Estuarine, Coastal and Shelf Science 92 (2011) 111e117
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Estuarine, Coastal and Shelf Science
journal homepage: www.elsevier.com/locate/ecss
Phosphorus release from coastal Baltic Sea sediments as estimated from sediment
profiles
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, Sweden
IVL Swedish Environmental Research Institute, P.O. Box 210 60, SE-100 31 Stockholm, Sweden
c
Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden
d
Department of Applied Environmental Science, Stockholm University, SE-106 91 Stockholm, Sweden
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2010
Accepted 16 December 2010
Available online 5 January 2011
We used the decline in total phosphorus (P) concentration with depth in sediment profiles from the
North-western Baltic Proper coastal zone to calculate the site-specific amount of sediment P eventually
to be released to the water column: The potentially mobile P. P fractionation revealed that iron bound P
dominated the potentially mobile P at sites with oxic surface sediment layers. Organic P forms were also
a major constituent of the potentially mobile P pool. We determined that 1e7 g P/m2 were potentially
mobile at our sites, and the turnover time of this P pool was considered short, i.e., less than a decade. To
determine long-term average P fluxes to and from the surface sediment layer, we first multiplied the
constant and relatively low P concentration in deeper sediment layers with the sediment accumulation
rate to gain the P burial rate. Then the average total P concentration in settling matter was multiplied
with the sediment accumulation rate to estimate the depositional P flux at each site. The difference
between the depositional and burial rates represents the long-term average release rate of sediment P
and varied between 1.0 and 2.7 g P/m2 yr among our sites. These rates are at the same order of magnitude
as values reported from other areas of the Baltic Sea, and constitute a major source of P to the water
column.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Phosphorus
sediment
release
burial
Baltic Sea
1. Introduction
Phosphorus (P) is a key limiting nutrient of summer cyanobacterial blooms in the Baltic Sea (Conley et al., 2009a), and P release from
sediments may be a major source of P to the water column, especially
in the Baltic Sea (Conley et al., 2002; Nausch et al., 2009) due to large
areas with anoxic surface sediments that retain P poorly (Vermaat
and Bouwer, 2009). The release of dissolved P from sediment accumulation bottom areas will occur if P is mobilized during sediment
diagenesis and if the dissolved P is not transformed and retained into
stable compounds that subsequently become permanently immobilized 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 be
permanently buried in Baltic Sea sediment. Our understanding of
permanent P immobilization processes within in the Baltic Sea
sediment profile is however poor. The transformation of “reactive
P” (Ruttenberg, 1992) into inorganic compounds in the sediment
* Corresponding author.
E-mail address: [email protected] (E. Rydin).
0272-7714/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ecss.2010.12.020
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 burial
process in anoxic sediments of the Baltic Sea. Phosphorus forms
present 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 mobilization
through diagenetic processes such as P associated to refractory
calcium and aluminum forms (Lukkari et al., 2009b; Mort et al.,
2010). Precipitation of dissolved P with iron in oxidized surface
sediment layers is a quantitatively important, but temporary
(Jensen et al., 1995), immobilization pathway of dissolved sediment
P (Jensen and Thamdrup, 1993; Jensen et al., 1995).
Degradation of labile organic P compounds, originating from
phytoplankton sedimentation is a major source to the dissolved P
pool in Baltic Sea sediment (Carman & Rahm, 1997; Hille et al., 2005;
Ahlgren et al., 2006). A pronounced decline pattern in total phosphorus (TP) concentration over increased sediment depth, until
a constant concentration has been reached, would reflect such
a mobilization of organic P forms and upwards migration (Carignan
and Flett, 1981) and subsequent release of the dissolved P. The
112
E. Rydin et al. / Estuarine, Coastal and Shelf Science 92 (2011) 111e117
decline pattern in TP concentration would also reflect the limited
capacity of sediment to retain the mobilized P. In such a system, the
stock of sediment P to be released in the future as dissolved P has
been referred to as “reactive P” (Lukkari et al., 2009b and references
therein) or “potentially mobile P” (Rydin, 2000). Potentially mobile P
will be the term heretofore used in this paper.
