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Estuarine, Coastal and Shelf Science 92 (2011) 111e117

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Estuarine, Coastal and Shelf Science

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Phosphorus release from coastal Baltic Sea sediments as estimated from sedimentprofiles

E. Rydin a,*, J.M. Malmaeus b, O.M. Karlsson b,c, P. Jonsson d

a Erken laboratory, Department of Ecology and Evolution, Uppsala University, Norr Malma 4200, 761 73 Norrtälje, Swedenb IVL Swedish Environmental Research Institute, P.O. Box 210 60, SE-100 31 Stockholm, SwedencDepartment of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, SwedendDepartment of Applied Environmental Science, Stockholm University, SE-106 91 Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 3 March 2010Accepted 16 December 2010Available online 5 January 2011

Keywords:PhosphorussedimentreleaseburialBaltic Sea

* Corresponding author.E-mail address: emil.rydin@ebc.uu.se (E. Rydin).

0272-7714/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.ecss.2010.12.020

a b s t r a c t

We used the decline in total phosphorus (P) concentration with depth in sediment profiles from theNorth-western Baltic Proper coastal zone to calculate the site-specific amount of sediment P eventuallyto be released to the water column: The potentially mobile P. P fractionation revealed that iron bound Pdominated the potentially mobile P at sites with oxic surface sediment layers. Organic P forms were alsoa major constituent of the potentially mobile P pool. We determined that 1e7 g P/m2 were potentiallymobile at our sites, and the turnover time of this P pool was considered short, i.e., less than a decade. Todetermine long-term average P fluxes to and from the surface sediment layer, we first multiplied theconstant and relatively low P concentration in deeper sediment layers with the sediment accumulationrate to gain the P burial rate. Then the average total P concentration in settling matter was multipliedwith the sediment accumulation rate to estimate the depositional P flux at each site. The differencebetween the depositional and burial rates represents the long-term average release rate of sediment Pand varied between 1.0 and 2.7 g P/m2 yr among our sites. These rates are at the same order of magnitudeas values reported from other areas of the Baltic Sea, and constitute a major source of P to the watercolumn.

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1. Introduction

Phosphorus (P) is a key limiting nutrient of summer cyanobacte-rial blooms in the Baltic Sea (Conley et al., 2009a), and P release fromsedimentsmay be amajor source of P to thewater column, especiallyin the Baltic Sea (Conley et al., 2002; Nausch et al., 2009) due to largeareas with anoxic surface sediments that retain P poorly (Vermaatand Bouwer, 2009). The release of dissolved P from sediment accu-mulation bottom areas will occur if P is mobilized during sedimentdiagenesis and if the dissolved P is not transformed and retained intostable compounds that subsequently become permanently immobi-lized and buried in the anoxic sediment (Gächter and Müller, 2003;Hille et al., 2005).

According to Mort et al. (2010), organic P is the major form to bepermanently buried in Baltic Sea sediment. Our understanding ofpermanent P immobilization processes within in the Baltic Seasediment profile is however poor. The transformation of “reactiveP” (Ruttenberg, 1992) into inorganic compounds in the sediment

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such as e.g. authigenic apatite, resisting early sediment diagenesis,has not been shown, except for a location in the Landsort Deep(Mort et al., 2010). Microbial mediated authigenic apatite formation(Goldhammer et al., 2010; Ingall, 2010) might be a potential P burialprocess in anoxic sediments of the Baltic Sea. Phosphorus formspresent at constant concentrations throughout a sediment profile,corresponding to a century or so of accumulation might, however,be considered to represent P forms resistant to mobilizationthrough diagenetic processes such as P associated to refractorycalcium and aluminum forms (Lukkari et al., 2009b; Mort et al.,2010). Precipitation of dissolved P with iron in oxidized surfacesediment layers is a quantitatively important, but temporary(Jensen et al., 1995), immobilization pathway of dissolved sedimentP (Jensen and Thamdrup, 1993; Jensen et al., 1995).

