Benthic mineralization and solute exchange on a Celtic Sea sand-bank (Jones Bank)

Progress in Oceanography 117 (2013) 64–75

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Benthic mineralization and solute exchange on a Celtic Sea sand-bank (Jones Bank) Morten Larsen a,b,c,⇑, Bo Thamdrup c, Tracy Shimmield a, Ronnie N. Glud a,b,c a

Scottish Marine Institute, Scottish Association for Marine Science, Oban, