), is characterized by high oxygenconcentrations on the order of 300 µM, and a year-round biological activity. The underlying 20–30 m layercontains steep gradients in chemical properties whereparticle remineralization causes considerable reduc-tion in oxygen concentration to the values of approxi-mately 10 µM, whereas nitrate increases to maximumconcentrations of 6–9 µM (Codispoti et al., 1991). Thesubsequent Suboxic Layer (SOL) of about 20–40 m ischaracterized by oxygen concentrations less than 10 µM, rapid decrease of nitrate concentrations to zerodue to the consumption in organic matter decomposi-tion (Murray et al., 1989). Below the SOL, sulfate isused to decompose organic matter, and hydrogen sul-fide is produced as a byproduct. The loss to the deepanoxic layer is not more than 10% of the total particu-late material remineralized, and is compensated by theriverine input. This highly delicate and efficient systemmaintains the current quasi-stable nature of the verticalbiogeochemical structure of the Black Sea, thus pre-venting the upward rise of the sub-surface sulfidicwaters. The upper boundary of the SOL, however,seems to have moved to shallower depth since the1960s, as reported by Konovalov and Murray (2001).

All these distinct features make the Black Sea aunique example for studying an array of natural andhuman impacts on a continuously changing (on

Figure 1. The geo-graphical settingand bathymetry ofthe Black Sea. Thelight blue zonearound the periph-ery signifies thecontinental shelfregion with depthsless than 200 m.

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model of oxygen and redox dynamics based on a set ofoxygen, nitrogen and sulfur reactions coupled with themanganese cycle, as suggested by Murray et al., (1995).Hydrogen sulfide, elemental sulfur, dissolved oxygen,dissolved and particular manganese constitute themain elements of the redox model (Oguz et al., 2001b).When dissolved oxygen is available, it oxidizes partic-ulate organic material and hydrogen sulfide, ammoni-um, and dissolved manganese transported upwardsfrom deeper levels. In oxygen depleted waters, dis-solved oxidized manganese reacts with nitrate to pro-duce settling particulate oxidized manganese andnitrogen gas. Hydrogen sulfide and ammonium arethen oxidized by particulate oxidized manganese tolocally form elemental sulfur, nitrogen gas and dis-solved manganese. Elemental sulfur is reduced back tohydrogen sulfide by bacteria; nitrogen gas escapes tothe atmosphere, and dissolved manganese is re-oxidized by nitrate.

species and/or groups at different periods of the tem-porally-changing ecosystem. Our modeling initiativestarted at the mid-1990s with a one-dimensional, verti-cally-resolved (with approximately a 3 m grid spac-ing), five compartment biological model coupled withupper ocean physics and the Mellor-Yamada order 2.5turbulence mixing parameterization (Oguz et al.,1996). This model has then been elaborated by intro-ducing sophistications, as well as carrying out extensivesensitivity, calibration and validation studies for thefood web (Oguz et al., 1999; 2001a), the nitrogen andredox cycles (Oguz et al., 2001b; Oguz, 2002), and thewater column oxygen dynamics (Oguz et al., 2000).

The main features of the biogeochemical model,which provides a unified description of the pelagicfood web, nitrogen cycle and oxidation-reduction reac-tions, are summarized in Figure 2. The food web struc-ture comprises two groups of phytoplankton (diatomsand phytoflagellates), two classes of herbivorous-omnivorous zooplankton (microzooplankton andmesozooplankton), nonphotosynthetic free living bac-terioplankton, medusae Aurelia aurita and ctenophoreMnemiopsis leidyi, as well as the opportunistic omnivo-rous dinoflagellate Noctiluca scintillans. These micro-bial and herbivorous-carnivorous food web structuresare coupled through particulate and dissolved organicmaterial fluxes to transformations among organic andinorganic (nitrate, nitrite and ammonium) forms ofdissolved nitrogen. They are further linked with a

Figure 2. Schematic representation of the major process-es and interactions between biogeochemical model com-partments. Pl, Ps denote large and small phytoplanktongroups, Hs is microzooplankton, Hl is mesozooplankton,Ca is jellyfish Aurelia aurita, Cm is ctenephoreMnemiopsis leidyi, Zn is opportunistic dinoflagellateNoctiluca scintillans, MnO2 is particulate manganese.Meanings of the other variables are as shown in the fig-ure. The biogeochemical model is coupled to the physicalmodel through vertical diffusivity and temperature.

