(North Sea) on nutrient exchange across the sediment ... - Springer Link

Biogeochemistry (2013) 113:37–52 DOI 10.1007/s10533-012-9710-7

Impact of resuspension of cohesive sediments at the Oyster Grounds (North Sea) on nutrient exchange across the sediment–water interface Fay Couceiro • Gary R. Fones • Charlotte E. L. Thompson Peter J. Statham • David B. Sivyer • Ruth Parker • Boris A. Kelly-Gerreyn • Carl L. Amos

•

Received: 16 March 2011 / Accepted: 2 February 2012 / Published online: 18 February 2012 Ó Springer Science+Business Media B.V. 2012

Abstract Benthic-pelagic exchange processes are recognised as important nutrient sources in coastal areas, however, the relative impact of diffusion, resuspension and other processes such as bioturbation and bioirrigation are still relatively poorly understood. Experimental shipbased data are presented showing the effects of diffusion and resuspension on cohesive sediments at a temperate shelf location in the North Sea. Measurements of diffusive fluxes in both spring (1.76, 0.51, -0.91, 17.6 lmol/m2/h) and late summer (8.53, -0.03, -1.12, 35.0 lmol/m2/h) for nitrate, nitrite, phosphate and dissolved silicon respectively, provided comparisons for measured resuspension fluxes. Increases in diffusive fluxes of nitrate and dissolved silicon to the water column

in late summer coincided with decreases in bottom water oxygen concentrations and increases in temperature. Resuspension experiments using a ship board annular flume and intact box core allowed simultaneous measurement of suspended particulate matter, water velocity and sampling of nutrients in the water column during a step wise increase in bed shear velocity. The resuspension of benthic fluff led to small but significant releases of phosphate and nitrate to the water column with chamber concentration increasing from 0.70–0.76 and 1.84–2.22 lmol/L respectively. Resuspension of the sediment bed increased water column concentrations of dissolved silicon by as much as 125% (7.10–15.9 lmol/ L) and nitrate and phosphate concentrations by up to 67%

F. Couceiro G. R. Fones (&) University of Portsmouth, School of Earth and Environmental Sciences, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK e-mail: [email protected]

D. B. Sivyer R. Parker Centre for Environment, Fisheries and Aquaculture Science, Cefas Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK e-mail: [email protected]

F. Couceiro e-mail: [email protected]

R. Parker e-mail: [email protected]

C. E. L. Thompson P. J. Statham C. L. Amos School of Ocean and Earth Sciences, University of Southampton, National Oceanography Centre Southampton, European Way, Southampton SO14 3ZH, UK e-mail: [email protected]

B. A. Kelly-Gerreyn National Oceanography Centre Southampton, European Way, Southampton SO14 3ZH, UK e-mail: [email protected]

P. J. Statham e-mail: [email protected] C. L. Amos e-mail: [email protected]

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(1.84–3.08 lmol/L) and 66% (0.70–1.15 lmol/L) respectively. Mass balance calculations indicate that processes such as microbial activity or adsorption/ desorption other than simple release of pore water nutrients must occur during resuspension to account for the increase. This study shows that resuspension is potentially an important pathway for resupplying the water column with nutrients before and during phytoplankton blooms and should therefore be considered along with diffusive fluxes in future ecosystem models. Keywords Resuspension Nutrients Cohesive sediment Flume Oyster Grounds North Sea

Introduction In temperate coastal areas such as the North Sea, nutrients are supplied to seawater from direct sources such as river inflow, land run-off, ground water seepage, the atmosphere, the ocean and seafloor sediments (Hopkinson 1987). The North Sea is thought not to receive sufficient nutrients from riverine and atmospheric sources alone to sustain the large phytoplankton blooms observed and productivity is likely maintained as much by recycling of nutrients within the system as by external inputs (Hydes et al. 1999). The potential for the supply of nutrients from sediments arises from the intensive remineralisation of particulate organic material deposited to and incorporated within the sediments and the subsequent release of a proportion of these nutrients back to the overlying water. The role of sediments in modifying nutrient concentrations in the overlying water is complex and it has been demonstrated that benthic sediments may act as either a nutrient source or sink (Nedwell et al. 1993). There are three key processes for exchange across the sediment water interface: diffusion, biota mediated processes and resuspension. Inorganic nutrients formed in the surface layer of the sediment by remineralisation may diffuse down into the sediments enhancing potential nutrient losses from the overlying water column. Conversely, nutrients may diffuse back into the water column or, in the case of nitrogen species, be converted to gaseous forms and lost from the marine system entirely. Biota mediated processes include microbial action converting one nutrient species to another e.g. nitrification converting

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Biogeochemistry (2013) 113:37–52

ammonium to nitrate. Temperature exerts a great control over remineralisation by changing the metabolic rates of the microbial community (Rabus et al. 2002), changing the composition of the microbial community itself (Ogilvie et al. 1997) and also influencing the speed with which chemical reactions take place. Enhanced respiration rates as temperatures rise and organic carbon is supplied to the sediment from bloom conditions will also directly impact oxygen concentrations. Finally, chemical transformations between forms of nutrients are likely to be affected by the physically dynamic nature of the upper sediment layers (Fanning et al. 1982). Exchangeability of the interstitial water with the overlying water column may also vary depending on the physical stability of the sediments (Vidal 1994). Studies of sediment–water column nutrient exchange mechanisms in cohesive sediments have primarily focused on diffusion (Serpa et al. 2007; Lohse et al. 1998). Bioturbation studies are generally species specific and are difficult to extrapolate to areas of mixed assemblages (e.g. Braeckman et al. 2010; Wood et al. 2009; Nizzoli et al. 2007). Our most limited understanding is of resuspension despite evidence that forcing from tidal action (Rocha and Cabral 1998; Sloth et al. 1996), wind regimes (Wainright and Hopkinson 1997), dredging (Nayar et al. 2007) and trawling (Dounas et al. 2007) may affect sediment–water nutrient exchange. In particular, resuspension can result in increased rates of remineralisation by the bacteria attached to the resuspended particles in the water column on exposure to increased oxygen concentrations (Wainright and Hopkinson 1997; Sloth et al. 1996). Resuspension may also enhance water column nutrient concentrations through release of pore water to the overlying water (Fanning et al. 1982) and enhanced adsorption/desorption processes with suspended particles (Tengberg et al. 2003). Previous resuspension studies using flumes have examined nutrient remobilisation from river sediments (Kleeberg et al. 2008; Rivera-Monroy et al. 2007; Davis et al. 2001), lake sediments (Luo et al. 2006; Sun et al. 2006; Scarlatos 1997) and partitioning of metals (Kalnejais et al. 2010; Couceiro et al. 2009; Kalnejais et al. 2007; Lansard et al. 2006) during resuspension. However the use of flumes for the investigation of dissolved nutrient fluxes associated with marine resuspension have been relatively few. Of these few Almroth et al. (2009), Tengberg et al. (2003) and Sloth et al. (1996) used only a single friction velocity (i.e. 1 level of resuspension)


during their experiments while more recently Kalnejais et al. (2010) applied multiple friction velocities. The authors have built on this earlier work by using an annular flume (Thompson et al. 2003) in which the hydrodynamics are fully constrained. A step wise increase in uni-directional water speed is applied ranging from no stirring (diffusive inputs only) to vigorous mixing conditions resulting in suspended sediment concentrations equivalent to those found under stormy conditions (Thompson et al. 2011). Unlike flumes used in the previously mentioned studies this allows the measurement of shear velocity (U*) and suspended particulate matter (SPM) directly alongside changes in nutrients in the water column. This approach provides a powerful method for studying resuspension events and associated changes in nutrient concentrations. The overall aims of this study were to assess (i) the seasonal variation in diffusive exchanges of nutrients across the sediment water interface from a temperate cohesive sediment and (ii) the influence of resuspension events on dissolved water column nutrient concentrations that are essential for primary production in the water column.

