The use of tracers to evaluate the importance of bioturbation in remobilising contaminants in Baltic sediments

Estuarine, Coastal and Shelf Science 66 (2006) 123e134 www.elsevier.com/locate/ecss

The use of tracers to evaluate the importance of bioturbation in remobilising contaminants in Baltic sediments C. Bradshaw*, L. Kumblad, A. Fagrell Department of Systems Ecology, Stockholm University, 10691 Stockholm, Sweden Received 21 February 2005; accepted 12 August 2005 Available online 26 September 2005

Abstract Large areas of the bottom sediments of the Baltic Sea are temporarily or permanently anoxic. These sediments are also an important sink for a variety of contaminants. Reoxygenation of bottom waters allows recolonisation by benthic infauna, which may have important implications for the fate of buried contaminants. This study used tracers to experimentally examine the role of bioturbation by benthic infauna in transporting sediment-associated contaminants in the Baltic Sea. Three different tracer methods were used in two experiments, using three key Baltic macrofaunal species: the amphipod crustacean Monoporeia affinis; the Baltic clam Macoma baltica; and the priapulid worm Halicryptus spinulosus. In the first experiment, a reoxygenationerecolonisation scenario was recreated in the laboratory, using hypoxic sediment cores collected in the field, to determine if there was remobilisation of buried 137Cs from the Chernobyl nuclear accident in 1986. The potential for the infauna to bury newly settled surface contamination was also investigated, using a fluorescent particle tracer. In the second experiment, artificially-created radiolabelled tracer layers (14C and 51Cr) were used to quantify both upward and downward movements of organic matter and sediment-associated contaminants by bioturbation. In both experiments there were clear visual differences between the sediment effects of the three species. Halicryptus spinulosus buried deepest into the sediment, creating a network of burrows, Monoporeia affinis burrowed actively in the upper few centimeters of the sediment, and Macoma baltica was quite stationary, but appeared to filter- and deposit feed at the sediment surface. Mixing depths in the hypoxic sediment varied from 4.0 G 3.5 cm for M. baltica to 7.8 G 2.1 cm for H. spinulosus. Biodiffusion rates (Db) were similar for all treatments but biotransport rates (r) were significantly different between treatments, mainly due to a high r value for H. spinulosus. In the experiment with radiolabelled tracer layers, 51Cr was transported more than 14C, and tracer originally at the surface transported more than tracer buried 4 cm below the surface. There was also transport of all tracers in treatments without added macrofauna. The most likely explanation is bioturbation by the meiofauna that were undoubtedly present in both experiments. Bioturbation by macrofauna both buries surface contaminants and remobilises those that are buried, but the effects are small and on a similar scale to transport caused by meiofauna. In addition, 137Cs profiles at the hypoxic site indicated that resuspension and redeposition of sediment by physical processes had occurred, and also showed that contaminants from the last 40 years were still present in the top 5e10 cm of the sediment, well within active mixing depths. At this site, as at many others in the Baltic, physical processes are likely to be far more important than biological processes in the redistribution of contaminants on a decadal timescale. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: benthic infauna; tracer techniques; bioturbation; remobilisation; contaminants; hypoxia; Baltic Sea

1. Introduction The Baltic Sea is a species-poor, semi-enclosed brackish sea, which owing to a densely inhabited and industrialised * Corresponding author. E-mail address: [email protected] (C. Bradshaw). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.08.002

drainage basin has extensive contamination and eutrophication problems (Jansson and Dahlberg, 1999; Elmgren, 2001). The bottom sediments of the Baltic contain a catalogue of past pollution, including organic contaminants (e.g., PCBs, PBDEs, PAHs), heavy metals (e.g., Hg, Cd), and radionuclides (e.g., 134Cs and 137Cs from the Chernobyl accident) (Jansson and Dahlberg, 1999; Elmgren, 2001; HELCOM,

