Estuarine, Coastal and Shelf Science (1989) 29,447-456

Adsorption Capacity of Phosphorus in Baltic Sea Sediments

R. Carman and F. Wulff Department of Geology, University of Stockholm, S-l 06 91 Stockholm and “Ask6 Laboratory, Institute of Marine Ecology, University of Stockholm, S-106 91

Stockholm, Sweden

Received 23une 1988 and in revisedform 173~1~ I989

Keywords: adsorption; phosphorus; sediment; budget; Baltic Sea

Nutrient budgets of the Baltic Sea indicate that an increased loading of phos- phorus to a great extent is counteracted by adsorption to the sediments, resulting in a comparatively small net increase of the phosphorus-concentration in the water column. The permanent salinity stratification and increasing eutrophi- cation of this sea has caused almost permanent anoxic conditions which limits the phosphorus-storage capacities in the deep basins below the halocline. The

phosphorus-loading has increased about eight-fold during this century and does not show any sign of declination.

The questions then arise-‘ what type of sediments store the phosphorus, what are the binding mechanisms and is the maximim storage capacity approached? ’

In an attempt to partly answer these questions, the phosphorus adsorption capacities of three different Baltic Sea sediments have been investigated. The greatest amount of adsorbed phosphorus, measured as mol g-’ dry substance, is found for an accumulation bottom from the deeper part of the Baltic Sea and the lowest for a transportation bottom above the halocline. However, if the rates are recalculated as phosphorus adsorption per area, which is more pertinent from an ecological point of view, i.e. most phosphorus is adsorbed in the shallow sediments.

A comparison of the experimental data and ambient conditions in the water and sediments indicate that adsorption of phosphate from the water column plays a minor role in the phosphorus budget of the Baltic Sea. Only the assimi- lation of phosphate and subsequent sedimentation and decomposition of organic matter at the sediment surface will result in a sufficiently high sediment pore water concentration to explain the large sink of phosphorus to the Baltic Sea sediments calculated from input and output data.

Introduction

The semi-enclosed brackish Baltic Sea represents a unique environment both in terms of

its impoverished fauna and flora, hydrography and biogeochemistry (see for instance Voipio, 1981; Kullenberg, 1981; Elmgren, 1984; Stigebrandt & Wulff, 1987).

Calculations of the overall nutrient budgets (Larsson et al., 1985; Anderson, 1986) show that, in spite of the large loading, the concentrations in the water mass do not increase in

0272-7714/89/050447 + 10 $03.00/O @ 1989 Academic Press Limited

448 R. Carman & F. Wulff

TABLE 1. Bottom water characteristics at the three sampling stations along the depth gradient in the Baltic Proper

Station

1 2 3

Latitude Longitude

Depth (4 Bottom water Temp (“C) Salinity Oxygen (mg 1-l) Nitrate (pm01 1-l) Nitrite (pm01 I-‘) Ammonia (pm01 1-l) Phosphate (pm01 I-‘)

NW40’88” N58”31’19” N58”29’73” E17”50’38” ElV02’60” E18”06’77”

47 82 130

4.1 4.8 4.9 6.99 8.28 9.46

13.7 6.3 1.8 1.9 8.9 10.5 0.2 0.3 >O.l 0.6 0.5 0.8 0.9 2.7 3.7

corresponding rate. The outllow of phosphorus (P) through the Danish Belts in the southern area can not explain the difference. Neither does removal of phosphorus by net sedimentation and fishing activities account for the discrepancy.