Degradation of labile organic P compounds, originating fromphytoplankton sedimentation is a major source to the dissolved Ppool in Baltic Sea sediment (Carman&Rahm,1997; Hille et al., 2005;Ahlgren et al., 2006). A pronounced decline pattern in total phos-phorus (TP) concentration over increased sediment depth, untila constant concentration has been reached, would reflect sucha mobilization of organic P forms and upwards migration (Carignanand Flett, 1981) and subsequent release of the dissolved P. The

Table 1Position and depth of the sediment cores analyzed from the Stockholm archipelagoarea in the NW Baltic Sea.

Station Latitude Longitude Water depth

Basin Abbreviation WGS 84 (m)

Torsbyfjärden Tor C 592040 N 182776 E 31Bulleröfjärden Bul I 591152 N 184952 E 47Gälnan Gäl Q 593150 N 184581 E 31Pilkobbsfjärden Pil A 591132 N 184521 E 58

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decline pattern in TP concentration would also reflect the limitedcapacity of sediment to retain the mobilized P. In such a system, thestock of sediment P to be released in the future as dissolved P hasbeen referred to as “reactive P” (Lukkari et al., 2009b and referencestherein) or “potentiallymobile P” (Rydin, 2000). Potentiallymobile Pwill be the term heretofore used in this paper.

Recently, the use of different methods to increase Baltic Seasediment P retentionhas beendebated (Conleyet al., 2009b). A basicparameter needed when evaluating different methods is a quantifi-cation of the potentially mobile P, i.e., the identification of sedimentareas with pronounced sediment P release. Lukkari et al. (2009b)estimated the long-term minimum average release of P fromdifferent sediment sites in the NE Baltic Proper and found that the Prelease varied by two orders of magnitude. This highlights the needto estimate the pool of potentially mobile P, including rates ofdeposition, release, and burial of P in specific areas. Such knowledgeis essential to understand long-term ecosystemP turnover and to beable to predict future sediment P release from specific bottom areasunder different conditions.

Our work has two goals. First, we quantified amounts ofpotentiallymobile sediment P, i.e., P that eventually will be releasedto the water column, using basic sediment parameters. To gainfurther insight into the P forms that contribute to potentiallymobile P, sediment P was divided into different forms usinga fractionation technique. Second, we estimated the long-termaverage release rates of the potentially mobile sediment P to thewater column. To obtain this, we used literature data on average Pconcentrations in settling matter from the region to estimate total Pdeposition using sediment accumulation rates from each site. Bysubtracting the permanent P burial rate in deeper sediment layersfrom the P deposition rate on the sediment surface, we calculateda site-specific measure of the long-term average P release rate atour sites in the Baltic Sea.

2. Material and methods

2.1. Sediment sampling

Sediments were collected from four accumulation bottom sitesrepresenting typical coastal areas of the Stockholm Archipelagosituated in the North-western Baltic Proper (Fig. 1, Table 1). In someof these bottom areas the oxygen situation has improved over thelast few years (Karlsson et al., 2010).

Sampling was performed in November 2008 using a Geminidouble corer (inner diameter 8 cm). Sediment cores were sliced in2 cm thick disks and selected layers were analyzed for watercontent, organic content, P fractions, and total P (TP).

Fig. 1. Sampling positions in th

2.2. Chemical analyses

Water content was determined after freeze-drying, and organiccontent using the Loss of ignitionmethod (LOI, 550 �C for 2 h). TotalP content in sediments was analyzed as phosphate after acidhydrolysis at high temperature (340 �C) according to Murphy andRiley (1962). Phosphorus forms were separated following, in prin-ciple, the sequential extraction scheme suggested by Psenner et al.(1988) within a few days after sampling. The following P formswere extracted, and the chemicals used in parenthesis: NH4Cl-rP(1 M NH4Cl at pH 7), BD-rP (0.1 M Na2S2O4/NaHCO3), NaOH-rP andNaOH-nrP (0.1 M NaOH), HCl-rP (0.5 M HCl). These fractions aredefined by the extraction method, but ideally each fraction corre-sponds to a specific phosphorus containing substance within thesediment. Generally, NH4Cl-rP is regarded as loosely-bound phos-phorus, BD-rP as phosphorus associated with iron hydroxides(Jensen and Thamdrup, 1993), NaOH-rP as phosphorus bound toaluminum, NaOH-nrP as organic phosphorus forms, and HCl-rP ascalcium bound phosphorus compounds. Residual P is calculated bysubtracting extracted and identified phosphorus from TP, and isconsidered to represent refractory organic P forms. P extracted willbe denoted as Loosely-sorbed P, FeeP, AleP, Org-P, CaeP, and Res-P,respectively. In one sample, Bul I (0e2 cm), fractions had to bemultiplied by a factor of 0.7 to calculate a positive Res-P.