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2003). Sediment-associated contaminants may be remobilised into aquatic food chains by processes such as sediment resuspension (Fileman et al., 1991; Matisoff, 1995), bioturbation (Matisoff, 1995) and bioaccumulation into benthic organisms (Fisher and Reinfelder, 1995; Kumblad et al., 2005). This may happen even after inputs from anthropogenic sources of contamination have been reduced or ceased. In recent decades, up to 100 000 km2 of the deep waters of the Baltic Proper have become semi-permanently hypoxic or anoxic as a result of reduced water inflow from the North Sea (Jansson and Dahlberg, 1999; Karlson et al., 2002). Eutrophication also leads to increased sedimentation and increased oxygen consumption at the seabed (Jonsson and Carman, 1994; Bonsdorff et al., 1997; Karlson et al., 2002). As a result, large areas are now completely devoid of macroscopic life (Bonsdorff and Pearson, 1999; Karlson et al., 2002). Improved water quality or an influx of oxygenated water from the North Sea through the Danish Straits, which occurs naturally, if irregularly (Jansson and Dahlberg, 1999), could increase oxygen concentrations in bottom waters and lead to rapid recolonisation by bioturbating benthos. Species that are tolerant of lowoxygen environments (e.g., Monoporeia affinis and Halicryptus spinulosus (Oeschger et al., 1992; Johansson, 1997) and the invasive polychaete worm Marenzelleria viridis (Schiedek, 1997) are especially likely to exploit these areas. Recolonisation and bioturbation have the potential to release stored pollutants through physical disturbance and reoxygenation of surface sediments (Peterson et al., 1996; Gunnarsson et al., 1999; Christensen et al., 2002). The Baltic benthic fauna is dominated by very few macrofaunal species (Elmgren and Hill, 1997). Although diversity is low, the densities of bioturbators can be very high (many thousand Monoporeia affinis or Macoma baltica mÿ2 (Aarnio et al., 1998; Ejdung and Elmgren, 2001; Bergstro¨m et al., 2002). As a result, their ability to turn over and oxidise large quantities of surface sediments is potentially large. This paper presents data from two experiments designed to investigate the importance of bioturbation in redistribution of contaminants in Baltic sediments. The three species used in the experiments were selected as they are key species in the Baltic and have different feeding and burrowing behaviours. The deposit-feeding amphipod Monoporeia affinis mostly burrows in the topmost 5 cm of the sediment (Hill and Elmgren, 1992), but feeds mainly on surface sediments (Byre´n et al., 2002). The facultative deposit-feeding clam Macoma baltica lives a few centimetres down in the sediment and primarily feeds on organic particles deposited on the sediment surface, although it also has the ability to filter feed (Lin and Hines, 1994). The omnivorous priapulid worm Halicryptus spinulosus may burrow down to a depth of at least 25 cm in search of infauna and detritus on which to feed (Powilleit et al., 1994; Aarnio et al., 1998). To investigate these processes, three different tracer methods were used in two experiments. The first experiment examines the extent to which reoxygenation of anoxic sediments and recolonisation by three key infaunal species affect the remobilisation of buried 137Cs from the Chernobyl nuclear

accident in 1986. The potential for these animals to bury newly settled surface contamination was also investigated for these species in the Baltic Sea Proper, using a fluorescent particle tracer. The second experiment used artificially created radiolabelled tracer layers to quantify both upward and downward movements of organic matter and sediment-associated contaminants by bioturbation. Tracers are a useful tool for describing and quantifying biological and physical processes in sediments. Different tracers can provide information about different types of processes and different timescales, depending on whether they are chemically or physically stable or unstable, associated with sediment particles, organic material or are in solution, or biologically available. Bioturbation and short-term sediment mixing have been studied by measuring the sediment depth distributions of the relatively short-lived radioisotopes 234Th, 7Be, 137Cs and 210 Pb in benthic samples (e.g., Feng et al., 1999; Berg et al., 2001; Santschi et al., 2001; Green et al., 2002; Shull and Mayer, 2002). Alternative methods involve adding tracers to experimental systems and following their distribution over time. A range of such tracers has been successfully used: radionuclides, such as 137Cs, fluorescent particles, glass beads, and bromide (Gerino et al., 1998). Organic molecules or particles, which may also be radiolabelled with 14C, can be used to follow biological processes such as feeding and mineralisation of organic matter (Sun et al., 1993; Byre´n et al., 2002). In the experiments described here, different tracers were used to examine the transport of sediment, organic material and a heavy metal. The dual characteristics of radionuclides, as tracer tools and pollutants, were also exploited, both to quantify bioturbation processes and to determine the importance of bioturbation in the transport of radionuclides, and other contaminants, in Baltic sediments. 2. Materials and methods 2.1. Experiment I: luminophores and 137Cs as tracers of bioturbation in naturally hypoxic sediments 2.1.1. Field sampling 2.1.1.1. Sediment cores. Sediment cores were taken with a Kajak-type gravity corer (Blomqvist and Abrahamsson, 1985) (8 cm internal diameter) from a hypoxic seabed (c. 55e60 m water depth) to the west of Landsort (58  45.52#N 17  50.81#E), NW Baltic Proper, Sweden. The cores were immediately sealed with rubber stoppers to prevent oxygenation of the water column. The upper sediment was black, and white sulphur bacteria were abundant on the sediment surface of most cores. The cores were taken immediately to the nearby Asko¨ Laboratory where natural conditions were maintained by keeping the cores in a thermoconstant room (7e8  C with green light/dark cycle) until the experiment began 4 days later. 2.1.1.2. Animals. Sediment was sampled with a benthic sled (Blomqvist and Lundgren, 1996) from the top 2 cm of the

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seabed at 22 m depth in Ha˚llsviken (58  49.70#N 17  31.67#E), NW Baltic Proper, Sweden, and sieved through a 1 mm mesh to collect the macrofauna. Macoma baltica, Monoporeia affinis and Halicryptus spinulosus were kept in fresh sieved sediment and aerated brackish Baltic water (c. 6&) in the same conditions as described above, until the experiment began.

sediment and the fluffy surface layer of the sediment (the top few mm) was removed with a syringe. The rest of the sediment was sectioned at 1 cm intervals by extruding it upwards from the core tube and sectioning it with a thin plastic slicer. A clean slicer was used for each sectioning to avoid contamination between slices.

2.1.2. Experimental set-up

2.1.3.2. Slice processing and sample analysis. A 5 ml sample was taken from the centre of each sediment slice, weighed wet, dried at 60  C for 48 h, weighed again, then burnt at 500  C for 6 h and re-weighed. These measurements were used to obtain values for water content, organic content and porosity ((wet weight ÿ dry weight)/bulk volume (Baskaran and Naidu, 1995)) of the sediment. The outer 2e3 mm of the remaining slice were removed and discarded, in order to eliminate smearing artefacts. The remaining sediment was dried at 60  C for 24 h and then mechanically disaggregated into a fine powder. A subsample (c. 3 g) was taken for fluorescent particle analysis, and the rest saved in plastic containers with airtight lids prior to gamma spectrometry analysis.