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(Fig. 1) that has been studied extensively over the last two decades (Greenwood et al. 2010; Teal et al. 2009; Weston et al. 2008; Reiss and Kroncke 2005; van Raaphorst et al. 1998; Everaarts and Fischer 1992). The water column has a maximum depth of 50 m, is well mixed in the winter and stratified during the summer when bottom water oxygen concentrations decline to 60% oxygen saturation (Greenwood et al. 2010). The OG provide benthic habitat to various macrofauna with Amphiura filiformis being the most abundant species (Teal et al. 2009). The site has been characterised using a variety of continuous and discreet measurements undertaken by the Centre of Environment, Fisheries and Aquaculture (Cefas, UK) throughout 2007 and 2008, including continuous in situ data collected using a benthic lander (mini-pod) deployed at the site. This lander measured water velocity and direction using an acoustic doppler current profiler (ADCP) and suspended particulate matter by optical backscatter sensors (OBS); see Greenwood et al. (2010) for instruments and calibration details. Sediment sampling

Materials and methods Sample site and water column sampling The Oyster Grounds (OG) is a large, depression in the central North Sea centred at 54°300 N and 4°300 E

Sediment samples were taken in April and August 2008 on the Cefas ship RV Endeavour (Cruises Cend 04-08 and Cend 08-08) as part of the Marine Ecosystem Connections programme. NIOZ (The Royal Netherlands Institute for Sea Research) box corers were used to retrieve intact cores from the OG

Fig. 1 Map of the North Sea and position of the sampling site, Oyster Grounds (OG)

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seabed with its overlying water. Burrowing fauna can destabilise sediments via bioturbation or biostabilise sediments through tunnel formation (Reise 2002), therefore cores containing macrofauna were excluded from sampling to focus the study on sedimentary biogeochemical processes rather than the influence of macrofauna. Of 8 cores collected only one was rejected. A 50 cm diameter core was collected for the resuspension experiment in April along with 30 cm diameter cores used for sediment characterisation samples and diffusive flux experiments in April and August. Three 30 cm box cores were subsampled, using 10 cm core tubes and sediment particle size and vertical pore water oxygen profiles were determined according to Sapp et al. (2010). Other parameters determined include organic matter content by loss on ignition (Luczak et al. 1997), and porosity calculated from computed tomography (CT) as described below. A further three 30 cm box cores were used to determine vertical inorganic nutrient profiles of pore water following the technique of Trimmer et al. (1998). This used a vacuum sipper system to draw pore water from a 1 cm deep section of the core through a 9 lm filter. The water was then filtered through a 0.2 lm filter into a storage vessel; samples were preserved with mercuric chloride (HgCl2) solution (see ‘‘Chemical analysis’’ section). Sediment densities were determined using CT scanning of syringe cores (2.8 cm diameter) that had been collected from the NIOZ box core and then immediately flash frozen with liquid nitrogen. The data collected was converted into sediment densities for the top 5 cm of sediment according to Amos et al. (1996). Using this technique the authors were able to determine densities in 0.1 cm horizons which they would otherwise be unable to do. The cores were scanned at Southampton General Hospital using a Siemens Sensation 64 scanner set at 140 kV, 140 mA, with 1 degree tilt to give 0.1 cm tomograms for the top 1 cm of sediment and 0.5 cm tomograms for the next 4 cm of sediment. Density data was used to calculate porosity in these narrow horizons according to Burdige (2006) qb ¼ uqw þ ð1 uÞqs where values are the densities of bulk sediment (qb), pore water (qw), sediment particles (qs) and u is porosity. qw and qs were assumed to be 1027 and 2650 kg/m3 respectively.

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Diffusive fluxes Diffusive fluxes of the measured dissolved inorganic nutrients (nitrate, nitrite, phosphate and dissolved silicon [DSi]) across the sediment–water interface were measured from 10 cm diameter sub-cores containing sediment and overlying water taken from the NIOZ box cores. The sediment used was intact and not sieved, thereby protecting the integrity of the sediment water interface. It is therefore likely that some meiofauna still resided within the cores and while the flux across the sediment water interface will be primarily driven by diffusion, some contribution by the meiofauna is expected. Under quiescent conditions this is closer to natural conditions than pure diffusion and for the rest of the article will be referred to as diffusive flux. The five sub-sampled cores were kept oxygenated, at ambient bottom water temperature and also in the dark to prevent photoproduction of inorganic nutrients (Southwell et al. 2010). Sub-samples (six) of the overlying water were removed over a 24 h period (Trimmer et al. 1998) and filtered through pre-rinsed and ashed (450°C for 4 h) Whatman GF/F filters, fixed with HgCl2 and stored for later analysis. A line of best fit was used on data collected from individual cores to provide an estimate of water column nutrient concentration change per hour. Resuspension experiment The physical and chemical effects of sediment resuspension were measured on the ship in April 2008. A 50 cm core of sediment was collected using a NIOZ box corer and a 30 cm outer diameter annular flume (Fig. 2) was placed on top of the sediment surface in the box core and pushed down until in penetrated to a depth of 5 cm. Once collected any suspended/resuspended matter in the box core was allowed to settle for 16 h (time until the OBS reading was constant—i.e. no further change in suspended sediment concentration) in a darkened atmosphere where photoproduction of inorganic nutrients was unlikely to occur (Southwell et al. 2010). The cores were considered intact when recovered, and the bed material itself was strong (Thompson et al. 2011), therefore it is felt that the majority of resuspended material observed on collection is likely to have come from a fluff layer that was present before recovery. Temperature remained the same throughout the resuspension experiment (9°C). The flume contained an acoustic doppler velocimeter (ADV) and


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Fig. 2 Elevation view diagram of the flume. A is the drive motor, B is the rotating lid, Cs are the paddles, Ds are the sample ports, Es are the OBS sensors and F is the ADV. All data provided in this study comes from the central sample port and OBS sensor E2. The hatched zone represents the water in the annular channel. Thompson et al. (2004)

optical backscatter sensors (OBS) to determine water velocity, turbulence, shear velocity (U*), bed shear stress, erosion rates and suspended particulate matter (SPM) concentrations. Flume dimensions and components are shown in Fig. 2 and further details are reported in Thompson et al. (2004). A full laser Doppler velocimeter calibration of the flume was carried out by Fung (1997), which showed that a bottom boundary layer was fully established at 1–2 cm above the bottom of the flume regardless of motor speed. Vertical velocities were low at low elevations, close to the bed. The influence of the walls and instrumentation were taken into account during this calibration to give a calibration between the velocity 8.5 cm above the bed, and the shear velocity at the bed. The flume was programmed to increase water column velocities incrementally in the channel and to remain at each velocity step for 11 min, 10 min to allow for characterisation of the type of resuspension as recommended by Amos et al. (2004) and 1 min in which to take the water sample. This gave an increasing bed shear stress in a step wise fashion over a period of approximately an hour. The range of conditions chosen for the experiment covers the typical averaged current velocities experienced at the OG as well as winter storms and extending upwards to conditions emulating resuspension due to dredging/ anthropogenic resuspension. The critical shear velocity

