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Estuarine, Coastal and Shelf Science 145 (2014) 9e21

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Estuarine, Coastal and Shelf Science

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Distribution and estimated release of sediment phosphorus in thenorthern Baltic Sea archipelagos

Irma Puttonen a,*, Johanna Mattila a, Per Jonsson b, O. Magnus Karlsson c, Tuula Kohonen a,Aarno Kotilainen d, Kaarina Lukkari e, J. Mikael Malmaeus c, Emil Rydin f

aÅbo Akademi University, Finlandb Stockholm University, Swedenc IVL Swedish Environmental Research Institute, SwedendGeological Survey of Finland, FinlandeMarine Research Centre/SYKE, Finlandf Erken Laboratory, Uppsala University, Sweden

a r t i c l e i n f o

Article history:Received 21 November 2013Accepted 19 April 2014Available online 28 April 2014

Keywords:biogeochemistrycoastal zoneeutrophicationphosphorus fractionationsedimentswater quality managementBaltic Sea

* Corresponding author.E-mail addresses: irmaputtonen@gmail.co

(I. Puttonen), johanna.mattila@abo.fi (J. Mattila), elismagnus.karlsson@ivl.se (O.M. Karlsson), tuula.kohaarno.kotilainen@gtk.fi (A. Kotilainen), kaarina.lukkamikael.malmaeus@ivl.se (J.M. Malmaeus), emil.rydin@

http://dx.doi.org/10.1016/j.ecss.2014.04.0100272-7714/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Phosphorus contents in the sediments were determined in archipelago areas of the northern Baltic Sea(Svealand in Sweden and Åland, SW Finland and W Uusimaa in Finland) during 2008e2012. Spatial andvertical distribution of phosphorus was studied by analysing sediment samples from 345 stations ofdifferent seabed substrate types. A sequential extraction method was applied to evaluate the pool of thepotentially mobile phosphorus, i.e., the amount of phosphorus that can be expected to be released fromsediments to water with time, and possibly support primary production. In addition, vertical distributionof immobile phosphorus forms in the sediments was used as a tool to assess phosphorus burial. Theuppermost 2 cm of sediments were calculated to contain 126,000 tonnes of phosphorus in the study areacovering 19,200 km2 of the seafloor. Subtracting the assumed average background content (i.e. thatassumed to be buried) of this total phosphorus content gave an estimation of 31,000�37,000 tonnes ofpotentially mobile phosphorus at the sediment surface. Redox sensitive iron-bound phosphorus accountsfor two thirds of this pool. Compared with the total phosphorus input from the catchment of the entireBaltic Sea e 29,000 tonnes in 2009 e it can be concluded that the store of phosphorus that can bereleased with time from the sediments is large, and that internal phosphorus recycling processes thusmay play a key role in phosphorus fluxes in the coastal zone. Spreading of hypoxia in the future, as recentmodelling and sediment proxy results suggest, is likely to severely deteriorate the water quality,particularly in the archipelago areas where the water exchange is slow.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Anthropogenic enrichment of the macronutrient elements, ni-trogen and phosphorus (P), has resulted in eutrophication with arange of harmful consequences in aquatic environments worldwide(Smith, 2003b; Diaz and Rosenberg, 2008). Coastal eutrophicationhas become a question of increasing global concern (e.g. Nixon,1995; National Research Council Staff et al., 2000; Andersen and

m, irma.puttonen@abo.fiaper@telia.com (P. Jonsson),onen@abo.fi (T. Kohonen),ri@ymparisto.fi (K. Lukkari),ebc.uu.se (E. Rydin).

Conley, 2009; Rabalais et al., 2009). Increased input of nitrogenand phosphorus from diffuse and point sources induces high pri-mary production and enhanced supply of organic matter to thesystem. Once nutrients from different sources are introduced in themarine system, they become a part of nutrient cycling within thesystem. A share of the nutrients may be removed from cycling inbiogeochemical processes, whereas the rest is recycled and par-ticipates in the biomass production. Sediments and variousbiogeochemical processes within them play an important role inthe nutrient cycling. The cycles of nutrient elements are coupled,affecting one another (Wollast et al., 1993). It has become evidentthat human activities have caused an increase in phosphorus andnitrogen loading to marine waters altering and disturbing thenatural element cycling processes in coastal ecosystems (Carpenter,2005). Increasing oxygen demand by organic matter degradation

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e2110

leads to spreading and worsening of seasonal hypoxia in the near-bottom waters (Pinckney et al., 2001).

Phosphorus is a growth-limiting nutrient for phytoplankton inmany coastal marine environments. Phosphorus removal from thenutrient cycling in the marine system takes place by permanentburial in the sediments during diagenesis. Burial of reactive phos-phorus in the sediments occurs principally in three ways: 1) byburial of organic matter 2) by adsorption on particle surfaces,mainly iron oxy(hydr)oxides, and co-precipitation with minerals,and 3) by authigenic fluorapatite mineral formation (Ruttenberg,2003; Hensen et al., 2006). So far it remains unclear to whatextent the processes mentioned above actually contribute to per-manent burial of phosphorus in the Baltic Sea coastal and archi-pelago sediments (Carman and Jonsson, 1991; Emeis et al., 2000;Malmaeus and Karlsson, 2012).

Oxygen deficiency is known to release phosphorus bound ontoiron oxy(hydr)oxides (Mortimer, 1941; Ingall and Jahnke, 1994).Iron-bound phosphorus is often considered the major internalsource of phosphorus in the Baltic Sea sediments (Jensen et al.,1995; Conley et al., 2002; Mort et al., 2010), but phosphorusfluxes from the sediment to the water have been observed also inoxic conditions in the coastal area of the Archipelago Sea of Finland(Suomela et al., 2005; Lukkari et al., 2009a). Phosphorus associatedwith organic phase is released by microbial activity during degra-dation of organic matter (Hupfer et al., 1995; Ahlgren, 2006), and itmay be a remarkable source of internal phosphorus loading in theBaltic Sea (Lukkari et al., 2009a; Jilbert et al., 2011).

Phosphorus input from the catchment to the Baltic Sea has beenreduced by 45% from 1990 to 2006 (HELCOM, 2011), but the waterquality has not improved as expected (HELCOM, 2009; Conley et al.,2011). Phosphorus release from sediments is one source of nutri-ents triggering harmful algal blooms, particularly for nitrogen-fixing cyanobacterial species (Vahtera et al., 2007; Walve andLarsson, 2007). Evidently, phosphorus recycling through sedi-ments provides a source of phosphorus to the primary productionsustaining eutrophication (Ingall and Jahnke, 1994; Maher andDeVries, 1994; Jensen et al., 1995).

