Particulate N and P characterizing the fate of nutrients along the estuarine gradient of the River Neva (Baltic Sea)

Estuarine, Coastal and Shelf Science 61 (2004) 275e287

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Particulate N and P characterizing the fate of nutrients alongthe estuarine gradient of the River Neva (Baltic Sea)

Jouni Lehtorantaa,), Anna-Stiina Heiskanenb, Heikki Pitkanena

aFinnish Environment Institute, P.O. Box 140, FIN-00251 Helsinki, FinlandbEuropean Commission Joint Research Centre, TP 290, I-21020 Ispra (VA), Italy

Received 16 May 2002; accepted 26 April 2004

Abstract

The estuary of the River Neva is eutrophied due to the high nutrient load from St. Petersburg. Previous studies have suggested

that a large part of the N and P load is efficiently retained in the estuary. However, the fate of particulate N and P in the estuarial seaarea has remained uncertain. Variations in the C, N and P sedimentation rate and in sediment organic matter (loss on ignition, LOI),total nitrogen (TN) and phosphorus (TP) were therefore studied in the recent sediment deposits along the estuarine gradient of the

River Neva. The high gross sedimentation rate of N and P indicated efficient retention of both N and P in the inner estuary. Anincrease in the concentrations of settling particulate C and N along the estuarine gradient was consistent with the increase in theconcentrations and burial of organic matter and TN in sediments. However, although the concentration of P in settling matter also

increased along the estuarine gradient, a decrease was noted in the sediment TP concentration. This decrease correlated significantlywith an increase in the concentration of sediment organic matter, but not with water depth along the estuarine gradient. The resultindicates that an increase in organic matter and changes in hydrodynamic conditions along the estuarine gradient can reduce thecapacity of the top 0e10 cm sediment layer to retain P. The average concentration of N in settling particulate matter was 23.3 mg

g�1 DW and in the sediment surface 6.5 mg g�1 DW. Corresponding values for P were much closer to each other, that is, 3.3 and2.8 mg g�1 DW, respectively. This suggests that, in the surface sediments (0e1 cm), settled P was more efficiently retainedand/or accumulated than N. In contrast, in the 0e10 cm sediment layer N seemed to be more efficiently retained than P.

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Keywords: phosphorus; nitrogen; sediment; cluster analysis; River Neva; Baltic Sea

1. Introduction

Eutrophication attributed to an excess supply ofnutrients leading to increased biological productivity isconsidered to be one of the main problems in the BalticSea (Ærteberg et al., 2001). In the eastern Gulf ofFinland, both primary productivity and biomasses ofautotrophic and heterotrophic organisms are among thehighest in the Baltic (Pitkanen et al., 1993; Kauppilaet al., 1995; Pitkanen and Tamminen, 1995), mainly dueto eutrophying effects of the high nutrient inflow fromthe St. Petersburg region (Fig. 1). The greater the

) Corresponding author.

E-mail addresses: [email protected] (J. Lehtoranta),

[email protected] (A.-S. Heiskanen), heikki.pitkanen@

ymparisto.fi (H. Pitkanen).

0272-7714/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2004.04.016

distance from the inner estuary of the Neva towards theopen Gulf, the lower are the concentrations of inorganicand total N and P (Pitkanen et al., 1993). In the Nevaestuary, this non-conservative behaviour of nutrientscan largely be explained by the effective biologicalfixation of nutrients into planktonic biomass followedby sedimentation during the productive season (Pitka-nen and Tamminen, 1995). Efficient biological bindingof N and P in the inner estuary is also supported by highchlorophyll a and algal biomass concentrations and bya decrease in these concentrations towards the openGulf (Kauppila et al., 1995). Thus, the role of the Nevaestuary is decisive in regulating the overall nutrientbalance of the entire Gulf of Finland.

The objective of the present study was to examine thefate of particulate N and P along the estuarine gradientof the River Neva in the eastern Gulf of Finland. To this

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276 J. Lehtoranta et al. / Estuarine, Coastal and Shelf Science 61 (2004) 275e287

Fig. 1. Sediment sampling and trap sites in study area. Grey denotes areas of recent sediment deposits (Winterhalter, Rybalko, Butylin and

Spiridonov, unpubl.). Increase in surface salinity (1 m) along estuarine gradient is presented in Fig. 1b.

end, we studied the regional relationships betweenorganic matter, N and P in settling matter and in thesurface sediment of recent sediment deposits. We alsostudied variations in the regional sediment retentioncapacity of P in order to identify potential areas for thebenthic outflux of P.