There are several mechanisms besides biological uptake that could be responsible for loss of phosphorus from the water column. One explanation may be that the sediment is a trap and adsorbs the P (Hingston et al., 1967, Jansson, 1978; Krom & Berner, 1980). Many studies suggest that P co-precipitates with iron and manganese oxides and hydroxides already in the water column (Jitts, 1959; Golterman, 1980). Direct sorption to suspended clay particles and organic matter may also be important (Carrit & Goodgal, 1954). The sedimentation of organic and inorganic aggregates to the bottom would then be the major mechanism responsible for reducing the total P-concentration in the water column. The oxidized top layer of the sediment would then act as a sink where mineralized phosphate rapidly precipitates, primarily with iron (Einsele, 1936; Mortimer, 1941, 1942); forms calcium-minerals like apatite (Atlas et al., 1977) or different iron-minerals like vivianite and strengite (Nriagu & Dell, 1974). It also appears likely that sorption to clay particles and organic aggregates in the water column could be an important mechanism for removal (Pomeroy et al., 1965). Large amounts are released when the sediment becomes anoxic. However, oxic sediments can also release phosphate (Bostriim et al., 1982). The role of these mechanisms vary in importance and have to be empirically determined in different environments. The aim of this study has been to determine the amount of phosphorus which could be adsorbed to the different sediment types in the Baltic Sea.

Materials and methods

Field methods Sediment cores were taken in the Ask&Landsort area in the northern Baltic Proper at three different depths (Table 1). We used a modified Kajak gravity corer (Blomqvist & Abrahamsson, 1985) equipped with an acrylic liner (8 cm diameter, 50 cm length). Temperature, salinity, oxygen and nutrient concentrations in the water overlying the sediment in the cores was determined. The redox condition of the sediment was measured with a platinum electrode which slowly, with a constant speed of 1 cm h-l, penetrated the

Phosphorus in Baltic Sea sediments 449

sediment down to 4 cm. The top 6 cm of the sediment was sliced in 2 cm sections, and analyzed further as described below. Adsorption-desorption experiments were done using the upper 2 cm, because we assumed that this was the maximum thickness of the oxidized layer active in nutrient exchange with the overlying water, even when higher ambient oxygen concentrations occur in the water column. The 2 cm layers thus represent a gradient from newly settled particles to very old material (Ignatius et al., 1981) and the description of the experiments that follow will yield an average estimate on the adsorption capacity of these particles.

Laboratory methods

The water content of the sediment samples was measured by drying the samples in an oven at 105°C for 6 h. Organic content was measured as the loss of weight by ignition after heating at 550°C for 2 h. Sediment subsamples representing the uppermost two cm were freeze-dried, homogenized and used in the adsorption experiments.

Ortho-phosphate standard solutions were prepared by dissolving KH,PO, in artificial sea water to concentrations ranging between 0.001-25 mM. The pH was adjusted to 7.5-7.8 in all standards in order to simulate natural Baltic Sea Water. Approximately 1 g of the freeze-dried sediments was added to 50.0 ml of the standard solution of each concen- tration (given in Figure 2). The suspensions in Erlen meyer flasks were then placed on a shaking table at 4°C to simulate actual bottom temperature. The experiment lasted for 3 weeks to establish an equilibrium between dissolved and adsorbed phosphate under oxidized conditions. The sediment-water mixture was centrifuged on a Beckman J2-21M at 1800 g for 20 min and the phosphate concentration in aliquots of the clear supernatant was determined in duplicate samples. Phosphorus was measured (before and after shaking) spectrophotometrically at 880 nm using the molybdenum-blue complex method (Murphy & Riley, 1962).

‘Total’ phosphorus and iron in the sediments was measured after leaching the sample in 5 M HNO, (Love, 1967; Hallberg, 1974). Analysis of iron was carried out on a Varian AA 475 atomic absorption spectrophotometer according to the manual supplied by the manufacturer.

Results

Field data The physical and chemical conditions at the three sampling stations as well as sediment characteristics are shown in Tables 1 and 2.

The phosphorus and iron content, calculated per g dry weight of the sediment (g dw ’ j has been recalculated to units per m*, using the bulk densities and a mean inorganic sediment density of 2.6 g cmm3 (from HQkanson & Jansson, 1983). Station 1 displayed the typical characteristics of a transport bottom and Stations 2 and 3 the characteristics of an accumulation bottom according to the classification by Hakanson and Jansson (1983).