The geomorphology in the shallow archipelagos is characterisedby a mosaic of heterogeneous seafloor environments (Kaskela et al.,2012). In the Archipelago Sea, water exchange is restricted due tothe abundance of islands (Mälkki et al., 1979; Winterhalter et al.,1981), and bottom water anoxia and seasonal hypoxia are com-mon in the shallow coastal areas in the Baltic Sea (Virtasalo et al.,2005). Repeated hypoxic events lead to phosphorus release fromsediments causing elevated phosphorus concentrations in thewater column. Some local studies of sediment phosphorus contentshave been made in the Finnish Archipelago Sea (Virtasalo et al.,2005; Virtasalo and Kotilainen, 2008), in the Gulf of Finland(Lukkari et al., 2009a, b) and in the Stockholm archipelago (Rydinet al., 2011; Malmaeus et al., 2012). These studies have focusedeither on accumulation areas or on sediment surface, while nocomprehensive information on general spatial and vertical distri-bution of phosphorus in the sediments exists. In coastal areas,temporal phosphorus fluxes from the sediments have beenmeasured in laboratory incubations (Pitkänen, 2001; Lehtorantaand Heiskanen, 2003; Viktorsson et al., 2012) and in-situ fluxmeasurements (Suomela et al., 2005; Viktorsson et al., 2012). Apartfrom that, an overall quantification of the pool of potentially mobilephosphorus in the sediments is needed for estimations of the long-term internal phosphorus loading in the coastal and archipelagoareas. The objectives of the present study were 1) to examine thephosphorus contents and their vertical distribution in differentsediment types in the northern Baltic Sea coastal and archipelagoareas, 2) to calculate the amount of phosphorus deposited in theupper sediment layers and 3) to estimate the pool of potentially

mobile phosphorus in the surficial sediments that could be releasedinto the overlaying water during different environmental condi-tions. Sediment phosphorus content was analysed with a chemicalsequential extraction technique.

2. Materials and methods

2.1. Study area

The study area in the northern Baltic Sea covers the ArchipelagoSea in south-western Finland and Åland, western Uusimaa archi-pelago in Finland, as well as the archipelagos along the coast ofSvealand in Sweden (Fig. 1). There are more than 50,000 islands inthe area of ca 20,000 km2. The coast of Svealand is separated fromthe Archipelago Sea by on an open sea area, the Åland Sea, locatedin a tectonic depression. The bedrock in the area is predominantlyPrecambrian granite, partly overlain by Late Quaternary glacial andpostglacial sediments. The area has undergonemultiple glaciations,but general topographic features are determined by the bedrock(Winterhalter et al., 1981). The seafloor geomorphic features in thearchipelagos and the coastal areas of northern Gulf of Finland arefragmented and patchy. Sea valleys, steep-sided small depressionsand sea troughs are common features, partly coinciding with faultlines and thrust zones (Kaskela et al., 2012). The glacio-isostaticland uplift is an on-going process at a rate of 3e5 mm per year inthe area (Mäkinen and Saaranen, 1998).

Spatial and vertical distributions of phosphorus were studied byanalysing sediment samples at 345 stations (Fig. 1). Samples werecollected during 2008e2012 on several cruises with differentresearch vessels. Water depth ranged from one to 111 m, with anaverage of 28 m.

2.2. Acoustic-seismic methods

In the Swedish study area, detailed seafloor sediment mapswere provided by the Geological Survey of Sweden (Sverigesgeologiska undersökning SGU, 2012) and from Jonsson et al.(2003). The maps were used for selecting the sampling stationsand as background data in estimating the coverage of differentsediment types. In Finland, the available seafloor mapping data areless detailed. Therefore, in the Finnish archipelago areas the sam-pling stations were chosen using the acoustic-seismic equipment ofR/V Geomari. The seafloor was investigated with Side scan sonar(Klein 3000) and sub-bottom profiler system (Meridata Pinger28 kHz and Chirp 4 kHz). With the Side scan sonar an image of theseabed surface was obtained with horizontal coverage of ca 280 munder the survey line. Sub-bottom profiler system creates acontinuous acoustic profile that shows soft sediment structuresbelow the seabed surface, under the sensor/vessel along the surveyline. A seismic profiler system ELMA at a frequency of 500 Hz en-ables to detect the upper boundary of hard bottom, generallybedrock or till. These acoustic-seismic surveys were used to deriveinformation of seafloor topography, bottom sediment type, sedi-ment thickness and internal structures.

Acoustic-seismic data were post-processed and interpreted us-ing Oy Meridata Finland Ltd’s MDPS software and ESRI ArcGIS�software. Hard bottom upper boundary (bedrock or till) wasidentified from ELMA seismic profile. Overlying sedimentary unitswere identified as sand, glacial clay/varved clay, (Ancylus) post-glacial clay, (Littorina) clayey gyttja/gyttja clay and modern gyttjausing the data from Pinger and Chirp together. In addition, side-scan sonar data and sediment samples were used to identify thesediment type in the surface layer. A graphic presentation of thesediment types in the surveyed areas was created. BALANCE sea-floor sediment map (Al-Hamdani and Reker, 2007) was used as

Fig. 1. Sediment samples were collected at 345 stations in the coastal and archipelago areas of Sweden and Finland during open water seasons 2008e2012.

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e21 11

background data, complemented with attained new informationderived from the acoustic profiles and the sediment samples.

2.3. Sediment samples

The final selection of sampling stations in Finland was donebased on the acoustic profiles of the seabed sediments. Sedimentsamples from soft bottoms at 264 sample stations were retrievedwith a gravity corer (Gemax, Niemistö, Gemini, Limnos andWillnercorers) to attain an undisturbed sediment profile. Sediment coreswere sliced into sub-samples of 2 cm. In 61 randomly selected coresthe sediment surface was separated in 0e1 and 1�2 cm slices to seeif there was a change in phosphorus content in the uppermostsediment layer. Box corers, van Veen and Ponar grab samplers wereused for sampling at hard clay bottoms at 81 sample stations, and asub-sample of the sediment surface layer, generally 0�2 cm, wasrecovered. Three sub-samples of 0e1 cm (two in the Uusimaa ar-chipelago and one in the Stockholm archipelago) and nine of 0e3 cm depth layers (four in the Stockholm archipelago and five in thenorthern part of the Archipelago Sea in Finland), above an evidentvisible change in the sediment type, were included to represent thesediment surface layer in the firm seabed substrates. One addi-tional two-cm thick sub-sample at 55 randomly selected stationswas obtained from a deeper layer, at 2�4 to 12�14 cm depths, torepresent the prevailing sediment type at the sample location. The

depth of the deeper sediment sub-sample was determined indi-vidually at each location based on the observed sediment structure.The sediments were photographed and the visible characteristicswere documented. Randomly selected sediment cores were splitvertically in the field for more detailed observations of the sedi-ment composition and structures. The sediments sampled for Pfractionation were stored in plastic bags or jars in a refrigeratorbelow 6 �C temperature until laboratory analyses, and were ana-lysed within two weeks from sampling.

