ORI GIN AL PA PER

Recolonisation by Macrobenthos Mobilises OrganicPhosphorus from Reoxidised Baltic Sea Sediments

Nils Ekeroth • Magnus Lindstrom • Sven Blomqvist • Per O. J. Hall

Received: 7 April 2011 / Accepted: 9 May 2012� Springer Science+Business Media B.V. 2012

Abstract In recent decades, eutrophication has increased the extent of hypoxic and

anoxic conditions in many coastal marine environments. In such conditions, the nutrient

flux across the sediment–water interface is a key process controlling the biogeochemical

dynamics, and thereby the level and character of biological production. In some areas,

management attempts to drive the ecosystem towards phosphorus (P) limitation, which

calls for reliable knowledge on the mechanisms controlling P-cycling. We report a well-

controlled laboratory experiment on benthic fluxes of P, when shifting from a state of

hypoxic and azoic sediments to oxic and zoic bottom conditions. Adding any of three types

of macrobenthic fauna (mysid shrimp, pontoporeid amphipod and tellinid clam) to oxy-

genated aquarium sections resulted in benthic P fluxes that differed consistently from the

azoic control sections. All species caused liberation of dissolved organically bound P

(DOP) from the sediment, in contrast to the azoic systems. The shrimp and the amphipod

also resuspended the sediment, which resulted in a release of P bound to particles

([0.45 lm). Dissolved inorganic phosphate (DIP) was released during hypoxic conditions,

but was taken up after oxygenation, irrespective of the presence or absence of bottom

fauna. In the presence of fauna, the uptake of DIP roughly equalled the release of DOP,

suggesting that the benthic efflux of DOP following oxygenation and bottom fauna

(re)colonisation might be considerable. This is an hitherto overlooked animal-controlled

nutrient flux, which is missing from coastal marine P budgets.

Keywords DOP � P retention � Bioturbation � Monoporeia affinis � Macoma balthica �Mysis mixta

N. Ekeroth (&) � S. BlomqvistDepartment of Systems Ecology, Stockholm University, 106 91 Stockholm, Swedene-mail: nils.ekeroth@ecology.su.se

M. LindstromTvarminne Zoological Station, University of Helsinki, J.A. Palmens vag 260, 109 00 Hango, Finland

P. O. J. HallDepartment of Chemistry and Molecular Biology, Marine Chemistry, University of Gothenburg,412 96 Gothenburg, Sweden

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Aquat GeochemDOI 10.1007/s10498-012-9172-5

1 Introduction

Globally, increased nutrient load and concomitant development of hypoxia or even

anoxia in coastal marine areas is a rapidly growing concern. Presently, some 400

coastal areas are known to be more or less affected (Diaz and Rosenberg 2008). Hence,

studies of the mechanisms controlling the dynamics and fate of nutrients in such areas

are needed. This applies to phosphorus (P), which is a key element in estuaries and

other coastal marine areas, because it can limit phytoplankton growth (Howarth et al.

2011) and the rate of diazotrophic nitrogen fixation (Moisander et al. 2007; Walve and

Larsson 2007). Therefore, eutrophication management in nutrient contaminated water

areas often attempts to reduce the P supply (e.g., Backer et al. 2010; Carstensen et al.

2006; HELCOM 2007). However, to be effective and cost-efficient, such P reductions

have to be carefully judged (cf., Wulff et al. 2001), which requires reliable biogeo-

chemical knowledge of P dynamics, including concentration levels (Larsson et al. 1985;

Savchuk 2005), crucial control mechanisms (Benitez-Nelson 2000; Blomqvist et al.

2004; Gachter and Meyer 1993) and functional cause–effect relationships (e.g., Conley

et al. 2002).