At Station 1 the sediment was well oxygenated down to 20 mm (redox > 220 mV). The fraction down to 20 mm was sand-slit, but the uppermost part of the sediment had a thin (1 mm) light brown layer of organic material/bacteria. Below 20 mm the colour was light grey, and the particle size decreased (silt-clay).

At Station 2 an oxidized and light brown layer was found down to ca 10 mm underlain by a light grey to black layer. Particle size was silt-clay. At Station 3 only the uppermost

450 R. Carman Q F. Wulff

TABLE 2. Sediment characteristics (down to 6 cm) at the three sampling stations along the depth gradient in the Baltic Proper

Station

1 2 3

Sediment Interval (cm) Water content

(%w) Organics (%dw) Bulk density

(g cm-3 Total-P

(pm01 g-l dw) Total-P (g mm*) Total-Fe

(mm01 g-’ dw) Total-Fe (g m-*)

O-2

28.6 54.2 2.7 2.6

1.75 I.38

29.5 27.7 32.9 50.3 35.2 33.2 41.6 37.1 37.4 19.0 10.9 13.2 6.5 5.8 8.4 2.3 4.5 6.5

0.13 0.29 0.52 0.35 0.41 0.39 0.34 0.39 0.44 181 204 375 82 125 178 35 85 133

2-4 4-6 O-2 2-4 4-6 O-2 2-4 4-6

53.2 81.1 4.9 15.6

1.38 I.11

76.3 66.8 91.1 12.2 8.2 20.7

1.15 1.23 I-05

82.5 76.4 11.7 8.8

1.11 1.15

few millimeters were oxygenated and showed light brown colour. Beneath that level the sediment was reduced and formed black sulfides (mainly iron sulfides). The particle size distribution was similar to Station 2 (i.e. silt-clay).

Experimental data Several adsorption isotherms for ortho-phosphate are presented in the literature, see for instance Stumm and Morgan (1981) and Atkins (1983). The most commonly used is the Langmuir equation (Langmuir, 1918) based on gas adsorption (Langmuir, 1918; Atkins, 1983). The Langmuir equation allows dilute equilibrium phosphorus concentrations and consequently smaller amounts of adsorbed phosphorus. However, the Langmuir equation is an approximation since it takes no account of surface charges (Bowden et al., 1974). The Langmuir equation is linear for low to moderate equilibrium concentrations of phosphor- us. Therefore, we use a simple linear regression to calculate the k-values of this study.

Figure 1 shows the various adsorption capacities, i.e. saturation limits of the three different sediment types. The plotted points are mean values from duplicate experiments. The variations between the replicates are in all cases less than 5%.

The sediments from the accumulation bottom reach saturation around 10 mM (Station 2) and 15 mM (Station 3), respectively, whereas the sediment from the shallow trans- portation bottom reach equilibrium below 10 mM (Station 1). The highest adsorption capacity, about 100 umol g-l, was found at Station 3.

The slopes (k-values) of phosphorus adsorption (pm01 PO,-P per g dry sediment) of the three sediments at phosphate concentrations between 0 and 50 PM, are clearly different (Figure 2). The relatively coarser sediment of the transportation bottom showed a lower adsorption coefficient than those of the two accumulation bottoms. The more shallow accumulation bottom (Station 2) had a higher background concentration of total phos- phorus than the deeper station (Station 3) resulting in lower adsorption capacity (fewer adsorption sites left).

The relative differences between the sediments, in terms of phosphorus equilibrium, are similar at both low and high concentrations. The iron content of the two accumulation types sediments are similar and considerably higher than in the shallower transport type

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452 R. Carman t3 F. Wulff

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sediment (Table 2). At low equilibrium concentration of phosphate (less than 8 pmol 1-l) at Station 2, there is a ‘ negative ’ adsorption, i.e. desorption from the sediment to the water phase. Desorption was also found at zero phosphate concentration in the Station 1 and 3 experiments.