Sediment water content was determined after freeze-dryingand is expressed as % of wet weight (ww). Weight loss on igni-tion (LOI, % of dry weight (dw)) was measured after ignition at550 �C for 2 h, with a sample size of 1 g dry sediment. LOI was usedas an approximation of the sediment organic content. A relativelygood correlation between organic carbon and LOI in the Baltic Seasurface sediments is well known (e.g. Leipe et al., 2011). Totalphosphorus (Tot-P) content in the sediments was analysed asphosphate after acid hydrolysis at high temperature (340 �C) ac-cording to Murphy and Riley (1962). Phosphorus forms wereseparated into NH4Cl-rP, BD-rP, NaOH-rP, NaOH-nrP, HCl-rP andresidual P following, in principle, the sequential extraction schemepresented by Psenner et al. (1988). The BD extractant was allowedto oxidize before adding the reagent in phosphate analysis to avoidinterference of dithionite. In the NaOH and HCl steps, pH wasadjusted to neutral before adding the reagent mixture. Further

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e2112

details of the procedure are not explained here, but can be found inthe original paper (Psenner et al., 1988). The extracted P fractionsare operationally defined but, ideally, each fraction is assumed torepresent certain binding or solubility form of phosphorus withinthe sediment. Generally, NH4Cl-rP is regarded as loosely bound andpore water phosphorus, BD-rP mainly as redox-sensitive phos-phorus associated with iron (and manganese) oxy(hydr)oxides(Jensen and Thamdrup, 1993), NaOH-rP as phosphorus bound tooxides of aluminium and non-reducible iron, NaOH-nrP as variousorganic phosphorus forms and (biogenic) polyphosphates (Hupferet al., 2004; Ahlgren et al., 2005; Baldwin, 2013), and HCl-rP asapatite- or calcium-bound phosphorus compounds. In this study,residual P is given by subtracting the sum of the extracted phos-phorus from Tot-P, and is considered to represent mainly refractoryorganic phosphorus forms. While the P fractions distinguishedrepresent only approximations of different binding forms of P in thesediments, for the sake of clarity, they will be denoted ‘looselybound P’,’ FeeP’, ‘AleP’, ‘unreactive P’, ‘CaeP’, and ‘Res-P’, respec-tively. Sediment phosphorus contents are expressed as mg P/g dryweight (dw). The phosphorus fractions were extracted from wetsediment samples, and their dry weights were determined from aparallel freeze-dried sample. Because the residual phosphorus wasnot determined separately but calculated as a difference betweenthe total P (determined from a parallel sample) and the sum of the Pfractions, in some cases, the residual phosphorus yielded negative

Fig. 2. Up to 65 cm long sediment cores were recovered from the accumulation areas (a acontinuous accumulation. At the bottoms with persistent hypoxia or anoxia, the sedimentsamples from the erosion or transportation (E/T)-areas (c and d) were old glacial and postgravel, and at some stations embedded with FeeMn concretions (d).

values. When the residual phosphorus was calculated to besignificantly negative (�10% of the Tot-P content), the results ofphosphorus fractionation were considered unreliable and incom-parable to the rest of the data, and were ignored. This appliedaltogether only to 15 of the 953 sub-samples.

The P fractionation was run in duplicate or in triplicate, and thedifference between replicates was usually very small. An indirectmeasure of the reliability of the P fractionation method is when thesum of the P extracted is subtracted from the total P, determined bya different method. The calculated difference (the so-called Resid-ual P) should be a positive, but small share of the total P content inthe sediment. The analyses were performed by Erken Laboratory,Uppsala University, Sweden, which has the P fractionation proce-dure accredited by the SWEDAC (the Swedish board for accredita-tion and conformity assessment).

The sampling stations were classified into two categories ac-cording to sediment type reflecting sedimentary conditions (Fig. 2):1) soft sediments characterised by relatively high organic matter(>10% dw in the sediment surface) and nearly continuous modernsediment accumulation (termed as A-areas), and 2) hard seafloorsubstrates, interpreted as erosion or transportation bottoms, wherethe sediment accumulation is disrupted either by wave action and/or currents (E/T-areas). Accumulation (A)-areas were identifiedbased on visual inspection of samples together with water contentand LOI values. Generally, sediments with a LOI value above 10% dw

nd b), where the sediments were soft, with relatively high organic content and nearlylayers were clearly visible since there were little or no traces of bioturbation (a). Theglacial clays, often covered by a thin transient organic-rich layer (c), or coarse sand or

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e21 13

and water content above 75% ww in the surface layer were classi-fied as A-areas (Håkanson and Jansson, 1983). Furthermore, grad-ually downwards declining LOI and water contents were assumedto characterise the sediments in the A-areas. In the shallow coastaland archipelago areas, true continuous sediment accumulation israre, and the classification was created specifically for this studyarea to separate these two distinctly different sedimentary envi-ronments. Phosphorus extraction scheme was applied to 953 sub-samples from 191 sampling stations (116 from the A-areas and 75from the E/T-areas). In addition, 131 total phosphorus analyseswere made from the surfaces of A-areas and 23 from the E/T-areas(from 154 stations altogether).

2.4. Statistical methods

In statistical analyses, parametric tests were used when the datamet the criteria of normal distribution and homogeneity of vari-ances. Normality was tested with KolmogoroveSmirnov test, and ifneeded, logarithmic transformation was used before the analyses.Homogeneity of variances was tested with Levene’s test. Sedimentphosphorus contents were assumed to be different in accumulationand erosion bottoms. Differences in water content, LOI, total phos-phorus content and the extracted phosphorus fractions between Aand E/T areas were tested with one-way ANOVA to characterisedifferences between the two groups. Extracted phosphorus frac-tions, total phosphorus, sedimentwater content and loss on ignitionwere tested with one-way ANOVA to compare means in 0e1 cm,1�2 cm and 0�2 cm sediment layers. Bivariate correlation test(Pearson correlation coefficient) was used to examine possible cor-relations betweenwaterdepthor exposure index in swm(simplifiedwave model, Isaeus, 2004) and total phosphorus and the extractedphosphorus fractions. For all statistical tests, a critical p-value<0.05was used to determine statistical significance. Variability in statis-tical parameters is expressed as � standard deviation (SD).

Principal Component Analysis (PCA) was applied to investigateif the variability in the sediment phosphorus contents in the sedi-ment surface is governed by certain factors included in this study.Variables entered in the analysis were water depth, exposure index(Isaeus, 2004), sediment water content, LOI and the extractedphosphorus fractions. All variables were logarithmically trans-formed and normalised prior to running the PCA.

2.5. Calculation of phosphorus in the sediments

The quantities of surface sediment P that can be expected to bereleased over time, termed here as potentially mobile P, aregenerally extracted as ‘loosely bound P’, ‘FeeP’ and ‘unreactive P’fractions. However, some remnants of these fractions are also foundin the deeper sediments (below 10 cm), apparently representing Pthat resists diagenesis enough to get buried in deeper sedimentlayers (Rydin et al., 2011) or that is transformed into an immobileform during the diagenesis (Ruttenberg, 2003). In areas of contin-uous sedimentation, the difference in phosphorus content betweenthe sediment surface and at the depth where the phosphoruscontent stabilizes can be used to estimate phosphorus release fromsediments (Williams and Mayer, 1972). In the A-areas, averagecontents of ‘loosely bound P’, ‘FeeP’ and ‘unreactive P’ below 10 cmdepth were assumed to represent the burial share of these P formsand were subtracted from their higher contents found in the0�2 cm layer. In some sediment cores, LOI decreased abruptly inthe deeper sediment layers and there was a clearly visible uncon-formity in the sediment profile (as in Fig. 2c, at 9�10 cm depth),indicating changes in hydrodynamic conditions and/or the qualityof the depositing material. When LOI was less than 5.5% dw in asediment core below 10 cm depth, it was interpreted as glacial or

postglacial clay (see: Virtasalo et al., 2007). Those layers wereassumed not to represent modern accumulated sediments afterdiagenesis, and were excluded from the calculations of P back-ground content in the A-areas.