Figure 3. (a) Observed, and (b) simulated annual distri-butions of total phytoplankton, mesozooplankton,Noctiluca, and Aurelia biomass for the eutrophic,Aurelia-dominated (pre-Mnemiopsis) phase of theecosystem. The data for phytoplankton and mesozoo-plankton biomass are taken from measurements carriedout at 2–4 weeks intervals during January–December1978 at a station, off Gelendzhik along the Caucasiancoast. The Noctiluca and Aurelia biomass data are takenfrom measurements on the Romanian shelf and the inte-rior basin, respectively, during the late 1970s and early1980s (after Oguz et al., 2001a).

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longer-lasting phytoplankton blooms due to strongertop-down control introduced into the ecosystem dur-ing the late 1980s and 90s. The resulting plankton dis-tribution possesses three successive bloom events dur-ing winter, spring and summer. An intense winterphytoplankton bloom emerged as a new feature of theannual phytoplankton structure after Mnemiopsis colo-nization. It is, in fact, a modified version of the latewinter event of the pre-Mnemiopsis era, shifted as aconsequence of the particular form of grazing pressureexerted by Mnemiopsis, which almost completelydepleted the microzooplankton, and Noctiluca stockstoward the end of autumn season. The lack of grazingpressure on the phytoplankton community then pro-moted earlier growth starting by the beginning ofJanuary. Winter blooms with that intensity were notreported until 1990s, but were persistently observed inboth satellite and in situ data during the 1990s (Yilmazet al., 1998; Oguz et al., 2002). The other two bloomsmay also be interpreted as the modified forms of latespring to early summer and autumn events.

Almost continuous particulate organic matter pro-duction associated with the year-long biological activi-ty supports an efficient nitrogen cycling within approx-imately the upper 75 m of the water column, wheredissolved oxygen generated photosynthetically and byair-sea interactions is depleted due to consumptionduring aerobic particulate matter decomposition(Oguz et al., 2000). The layer below could not be venti-lated even for the conditions of exceptionally high win-ter cooling due to the presence of a strong density strat-ification. Even with a highly simplified representationof the redox processes, the model provided a quasi-steady state suboxic-anoxic interface zone structuresimilar to observations (Figure 4). It was able to givequantitative evidence for the presence of an oxygendepleted and non-sulfidic suboxic zone. This modelpointed out the crucial role of the downward supply ofnitrate from the overlying nitracline zone and theupward transport of dissolved manganese from theanoxic pool below for maintenance of the suboxiclayer. The model is also used to test the assumption ofisopycnal homogeneity of the SOL properties and theirindependence from the circulation features, as assertedpreviously (Tugrul et al., 1992). It is found that the SOLproperties do not possess isopycnal uniformitythroughout the basin, and vary depending on theintensity of vertical diffusive and advective oxygenfluxes across the oxycline (Oguz, 2002). Anticyclones,with downwelling and downward diffusion (i.e., withstronger net downward supply of oxygen), attain athinner suboxic layer at a deeper part of the water column relative to cyclones. The position of the upperboundary of the SOL changes from �t~15.6 kg m-3 incyclonic to 15.9 kg m-3 in anticyclonic regions, whereasits position in the peripheral Rim Current transitionzone occurs at intermediate density values. Re-analysisof the existing data provides firm evidence for such differences.

A set of model simulations demonstrated how thesimultaneous controls imposed by bottom-up forcingassociated with anthropogenic-based nutrient enrich-ment and top-down grazing pressure introduced bygelatinous predators on mesozooplankton could leadto the changes in functioning of the Black Sea ecosys-tem during its transformation from a healthy state inthe 1960s to a eutrophic state in the 1980s (Oguz et al.,2001a). Much smaller and almost uniform planktondistributions characterized the unperturbed ecosystemfor most of the year. The increased anthropogenicnutrient load (bottom-up control) together with popu-lation explosions in medusa Aurelia during the 1970sand 80s (top-down control) were shown to lead topractically uninterrupted phytoplankton blooms and amajor reduction in microzooplankton and omnivorousmesozooplankton biomass (Figure 3). The major bloomevent of the year took place during late winter to earlyspring as a consequence of nutrient accumulation inthe surface waters at the end of the winter mixing sea-son. This was followed by two successive and longerevents during spring-early summer, and autumn. Theearly spring bloom was followed first by a mesozoo-plankton bloom of comparable intensity, whichreduced the phytoplankton stock to a relatively lowlevel, and then by an Aurelia bloom that similarlygrazed down the mesozooplankton. The phytoplank-ton recovered and produced a weaker late springbloom, which triggered a steady increase in Noctiluca biomass during the mid-summer. As theAurelia population decreased in August, first mesozoo-plankton and then phytoplankton, Noctiluca andAurelia gave rise to successive blooms duringSeptember–November period.