Recently, the use of different methods to increase Baltic Sea
sediment P retention has been debated (Conley et al., 2009b). A basic
parameter needed when evaluating different methods is a quantification of the potentially mobile P, i.e., the identification of sediment
areas with pronounced sediment P release. Lukkari et al. (2009b)
estimated the long-term minimum average release of P from
different sediment sites in the NE Baltic Proper and found that the P
release varied by two orders of magnitude. This highlights the need
to estimate the pool of potentially mobile P, including rates of
deposition, release, and burial of P in specific areas. Such knowledge
is essential to understand long-term ecosystem P turnover and to be
able to predict future sediment P release from specific bottom areas
under different conditions.
Our work has two goals. First, we quantified amounts of
potentially mobile sediment P, i.e., P that eventually will be released
to the water column, using basic sediment parameters. To gain
further insight into the P forms that contribute to potentially
mobile P, sediment P was divided into different forms using
a fractionation technique. Second, we estimated the long-term
average release rates of the potentially mobile sediment P to the
water column. To obtain this, we used literature data on average P
concentrations in settling matter from the region to estimate total P
deposition using sediment accumulation rates from each site. By
subtracting the permanent P burial rate in deeper sediment layers
from the P deposition rate on the sediment surface, we calculated
a site-specific measure of the long-term average P release rate at
our sites in the Baltic Sea.
2. Material and methods
Table 1
Position and depth of the sediment cores analyzed from the Stockholm archipelago
area in the NW Baltic Sea.
Station
Latitude
Basin
Abbreviation
WGS 84
Torsbyfjärden
Bulleröfjärden
Gälnan
Pilkobbsfjärden
Tor C
Bul I
Gäl Q
Pil A
592040
591152
593150
591132
Longitude
Water depth
(m)
N
N
N
N
182776
184952
184581
184521
E
E
E
E
31
47
31
58
2.2. Chemical analyses
Water content was determined after freeze-drying, and organic
content using the Loss of ignition method (LOI, 550 C for 2 h). Total
P content in sediments was analyzed as phosphate after acid
hydrolysis at high temperature (340 C) according to Murphy and
Riley (1962). Phosphorus forms were separated following, in principle, the sequential extraction scheme suggested by Psenner et al.
(1988) within a few days after sampling. The following P forms
were extracted, and the chemicals used in parenthesis: NH4Cl-rP
(1 M NH4Cl at pH 7), BD-rP (0.1 M Na2S2O4/NaHCO3), NaOH-rP and
NaOH-nrP (0.1 M NaOH), HCl-rP (0.5 M HCl). These fractions are
defined by the extraction method, but ideally each fraction corresponds to a specific phosphorus containing substance within the
sediment. Generally, NH4Cl-rP is regarded as loosely-bound phosphorus, BD-rP as phosphorus associated with iron hydroxides
(Jensen and Thamdrup, 1993), NaOH-rP as phosphorus bound to
aluminum, NaOH-nrP as organic phosphorus forms, and HCl-rP as
calcium bound phosphorus compounds. Residual P is calculated by
subtracting extracted and identified phosphorus from TP, and is
considered to represent refractory organic P forms. P extracted will
be denoted as Loosely-sorbed P, FeeP, AleP, Org-P, CaeP, and Res-P,
respectively. In one sample, Bul I (0e2 cm), fractions had to be
multiplied by a factor of 0.7 to calculate a positive Res-P.
2.1. Sediment sampling
2.3. Sediment accumulation
Sediments were collected from four accumulation bottom sites
representing typical coastal areas of the Stockholm Archipelago
situated in the North-western Baltic Proper (Fig. 1, Table 1). In some
of these bottom areas the oxygen situation has improved over the
last few years (Karlsson et al., 2010).
