Input of organic carbon as determinant of nutrient !uxes, light climate andproductivity in the Ob and Yenisey estuaries

Dag O. Hessen a,*, JoLynn Carroll b, Berit Kjeldstad c, Anton A. Korosov d,e, Lasse H. Pettersson e,Dmitry Pozdnyakov d,e, Kai Sørensen f

aUniversity of Oslo, Department of Biology, CEES, P.O.Box 1066 Blindern, 0316 Oslo, NorwaybAkvaplan-niva, Polar Environmental Centre, 9296 Tromsø, NorwaycNorwegian University of Science and Technology, Department of Physics, 7491 Trondheim, NorwaydNansen International Environmental and Remote Sensing Centre, 14 Line V.O. 7, 199034, St. Petersburg, RussiaeNansen Environmental and Remote Sensing Center, Thormøhlensgate 47, N-5006, Bergen, NorwayfNorwegian Institute for Water Research (NIVA), Gaustadalléen 21, 0349 Oslo, Norway

a r t i c l e i n f o

Article history:Received 22 December 2009Accepted 8 March 2010Available online 16 March 2010

Keywords:Arctic riversDOCnutrientsirradiationKara Seaproduction

a b s t r a c t

Spectral light attenuation pro"les and concentrations of total and dissolved carbon (C), nutrients andchlorophyll a (Chla) were studied along transects running from the river mouth to the Kara Sea duringlate summer 2003 for the Yenisey and fall 2005 for the Ob estuaries. Earth Observation data were used togenerate composite images of water color and Chla distribution over the estuaries and the Kara Sea toreveal the spatial impact of the river ef!ux in terms of optical properties.

High levels of total nitrogen (N), total phosphorus (P), silicate (Si) and iron (Fe), but low levels ofinorganic N and P and Chla were found in the estuaries. More than 90 % of total organic C was in dis-solved form (DOC). The high concentrations of DOC, mostly terrigenous, humic compounds, gaveextremely high attenuation coef"cients for both visible and ultraviolet light. For UV-B, Z10% (the depth atwhich 10% of surface light remains) was <10 cm, while Z10% for visible light (PAR) generally rangedbetween 1 and 3 m for both transects. The light attenuation rapidly decreases when the freshwater ismixed with the coastal water outside off the coast. This leads to a strong light limitation and lowproductivity in the inner estuaries, while the high load of N and P associated with DOC eventually couldpromote primary production in the Kara Sea and further upstream the coastal current in the Arctic Oceanas the organic matter becomes diluted and photooxidized. On the other hand, the high inputs of coloreddissolved organic matter (CDOM) provide an ef"cient screening of potential harmful UV-radiation overvast areas of the Arctic Ocean. A rising trend of riverine ef!ux to the Arctic seas is observed, and furtherincreases in freshwater runoff as well as eventual permafrost thawing, will accentuate the freshwaterimpact in the estuaries and the Kara Sea.

! 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The Arctic Rivers drain the tundra and taiga areas, and haveamong the world’s highest concentrations of dissolved organicmatter (DOM) (Opsahl et al., 1999; Fütterer and Galimov, 2003). Theareas drained by these rivers are estimated to hold a third of theworld’s soil carbon in the permafrost tundra mires (Gorham, 1991;Dixon et al., 1994), and the Arctic shelf seas receive a freshwaterinputof approximately4200 km3/yr (AMAP,1998), corresponding toabout 10% of the global river runoff (Aagaard and Carmack, 1989;

Peterson et al., 2002) and hence also a signi"cant !ux of carbon ina global context. . The Russian rivers Ob and Yenisey are two of thefour largest rivers!owing into theArctic. TheYeniseyRiver is ranked1st (603 km3/yr) and theObRiver is ranked 3rd (404 km3/yr) amongtheArctic rivers in termsof freshwaterdischarge rates (AMAP,1998).These rivers supply the shelf seas with nutrient-rich terrestrial andriverinematter. TheYenisei andObaccount for about34%of the totalArctic river discharge and nearly 40 % of the DOM output, corre-sponding to some8e9 Tg C year!1 (Stein et al., 2003; Raymond et al.2007). This allochthonousDOM is a carrier of organic carbon (C), butalso of organic nitrogen (N) and phosphorus (P) as well as iron (Fe).The rivers are also the major source of silicate (Si) for marinerecipients, yet Si is less linked with DOM compared to the othermajor nutrients. These rivers are also a major source of dissolved,

* Correpsonding author.E-mail address: dag.hessen@bio.uio.no (D.O. Hessen).

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Estuarine, Coastal and Shelf Science 88 (2010) 53e62

inorganic N and P from agricultural runoff and sewage. While the!uxes of N, P Fe and Si primarily support primary production inmarine recipients, organic C fuels the heterotrophic bacterialactivity. Hence, the balance between the inputs of organic C andother nutrients has a strong bearing on the net CO2-balance of theecosystem.