2. Material and methods

2.1. Geomorphology, hydrography and characteristicsof sedimentation in the study area

The Gulf of Finland (area 30,000 km2, average depth37 m) is geomorphologically a direct continuation of theBaltic proper without any sills. The borderline betweenthe eastern (area 13,000 km2) and western Gulf can bedefined as the belt of reefs and shallows that extendsfrom the coast of Finland to the coast of Estonia via theislands of Kaunissaari, Gogland and Tso-Tytarsaari(Fig. 1).

Owing to its topography and to the mean freshwaterinflow rate (2460 m3 s�1; Carlsson and Bergstrom, 1993)to the easternmost end of the Gulf from the River Neva,

the Gulf of Finland can be regarded hydrographically asa partially mixed estuary. The Neva, the largest riverflowing into the Baltic Sea, produces 75% of the totalinflow into the Gulf of Finland (Ehlin, 1981). Due to theCoriolis effect and prevailing wind directions, theaverage circulation of surface water in the Gulf is anti-clockwise. As a result, the northern part of the easternGulf receives the bulk of the load from the Neva. Thesurface salinity of the study area increases from 0�0 inNeva Bay to about 4�5 in the open Gulf (measured onthe Practical Salinity Scale, Fig. 1).

In addition to these hydrodynamic conditions, sedi-ment accumulation in the eastern Gulf of Finland ischaracterized by the highly variable distribution ofbottom types due to the irregular topography, whichincludes numerous depressions filled with pelitic mud(Emelyanov, 1988). Thus, the critical depth for perma-nent sediment accumulation varies greatly. Mosaic-likeareas are common, particularly in the northern part ofthe study area, where rock outcrops alternate with tilland clay sediments. There, recent sediment deposits aremainly located in small, steeply sloping basins sur-rounded by large, shallow areas. The topmost layers ofthese deposits are covered with a layer of muddy clays or

277J. Lehtoranta et al. / Estuarine, Coastal and Shelf Science 61 (2004) 275e287

silts (Winterhalter, 1992). The thickness of the recentsediment ranges from a couple of centimetres to severalmetres between, and even within, the basins (Vallius,1999).

The benthic animals mainly responsible for biotur-bation of the soft surface sediments in the Gulf ofFinland are the amphipods Monoporeia affinis (formerlyPontoporeia), Pontoporeia femorata and Saduria ento-mon and the mollusc Macoma baltica (Andersin andSandler, 1991). However, the effect of bioturbation isinsignificant due to the low abundance of burrowingbenthic animals in the sedimentation areas of the easternGulf. A high abundance of P. femorata and M. affinishas been observed at only two sites in the study area(Gran and Pitkanen, 1999).

2.2. Sampling and analyses

The structure of the bottom deposits was studied withan Atlas Deso 10 echosounder (30 kHz), an instrumentthat provides sufficient deep penetration and resolutionfor soft sediments. As well as with echographs, thebottoms of active sedimentation were located with theaid of bathymetric maps and recent sediment depositmaps (Winterhalter, Rybalko, Butylin and Spiridonov,unpubl.).

The sediment samples were taken with a gravity corer(polycarbonate tube, diameter 94 mm) at 28 sites alongthe estuarine gradient during three research cruises, inJuly 1992, July 1993 and August 1994 (Fig. 1). Thesediment cores were sectioned at the following depths:0e1, 1e2, 2e3, 3e4, 4e5, 5e6, 6e7, 7e8, 8e9 and9e10 cm. The sediment samples were frozen in plasticglove bags onboard the research vessel. The dry weight(DW) of sediment samples was measured by drying thewet samples for 24 h at C105(C. The dried sampleswere stored in air-tight glass jars for further sedimentanalyses. Loss on ignition (LOI) was analysed byigniting the samples for 24 h at C550(C. Total nitrogen(TN) and total phosphorus (TP) were analysed by co-digestion using the method of Zink-Nielsen (1975).Organic matter was decomposed with strong sulphuricacid. Nitrate and nitrite were reduced to ammonia withDevarda solution. The resulting ammonium sulphatewas distilled and ammonium was titrated (Starck andHaapala, 1984). Inorganic phosphate complexes andorganic P were converted to orthophosphate by sul-phuric acid, and orthophosphate was analysed with themolybdenum blue method (Murphy and Riley, 1962).