Sampling was performed in November 2008 using a Gemini
double corer (inner diameter 8 cm). Sediment cores were sliced in
2 cm thick disks and selected layers were analyzed for water
content, organic content, P fractions, and total P (TP).
We determined the average yearly sediment accumulation at
our sites by lamina counting over a 10e20 cm section and got
a good agreement with earlier sediment accumulation rates using
137
Cs dating (Jonsson et al., 2003). The resulting thickness of an
average yearly sediment deposition load was converted to dry
matter accumulation using water content and sediment density. In
this way, sediment compaction was indirectly corrected for as the
water content decreased with increasing sediment depth.
Fig. 1. Sampling positions in the Stockholm archipelago.
E. Rydin et al. / Estuarine, Coastal and Shelf Science 92 (2011) 111e117
3. Calculations
To estimate potentially mobile sediment P, we first determined
the “stabilization depth” at which the sediment TP concentration
became constant, and where sediment diagenesis apparently at
large had ceased. We measured the average TP concentration in the
sediment profile below the stabilization depth (Table 2), and subtracted it from the TP concentration in the layers above. The
resulting P concentrations (Fig. 2) were multiplied with the dry
matter content in each layer to calculate the P amount. By depth
113
integration the amount of potentially mobile P per square meter
was obtained. To identify the forms of P responsible for the decline
in TP concentration with increased sediment depth, we repeated
this calculation for all P fractions.
To calculate gross and net P fluxes, we used a value from the
literature, 1.7 mg P/g sediment DW (dry weight) (Blomqvist and
Larsson, 1994), as a measure of the average P concentration in
settling matter in this area of the Baltic, and multiplied it by the
obtained sediment accumulation (g DW/m2) to gain gross deposition rate. Removal of P from the system (permanent burial) was
Table 2
Phosphorus fractions (see Methods), total phosphorus concentration, water and organic content. The horizontal line in the TP column indicates the assumed stabilization of the
sediment TP concentration.
Station
Layer
NH4Cl-rP
BD-rP
NaOH-rP
NaOH-nrP
HCl-rP
Loosely-bound P
FeeP
AleP
Org-P
CaeP
(mg P/g DW)
(mg P/g DW)
Res-P
Total phosphorus
Water content
LOI
TP
(cm)
(mg P/g DW)
(mg P/g DW)
(mg P/g DW)
(mg P/g DW)
(mg P/g DW)
(%)
(%)
Bul I
0e2
2e4
4e6
6e8
8e10
10e12
12e14
14e16
16e18
18e20
28e30
38e40
19
5
1
0
0
5
0
0
0
0
3
10
511
110
110
100
110
120
120
110
130
140
120
120
77
58
52
54
56
58
64
55
62
67
59
69
525
470
450
420
390
400
390
340
360
380
310
310
357
390
370
370
400
390
400
360
390
360
400
370
291
280
290
210
200
170
150
260
190
180
240
190
1.77
1.31
1.26
1.15
1.16
1.14
1.12
1.12
1.13
1.12
1.13
1.07
92
88
87
86
84
86
85
84
84
83
81
80
19
18
16
15
22
17
17
17
17
16
17
15
Gäl Q
0e2
2e4
4e6
6e8
8e10
10e12
12e14
14e16
16e18
18e20
28e30
38e40
48e50
58e60
6
2
2
0
0
1
0
1
0
0
9
15
12
27
210
83
86
75
78
62
70
78
79
81
84
215
92
90
80
69
61
55
56
52
50
55
53
52
67
71
67
85
472
375
364
312
339
327
341
324
313
300
328
337
332
313
392
346
372
347
374
302
376
382
356
400
371
367
392
401
139
209
149
219
159
280
182
151
172
153
161