2.1.2.1. Cores. The experiment consisted of five treatments, each of four replicate cores (Table 1), which were arranged randomly in racks in the thermoconstant room. The water volume above the sediment (0.75 l) was equalised with Baltic seawater in all cores, except cores in the anoxic treatment, which were kept sealed throughout the whole experiment. An aeration system, aerating each core individually, was connected to all the oxic treatments and left to bubble for 3 days before the start of the experiment. Oxygen, pH, salinity and temperature of the water of the oxic cores were measured regularly, and within 2 days the O2 concentration reached a constant level of about 12 mg O2 lÿ1. Throughout the experiment, there were no differences in these 4 parameters between treatments. 2.1.2.2. Fluorescent tracer. A suspension of fluorescent particles (2.5 ml suspension containing 42 mg dw tracer per core) was added with a syringe just below the water surface to all the oxic cores, and allowed to settle out on to the sediment surface. 2.1.2.3. Animals. The animals were carefully added to the sediment surface of the cores. Animal densities used in the experiment (40 Monoporeia affinis, 5 Macoma baltica and 4 Halicryptus spinulosus per replicate) were chosen to be equivalent to high natural densities (8000 M. affinis mÿ2, 1000 M. baltica mÿ2, 800 H. spinulosus mÿ2) (Aarnio et al., 1998; Ejdung and Elmgren, 2001; Bergstro¨m et al., 2002; Kotta and Olafsson, 2003), and to generate approximately the same wet biomass in each experimental container. No macrofauna were added to the azoic or anoxic treatments. 2.1.3. Sampling and sample analyses 2.1.3.1. Core slicing. The experiment was terminated 22e25 days after the animals had been added. The water above the Table 1 Summary of the experimental design (Experiment I) Treatment

M. affinis

M. baltica

H. spinulosus

Azoic

Anoxic

Replicates Oxygenation Fluorescent tracer Added animals (ind coreÿ1) Abundance (ind mÿ2) Biomass (g wet weight)

4 Yes Yes 40

4 Yes Yes 5

4 Yes Yes 4

4 Yes Yes None

4 No No None

8000

1000

800

e

e

0.28

0.56

0.44

e

e

2.1.3.3. Fluorescent particle analysis. Two milligrams of dried ground sediment was taken from each sample and mounted on a microscope slide with a drop of water. The fluorescent particles were counted at 10! magnification using an Olympus BH-2 RFC fluorescence microscope with a mercury burner. Ten fields of view (total area 28 mm2) were counted on each slide. When more than 100 fluorescent particles were visible in a field of view, counting became difficult, so the maximum count was set to 100 particles. The average amount of background fluorescence (caused by small amounts of fluorescing organic material) was estimated by identifying the depth at which there was a sharp decrease in the number of fluorescent particles in the four azoic cores and averaging the number of fluorescent particles found below this point. This figure (6 particles per field of view) was subtracted from all counts. The mixed depth in the other three treatments was then defined as the depth where no fluorescent particles were found. The luminophore distributions were used in a gallery-diffusor model (Francois et al., 2002) to quantify both biodiffusive processes (biodiffusion coefficient Db), which are predominant in the upper sediment, and non-local mixing (biotransport coefficient r), which occurs in the deeper sediment. These coefficients were compared between treatments using one-way ANOVAs and mean values. Note that Halicryptus spinulosus replicate 1 was omitted from all luminophore comparisons, as the tracer distribution was so different to the observed bioturbation that it was thought to be unreliable. Macoma baltica replicate 3 was omitted from Db comparisons as the abnormally low surface counts severely distorted the Db value. 2.1.3.4. Gamma spectrometry. Two gamma spectrometers were used for the 137Cs analyses: an Ortec PB-210 LB-SV30 (Swedish Museum of Natural History, Stockholm, Sweden) and an EG&G Ortec Model 919 (Ukrainian Hydrometeorological Research Institute, Kiev, Ukraine). 137Cs activity was

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determined via its gamma emissions at 662 keV. Counts were standardised for time and weight of sample. The absolute efficiencies of the detectors for different sample mass and volume were determined using calibrated sources and sediment samples of known activity. Corrections were made for the effect of self-absorption of low energy g-rays within the sample. 2.2. Experiment II: 51Cr- and in experimental microcosms