Phosphorus in Baltic Sea sediments 453

If adsorption per gram dry substance is recalculated into adsorption per area of sea bed, the relative importance of the different stations in the sediment-water nutrient exchange becomes apparent (Figure 2). The shallow transportation bottom (Station 1) has the highest adsorption capacity per unit area. In spite of the higher adsorption capacity per unit weight of sediment at the deeper stations, the high water content results in a low adsorption per unit area.

The regression lines (y = Izx + Q, for the interval shown in Figure 2) are:

Station 1: y= 9.2~ Station 2: y=3O.lx-0.23 Station 3: y = 42.3x- 0.04

where y is adsorption (urn01 PO,-P per g dry sediment) at different equilibrium concentrations (l.tmol ml-’ PO, - P).

Discussion

The adsorption capacity is dependent on many factors e.g. pH (Jitts, 1959; Hingston, et al.,

1967; Parfitt, 1977; Ku et al., 1978), Eh (Ku et al., 1978; Krom & Bemer, 1980), tempera- ture (Ku et al., 1978), chemistry of the sediments (Shukla et al., 1971; Berner, 1973; Parfitt, 1975, 1977) duration of incubation (Berner, 1973; Hwang et al., 1976), equilib- rium concentration and particle size (specific area) (Borggaard, 1983). Loss of phosphorus due to potential biological activity or adsorption on the glass walls of the bottles is, however negligible (Berner, 1973).

Pure minerals, e.g. iron- and aluminium-oxides/hydroxides have large adsorption capacities (Hingston et al., 1974; Lijklema, 1980; Borggaard, 1983; Lucotte & d’Anglejan, 1988). In oxic estuarine sediments k values (ml g-l, the slope in the Langmuir or regression equations) varies between 50 (Krom & Berner, 1980) to nearly 4000 (Jitts, 1959) and in oxic iron rich oceanic sediments the K-value can reach almost 5000 (Berner, 1973). For oxic lake sediments values between 20-40 (Hwang et al. 1976; Li et al., 1972) are reported. In anoxic environments the k-value becomes much lower (Li et al., 1972; Krom & Berner, 1980) which probably results from reduction of high adsorbing iron (III) oxide-hydroxides to more water soluble ferrous ion.

Although it seems like oxic estuarine and oceanic sediments have the highest adsorption capacities, few studies are available for an explanation that is satisfactory to the variations found. In most oxic environments, the iron content of the sediment seems to be the prime factor that determines the sorption capacity (Berner, 1973; Khalid et al., 1977; Lucotte & d’Anglejan, 1988). However, in eutrophic systems, most of the available adsorption sites may be occupied, which explains the low K-values reported (Bostrom & Pettersson, 1982). This may also be the explanation for the comparatively low values reported here for the Baltic Sea.

How important is the adsorption of phosphate from the overlying water to oxidized sediments? We have calculated the total phosphate uptake for sediments in the Baltic Proper using the following assumptions: Station 1 represents transport bottoms which include all bottoms down to 75 m. These sediments above and below the halocline (situ- ated at 60-70 m) are usually well oxidized. Station 2 represents accumulation bottoms in the area between 75-l 10, an area which is usually oxidized. Station 3 represents all deeper bottoms which are exposed to alternating reduced and oxidized conditions.

454 R. Carman & F. Wulff

TABLE 3. Calculated phosphate sediment uptake in the Baltic Proper, using experimen- tally determinated uptake rates of two ambient concentrations