Increases of the biomass of the ctenophoreMnemiopsis leidyi led to even more pronounced and

Figure 4. (a) Observed, and (b) simulated profiles of dis-solved oxygen, hydrogen sulfide, nitrate and ammoniumwithin the Suboxic Layer and onset of the sulfide zone.

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the Rim Current meanders. The nearly four year timeseries of the SeaWIFS chlorophyll data from September1997 to the end of December 2000 provides examples ofvarious forms of the northwestern shelf circulation andits mesoscale and sub-mesoscale structures (Oguz etal., 2002).

Role of Circulation on BiogeochemicalTransports

The SeaWiFS images also suggest evidence for aclose link between biological production along theAnatolian coast and on the northwestern shelf (Oguz etal., in press). The Anatolian coastal waters are shown tosupport production during most parts of the year aslong as they receive sufficient nutrient supply from thenorthwestern shelf through the coastal current systemas in the spring-summer 1999 (Figure 6). The offshoremeanders and filaments of the Rim Current systemsupply part of this production into the interior of thebasin. Thus, in addition to vertical processes (entrain-ment, diffusion, upwelling), the interior basin produc-tion within the mixed layer can be maintained orenhanced by lateral advective transports as well. Attimes, when the fresh water induced coastal flow is

Features of the Circulation FieldOur conception of the Black Sea circulation has had

significant progress during the last decade throughrealization of several basin-wide, multi-ship field cam-paigns, complemented by different types of satellitedata. When compared with the classical concept of thetwin-gyre circulation inferred from the coarse resolu-tion hydrographic data, the circulation deduced by therecent medium resolution synoptic surveys (Oguz etal., 1994; Oguz and Besiktepe, 1999; Besiktepe et al.,2001) and satellite imagery (Oguz et al., 1992; Sur et al.,1994; Korotaev et al., 2001) indicated a mesoscale-dominated circulation system superimposed on thequasi-permanent basin and sub-basin scale elements.The building blocks of the upper layer circulation areidentified as the Rim Current system around theperiphery, an interior cell involving different types ofstructural organizations from two sub-basin scalecyclonic gyres to a series of interconnected largemesoscale cyclonic eddies, and some quasi-stable/recurrent anticyclonic eddies on the coastal sideof the Rim Current. This system of circulation is furtheraccompanied by meanders, filaments, offshore jets aswell as cyclonically propagating highly transient fea-tures of the Rim Current.

Common to all data sets is the bi-modal characterof the circulation during the year. The winter circula-tion is persistently dominated by a two cyclonic gyresystem in the western and eastern basins, encircled by aweakly meandering, organized and strong Rim Currentjet (Figure 5a). During the spring and summer seasons,this system transforms into one composite basin-widecyclonic cell comprising multi-centered cycloniceddies, surrounded by a weaker but broader and morevariable Rim Current zone (Figure 5b). During theautumn, the upper layer circulation attains its mostdisorganized form, identified by a series of cycloniceddies within the interior cell and accompanying pro-nounced lateral variabilities and larger coastal anticy-clonic eddies around the periphery. A compositecyclonic peripheral Rim Current circulation is hardlyidentified during this period. On the other hand, thenorthwestern shelf exhibits a highly variable anddynamic circulation structure throughout the year. It isformed by a combination of the Danube outflow, windstress forcing and the Rim Current structure of thebasin-wide circulation system along its offshore side.The southward direction of the coastal current emergesas the predominant path of the fresh water-inducedflow regime within the inner shelf along the west coast.It may be deflected occasionally northward towardsthe upstream, and forms an anticyclonic circulation cellconfined within the northern part of the shelf. Thesouthward coastal flow largely diminishes under suchcases. Moreover, the outer flank of shelf flow systemoften has an unstable structure; it exhibits meanders,spawns filaments toward the wide topographic slopezone, and is also modulated by on-shelf intrusions of

Figure 5. The upper layer circulation pattern for (a)February 1998, (b) July 1998, shown as examples of thewinter and summer horizontal flow structures, respec-tively. They are derived by assimilation of the Topex-Poseidon and ERS altimeter data into a 1.5 layer reduced-gravity model. The background colors show thecorresponding sea level anomaly patterns given by thealtimeter data.