14

C-labelled tracer layers

2.2.1. Experimental set-up This experiment used small (1 l) microcosms containing sediment and radiolabelled tracer layers. The sediment and animals were collected on the 15th September 2003 at the same location and using the same methods as described for Experiment I. In this case, the sieved sediment was also used in the experiment, as follows. Each microcosm was set up to contain (from the bottom up) 1 cm sediment, a tracer layer, 4 cm sediment, a second tracer layer, 4 cm Baltic seawater. The tracer layers were 1e2 mm thick frozen discs of either 51Cr-labelled sediment, or a 14C-labelled algaeesediment mixture, and each microcosm contained one of each type, i.e. either 51Cr on top and 14C below or vice versa (Table 2). Animals were then added to each microcosm (Table 2), except for the ‘Azoic’ treatment, and all microcosms were then individually aerated. The experiment ran for 20 days in a thermoconstant room at 8  C with a 14 h green light: 10 h dark cycle. 14C-labelled algae was chosen as a tracer, as its redistribution could reflect feeding and other biological processes. Chromium was chosen as an example of an important metal pollutant in the Baltic, and because it is available as a radioisotope (51Cr) whose half life is sufficiently short (27.7 d) that radioactive decay would effectively remove it from the samples after several months, enabling measurement of 14C in the same samples. The hexavalent form, Cr(VI), was chosen as it is more bioavailable than the trivalent form, and therefore a more important toxin for aquatic organisms (e.g., Wang et al., 1997). 2.2.2. Sampling and analysis After 20 days, a water sample was taken from each beaker and a small sediment core was taken from each beaker using a 50 ml open-ended plastic syringe. The small cores were sliced into 6 ! 1 cm thick pieces and subsamples taken from these for radioisotope analysis by scintillation counting. All animals were collected from each beaker and allowed to purge their gut contents for 24 h in clean water. 14C and Table 2 Experimental set-up (Experiment II) with various combinations of animals and tracer treatments. All treatment combinations had three replicates Treatment

51 Cr at surface and 14C buried

14 C at surface and 51Cr buried

C80 Monoporeia affinis C10 Macoma baltica C10 Halicryptus spinulosus No added macrofauna (azoic)

3 3 3 3

3 3 3 3

51

Cr activities in all samples were measured by liquid scintillation analysis (for detailed methods see Kumblad et al., 2005). All samples were run once in October 2003 and again in August 2004. In the second run it was assumed that there was no 51Cr present, as 10 half lives had passed. The August 2004 counts were, therefore, subtracted from the October 2003 counts in order to calculate the amount of 51Cr originally present in each sample. Redistribution of the original tracer layers into the surrounding sediment is expressed as percentages of the total activity detected in each microcosm in order to allow comparisons between treatments. A two-way ANOVA was performed for both 14C and 51Cr to compare the % tracer that had been removed from the original layer. For each microcosm, the activity in the tracer layer at the end of the experiment was subtracted from total activity detected and the resulting % data were fourth root transformed to achieve homogeneous variance. Microcosms where the position of the original tracer layer was unclear were not included in the analysis. 3. Results 3.1. Experiment I 3.1.1. Bioturbation effects estimated from fluorescent particles There were clear visual differences in bioturbation between the treatments (see Section 3.3), but fluorescent particle distributions only partly reflected these observed differences (Fig. 1). Two of the four Macoma baltica cores (1 and 4) showed almost no downward mixing of tracer, while the tracer distribution was highly patchy in the Halicryptus spinulosus cores, as might be expected from an animal creating distinct burrows. Monoporeia affinis cores generally showed a gradual decrease of tracer to around 5 cm, illustrating thorough mixing. Azoic cores also showed downward transport of tracer. The shallowest mixing depth was found for Macoma baltica (4.0 G 3.5 cm), while the mixing depths for the rest of the treatments were 7.3 G 0.5 cm for azoic, 6.0 G 1.3 cm for Monoporeia affinis and 7.8 G 2.1 cm for Halicryptus spinulosus (Table 3), although these depths were not significantly different. Biodiffusion rates (Db) were similar for all treatments and there was high within-treatment variability (Table 3). Biotransport rates (r) were significantly different between treatments (One-way ANOVA: p Z 0.01). A post hoc Tukey test indicated that this difference was mainly due to a significant difference between the Macoma baltica and Halicryptus spinulosus treatments ( p Z 0.006). 3.1.2. 137Cs profiles In an undisturbed Baltic sediment core, 137Cs shows a peak in concentration at a depth below the sediment surface that is dependent on the sedimentation rate, since it originates from a single event, the Chernobyl accident, in 1986 (Ilus et al., 2003). Sometimes a smaller peak deeper in the sediment can also be detected, corresponding to global fallout from the

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Number of fluorescent particles Azoic

0

20 40 60 80 100

0

20 40 60 80 100

0

20 40 60 80 100

0

20 40 60 80 100

depth (cm)

0 4 8 12 16 20

depth (cm)

M. affinis

depth (cm) depth (cm)

3

4

1

2

3

4

1

2

3

4

1

2

3

4

4 8 12 16

0 4 8 12 16 20

H. spinulosus

2

0

20

M. baltica

1

0 4 8 12 16 20

Fig. 1. Depth profiles of fluorescent particles in Experiment I. Each row shows a different treatment, each with four replicate sediment cores. The anoxic cores are not shown, as no tracer was added to that treatment. The x-axis shows the number of particles counted per subsample, minus background fluorescence (6 counts) from organic particles.

1963 weapons tests. In this study, the 137Cs profiles (Fig. 2) have more or less constant concentrations in the upper 5e 10 cm (depending on the core), clearly indicating that the sediment in these layers had been disturbed, despite being laminated and anoxic. This shape of profile was found for all cores (Fig. 2), even the anoxic and azoic cores that had been left untreated throughout the experiment. This indicates that the surface layers were already mixed when taken from the seabed, and thus 137Cs profiles in cores from this area cannot be used as an indicator of bioturbation in this experiment.