The Baltic Sea is a eutrophicated marine area where the adopted remediation strategy

(HELCOM 2007) is based on the ecosystem approach (Backer et al. 2010), supported by

biogeochemical large-scale budgeting and modelling (Savchuk and Wulff 2007). It has

become increasingly apparent that the internal load of P in the Baltic proper is presently

very large in comparison with the external supply (Conley et al. 2002; Pitkanen et al. 2001;

Savchuk 2005; Viktorsson et al. 2012), which suggests that management of external P

sources alone may result in a very long recovery time (Savchuk and Wulff 2007). This

projection is, however, based on budget arguments rather than detailed understanding of

benthic–pelagic P coupling in the Baltic, stressing the need for improved understanding of

this process.

The soft bottom sediment–water interface in the aphotic zone is a major potential source

of internally regenerated P (for review see; Valiela 1995). Benthic P fluxes are traditionally

considered strongly redox dependent (e.g., Gunnars and Blomqvist 1997; Sundby et al.

1986) and linked to microbial assimilation/dissimilation processes in the sediment

(Gachter and Meyer 1993). Benthic macrofauna activity (bioturbation) can also influence

sedimentary P fluxes. Previous studies of Baltic Sea bottom dwelling animals show that the

effect of their bioturbation is species specific and can either increase (Karlson 2007;

Viitasalo-Frosen et al. 2009) or decrease (Hietanen et al. 2007; Karlson et al. 2005;

Viitasalo-Frosen et al. 2009) the sedimentary retention of ortho-P. However, chemical

studies considering also other P fractions are few.

We report a mechanistic study of P exchange across the sediment–water interface when

shifting from hypoxic–anoxic to oxic bottom conditions. We added different bottom fauna

species common in the Baltic Sea to the reoxidised sediment and found highly divergent,

species–specific effects. Our results suggest a need for renewed study of how P is cycled in

the many transient, euxinic coastal environments worldwide.

2 Study Area

In the brackish Baltic Sea (Fig. 1), almost 60.000 km2 of the bottom area in the

Baltic proper and the Gulf of Finland (totalling 240.000 km2; Ehlin and Mattisson

1976) shifts regularly between hypoxic/anoxic and oxic conditions (Conley et al.

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2002). During an anoxic–oxic cycle, as much as 120,000 metric tonnes of P can be

first mobilised and then immobilised within this system (Stigebrandt and Gustafsson

2007), with bottom sediments inferred to play a pivotal role (Conley et al. 2002). The

oxygen (O2 gas) conditions also control the abundance of macrozoobenthos in sub-

halocline depths of the Baltic Sea (Laine et al. 1997). The benthic macrofauna

community deteriorates during periods of bottom water hypoxia and is completely

wiped out by prolonged anoxia (Karlson et al. 2002). However, macrofaunal recol-

onisation after reoxygenation of previously anoxic/hypoxic sediments starts often

within 1 year (Laine et al. 1997).

Fig. 1 Map of study area and geographical names mentioned in the text. The X denotes the sedimentcollection station in the open Baltic Sea proper. Sampling sites for the amphipod Monoporeia affinis and themysid shrimp Mysis mixta are marked by open dots in the Bothnian Sea and in the Baltic proper,respectively, and the tellinid bivalve Macoma balthica was collected in the bay of Braviken. The experimentwas conducted at the Tvarminne Zoological Station, Finland (TZ Stn)

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3 Methods

3.1 Field Sampling

We collected sediment that was euxinic, laminated in black and grey and free of macro-

scopic fauna at a water depth of 148 m in the open north-western Baltic proper

(N58�26021000 E18�25037500; Fig. 1), in a calm sea on 12 January, 2010, using a 0.04 m2

Ekman grab, equipped with a supporting frame (Blomqvist 1990). The top five cm of

sediment was transferred from several grab collections to 10 L polypropylene containers,

where it was mixed and stored cold and dark until the experiment started 1 day later.