The output of colored terrigenous DOM impacts both thespectral properties and attenuation of light of the recipient marinewaters, and the balance between nutrient availability and lightattenuation in the estuaries governs the productivity of thesesystems. Further offshore, the major currents divert freshwaterinputs eastwards and northwards along the Siberian coast and intothe Arctic Ocean proper. Thus, the !ux and dispersion of theseelements are of vital importance for the ecosystem processes notonly in the Kara Sea, but also in large upstream areas of the ArcticOcean. The extent to which the effects of both nutrients and DOCwill affect marine areas outside the estuaries depends on thenutrient uptake and production in the estuaries themselves as wellas on the rates of bacterial utilization, photooxidation and DOMsedimentation. The spectral properties of the Arctic Ocean watersuggest a pronounced in!uence of colored organic C over wideareas (Aas et al., 2001), re!ecting that a major fraction of terrestrialDOC is rather recalcitrant to microbial breakdown and photo-oxidation (Amon and Meon, 2004). DOM is also to a large extenta carrier of organic-bound nutrients and the oxidation of DOMmayalso constitute an evolving source of primary production througha gradual release of bioavailable nutrients (Vähätalo and Zepp,2005; Stedmon et al., 2007).

The !ux of DOC from boreal catchments may range from 1 to8 tonnes km!2 yr!1 (Hessen, 1999) depending on productivity andhydrology. Since the riverine DOM also is the main determinant ofannual !uxes of N, P and to some extent also of Fe and Si, changes inhydrology per se as well as changes in the DOM-!ux should havestrong impacts on marine productivity. In addition, river dischargeis crucial to the establishment of the Arctic Ocean’s halocline(Aagaard et al., 1981; Steele et al., 1995; Rudels et al., 1996; Schaueret al., 1997) and in the inter-hemispheric transport of freshwater(Wijffels et al., 1992) as well as bicarbonate for buffering againstacidi"cation (Riebesell et al., 2007). Nutrient !uxes from land toocean integrate changes in terrestrial ecosystems, in land use, andin other human activities. The Ob and Yenisei rivers provide a hugesupply of bio-reactive substances (nutrients, trace elements, DOC)that together with sunlight intensity regulates primary productionin the adjacent coastal seas (Holmes et al., 2000). As yet, we haveonly an incomplete understanding of the !uxes of nutrients andother materials brought into the Arctic Ocean by rivers as well as byprocesses of coastal erosion (Reimnitz et al., 1994; Gordeev et al.,1996; Rachold et al., 2000).

During the past two decades, a major increase of freshwaterrunoff has been seen in the main Siberian Rivers (Peterson et al.,2002; Wu et al., 2005). This could further increase the overallimpact of riverine outputs on the Kara Sea and further on the ArcticOcean. The actual impact of the increased freshwater !ux willhowever depend on whether it is yielding only a proportionaldilution of nutrients or organic matter, or a real increase in thesesubstances. Climate models predict signi"cant warming in theArctic in the 21st century, which will impact the functioning ofterrestrial and aquatic ecosystems as well as alter land-oceaninteractions in the Arctic (IPCC, 1998; Zwiers 2002). It has beenpredicted that the regionwill experience ampli"ed effects of globalclimatic changes (Manabe and Stouffer, 1994). Estimated over thepast decades (Peterson et al., 2002; Wu et al., 2005), a pronouncedincrease in freshwater discharge from the Arctic rivers can be, atleast partially, linked to increased atmospheric concentrations ofCO2 causing a decrease of stomata density in plants and hence

reduced evapotranspiration (Gedney et al., 2006). Global temper-ature increases will also cause decreases in snow cover albedo dueto enhanced melting with increased heat absorption and perma-frost thawing. This again would most likely cause not only inten-si"ed oxidation in the tundra peatlands and soils, but alsoa growing export of organic C as well as changes in the biologicaland chemical properties of the DOC (Kawahigashi et al., 2004).

To study the ef!ux of organic C and major nutrients from theestuaries of the major rivers Ob and Yenisei to off-coastal areas, weconducted two transect cruises from the river outlets to the openwaters in both estuaries. By studying not only the concentrations oforganic matter and nutrients but also solar attenuation and thespectral properties (radiometry) as well as the Chla concentrations,we aimed to assess the role of river ef!ux in the productivity of theestuaries. The riverine impacts further off the coast were assessedby remote sensing. Previous MODIS imageries over the Kara Sea(Pozdnyakov et al., 2005a) have veri"ed that this semi-enclosedArctic Oceans shelf sea is strongly in!uenced by river dischargefrom the Ob and Yenisei rivers.