In the E/T-areas, where the real P burial cannot be assumed butfrom where some of the phosphorus is eventually released beforefurther transportation to other areas, estimations of lower andupper boundaries of potentially mobile P were calculated. Forestimation of the lower boundary, the average background contentin the A-areas was subtracted from the phosphorus content in thesediment surface layer, and for estimation of the upper boundary,the percentage of the sum of ‘loosely bound P’, ‘FeeP’ and‘unreactive P’ estimated to be potentially mobile in the A-areas wasused.

To estimate the pool of potentially mobile phosphorus, theassumed background contents were subtracted from the extracted‘loosely bound P’, ‘FeeP’ and ‘unreactive P’ fractions in the sedimentsurface layer (0�2 cm). For samples where the topmost 2 cm wasseparated into two sub-samples, the results for sediment surfacelayer (0�2 cm) were calculated as the average of the results fromthe 0�1 and 1�2 cm layers. Spatial distributions of total andpotentially mobile phosphorus in the sediments were calculated inGIS software ArcGIS�. The sediments in the study area werereclassified into two categories (A- and E/T-areas, as describedabove) based on the SGU seafloor sediment maps (Sverigesgeologiska undersökning SGU, 2012) and BALANCE seabed sedi-ment map (Al-Hamdani and Reker, 2007), complemented with thenew acoustic data. Sediment bulk density r (g/cm3) was calculatedfrom the equation after Håkanson and Jansson (1983) and dry bulkdensity from the bulk density, sediment water content and LOI afterDadey et al. (1992). Potentially mobile phosphorus values wereconverted to mg P/cm3 and interpolated subsequently for sedimentsurface layer of 0e2 cm thickness over the study area using ordi-nary kriging method. Measured values of P were interpolatedseparately in the A- and E/T-areas. The interpolationwas conductedusing spherical semivariogram model and variable search radius,including 12 nearest sample stations. The sediments in the openparts of the Baltic Sea differ from coastal sediments, both in termsof settling rates, distribution of grain sizes (Jonsson et al., 1990) andwith respect to its phosphorus content and dynamics (Carman andCederwall, 2001; Mort et al., 2010; Malmaeus and Karlsson, 2012).Therefore it was decided to not include the open parts of the Sea ofÅland in the presentation, calculation an interpolation of phos-phorus store within the study area. The borderline between thecoastal zone and the open sea was drawn using an approach sug-gested by Håkansson (1990).

3. Results

Our results show that, at the sediment surface, contents of Tot-Pand all the extracted P fractions (one-way ANOVA, p < 0.001),except ‘CaeP’ (p > 0.05), were significantly higher in the organic-rich soft sediments in the accumulation (A)-areas than in the firmseabed substrates of erosion or transportation (E/T)-areas (Tot-P x(A) ¼ 1760 � 810, x (E/T) ¼ 1000 � 480, F(2,187) ¼ 101.295; ‘looselybound P’ x (A) ¼ 30 � 30, x (E/T) ¼ 8 � 7, F(2,187) ¼ 54.738; ‘FeeP’ x(A)¼ 640� 640, x (E/T)¼ 200� 180, F(2,187)¼ 78.020; ‘unreactive P’ x(A) ¼ 360 � 110, x (E/T) ¼ 150 � 90, F(2,187) ¼ 188.939; ‘AleP’ x(A) ¼ 180 � 180, x (E/T) ¼ 120 � 200, F(2,187) ¼ 39.804; ‘CaeP’ x(A) ¼ 390 � 55, x (E/T) ¼ 370 � 120, F(2,187) ¼ 0.707). The resultconfirms the relevance of separating the sediments in A- and E/T-groups.

Total phosphorus contents in the sediment surface layer variedbetween 570 and 6420 mg P/g dw in the A-areas (0�2 cm, n ¼ 247)and between 180 and 3500 mg P/g dw in the E/T-areas (0�1/0�2/

surface >10 cm surface >10 cm0

500

1000

1500

2000

Phos

phor

us c

onte

ntµg

P/g

dw

Loosely bound P Fe-P Unreactive P Al-P Ca-P Res-P

A E/T

Fig. 4. Mean distribution of sediment phosphorus forms (mg P/g dw) in the A-areas atthe sediment surface (n ¼ 116) and below 10 cm depth in the sediment (n ¼ 75), aswell as in the E/T-areas at the sediment surface (n ¼71) and below 10 cm depth (n ¼27). Al-P, Ca-P and Res-P are considered to be immobile phosphorus fractions.

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e2114

0�3 cm; n ¼ 94). Remarkably high, over 4000 mg P/g dw, totalphosphorus contents were observed at five stations near Stockholmand further to the north on the Swedish coast. Tot-P contents over2500 mg P/g dw were observed also in the archipelago near Turku.Phosphorus contents in the sediment surface layer (0�2 cm) didnot seem to depend on the water depth (the highest r2 ¼ 0.284,p < 0.01 for Tot-P) or the wave exposure index (correlations be-tween the variables were non-significant). Sediment total phos-phorus contents in the A-areas around mainland Åland weregenerally low,x¼1480� 370 mg P/g dw for 16 stations closest to theisland. Also in the western Uusimaa archipelago, the easternmostpart of the study area, phosphorus contents were comparativelylow: themeanTot-P value in the sediment surface layer (0�2 cm) ofthe A-areas in the western Uusimaa archipelago (n ¼ 10) was1570 � 430 mg P/g dw, with a maximum of 2400 mg P/g dw at onestation.

Three components in the PCA for the sediment surface layer (0e2 cm), both A and E/T-samples included, explained 74% of the totalvariation in the data. The first principal component (PC1) consistedmainly of water content (coefficient 0.410), LOI (0.394), ‘looselybound P’ (0.339), ‘FeeP’ (0.404), ‘unreactive P’ (0.406) and ‘AleP’(0.340), explaining 47% of the total variation. PC2 comprisedmainlyof ‘Res-P’ (0.731) and ‘AleP’ (�0.397), explaining 14% of the totalvariation. PC3 explained 13% of the total variationwith water depth(0.518) and exposure index (0.628) as major constituents. Samplesin the A and E/T-areas were divided in clear groups along PC1(Fig. 3a). Both A and E/T samples were scattered evenly along thethird component.

‘Loosely bound P’, ‘FeeP’ and ‘unreactive P’ together comprised60% of Tot-P in the sediment surface layer (0�2 cm) in the A-areas,while in the E/T-areas, 35% of the total phosphorus in the sedimentsurface layer was extracted in those fractions (Fig. 4). The majorpart of these potentially mobile fractions was ‘FeeP’, on average62% in the A-areas and 57% in the E/T-areas.