66
129
158
1.30
1.08
1.03
1.01
1.01
1.03
1.02
0.99
0.97
0.99
1.02
1.07
1.02
1.07
88
84
83
82
82
81
83
81
82
81
79
79
78
77
17
15
14
14
14
14
14
14
13
13
13
13
13
12
Tor C
0e2
2e4
4e6
6e8
8e10
10e12
12e14
14e16
16e18
18e20
28e30
38e40
48e50
58e60
73
3
0
0
0
0
0
0
0
0
5
2
13
31
1700
250
82
73
85
84
95
96
99
82
92
68
100
81
170
100
73
64
60
50
55
62
63
59
68
54
100
150
390
330
330
300
310
300
340
320
330
230
330
210
160
240
390
400
350
280
320
300
290
280
320
330
300
210
270
320
130
67
320
220
74
100
100
150
110
140
180
160
160
150
2.84
1.16
1.16
0.93
0.85
0.84
0.89
0.90
0.91
0.84
0.98
0.70
0.81
0.98
88
85
85
86
86
87
88
89
88
83
85
78
77
71
14
13
13
13
13
14
14
15
15
12
14
9
9
9
Pil A
0e2
2e4
4e6
6e8
8e10
10e12
12e14
14e16
16e18
18e20
30e32
40e42
50e52
60e62
0
0
0
0
0
0
9
0
0
0
3
9
11
15
170
120
110
99
84
110
95
97
90
110
110
110
110
110
71
61
64
53
58
60
49
54
56
53
55
62
65
73
500
420
460
390
430
410
360
370
370
370
360
310
300
260
350
340
330
320
310
330
320
320
330
310
370
330
350
350
280
340
220
320
510
240
240
260
270
280
200
240
110
180
1.37
1.27
1.18
1.17
1.39
1.15
1.07
1.10
1.11
1.12
1.09
1.06
0.95
1.00
91
90
91
88
88
88
86
86
86
85
83
81
79
77
23
22
21
22
22
19
23
20
19
18
18
16
16
14
114
E. Rydin et al. / Estuarine, Coastal and Shelf Science 92 (2011) 111e117
Fig. 2. Distribution of iron bound phosphorus “FeeP”, extractable organic phosphorus “Org-P”, and total phosphorus “TP” in the four investigated sediment cores from the
Stockholm archipelago. Shaded areas represent phosphorus that is expected to eventually be released from the sediment.
calculated as sediment accumulation multiplied by the P concentration below the stabilization depth in the sediment. The loss of
dry matter was assumed to be limited to the respiration of organic
carbon, and was represented by the decline of LOI with sediment
depth.
To estimate the average release rate of P from the Baltic sediments, we subtracted the burial rate of P from the gross deposition
of P. The difference represents the average long-term P release rate
from the sediments.
4. Results
Sediment surfaces were judged oxic, except for the sediment
collected at the Pilkobbsfjärden (Pil A) site (Table 1, Fig. 1), which
showed symptoms of anoxia, such as black colour and hydrogen
sulphide scent. The sediment from Torsbyfjärden (Tor C) was oxic at
the surface and exhibited indicators that the surface had apparently
been oxic for about 3 years (Karlsson et al., 2010), judging by the
mixed character and gray colour of the top 5 cm in the sediment.
Below 5 cm depth the sediment was laminated. All four cores
appear to represent sites of undisturbed sediment accumulation, as
judged from both present laminas below the oxidized zone, as well
as from the slowly declining water and organic content development with depth in the core (Table 2). The average thickness of one
year of deposition varied from 0.9 to 2.0 cm (Table 3). The age of the
stabilized TP concentration was roughly determined from the
sediment accumulation rate and TP stabilization depth.
Surface sediment TP concentrations differed considerably between
locations, 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
(0.17 mg/g DW), resulting in half the TP concentration (1.37 mg/g DW)
compared to Tor C.