can be related to natural conditions as discussed later or in Thompson et al. (2011). Sediment bed erosion thresholds were calculated using a linear regression of SPM against applied friction velocity following the methodology of Amos et al. (2003), Widdows et al. (2007) and Sutherland et al. (1998). This has been found to be accurate even in cohesive sediments with high proportions of fine sands (Sutherland et al. 1998). Increases in water column SPM occurring significantly before the erosion threshold of the bed sediment were attributed to resuspension of fluff material. The erosion threshold fluff material was calculated from a linear regression of this portion of the data. Water samples were removed from the flume from the central sampling port (Fig. 2) via a 60 ml BD Plastipak syringe at the end of each period of constant velocity and prior to the next step change in velocity. Water samples from the syringes were then filtered through pre-washed and ashed Whatman GF/F filters for inorganic nutrient analysis. The percentage change in water column nutrient concentrations were calculated as % change ¼ ð½Nt ½Nt0 Þ=½Nt0 100 where [N]t is water column nutrient concentration at time t and [N]t0 is water column nutrient concentration at time 0.

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The amount of pore water released to the water column during resuspension and its contribution to the total observed change in water column nutrient concentrations was also estimated using SPM, density/porosity and pore water nutrient concentration data. Erosion depth and volume of pore water released were calculated as below (Thompson et al. 2011); E ¼ SPM/(As qb Þ Vpwr ¼ E*As u ½N]pwr ¼ ½N]pw Vpwr % contribution of pore water to change ¼ [N]pwr =ð[N]t [N]t0 Þ 100 where E is the erosion depth, SPM as dry mass, As is the surface area of the sediment bed, Vpwr is the volume of pore water released, [N]pw is nutrient concentration in the pore water and [N]pwr is total number of lmol of nutrient released from pore waters. Chemical analysis All inorganic nutrient water samples were filtered as described above and fixed with 5 lL 4 g/L HgCl2 per 1 ml sample and stored for later analysis. Addition of mercuric chloride has been shown to be effective at preserving nitrate, nitrite, phosphate and DSi (Kirkwood 1999). Nitrate, nitrite, phosphate and DSi in the pore waters and water column were determined on shore at the University of Portsmouth laboratory using a QuAAtro segmented flow analysis system (Seal Analytical, UK). Detection limits of nutrients were calculated as three times the standard deviation of analyses of 5 Milli-Q (Millipore) ultra-pure water samples; these Milli-Q samples were filtered and mercuric chloride added at the time the samples were collected. For nitrate, nitrite, phosphate, and DSi limits of detection were 0.06, 0.01, 0.01 and 0.05 lmol/L, respectively.

Results and discussion Background sediment and water column conditions General sediment characteristics for the OG in April and August 2008 show that the OG sediments were muddy sand with up to 15% silt (Table 1). Sediment

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densities and porosities in cores show typical vertical profiles (Fig. 3) where porosity decreases and density increases with depth as compaction occurs (Burdige 2006). However, porosity appears to decrease in the top 1 cm between April and August 2008 (Fig. 3), possibly due to destabilisation of sediment by increased bioturbation as infauna abundances and activities change with temperatures and food supply. Variations of this type have been observed in studies of vertical distribution of foraminifera at the OG (de Nooijer et al. 2008) and with mean macrofaunal abundances (Reiss and Kroncke 2005). Cefas data collected from SmartBuoys and landers in 2008 suggest that the phytoplankton bloom had begun at the OG during April but had yet to crash and sink to the sediments at the time of sampling (Greenwood et al. 2010) and that the water column was still well mixed at this point. Greenwood et al. (2010) also showed that following the 2008 phytoplankton crash the water column became stratified and bottom water oxygen concentrations rapidly decreased in mid May and remained low throughout August. The observed temperature increase at the OG alone is not enough to account for the reduced oxygen concentration as a change in physical solubility, indicating that the bottom 20 m of the stratified water column was responding to the turnover of carbon (Greenwood et al. 2010). The decrease in bottom water oxygen coincides

Table 1 Bottom water (1 m above sediment water interface) and mean OG sediment characteristics (0–1 cm depth) ± standard deviations (number of replicates) April 2008

August 2008

Units

Particle size

0.109

0.103

mm

Silt

8.3

12.5

% °C

Bottom water Temperature

7.6

17.0

Oxygen

9.5

6.2

mg/L

Oxygen penetration

0.83 ± 0.06 (3)

0.34 ± 0.07 (3)

cm

Organic matter (loss on ignition)

1.45

1.57

%

Nitrate

1.84

1.20

lmol/m2

Nitrite

0.42

0.13

lmol/m2

Phosphate

0.70

0.75

lmol/m2

DSi

7.07

11.9

lmol/m2

Bottom water


Porosity 0.5

0.6

0.7

0.5

0.6

0.7

0

Depth (cm)

Fig. 3 Porosity (open symbols) and density (closed symbols) data with standard deviations (2800 \ n \ 10500, pixel number from tomographs) for the OG sediment in a April 2008 and b August 2008

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0.5 1 1.5 2

(a)

(b)

2.5 3 3.5 4 4.5 5 1550

1550

1800

1850

1550

1550

1800

1850

3

Density (Kg/m )

with an almost 60% decrease in sediment oxygen penetration depth (Table 1). Sediment organic matter content increases when detrital material falling from the water column settles on the sea bed and is incorporated at a greater rate than it is remineralised. The data show little change in organic matter content with season (Table 1) suggesting remineralisation of any organic matter accrued between April and August after the phytoplankton bloom crash had already occurred and microbial activity was slowing due to depletion in the organic material abundance. Stoeck et al. (2002) similarly observed no changes in sediment chlorophyll a concentrations between samples taken in May and September at the OG in 1999. This is corroborated by bottom water chlorophyll a data (Greenwood et al. 2010) where high bottom water concentrations were observed throughout May 2008 followed by a decrease in August, suggesting a reduction in carbon export to the sediment. The authors theorise that sediment organic matter concentration did change between April and August, but the change was not detected due to the length of time between sampling. This theory is reinforced by de Nooijer et al. (2008) who indicate sediment organic carbon concentrations at the OG across 2002–2005 begin to increase in April, continue to increase until June/July after which they then decrease again. Changes in sediment organic carbon concentrations and profiles will relate to changes in the rate of remineralisation occurring in the sediments. The summer decrease in sediment oxygen penetration depth is a response to the decrease in bottom water

oxygen concentrations and increased remineralisation of labile organic carbon from the spring bloom and has been noted in the North Sea by others (Gao et al. 2009; Glud et al. 2003; Lohse et al. 1993) with sediment oxygen penetration being deepest in winter and shallowest just prior to stratification breakdown in October. The sediment surface layer (0–1 cm) in April was mainly oxic, with oxygen penetrating to 0.83 cm. This is in contrast to August when oxygen only penetrated to 0.34 cm (Fig. 4). The pore water concentrations of nitrate, phosphate and DSi in the surface sediment (*2 cm) were all found to be greatly enriched compared to the overlying waters (Fig. 4). Excepting DSi, pore water concentrations of other nutrients were generally greater in the cooler month of April than in August (Fig. 4). This seasonal pattern for pore water nitrate has been previously observed in North Sea sediments by Lohse et al. (1993) and is likely due to greater oxygen penetration and concentration in the overlying water stimulating nitrification over denitrification (Rysgaard et al. 1994). The increase in pore water nitrate concentrations at the 0–1 cm sediment horizon in both April and August is a common phenomenon explained by nitrification (Hall 1986). The subsequent decrease in nitrate concentrations deeper in the sediments is then likely due to denitrification which occurs under suboxic or anoxic conditions and results in the conversion of nitrate to nitrogen or nitrous oxide gas (Rysgaard et al. 1994). Phosphate concentrations, however, increase with depth as phosphate is released by the remineralisation of organic matter (Ingall and