Phosphorus content declined rapidly immediately below theuppermost sediment surface layer in the A-areas. Sediment watercontent and Tot-P, ‘loosely bound P’ and ‘FeeP’ contents weresignificantly higher in the 0�1 cm than in the 1�2 cm layer(Table 1; ANOVA: water content: F(2,182) ¼ 9.371, p < 0.0001; Tot-P:F(2,182) ¼ 5.920, p < 0.01, ‘loosely bound P’: F(2,182) ¼ 5.558, p < 0.01,‘FeeP’: F(2,182) ¼ 8.203, p < 0.001). However, there was no signifi-cant difference between the samples from 0�1 cm and 0�2 cmlayers for any tested variables, except for the water content, whichwas higher in the 0�1 cm than in the 0�2 cm layer (ANOVAF(2,182) ¼ 4.488, p < 0.05). Differences in organic content, ‘unreac-tive P’, ‘AleP’, ‘CaeP’ and ‘Res-P’ were non-significant between the

Fig. 3. The first principal component in principal component analysis separates A (n ¼ 11content, LOI and potentially mobile phosphorus fractions, in PC2 immobile phosphorus fractiof the total variation. The eigenvectors indicate the direction and extent of each variable in

0�1 cm, 1e2 cm and 0�2 cm sediment depths (LOI: F(2,182) ¼ 2.889,‘unreactive P’: F(2,182) ¼ 1.929, ‘AleP’: F(2,182) ¼ 0.052, ‘CaeP’:F(2,182) ¼ 0.160 and ‘Res-P’: F(2,182) ¼ 2.160; p > 0.05).

In the A-areas in general, Tot-P contents declined successivelyfrom the surface down to 10�20 cm, most rapidly above 10 cm,stabilising to an approximately constant level below that depth.However, the amount of ‘loosely bound P’ seemed to increase below40 cm. The downward declinewas less pronounced in ‘unreactive P’contents, and the variations were relatively high throughout thevertical profile of the sediment (Fig. 5). The proportion of ‘FeeP’ranged from 3 to 51% and ‘unreactive P’ from 13 to 39% of the Tot-Pbelow 10 cm depth in the sediment. On average, 39% of totalphosphorus was extracted as ‘loosely bound P’, ‘FeeP’ and‘unreactive P’ (Fig. 4), and the most abundant of these forms insamples below 10 cm depth was ‘unreactive P’. In the E/T-areas, 82%of the total phosphorus was extracted as immobile P forms, ‘AleP’,‘CaeP’ and ‘Res-P’, in samples below 10 cm depth of the sediment(Fig. 4).

The mean total phosphorus content in the E/T-areas was1000 � 480 mg P/g dw at the sediment surface (n ¼ 71),830� 200 mg P/g dw in 2e6 cm depth (n¼ 29) and 720�140 below10 cm depth in the sediment (n¼ 27; Fig. 6). ‘Loosely bound P’, ‘FeeP’ and ‘unreactive P’ comprised 15% of Tot-P in 2e6 cm and 15%

6) and E/T (n ¼ 71) samples into two groups (a). Main constituents in PC1 are waterons, and in PC3 water depth and wave exposure. PC1 explains 47%, PC2 14% and PC3 13%the analysis (b).

150 500 1000 1500200 500 1000 1500

10 500 500 1000 1500

Unreactive PFe-P

Loosely bound PTot-P

Phosphorus content, g P/g dw

Sedi

men

t dep

th, c

m

0-2

2-6

10-40

0-2

2-6

10-40

Fig. 6. Total phosphorus and ‘loosely bound P’, ‘Fe-P’ and ‘unreactive P’ fractions in thesediment surface layer (mg P/g dw) in the E/T-areas (0e1/0e2/0e3 cm, n ¼ 71) in 2e6cm (n ¼ 29) and below 10 cm (n ¼ 21) depth in the sediment, mean � SD. Note thedifferent scale in the x-axis for loosely bound P.

Table 1Water content (% ww), LOI (% dw), total P and the extracted phosphorus fractions (mgP/g dw) in the sediment surface layers 0�1 cm and 1�2 cm in the accumulationareas.

n ¼ 61 Sediment depth (cm) Mean � SD Min�Max

Water 0�1 85 � 5 68�96Content 1�2 81 � 5 68�94LOI 0�1 15.3 � 5.4 9.4e47.5

1�2 13.5 � 3.1 7.2e22.8Tot-P 0�1 1920 � 790 910�5200

1�2 1500 � 540 910�3900Loosely 0�1 38 � 44 2�240Bound P 1�2 17 � 26 1�190FeeP 0�1 730 � 700 47�4000

1�2 350 � 290 48�1800Unreactive-P 0�1 390 � 120 180�660

1�2 350 � 110 150�610AleP 0�1 160 � 130 44�790

1�2 150 � 190 47�1500CaeP 0�1 400 � 65 240�620

1�2 400 � 52 270�570Res-P 0�1 190 � 200 �318�710

1�2 240 � 190 �6�1100

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e21 15

below 10 cm depth. ‘FeeP’ and ‘unreactive P’were found in roughlyequal amounts at both depths. The mean values of sediment watercontent, LOI, total phosphorus contents and phosphorus fractionswere compared at the sediment surface (0�1/0�2/0�3 cm),2�6 cm and below 10 cm depth in the sediment with one-wayANOVA. Differences in water content and LOI were statisticallynon-significant (ANOVA F(2, 126) ¼ 1.531 and 1.163, respectively,p > 0.05). Tot-P contents declined with increasing depth in thesediment (F(2, 126) ¼ 5.055, p < 0.05), and the decline was clear in‘loosely bound P’ (F(2, 126) ¼ 24.470, p < 0.0001), ‘FeeP’ (F(2,126) ¼ 22.394, p < 0.0001) and ‘unreactive’ P (F(2, 126) ¼ 8.622,p < 0.001) (Fig. 6).

The spatial pattern of phosphorus distribution at the sedimentsurface followed roughly that of the sediment types (Figs. 7 and 8).Exceptionally high contents of ‘FeeP’ were found in oxygenatedsediments in the Stockholm archipelago. Relatively high, abovemean phosphorus contents appeared to exist in the proximity ofthe Swedish coast and in the Archipelago Sea south-southwestfrom the city of Turku (red and dark orange colours in Figs. 7 and8). Also in the more open part of the archipelago between Ålandand Turku, the phosphorus contents were high compared to thosenear the Åland islands and outer parts of the archipelago, south ofTurku. The lowest phosphorus contents were measured in firmseabed substrates where erosional conditions prevail. Three of the

6

5

4

3

2

1

60

50

40

30

20

10

00 200 400

60

50

40

30

20

10

00 1000 2000

Sedi

men

t dep

th, c

m

Phosphorus con

a) Tot-P b) Loosely bound

Fig. 5. Vertical distribution of total phosphorus (Tot-P) and ‘loosely bound P’, ‘Fe-P’ and ‘unrthe x-axes. In the sub-samples 0e10 cm, 20e40 cm and 16e18 cm n varies from 53 to 116, aand 13. Sub-samples below 20 cm are pooled together into 20e30 cm, 30e40 cm and 40e

eight smallest measured total phosphorus contents (less than500 mg P/g dw) were located on the Swedish coast east fromStockholm, four were near Åland island, and one in the easternmostpart of the study area.