During cruises on 9e15 August 1992 and 11e17August 1995 duplicate polycarbonate cylinders (diam-eter 10 cm, height 50 cm) were moored with surfacefloats at nine sites below the mixed surface water layer(generally 10e15 m from the bottom surface) (Fig. 1).Subsurface buoys provided up-lift for the traps,and anchors kept the moorings at their location.

Concentrated formaldehyde was used as preservativein the trap cylinders (Gundersen, 1991). The water in theupper part of the cylinder was discarded and the rest wascollected through an opening in the bottom of thecylinder. The total volume of the sample was measured,and sub-samples for chemical analysis were taken fromthe homogenous suspension. Samples were filtered onWhatman GF/F glass fibre filters for total particulatematter (TPM), organic carbon (POC), nitrogen (PON)and phosphorus (PTP). The TPM samples were filteredin duplicate on precombusted (4 h at 450(C) andpreweighed Whatman GF/F glass fibre filters. The POC,PON and PTP samples were filtered in duplicate onprecombusted (4 h at 450(C) and acid-washed What-man GF/F glass fibre filters. The POC and PON filterswere dried and analysed with an elemental CHNanalyser (LECO and Leeman Labs CHN analyzers)and PTP using acid hydrolysis after high temperaturecombustion (Solorzano and Sharp, 1980). The ortho-phosphate produced was determined according toKoroleff (1983).

2.3. Statistical analyses of the data

Pearson product-moment correlation coefficientswere calculated using SAS software (SAS InstituteInc., 1989, 1990). The data were clustered with theaverage distance method (Sokal and Michener, 1958),which tends to join clusters with small variances andis slightly biased towards producing clusters with thesame variance. SAS (PROC CLUSTER; SAS Institute,Inc., 1989) was used for statistics. Cluster analysis wasperformed by sampling sites (individual directed anal-ysis) using each measurement (DW, LOI, TN and TP) ofeach sectioned sediment depth as a discrete variable;thus, the total number of variables per sampling sitewas 40.

3. Results

3.1. The general character of sediment

The surface sediments were rich in organic matter(LOI 12.7e24.0% DW). The innermost site of the Nevaestuary (S1) was classified as the transport/depositionbottom of fine particulate matter, since sand-sizeparticles were found in the sediment cores. A brownoxidized surface layer, evidently coloured by Fe(III)oxides, was usually found in the upper surface sediment.This layer was thicker in the inner and outer NevaEstuary (10e20 mm) than in the open Gulf (5e10 mm)and northern regions (0e3 mm). Beneath the oxidizedsurface layer, a black or dark grey layer, evidentlycoloured by sulphides, was observed in all the sediment


samples, and the smell of hydrogen sulphide (H2S) wasaccordingly always present. In the organic-rich sediment(LOI 17.9e24.0% DW), the black layer often reachedthe very surface of the sediment and was colonized bywhite bacteria (probably filamentous sulphur oxidizingBeggiatoa spp).

3.2. N and P concentrations in settling matterand in sediments along the estuarine gradient

The gross sedimentation rates of TPM, POC, PONand PTP were highest in the inner estuary, decreasingtowards the open Gulf (Fig. 2AeD). The concentrationsof POC (5e35% of TPM), PON (7.5e35 mg g�1) andPTP (1.6e4.6 mg g�1) in settling matter increased withdistance from the river mouth (Fig. 3A,C,E). The meanLOI and TN concentrations calculated for the 0e10 cmsediment layer also increased, whereas the correspond-ing TP concentration decreased from 2.2 to 1.6 mg g�1

DW along the estuarine gradient (Fig. 3B,D,F).Furthermore, sediment surface (0e1 cm) LOI and

TN concentrations also increased significantly withdistance (for LOI r2=0.428 and for TN r2=0.444,P!0.001, data not shown), but a corresponding

relationship for surface TP was not observed(r2=0.021, data not shown). However, water depth didnot correlate significantly with sediment surface LOI(r2=0.038), TN (r2=0.057) or TP (r2=0.005), nor did itcorrelate with the mean LOI, TN and TP concentrations(Table 1).