Three macrofauna species differing in feeding mode were studied, namely the sus-

pension feeding, epibenthic–pelagic mysid shrimp Mysis mixta Liljeborg, the burrowing,

suspension/deposit feeding tellinid bivalve Macoma balthica (L.) and young-of-the-year of

the benthic deposit feeding, pontoporeid amphipod Monoporeia affinis (Lindstrom). They

were collected in the southern Stockholm Archipelago by means of a Tucker 0.25 m2 trawl

(Hopkins et al. 1973), in the bay of Braviken by a van Veen grab (Veen 1933), and in the

Bothnian Sea by a benthic sled (Blomqvist and Lundgren 1996), respectively (Fig. 1). The

mysids were collected 2 weeks before the other macrofauna, kept cold and fed a mixture of

brine shrimp Artemia sp. and commercial fish food (TetraRubin, Tetra Europe, Melle,

Germany). The amphipods and the clams were kept cold during 2 days of transport and

storage before the start of the experiment.

3.2 Experimental Set-up

The experiment was conducted at the Tvarminne Zoological Station (Helsinki University),

southern Finland (N59�500 E23�150), on the Gulf of Finland (Fig. 1). It was performed in a

long narrow acrylic aquarium (300 9 35 9 4.6 cm; Fig. 2), permitting visual inspection,

in a temperature-controlled room (5–8 �C). The aquarium was divided into 12 consecutive

sections by insertion of net poly-vinyl chloride (PVC) walls (0.5 mm mesh size), which

retained the animals in the sections, while permitting water to flow through the aquarium.

The aquarium system (Fig. 2) was supplied with brackish water (salinity *5 PSU)

from an intake at 8–9 m depth about 1 km off the Station. A *23 L reservoir (W in Fig. 2)

permitted adjustment of the oxygen concentration before supply to the aquarium at

*1 L min-1. The water was recirculated to the reservoir after passing through the series of

aquarium sections and back, resulting in that no horizontal gradients were formed in the

aquarium. One tenth of the water was automatically renewed every 24 h, except during

flux incubations. This flow-through aquarium system has been used successfully in pre-

vious studies (e.g., Lindstrom and Sandberg-Kilpi 2008).

3.3 Timeline

The sediment was carefully homogenised and placed between the net walls of the aquarium

(day 0, i.e. 13 January, 2010). Average initial sediment depth in each subsection was

3.95 ± 0.16 cm (mean ± SD), which corresponds to the approximate bioturbation depth

(4 cm) in the Baltic proper soft sediments, as revealed from the vertical distribution of live

Bosmina (Cladocera) resting eggs (Kankaala 1983). The system was left to stabilise for

4 days.

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Fig. 2 (I) Plan of the experimental set-up. The conditioned water flowed from the water reservoir(W) through the aquarium (A) and back in a closed circuit. 10 % of the water volume in the system wasrenewed every 24 h. The oxygen concentration in the reservoir water was adjusted by bubbling withnitrogen gas (N2) or air. The sections in the aquarium were separated by net walls (0.5 mm mesh size) (1) toconfine the macrofauna. During flux incubations, solid walls (2) were inserted between the sections torestrict water exchange and gas bubbling (air for the oxic and N2 for the hypoxic period) via sintered glassdiffusers (3) in each section gently mixed the supernatant water. A remotely operated robot (R) wasequipped with a microelectrode (O) for oxygen measurements. A light rhythm of 12-h daylight and 12-hdarkness was provided by a light box (L) controlled by a preset timer. Further details: (B) magnetic valve forwater supply, (C) N2 gas tube, (D) N2 pressure regulator, (E) magnetic valve for N2 supply, (F) oxygenmeter, (G) unit regulating N2 bubbling by opening (E) when signal from (F) deviates from preset oxygenvalue, (H) air stone in (W), (I) water intake to aquarium, (J) oxygen electrode in (W), (K) water overflow todrain (M), (N) valve for regulating water flow through aquarium, (P) circulation pump, (Q) water flow meter,(S) containers for replacement water, acting as air traps, (T) valve used to remove air, (U) rail for themicroelectrode robot, (X) incandescent light tubes, (Y) acrylic diffuse and dark filters. Modified afterLindstrom and Sandberg-Kilpi (2008). (II) Typical plots of the concentration changes of DIP (open circles),DOP (filled circles) and PP (crosses) in one section during a flux incubation

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Oxygen saturation was regularly measured in the supernatant water and porewater of all

sections, using an oxygen microelectrode (OX 100, Unisense, Aarhus, Denmark), and

converted into molar concentration (Weiss 1970). The water temperature was recorded

(5.4–7.8 �C) before the oxygen measurements.