Of the total study area, 41% is covered by organic-rich softsediments, and 61% of the total phosphorus at the sediment surfacewas calculated to exist in these areas. After subtracting the averagebackground contents, the mean potentially mobile phosphoruscontent in the sediment surface layer (0�2 cm) of the A-areas was630 � 670 mg P/g dw. At the sediment surface of the E/T-areas, themean content of potentially mobile phosphorus was230� 290 mg P/g dw.Within all of the A-areas, 60% of the calculatedtotal amount of 77,000 tonnes of phosphorus in the sedimentsurface layer (0�2 cm) was estimated to exist in potentially mobileforms (i.e. extracted as ‘loosely bound P’, ‘FeeP’ and ‘unreactive P’fractions; Table 2). After subtracting the average background con-tents of these P forms, 61% of their sumwas estimated to representthe actual potentially mobile P in the A-areas. Thus, 36% of the totalphosphorus in the A-areas was estimated to be potentially mobile.The E/T-areas were calculated to contain 49,300 tonnes of totalphosphorus, out of which 7.5e19% was estimated to be potentiallymobile. In the entire study area of 19,200 km2, the total amount ofpotentially mobile phosphorus in the sediment surface layer(0�2 cm) was estimated to be 31,700�37,300 tonnes.

0

0

0

0

0

0

00 500 1000

60

50

40

30

20

10

00 500 1000

tent, g P/g dw

c) Fe-PP d) Unreactive P

eactive P’ fractions in the A-areas; (mg P/g dw), mean � SD. Note the different scales innd in the sub-samples 10e12 cm, 14e16 cm, 18e20 cm and 40e60 cm n lies between 660 cm. All the sub-samples are 2 cm thick.

Fig. 7. Spatial distribution of total phosphorus content (mg P/g dw) in the sediment surface layer (0e2 cm). Interpolated values were obtained using Ordinary Kriging.

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e2116

4. Discussion

The average total phosphorus content in the sediment surfacelayer in the A-areas of our study (1750 � 780 mg P/g dw in the0�2 cm layer) is of the same order of magnitude as reported pre-viously in the Baltic Sea archipelagos (Virtasalo et al., 2005; Lukkariet al., 2009a, b; Rydin et al., 2011; Malmaeus et al., 2012). Also in theeutrophic Chesapeake Bay estuary, on the eastern coast of NorthAmerica, similar levels of total P contents (1040�2320 mg P/g dw)have been measured in the surface sediment (Hartzell et al., 2010).In the open Baltic Sea and Danish straits, total phosphorus contentsof w600e2500 mg P/g dw have been reported (Jensen et al., 1995;Mort et al., 2010; Malmaeus and Karlsson, 2012). In comparisonto other estuarine areas in the world, the P content in our researcharea is high as Tot-P values have varied between 169 and 1026 mg P/g dw in Peel-Harvey estuarine system, Western Australia(169�711 mg P/g dw, McComb et al., 1998), the eutrophic Montegoestuary, in the western coast of Portugal (460�840 mg P/g dw,Coelho et al., 2004) and Delaware River estuary in the east coast ofthe USA (341�1026 mg P/g dw, Strom and Biggs,1982). This could bedue to different sedimentation conditions (e.g. lack of tides andstrong underwater currents in the Baltic Sea) and differences incoastal topography (e.g. most other estuaries lack the mosaic ar-chipelago). Also in other coastal sea areas with higher salinity, thetotal phosphorus content seems to be lower compared to our

results, for example,130�1400 mg P/g dw inwestern coast of France(Andrieux-Loyer and Aminot, 2001), and around 430 mg P/g dw insand�silt sediments in the Bay of Brest, France (Tallberg, 2008).

Fe-bound phosphorus dominated the potentially mobile phos-phorus pool in the sediment surface layer (36% of total phosphorusin the A-areas and 20% in the E/T-areas) in our study area, while‘loosely bound P’ was less than 2%. This is also in accordance withprevious studies in both deep (‘loosely bound P’ and ‘FeeP’ togetherca 30%, Mort et al., 2010) and shallow (33% as ‘FeeP’, Jensen et al.,1995) areas of the Baltic Sea. In Chesapeake Bay, ‘FeeP’ accountedfor 58% and ‘loosely bound P’ 5.7% of the total extracted phosphorusin the sediment surface (Hartzell et al., 2010). In other sea areas theproportion of ‘loosely bound P’ and ‘FeeP’ in the sediment surfaceseems to vary rather much, e.g. the sum of ‘FeeP’ and ‘looselybound P’ was 7e39% in the west coast of France (Andrieux-Loyerand Aminot, 2001; Tallberg, 2008), while 35�52% of “non-apatiteinorganic phosphorus” has been measured in Western Australia(McComb et al., 1998) and 20�40% in the coast of Spain (Avilés andNiell, 2005). Although the release of iron-bound phosphorus undernegative redox-turnover conditions has often been considered thedominant source of phosphorus in marine sediments, also organicphosphorus may have a significant role in the internal phosphorusloading (Benitez-Nelson, 2000; Ahlgren et al., 2006; Lukkari et al.,2009b; Jilbert et al., 2011; Sinkko et al., 2013). ‘Unreactive P’ wasassumed to represent mainly organic and biogenic P (Psenner et al.,

Fig. 8. Spatial distribution of potentially mobile phosphorus content (mg P/g dw) in the sediment surface layer (0e2 cm). Interpolated values were obtained using Ordinary Kriging,and in the E/T-areas the mean phosphorus contents between the lower and upper boundaries of the estimated potentially mobile phosphorus were used.

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e21 17

1988; Ahlgren et al., 2005). Our results suggest that, in the archi-pelago area, approximately one fourth of the ‘unreactive P’, in thesediment surface will be recycled, while the major part of it will beburied with the sediment. However, the observed burial of‘Unreactive P’ varied considerably in our research area. The varia-tions may partly be attributed to fluctuations in the compositionand origin of the depositing material and the physico-chemicalconditions during sedimentation. The fractionation method usedin this study does not allow more detailed characterization of the‘unreactive P’ but it can be assumed that its reactivity and sensi-tivity against microbial mineralization is affected by its origin:organic P produced in the sea can be easier to decompose whilethat transported from terrestrial environment could be more re-fractory (Ruttenberg and Goni, 1997). More specific techniques,such as nuclear magnetic resonance (NMR) spectroscopy areneeded to acquire more detailed information about the composi-tion and the degradability of organic phosphorus forms in thesediments (Hupfer et al., 1995; Ahlgren et al., 2006).