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Nutrient concentration (µmol/L) -2 0 0

2

4

6

8

Nitrate

2

0 0.2 0.4 0.6 0.8 Nitrite

0

2

4

6

8

0

50 100 150 200

Phosphate

DSi

4

Depth (cm)

Fig. 4 Vertical pore water profiles with standard deviations (n = 3) of oxygen and dissolved inorganic nutrients nitrate, nitrite, phosphate, DSi at the OG in April 2008 (closed symbols) and August 2008 (open symbols). The 0 indicates the sediment water interface and negative values height above sediment in the water column


6 8 10 12 14 16 18 20 Oxygen concentration (µmol/L) -0.2

0

50

100 150 200 250 300

350

Depth (cm)

-0 0.2 0.4 0.6 0.8 1.0

Jahnke 1997). It seems likely that the secondary increase in pore water phosphate concentrations at 10 cm in April is related to iron reduction and corresponds with the increase in dissolved iron concentrations observed by Teal et al. (2009) at a similar depth at the OG in 2007. DSi concentrations increased by an order of magnitude from overlying water concentrations of *10 (7.1–11.9) lmol/L across the sediment–water interface to over 100 lmol/L at 4–5 cm in the surface sediment (Fig. 4). This was attributed to the burial and dissolution of biogenic opal in the surface sediment (Dixit et al. 2001). The DSi pore water profile seen here is typical of temperate coastal waters, with similar concentrations to those observed in the nearby Irish Sea by Gowen and Stewart (2005). Concentrations increase asymptotically and reach a constant value at a depth where silica dissolution ceases as a result of solution saturation, in this case around 10 cm. Interestingly DSi is the only nutrient found to have consistently greater pore water concentrations in

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August than April all through the vertical profile. This can be explained by the increased export of detrital material containing diatoms from the surface to the sediment between these dates and greater temperatures in August increasing opal dissolution (Kamatani 1982). This increase between the seasons is not observed for the other nutrients as they are actively used microbially or removed via adsorption while DSi is allowed to accumulate in the pore waters. Impact of diffusive and resuspension processes on water column nutrients Changes in water column concentrations above undisturbed sediment resulting from diffusive inputs are shown in Fig. 5. An increased DSi efflux from the sediment in August reflected increased sediment pore water concentrations when there was more opal present after the phytoplankton bloom crash and higher temperatures induced increased opal dissolution (Kamatani 1982). Whilst temperature is


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considered the main factor in increasing dissolution, increased bacterial numbers in summer at the OG (as observed by Osinga et al. 1996) may be responsible for the release of hydrolytic enzymes. These enzymes are capable of degrading the organic matrix protecting the frustules from dissolution (Bidle and Azam 2001) which would also enhance dissolved silicon release. Diffusive inputs of nitrate to the water column also increased with increased temperatures at the OG. Similar seasonal patterns were observed by Lohse et al. (1993) at the OG and van Raaphorst et al. (1992) at other North Sea sites. This increase in summer efflux also coincides with an increase in total bacterial numbers and biomass observed at the OG by Osinga et al. (1996) and further work by Caffrey et al. (2003) in California shows increased summer nitrate efflux from sediments with increased bacterial abundance, bacterial productivity and potential nitrification. Therefore despite expected increases in denitrification at the OG due to the decrease in oxygen concentrations in August, remineralisation and nitrification are also enhanced and nitrate fluxes out of the sediment increase. Similar rates of phosphate removal from the water column were observed in April and August under diffuse conditions (Fig. 5). The similarity in flux may be expected due to the similarity in the gradient of phosphate across the sediment water interface. Changes in physical parameters over the resuspension experiment in April are shown in Fig. 6 where the stepwise increase in shear velocity over time and April August

40 30 20

1

10 0

0.5 Phosphate DSi

Nitrate

Nitrite

0 -0.5 -1 -1.5

Nitrite and Phosphate 2 µmol/m /h

DSi and Nitrate 2 µmol/m /h

50

-2

Fig. 5 Diffusive changes in water column nutrient concentrations above undisturbed sediment for April and August 2008 in lmol/m2/h with standard errors shown (n = 5). DSi and nitrate are shown on the left y axis and nitrite and phosphate on the right y axis. Negative values indicate removal from the water column

resulting changes in SPM concentration are shown. Shear velocity values were 0.015, 0.021, 0.028, 0.040, 0.05 m/s, and corresponding mean SPM concentrations ± standard deviations for each of the steps were 0.25 ± 0.20, 0.33 ± 0.19, 0.79 ± 0.24, 3.1 ± 0.98 and 17 ± 7.2 g/L. Figure 6 only shows SPM values up to 5 g/L so changes at the lower shear velocity values can be distinguished. Remobilisation of a benthic fluff layer, a layer of disaggregated material found on top of a consolidated bed, was determined to begin at U* = 0.008 m/s using a linear regression of SPM and applied frictional velocity as described in the methods. Benthic fluff is produced when phytodetritus derived from algal blooms in surface waters combines with other suspended particles in the water column to form a high carbon, low density deposit immediately above the sediment surface (Jago et al. 2002) which is much more susceptible to resuspension than the denser sediments in the seabed. The second velocity step in the experiment was at the sediment resuspension threshold, U* = 0.02 m/s and the following steps then showed increasing sediment resuspension. This value corresponds well with data from Mehta et al. (1982) (0.05–0.4 Pa) and Winterwerp (1989) (0.1–0.5 Pa). The last three steps resuspended the sediment bed to differing degrees; the first was a low resuspension event eroding only 0.54 mm of the sediment surface, the next a mid-level resuspension event eroding to a depth of 1.56 mm while the final step was an extreme resuspension event with a total of 7.1 mm of the bed eroded (Table 2), which is representative of an anthropogenic impact such as dredging or trawling. The impacts of resuspension on nutrient concentrations are shown as percentage changes relative to concentrations prior to the resuspension in Fig. 7. Concentrations of nitrate, nitrite, phosphate and DSi started and ended at 1.84–3.08, 0.42–0.70, 0.70–1.15, and 7.07–15.9 lmol/L respectively (Table 1). Significant changes in water column nutrient concentrations due to resuspension have been observed by Kalnejais et al. (2010), Tengberg et al. (2003) and Fanning et al. (1982), while Sloth et al. (1996) and Almroth et al. (2009) have reported only small changes resulting from resuspension events. However, Tengberg et al. (2003), Fanning et al. (1982), Sloth et al. (1996) and Almroth et al. (2009) used only a single water column velocity and therefore processes occurring at lower or higher shear stresses were not uniquely identified. In the present study sediment shear stress was