In the experiment, P fluxes were measured during a shift from an 8-day period of anoxia

(days 5–13) at the sediment–water interface, recorded 0.5 mm above the surface. This time

period is hereafter referred to as the ‘‘hypoxic period’’, as there was some oxygen present

in the supernatant water (Fig. 3, lower left panel). The hypoxic period was followed by a

28-day oxic period (Fig. 3, lower right panel). Macrofauna species (single species treat-

ments) were added day two of the oxic period. Flux measurements comprising all P

fractions (defined in Sect 3.6) were conducted on day 13 (hypoxic period) and day 28 (oxic

period). In order to detect temporal changes in the relative magnitude of postoxygenation

DIP fluxes, a complementary flux measurement of DIP was performed at the end of the

oxic period (day 41).

3.4 Introduction of Animals

Animals to be introduced to the aquarium were gently picked out, using a small strainer for

the mysids and the amphipods, and tweezers for the clams. The control sections without

animals (n = 3 replicates) were distributed four sections apart, and the animal treatments

were positioned in different order between the control sections. The animal treatments

sections (n = 3 replicates each) held 10 clams, 50 amphipods or 7 mysids, corresponding

to approximately 1,500, 6,500 and 950 individuals m-2, respectively. In order to

Fig. 3 Fluxes of DIP, DOP and PP (in lmol m-2 d-1) during the hypoxic period (left), and first fluxmeasurement during the oxic period (right), along with typical oxygen profiles during these periods (below).Error bars of the histogram denote ± SD

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standardise the replicates within species, only individuals of very similar size were used.

The macrofaunal densities were chosen to represent late stages in the recolonisation suc-

cession, that is, after prolonged reoxygenation. The experimental density of the clam M.balthica is within the upper range of natural densities in the southern Baltic Sea (Ankar

1977; Ankar and Elmgren 1976; Lehtoranta and Heiskanen 2003). Densities of the

amphipod M. affinis vary greatly and range up to 5,000 individuals m-2 in the Baltic proper

(Ankar 1977; Ankar and Elmgren 1976) and up to 25,000 individuals m-2 in adjoining

freshwater Lake Malaren (Goedkoop and Johnson 2001; Fig. 1). Few data are available on

the abundance of the mysid M. mixta in the Baltic, but densities of more than 300 indi-

viduals m-2 occur in oxic environments (Salemaa et al. 1990).

3.5 Flux Measurements

The flux incubations were made as follows: (1) The water flow through the aquarium was

turned off. (2) Solid, water-tight PVC walls (2 in Fig. 2) were inserted into the spaces

between the net wall-pairs to stop water flow and exchange of material between sections.

Fig. 4 Change over time in concentration of TP, TDP and DIP in all sections during the hypoxic period fluxincubation. Linear regression lines (solid black lines) are based on pooled data from all sections. See Table 1for regression statistics and slope coefficients

Table 1 Linear regression statistics and slope coefficients for the concentration change versus time duringthe hypoxic period flux measurement

Hypoxic

Slope coefficient R2 p

DIP 0.061 0.14 **

DOP 0.013 0.02 ns

TDP 0.054 0.11 **

PP 0.008 0.00 ns

TP 0.062 0.06 *

All figures are based on pooled data from all aquarium sections. See Fig. 4 for visual representations of thelinear regression lines

* p \ 0.05; ** p \ 0.001

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(3) To each isolated section, air (oxic period) or N2 gas (hypoxic period) were carefully led

down through a gas distributor (3 in Fig. 2, and Fig. 7) to maintain the adequate oxygen

condition and create mixing. (4) Water was collected in each sampling section five times

during each of the approximately 24-h long flux incubations, and net P fluxes were cal-

culated from the concentration change over time of the P fractions in the supernatant water

(II in Fig. 2). When our flux concentration values versus time changed rectilinearly

(p \ 0.05 by least square linear regression analysis), the obtained regression was used to

calculate the P flux.