The sediment profiles exhibited a pronounced decline in TP
concentration from the surface until between 6 and 12 cm sediment depth, at which the TP concentration stabilized at approximately 1 mg P/g DW (Table 2). The age of the sediment at this
stabilization depth corresponded to between 4 and 8 years (Table
3). Org-P decreased with sediment depth in all profiles, but less
exponentially than FeeP. In addition, the Res-P fraction also
exhibited a declining pattern over sediment depth. Some Looselybound P was detected in the all of the cores’ surface layers except
for Pil A. The remaining P fractions (AleP and CaeP) did not show
any 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.,
the difference between TP concentration and the concentration stable
with sediment depth; Table 2). The contribution from the Looselybound P fraction was calculated to be less than 0.2 g/m2, while the
FeeP was the dominant fraction in Tor C. P extracted as Org-P was the
largest fraction in the surface layers of Bul I and Pil A (Table 4).
Our results show that 51e66% of the calculated gross P deposition will be permanently buried in deeper sediment layers.
Consequently, between 34 and 49% of the gross deposition will be
released back to the water column, corresponding to long-term
average release rates of between 1.0 and 2.7 g P/m2 yr, respectively
(Table 3).
E. Rydin et al. / Estuarine, Coastal and Shelf Science 92 (2011) 111e117
115
Table 3
Sediment accumulation and age of sediment when phosphorus concentrations ceased to drop further. Calculated rates of deposition, release, and permanent burial of
phosphorus at accumulation sites in the Stockholm archipelago.
Sampling
site
Sediment
accumulation
Dry matter
deposition
Stabilized
TP
(cm/yr)
(g/m2 yr)
(yr)
(g/m2 yr)
(g/m2 yr)
(g/m2 yr)
(%)
Bul I
Gäl Q
Tor C
Pil A
1.2
0.9
2
1.1
1980
1720
3212
1597
8
6
4
8
3.4
2.9
5.5
2.7
1.2
1.2
2.7
1.0
2.2
1.7
2.8
1.7
66%
60%
51%
63%
5. Discussion
In all of our cores, we observed a decline in TP concentration
until stabilization at a lower concentration in deeper sediment
layers. This pattern is also found in nutrient-rich lakes, such as Lake
Erken, Sweden (Rydin, 2000). In these shallow systems, only
a fraction of organic matter is mineralized during sedimentation
and the upper layers of the sediment therefore represent an
important site for the continued degradation of organic matter. We
suggest that the decline in TP concentration during this process
reflects the limited ability of the sediment to permanently retain all
the deposited P (Fig. 2).
During the 20th century, P loading to the Baltic Sea increased 8fold until the 1980’s (Larsson et al., 1985). During the last two
decades, external loading has stabilized, both from the major
freshwater 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 to
have been fairly constant over the time period representing the
sediment layers investigated in this study (1980s-present). Therefore, we predict that the surface sediment layers presently rich in P
will loose the potentially mobile P and end up with the same low
concentration found in older sediment layers.
5.1. Limitations in P fractionation
Our data indicate that the potentially mobile P observed in the
top layers of the sediment cores was composed of Loosely-sorbed P,
FeeP, Org-P, and as Res-P fractions. These fractions decreased
initially with increasing sediment depth (age), indicating mobilization and transport to the overlying water column. It should be
noted that some portion of these fractions was found also in deeper
layers, indicating that not all of the P extracted in these steps should
be considered potentially mobile. In deeper sediments, the P
concentration is typically constant with depth, indicating that P
mobilization has ceased and only inert P forms remain. We also
observed that P extracted as AleP (NaOH-rP) showed a higher
concentration in surface sediment when the previous extractions
step (BD-rP) was elevated (Table 2). This phenomenon can be
explained methodologically: remnants of the P-rich solution used
for BD-rP extraction were not completely removed before the next
Table 4
Pools of phosphorus fractions (see Methods) and total amount (TP) of potentially
mobile phosphorus in sediment cores from accumulation areas in the Stockholm
archipelago.