2.3. Sediment accumulation

We determined the average yearly sediment accumulation atour sites by lamina counting over a 10e20 cm section and gota good agreement with earlier sediment accumulation rates using137Cs dating (Jonsson et al., 2003). The resulting thickness of anaverage yearly sediment deposition load was converted to drymatter accumulation using water content and sediment density. Inthis way, sediment compaction was indirectly corrected for as thewater content decreased with increasing sediment depth.

e Stockholm archipelago.

E. Rydin et al. / Estuarine, Coastal and Shelf Science 92 (2011) 111e117 113

3. Calculations

To estimate potentially mobile sediment P, we first determinedthe “stabilization depth” at which the sediment TP concentrationbecame constant, and where sediment diagenesis apparently atlarge had ceased. Wemeasured the average TP concentration in thesediment profile below the stabilization depth (Table 2), and sub-tracted it from the TP concentration in the layers above. Theresulting P concentrations (Fig. 2) were multiplied with the drymatter content in each layer to calculate the P amount. By depth

Table 2Phosphorus fractions (seeMethods), total phosphorus concentration, water and organic cosediment TP concentration.

Station Layer NH4Cl-rP BD-rP NaOH-rP NaOH-nrP

Loosely-bound P FeeP AleP Org-P

(cm) (mg P/g DW) (mg P/g DW) (mg P/g DW) (mg P/g DW

Bul I 0e2 19 511 77 5252e4 5 110 58 4704e6 1 110 52 4506e8 0 100 54 4208e10 0 110 56 39010e12 5 120 58 40012e14 0 120 64 39014e16 0 110 55 34016e18 0 130 62 36018e20 0 140 67 38028e30 3 120 59 31038e40 10 120 69 310

Gäl Q 0e2 6 210 80 4722e4 2 83 69 3754e6 2 86 61 3646e8 0 75 55 3128e10 0 78 56 33910e12 1 62 52 32712e14 0 70 50 34114e16 1 78 55 32416e18 0 79 53 31318e20 0 81 52 30028e30 9 84 67 32838e40 15 215 71 33748e50 12 92 67 33258e60 27 90 85 313

Tor C 0e2 73 1700 170 3902e4 3 250 100 3304e6 0 82 73 3306e8 0 73 64 3008e10 0 85 60 31010e12 0 84 50 30012e14 0 95 55 34014e16 0 96 62 32016e18 0 99 63 33018e20 0 82 59 23028e30 5 92 68 33038e40 2 68 54 21048e50 13 100 100 16058e60 31 81 150 240

Pil A 0e2 0 170 71 5002e4 0 120 61 4204e6 0 110 64 4606e8 0 99 53 3908e10 0 84 58 43010e12 0 110 60 41012e14 9 95 49 36014e16 0 97 54 37016e18 0 90 56 37018e20 0 110 53 37030e32 3 110 55 36040e42 9 110 62 31050e52 11 110 65 30060e62 15 110 73 260

integration the amount of potentially mobile P per square meterwas obtained. To identify the forms of P responsible for the declinein TP concentration with increased sediment depth, we repeatedthis calculation for all P fractions.