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Fig. 6 Changes in SPM concentration (black) and U* (grey) during the resuspension experiment. The experiment was stopped briefly early in the run to repair a problem with the flume lid, no data is shown for that time. On the 5th stepwise increase at a U* of approximately 0.05 m/s the ADV lost the signal due to the high quantity of SPM in the water column although the lid continued to rotate at the same speed so an assumed value of U* has been used. Arrows indicate when water samples were taken from the flume

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Table 2 Percentage change in water column concentration due to release of pore water nutrients during resuspension Eroded (mm)

Nitrate

Phosphate

DSi

Low sediment resuspension

0.028

0.54

2.4

5.3

3.8

Mid sediment resuspension

0.040

1.56

5.1

4.3

2.7

High sediment resuspension

0.050

7.10

9.0

6.2

6.3

% nutrient change

U* (m/s)

123

sediment resuspension

100 80 60 40 20 0 -20 -40

incrementally increased to allow chemical changes occurring at differing shear stresses to be followed as in Kalnejais et al. (2010) and mass balances for each nutrient were calculated. The advantages to using the annular flume in this study are the more uniform shear stresses across the sediment bed and the simultaneous measurements of water velocities and SPM during the experiment rather than using separate calibrations to determine them. The mass balance was calculated from the water column concentrations, the pore water concentrations, the erosion depth and the porosity of the sediment giving a volume of pore water released to the overlying water column during resuspension. Using the method described here it is evident that at low friction velocities fluff remobilisation prior to sediment resuspension led to enhanced water column concentrations of nitrate and phosphate (11 and 7% respectively) as shown in Fig. 7. Fluff is an important

fluff resuspension

120

0

0.015

0.021

0.028

0.040

0.050

U* (m/s)

Fig. 7 Changes in water column concentrations of nitrate (squares), nitrite (diamonds), phosphate (triangles) and DSi (circles) as percentage of starting concentration against U* during the resuspension experiment. Negative values indicate a loss of that nutrient from the water column. Vertical bars indicate cumulative standard deviations for measurements shown. An arrow indicates the critical erosion threshold for bed resuspension

niche for microbial processes. When this material is resuspended its large surface area on exposure to the water column favours adsorption/desorption processes and microbial remineralisation by bacteria adsorbed to the fluff; both processes can change water column nutrient concentrations (Dale and Prego 2002). Little quantitative data is available on the effect of fluff resuspension on nutrient concentrations although a similar pattern concerning increasing water column


phosphate concentrations and fluff resuspension was observed by Kleeberg et al. (2008) using riverine water and sediment. In marine sediments, Kalnejais et al. (2010) show a similar increase in phosphate concentrations at low shear velocities in summer but not nitrate concentrations and they do not discuss whether the increase is thought to be from resuspension of a fluff layer or not. As resuspension of the sediment bed begins, water column concentrations of inorganic nutrients increase further with DSi showing the greatest increase (Fig. 7). DSi concentrations increased by 20% at an erosional depth of only 0.54 mm and then increased almost linearly up to 125% at the maximum shear velocity. DSi concentrations in the water column are controlled by the physical rate of dissolution of biogenic opal which is known to be affected by temperature (Kamatani 1982), aluminium ions (van Bennekom et al. 1991) and bacterial protease activity (Bidle and Azam 2001). During the resuspension experiment the temperature remained constant while Table 2 shows that the release of available DSi in the pore waters (calculated from the pore water profiles) contributed only 6.3% to the increase in water column concentrations attributed to resuspension. This more than doubling of the water column concentrations of DSi indicates dissolution is greatly increased during resuspension and/or possible increases due to bacterial activity (Bidle and Azam 2001) or desorption processes. If sediment resuspension were to occur in August at the OG the data suggests that an even larger quantity of DSi would be released to the water column than that seen in the April experiment as there would be both more DSi in surface pore waters and warmer temperatures increasing the rate of dissolution. The caveat to this is that less resuspension is likely to take place during the summer when there are fewer storms. Water column concentrations of nitrate and phosphate follow a similar pattern to one another and while unlike DSi they changed during fluff resuspension, they also increased by up to 67 and 66% respectively during resuspension of bed sediment (Fig. 7). Increases in nitrate concentrations are most likely caused by stimulation of nitrification during resuspension, which increases the available sediment surface area nitrifying bacteria can use, together with pore water injection into the water column. According to Rysgaard et al. (1994) nitrification rates in lake sediment can reach 200–600 lmol N/m2/h at oxygen

47

concentrations of 300 lmol/L. Such a rate applied to the experiments reported here would result in an increase of 37–110 lmol N/m2 over a resuspension step, which is enough to account for the changes observed (Table 2). Pore water input accounts for 9% of the water column increase in nitrate observed at the greatest rate of resuspension and accounts for less at lower shear stresses suggesting that pore water injection into the overlying water column becomes proportionally more important as deeper, less oxygenated and higher nutrient content pore water is mixed into the water column. If the resuspension experiment had been performed in August when water column oxygen concentrations and oxygen penetration depth decreased, anoxic sediments and pore waters would have been released to the water column, which would contain higher concentrations of nutrients, thus leading to a more significant input. Fluff resuspension in the summer would also be expected to result in greater release of nitrate and phosphate to the overlying water as there would be a greater reservoir of fluff and greater concentrations of organic matter available from detrital plankton material. Nitrite unlike the other macronutrients does not show a definite trend with increasing shear stress. This may be due to it being an intermediary product of both nitrification and denitrification and is therefore constantly being released and removed to and from the water column by different bacterial assemblages. In the present study a striking feature is that for phosphate the sediment changes from a sink under calm (diffusive) conditions to a source during fluff and/or sediment resuspension. There are conflicting reports about the impact of resuspension on the flux of phosphate across the sediment water interface. Tengberg et al. (2003) observed a decrease in water column phosphate concentrations with resuspension, Almroth et al. (2009) saw no significant impact, while Kalnejais et al. (2010) and Sondergaard et al. (1992) using coastal and lake sediments respectively, concluded from laboratory studies that resuspension significantly increases water column phosphate concentrations. Tengberg et al. (2003) suggested that phosphate concentrations decreased as phosphate was adsorbed onto iron oxides that were formed as ferrous iron mixed with the oxic water. In the present study phosphate concentrations in the water column rose by 66% at the end of the resuspension experiment, indicating that phosphate inputs as a result of release

