Boreal Environment Research 17: 425–436
© 2012
ISSN 1239-6095 (print) ISSN 1797-2469 (online)Helsinki 30 November 2012
Estimating the amount of mobile phosphorus in Baltic coastal
soft sediments of central Sweden
J. Mikael Malmaeus1)*, Emil Rydin2), Per Jonsson3), Dan Lindgren4) and
O. Magnus Karlsson1)
IVL Swedish Environmental Research Institute, P.O. Box 210 60, SE-100 31 Stockholm, Sweden
(*corresponding author: [email protected])
2)
Erken laboratory, Institute of ecology and evolution, Uppsala University, Norr Malma 4200, SE-761 73
Norrtälje, Sweden
3)
Department of Applied Environmental Science, Stockholm University, SE-106 91 Stockholm, Sweden
4)
Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden
1)
Received 25 Nov. 2010, final version received 10 Apr. 2012, accepted 6 July 2011
Malmaeus, J. M., Rydin, E., Jonsson, P., Lindgren, D. & Karlsson, O. M. 2012: Estimating the amount of
mobile phosphorus in Baltic coastal soft sediments of central Sweden. Boreal Env. Res. 17: 425–436.
A new dataset based on 102 sediment cores was examined to estimate the amount of phosphorus (P) that will eventually be released to the water column from the Baltic coastal
sediments along the Swedish coast between Öregrund and Oxelösund. Total P (Ptot) concentration in the surface sediments varied between 840 and 7100 µg g–1 dry weight (dw)
with an average of 1650 µg g–1 dw. In deep sediments, the Ptot concentration was around
1000 µg g–1 with small variation. The difference between surface concentration and the
stable, deeper, concentration represents P to be released, i.e. the mobile P. Pools of mobile
P varied between 1.5 and 18.2 g m–2. Correlations between surface Ptot concentrations and
amounts of mobile P were strong (r2 = 0.88). We estimate the amount of mobile P in the
coastal sediments of the investigated region to be between 1000 and 4000 tonnes. Assuming a turnover time of the mobile P between three and ten years gives an average annual P
release of 100–1300 tonnes yr–1.
Introduction
The trophic status of the Baltic Sea is determined by the availability of phosphorus (P) and
nitrogen (N) in the water column (e.g. Larsson et al. 1985, Granéli et al. 1990, O’Melia
et al. 2006). The role of the sediments in the
cycling of P is often highlighted (e.g. Eilola
et al. 2009, HELCOM 2009). In a number of
studies, P fluxes between the sediments and the
water column are indirectly quantified by massbalance calculations or by monitoring changes in
water-column concentrations. Such calculations
Editor in charge of this article: Johanna Mattila
indicate that internal P loads are of the same
order of magnitude as the external ones (Emeis
et al. 2000, Conley et al. 2002, Savchuk 2005).
Redistribution of sediments through wind/wave
action and land uplift may mobilize even larger
amounts (Jonsson et al. 1990, Blomqvist and
Larsson 1994, Eckhéll et al. 2000, Håkanson and
Bryhn 2008). From an ecological perspective,
however, release of molecular phosphate from
the sediments represents a paramount source
since it is all bioavailable. In contrast, resuspension of particulate P forms and external P load
consist of a mixture of labile and inert P forms.
426
Total phosphorus (Ptot) concentrations in the
Baltic Sea sediments range between 300 and
2500 µg g–1 dw (Jonsson et al. 1990, Virtasalo
et al. 2005). According to Carman and Cederwall (2001), Ptot concentrations in surface sediments (0–1 cm) of different parts of the Baltic
Sea and in different bottom types are 700–2500
µg g–1 dw (median 1300 µg g–1 dw). The highest concentrations were found in accumulation
areas and the lowest in erosion areas. Emelyanov
(2001) collected at least 386 samples of surface
sediments (0–5 cm) from different parts of the
Baltic Sea and found average P concentrations
in different bottom types to be between 600 and
1400 µg g–1 dw. Deep sediments (below 5 cm)
in accumulation areas typically contain around
1000 µg P g–1 dw (Lehtoranta and Heiskanen
2003, Lukkari 2008).