To calculate gross and net P fluxes, we used a value from theliterature, 1.7 mg P/g sediment DW (dry weight) (Blomqvist andLarsson, 1994), as a measure of the average P concentration insettling matter in this area of the Baltic, and multiplied it by theobtained sediment accumulation (g DW/m2) to gain gross deposi-tion rate. Removal of P from the system (permanent burial) was

ntent. The horizontal line in the TP column indicates the assumed stabilization of the

HCl-rP Res-P Total phosphorus Water content LOI

CaeP TP

) (mg P/g DW) (mg P/g DW) (mg P/g DW) (%) (%)

357 291 1.77 92 19390 280 1.31 88 18370 290 1.26 87 16370 210 1.15 86 15400 200 1.16 84 22390 170 1.14 86 17400 150 1.12 85 17360 260 1.12 84 17390 190 1.13 84 17360 180 1.12 83 16400 240 1.13 81 17370 190 1.07 80 15

392 139 1.30 88 17346 209 1.08 84 15372 149 1.03 83 14347 219 1.01 82 14374 159 1.01 82 14302 280 1.03 81 14376 182 1.02 83 14382 151 0.99 81 14356 172 0.97 82 13400 153 0.99 81 13371 161 1.02 79 13367 66 1.07 79 13392 129 1.02 78 13401 158 1.07 77 12

390 130 2.84 88 14400 67 1.16 85 13350 320 1.16 85 13280 220 0.93 86 13320 74 0.85 86 13300 100 0.84 87 14290 100 0.89 88 14280 150 0.90 89 15320 110 0.91 88 15330 140 0.84 83 12300 180 0.98 85 14210 160 0.70 78 9270 160 0.81 77 9320 150 0.98 71 9

350 280 1.37 91 23340 340 1.27 90 22330 220 1.18 91 21320 320 1.17 88 22310 510 1.39 88 22330 240 1.15 88 19320 240 1.07 86 23320 260 1.10 86 20330 270 1.11 86 19310 280 1.12 85 18370 200 1.09 83 18330 240 1.06 81 16350 110 0.95 79 16350 180 1.00 77 14

Fig. 2. Distribution of iron bound phosphorus “FeeP”, extractable organic phosphorus “Org-P”, and total phosphorus “TP” in the four investigated sediment cores from theStockholm archipelago. Shaded areas represent phosphorus that is expected to eventually be released from the sediment.

E. Rydin et al. / Estuarine, Coastal and Shelf Science 92 (2011) 111e117114

calculated as sediment accumulation multiplied by the P concen-tration below the stabilization depth in the sediment. The loss ofdry matter was assumed to be limited to the respiration of organiccarbon, and was represented by the decline of LOI with sedimentdepth.

To estimate the average release rate of P from the Baltic sedi-ments, we subtracted the burial rate of P from the gross depositionof P. The difference represents the average long-term P release ratefrom the sediments.

4. Results

Sediment surfaces were judged oxic, except for the sedimentcollected at the Pilkobbsfjärden (Pil A) site (Table 1, Fig. 1), whichshowed symptoms of anoxia, such as black colour and hydrogensulphide scent. The sediment from Torsbyfjärden (Tor C) was oxic atthe surface and exhibited indicators that the surface had apparentlybeen oxic for about 3 years (Karlsson et al., 2010), judging by themixed character and gray colour of the top 5 cm in the sediment.Below 5 cm depth the sediment was laminated. All four coresappear to represent sites of undisturbed sediment accumulation, asjudged from both present laminas below the oxidized zone, as wellas from the slowly declining water and organic content develop-ment with depth in the core (Table 2). The average thickness of oneyear of deposition varied from 0.9 to 2.0 cm (Table 3). The age of thestabilized TP concentration was roughly determined from thesediment accumulation rate and TP stabilization depth.

Surface sedimentTPconcentrationsdifferedconsiderablybetweenlocations, largely due to the difference in FeeP concentrations

(Table 2). Tor C exhibited high concentrations of FeeP in the surface(0e2 cm) sediment (1.70 mg/g DW), resulting in a TP of 2.84 mg/g,while FeeP concentration in Pil A was an order of magnitude lower(0.17mg/gDW), resulting inhalf the TP concentration (1.37mg/gDW)compared to Tor C.

The sediment profiles exhibited a pronounced decline in TPconcentration from the surface until between 6 and 12 cm sedi-ment depth, at which the TP concentration stabilized at approxi-mately 1 mg P/g DW (Table 2). The age of the sediment at thisstabilization depth corresponded to between 4 and 8 years (Table3). Org-P decreased with sediment depth in all profiles, but lessexponentially than FeeP. In addition, the Res-P fraction alsoexhibited a declining pattern over sediment depth. Some Loosely-bound P was detected in the all of the cores’ surface layers exceptfor Pil A. The remaining P fractions (AleP and CaeP) did not showany clear trend with sediment depth or age.