The spatial distribution of the potentially mobile phosphorus inthe sediment surface layer was, as also expected, closely related tothe sediment type. Previous results in the Baltic Sea show a trend ofdeclining phosphorus contents from shallow coastal area towardsdeeper open sea (Lukkari et al., 2009b). In our results, statisticalanalyses did not reveal any significant relationship between

geographic location, water depth or exposure to waves, and thedistribution of phosphorus contents. Moreover, the spatial distri-bution of phosphorus in the sediment surface layer did not showany particular pattern, probably because of the fractured topog-raphy in the archipelago. High phosphorus contents were found inthe central area of the Finnish Archipelago Sea, and the highestvalues were determined in the coastal region of the Stockholmarchipelago. In the eastern part of our study area, the phosphoruscontents were generally low, despite of the accumulation oforganic-rich sediment. This may be due to lifted redox-cline in thesediment and seasonal hypoxia, which has led to depletion of iron-bound phosphorus at the sediment surface. In the sediment surfacelayers both in the A- and E/T-areas, ‘FeeP’ dominated the poten-tially mobile phosphorus pool. Although high phosphorus contentsin the sediments may indicate high potential for internal phos-phorus loading, the environmental conditions during and aftersedimentation, as well as the properties of the depositing material,are crucial for the nutrient recycling and burial processes (Jilbertet al., 2011; Jilbert and Slomp, 2013; Slomp et al., 2013). In coastalareas where seasonal hypoxia is common (Conley et al., 2011), theareas of intense phosphorus release do not necessarily coincidewith high phosphorus contents in the sediment surface layer.Firstly, even though some areas are rich in ‘FeeP’ in the surface, itmay not be released if the conditions remain oxic and diffusion of

Table 2The amount of phosphorus store in the sediment surface layer (0�2 cm) in the studyarea. The seafloor areas covered by bedrock outcrops (4700 km2) were excludedfrom phosphorus calculations.

Area Total P ‘Loosely bound P’þ ‘FeeP’ þ ‘unreactive P’

Potentially mobile P

km2 Tonnes P g P/m2 Tonnes P g P/m2 Tonnes P g P/m2

A 7900 77,000 9.7 46,100 5.8 28,000 3.5E/T 6600 49,300 7.5 16,800 2.5 3700�9300 0.6e1.4Total 14,500 126,000 8.7 62,900 4.3 31,700�37,300 2.2e2.6

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e2118

pore water P from deep sediments is effectively captured to the Fe-oxides. Secondly, in seasonally hypoxic/anoxic areas, the P fluxfrom the sediment to the water column may be intense, even if thesediment phosphorus content is moderately low due to dissolutionof ‘FeeP’ from the surface. In the eastern part of our study area, theUusimaa archipelago, severe seasonal hypoxia and anoxia areknown to occur (HELCOM, 2009; Ahlman and Lappalainen, 2012),and the contents of potentially mobile phosphorus were compar-atively low, probably due to depletion of ‘FeeP’. If there is miner-alization- and reduction-induced P release from deep, reducedsediment layers, the released P in the pore water migrates upwardsto the surface sediment through diffusion (Krom and Berner, 1981),and the enriched ‘FeeP’ at the surface may or may not be detectedin the sampling, depending on the presence of seasonal hypoxia atthe time of the sampling. Relatively low contents of ‘FeeP’ in thedeep sediments of our study area suggest that most of iron-boundphosphorus contributes to phosphorus recycling and only little isburied permanently in the coastal accumulation areas.

In our study area, the estimated annual waterborne input ofphosphorus from the catchment is approximately 1700 tonnes(Suomela, 2011; Swed. Univ. of Agriculture unpubl., Finnish Envi-ronment Institute unpubl.). The comparable input to the entireBaltic Sea is approximately 29,000 tonnes, accounting ca. 95% of thetotal phosphorus input to the sea (HELCOM, 2011). The calculatedamount of potentially mobile phosphorus in the sediment surfacelayer (0�2 cm) of the A-areas in our study alone exceeds theexternal annual phosphorus input to the entire Baltic Sea. Thisunderlines the importance of internal phosphorus loading ofshallow archipelago areas. Content of potentially mobile phos-phorus in the sediment surface layer of the E/T-areas was only12�28% of that in the A-areas. Nevertheless, as much as ca. 37% ofthe total area of the Baltic Sea (Al-Hamdani and Reker, 2007) iscovered by these hard clays and tills, and seafloor erosion of theseareas may cause mobilisation of phosphorus. Thus, erosion andtransportation areas should not either be neglected in estimationsof sediments as an internal source of phosphorus in the Baltic Sea.

The phosphorus load from the bottom needs to come to thephotic zone in order to increase primary production. The calcula-tions presented here assume that all P lost from the sediment is lostto thewater column. However, part of the dissolved P released fromsediments may be bound again to resuspended particulate materialand deposited back to the seafloor, possibly after horizontaltransportation. Resuspension and transportation of particulatematerial, and P bound to it, is poorly known in the study area.Excess of P in the productive layer turns the production towardsnitrogen limitation. Hence, high phosphorus concentrations in thewater induce high algal growth, favouring particularly nitrogen-fixing cyanobacteria (Tyrrell, 1999; Vahtera et al., 2007). N2 fixa-tion brings more nitrogen into the system enhancing phyto-plankton growth, which enhances secondary production andincreases the supply of organic matter to the system. Elevated Pconcentrations in the water and increasing organic matter supplyto the sediments might alter cycling of other nutrients. The

interactions are complex and not completely understood, and mayvary considerably in different environments. Nevertheless, the in-ternal phosphorus loading is likely to maintain high productivity inthe archipelagos, especially with assumed enlarging hypoxic bot-tom areas, and extensive reductions in the external phosphorussupply are required to reduce further accumulation and recycling ofbioavailable phosphorus in the sediments. Due to the heteroge-neous morphology, the water exchange between adjacent basins isrestricted, and most of the phosphorus remains in the archipelagowaters. The archipelago sediments may have acted as a filtertrapping the increased nutrient load from the catchment(Jumppanen and Mattila, 1994). The elevated phosphorus contentsin the sediments may thus cause intense and long-lasting internalphosphorus loading deteriorating the water quality.

Our results indicate that internal phosphorus loading in shallowarchipelago areas, primary recipients of land-based loading ofphosphorus, is most likely important in sustaining high nutrientlevels in these areas and thus considerably hampers effects ofongoing measures to improve the water quality. Internal loading ofshallow areas may partially explain the slow recovery of coastalareas even after considerable mitigation work during the past de-cades (Jumppanen and Mattila, 1994; Vahtera et al., 2007). Inseasonally hypoxic areas, the importance of internal loadingmay beeven more decisive, when new organic material with high contentof potentially mobile phosphorus is deposited during oxygenatedperiods and when the new supplies of phosphorus will temporallybe released in big pulses to the water column during hypoxic/anoxic periods. Our results support the previous assumptions thatnot only the deep anoxic Baltic Sea areas (Mort et al., 2010;Viktorsson et al., 2013), but also the shallow ones (Conley et al.,2011) may be very important sources of internal loading. As acomplement to measurements of the sediment phosphorus con-tent, frequently repeated in situ flux studies should be used fromlarge areas and different seasons. Furthermore, as human-inducedeutrophication is a global problem (Smith, 2003b; Rabalais et al.,2009), research on sediment phosphorus contents also in othercoastal and estuarine systems is required to increase our under-standing of nutrient cycling in those environments.