2. Materials and methods

2.1. Sampling programme and cruises

During the cruise from 14th to 29th of August 2003, 19 stationswere sampled on the Ob-Yenisey shelf (Fig. 1). Emphasis was givento covering a transect from the Yenisey river mouth northwardsacross the salinity gradient from 0 to 35&. During September 2005a corresponding transect was made from the inner to the outer part

Fig. 1. Sampling locations in the Ob and Yenisei estuaries and Kara Sea shelf. Samplingprograms were carried out in 2005 (Ob estuary) and 2003 (Yenisei estuary).

D.O. Hessen et al. / Estuarine, Coastal and Shelf Science 88 (2010) 53e6254

of the Ob estuary (Fig. 1). For both transects we obtained watersamples for analysis of nutrients, Chla, suspended matter andorganic carbon as well as assessing the spectral properties of thewater column. For reference purposes, CTD pro"les and Secchi-depths were recorded at each station. These samples also served as“ground truth” data for the satellite based parameters. Variouscontaminants were also sampled along the same transects, butthese results are reported elsewhere (Carroll et al., 2008)

2.2. Parameters and analysis

The hydrochemistry sampling programme consisted of col-lecting samples for total and dissolved nutrient concentrationsand total organic carbon (TOC). Samples were collected fromthree different water layers (surface, middle depth and bottom).Exact sample depths were chosen based on water mass changeswith depth as identi"ed in the CTD pro"les. Water samples werethen collected directly from Niskin bottles mounted on the CTDunit. For Yenisey, both "ltered and un"ltered samples werecollected for the analysis of NH4, NO3"NO2 PO4, SiO2 and Fe,while for Ob no fractionation was performed and hence only totalN, P, Si and Fe was analyzed. To obtain the dissolved samplefractions from the Yenisey estuary, sub-samples were immedi-ately "ltered through individual 47 mm, 0.2 mm PCMB "lters. Thevolume of "ltered water varied between 1000 and 2000 ml.Sample "lters were then dried and stored at room temperature.Fe samples were preserved with HNO3, SiO2 samples werepreserved with chloroform, PO4, NH4 and NO3"NO2 sampleswere preserved with H2SO4. The Fe, SiO2, PO4, NH4 andNO3"NO2 were stored at 5e10 #C. For measuring dissolvedorganic carbon (DOC), 3 ampoules (2 ml) of "ltered surface waterwere collected and stored frozen for later analysis. As a back-up,250 ml of surface water were "ltered through a 0.2 mm Al2O3-"lter and stored for later analysis at 4 #C.

Total suspendedmatter (SM) samples were prepared by "ltering2000 ml of water through a pre-weighted 47 mm GFF "lter. These"lters were also stored at 4 #C. Samples for Chla analysis were takenfrom the sea surface only. These 2000 ml sub-samples were "lteredthrough 47 mm GFF "lters and stored in liquid nitrogen. Yellowsubstance, which is a good proxy of the presence of humic materialin seawater largely controlling the optical properties of the watercolumn, were quanti"ed from un"ltered water samples (100 ml)taken from the surface only. Samples for yellow substance werestored at 5e10 #C. In addition, water samples for the analysis ofparticulate C, N and P were "ltered through 25 mm GFF "lters. Thevolume "ltered varied between 500 and 1500 ml depending on theparticle load at each station. The "lter was dried and stored at roomtemperature, and analyzed by standard procedures at the accredi-ted laboratory of the Norwegian Institute for Water Research.

The spectral properties and sunlight attenuation in the watercolumn was assessed by a four-channel radiometer (PUV Bio-spherical instruments, San Diego, US) at 4 UV wavelengths(308 nm, 320 nm, 340 nm, and 380 nm) and photosynthetic activeradiation (PAR). Attenuation coef"cients (Kd) were calculated fordownwelling irradiance at different depths.

Downwelling irradiance at a certain depth z (Ed(z))was given byEd(z)$ Ed(0)% e!Kd*z. Care was taken to do the readings on thesame side of the ship at all stations, and then avoiding shadingeffects. Kd’s were estimated relative to surface light, and it wasgenerally overcast during all samplings.

2.3. Satellite images

High quality satellite ocean color images (MODIS-Aqua data) forthe Kara Seawere available from the NASA Ocean Color web site for

August 2003 and also for the period prior to and right after theexpedition. The MODIS-Aqua spectrometer sensor acquires imagesin the visible, and at high latitudes the revisiting frequency is oneday. However, ice cover in early summer, dense cloudiness and iceedge haze as well as low sun zenithal angle in late summerconstrains the number and quality of the remote sensing dataavailable for analysis.