In settling matter, there was a strong positivecorrelation between POC and PON and also betweenPOC and PTP (Fig. 4A,C). In sediments, the correlationbetween LOI and TN was still strong and positive(Fig. 4B), whereas between sediment LOI and TPa significant negative correlation was observed (Fig. 4D).Only an increase in sediment organic matter couldexplain significantly the decrease in the mean TPconcentration along the estuarine gradient (compareFigs. 3F and 4D).

At the trap sites studied, the average concentration ofsettling PON was clearly higher (23.3 mg g�1 DW) thanthe average value (6.5 mg g�1 DW) in the sedimentsurface. The corresponding values for P were muchcloser to each other (3.3 and 2.8 mg g�1 DW, Fig.5A,B). Thus, the average PON:PTP ratio in settlingmatter was clearly higher than the corresponding TN:TPratio in sediments (Fig. 5C).

Fig. 2. Gross sedimentation of (A) suspended particulate matter (SPM); (B) organic C (POC); (C) organic N (PON); and (D) total P (PTP) vs.

distance from mouth of the Neva.


Fig. 3. Concentrations of particulate (A) POC; (C) PON; and (E) PTP in settling matter and (B) average concentrations of organic matter as loss on

ignition; (D) TN; and (F) TP in 0e10 cm sediment layer vs. distance from mouth of the Neva. Number beside symbol denotes (A) POC:PON ratio by

mass and concentration of (C) TN and (E) TP as mg g�1 DW in 0e1 cm sediment layer. Reference concentration in settling matter from coastal

western Gulf of Finland is marked with an open circle (Heiskanen and Tallberg, 1999). For sediment measurements (B, D and F), symbols denote the

different clusters (C2eC4) presented in Fig. 6. Sediment sampling site S1 (black dot) was not included in calculations.

3.3. Grouping of sediment N and P concentrationsalong the estuarine gradient

To examine the regional similarity of DW, LOI, TNand TP in the sediment column studied, we divided thesampling sites into four groups on the basis of thecluster analysis (Fig. 6).

Cluster C1 contained only one site: this represents theinnermost estuary, classified as the transport/depositionbottom of fine particulate matter. Cluster C2

corresponded well to the basin of the inner NevaEstuary (IE), whereas the sampling sites of cluster C3were located in the outer estuary (OE) and in the openGulf (OG). Cluster C4 corresponded well to thenorthern Gulf (NG), but some of the sites were locatedin the outer estuary. Thus, the spatial subdivision ofclusters C1eC4 coincides rather well with distance fromthe mouth of the Neva and the subdivision made on thebasis of geomorphological and hydrographical features(Pitkanen et al., 1993; Pitkanen and Tamminen, 1995).


According to the cluster analysis, the water content aswell as the mean concentrations of LOI and TN insediments increased in the order C1!C2!C3!C4from the mouth of the Neva towards the west (Fig.7AeC). In the inner estuary (C2) and in the open Gulf

Table 1

Pearson product-moment correlations between distance from mouth of

the Neva (km) and water depth of sampling site (m) and mean

concentration of organic matter (LOI), TN, TP and TN:TP ratio in

0e10 cm layer (n=27)

km m

km 1

m 0.595* 1

LOI 0.651* 0.318

TN 0.654* 0.280

TP �0.608* �0.009

TN:TP 0.716* 0.108

Symbol * denotes P! 0.001.

(C3), the mean concentrations of surface TN wereclearly lower (6.5 and 7.2 mg g�1 DW) than those in thenorthern Gulf (C4, 11.5 mg g�1 DW).