Knowledge of permanent immobilization
processes for P in brackish environments is poor
(Conley et al. 2009). Settling P is associated
with either organic or inorganic particles. Some
fractions of inorganic P are inert and are consequently not affected by chemical processes in the
sediment. These stable P forms, including apatite
and P bound to aluminium, will therefore stay
and be buried in the sediment after settling from
the water column, as observed in lake sediments
(e.g. Boström et al. 1982, Rydin 2000, Reitzel
et al. 2005). Other forms of P are influenced by
chemical conditions in the sediment. Redox-sensitive P fractions are mostly associated with iron
compounds, although e.g. redox-sensitive manganese compounds may also bind P under oxic
conditions (Mortimer 1941, 1942, Boström 1984,
Ingri et al. 1991, Gunnars and Blomqvist 1997).
Even if surface sediments are oxidized, the
redox potential a few centimeters down in fine
sediments is always low and consequently the
concentration of iron-bound P in deep sediments
is generally low. Organic matter may be more
or less degraded which also includes P present
in this material. The amount of easily degradable material decreases with sediment depth.
At greater sediment depths, non-degradable or
refractory material dominates the organic fraction. The amount of organically-bound P generally decreases with sediment depth together with
the organic content (Gächter et al. 1988, Ahlgren
et al. 2006).
Malmaeus et al. • Boreal Env. Res. Vol. 17
Dissolved P in sediment pore-water can diffuse upward in the sediment and reach the water
column or become trapped by sediment particles,
in particular Fe(III) compounds, in oxidized surface sediment. Dissolved iron may be scarce in
marine sediments due to formation of insoluble iron sulphides (e.g. Matthiesen et al. 1998,
Blomqvist et al. 2004). P which is potentially
released to the water column is mostly present
in the top 2–3 centimeters of sediment (Carman
and Jonsson 1991, Lukkari 2008).
Carman and Jonsson (1991) attempted to calculate total amounts of P in Baltic Sea sediments
and concluded that a major proportion (~95%)
of the potentially mobile P fractions is present
in near-shore and archipelago areas. One reason
for this is the higher P binding capacity of the
permanently oxidized surface sediments in shallow areas. Their estimate, however, was based on
the top 2 cm only. Lukkari (2008) studied the P
composition in 20 sediment cores from the Gulf
of Finland and the Archipelago Sea and quantified mobile pools and burial of P. Rydin et al.
(2011) estimated long-term average of gross and
net loading, including the release of sediment P
at four stations in the Stockholm Archipelago.
The difference between the generally higher Ptot
concentrations found in surface sediments and
the lower concentrations found in older, deeper
layers indicates the magnitude of the pool of P to
be eventually released (Rydin et al. 2011). The
stock of sediment P that in the future will eventually be released as dissolved P has been referred to
as “reactive P” (Lukkari et al. 2009 and references
therein) or “potentially mobile P” (Rydin 2000). It
may also be referred to as bioavailable P although
it is only bioavailable after it is released from the
sediment. It is the potentially-mobile P (Rydin
2000) that we attempted to estimate in this study,
and we will hereafter refer to it as “mobile P”.
Given the key role of the sediments for the P
cycling and the nutrient status in the Baltic Sea
there is a need to quantify the pool of mobile P
in the Baltic Sea sediments, and to improve the
understanding of the mechanisms determining
the fate of different P forms in sediments and the
response to environmental forcings in terms of,
e.g. external P load or climate change.
In this study, we present a new set of data on P
composition and Ptot concentrations in the Swed-
Boreal Env. Res. Vol. 17 • Mobile phosphorus in Baltic coastal soft sediments
427
ish east coast around the Stockholm Archipelago.
The aim was to estimate the pool of mobile P, and
to improve the understanding of the P dynamics
including quantification of different P forms in
the sediments of the studied area.