The images were processed with an advanced algorithmdeveloped for a simultaneous retrieval of three water qualityparameters: concentrations of phytoplankton Chla, suspendedminerals and dissolved organic carbon (Pozdnyakov et al., 2005b).The algorithm is based on the Levenberg-Marquardt multivariateoptimization procedure and employs the relevant hydroopticalmodel. The model represents spectral cross sections of absorptionand backscattering of pure water, phytoplankton, suspendedmatter (SM) and dissolved organic carbon (DOC).

To avoid inadequate retrieval results, the algorithm identi"esand eventually discards the pixels with inaccurate atmosphericcorrection and/or pixels with the water properties incompatiblewith the applied hydro-optical model (Pozdnyakov et al., 2005b;Pozdnyakov et al., 2007)

The hydro-optical model initially established for Lake Ladoga(cf. Korosov et al., 2007) was used in this study. The model wasassumed to be applicable on the following grounds: the water-sheds of both water bodies are similar in terms of boggy areas,and the mineral and granularmetric composition of sediments. Itwas not possible to develop a speci"c hydro-optical model for theKara Sea due to lack of concurrent in situ optical and hydro-chemical measurements. The speci"c feature of the appliedalgorithm do however allow for a separation between DOC, Chlaand SM due to the multivariate optimization. The accuracy of thisdifferentiation and hence the accuracy of the respectiveconcentration retrieval is determined by the adequacy of theapplied hydro-optical model, however. The model is developed insuch a way that the actual retrieved value is the concentration ofDOM in mg C l!1. This was based on the assumption that therelationship between colored DOM in DOC is invariant for thiswater body. This assumption holds for Lake Ladoga, for which themodel has been developed, and given the close correlationbetween CDOM and DOC also in the estuary (se below) it shouldhold here as well.

The retrieval accuracy of the algorithm was estimated in ourseveral studies (Pozdnyakov et al., 2005b; Korosov et al., 2007,2009). It was shown that the difference between satellite esti-mates of water quality parameters in coastal waters and ground-truth measurements is less than 50%. The percentage has beenestimated as the module difference between measured andretrieved concentrations of the target water constituent (Chla SM,DOC) normalized to the in situ concentration. Only quali"edmatchups were used for collecting this statistics. This has beendone for many water bodies (see Korosov et al., 2009). The differ-ence value is a statistical mean for each of the studied water bodies,which is then averaged over all water bodies. This indicates that thealgorithm performs much better than the NASA standard algorithmfor MODIS for case 2 waters, i.e. optically complex waters rich inChla, SM and DOC, as opposed to clear oceanic waters (case 1waters), largely void of SM and DOC (Folkestad et al., 2007). Thestandard NASA SeaDAS algorithm OC4 retrieves the concentrationof Chla, and is not capable of yielding the other two products, viz.concentrations of SM and DOC; moreover, the concentrations ofChla retrieved by OC4 for August 2003 proved to be unrealisticallyhigh (e.g. 30e60 mg/l in the Ob Bay and adjacent areas in the KaraSea), while our algorithm restores the concentrations of Chla,which are supported by the available historical data (e.g. Nöthiget al., 2003).

D.O. Hessen et al. / Estuarine, Coastal and Shelf Science 88 (2010) 53e62 55

3. Results

3.1. Carbon and nutrients

Total organic carbon (TOC) was mainly made up by dissolvedorganic C (Yenisey: 95 & 8 (StDev) %; Ob: 92& 9%), and, whenexcluding the freshwater (<0.1&) samples, decreased mono-tonically with salinity in both the Ob and the Yenisey transects(Fig. 2). This TOC was almost entirely composed of terrestriallyderived humic matter, re!ected by the strong correlation betweenTOC and absorbance at 380 nm (p< 0.0001, r2$ 0.99). Peakconcentrations of TOC at both river outlets were close to10 mg C l!1. Both estuaries displayed high and similar concentra-tions of SiO2 (close to 3 mg SiO2l l!1) and similar levels of dissolvedFe (mean 23 and 30 mg Fe l!1) for Ob and Yenisey respectively, whilethe concentrations of total N, and especially total P were generallyhigher at the outlet of the Ob compared to the Yenisey. Averagetotal P was 83 mg P l!1 (range 34e163) for the Ob while 26 mg P l!1

(range 10e48) for the Yenisey. Correspondingly, average total Nwas306 mgN l!1 (range 185e385) for the Ob and 270 (range 123e415)for the Yenisey. As for C, the major pool of N was in the dissolvedfraction (Yensiey: 97&11% for N and 67& 21% for P, Ob: 84&1%).