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endowed with active thermodynamics and bulk mixedlayer dynamics. It is coupled with a four layer biologi-cal model involving a food web structure similar to theone shown in Figure 2 except for the bacterial dynam-ics. Three of these four layers represent the euphoticzone resolved by the mixed layer at the surface andtwo intermediate layers below. The fourth layer corre-sponds to the aphotic part of the water column up tothe anoxic interface. As demonstrated by Oguz et al.,(2001c), representation of the upper layer water col-umn in the form of four layers can simulate reasonablywell the annual phytoplankton biomass structure, con-sistent with its computationally more demandingmulti-level counterpart. The model is able to success-fully reproduce major bloom events of the yearinferred from the SeaWIFS chlorophyll data as well asavailable composite in situ measurements performedduring the last decade. In particular, the model pro-vides the extended, almost two month-long, autumnand winter phytoplankton blooms, separated fromeach other by a short period of transition in January(Figure 8). The presence of complex top-down controlsoperating simultaneously by jellyfish Aureliaand ctenophore Mnemiopsis on heterotrophs andautotrophs is responsible for the presence of suchextended autumn-winter bloom events. Eddy-inducedlateral transports, asymmetries in the mixed layerdepth and its layer-averaged light intensities, togetherwith a complex and highly variable nutrient supplyinto the euphotic zone controlled by the combinationof wind-driven coastal upwelling, eddy-pumping,entrainment due to mixed-layer deepening, and verti-cal diffusion manifest complex patterns of phytoplank-ton blooms of both new and regenerated production.

The autumn and the early winter productionevents begin during late October near the base of the

trapped inside the northwestern shelf due to itsupstream deflection, productivity within the Anatoliancoastal zone and consequently supply into the interiorof the western basin rapidly ceases. This was the caseobserved during the summer of 1998.

SeaWiFS-normalized back radiance data providenew details on the duration, persistence, and spatialextent of the Emiliania huxleyi bloom events in the BlackSea, as well as their connection to circulation dynamics.The data analyzed for the 1997–2000 period (Cokacaret al., 2001) suggest high reflectance patches of E. hux-leyi coccolith platelets throughout the basin during theMay–July period in each of these years (Figure 7). Thecyclonic cell occupying the interior part of the sea ischaracterized by a relatively stronger signature ofwater leaving irradiance as compared to the coastalanticyclonic eddies around the periphery. The separa-tion of these two regions with different irradiance char-acteristics coincides closely with the structure of theRim Current. Because of its relatively shallow mixedlayer thickness, and thus stronger mixed layer averagewater leaving radiances, the cyclonic cell appears as afavorable site for more intense coccolith blooms rela-tive to the anticyclonic peripheral zone. The presenceof a persistent large scale cyclonic circulation systemtherefore gives the E. huxleyi blooms a basin-wide char-acter in the Black Sea. The contribution of these bloomsto increases in the methanesulfonate (MSA) concentra-tions at two stations along the Turkish and Cretancoastline of the Eastern Mediterranean has recentlybeen pointed out by Kubilay et al. (in press).

The three-dimensional, physical-ecosystem modelstudies elucidate how the patchiness of phytoplanktonblooms is linked with spatial and temporal features ofthe circulation system. The physical model involves atwo and half layer reduced gravity circulation model

Figure 6. Monthly composite SeaWiFS image of the surface chlorophyll concentration (in mg m-3) for August 1999.

Figure 7. SeaWiFS true color image on 16 June 2000.The turquoise color covering the Sea of Marmara and majority of the Black Sea indicates dense coccolithaccumulations as a byproduct of the Emiliania huxleyibloom event.

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Cokacar, T., N. Kubilay and T. Oguz, 2001: Structure ofEmiliania huxleyi blooms in the Black Sea surfacewaters as detected by SeaWIFS imagery. Geophys.Res. Letts., 28(24), 4607–4610.

Kideys, A.E., A.V. Kovalev, G. Shulman, A. Gordinaand F. Bingel, 2000: A review of zooplankton inves-tigations of the Black Sea over the last decade. J.Mar. Syst., 24, 355–371.

Konovalov, S.K. and J.W. Murray, 2001: Variations inthe chemistry of the Black Sea on a time scale ofdecades (1960–1995). J. Mar. Syst., 31, 217–243.