3.1.3. Physical properties of the sediment cores The organic carbon content, water content and porosity decreased with increasing depth in all cores, whereas the dry density increased with depth (Fig. 3). Trends were very similar in all cores, regardless of treatment, implying that bulk properties of the sediment were not affected by the various experimental treatments. In all cores, there was a sharp change in sediment characteristics from the upper sediment (higher organic carbon and water content, dark colour and sulphide rich) to deeper dense, grey, odourless clay. The depth of this change ranged between 4 and 12 cm in different cores (in Fig. 3 between slices 4 and 5).

3.2. Experiment II Surface 14C tracer was transported downwards in all treatments, with concentrations generally becoming less with depth (Fig. 4). Surface 51Cr was transported downwards to all layers in all treatments in a more homogeneous pattern than 14C, leaving much less of the tracer at the surface (Fig. 4). The buried tracer layer was mixed less than the surface layer (Figs. 4 and 5), a difference that was significant in the case of 51 Cr (two-way ANOVA, p ! 0.05). The 14C tracer initially placed 4 cm below the sediment surface showed little transport in the azoic treatment. There was limited spreading of tracer from the original layer in some replicates of the other three treatments. Where Monoporeia affinis was present, there was clear evidence of upward mixing (Fig. 4). Buried 51Cr showed little mixing in the case of azoic and Halicryptus spinulosus, but some spreading to the surrounding sediment in both Macoma baltica and M. affinis treatments. The amount of tracer removed from the original layer was not significantly different between the four different animal treatments for either of the tracers (two-way ANOVA, p O 0.05) (Fig. 5), however H. spinulosus showed a particularly low degree of mixing in the case of buried 51Cr.

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Table 3 Summary table of results from Experiments I and II

Experiment I Oxygenated layer (mm) Sulphur-reducing bacteria on surface Appearance of overlying water Fluorescent particles on surface (% of total number found per core, and observations) Max mixing depth of tracer (cm) (average G st dev) Sediment laminations Biodiffusion coefficient (Db) (average G st dev) Biotransport coefficient (r) (average G st dev) Experiment II Fate of 14C from surface layer (0 cm) Fate of 14C from buried layer (4 cm deep) Fate of 51Cr from surface layer (0 cm) Fate of 51Cr from buried layer (4 cm deep) Appearance of overlying water Algae visible at surface at end (14C surface treatment) a b

M. affinis

M. baltica

H. spinulosus

Azoic

Anoxic

3e10 None Turbid 23e35% (little colour visible)

3 None Clear 16e61% (sediment surface still appeared coloured)

1.5 Little Clear 31e45% (sediment surface still appeared coloured)

0 Much Clear e

6.0 G 1.3

2e15 (patchy) Little Clear 22e56% (patchy colouration due to faecal pellets at the surface) 4.0 G 3.5

7.8 G 2.1b

7.3 G 0.5

e

Heavy disruption in top 3 cm 1.55 G 0.21

Slight disruption in top 3 cm 0.82 G 0.73a

Heavy disruption down to 10 cm 1.87 G 1.42b

Unchanged

Unchanged

0.85 G 0.47

e

6.53 G 2.25

2.85 G 1.92

9.87 G 3.44b

6.68 G 0.67

e

Transport down to 2e3 cm Some transport up to 1 cm depth. Some transport all the way down Some spreading of layer Turbid None

Transport down to 1e2 cm Limited spreading of layer Some transport all the way down Limited spreading of layer Clear None

Transport down to 1e2 cm Limited spreading of layer Some transport all the way down Limited spreading of layer Clear Little

Patchy downward transport Very little transport Some transport all the way down Very little transport

e

Clear Half

e e e e e

Macoma baltica replicate 3 is not included in this calculation. Halicryptus spinulosus replicate 1 is not included in these calculations.

3.3. Observations from both experiments Surface features initially visible, such as the presence of white sulphur-reducing bacteria and fluorescent tracer (Experiment I) and algal fragments (where 14C-labelled algae was present in the upper tracer layer, Experiment II), were modified during the experimental period, usually to a greater extent in the animal treatments than in the azoic and anoxic treatments. There were also some differences between the three animal treatments. In Experiment I, an oxygenated layer developed in all aerated cores, but the average depth of this layer was deeper in the cores with animal treatments than in the azoic treatment. Initial laminations were also disrupted by bioturbation. Detailed observations are provided in Table 3. The majority of animals were still alive at the end of both experiments. In both cases, the sediment surface of the Monoporeia affinis cores was pitted with small burrows and the uppermost 1e2 cm were completely mixed. During the experiments the amphipods were very active and were seen swimming, digging and feeding at the sediment surface, resulting in a turbid water column. Macoma baltica were buried into the sediment but their siphons were often visible. The water was very clear compared to the M. affinis treatments, presumably due to their respiratory currents. Faecal pellets were often found at the surface in the M. baltica treatments. A network of burrows was found in all Halicryptus spinulosus replicates,

down to a maximum of 10 cm in Experiment I, and all the way to the bottom of the microcosms in Experiment II. The brown aerated linings of these burrows contrasted clearly against the black anoxic sediment in the cores. Some H. spinulosus were occasionally visible on the sediment surface but were usually down in their burrows. 4. Discussion 4.1. Bioturbation and ecology 4.1.1. Macrofauna Visual differences in bioturbation activity were clear in both experiments, in the type and maximum depth of animal activity, changes in the appearance of the sediment surface and water column, and in patterns of sediment oxygenation (Table 3). Differences in the redistribution of the various tracers were not so apparent, but generally supported the patterns seen visually. The greatest differences were observed in biotransport, or non-local mixing. In summary, Monoporeia affinis caused both upward and downward mixing in the top 5 cm, although mostly in the top 3 cm. This seems to be due to mixing of particles through burrowing and/or feeding activity. In contrast, little transport of sediment particles or organic carbon (14C) was observed by Macoma baltica. Sediment transport may be hard to detect as particles eaten at the sediment surface