For both estuaries, nutrient concentrations decreased withsalinity, yet with far more scatter and less obvious linearitycompared with TOC (Table 1), strongly indicating that various loss

rates and transformations operated in addition to the dilution, andthat these mechanisms also were different for the two estuaries.The relationship between total N and P and salinity was onlymarginally signi"cant for both estuaries, and the concentrations ofdissolved nutrients (Yenisey) were too low and scattered to providestrong trends over salinity. Both Si and Fewere negatively related tosalinity, yet not in a linear fashion. The concentrations of Fe drop-ped rapidly from peak concentrations around 60 mg l!1 at the innerstations of both estuaries, and levelled off around 20 mg l!1 >5&(Fig. 3). SiO2, on the contrary gave a bimodal response over salinitywith peak concentrations close to 4 mg l!1 around 10& for both

Fig. 2. Relation between total organic carbon (TOC) and salinity in the Ob (upper) andYenisey estuary. Open symbols represent strictly freshwater samples (salinity belowdetection levels) and are omitted from the regression line.

Table 1Linear regression statistics for core parameters versus salinity.

Ob r2 p

TOC$ 7.8e0.21Sal 0.73 <0.001TotN$ 345e4.46Sal 0.28 0.003TotP$ 99.0e1.78Sal 0.31 0.001SiO2$ 3.0e42.4Sal 0.29 0.004FiltFe$ 36.5e0.2Sal 0.38 <0.001Chla$ 1.93e0.07Sal 0.4 0.05

Yenisey r2 p

TOC$ 7.8e0.17Sal 0.81 <0.001TotN$ 308e2.08Sal 0.13 0.03TotP$ 19.5.0e0.408Sal 0.22 0.004SiO2$ 3161e19.8Sal 0.01 NSTotFe$ 39.3e1.2Sal 0.32 0.002Chla$ 1.92e0.07Sal 0.4 <0.001

Fig. 3. Fe ("ltered samples) versus salinity. Smoothing spline "t (lambda$ 100). Opensymbols are freshwater samples.

D.O. Hessen et al. / Estuarine, Coastal and Shelf Science 88 (2010) 53e6256

estuaries (Fig. 4). These different responses for various elements arestriking and is re!ected both by their different relationships withsalinity but also their co-variation e or lack of such e suggestingthat there are different fates and !uxes for these elements (Table 2).

Concentrations of dissolved nutrients (Yenisey estuary) werelow, generally <20 mg l!1 for both NO3"NO2 and NH4, and<5 mg l!1 of PO4 in the surface layers, while both NO3"NO2 andPO4 (but not NH4) increased strongly with depth (Appendix 1). ForSiO2, there were no distinct depth pro"les for the inner stations,while there was a distinct dilution with increasing salinity at thedeeper, offshore stations. Si remained high in the surface layeralong the entire transect.

Particulate (seston) concentrations of nutrients were alsomeasured along with the transect throughout the Yenisey estuary,where the mean values of particulate C, N and P were 416 (&204),64 (& 22) mg and 10 (&4) mg l!1, respectively. The elemental ratioswere close to Red"eld ratio (106:16:1) with molar ratios of 7.9(&2.9) for C:N, 107.8 (&29.7) for C:P and 15.4 (&7.9) for N:P. There

were no consistent patterns for elemental ratios along the transect.Concentrations of total suspended matter (TSM) in the Ob estuaryranged from 7 to 37 mg l!1 but showed no discernable patternwithsalinity or TOC. Based on the concentrations in the inner estuaryand the average annual discharge of water from Ob and Yenisey, wecalculated approximate annual !uxes of all elements (Table 3).

Levels of Chla were low for both estuaries, 1.47 (&0.68) l!1 forthe Yenisey and 1.41 (&1.03) l!1 for the Ob. Peak concentrations(close to 3 mg Chla l!1) in both estuaries were recorded near theriver mouth, suggesting that most of the phytoplankton biomass infact was of freshwater origin.

3.2. Sea optics

Sea surface light penetration was extremely low at all stationsalong the transects. Z10, the depth at which 10% of surface (air)radiation remained, ranged from near 10 cm at 320 nm, to0.3e0.4 m at 380 nm (Fig. 5). Even for the spectrally integrated PAR,the average Z10 was <2.5 m for the Yenisey and only 1.5 m for theOb. The diffuse attenuation coef"cients were extremely high,ranging from about 25 m!1 at 320 nm to 1e1.7 m!1 for PAR withsomewhat higher attenuation coef"cients for the Ob estuary. Thelight pro"le remained fairly constant from the river mouth towardsopen water for the Yenisey transect (Fig. 6), and only at the outerstation close to Novaja Semlja, did the water transparency increasesubstantially. For the Ob estuary, there was an apparent increase intransparency along the inner station, followed by a marked drop atthe outer station, re!ecting that the optical properties in this closedestuary are also determined by mixing processes. The strongFig. 4. 2nd order polynomial "t of SiO2 versus salinity for un"ltered and "ltered

samples for Ob (upper and mid panel) and total samples for Yenisey.

Table 2Correlation matrix for key parameters of Ob and Yenisey.