Bul I
Gäl Q
Tor C
Pil A
Loosely-bound P
FeeP
Org-P
TP
(g/m2)
(g/m2)
(g/m2)
(g/m2)
0.1
0.0
0.2
0.0
0.7
0.3
4.6
0.2
1.7
0.9
1.4
1.6
2.5
1.0
7.2
2.9
Deposition
Release
Burial
extraction step (NaOH) was added. A washing step in between
extractions would be necessary to accomplish this, as suggested in
the Psenner et al. (1988) fractionation procedure. Also another
phenomenon can be explained methodologically: the initial
extraction step is performed under oxidized conditions. A shift
from dissolved P to FeeP will therefore occur in anoxic sediment
samples if dissolved iron is available, overestimating FeeP on
behalf of Loosely-bound P (Lukkari et al., 2007a,b). In this context it
is of minor importance since P measured as Loosely-bound P and
FeeP are closely connected. Under oxic conditions, equilibrium
processes maintain a certain portion of the FeeP to be measured as
Loosely-bound P (Rydin and Welch, 1998). Other potential transformations might, however, also occur when exposure low-redox
sediment to air over several weeks (Lukkari et al., 2007b).
5.2. Settling P
The P concentration in the surface sediment layer cannot be
assumed to represent the P concentration in settling matter
(Carignan and Flett, 1981). Dissolved P migrating upward from
anoxic layers in the sediment profile might precipitate together
with iron under oxic conditions and subsequently increase sediment P content. Alternatively, under anoxic conditions, the P
concentration in settling matter might be underestimated due to
a considerable release of dissolved P (Krom and Berner, 1981) from
recently settled labile organic P compounds (Ahlgren et al., 2006;
Reitzel et al., 2007). The assumed TP concentration in settling
matter, 1.7 mg P/g DW, was a yearly average based on Blomqvist and
Larsson (1994) and represents a concentration in between the
measured TP values from the four cores’ surface sediments. The
settling matter concentration is less than the TP concentration
measured in the surface layers of Tor C (2.84) or Bul I (1.77) (the two
sites 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 show
a substantial accumulation of redox-sensitive P (Table 2). To better
constrain the P concentrations in settling matter is critical for the
quantification of P fluxes over the sediment surface.
5.3. Organic P
Some part of settled organic phosphorus will eventually be
mineralized (Ahlgren et al., 2006) and constitutes the source of
dissolved P to the pool of Loosely-bound P and FeeP (Hille et al.,
2005). Although the TP concentrations in our cores stabilized
already after 4e8 years, Org-P actually showed a slow but continuous decrease also in deeper (older) sediment layers. Although the
degradation of organic matter apparently continues also in deeper
sediment layers, it does not seem to be of quantitative importance
for the P budget, as judged from the general lack of TP concentration decline with sediment depth (Table 2). We noted that the
potentially mobile P concentration at Tor C (7.2 g/m2, Table 4) was
similar to potentially mobile P concentrations measured in
moderately eutrophic Lake Erken sediments (5 g/m2). However, in
116
E. Rydin et al. / Estuarine, Coastal and Shelf Science 92 (2011) 111e117
Lake Erken, most potentially mobile P was organic (4 g/m2) (Rydin,
2000; Ahlgren et al., 2005), and data from hypertrophic Lake
Sønderby (Denmark) also indicate that the potentially mobile
sediment P pool (8 g/m2) was dominated by organic P forms
(Reitzel et al., 2005). In the cores investigated in this study, the pool
of potentially mobile organic P was smaller, w1 g P/m2 (Table 4).
The difference in mobile P fractions between the Baltic and the
Lakes Erken and Sønderby may be due to that organic P degradation
is faster in marine than limnic systems. It only took w5 years for TP
concentrations in our Baltic cores to stabilize (Table 3), in
comparison to Lake Erken, where TP concentration stabilized only
after about two decades (Ahlgren et al., 2005). Furthermore, more
complete mineralization of the autochthonous organic matter
might also be expected in brackish sediments compared to lake
sediments according to Caraco et al. (1990). Regardless, P extracted
as organic P was present also in deeper sediment layers in the Baltic
cores, representing P to be permanently buried (Mort et al., 2010).