The lowest statistically significant flux record for each P fraction was assumed to

represent the detection limit of this particular experimental set-up. These regression

coefficient records were used to separate zero-fluxes (i.e. constant supernatant concen-

tration over time) from near significant or ambiguous nonlinear relationships without

committing Type-II error (e.g., Sokal and Rohlf 1995). In the data analysis, fluxes with

non-significant (p [ 0.05) regression coefficients, having lower absolute values than the

lowest statistically significant regression coefficient, were set to zero, while those with

higher absolute values were rejected and treated as missing values. We compensated the

flux calculations for the decreased volume of withdrawn sample. Also, as a precautionary

control, flux measurements were also conducted in sections without sediment or animals.

3.6 Phosphorus Species Definitions and Analytical Determinations

On each sampling occasion, one unfiltered and two filtered water samples (using 0.45 lm

pore size, presoaked and washed cellulose acetate filters, Filtropur S 0.45 by Sarstedt AG

& Co., Numbrecht, Germany) were collected from every section in the aquarium. The

unfiltered samples were analysed for total P (TP). The filtered samples were analysed for

total (operationally) dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP),

respectively. To avoid postsampling alteration of the filtrate, SRP samples were acidified

immediately by adding 0.5 mL blank tested H2SO4 (5 %) to 9.5 mL of sample. All

samples were stored chilled and analysed within 72 h.

The SRP fraction was assumed to represent the maximum dissolved inorganic phos-

phate (DIP: ortho-P, pyro-P and poly-P) concentration—the latter two due to the acid

preservation. The particle bound P (PP) fraction was defined as the concentration differ-

ence between TP and TDP samples. The concentration difference between TDP and SRP

samples was operationally defined as ‘‘dissolved organically bound P’’ (DOP), although

this matter might also contain some colloidal or inorganic P species (Benitez-Nelson 2000;

Hollibaugh et al. 1991).

All analyses were conducted calorimetrically by segmented flow analysis on an AL-

PKEM O I Analytical Flow Solution IV system at the Department of Systems Ecology,

Stockholm University, Stockholm. Prior to analysis, TP and TDP samples were digested by

acid-persulphate at high temperature (modified after Valderrama 1981). Throughout, SRP

was measured by the molybdenum blue method (modified after Koroleff 1983).

4 Results and Discussion

We found a consistent pattern of fluxes of the different P fractions (Fig. 3). During the

hypoxic period, DIP was released from the bottom sediment (Figs. 3, 4). This follows the

classical pattern, first recognised more than a half century ago (Einsele 1936, 1938).

Conversely, and also in line with previous reports (Gunnars and Blomqvist 1997; Sundby

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et al. 1986), in our experiment, DIP was consistently taken up by the sediment during the

oxygenated period, whether or not animals were present. Two weeks after oxygenation,

DIP uptake was significantly higher in sections with amphipods, than in the other animal

sections and the controls (one-way ANOVA, F(3,8) = 16.3, p = 0.0009; Fig. 3). However,

this difference disappeared in the last DIP flux measurement, 13 days later. For the oxic

period as a whole, there was a significant difference only between sections with amphipods

and the clams, with lower DIP uptake in the latter (one-way ANOVA, F(3,19) = 5.15,

p = 0.009). This is consistent with previous studies reporting relatively low DIP retention

in sediments inhabited by this clam (Karlson et al. 2005; Viitasalo-Frosen et al. 2009).