The results of the cluster analysis revealed that e withthe exception of cluster C1 and the surface layer e theaverage concentration of TP in clusters decreased alongthe estuarine gradient in the order C2OC3OC4(Fig. 7D). The vertical profiles of TP concentrations inthe regional clusters showed that, in the inner estuary(C2), TP decreased gradually from the sediment surface(3.7 mg g�1 DW) to a depth of 8 cm (1.5 mg g�1 DW).As in C2, the TP concentration decreased gradually withdepth in the outer estuary and in the open Gulf (C3); inthe whole sediment profile, however, it was consistentlylower than in the inner estuary (C2). In contrast to C2and C3, the concentration of TP in C4 decreased sharply(from 3.3 to 1.5 mg g�1 DW) in the 0e3 cm layer butonly slightly below 3 cm. To summarize, the greatestdifference in average TP concentration profiles between

Fig. 4. Relationship between concentrations of (A) POC and PON in settling matter, (B) sediment LOI and TN in 0e10 cm sediment layer and

corresponding relationship between (C) settling matter POC and PTP and (D) sediment LOI and TP. In (B) and (D), symbols denote the different

clusters (C2eC4) and corresponding area, i.e. IEZInner Neva Estuary, OEZOuter Neva Estuary, OGZOpen Gulf and NGZNorthern Gulf,

formed in cluster analysis.


Fig. 5. Average concentrations of (A) settling particulate organic N (PON) and sediment TN and (B) settling particulate P (PTP) and sediment TP at

trap sites and at all studied sites (TNall and TPall) in 0e1 cm sediment layer and (C) corresponding PON:PTP and TN:TP ratios at trap sites (TN

and TP) and at all studied sites (TN:TPall). Box plot shows the median, 25th and 75th percentiles, and standard deviation.

clusters occurred in the 1e8 cm layer; in deeper layersthe average concentrations were closer to each other,ranging only from 1.2 to 1.5 mg g�1 DW. Thus, thedecrease in mean TP calculated for the 0e10 cm layeralong the estuarine gradient (Fig. 3F) was related to thehigh concentrations of TP in deeper sediment layers inthe inner and outer estuary.

The average TN:TP ratio of the groups increasedfrom the inner estuary towards the west in the wholesediment column due to the increase in TN and thegeneral decrease in TP along the estuarine gradient(Fig. 8). In contrast to the clear decrease in the N:P ratioof settling matter and surface sediment (Fig. 5C), theaverage TN:TP ratio increased with sediment depth inall clusters.

There was only slight variation in the proportionaldecrease in TN and TP concentrations with sedimentdepth between the regional clusters, C2, C3 and C4. Inthese clusters, the average surface layer TN concentra-tion was from 42 to 46% higher than that in the deeplayer (9e10 cm). The corresponding values for TP

ranged from 59 to 62% between the clusters. Further,the proportional decreases in both TN and TP in theclusters are close to the values (44 and 61%) calculatedfor the whole sediment data.

The above figures (44 and 61%) were used to predictthe TN and TP concentrations in the 9e10 cm layer asfollows: the surface concentration was multiplied by theconstant of 0.56 for TN and 0.39 for TP. In this cal-culation a strong correlation was found between themeasured TN concentration in the deep layer and thepredicted deep layer TN (Fig. 9). However, there was nosignificant correlation between the measured TP and thepredicted deep layer TP (r2=0.09, n=27).

4. Discussion

4.1. Differences between settled and buried N and P

The strong positive relationship between settling mat-ter POC, PON and PTP indicates that the sedimentation


Fig. 6. Locations of clusters C1, C2, C3 and C4 in sedimentation areas and sub-division of study area based on cluster analysis.

of organic particulate matter is an important mechanismfor transporting both N and P into the sediment in latesummer, although the chemical precipitation of P withFe(III) oxides may also play a role (Gunnars et al.,2002). Similar relationships between settling matterfactors are found in the coastal western Gulf of Finland(Laakkonen et al., 1981).

The results of this study suggest that a much largerportion of settling PON than of PTP was lost in thesurface sediment after settling. In other words, retentionof sedimented P may be more efficient than that of N insurface sediments. Settling PTP and sediment surface TPconcentrations comparable to those found here havebeen measured in other sea areas, including the centralGulf of Finland (Sundby et al., 1992; Jensen et al., 1995;Leivuori and Vallius, 1998). The difference in thebinding of N and P after settling was supported by theopposite correlations between sediment TN and TP andorganic matter, although both PON and PTP correlatedstrongly and positively with POC in settling matter. Thechange in the correlation from positive to negativebetween organic matter and P is explained by the factthat, after the settling and mineralization of organic P,the inorganic compounds participate in the binding of P.In contrast to P, N is mostly bound to organic matter in

sediments, even after the mineralization processes. Theresult is in accordance with the finding that organic Nconstitutes 99% of the N in marine sediments (Keefe,1994).