The topmost 2 cm of the sediment surface was included in thecalculations of the pool of potentially mobile P. How much phos-phorus and how deep from the sediments eventually will bereleased depends also on the chemical character of phosphorus,biogeochemical processes in the sediment as well as on the pre-vailing environmental conditions. Benthic fauna and flora, oxygenconditions, microbial activity, currents, waves, temperature, pH,etc. affect nutrient dynamics in the sediments. For example, activityof benthic fauna, bioirrigation and bioturbation, affect nutrientdynamics between the sediments and the water column (Koretskyet al., 2002; Middelburg and Levin, 2009). For example, the invasivepolychaete Marenzelleria spp. in the Baltic Sea can create burrowsas deep as 30 cm (Hedman et al., 2011; Norkko et al., 2012), andbioturbation by these species has been shown to relocate con-taminants from sediments in laboratory tests at least down to10 cm depth (Josefsson et al., 2010). Furthermore, human activitieslike boat traffic induced erosion, underwater construction work,dredging and dumping of dredged waste, etc. bring deep sedimentto interaction with the water column, which may lead to furtherphosphorus release, depending on the quality of the material. Alsoland uplift brings slowly, but continuously, new seafloor areasabove the wave base causing sediment erosion. The observed in-crease of ‘loosely bound P’with sediment depth could be explainedby mineralization-induced release of P from ‘unreactive P’.Although a part of organic P in the recently deposited material ismineralized fast (within a few weeks; Benitez-Nelson, 2000), therelease of P from organic phase can be a slow process continuing for

I. Puttonen et al. / Estuarine, Coastal and Shelf Science 145 (2014) 9e21 19

a long period of time (Ahlgren et al., 2006). Hence, including onlythe top 2 cm will probably underestimate the total amount ofpotentially mobile P.

Various chemical phosphorus extraction methods are widelyused in assessing bioavailable phosphorus pool in sediments (e.g.Van Eck, 1982; Psenner et al., 1984; Ruttenberg, 1992; Jensen andThamdrup, 1993; Ruban et al., 1999; Rydin, 2000), and the resultsdepend on the method used. Oxidation of ferro-iron into ferri-formis fast (Davison and Seed, 1983) and, since in this study, it was notpossible to shield the sediment samples from atmospheric oxygenduring sampling, exposure of the samples to oxygen has probablycaused some transformations between phosphorus forms, partic-ularly loosely bound and pore water P binding onto iron oxy(hydr)oxides (Lukkari et al., 2007a). However, as our target was todetermine the amount of bioavailable phosphorus, which includesthe both of these fractions, this is of minor importance in our study.Furthermore, a small proportion of Ca-bound phosphorus may bedissolved in BD-extraction, thus resulting in over-estimation of the‘FeeP’ fraction (Jensen and Thamdrup, 1993), but in Ca-poor sedi-ments in the northern Baltic Sea this is negligible (Lukkari et al.,2007b). In our study ‘unreactive P’ is assumed to consist ofmainly organic or biogenic P (Psenner et al., 1988; Ahlgren et al.,2005). Although ‘unreactive P’ can be assumed to include poly-phosphates (Hupfer et al., 2004; Baldwin, 2013), whilst inorganic,they are predominantly of biogenic origin (Hupfer et al., 2004;Ahlgren et al., 2005). The residual phosphorus fraction is consid-ered to consist mainly of refractory organic phosphorus, but it mayalso include some inorganic phosphorus (Ruttenberg, 1992). Pfractions distinguished in a sequential extraction scheme areoperationally defined, and as such, only approximations of P forms.Our estimations of the pool of potentially mobile P are based onaverage background content, which was reasoned to excludeconceivable overestimations in potentially mobile P fractions.

Weight loss-on-ignition (LOI) is a widely used method for esti-mating organic content in sediments and soils, since it is a simpleand inexpensive technique (Bengtsson and Enell, 1986). Concernsof loss of structural water from clay minerals may cause over-estimation of organic matter in clay-rich sediments (Heiri et al.,2001; Barillé-Boyer et al., 2003). The loss of structural water de-pends on, besides the mineral composition, also on exposure timeto ignition temperature, sample size and even the position in thefurnace (Heiri et al., 2001). J. Smith (2003a) demonstrated that lossof structural water was insignificant after 2 h ignition in 550 �Cfrom clay-rich glaciolacustrine sediments, comparable with oursamples. Hence, considering the aim and scale of our study, LOI wasassumed to provide an adequate estimation of organic mattercontent for the studied sediments.

In our study, it was assumed that phosphorus content below10 cm depth in the sediment of the A-areas represents the contentof phosphorus that will remain in the sediments, unless erosionexposes these deposits. Possible variations in the phosphorusaccumulation rates in the sediments or in the quality and compo-sition of the depositing material through time, as well as trans-formations of phosphorus forms in the deep sediment layers anddiffusive transportation upwards may, however, cause errors in theestimates of phosphorus burial. A thin transient layer of fluffy,organic-rich sediment was often observed at the surface of the E/T-areas, but this layer is likely to be resuspended and transportedaway from the sites to be further processed elsewhere. The releaseof phosphorus in the E/T-areas cannot be estimated similarly as inthe A-areas, since the sediments below the surface layer weredeposited in an environment and conditions completely differentfrom the current ones. The lower and upper boundaries of the poolof potentially mobile phosphorus in the E/T-areas presented herecan be considered as an indicative approximation of the amount of

phosphorus to be released over time in those areas. Bearing theseuncertainties in mind, the results are considered to give a reason-ably good estimate of the store of the potentially mobile phos-phorus that can be released with time from these coastal andarchipelago sediments.

5. Conclusions

Phosphorus contents are high in large areas in the studiedshallow coastal and archipelago sediments. The highest potentialfor internal phosphorus loading is bound in organic rich accumu-lation sediments. Moreover, even erosion and transportation bot-toms, where no continuous modern sedimentation occurs, may actas sources of bioavailable phosphorus. The coastal and archipelagosediments have accumulated nutrients transported from land fordecades, and now comprise a remarkable source of phosphorus inthe northern Baltic Sea archipelagos. The role of internal phos-phorus loading in these areas has probably been seriously under-estimated. Our results show that not only the deep anoxic Baltic Seaareas, but also the shallow archipelago areas, should be consideredas important sources of phosphorus. This is especially important asrecent modelling and sediment proxy results suggest that underthe IPCC scenario of a global warming there is likely no improve-ment of bottom water conditions in the future (Meier et al., 2011;Kabel et al., 2012; Neumann et al., 2012; Kotilainen et al., 2014).Moderation of the phosphorus fluxes from sediments requiresconsiderable reductions of the phosphorus load from land basedsources. To achieve a good state of the Baltic Seawaters, muchmorestringent water protection measures are needed in the near futurethan what are presently planned.

Acknowledgements

The research was funded by the EU Central Baltic INTERREG IVAProgramme (SEABED A13) (SEABED-project 2009e2012), TheNordic Council of Ministers (2010e2012) (NMR AEG-11323) andK.H. Renlund Foundation (2010e2012). The authors wish to thankthe crews of R/V Sunbeam, R/V Muikku and R/V Geomari, and theGeological Survey of Finland. We would also like to thank Aqua-biota Water Research for providing the wave exposure model, andthe Geological Survey of Sweden for the sediment maps.

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