More detailed knowledge of the origin and degradability of
different kinds of organic P forms are needed to understand the
source and turnover of potentially mobile P.
5.4. FeeP
In Tor C, where surface sediment layers were oxic, the potentially mobile P pool was larger than in other cores. This difference is
explained by a larger FeeP pool (Table 4) that apparently result
from dissolved iron and P migrating upwards from deeper, anoxic
layers that precipitates in the oxic surface layer (Carignan and Flett,
1981; Jensen et al., 1995). A 2-year period of accumulated P
mobilized at a rate of 2.7 g P/m2 yr (Table 3) would roughly
correspond to the observed amount of FeeP (4.6 g/m2) at the Tor C
site (Table 4). Eventually, however, the oxidized surface sediment
layer will become saturated with FeeP (Jensen and Thamdrup,
1993), and P release from oxic and hypoxic sediment can be
expected to gradually reach the same rate as that of constantly
anoxic surface sediments as observed by e.g. Hille et al., 2005. The
transition of oxidized surface sediment, with an accumulated pool
of FeeP, into anoxic sediments will result in a period of pronounced
P release due to dissolution of the accumulated FeeP. In the long
run, however, the release will be determined by the settling rate of
organic P compounds, e.g. phytoplankton, that will degrade,
dissolve and eventually mobilize P. Phosphate adsorption into iron
oxyhydroxides in oxidized surface sediment layers will delay
release, 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 Sea
has been reported. In the coastal zone, at 15 m depth in the Aarhus
Bay (southern Baltic Sea) the pool of redox-sensitive iron bound
sediment P was 5.4 g/m2, and deposition, release and permanent
burial of P was 1.6, 1.0, and 0.6 g/m2 yr, respectively, with
a permanent burial efficiency of 37% (Jensen et al., 1995). Our data
(Table 3) are at the same order of magnitude except for a higher
share of deposited P to get permanently buried. Unlike Aarhus bay,
ongoing land rise after the last glacial period in the northern Baltic
proper cause’s erosion of new bottom areas most likely dominated
by more refractory P forms that settles out together with labile P
forms, such as P in phytoplankton, resulting in a larger share of the
deposited P to get permanently buried. This phenomenon might
explain higher burial efficiencies also reported from the north
eastern part of the Baltic proper that varied between 41 and 93%
(Lukkari et al., 2009a,b) resembling burial efficiencies between 51
and 66% found in this study (Table 3).
We calculated long-term average P release rates between 1.0
and 2.7 g P/m2 yr (Table 3). These figures are comparable to the rate
obtained from organic P degradation in a sediment profile offshore
of the Stockholm archipelago, where the long-term average release
was estimated to be 0.6 g P/m2 yr (Ahlgren, 2006). They are also
comparable to rates reported by Mort et al. (2010) who calculated
diffusive fluxes of 0.5 and 3 g P/m2 yr from north of Gotland and the
Landsort Deep, respectively. In the Eastern Gotland Basin, however,
at depths >150 m, P deposition averaged at 0.20 g P/m2 yr of which
only one-third was judged to get permanently buried, and subsequently two-third was released as phosphate (Hille et al., 2005).
One explanation for the higher release rates obtained in coastal
areas seems to be higher sediment accumulation rates compared to
the open Baltic.
6. Conclusion
We observed a decline in sediment TP concentration with depth
in Baltic Sea coastal sediment cores, indicating the limited ability of
the sediment to retain deposited P. This difference in P concentration
represents the total pool of P to be released, and the long-term
average sediment P release can be calculated using the sediment
accumulation rate. At sites where surface sediments turned oxic
a few years ago, we measured substantial amounts of P bound to
iron, suggesting that mobilized phosphate was temporary trapped in
the surface sediment instead of being released to the water column.
Acknowledgments
This study was funded by the Swedish Environmental Protection
Agency and FORMAS. We thank the Erken Lab for analyses, Dan
Lindgren for drawing the map, and Cayelan Carey for her edits on
the manuscript.
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