Settled organic matter rich in P can be partlymineralized and partitioned between pore water andsurface adsorption sites in sediments. For example, inArhus Bay, P sedimented in spring has for the most partbeen mineralized in the surface of the sediment andsubsequently largely retained in the pool of Fe-bound P(Jensen et al., 1995). In the study of Sundby et al. (1992),the settling matter also had a much higher proportion oforganic P (35% of TP) and a lower proportion of Pbound to metal oxides (25% of total P) than was mea-sured on the sediment surface (6 and 50% of TP, res-pectively). However, in addition to the adsorption ofnewly mineralized P, the surface TP may be increaseddue to the binding of P diffusing upwards from deeperlayers onto metal oxides present in the surface layer(Sundby et al., 1992).

The processes in sediments during burial seem to leadto more efficient dissolution of P than of N although,after settling, the surface sediment retains P moreeffectively than N. The average sediment surface TNand TP concentrations were, respectively, 44% and 61%


Fig. 7. Vertical profiles of average concentrations of (A) dry weight; (B) loss on ignition; (C) TN; and (D) TP in clusters. Bars denote the standard

deviation. C1Zsite S1, C2ZInner Estuary (IE), C3ZOuter Estuary (OE) and open Gulf (OG) and C4ZNorthern Gulf (NG).

Fig. 8. Vertical profiles of N:P ratios in clusters. C1Zsite S1,

C2ZInner Estuary, C3ZOuter Estuary and open Gulf, C4ZNorthern

Gulf. Bars denote standard deviation.

higher than those of the deep layer (9e10 cm). Thus, inthe sediment layers studied, the average TP concentra-tion decreased relatively more with sediment depth thandid that of TN. Further, except in the inner Nevaestuary, the ratio of TN to TP (ranging from 2.4 to 5.8by mass) was higher than the pore water DIN:DIP ratio(ranging from 0.8 to 2.0 by mass; re-calculated fromdata of Lehtoranta, 1998) in the deep layers of thesediments, suggesting that P is dissolved more efficientlythan N during burial and thus leads to low pore waterDIN:DIP ratio in sediments.

4.2. Role of settled organic matter in bindingof N and P along estuarine gradient

The decrease in the gross sedimentation rate of C, Nand P along the estuarine gradient suggests that N and Psettle efficiently in the inner estuary during late summer.The observation is consistent with the non-linear


dependencies between the total and inorganic N and Pconcentrations and salinity in the surface water layer(Pitkanen et al., 1993; Pitkanen and Tamminen, 1995)and the decrease in chlorophyll-a and phytoplanktonbiomass along the estuarine gradient during the summer(Kauppila et al., 1995; Pitkanen and Tamminen, 1995).The increase in the proportion of settling POC, PONand PTP in particulate matter along the estuarinegradient is attributed to the decrease in the sedimenta-tion rate of inorganic particulate matter supplied by theNeva and the increase in that of autochthonous par-ticulate organic matter.

Resuspension of particulate matter from littoralregions and the slopes surrounding basins is a commonphenomenon in shallow coastal areas and estuaries. Forexample, in the coastal area of the Baltic Proper, theproportion of resuspended settling matter frequentlyexceeds 50% at water depths similar to those in the Gulfof Finland (Blomqvist and Larsson, 1994). Further,the concentration of suspended solids in the water ofthe Neva is low (annual average ranges from 5.1 to7.3 mg l�1, P. Ekholm, unpublished data); satelliteimages, however, indicate high turbidity in Neva Bayand the inner estuary (Victorov, 1996). In addition, themeasured concentration of POC in settling matter wasat its lowest in the inner Neva estuary. Therefore, a largepart of the measured gross sedimentation consists ofresuspended matter, especially in the inner Nevaestuary. In the outer Neva Estuary and the open Gulf,however, the high concentration of POC (w20%,Ahlgren, 1983) and the clearly higher concentration of

Fig. 9. Relationship between measured concentration of TN in 9e10

cm layer and predicted concentration (surface TN*0.56). Thin line

denotes regression line. Symbols denote clusters (C2eC4) formed in

cluster analysis.