Korotaev, G. K., O.A. Saenko and C.J. Koblinsky, 2001:Satellite altimetry observations of the Black Sealevel. J. Geophys. Res., 106, 917–933.

Kubilay, N., M. Kocak, T. Cokacar, T. Oguz, G.Kouvarakis and N. Mihalopoulos, in press. TheInfluence of Black Sea and Local Biogenic Activityon the Seasonal Variation of Aerosol Sulfur Species

euphotic zone (i.e., the third layer) following accumu-lation of a sufficient amount of nitrate by diffusion andupwelling from the subsurface nitrate pool. Towardthe end of November, the subsurface bloom extends tothe entire euphotic layer as a result of the convectivemixing produced by atmospheric cooling. This bloomevent is essentially driven by the upwelling and thusdeveloped preferentially from the cyclonic regions,generally occupying the interior part of the basin. Itmaintains itself until the first week of January as aresult of a continuous supply of nitrate from the sub-surface pool as well as its internal recycling inside theeuphotic layer. The bloom is then terminated as thephytoplankton stocks are consumed by mesozoo-plankton. The nutrient supply and recycling howevercontinue throughout January and February. They sup-port another major production event during lateFebruary and March once the mesozooplankton graz-ing pressure is released on phytoplankton due to theirpredation by gelatinous carnivores, and the mixedlayer receives sufficiently strong photosyntheticallyavailable solar radiation to trigger primary production.The mixed layer deepens over the euphotic zone every-where in the basin during the second half of February.Thus, the anticyclones and the peripheral zone of thebasin characterized by deeper mixed layers store morenutrients entrained from the chemocline layer, andthus constitute preferential sites for the developmentof the late winter bloom. The subsequent spring pro-duction takes place initially in the mixed-layer andthen continues within the subsurface layers after themixed-layer is depleted in nutrients. They are main-tained largely by the regenerated production and grad-ually loose their intensity toward the summer months.The present version of the coupled physical-ecosystemmodel serves as a basis to carry out more specific inter-disciplinary studies by the regional oceanographiccommunity.

AcknowledgementsThis work is a contribution to the Black Sea-ODBMS

Project sponsored by the NATO Science for PeaceProgram. It is supported in part by the NSF Grant OCE-9906656 to T. Oguz and P. Malanotte-Rizzoli, OCE-9908092 to H. W. Ducklow, and OCE-0081118 to J. W.Murray. T. Oguz also acknowledges the NATO LinkageGrant EST.CLG975821, and the Turkish Scientific andTechnical Research Council Grant 101Y005.

ReferencesBesiktepe, S.T., C.J. Lozano and A.R. Robinson, 2001:

On the summer mesoscale variability of the BlackSea. J. Mar. Res., 59, 475–515.

Codispoti, L.A., G.E. Friederich, J.W. Murray and C.M.Sakamoto, 1991. Chemical variability in the BlackSea: implications of continuous vertical profilesthat penetrated the oxic/anoxic interface. Deep-SeaRes. I, 38, Suppl.2A, S691-S710.

Figure 8. Snapshots of model simulated phytoplanktonbiomass distributions (in mmol N m-3) at three times ofthe year corresponding to (a) October 30, (b) December20, (c) March 10. They represent, respectively, theautumn subsurface, the early winter and the late winter–early spring mixed layer phytoplankton bloom events.Contours are drawn at an interval of 0.05 mmol N m-3.

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Oguz, T., P. Malanotte-Rizzoli and H.W. Ducklow,2001c: Simulations of phytoplankton seasonal cyclewith multi-level and multi-layer physical-ecosys-tem models: The Black Sea example. EcologicalModelling, 144, 295–314.

Oguz, T., A.G. Deshpande and P. Malanotte-Rizzoli,2002: On the Role of Mesoscale ProcessesControlling Biological Variability in the Black Sea:Inferrences From SeaWiFS-derived SurfaceChlorophyll Field. Continental Shelf Res., 22,1477–1492.

Sur, H.I., E. Ozsoy and U. Unluata, 1994: Boundarycurrent instabilities, upwelling, shelf mixing andeutrophication processes in the Black Sea. Progr.Oceanogr., 33, 249–302.

Tugrul, S., O. Basturk, C. Saydam and A. Yilmaz, 1992:The use of water density values as a label of chem-ical depth in the Black Sea. Nature, 359, 137–139.