The pool of potentially mobile P varied between 1.0 and 7.2 g/m2

(Table 4) as determined from the TP concentration development (i.e.,thedifferencebetweenTPconcentrationand the concentration stablewith sediment depth; Table 2). The contribution from the Loosely-bound P fraction was calculated to be less than 0.2 g/m2, while theFeePwas the dominant fraction inTor C. P extracted asOrg-Pwas thelargest fraction in the surface layers of Bul I and Pil A (Table 4).

Our results show that 51e66% of the calculated gross P depo-sition will be permanently buried in deeper sediment layers.Consequently, between 34 and 49% of the gross deposition will bereleased back to the water column, corresponding to long-termaverage release rates of between 1.0 and 2.7 g P/m2 yr, respectively(Table 3).

Table 3Sediment accumulation and age of sediment when phosphorus concentrations ceased to drop further. Calculated rates of deposition, release, and permanent burial ofphosphorus at accumulation sites in the Stockholm archipelago.

Samplingsite

Sedimentaccumulation

Dry matterdeposition

StabilizedTP

Deposition Release Burial

(cm/yr) (g/m2 yr) (yr) (g/m2 yr) (g/m2 yr) (g/m2 yr) (%)

Bul I 1.2 1980 8 3.4 1.2 2.2 66%Gäl Q 0.9 1720 6 2.9 1.2 1.7 60%Tor C 2 3212 4 5.5 2.7 2.8 51%Pil A 1.1 1597 8 2.7 1.0 1.7 63%

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5. Discussion

In all of our cores, we observed a decline in TP concentrationuntil stabilization at a lower concentration in deeper sedimentlayers. This pattern is also found in nutrient-rich lakes, such as LakeErken, Sweden (Rydin, 2000). In these shallow systems, onlya fraction of organic matter is mineralized during sedimentationand the upper layers of the sediment therefore represent animportant site for the continued degradation of organic matter. Wesuggest that the decline in TP concentration during this processreflects the limited ability of the sediment to permanently retain allthe deposited P (Fig. 2).

During the 20th century, P loading to the Baltic Sea increased 8-fold until the 1980’s (Larsson et al., 1985). During the last twodecades, external loading has stabilized, both from the majorfreshwater input to this part of the archipelago, Lake Mälaren(Karlsson et al., 2010), as well as to the Baltic Proper (HELCOM,2005). Phosphorus deposition can therefore be considered tohave been fairly constant over the time period representing thesediment layers investigated in this study (1980s-present). There-fore, we predict that the surface sediment layers presently rich in Pwill loose the potentially mobile P and end up with the same lowconcentration found in older sediment layers.

5.1. Limitations in P fractionation

Our data indicate that the potentially mobile P observed in thetop layers of the sediment cores was composed of Loosely-sorbed P,FeeP, Org-P, and as Res-P fractions. These fractions decreasedinitially with increasing sediment depth (age), indicating mobili-zation and transport to the overlying water column. It should benoted that some portion of these fractions was found also in deeperlayers, indicating that not all of the P extracted in these steps shouldbe considered potentially mobile. In deeper sediments, the Pconcentration is typically constant with depth, indicating that Pmobilization has ceased and only inert P forms remain. We alsoobserved that P extracted as AleP (NaOH-rP) showed a higherconcentration in surface sediment when the previous extractionsstep (BD-rP) was elevated (Table 2). This phenomenon can beexplained methodologically: remnants of the P-rich solution usedfor BD-rP extraction were not completely removed before the next

Table 4Pools of phosphorus fractions (see Methods) and total amount (TP) of potentiallymobile phosphorus in sediment cores from accumulation areas in the Stockholmarchipelago.