PON than of sediment surface TN indicated that a largefraction of the TPM was primary settling matter ratherthan resuspended sediments. Moreover, the C:N ratio inthe settling matter was generally below 7.0, althoughhigher ratios (7.6e9.7) were also observed.

The increase in settling POC and PON seemed toresult in an increase in the concentration of sedimentorganic matter and TN along the estuarine gradient.However, the increase in PTP along the estuarinegradient resulted in a decrease rather than an increasein the TP concentration in sediment. Further, neitherTN nor TP concentrations correlated with water depth,even though this increases significantly, from 20 to 60 m,from the inner estuary towards the west. The poorcorrelation between sediment TN and TP with waterdepth along the estuarial gradient is evidently due to thegreat variation in topography and water currents in theopen Gulf and the northern areas. This produces vari-able conditions for the sedimentation of fine-grainedparticles and water residence times, leading to, amongother things, variable O2 conditions in the sediment-water interface, regardless of the variation in waterdepth.

The increase in the sediment TN concentration alongthe estuarine gradient appeared to result in enhancedburial of N, because the deep layer TN concentrationcould be predicted well by the surface TN concentration(r2=0.73, P!0.0001). It is therefore probable that thesurface TN concentration largely controls the concen-tration of buried N. Henrichs and Reeburgh (1987)suggested that the burial efficiency of organic matterincreases with its deposition rate; the same can beexpected to hold for TN.

Unlike TN, however, the deep layer TP concentrationcould not be predicted by the surface sediment TPconcentration (r2=0.09) along the estuarine gradient.This poor dependency is due to the fact that, despiteconsiderable variation in the surface TP concentration,the concentration of TP has remained fairly constant inthe deep layer; for example, the average TP concentra-tion with a 95% confidence interval varied considerablyin the surface layer (TP 3.2G 0.4 mg g�1 DM) but onlyslightly in the deep layer (TP 1.2G 0.1 mg g�1 DM). Wetherefore conclude that, in the sediments, the dissolutionof the mobile P pool during burial eventually leads tofairly similar TP concentrations in the deep layer thatare not related to the surface concentration.

In the present study, only the increase in the sedimentorganic matter concentration explained significantly thedecrease in the mean TP concentration. The relationshipobtained is analogous to the decrease in the mean TPconcentration along the estuarine gradient. In theDelaware estuary, the decrease in the sediment Fe(III)oxide concentration explains the decrease in thesediment TP concentration along the estuarine gradient(Strom and Biggs, 1982). Here, Fe(III) oxides were not


measured, but we hypothesize that the organic matterconcentration is related to the efficiency of Fe(III) oxidesto retain P in sediments along the estuarine gradient asfollows:

The increase in the sediment organic matter concen-tration along the estuarine gradient together withchanges in hydrodynamic conditions may shift thebalance of oxidants towards anaerobic mineralizationcloser to the sediment-water interface. Thus, it is pos-sible that in organic-rich sediments Fe(III) oxides areefficiently reduced by Fe(III) oxide-reducing bacteriaand by H2S in the surface layer of the sediments,whereas in organic-poor sediments efficient Fe(III) oxidereduction does not occur until deeper in the sediment.Accordingly, a decrease in the ability of sediment toretain P with an increase in organic matter was observedalong the estuarine gradient. The decrease in the abilityof sediment to retain P along the estuarine gradient issupported by the following factors:

� the vertical TP concentration profiles produced bycluster analysis show that in the inner estuary the0e7 cm layer, and in the outer estuary and the openGulf the 0e5 cm layer, can retain P well, whereas inthe organic-rich northern areas the layer able toretain P is only 0e2 cm thick;

� both the pore water DIP concentrations and thepool of loosely-adsorbed P increase along theestuarine gradient (Table 2);

� the sediment Fe-P concentration suggests that Femaintains its ability to bind P deeper in the innerNeva Estuary and the open Gulf than in theorganic-rich areas (Lehtoranta, 1998); and

� the benthic flux of DIP from the sediment to watermeasured in early autumn appeared to increaserather than decrease with the sediment organicmatter concentration and with the estuarine gradi-ent (Lehtoranta, 2003).