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48


at this site significant wave action would be required. Wave and current data collected by Cefas (Cefas Wavenet, http://www.cefas.defra.gov.co.uk/) near the OG, combined with bed sediment resuspension data gathered from the flume suggest the same conditions existed at the OG, with sediment resuspension only occurring during stormy conditions and fluff resuspension occurring more frequently. The occurrence of fluff and sediment resuspension between January and August 2007 (Table 4) was estimated from a comparison of applied bed shear stresses calculated from benthic lander ADCP data (Thompson et al. 2011), and critical shear stresses found in the flume experiments. Significant bed sediment resuspension and thus release of nutrients occurs over the winter period up to March. Considering just diffusive fluxes it can be estimated that 1.6 mmol/m2 of nitrate was released from the sediments and 0.65 mmol/m2 of phosphate was removed from the water column by the sediments at the OG for the month of April in 2008. Currently North Sea biogeochemical models, such as ERSEM (Baretta et al. 1995), use only diffusive fluxes to calculate the sediment input or removal of nutrients from the water column. However, if the effects of fluff resuspension alone were added to the calculations for April 2008 our data indicate nitrate would be released to the water column and phosphate would also be released rather than removed. Our data indicates that water column concentrations of nitrate and particularly phosphate will be underestimated almost every month of the year if diffusive fluxes alone are used. As

through particle remineralisation and release from the sediment pore waters were greater than any adsorption that occurred. The difference between the quantitative amount of nutrients passed across the sediment water interface by diffusive flux and that by resuspension cannot be shown graphically due to different time scales. An indication of this difference is given in Table 3, where the number of days of diffusive flux required to exchange the same quantitative amount of instantaneously released nutrients by resuspension has been calculated. Cumulative nutrient load (Cumulative resus release (lmol/m2)) used in Table 3 is a measure of the combined nutrient load for that shear stress i.e. U* 0.028 shows the release from the sediment over shear stresses U* 0.015 ? 0.021 ? 0.028. Phosphate shows the most dramatic change with it taking almost 3 days of diffusive flux in April to remove the quantity of phosphate released to the water column from high resuspension. Nitrate, nitrite and DSi releases to the water column by high resuspension equate to 4.21, 3.23 and 3.19 days of diffusive flux respectively.

Environmental consequences of resuspension Tidally induced bottom shear stresses do not exceed the erosion threshold of the bed sediment at a deeper (110 m) North Sea site but were often high enough to resuspend fluff material (Jago et al. 2002), suggesting that for any resuspension of the bed sediment to occur

required to deliver that quantity of nutrient to the water column (April Equivalent DF, days)

Table 3 Changes in cumulative water column nutrient load during resuspension steps (Cumulative resus release, lmol/m2) and the number of days of equivalent diffusive flux in April U* (m/s)

Erosion depth (mm)

Nitrate Cumulative resus release (lmol/m2)

Nitrite April equivalent DF (days)

Cumulative resus release (lmol/m2)

Phosphate April Equivalent DF (days)


DSi April equivalent DF (days)


April equivalent DF (days)

0.015

0

58.3

1.38

-18.7

(1.53)

6.8

(0.31)

-52.5

(0.12)

0.021

0

54.4

1.29

-22.4

(1.83)

5.2

(0.24)

-47.9

(0.11)

0.028

0.54

58.1

1.38

11.9

0.97

6.3

(0.29)

191

0.45

0.040

1.56

116

2.76

-24.3

(1.99)

22.0

(1.01)

771

1.83

0.050

7.10

177

4.21

39.5

3.23

63.5

(2.91)

1347

3.19

Negatives indicate removal from the water column. Brackets show a reversal in the direction of nutrients across the sediment water interface between diffusive flux and resuspension (i.e. phosphate resuspension released 63.5 lmol/m2 and it would take 2.91 days to remove that from the water column via diffusion as diffusive flux is negative; it would take 4.21 days of diffusive flux to release as much nitrate as released by resuspension as diffusive flux is positive)

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49

Table 4 Percentage of each month when benthic fluff and bed sediment were resuspended in 2007 Month

Fluff

Bed sediment

January

100

100

February

ND

ND

March

58.63

19.4

April

43.75

0

May

4.27

0

June

3.40

0

July

ND

ND

August

0.98

0

ND no data available

the water column is shown to be well mixed at the OG in April the availability of these nutrients to phytoplankton would increase. Fluff and pore water nutrients are finite resources however and if the available nutrients are rapidly released, over prolonged periods of resuspension, inputs of nutrients to the water column will rapidly reduce unless the re-equilibration of pore-water and sediment is fast. Nevertheless the data indicates that resuspension can play an important and vital role in resupplying nutrients to the water column during bloom periods while shear stresses on the bed are too low to suspend bed sediment. Water column increases of nitrate and phosphate will be expected to follow sediment resuspension and be most pronounced during autumn/winter storms. The wave data collected for January 2007 suggests that the sediment bed is never entirely settled during this period and thus the issue of replenishing pore waters nutrients may become important as indicated above. Water column SPM data from Greenwood et al. (2010) clearly shows there are resuspension events during storms that are evident through the breakdown of water column stratification; these events are of the same magnitude as the mid-level flume conditions described here. Under these conditions sediment resuspension will be an important part of recycling nutrients from the sediments to the upper water column in the winter months, replenishing them for the phytoplankton bloom in the following spring. This is particularly important for the resupply of DSi to the water column that is needed for diatom growth. The observations given here support the theory of Hydes et al. (1999) that productivity is at least partially

maintained by recycling of nutrients from sediments in this part of the North Sea.

Conclusions and further work Sediment resuspension appears to be a significant process at the North Sea site studied and is also expected to be important for other temperate systems with similar cohesive sediment systems. The magnitude of inputs will be influenced by changes in temperature, organic matter supply and oxygen availability. These resuspension inputs are in addition to the diffusive inputs which show a clear difference between seasons with higher summer releases when the content of organic carbon and degrading phytoplankton debris is higher, and oxygen concentrations are lower. In the resuspension experiments reported here using freshly collected sediments, nutrient fluxes from the sediment were greater than could be accounted for by pore water mixing alone. Thus indicating that sediment resuspension enhances the release of nutrients via increased remineralisation, dissolution (Si) or desorption. Fluff and sediment resuspension should therefore both be taken into account when modelling benthic fluxes of nutrients. Further work is required to fully understand seasonal effects of oxygen, carbon and temperature variations on nutrient release and the effects of long term resuspension on water column nutrient concentrations. In particular, resuspension effects on DSi should be studied further to better understand these processes which may be a significant pathway for resupplying DSi to the water column and the diatom bloom in the spring. Acknowledgments We thank the crew and scientists of the RV Endeavour (Centre for Environment, Fisheries and Aquaculture Science (CEFAS), cruise Cend 04-08) and Sue Atkins for all her help with equipment preparation. This partnership project was funded equally by the UK Natural Environment Research Council (NERC NE/F003293/1 & NE/F 003552/1) and Defra as part of the Marine Ecosystem Connections (MEC, ME3205) project.