In our study, no fluxes of DOP or PP were detected during the hypoxic period or in

azoic control sections (Fig. 3). Conversely, under these circumstances, the concentration

increase of DIP in the overlying water should equal the concentration increase of TDP as

well as TP. TDP and TP concentrations showed, however, more fluctuation than DIP

during the flux incubations, particularly during the hypoxic period (Fig. 4). Due to this

heterogeneity, fewer significant fluxes of TDP and TP than fluxes of DIP were detected

during the hypoxic period. Linear model regression analyses of pooled data from all

sections during the hypoxic period show, however, that the concentration increase of TDP

and TP versus time was very similar to that of DIP, but that no such pattern exists for DOP

or PP (Fig. 4, Table 1). The same is valid when combining data from all azoic sections

during the oxic period (Figs. 5, 6, Table 2). Thus, it can be concluded that in the absence of

macrofauna, DIP is the main form of P exchanged across the sediment–water interface.

Conversely, when benthic macrofauna was introduced, the concentration evolution of

TP and TDP clearly deviated from that of DIP (Figs. 5, 6, Table 2), due to pronounced

fluxes of PP and/or DOP (Fig. 3). The flux of PP dominated in systems with the amphipods

and the mysids, whereas in systems with clams the flux of PP was close to zero (Fig. 3).

Resuspension was visually observed in both the mysid and amphipod systems, in the

latter in connection with their plunge diving into the sediment. The burrowing behaviour

during the light hours by this species is well known (Lindstrom and Lindstrom 1980), but

has not been well pictorially documented. The evenly distributed holes in the sediment at

the end of the experiment (Fig. 7) were more numerous than the number of individuals,

suggesting that each amphipod produces many burrows over a relatively short time span.

The relatively small differences in DIP uptake among the treatments indicate, in contrast to

the findings of Viitasalo-Frosen et al. (2009), that fauna-mediated resuspension appeared

not to have significantly enhanced the sedimentary retention of DIP.

All investigated macrofauna caused a release of DOP from the sediment, but this was

smaller for the bivalve (Fig. 3). These animal-dependent benthic DOP effluxes suggest that

an important P flux has been overlooked in many previous flux measurements in the Baltic

Sea (e.g., Hietanen et al. 2007; Karlson 2007; Karlson et al. 2005, 2007a; Viitasalo-Frosen

et al. 2009, but see Lehtoranta and Heiskanen 2003) and therefore also when modelling P

dynamics in coastal marine environments. The benthic DOP release during the oxygenated

period appears to be a potentially significant flux, since it equals the amount of DIP

concurrently immobilised in the sediment (Fig. 3). This thought-provoking comparison

calls for further research on other benthos, as well as studies in situ (e.g., Tengberg et al.

1995; Viktorsson et al. 2012). An earlier study from a shallow (16 m depth) bay in the

Kattegat reports relatively low benthic fluxes of DOP compared to DIP (Jensen et al.

1995), except shortly after the sedimentation of a spring diatom bloom when the efflux of

DOP and DIP were of equal magnitudes. The macrofauna abundance was, however, not

assessed in Jensen et al. (1995), and low abundance of benthos may explain the low fluxes

of DOP.

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Previous studies on the chemical composition of surficial anoxic Baltic proper sedi-

ments have reported organic P mono- and diesters together make up *60 % of NaOH–

EDTA-extractable P in the top 4 cm layer of anoxic sediment (Ahlgren et al. 2006).

P-esters are known to be utilised during phosphate depletion by eukaryotic as well as

prokaryotic plankton (reviewed by Paytan and McLaughlin 2007). Our findings suggest

that the activity of benthic macrofauna might mobilise a fraction of the organic P stored in

the sediment and release it as DOP to the supernatant water. Due to its submicron size, the

released DOP might easily disperse widely in the overlaying water column (cf., Buffle and

Leppard 1995).

Further studies are desirable regarding DOP mobilisation from sediments and its

importance as a source of water column DOP. For example, Nausch and Nausch (2006)

showed that up to about 60 % of the bulk DOP in May–June in the surface water of the

Baltic proper was bioavailable to free-living heterotrophic bacteria. Moreover, DOP has

been proposed to constitute a significant P source for growing diazotrophic photosyn-

thesising cyanobacteria during P-limited bloom conditions in the Baltic Sea (Poder et al.