Clearly, then, the sediments of the inner NevaEstuary retain P better than do the organic-richsediments of the open eastern Gulf. According to clusteranalysis, the majority of the bottoms with high organicmatter concentrations are situated in the northern partof the open eastern Gulf.

5. Conclusions

Organic particulate matter is clearly an importantcarrier of both N and P into the sediment of the Nevaestuary in late summer. However, settled P is moreefficiently retained than N in the surface layer of thesediment, most likely because settled P is partly boundto redox-sensitive inorganic particulate matter aftermineralization. Deeper in the sediments, however, thisP pool appears to be largely depleted. Thus, rather
T
able

2

Averageconcentrationofsurface

sedim

entTN,TPandTN:TPin

clustersalongestuarinegradientoftheNeva

Region

TN

(mgg�1DW)

TP

(mgg�1DW)

TN:TP

(w:w)

NH

4

(mgl�

1)

DIP

(mgl�

1)

NH

4:D

IP

(w:w)

Loosely

adsorbed

P

(NH

4Cl-RP)

%ofTP

Fe-P

(NaOH-R

P)

%ofTP

DM

acc.-rate

(gcm

�2a�1)

Diagenesis

ofP

Efficiency

toretain

P

IE(C

2)

6.5

(6)

3.7

(6)

1.8

(6)

0.6

(4)

0.4

(4)

1.5

(4)

0.2

(1)

40.6

(1)

0.167(2)

Slow

High

OE,O

G-O

F(C

3)

7.2

(10)

2.9

(10)

2.5

(10)

0.6

(9)

0.7

(9)

0.9

(9)

1.8

(1)

38.8

(1)

0.138(6)

Moderate

Moderate

NG

(C4)

11.5

(11)

3.3

(11)

4.0

(11)

1.5

(11)

1.3

(11)

1.2

(11)

10.7

(1)

8.0

(1)

0.195(3)

Fast

Low

Pore

waterammonium

(NH

4+)anddissolved

inorganicP(D

IP)concentrationsandNH

4+:DIP

ratiosin

0e1cm

layer

andproportionsofloosely-adsorbed

PandFe-PpoolsofTPin

0e4cm

layer

are

from

Lehtoranta

(1998).Averagedry

matter

accumulationratesin

clustersare

basedon

137Csmeasurements

from

Kankaanpaaet

al.(1997)andMattilaJ.

(unpubl.).Number

ofsitesusedin

calculationsisin

parentheses.Explanationsforabbreviations:IE

=inner

estuary

(cluster

C2),OE=

outerestuary,OGOF=

open

GulfofFinland(cluster

C3)andNG=

northernGulf(cluster

C4).


similar sediment TP concentrations are found in thedeep layer of the sediment throughout the study area incontrast to organic matter and N, whose burial iscontrolled by the concentration of organic matter and Nin the settled organic matter.

The decrease in the efficiency of sediment to retain Pin sediments along the estuarine gradient was signifi-cantly related to the increase in organic matter concen-trations in sediments. We can therefore hypothesize thatan increase in organic matter can shift the reduction ofFe(III) oxides upwards towards the sediment-waterinterface and, thus, the sediment P binding capacitydecreases along the estuarine gradient. It is also evidentthat the sensitive areas for benthic outflux of P are theorganic-rich sedimentation areas located mainly in thenorth of the eastern Gulf of Finland. Due to the rathersimilar regional accumulation rates of dry matter, ourresults suggest that the turnover rate of P in sediments isaccelerated along the estuarine gradient.

Acknowledgements

We thank O. Sandman, N. Ignatieva, V. Gran andthe crew of RV Muikku for their help with sampling.We appreciate the work of E. Leskinen, P. Piipari, M.Pokki, A. Ryynanen, M. Sjoblom, and S. :ygarden indoing the nutrient analyses. We also gratefully acknowl-edge the financial support of the Ministry of theEnvironment, the Maj and Tor Nessling Foundationand the Academy of Finland.

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Documents

Particulate N and P characterizing the fate of nutrients along the estuarine gradient of the River Neva (Baltic Sea)