References Almroth E, Tengberg A, Andersson JH, Pakhomova S, Hall POJ (2009) Effects of resuspension on benthic fluxes of oxygen, nutrients, dissolved inorganic carbon, iron and manganese

123

50 in the Gulf of Finland, Baltic Sea. Cont Shelf Res 29:807–818 Amos CL, Sutherland TF, Radzijewski B, Doucette M (1996) A rapid technique to determine bulk density of fine-grained sediments by X-ray computed tomography. J Sediment Res 66:1023–1025 Amos CL, Droppo IG, Gomez EA, Murphy TP (2003) The stability of a remediated bed in Hamilton Harbour, Lake Ontario, Canada. Sedimentology 50:149–168 Amos CL, Bergamasco A, Umgiesser G, Cappucci S, Cloutier D, DeNat L, Flindt M, Bonardi M, Cristante S (2004) The stability of tidal flats in Venice Lagoon—the results of in situ measurements using two benthic, annular flumes. J Mar Syst 51:211–241 Baretta J, Ebenho¨h W, Ruardij P (1995) The European Regional Seas Ecosystem Model, a complex marine ecosystem model. Neth J Sea Res 33:233–246 Bidle KD, Azam F (2001) Bacterial control of silicon regeneration from diatom detritus: significance of bacterial ectohydrolases and species identity. Limnol Oceanogr 46: 1606–1623 Braeckman U, Provoost P, Gribsholt B, Van Gansbeke D, Middelburg JJ, Soetaert K, Vincx M, Vanaverbeke J (2010) Role of macrofauna functional traits and density in biogeochemical fluxes and bioturbation. Mar Ecol-Prog Ser 399:173–186 Burdige DJ (2006) Geochemistry of marine sediments. Princeton University Press, New Jersey, USA Caffrey JM, Harrington N, Solem I, Ward BB (2003) Biogeochemical processes in a small California estuary. 2. Nitrification activity, community structure and role in nitrogen budgets. Mar Ecol-Prog Ser 248:27–40 Cefas Wavenet (nd) http://www.cefas.co.uk/our-science/observingand-modelling/monitoring-programmes/wavenet.aspx Couceiro F, Rauen W, Millward GE, Lin B, Turner A, Falconer R (2009) Transport and reactivity of nickel in estuarine sediments: results from a high capacity flume. Mar Chem 117:71–76 Dale AW, Prego R (2002) Physico-biogeochemical controls on benthic-pelagic coupling of nutrient fluxes and recycling in a coastal upwelling system. Mar Ecol-Prog Ser 235: 15–28 Davis SE, Childers DL, Day JW, Rudnick DT, Sklar FH (2001) Nutrient dynamics in vegetated and unvegetated areas of a southern Everglades mangrove creek. Estuar Coast Shelf Sci 52:753–768 de Nooijer LJ, Duijnstee IAP, Bergman MJN, van der Zwaan GJ (2008) The ecology of benthic foraminifera across the Frisian Front, southern North Sea. Estuar Coast Shelf Sci 78:715–726 Dixit S, Van Cappellen P, van Bennekom AJ (2001) Processes controlling solubility of biogenic silica and pore water build-up of silicic acid in marine sediments. Mar Chem 73:333–352 Dounas C, Davies I, Triantafyllou G, Koulouri P, Petihakis G, Arvanitidis C, Sourlatzis G, Eleftheriou A (2007) Largescale impacts of bottom trawling on shelf primary productivity. Cont Shelf Res 27:2198–2210 Everaarts JM, Fischer CV (1992) The distribution of heavymetals (Cu, Zn, Cd, Pb) in the fine fraction of surface sediments of the north-sea. Neth J Sea Res 29:323–331

123

Biogeochemistry (2013) 113:37–52 Fanning KA, Carder KL, Betzer PR (1982) Sediment resuspension by coastal waters: a potential mechanism for nutrient re-cycling on the ocean’s margins. Deep-Sea Res Pt A 29:953–965 Fung A (1997) Calibration of flow field in MiniFlume. Natural Resources of Canda report L 23420-7-M339. 8 pp Gao Y, Lesven L, Gillan D, Sabbe K, Billon G, De Galan S, Elskens M, Baeyens W, Leermakers M (2009) Geochemical behaviour of trace elements in sub-tidal marine sediments of the Belgian coast. Mar Chem 117:88–96 Glud RN, Gundersen JK, Roy H, Jorgensen BB (2003) Seasonal dynamics of benthic O-2 uptake in a semienclosed bay: importance of diffusion and faunal activity. Limnol Oceanogr 48:1265–1276 Gowen RJ, Stewart BM (2005) The Irish Sea: nutrient status and phytoplankton. J Sea Res 54:36–50 Greenwood N, Parker ER, Fernand L, Sivyer DB, Weston K, Painting SJ, Kroger S, Forster RM, Lees HE, Mills DK, Laane RWPM (2010) Detection of low bottom water oxygen concentrations in the North Sea; implications for monitoring and assessment of ecosystem health. Biogeosciences 7:1357–1373 Hall GH (1986) Nitrification in lakes. In: Prosser JI (ed) Nitrification. IRL Press, Oxford, pp 127–156 Hopkinson CS Jr (1987) Nutrient regeneration in shallow water sediments of the estuarine plume region of the nearshore Georgia Bight, USA. Mar Biol 94:127–142 Hydes DJ, Kelly-Gerreyn BA, Le Gall AC, Proctor R (1999) The balance of supply of nutrients and demands of biological production and denitrification in a temperate latitude shelf sea—a treatment of the southern North Sea as an extended estuary. Mar Chem 68:117–131 Ingall E, Jahnke R (1997) Influence of water column anoxia on the elemental fraction of carbon and phosphorus during sediment diagenesis. Mar Geol 139:219–229 Jago CF, Jones SE, Latter RJ, McCandliss RR, Hearn MR, Howarth MJ (2002) Resuspension of benthic fluff by tidal currents in deep stratified waters, northern North Sea. J Sea Res 48:259–269 Kalnejais LH, Martin W, Signall RP, Bothner MH (2007) Role of sediment resuspension in the remobilization of particulate-phase metals from coastal sediments. Environ Sci Technol 41:2282–2288 Kalnejais LH, Martin W, Bothner MH (2010) The release of dissolved nutrients and metals from coastal sediments due to resuspension. Mar Chem 121:224–235 Kamatani A (1982) Dissolution rates of silica from diatoms decomposing at various temperatures. Mar Biol 68:91–96 Kirkwood DS (1999) Stability of solutions of nutrient salts during storage. Mar Chem 38:151–164 Kleeberg A, Hupfer M, Gust G (2008) Quantification of phosphorus entrainment in a lowland river by in situ and laboratory resuspension experiments. Aquat Sci 70:87–99 Lansard B, Grenz C, Charmasson S, Schaaff E, Pinazo C (2006) Potential plutonium remobilisation linked to marine sediment resuspension: first estimates based on flume experiments. J Sea Res 55:74–85 Lohse L, Malschaert JFP, Slomp CP, Helder W, Vanraaphorst W (1993) Nitrogen cycling in north-sea sediments— interaction of denitrification and nitrification in offshore and coastal areas. Mar Ecol-Prog Ser 101:283–296