Loosely-bound P FeeP Org-P TP

(g/m2) (g/m2) (g/m2) (g/m2)

Bul I 0.1 0.7 1.7 2.5Gäl Q 0.0 0.3 0.9 1.0Tor C 0.2 4.6 1.4 7.2Pil A 0.0 0.2 1.6 2.9

extraction step (NaOH) was added. A washing step in betweenextractions would be necessary to accomplish this, as suggested inthe Psenner et al. (1988) fractionation procedure. Also anotherphenomenon can be explained methodologically: the initialextraction step is performed under oxidized conditions. A shiftfrom dissolved P to FeeP will therefore occur in anoxic sedimentsamples if dissolved iron is available, overestimating FeeP onbehalf of Loosely-bound P (Lukkari et al., 2007a,b). In this context itis of minor importance since P measured as Loosely-bound P andFeeP are closely connected. Under oxic conditions, equilibriumprocesses maintain a certain portion of the FeeP to be measured asLoosely-bound P (Rydin and Welch, 1998). Other potential trans-formations might, however, also occur when exposure low-redoxsediment to air over several weeks (Lukkari et al., 2007b).

5.2. Settling P

The P concentration in the surface sediment layer cannot beassumed to represent the P concentration in settling matter(Carignan and Flett, 1981). Dissolved P migrating upward fromanoxic layers in the sediment profile might precipitate togetherwith iron under oxic conditions and subsequently increase sedi-ment P content. Alternatively, under anoxic conditions, the Pconcentration in settling matter might be underestimated due toa considerable release of dissolved P (Krom and Berner, 1981) fromrecently settled labile organic P compounds (Ahlgren et al., 2006;Reitzel et al., 2007). The assumed TP concentration in settlingmatter,1.7mg P/g DW,was a yearly average based on Blomqvist andLarsson (1994) and represents a concentration in between themeasured TP values from the four cores’ surface sediments. Thesettling matter concentration is less than the TP concentrationmeasured in the surface layers of Tor C (2.84) or Bul I (1.77) (the twosites with a pronounced pool of FeeP), but above that of Gäl Q(1.30) and Pil A (1.37), where the surface sediments do not showa substantial accumulation of redox-sensitive P (Table 2). To betterconstrain the P concentrations in settling matter is critical for thequantification of P fluxes over the sediment surface.

5.3. Organic P

Some part of settled organic phosphorus will eventually bemineralized (Ahlgren et al., 2006) and constitutes the source ofdissolved P to the pool of Loosely-bound P and FeeP (Hille et al.,2005). Although the TP concentrations in our cores stabilizedalready after 4e8 years, Org-P actually showed a slow but contin-uous decrease also in deeper (older) sediment layers. Although thedegradation of organic matter apparently continues also in deepersediment layers, it does not seem to be of quantitative importancefor the P budget, as judged from the general lack of TP concentra-tion decline with sediment depth (Table 2). We noted that thepotentially mobile P concentration at Tor C (7.2 g/m2, Table 4) wassimilar to potentially mobile P concentrations measured inmoderately eutrophic Lake Erken sediments (5 g/m2). However, in

E. Rydin et al. / Estuarine, Coastal and Shelf Science 92 (2011) 111e117116

Lake Erken, most potentially mobile P was organic (4 g/m2) (Rydin,2000; Ahlgren et al., 2005), and data from hypertrophic LakeSønderby (Denmark) also indicate that the potentially mobilesediment P pool (8 g/m2) was dominated by organic P forms(Reitzel et al., 2005). In the cores investigated in this study, the poolof potentially mobile organic P was smaller, w1 g P/m2 (Table 4).The difference in mobile P fractions between the Baltic and theLakes Erken and Sønderbymay be due to that organic P degradationis faster in marine than limnic systems. It only tookw5 years for TPconcentrations in our Baltic cores to stabilize (Table 3), incomparison to Lake Erken, where TP concentration stabilized onlyafter about two decades (Ahlgren et al., 2005). Furthermore, morecomplete mineralization of the autochthonous organic mattermight also be expected in brackish sediments compared to lakesediments according to Caraco et al. (1990). Regardless, P extractedas organic P was present also in deeper sediment layers in the Balticcores, representing P to be permanently buried (Mort et al., 2010).More detailed knowledge of the origin and degradability ofdifferent kinds of organic P forms are needed to understand thesource and turnover of potentially mobile P.