Methods
Sediment sampling
In total, 102 cores of sediments were collected
with a Gemini double corer (inner diameter
80 mm) or with a Willner sampler (inner diameter 63 mm) from 33 coastal areas of the southern Bothnian Sea and the northwestern Baltic
Proper from Öregrund to Oxelösund including
the Stockholm Archipelago (Fig. 1) between
November 2008 and February 2010. Core samples were sliced in the field in 2-cm-thick layers
(a few 1-cm layers from surface sediments were
also prepared, see Appendix). Surface sediments
(0–2 cm) and deep sediments (between 2- and
5-cm-thick layers located between 10 and 70 cm
below the sediment surface depending on the
sediment core structure) were analyzed for water
content, organic content and Ptot. Twelve sediment cores were selected for P fractionation at
several depth levels (1–2 cm slices) with close
intervals above 10 cm sediment depth and more
sparsely at deeper levels (Table 1). In addition
to the 12 fractionated sediment cores, 90 surface
samples and 73 deep sediments from our collected cores were used in this work.
Fig. 1. Locations of sediment sampling stations. Numbered crosses indicate fractionated cores. Black dots
indicate locations for which Ptot was analysed.
Chemical analyses
Chemical analyses were performed at the Erken
laboratory (Uppsala University). All the cores
were stored in darkness at 4 °C until preparation
in the laboratory. Water content was determined
after freeze drying, and organic content (loss on
ignition, LOI) after ignition at 550 °C for two
hours.
Table 1. Labels (location), positions (WGS-84) and characteristics of sediment cores collected for P fractionation.
Latitude (N)Longitude (E) Depth (m)
01 Norr A (Norrtäljeviken)
02 Norr B (Norrtäljeviken)
03 Sing D (Singöfjärden)
04 Pil A (Pilkobbsfjärden)
05 Bull I (Bulleröfjärden)
06 Bull J (Bulleröfjärden)
07 Gäl D (Gälnan)
08 Gäl Q (Gälnan)
09 Tor C (Torsbyfjärden)
10 Tor P (Torsbyfjärden)
11 Stu B (Stussviken)
12 Him B (Himmerfjärden)
5947
5947
6009
5911
5912
5912
5931
5932
5920
5922
5845
5900
1853
1854
1841
1845
1849
1850
1845
1846
1828
1827
1724
1745
34
28
41
58
47
44
27
31
31
52
20
32
Water contentLoss on ignition
(%)
(%)
98
92
91
91
90
92
88
88
88
88
84
80
23
21
17
23
19
19
16
17
14
16
16
10
428
Content of Ptot in sediments was analyzed as
PO43– after acid hydrolysis at high temperature
(340 °C) according to Murphy and Riley (1962).
Phosphorus forms were separated into
NH4Cl-rP, BD-rP, NaOH-rP, NaOH-nrP, HCl-rP
and residual P following, in principle, the sequential extraction scheme suggested by Psenner et
al. (1988). These fractions are defined by the
extraction method, but ideally each fraction corresponds to a specific P containing substance in
the sediment. Generally, NH4Cl-rP is regarded as
loosely bound P, BD-rP as P associated with iron
hydroxides, NaOH-rP as P bound to aluminium,
NaOH-nrP as organic P forms, and HCl-rP as
calcium bound P compounds. Residual P is given
by subtracting extracted and identified P from Ptot.
P fractionations were started within a week after
sampling.
Quantifying the pool of mobile P
To compute the amount of mobile P per unit area
from the sediment core data, the burial concentration, i.e. the P concentration in deep sediment in
each core where sediment diagenesis has ceased,
was subtracted from the higher concentrations in
more shallow layers. Burial concentrations were
taken as the average concentrations below the
stabilization depth. Concentration differences (in
µg g–1 dw) were multiplied by dry matter content
to obtain the amount of mobile P in each layer.
Dry matter content was determined by calculating the bulk density in each layer adjusting for
water content and organic content according to
Håkanson and Jansson (1983). The surpluses of
P in each sediment layer were taken as mobile P
and integrated over the depth profile to obtain the
amount of mobile P per square meter accumulation bottom area (missing layers were filled by
linear interpolation). This procedure was followed
for Ptot as well as for the different P fractions.
Statistical analyses
A Mann-Whitney U-test was used to determine
if average P concentrations differed in different subsamples. A least-square linear regression
analysis was also carried out.