Biogeochemistry (2013) 113:37–52 Lohse L, Helder W, Epping EHG, Balzer W (1998) Recycling of organic matter along a shelf-slope transect across the N.W. European Continental Margin (Goban Spur). Prog Oceanogr 42:77–110 Luczak C, Janquin MA, Kupka A (1997) Simple standard procedure for the routine determination of organic matter in marine sediment. Hydrobiologia 345:87–94 Luo LC, Qin BQ, Zhu GW, Sun XJ, Hong DL, Gao YJ, Xie R (2006) Nutrient fluxes induced by disturbance in Meiliang Bay of Lake Taihu. Sci China Ser D 49:186–192 Mehta AJ, Parchure TM, Dixit JG, Ariathuri R (1982) Resuspension potential of deposited cohesive sediment beds. In Kenedy VS (ed) Estuarine comparisons. Academic Press, New York, pp 591–609 Nayar S, Miller DJ, Hunt A, Goh BPL, Chou LM (2007) Environmental effects of dredging on sediment nutrients, carbon and granulometry in a tropical estuary. Environ Monit Assess 127:1–13 Nedwell DB, Assinder DJ, Parkes RJ, Upton AC (1993) Seasonal fluxes across the sediments-water interface, and processes within sediments. Philos T R Soc S-A 343: 519–529 Nizzoli D, Bartoli M, Cooper M, Welsh DT, Underwood GJC, Viaroli P (2007) Implications for oxygen, nutrient fluxes and denitrification rates during the early stage of sediment colonisation by the polychaete Nereis spp. in four estuaries. Estuar Coast Shelf Sci 75:125–134 Ogilvie BG, Rutter M, Nedwell DB (1997) Selection by temperature of nitrate-reducing bacteria from estuarine sediments: species composition and competition for nitrate. FEMS Microbiol Ecol 23:11–22 Osinga R, Kop AJ, Duineveld GCA, Prins RA, VanDuyl FC (1996) Benthic mineralization rates at two locations in the southern North Sea. J Sea Res 36:181–191 Rabus R, Bruchert V, Amann J, Konneke M (2002) Physiological response to temperature changes of the marine, sulfate-reducing bacterium Desulfobacterium autotrophicum. FEMS Microbiol Ecol 42:409–417 Reise K (2002) Sediment mediated species interactions in coastal waters. J Sea Res 48:127–141 Reiss H, Kroncke I (2005) Seasonal variability of infaunal community structures in three areas of the North Sea under different environmental conditions. Estuar Coast Shelf Sci 65:253–274 Rivera-Monroy VH, de Mutsert K, Twilley RR, CastanedaMoya E, Romigh MM, Davis SE (2007) Patterns of nutrient exchange in a riverine mangrove forest in the Shark River Estuary, Florida, USA. Hidrobiologica 17:169–178 Rocha C, Cabral AP (1998) The influence of tidal action on porewater nitrate concentration and dynamics in intertidal sediments of the Sado Estuary. Estuaries 21:635–645 Rysgaard S, Risgaard-Peterson N, Sloth NP, Jensen K, Nielsen LP (1994) Oxygen regulation of nitrification and denitrification in sediments. Limnol Oceanogr 39:1643–1652 Sapp M, Parker ER, Teal LR, Schratzberger M (2010) Advancing the understanding of biogeography-diversity relationships of benthic microorganisms in the North Sea. FEMS Microbiol Ecol 74:410–429 Scarlatos PD (1997) Experiments on water-sediment nutrient partitioning under turbulent, shear and diffusive conditions. Water Air Soil Pollut 99:411–425

51 Serpa D, Falca˜o M, Duarte P, Cancela da Fonseca L &Vale C (2007) Evaluation of ammonium and phosphate release from intertidal and subtidal sediments of a shallow coastal lagoon (Ria Formosa-Portugal): a modelling approach. Biogeochemistry 82:291–304 Sloth NP, Riemann B, Nielsen LP, Blackburn TH (1996) Resilience of pelagic and benthic microbial communities to sediment resuspension in a coastal ecosystem, Knebel Vig, Denmark. Estuar Coast Shelf Sci 42:405–415 Sondergaard M, Kristensen P, Jeppesen E (1992) Phosphorus release from resuspended sediment in the shallow and wind-exposed Lake Arreso, Denmark. Hydrobiologia 228:91–99 Southwell MW, Kieber RJ, Mead RN, Avery GB, Skrabal SA (2010) Effects of sunlight on the production of dissolved organic and inorganic nutrients from resuspended sediments. Biogeochemistry 98:115–126 Stoeck T, Kroncke I, Duineveld GCA, Palojarvi A (2002) Phospholipid fatty acid profiles at depositional and nondepositional sites in the North Sea. Mar Ecol-Prog Ser 241:57–70 Sun XJ, Zhu GW, Luo LC, Qin BQ (2006) Experimental study on phosphorus release from sediments of shallow lake in wave flume. Sci China Ser D 49:92–101 Sutherland TF, Amos CL, Grant J (1998) The erosion threshold of biotic sediments: a comparison of methods. In: Black KS, Paterson DM, Cramp A (eds) Sedimentary processes in the intertidal zone. Special Publications 139. Geological Society, London, pp 295–307 Teal LR, Parker R, Fones G, Solan M (2009) Simultaneous determination of in situ vertical transitions of color, porewater metals, and visualization of infaunal activity in marine sediments. Limnol Oceanogr 54:1801–1810 Tengberg A, Almroth E, Hall P (2003) Resuspension and its effects on organic carbon recycling and nutrient exchange in coastal sediments: in situ measurements using new experimental technology. J Exp Mar Biol Ecol 285:119–142 Thompson CEL, Amos CL, Jones TER, Chaplin J (2003) The manifestation of fluid-transmitted bed shear stress in a smooth annular flume—a comparison of methods. J Coast Res 19:1094–1103 Thompson CEL, Amos CL, Lecouturier M, Jones TER (2004) Flow deceleration as a method of determining drag coefficient over roughened flat beds. J Geophys Res 109:12 pp Thompson CEL, Couceiro F, Fones GR, Helsby R, Amos CL, Black K, Parker ER, Greenwood N, Statham PJ, KellyGerreyn BA (2011) In situ flume measurements of resuspension in the North Sea. Estuar Coast Shelf Sci 94:77–88 Trimmer M, Nedwell DB, Sivyer DB, Malcolm SJ (1998) Nitrogen fluxes through the lower estuary of the river Great Ouse, England: the role of the bottom sediments. Mar EcolProg Ser 163:109–124 van Bennekom AJ, Buma AGJ, Nolting RF (1991) Dissolved aluminium in the Weddell-Scotia confluence and effect of Al on the dissolution kinetics of biogenic silica. Mar Chem 35:423–434 van Raaphorst W, Kloosterhuis HT, Berghuis EM, Gieles AJM, Malschaert JFP, Van Noort GJ (1992) Nitrogen cycling in two types of sediments of the southern North Sea frisian front broad fourteens field data and mesocosm results. Neth J Sea Res 28:293–316

123

52 van Raaphorst W, Malschaert H, van Haren H (1998) Tidal resuspension and deposition of particulate matter in the Oyster Grounds, North Sea. J Mar Res 56:257–291 Vidal M (1994) Phosphate dynamics tied to sediment disturbances in Alfacs Bay (NW Mediterranean). Mar Ecol-Prog Ser 110:211–221 Wainright SC, Hopkinson CS (1997) Effects of sediment resuspension on organic matter processing in coastal environments: a simulation model. J Mar Syst 11:353–368 Weston K, Fernand L, Nicholls J, Marca-Bell A, Mills D, Sivyer D, Trimmer M (2008) Sedimentary and water column processes in the Oyster Grounds: a potentially hypoxic region of the North Sea. Mar Environ Res 65:235–249

123

Biogeochemistry (2013) 113:37–52 Widdows J, Friend PL, Bale AJ, Brinsley MD, Pope ND, Thompson CEL (2007) Inter-comparison between five devices for determining erodability of intertidal sediments. Cont Shelf Res 27:1174–1189 Winterwerp JC (1989) Flow-induced erosion of cohesive beds: a literature study. Rijkswatersaat—Delft Hydraulics Report No. 25 Wood HL, Widdicombe S, Spicer JI (2009) The influence of hypercapnia and the infaunal brittlestar Amphiura filiformis on sediment nutrient flux—will ocean acidification affect nutrient exchange? Biogeosciences 6:2015–2024