5.4. FeeP

In Tor C, where surface sediment layers were oxic, the poten-tially mobile P pool was larger than in other cores. This difference isexplained by a larger FeeP pool (Table 4) that apparently resultfrom dissolved iron and P migrating upwards from deeper, anoxiclayers that precipitates in the oxic surface layer (Carignan and Flett,1981; Jensen et al., 1995). A 2-year period of accumulated Pmobilized at a rate of 2.7 g P/m2 yr (Table 3) would roughlycorrespond to the observed amount of FeeP (4.6 g/m2) at the Tor Csite (Table 4). Eventually, however, the oxidized surface sedimentlayer will become saturated with FeeP (Jensen and Thamdrup,1993), and P release from oxic and hypoxic sediment can beexpected to gradually reach the same rate as that of constantlyanoxic surface sediments as observed by e.g. Hille et al., 2005. Thetransition of oxidized surface sediment, with an accumulated poolof FeeP, into anoxic sediments will result in a period of pronouncedP release due to dissolution of the accumulated FeeP. In the longrun, however, the release will be determined by the settling rate oforganic P compounds, e.g. phytoplankton, that will degrade,dissolve and eventually mobilize P. Phosphate adsorption into ironoxyhydroxides in oxidized surface sediment layers will delayrelease, but is not likely to affect the burial rate of P in the Baltic Sea(Jensen et al., 1995).

5.5. P fluxes

A few attempts to quantify the fate of settled P in the Baltic Seahas been reported. In the coastal zone, at 15 m depth in the AarhusBay (southern Baltic Sea) the pool of redox-sensitive iron boundsediment P was 5.4 g/m2, and deposition, release and permanentburial of P was 1.6, 1.0, and 0.6 g/m2 yr, respectively, witha permanent burial efficiency of 37% (Jensen et al., 1995). Our data(Table 3) are at the same order of magnitude except for a highershare of deposited P to get permanently buried. Unlike Aarhus bay,ongoing land rise after the last glacial period in the northern Balticproper cause’s erosion of new bottom areas most likely dominatedby more refractory P forms that settles out together with labile Pforms, such as P in phytoplankton, resulting in a larger share of thedeposited P to get permanently buried. This phenomenon mightexplain higher burial efficiencies also reported from the northeastern part of the Baltic proper that varied between 41 and 93%(Lukkari et al., 2009a,b) resembling burial efficiencies between 51and 66% found in this study (Table 3).

We calculated long-term average P release rates between 1.0and 2.7 g P/m2 yr (Table 3). These figures are comparable to the rateobtained from organic P degradation in a sediment profile offshoreof the Stockholm archipelago, where the long-term average releasewas estimated to be 0.6 g P/m2 yr (Ahlgren, 2006). They are alsocomparable to rates reported by Mort et al. (2010) who calculateddiffusive fluxes of 0.5 and 3 g P/m2 yr from north of Gotland and theLandsort Deep, respectively. In the Eastern Gotland Basin, however,at depths>150 m, P deposition averaged at 0.20 g P/m2 yr of whichonly one-third was judged to get permanently buried, and subse-quently two-third was released as phosphate (Hille et al., 2005).One explanation for the higher release rates obtained in coastalareas seems to be higher sediment accumulation rates compared tothe open Baltic.

6. Conclusion

We observed a decline in sediment TP concentration with depthin Baltic Sea coastal sediment cores, indicating the limited ability ofthe sediment to retain deposited P. This difference in P concentrationrepresents the total pool of P to be released, and the long-termaverage sediment P release can be calculated using the sedimentaccumulation rate. At sites where surface sediments turned oxica few years ago, we measured substantial amounts of P bound toiron, suggesting thatmobilized phosphatewas temporary trapped inthe surface sediment instead of being released to the water column.

Acknowledgments

This studywas funded by the Swedish Environmental ProtectionAgency and FORMAS. We thank the Erken Lab for analyses, DanLindgren for drawing the map, and Cayelan Carey for her edits onthe manuscript.

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