Vinogradov, M.E., E.A. Shushkina, A.S. Mikaelyan andN.P. Nezlin, 1999: Temporal (seasonal and interan-nual) changes of ecosystem of the open waters ofthe Black Sea. In: Environmental degradation of theBlack Sea: Challenges and Remedies. S.T. Besiktepe, U.Unluata and A.S. Bologa, eds., NATO Sci.Partnership Sub-ser., 2, Vol. 56, Kluwer AcademicPublishers, 109–129.

Yilmaz, A., O.A. Yunev, V.I. Vedernikov, S. Moncheva,A.S. Bologa, A. Cociasu and D. Ediger, 1998:Unusual temporal variations in the spatial distri-bution of chlorophyll-a in the Black Sea during1990–1996. In: Ecosystem Modeling as a ManagementTool for the Black Sea. L. Ivanov and T. Oguz, eds.,NATO ASI Series, 2, Vol. 1, Environment-Vol. 47,Kluwer Academic Publishers, 105–120.

Zaitsev, Yu. and V. Mamaev, 1997: Marine BiologicalDiversity in the Black Sea: A Study of Change andDecline. GEF Black Sea Environmental Programme,United Nations Publications, 208 pp.

in the Eastern Mediterranean Atmosphere. GlobalBiogeochem. Cycles.

Mee, L. D., 1992: The Black Sea in crisis: A need for con-certed international action. Ambio, 21, 278–286.

Murray, J.W., H.W. Jannash, S. Honjo, R.F. Anderson,W.S. Reeburgh, Z. Top, G.E. Friederich, L.A.Codispoti and E. Izdar, 1989: Unexpected changesin the oxic/anoxic interface in the Black Sea.Nature, 338, 411–413.

Murray, J.W., L.A. Codispoti and G.E. Friederich, 1995:Oxidation-reduction environments: The suboxiczone in the Black Sea. In: Aquatic chemistry:Interfacial and interspecies processes. C.P. Huang, C.R.O’Melia and J.J. Morgan, eds., ACS Advances inChemistry Series No.224, 157–176.

Oguz, T., 2002: The role of physical processes control-ling the Oxycline and Suboxic Layer structures inthe Black Sea. Global Biogeochem. Cycles, 16(2),10.1029/2001GB001465.

Oguz, T., P. La Violette and U. Unluata, 1992: Upperlayer circulation of the southern Black Sea: Its vari-ability as inferred from hydrographic and satelliteobservations. J. Geophys. Res., 97, 12569–12584.

Oguz, T., D.G. Aubrey, V.S. Latun, E. Demirov, L.Koveshnikov, H.I. Sur, V. Diacanu, S. Besiktepe, M.Duman, R. Limeburner and V. Eremeev, 1994:Mesoscale circulation and thermohaline structureof the Black Sea observed during HydroBlack ’91.Deep-Sea Res. I, 41, 603–628.

Oguz, T., H. Ducklow, P. Malanotte-Rizzoli, S. Tugrul,N. Nezlin and U. Unluata, 1996: Simulation ofannualplankton productivity cycle in the Black Seaby a one-dimensional physical-biological model. J.Geophys. Res., 101, 16585–16599.

Oguz, T., H.W. Ducklow, P. Malanotte-Rizzoli, J.W.Murray V.I. Vedernikov and U. Unluata, 1999: Aphysical-biochemical model of plankton produc-tivity and nitrogen cycling in the Black Sea. Deep-Sea Res. I, 46, 597–636.

Oguz, T. and S. Besiktepe, 1999: Observations on theRim Current structure, CIW formation and trans-port in the western Black Sea. Deep-Sea Res. I, 46,1733–1753.

Oguz,T., H.W. Ducklow and P. Malanotte-Rizzoli, 2000:Modeling distinct vertical biogeochemical struc-ture of the Black Sea: Dynamical coupling of theoxic, suboxic and anoxic layers. Global Biogeochem.Cycles., 14, 1331–1352.

Oguz, T., H.W. Ducklow, J. E. Purcell and P. Malanotte-Rizzoli, 2001a: Modeling the response of top-downcontrol exerted by gelatinous carnivores on theBlack Sea pelagic food web. J. Geophys. Res., 106,4543–4564.

Oguz, T., J. W. Murray, and A.E. Callahan, 2001b:Modeling redox cycling across the suboxic-anoxicinterface zone in the Black Sea. Deep-Sea Res. I., 48,761–787.