C. Bradshaw et al. / Estuarine, Coastal and Shelf Science 66 (2006) 123e134 137Cs-concentration

Anoxic

0

200

400

600

0

200

129

(Bq kg dry weight-1) 400

600

0

200

400

600

0 depth (cm)

4 8 12 16 1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

20

Azoic

0

depth (cm)

4 8 12 16 20

M. affinis

0

depth (cm)

4 8 12 16 20

M. baltica

0

depth (cm)

4 8 12 16 20

H. spinulosus

0

depth (cm)

4 8 12 16 20

Fig. 2. Depth profiles of 137Cs in Experiment I. Each row shows a different treatment. Only three of each of the four replicates were analysed for each treatment.

are returned there in the faeces. The deep burrowing and open burrow construction of Halicryptus spinulosus resulted in disruption of sediment laminations and patchy deep distribution of fluorescent tracer in Experiment I, evidently due to biotransport, as reflected in the high r values (9.87 G 3.44). The limited movement of 14C tracer in Experiment II is perhaps to be expected, as H. spinulosus feeds infrequently, shown by high frequencies of empty guts in food composition studies (Ankar and Sigvaldadottir, 1981; Aarnio et al., 1998). Several processes can cause the redistribution of tracers. Movement of the animals in the sediment can cause physical transport of tracer particles, but they may also feed on the labelled particles, either selectively (in the case of 14C-labelled

algae) or unselectively (51Cr-labelled sediment, fluorescent tracer). Subsequent gut purging may occur in a different area to feeding, resulting in transport of tracer. Few other tracer studies have been carried out to investigate bioturbation by Baltic infauna. Byre´n et al. (2002) used 14C-labelled diatoms to demonstrate that Monoporeia affinis was mainly a surface feeder and could bury surface material to a depth of 2.5 cm. Powilleit et al. (1994) also showed downward transport of fluorescent tracer by Halicryptus spinulosus, which was correlated to burrow density and animal abundance. Physical processes, such as diffusion, may also be important tracer transport mechanisms. The more extensive redistribution of 51Cr than 14C in Experiment II may be indicative of

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130 0.0

0.5

1.0

1.5

2.0

2.5

0

depth (cm)

2 4 6 8 10 12 organic carbon (%)

water porosity content (g cm -3) (%)

density (g dry weight cm -3)

Fig. 3. Depth profiles of organic carbon (% of dry weight), water content (% of wet weight), porosity (g cmÿ3) and density of dry sediment (g dry weight cmÿ3) for replicate 1 of the anoxic treatment. The trends seen in this core are typical of all cores sampled.

movement of dissolved 51Cr in the porewater. The Cr added to the tracer sediment in this experiment was in the hexavalent form, which is very soluble but is also readily removed from solution as Cr(III) in the presence of dissolved humic acids, colloids and other organic molecules and fine sediment particles (Mayer and Schick, 1981; Lam et al., 1997) In this experiment, chromium is likely to have been distributed in the sediment both as dissolved Cr(VI) and complexed as Cr(III) to organic matter and sediment particles, so its distribution will reflect both particle and solute transport. Bioturbation has been shown to have a greater effect on solute transport than on solids in some systems (e.g., Berg et al., 2001; Pelegri et al., 1994) and this may also be the case in this study. If so, bioturbation is likely to have a greater impact on the redistribution of contaminants that are only weakly bound to particles. For example, the activities of Monoporeia affinis could be expected to increase porewater flow substantially in the upper few centimetres and to enhance partitioning of certain contaminants into the dissolved phase. Halicryptus spinulosus burrows provide water-filled channels from the

Tracer layer originally at the surface 14C

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Fig. 4. Depth profiles of tracer ( C and 51Cr) after 20 days (Experiment II). Each row shows a different treatment. The first two columns show the transport of tracer that was originally at the sediment surface; the third and fourth columns show transport of the buried tracer layer (originally at 4 cm sediment depth). The amount of radionuclide is expressed as a % of the total amount found in the sediment for each replicate.

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Although anoxic conditions prevent the survival of most infauna, some meiofaunal taxa (particularly nematodes) are known to be able to survive temporary or even prolonged hypoxia in the Baltic (Modig and Olafsson, 1998) and are likely to have been present in the hypoxic cores in Experiment I. Nematodes are quite mobile and move in the interstitial porewater, increasing the transport of solutes (Rysgaard et al., 2000) and partitioning of some elements (Green and Chandler, 1994), and creating burrows that increase sediment water content (Jensen, 1996) and contact with the overlying water column. The movement of these animals could have been a vector for the downward transport of both 51Cr and smaller fluorescent tracer particles in this study. Aller and Aller (1992) found meiofaunal bioturbation to increase fluxes of dissolved compounds in the sediment by a factor of 1.7e2.3. In the Baltic environment, bioturbation by meiofauna and transport by physical processes may thus be as important as macrofauna.