Malmaeus et al. • Boreal Env. Res. Vol. 17
Results
Basic sediment parameters
Basic information about our sampling sites is
given in the Appendix. In our dataset, all surface samples were categorized as accumulation
sediments according to Håkanson and Jansson
(1983). Water content varied between 75.0%
and 97.9% (mean = 86.6%) and LOI (≈ organic
content) varied between 9.1% and 54.7% (mean
= 16.5%).
Ptot in surface and deep sediments
In the deep sediments, the Ptot concentration was
typically around 1000 µg g–1 and the variation was
relatively small (Fig. 2). Its standard deviation
was 266 µg g–1 but excluding one outlier (3000
µg g–1; the second highest value was 1700 µg g–1)
the standard deviation was reduced to 154 µg g–1.
In the surface sediments, variation in Ptot concentration was considerably greater. The dataset
included 11 coastal areas with 4–20 samples,
making statistics meaningful within these areas.
The within-site coefficient of variation was on
average 0.28. Ptot concentration was not correlated with water depth, depth below wave base
(calculated according to Håkanson and Eklund
2007), water content or organic matter content
(r2 < 0.25, n = 102).
P fractions in sediment profiles
Concentrations of Ptot, iron-bound P and organic
P decreased with depth (Fig. 3). It is apparent
that the loosely bound P fraction generally constitutes a quantitatively minor fraction (Fig. 4).
The iron-bound P decreased with depth towards
a well defined burial concentration of around 100
µg g–1 dw. The relatively constant concentrations
with depth of P associated with calcium and
aluminium indicate that these fractions are more
or less inert. The aluminium-bound P, however,
declined slightly with depth close to the surface
which is probably due to imperfections in the
chemical extraction rather than sediment dynamics (Rydin et al. 2011). Organic P declined with
Boreal Env. Res. Vol. 17 • Mobile phosphorus in Baltic coastal soft sediments
429
8000
Fig. 2. Ptot concentration
in surface sediments (0–2
cm) and deep sediments
(2–5 cm layers collected
from 11–70 cm sediment
depth). Lines indicate
the whole range of data
while boxes indicate 25th
and 75th percentiles. All
= all data, By area = data
grouped into 33 areas.
Ptot (µg g–1 dw)
7000
6000
5000
4000
3000
2000
1000
0
All
By area
Surface sediments
depth, and in some profiles the decline continued
throughout the cores indicating that degradation
of organic material in some cases may proceed
decades after deposition. However, Ptot concentrations generally stabilized below 5 cm or even
shallower depths, hence most mobile P occurs
above this level. Deeper sediment layers typically contained around 1000 µg P g–1 dw. In one
sediment core — Norr A — the concentration
profile deviated from the general pattern found
in the other cores. Some exceptional Ptot concentrations were found in layers Pil A (8–10 cm),
Bull J (6–8 cm) and Tor P (48–50 cm). The Ptot
decline with depth is larger than the combined
decline of iron-bound and organic P, indicating
that some of the mobile P was present in the
residual fraction.
Mobile P in sediment profiles
Burial concentrations used to estimate the mobile
P (see Methods) were obtained from average
concentrations below 10 cm depth for each fraction. In Norr A, the concentrations at 28–30 cm
depth were taken as the burial concentrations. In
Tor P, the value at 48–50 cm depth was excluded
due to the fact that it was three SD above the
mean for the rest of the core below 10 cm depth.
When summarizing amounts of mobile P, negative values (i.e., actual concentrations lower than
burial concentrations) were excluded.
The total pools of mobile P varied between
1.5 and 18.2 g m–2 (Fig. 5). Mobile P constituted
19%–79% (mean = 47%) and 8%–51% (mean =
26%) of Ptot in the top 2 and 10 cm of the sedi-
All
By area
Deep sediments
ment, respectively. Typically the concentration
of mobile P decreased rapidly with depth. In
four cores, the 0–1-cm and the 1–2-cm layers
were analyzed separately, and the Ptot concentration was between 1.3 and 2.9 times higher in the
0–1 cm layer. On average 57% of the mobile P
occurs in the top 2 cm of the sediment (n = 12).