Ob Salinity TOC Total N Total P SiO2

Salinity 1TOC !0.866 1Total N !0.622 0.377 1Total P !0.546 0.539 0.814 1SiO2 !0.538 0.341 0.233 0.058 1Fe !0.325 0.276 0.778 0.883 0.011

Yenisey Salinity TOC Total N Total P SiO2

Salinity 1TOC !0.542 1Total N !0.467 0.788 1Total P !0.305 0.379 0.771 1SiO2 !0.105 0.426 0.823 0.748 1Fe !0.566 0.559 0.305 0.348 !0.053

Table 3Estimated annual !uxes in terragrams for TOC andmajor minerals based peak values(inner part of estuaries) and published data on discharge.

Cons. mg l!1 Discharge km3 Annual !ux tg

ObTOC 10 400 4Total N 0.45 400 0.18Total P 0.12 400 0.048Total Si 3 400 1.2Total Fe 2 400 0.8

YeniseyTOC 10 600 5.6Total N 0.3 600 0.18Total P 0.025 600 0.015Total Si 3 600 1.8Total Fe 2 600 1.2NH4 0.012 600 0.0072NO3"NO2 0.012 600 0.0072Dissolved P 0.019 600 0.0114PO4-P 0.003 600 0.0019

D.O. Hessen et al. / Estuarine, Coastal and Shelf Science 88 (2010) 53e62 57

attenuation also for PAR imply that the primary production isrestricted to the very uppermost layers.

3.3. Remote sensing

The results of the retrievals of Chla, DOC and suspended matterfrom satellite data are illustrated in Figs. 7e9. Using Chla as a proxyof primary production in the Kara Sea (summer 2003), we foundthat the predominant wind patterns yield a distinct eastern spreadof river runoff from the Ob and Yenisei Rivers that boosts autotrophproduction. The concurrent spatial distributions of suspendedmatter (SM) and DOC comply very well with this observation asboth constituents weremost abundant to the east of the Ob estuary,and along the coastal zone between the Yamal Peninsula and theSevernaya Zemlya Islands. Accordingly, the northern area of thesouth-western region of the Kara Sea is less in!uenced by fresh-water inputs, and is also less productive. At the same time, as seenin the southeast part of the Kara Sea (Figs. 7 and 8), the warmBarents Sea waters appear as rich in Chla, and contain appreciableamounts of DOC, thus suggesting a positive impact of riverineinputs on the autotroph production in spite of the extremely lowtransparency.

While the range and distribution of DOC assessed from MODISand ground measurements were in close agreement, the MODIS-based estimates of Chlawere almost an order of magnitude higherthan the measured concentrations, re!ecting the fact that thetransect sampling was performed in late fall (hence low phyto-plankton biomass) while the MODIS data represents compositeimages over the ice-free season including spring and summerblooming periods. Nevertheless the MODIS data clearly visualizedthe decreasing concentrations of Chla along the transects.

4. Discussion

Our data con"rm the "ndings of previous studies with regard toboth concentrations and annual !uxes of TOC and DOC (Köhleret al., 2002; Dittmar and Kattner, 2003; Stein et al., 2003). Esti-mated !uxes based on such a brief time span of sampling shouldadmittedly be judged with some caution, but estimated !uxes oftotal C (4 Tg yr!1 and 6 Tg yr!1 for the Ob and the Yenisey,

respectively) are within the range of previous reports (Stein et al.,2003); also in Raymond et al., 2007 for !ow-weighted discharge2004-2005 (4.7Tg/yr!1 Yenisey, 3.1Tg/yr!1 Ob), yet somewhathigher than those given by Dittmar and Kattner (2003). Although,for the !uxes of total N our data exceeds those of Dittmar andKattner (2003), and SiO2-Si are in the upper range of previousestimates. Dissolved inorganic N (DIN: NO3"NO2"NH4) and PO4,however, are in the lower range of previous estimates.

For the Ob, there is a positive correlation between TOC and SiO2as well as total N and total P, while for the Yenisey the picture ismore scattered (Fig. 5): there was a signi"cant positive correlationbetween TOC and SiO2 and TOC and total N, while a rathersurprising negative correlation between TOC and Total P (or salinityand total P) in this estuary. Yet, all nutrients were assumed to beprimarily of freshwater origin. Differential sedimentation rates,nutrient uptake and regeneration plus the turbulent dynamics inthe estuary exhibited contrasting patterns for various nutrients anddissolved versus total pools of speci"c nutrients. Although SiO2 isclearly of freshwater origin, peak concentrations of SiO2, werefound at medium salinity levels (Fig.5) while for dissolved Fe therewas an immediate drop, probably re!ecting the turbulent dynamicsin the estuary (Volkov et al., 2002).