100%

% of tracer transported away from the original tracer layer

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a 75%

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25%

4.2. Implications for remobilisation of contaminants

0% Azoic

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Fig. 5. The amount of tracer transported out of the tracer layers for (a) 14C and (b) 51Cr. Values are percentages of the tracer in the sediment found outside the original layer at the end of the experiment, expressed as means for each treatment G standard deviation. Black bars show tracer originally at the sediment surface, white bars show buried tracer.

sediment surface to deeper sediment, and increase the surface area of the sedimentewater interface (Powilleit et al., 1994), which could be an important pathway for contaminants to leach out of the sediment into the water. This is supported by data from Experiment II, where concentrations of both 14C and 51Cr in the overlying water were highest in the H. spinulosus treatment, when the tracer layer was buried at 4 cm below the surface. 4.1.2. Meiofauna In both experiments, tracer transport was often of a similar magnitude in the azoic treatment as in the three faunal treatments. Transfer of tracer between slices during sample processing is an unlikely cause, as the smeared outer edges of the core were removed and great care was taken to avoid cross contamination. There are two possible explanations for the tracer movement in the azoic treatments, although both apply only to the smallest particle sizes. The first is diffusion and mass flow of interstitial water and suspended small particles. In Experiment I, reoxygenation of the water column will likely have caused biogeochemical changes in the sediment and altered diffusion gradients. In Experiment II, the sieved sediment was probably settling throughout the experiment, causing porewater flow. Second is the presence of mobile meiofauna, such as nematodes and ostracods, in the sediment. These could have been present from the start of the experiments or have settled out from the water column as larvae or eggs and developed during the course of the experiments. In Experiment II, sediment was sieved through a 0.5 mm sieve, but all meiofauna will have passed through this mesh size.

Many studies in the Baltic have shown the presence of a cocktail of various pollutants in bottom sediments, including metals, persistent organic pollutants, and radionuclides (Elmgren, 2001). The majority of contaminants were released into the environment during the last 50 years, which is reflected in Baltic sediment profiles (Borg and Jonsson, 1996; Jonsson et al., 2000). In Experiment I, the manmade radionuclide 137Cs was found at concentrations of 300e350 Bq kg dry weightÿ1 throughout the upper 5e10 cm of sediment, a similar range to that found in other recent studies in the southern Baltic (Meili et al., 1998; Vallius et al., 1998). Most 137Cs in the Baltic originates from the accident at Chernobyl in 1986, although some also remains from the 1963 weapons tests (Holby and Evans, 1996; HELCOM, 2003). 210Pb profiles from the same cores were used to independently date the 137Cs layer, and imply that the deepest point at which 137Cs is found corresponds to 1963 (average age 42.5 y G 12.5, calculated using the Constant Rate of Supply (CRS) model (Appelby and Oldfield, 1978)), while the depth at which 137Cs concentrations steeply increase represents 1986 (average age 20.4 y G 5.1). These depth profiles demonstrate that contaminants from single events 18 and 40 years ago (and by implication all other contaminants that have entered the sediment since that time) are still present in the top 5e10 cm, a depth that is susceptible to biological and physical reworking and remobilisation. The shape of the 137Cs depth profiles, with an upper mixed layer, is not uncommon in the Baltic. For example, it is also described by Meili et al. (1998) in the Stockholm Archipelago, Holby and Evans (1996) in Ga¨vle Bay, and Andre´n et al. (2000) in the Gotland Basin. This type of profile can derive from continuous mixing (biological or physical) and a slow sedimentation rate, or from less constant, but deeper, mixing events. In this case, it is difficult to distinguish which is more likely. Erosion and resuspension are thought to be important processes in Baltic sedimentation (Persson and Jonsson, 2000), and are likely to have occurred in the cores in this experiment, where a layer of coarser material between the

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upper mixed layer and the underlying clay suggests an erosion event (T. Andre´n, pers. comm.). It is, therefore, clear that Baltic sediments are not necessarily a permanent sink for contaminants, but rather a dynamic system, where remobilisation by physical and biological processes is common. Large areas of Baltic bottom water and sediment are currently anoxic or hypoxic, and these areas are commonly assumed to be devoid of life, serving as sinks for sediment and contaminants (e.g., Elmgren, 2001). Nevertheless, this study (Experiment I) suggests that meiofauna are able to survive in hypoxic sediments either as active adults or as resting stages that hatch out under more oxic water conditions, and that they play an important role in the transport of particles and/or contaminants, at least in the top few centimeters of sediment. In addition, with reoxygenation of the water column, which could happen relatively quickly with an inflow of water from the North Sea (Jansson and Dahlberg, 1999), conditions are also suitable to support a macrofaunal community, even if the sediment itself is hypoxic. This is not surprising in the case of Halicryptus spinulosus, which is known to be resistant to severe hypoxia (Oeschger et al., 1992; Aarnio et al., 1998). Monoporeia affinis is more sensitive, but Johansson (1997) showed that it may cope with reduced O2 conditions by swimming less and burrowing less deeply in the sediment, and Modig and Olafsson (1998) found that it created microhabitats where sulphide is rapidly oxidised, that were visible as pockets of light-brown, oxygenated sediment. Monoporeia affinis could, therefore, be one of the first colonisers to a newly aerated hypoxic sediment, together with larvae of other benthic species that settle out from the water. Recovery of a full benthic community in previously anoxic or hypoxic sediment is likely to take several years, depending on the length of time the sediment was anoxic and the amount of unmineralised organic matter in the sediment (Pearson and Rosenberg, 1978; Diaz and Rosenberg, 1995). Another pathway by which buried contaminants can be remobilised is by uptake into infauna. Other studies have shown bioaccumulation of sediment-associated trace elements and radionuclides by infauna (Reinfelder et al., 1998; Ryan, 2002; Schlekat et al., 2002; Kumblad et al., 2005), with likely trophic transfer up the food chain and subsequent export from the area. Recolonisation of anoxic bottoms could also lead to remobilisation of contaminants in this way. Samples of the animals in Experiment I were analysed for 137Cs content, but none was detected, despite a detection limit as low as 3 Bq kg dry weightÿ1. In the case of 137Cs, concentrations in the sediment may be too low to cause measurable uptake, and/or the experiment was not run long enough to detect an effect and/or it is not taken up by these species. In Experiment II, high body concentrations of 14C were detected in Macoma baltica and Monoporeia affinis, but this was not the case for 51 Cr, a finding supported by Kumblad et al. (2005).