Correlations between surplus Ptot concentrations
and the sums of mobile fractions (Fig. 6a) and
between surface Ptot concentrations and amounts
of mobile Ptot (Fig. 6b) were strong (r2 = 0.90, n
= 12; and r2 = 0.88, n = 12; respectively).
A dataset for comparison
We had the opportunity to compare our data
with 83 surface samples (0–2 cm) collected by
the Swedish Geological Survey (SGU) between
July 1996 and October 2002 from the same area
as ours. The samples in the SGU database were
a sub-selection from a larger data set rejecting
data from sediments which were considered to
be from non-accumulation areas (based on water
depth and sediment water content). The mean Ptot
concentration in the SGU data was 1550 µg g–1
dw. The average Ptot concentration in the surface
sediments found by us did not differ statistically
(Mann-Whitney U-test) from the values in the
SGU database.
Discussion
This study estimates mobile P in accumulation
sediments. Hence, the pool of mobile P in ero-
Malmaeus et al. • Boreal Env. Res. Vol. 17
430
0
0
Norr A
1000
1500
500
2000
2500
0
5
5
10
10
15
15
20
20
25
25
30
0
1000
0
2000
6000
0
5
10
20
15
30
20
40
25
50
30
0
5
10
15
20
25
30
35
40
0
0
500
1000
1500
60
0
Bull I
1000
500
1500
2000
0
0
Bull J
1000
500
1500
2000
10
20
30
40
50
0
500
Gäl D
1000
1500
Gäl Q
2000
2500
0
0
500
0
2000
0
1000
1000
1500
4000
6000
2000
3000
10
20
30
40
50
60
0
1000
Tor C
2000
3000
0
10
10
20
20
30
30
40
40
50
50
60
60
0
4000
Pil A
4000
10
0
5
10
15
20
25
30
35
40
3000
30
Sing D
0
Norr B
2000
0
1000
Stu B
2000
3000
0
5
5
10
10
15
15
20
20
25
25
Loosely bound P
Aluminium bound P
Iron bound P
Calcium bound P
sion and transportation areas is not considered
although it could be substantial. However, sediment dynamics above the wave base are different
than in accumulation areas, since sediment resus-
Tor P
Him B
Organic P
Ptot
Fig. 3. Vertical concentration profiles of different P
fractions in 12 sediment
cores.
pension interrupts P diagenesis and creates a different chemical environment. For this reason, in
erosion and transportation areas all P is “mobile”
(but only partly bioavailable). As stated in the
Boreal Env. Res. Vol. 17 • Mobile phosphorus in Baltic coastal soft sediments
Sediment depth (cm)
–50
0
Sediment depth (cm)
Sediment depth (cm)
0
10
20
30
40
50
60
20
18
Mobile P (g m–2)
16
Fig. 5. Pools of different
fractions of mobile P at 12
different sampling locations.
150
–1000
10
20
30
40
50
60
0
Aluminium bound P
100
200
300
400
10
20
30
40
50
60
0
Iron bound P
1000
2000
3000
Calcium bound P
200
300
400
500
0
10
20
30
40
50
60
–100
Fig. 4. Mean ± SD concentrations of different P
fractions in sediment profiles (n = 12).
Loosely bound P
50
100
431
200
Organic P
400
600
800
0
10
20
30
40
50
60
0
10
20
30
40
50
60
0
0
100
1000
Total P
2000
3000
4000
Residual P
Organic P
Iron bound P
Loosely bound P
14
12
10
8
6
4
2
0
Norr A Norr B Sing D Pil A
introduction, typical P concentrations are lower
in erosion and transportation sediments than in
accumulation sediments.
Total amounts of P differ considerably
between locations, which was largely explained
by the iron-bound fraction. In fact, variation in
the iron-bound fraction explained 95% of the
variation in Ptot. For example in Tor P, concentration of iron-bound P in the surface sediment
(0–2 cm) was very high (3600 µg g–1 dw), while
the iron-bound P concentration in Pil A was very
low (170 µg g–1 dw). The extremely high surface
concentrations of iron-bound P may either be
upward migrating P trapped in the oxidized sedi-
Bull I
Bull J Gäl D Gäl Q Tor C
Tor P Stu B Him B
ment surface or phosphate precipitating from the
deep water column at the sediment surface.