Depth (4 km* 10-3

Area Uptake

% Cone (PM P) kg P kn-2 tonnes P total

O-75 195 73 1.4 9.2 1794 14.0 99.0 19 300

75-110 32 12 1.4 - 12.2 -391 14.0 12.4 398

110-456 40 15 1.4 0.3 11 14.0 8.0 320

Total 267 100 1.4 - 1414 14.0 - 20018

The percentages of the total bottom areas covered by the three depth intervals have been calculated from the hypsographic curve in Stigebrandt and Wulff (1987) (Table 3). We have then calculated sediment uptake rates using two concentrations of phosphate in the overlying water column and the experimental adsorption results of this work (Table 3). For the shallow depth interval (Station 1) we have assumed that the first 2 cm of the sediment is well oxidized and participate in the phosphate uptake. For the middle range (Station 2) we assumed a 1 cm sediment oxidized layer and for the deepest part (Station 3) a 0.5 cm layer. Calculations of total amount of phosphate in the Baltic Proper (Wulff & Rahm, 1987) gives a mean concentration of 1.4 pmol 1-l during winter time. However, seasonal and temporal variations are great, between 04-0.8 pmol 1-l in surface waters in winter time and less than 0.01 pmol 1-l in summer time due to primary production. Below the halocline the concentration increases with depth to 2-4 pmol 1-l (Nehring et al., 1984).

The phosphorus concentration close to sediment surface and in the pore waters can be considerably higher (Holm, 1978). Therefore, we also calculated uptake rates using a concentration one order of magnitude higher, 14 umol l-l, to get a maximum estimate of sediment adsorption.

These tentative calculations show that the two ambient phosphate concentrations yield a total uptake to the sediments of 1414 and 20 018 tons yr-‘, respectively (Table 3). A comparison with the phosphorus budget in the Baltic Proper compiled by Wulff and Stigebrandt (1989) which suggest an annual loss of 36 700 tons. Thus, none of our calcu- lated estimates, based on the chemical sorption capacities, are large enough to explain losses to the sediments.

One possible explanation for this discrepancy would be errors in the phosphorus budget calculation by Wulff and Stigebrandt (1988) where the phosphorus-losses to the sediment have been calculated as the difference between inputs, advective outputs and concen- tration changes. However, even if we allow an error of 50% in these calculation, the residual sink to the sediment is still to large to be explained by adsorption of phosphate from the water column. The ambient concentrations are too low. Only pore waters reach a concentration level where phosphate adsorption would be sufhcient to account for the accumulation that occur in Baltic sediments.

Although rough, these calculations indicate the overwhelming importance of shallow sediments as the major sink for phosphorus, contrary to the classical view. From a geological view, they are to a large extent ‘ erosion ’ or ‘ transport ’ bottoms whereas the

Phosphorus in Baltic Sea sediments 455

deeper areas are ‘ accumulation ’ bottoms (HHkanson & Jansson, 1983) where permanent storage of phosphorus would occur. Our calculations of the phosphorus uptake of shallow bottoms, based on only one sample, could be greatly erroneous but it does not change the overall conclusions. For the deeper situated sediments, the regional variations are less (Holm, 1978) and the extrapolations from two cores, which represent only about 25 O. of the total area, are likely to be more accurate. In the Baltic Sea deep basins, where anoxic conditions now prevail, accumulation of phosphate in the sediments is limited, because most areas below the halocline are anoxic or have a very thin oxidized layer. We have also showed that the actual storage and uptake of phosphorus per unit area in these sediments is rather small, due to high water content compared to more shallow bottoms.

The experiments also indicate that shallow oxidized sediments are capable to continue to act as traps for phosphorus and that higher ambient concentrations would increase adsorption. Saturation level is not reached until the concentration is in the mM rather than the PM range (Figure 2). Furthermore, the sediments are subjected to a continuous input of not only phosphorus but also iron and other adsorbing agents. On the other hand, if phosphorus concentration decreases, the experimental evidence indicate that a loss of phosphate will occur into the water column (at concentration < 1,2 and 8 PM at Stations 1, 2 and 3, respectively; Figure 2). Thus, a reduction of the phosphorus load will not immediately result in reduced phosphorus concentrations. However, the net exchange due to chemical equilibria between dissolved and adsorbed phases of phosphate, will be very small at concentrations that normally prevail above the sediment.

To conclude, the main mechanism responsible for the depletion of phosphate in the water column is likely to be biological assimilation and sedimentation of organic phosphorus. Only decomposition of organic matter at the sediment surface can increase phosphate concentrations in the pore water, sufficient to cause an adsorption to the oxidized layer of the sediment.

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