not only a valuable sedimentary dating tool in many areas, but potentially a useful tracer layer which could be used to monitor upward mixing by bioturbation, without the need for disruptive emplacement of experimental tracer. The 1963 weapons fallout peak is of less use for this purpose, as it is a weaker signal and in some areas is now buried below the active mixing depth. This method was not possible in this study, as it requires a discrete 137Cs layer within biologically active depths, in areas where physical reworking is not important, conditions that were not fulfilled. Using tracers that occur in the field can be a useful technique, as it allows natural sediments and benthic communities to be studied undisturbed, providing a more realistic, if more variable, picture of processes occurring. The naturally occurring isotopes 234Th, 210Pb and 7Be have been used successfully to study downward mixing (Feng et al., 1999; Berg et al., 2001; Santschi et al., 2001; Green et al., 2002; Shull and Mayer, 2002) but, due to their rapid disintegration, they require immediate measurement, which was not possible in this study. Another approach is to add tracers to the system being studied. Experiment I used the common method of adding a fluorescent particle tracer to the sediment surface, allowing minimal disruption to the natural sediment structure. In Experiment II, tracer layers were embedded in sieved sediment. This created an artificial set-up where conditions were of course quite different to those found in the field, however this allowed full control of the experimental design, and the creation of good replicates and controls, something that is more difficult with natural sediment taken from the field. This experiment also enabled simultaneous measurement of upward and downward transport of two different types of tracer (a 14Clabelled food source and the heavy metal 51Cr) and showed that these processes were quite different. Quantification of tracers of all types in bulk sediment can be difficult as it tends to either be mixed by bioturbation to the extent where it is diluted to the limits of detection, or is mixed in such an extremely patchy way (e.g., localised to burrow walls, faecal pellets etc.) that either huge variability is found, or averaging concentrations across a particular depth horizon causes low concentrations. Targeted sampling of faecal pellets and burrow linings is likely to yield higher activities than bulk sediment sampling. It can also be difficult and time consuming to analyse tracers such as fluorescent particles, although photographic and digital methods are being developed (Gilbert et al., 2003; Solan et al., 2004) which help to overcome these problems. Radioisotope tracers are generally easier to analyse, and even very low concentrations can be detected with most scintillation counters or gamma spectrometers.

4.3. The use of the tracer methods for studying bioturbation

A combination of different types of tracer in the same experiment is a useful approach, as the different tracers provide information about different processes that occur simultaneously (e.g., movement of sediment, solutes, water, organic matter, contaminants).

The manmade isotope 137Cs, released from the Chernobyl accident in 1986, is now found in the environment and is

5. Conclusions

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Macrofauna and meiofauna seemed to have an equally important effect on tracer transport in these experiments. All species had no difficulty surviving hypoxic sediment conditions, as long as the water column was aerated. This implies that reoxygenation of bottom waters can be quickly followed by recolonisation of the benthos. On decadal or longer timescales, physical events such as resuspension and sediment focussing are likely to be more important than biological processes in shaping the sedimentary record and in redistributing buried contaminants in the Baltic Sea. Although bioturbation by the species in this study may be a detectable process at the scale of an individual animal or burrow, from a bulk transport perspective the effects are not significant, at least on the timescale of this experiment.

Acknowledgements The authors would like to thank the staff of the Asko¨ Lab¨ rjan Gustafsson for the use of the oratory, Per Andersson and O gamma spectrometer at the Swedish Museum of Natural History (NHM) (Stockholm) and Marie Elmquist (NHM), Mats Jonsson and Kjell Sva¨rdstro¨m (KTH, Stockholm) for much help and advice on gamma counting. Also Oleg Voitsekhovitch and Alexander Kostezh at the Ukrainian Hydrometeorological Research Institute, Kiev. Franck Gilbert and Jean-Christophe Poggiale (Centre D’Oce´anologie de Marseille) very kindly calculated the Db and r coefficients. Markus Meili, Thomas Andre´n and Sven Blomqvist provided helpful discussions. The project was funded by an EC Marie Curie Fellowship to CB, PhD funding by SKB (Swedish Nuclear Waste Management Co) to LK, and the Swedish Research Council for the Environment (Formas).

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