In this work, temporal variability in sediment
P storage was effectively disregarded. Deposition of organic material is higher during the
growing season which enhances P accumulation
in surface sediments. In lakes, the accumulation
of organic material in surface sediments typically
decreases the redox potential and causes release
of iron-bound P. It is reasonable to expect that
temporal dynamics also affects the P dynamics
in the coastal sediments of this study, although
seasonality is less predictable than in lakes due
to e.g. events of water exchange.
Malmaeus et al. • Boreal Env. Res. Vol. 17
432
Sum of mobile fractions (g m–2)
25
a
20
y = 0.0034x + 0.4938
r2 = 0.90
15
10
5
0
1000
25
2000
3000
4000
5000
Surface – burial Ptot concentration (µg g–1 dw)
–5
b
Mobile Ptot (g m–2)
20
y = 0.0036x – 3.4264
r2 = 0.88
15
10
5
0
1000
–5
2000
3000
4000
5000
Surface (0–2 cm) Ptot concentration (µg
Amounts of mobile P in different sediment
cores reflect spatial variability in sediment accumulation, P content in settling matter and diagenesis/turnover rates in the sediments. Sediment
cores contain valuable information about these
processes, but the quantification of different rates
requires additional information. Average and
median P concentrations in our sediment data are
consistent with values in the Baltic Sea reported
in the literature, however, in the upper part of
the range given in e.g. Jonsson et al. (1990)
and Emelyanov (2001). The reason for this is
probably that our dataset includes samples from
accumulation sediments only. Our median deepsediment P concentration of 1000 µg g–1 dw is
consistent with that of Lehtoranta and Heiskanen
(2003) and Lukkari (2008). The average Ptot concentration in surface sediments found by us did
not differ from the values in the SGU database.
The sampling stations used in this study
included both oxic and anoxic areas. In some
cases it is likely that during recent years the sediments had undergone regime shifts moving from
g–1
dw)
6000
Fig. 6. Regressions with
95% confidence limits to
estimate mobile phosphorus pools from (a) surface
minus burial concentrations and (b) surface concentrations.
anoxic towards oxic conditions (Karlsson et al.
2010). This clearly affects the pool of mobile P
in the surface layers of the sediments. However,
we did not find any significant differences (MannWhitney U-test) when comparing deep-sediment
Ptot concentrations from sampling stations with
anoxic surface sediments with those from stations
with oxic surface sediments. The average deepsediment Ptot concentrations in the anoxic and oxic
sediments were 1001 and 1010 µg g–1 dw, respectively. Studies of coastal sediments in the southern
Baltic Sea have shown that precipitation of dissolved P with iron in oxidized surface sediment
layers is a quantitatively important but transitory
immobilization pathway of dissolved sediment P
(Jensen and Thamdrup 1993, Jensen et al. 1995).
Persson et al. (1993) and Jonsson et al.
(2003) used results from seafloor mapping by
means of side scan sonar, sediment echo sounder
and common echo sounder in combination with
sediment sampling of cores to estimate sediment
accumulation areas along the Swedish east coast.
Using the results from Persson et al. (1993) and
Fig. 7. Mobile P pools
estimated using the two
regressions given in Fig.
6. ‘Reg 1’ corresponds to
Fig. 6a and ‘Reg 2’ corresponds to Fig 6b. Results
are shown for all data
(‘All’) and for data grouped
into 33 areas (‘By area’).
Lines indicate the whole
range of data while boxes
indicate 25th and 75th
percentiles.
Predicted mobile Ptot (g m–2)
Boreal Env. Res. Vol. 17 • Mobile phosphorus in Baltic coastal soft sediments
433
25
20
15
10
5
0
Reg 1
Jonsson et al. (2003) the area of accumulation
bottoms in the 33 areas where surface sediment
were collected for this study was estimated to
equal 253 km2. Using hypsographic information
(SMHI 2003) in combination with algorithms for
predicting the depth of the wave base (Håkanson
and Eklund 2007), we estimated the coastal sediment accumulation area between Öregrund and
Oxelösund to be 983 km2.