By and large the Ob and Yenisey estuaries are rather similar withregard to carbon and nutrient concentrations, hence their relativeexport of elements to the Kara Sea can basically be scaled withannual discharge. The Ob has, however, twice as high concentra-tions of total P as Yenisey, probably re!ecting somewhat differentactivities in the catchment, although this could also re!ect thatsampling in the Ob occurred in late fall when biological uptake hadceased. An overall higher concentration of PO4 in the Ob relative tothe Yensiey (Dittmar and Kattner, 2003; Amon andMeon, 2004) do,however, indicate a higher relative P-export from this river.

While almost the entire fraction of TOC and SiO2 is in dissolvedform, this is somewhat different for P and N. Close to 70 % of total P(Ob estuary) was in dissolved form, however while the pool ofdissolved P was on average 19 mg P!1, PO4 accounts for no morethan 2.8 mg P!1, suggesting that some 85 % of dissolved P was in theorganic form. Almost the entire pool of N was in dissolved form(97%), while oxidized DIN (NO3"NO2) and reduced DIN (NH4)together accounted for up to 23.7 mg l!1 or<10 % of dissolved N. ForFe, on the contrary, the major fraction was in dissolved form. The!ux and fate of the various elements play a major role in produc-tivity, light climate and the balance between autrophic andheterotrophic processes in the estuary. While Fe rapidly precipi-tates at elevated salinity, SiO2 and the dissolved organic fractions of

Fig. 5. Light responses of different wavelengths represented as depths where 10 % ofsurface light remains (Z10, upper panel) and vertical attenuation coef"cient (Kd ,lower).

Fig. 6. Z10 for PAR along the transects from the river mouth to the oceanic stations.

D.O. Hessen et al. / Estuarine, Coastal and Shelf Science 88 (2010) 53e6258

C, N and P can be transported over vast areas, gradually becomingavailable to autotrophs after photo-oxidation and microbialdegradation.

The relationship between CDOM and salinity in estuaries willdepend both on !ushing rate and internal processes (cf. Bowers andBrett, 2008), yet in the large estuaries of Ob and Yenisey DOC or

humic substances decrease monotonously with increased salinitywith only limited losses by !occulation (cf. Amon andMeon, 2004),re!ecting a straightforward dilution of this assumed recalcitrantorganic C. Within the estuaries, the DOC-concentrations are ratherconservative, and long-term incubation experiments indicate lowbacterial mineralization rates (Köhler et al., 2002; Amon and Meon,

Fig. 7. The spatial averaged distribution of chlorophyll a (m g l!1) as retrieved from the composite MODIS images over the Kara Sea, taken during the ice-free season 2003.

Fig. 8. The spatial averaged distribution of dissolved organic carbon (mg C l!1) as retrieved from the MODIS images over the Kara Sea, taken during the ice-free season 2003.

D.O. Hessen et al. / Estuarine, Coastal and Shelf Science 88 (2010) 53e62 59

2004), while photooxidation at times may represent signi"cantlosses of DOC. Under clear sky, Amon and Meon (2004) reportedlosses of 40 mM DOC or 6% of total DOC over a course of 290 h. Itshould be kept in mind, however, that owing to the extremely highattenuation of short-waved light, suchmineralization rates are onlyvalid for the upper few centimetres. Also, for most of the year,conditions for photooxidation are poor or absent.

Even though a certain fraction of DOC certainly is lost bycombined microbial breakdown and photooxidation, most of theKara Sea and the entire Arctic ocean are characterized by highattenuation of short-wave radiation. In the inner part of the estu-aries, the high attenuation coef"cients for PAR as well will poseconstraints on primary production. In the offshore areas, however,the elevated levels of DOC will provide an ef"cient sunscreen forharmful UV radiation. Z10% level at 380 nm are typically around10e20 m formost of the Norwegian Sea, Greenland Sea and BarentsSea, and rarely <5 m even for coastal areas (Aas et al., 2001). Incontrast, Z10% 380 nm for the estuaries was on average 0.3 m,exceeding 0.5 m only at the outer stations. This "ts well withprevious estimates of mean Z10% 380 nm for the Ob and Yeniseyestuaries (0.2 m) reported by Aas et al. (2001). These authors alsoreported 2.2 m as the average value of Z10% 380 nm for the Kara Seaas a whole.

The Chla a concentrations found in our study (0.4e3.4 for theOb, 0.6e2.9 for the Yenisei) "ts well those of Nöthig et al. (2003),and themarked drop in Chla along the transect suggest that most ofthe autotrophs in the estuary are of freshwater origin (cf.Makarevich et al., 2003). Both the salinity stress and low sunlightlevels could thus constrain primary production in the estuary, andalso pose constraints on bacterial mineralization of DOC(Langenheder et al., 2003). Further off coast, a gradual oxidation ofhumus-bound, organic N and P could stimulate both autroph andheterotroph production, as indicated by the remote sensing data,and also shield vast areas from harmful UV-R.