2003), as well as in the global Ocean (e.g., Dyhrman et al. 2007).

4.1 Final Remarks

Oxygenation of anoxic Baltic Sea sediments showed the expected redox-controlled DIP

dynamics, namely increased sedimentary retention (Fig. 3). Initially, the amphipods

speeded up the DIP uptake, perhaps due to surface expansion of oxic sediment (Kristensen

Fig. 5 Change over time in concentration of TDP, DOP and DIP during the oxic period flux incubation.Linear regression lines (solid black lines) are based on pooled data from each triplicate treatment. SeeTable 2 for regression statistics and slope coefficients

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Fig. 6 Change over time in concentration of TP, TDP and PP during the oxic period flux incubation. Linearregression lines (solid black lines) are based on pooled data from each triplicate treatment. See Table 2 forregression statistics and slope coefficients

Table 2 Linear regression statistics and slope coefficients for the concentration evolution versus timeduring the oxic period flux measurement

Control M. affinis

Slope coefficient R2 p Slope coefficient R2 p

DIP -0.085 0.92 *** -0.167 0.92 ***

DOP 0.004 0.00 ns 0.099 0.93 ***

TDP -0.071 0.63 *** -0.068 0.49 **

PP -0.018 0.04 ns 0.165 0.42 **

TP -0.099 0.34 * 0.097 0.14 ns

M. mixta M. balthica

Slope coefficient R2 p Slope coefficient R2 p

DIP -0.085 0.66 *** -0.063 0.60 ***

DOP 0.085 0.67 *** 0.070 0.40 *

TDP 0.000 0.00 ns 0.006 0.00 ns

PP 0.234 0.37 * -0.043 0.05 ns

TP 0.234 0.46 ** -0.037 0.02 ns

All figures are based on pooled data from each triplicate treatment. See Figs. 5 and 6 for visual represen-tations of the linear regression lines

* p \ 0.05; ** p \ 0.001; *** p \ 0.001

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2000) by the formation of holes from their plunge diving (Fig. 7), but this uptake effect of

macrofauna activity appeared quantitatively low in a longer time span.

When animals were present during the oxygenated period, the amount of P released as

DOP at the sediment–water interface roughly equalled the uptake of DIP (Fig. 3), while no

DOP efflux was found during hypoxia, or in azoic aquarium sections. This suggests that

anoxic/hypoxic sediments subjected to oxygenation and (re)colonisation by macrofauna

might act as a significant DOP source to the pelagic ecosystem of coastal marine envi-

ronments. Hence, animal colonisation counteracts the sedimentary P uptake upon oxy-

genation of reduced sediment. This is in contrast to the view that benthic macrofauna

generally stimulate sedimentary P retention (Karlson et al. 2007b). We expect that the

released DOP is potentially biologically available (cf., Cembella et al. 1984a, b) and can

affect the productivity of these environments. Hence, this benthic efflux might be a con-

siderable, but overlooked P source, which hitherto not has been included in coastal marine

budgets and models of sediment–water exchange processes.

Acknowledgments The Tvarminne Zoological Station provided excellent laboratory facilities and agenerous atmosphere. We are grateful to the staff of the Chemical Laboratory at the Department of SystemsEcology, Stockholm University, for skilful determination of different P fractions. Ragnar Elmgren providedconstructive comments and linguistic improvements. The Stockholm University Marine Research Centresubsidised ship time. ML received a grant from the Walter and Andree de Nottbeck Foundation. This studyis a contribution from the BOX project, which is financially supported by the Swedish EnvironmentalProtection Agency. Financial support was also provided to PH by the EU through the FP7 project HYPOX.Finally, PH sincerely enjoyed and benefited from the years of scientific collaboration with Bjørn Sundby.Those years were special. The topics of this paper, P-cycling and bioturbation, are among the favouriteresearch themes of Bjørn, and we dedicate this paper to him.

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