The strong correlations between the surface
Ptot concentrations and amounts of mobile P probably reflect the fact that surface conditions typically determine how much P can be retained in
the profile. We may use the regressions given
in Fig. 6 to estimate amounts of mobile P at the
sampled sites. The outcomes of both regressions
when the deep-sediment data (n = 33) were used
in calculations were comparable (Fig. 7). Our
extensive dataset allowed us to tentatively extrapolate our estimate of potentially mobile P in accumulation areas in our study area. Multiplying the
average mobile P concentration in our 33 recently
sampled areas (2.5 g m–2) by the sediment accumulation area as mapped by Jonsson et al. (2003)
we found that this area contained ~630 tonnes of
mobile P. For the entire area, the corresponding
amount of mobile P would be 2460 tonnes.
Confidence limits around the total amount of
mobile P are not so much determined by uncertainties in the average mobile P concentration or
the area estimate. The number of observations
reduces the width of the confidence bands by
1/√n (for more than 100 surface-sediment Ptot
measurements). Instead, predictions made by the
regressions based on 12 samples (see Fig. 6)
will determine total uncertainty. The standard
deviation around these regressions is ~1.5 g m–2.
Reg 2
All
Reg 1
Reg 2
By area
Hence, it is fair to say that the amount of mobile
P in the coastal accumulation sediments of the
investigated region is probably between 1000
and 4000 tonnes.
Typical sediment accumulation in this region
is between 5 and 20 mm yr–1 (Jonsson et al.
2003) and mobile P concentrations below 5 cm
are generally close to zero (see Fig. 2). We may
thus conclude that the turnover time for mobile
P is between 3 and 10 years (5 cm divided by 5
to 20 mm). This in turn means that the release of
P from the accumulation sediments in our region
will be between 100 and 1300 tonnes yr–1 (or
between 0.1 and 1.3 g m–2 yr–1 which is comparable to experimental data from various sites
in the Baltic Proper; see e.g. Matthiesen et al.
1998, Emeis et al. 2000, Hille et al. 2005, Mort
et al. 2010). This is admittedly a very rough
calculation but a reasonable order-of-magnitude
long-term estimate that allows us to assess the
quantitative importance of the mobile P pool.
These values are comparable to the P transport
of around 150 tonnes yr–1 in the Norrström River
— the outflow of Lake Mälaren with a catchment
area of 22 650 km2 constituting the major terrestrial water inflow to the Stockholm Archipelago.
For comparison, the P release estimates from the
Gotland Basin sediments and the anoxic parts of
the Baltic Proper are 14 000 tonnes yr–1 (Emeis
et al. 2000) and 33 000 tonnes yr–1 (Ahlgren et
al. 2006), respectively. In any case the sediment
P release from the investigated region has some
quantitative importance.
The storage of potentially mobile P in the
Baltic Sea accumulation sediments is comparable to the P amount in the water column, and the
output from the sediments is in the same order of
434
magnitude as the input from land (Carman and
Jonsson 1991, Conley et al. 2002). This study
is an effort to quantify the mobile amount of P
in the sediments in one Baltic coastal region.
Further efforts to improve the knowledge of
the P storage in other parts of the Baltic Sea are
motivated by the quantitative importance of the
P fluxes between sediment and water. Improved
knowledge about factors and processes regulating burial and release of sediment P is crucial for
the management of the Baltic Sea ecosystem.
Conclusions
Based on 102 coastal sediment surface samples
from the Swedish east coast between Öregrund and Oxelösund we found that surface Ptot
concentrations varied between 840 and 7100
µg g–1 dw with an average concentration of 1650
µg g–1 dw and a median concentration of 1400
µg g–1 dw. The standard deviation for the surface
Ptot concentrations was around 1250 µg g–1 dw.
In deep sediments the Ptot concentration was
typically around 1000 µg g–1 (n = 85) and the
variation was remarkably small. Excluding one
outlier from the dataset, the standard deviation in
the burial Ptot concentration was 154 µg g–1.
Phosphorus fractions were analyzed in detail
in 12 sediment cores. On average 57% of the
mobile P occurred in the top 2 cm of sediment.