Division of the marine and the riverine water masses are clearlyseen in the satellite images. The frontal zones are located across the

gradients of TOC and suspended matter, which can be used asa passive tracer for extent of the riverine waters (Figs. 8 and 9). It isseen in Fig. 7 that this frontal zone limits the extent of the areaswith high Chla concentrations. Importantly, the maximumblooming occurs not in the entire Ob Bay, but only in the mixingzone where turbid brackish waters are diluted with cleaner salinewaters. Further south, where waters are almost opaque due to theextremely high DOC and SM content, Chla concentrations are lower.DOC and suspended matter exhibits similar behaviour in the outerpart of the Ob bay and in the open front of the estuaries e itsconcentrations are high only within the area of riverine watersinvasion (Fig. 8 and 9). Clearly, the TOC content in the Ob Bay onlyincreases towards the south. The adequacy of the retrievals for TOCwas con"rmed by our ground truth data, which also compare wellwith previous shipborne determinations (Köhler et al., 2002; Gaye-Haake et al., 2003; Nöthig et al., 2003). For Chla, however, theremote sensing estimate should be read with some caution,although themajor reason for the inconsistency betweenmeasuredchla and MODIS-derived chla is likely that the transect samplingwas performed in late fall (hence low phytoplankton biomass)while the MODIS data represents binned, composite images overthe ice-free season including spring and summer blooming periods.

The huge drainage basins of these rivers are under change.Increased levels of CO2 and permafrost thawing may in concertincrease both the discharge and also levels of TOC, and thus asso-ciated nutrients. Permafrost thawing will most likely increasesubstantially the concentrations of TOC (e.g. Freeman et al., 2004;Frey and Smith, 2005), as well as accompanied by increasedrunoff (Peterson et al., 2002;Wu et al., 2005). The net effect of thesechanges on productivity and C-sequestration in the Arctic ocean isnot straightforward, since TOC per se will likely stimulate theheterotrophic activity: increased nutrient loads lead to a stimula-tion of the primary production, but can also enhance TOC miner-alization rates. These conditions will likely cause an increase in heatabsorption in the top-most layer of the water column, increasingthermal stability and producing a shallower location of the

Fig. 9. The spatial averaged distribution of suspended matter (mg l!1) as retrieved from the MODIS images over the Kara Sea, taken during the ice-free season 2003.

D.O. Hessen et al. / Estuarine, Coastal and Shelf Science 88 (2010) 53e6260

thermocline. Together these conditions are expected to strengthenthe “sun-glass” protection of marine biota from shortwave radia-tion. Finally it should be kept in mind that in the long term, newproduction in marine areas ultimately depends on freshwaterinputs of P, Si and Fe which again regulates the marine C-seques-tration (cf. Lenton and Watson, 2000). These nutrient suppliesconnected to freshwater inputs also enhance alkalinity and hencebuffer against CO2-induced marine acidi"cation.

Acknowledgements

Funding for this project was provided by the NorwegianResearch Council’s East European Programme (Grant # 152710)within the Environment and Development (Miljø og Utvikling)Programme. For completion of the paper the Nansen Centresacknowledge the grant to the scienti"c foundation Nansen Inter-national Environmental and Remote Sensing Center from ResearchCouncil of Norway (Grant # 179125/S30) and The NorwegianMinistry of Education and Research. The authors gratefullyacknowledge the assistance and support of Dr. Oleg Stepanets,Vernadsky Institute of Geochemistry and Analytical Chemistry(GEOKHI), Russian Academy of Sciences, Moscow. The project hasbene"ted from collaboration with the RussianeGerman Researchprogramme, Siberian River Run-off (SIRRO), coordinated by Acad.Prof. Dr. E. Galimov (GEOKHI), and Prof. Dr. D. K. Fütterer, AlfredWegener Institute for Polar and Marine Research (AWI), Bre-merhaven. We thank the captain and crew of the Russian ResearchVessel ‘Academician Boris Petrov’ for their experience and help-fulness throughout the 2003 expedition. Dr. Robert McCrea,Dr. Derek Muir, and Dr. Xiaowa Wang of Environment Canadakindly provided equipment and valuable advice to assist us in thecontaminant programme. Vladimir Savinov, Tatiana Savinova, KnutEitrheim and Guttorm Christensen provided valuable assistance inthe planning and execution of "eld sampling activities. Finally weacknowledge the highly valuable and helpful comments from Dr. D.G. Bowers and an anonymous reviewer.

Appendix A. Supplementary data

The supplementary materials associated with this article can befound, in the online version, at doi:10.1016/j.ecss.2010.03.006.

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