Total pools of mobile P varied between 1.5 and
18.2 g m–2. Correlations between surface Ptot
concentrations and amounts of mobile P were
strong (r2 = 0.88). We estimate the amount
of mobile P in the coastal sediments of the
investigated region to between 1000 and 4000
tonnes. Assuming a turnover time of the mobile
P between three and ten years gives an average annual release of between 100 and 1300
tonnes yr–1, which is in the same order of magnitude as the regional P input from land.
Acknowledgements: The Swedish Environmental Protection
Agency, Svealands Coastal Water Management Association,
The Swedish Water & Wastewater Association and the Foundation for IVL Swedish Environmental Research Institute
are acknowledged for financial support. We also thank the
Swedish Geological Survey (SGU) for sharing their sediment
data with us. Finally, we thank the crew of r/v Sunbeam for
cooperation during the field work.
Malmaeus et al. • Boreal Env. Res. Vol. 17
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Askrikefjärden
Baggensfjärden
Bergöområdet
Bulleröfjärden
Edöfjärden
Erstaviken
Farstaviken
Galtfjärden
Gupafjärden
Gälnan
Hargsviken
Himmerfjärden
Höggarnsfjärden
Lilla Värtan
Långholmsfjärden
Möja Söderfjärd
Norrtäljeviken
Pilkobbsfjärden
Risöområdet
Sandemarsfjärden
Singöfjärden
Solöfjärden
Stora Värtan
Strömmen
Stussviken
Torsbyfjärden
Trälhavet
Ägnöfjärden
Älgöfjärden
Östhammarsfjärden
Östra Saxarfjärden
59 23
59 18
58 44
59 12
59 28
59 14
59 20
60 11
58 48
59 31
60 11
59 00
59 22
59 20
59 23
59 22
59 47
59 11
58 44
59 07
60 09
59 23
59 24
59 19
58 44
59 22
59 26
59 15
59 20
60 15
59 26
18 13
18 20
17 29
18 49
18 40
18 22
18 22
18 37
17 30
18 45
18 28
17 45
18 20
18 11
18 16
18 52
18 53
18 45
17 18
18 20
18 42
18 27
18 08
18 06
17 25
18 27
18 23
18 25
18 40
18 24
18 31
30
39
9
45
23
61
10
9
21
29
7
31
34
30
23
105
31
52
6
29
40
47
20
31
22
41
60
33
23
6
62
16
7
3
15
22
12
7
10
9
15
6
12
16
10
16
21
8
22
9
10
16
9
12
7
8
12
15
23
8
5
20
oxic
varying
oxic
slightly oxic
varying
oxic
reduced
oxic
oxic
oxic
oxic
oxic
oxic
oxic
oxic
reduced
varying
varying
oxic
oxic
varying
oxic
reduced
slightly oxic
oxic
oxic
oxic
oxic
reduced
varying
oxic
AreaLat.Long.
Depth
WaveOxic
WGS-84
WGS-84
(m)
base
status
(m)
Appendix. Information about sampling stations.
86
84
90
87
86
83
90
81
83
87
86
79
84
85
87
91
92
89
86
85
89
84
86
79
85
85
84
81
88
95
86
14
15
18
18
15
14
19
12
12
16
16
10
13
15
15
20
20
21
17
14
16
13
13
16
19
13
14
29
17
28
14
WaterLoss
content
on ignition
0–2 cm
0–2 cm
(%)
(%)
2600
1597
1100
1520
1190
2000
1300
1000
1400
1425
1300
1665
1500
1600
1200
1300
1750
1300
1100
1000
3487
2000
870
2250
1207
3300
4900
1900
1023
1467
1050
Ptot 0–2 cm
(µg g–1 dw)
880
1035
880
1021
980
1075
1160
750
810
1081
820
880
1000
1200
913
1100
1100
1017
800
1205
1000
1000
830
3000
878
910
1500
1150
910
1100
1050
1
3
2
5
20
2
1
1
1
4
1
2
2
1
2
1
4
4
2
2
15
1
1
2
6
5
1
2
3
3
2
Ptot deepNumber
(µg g–1 dw)
of samples
436
Malmaeus et al. • Boreal Env. Res. Vol. 17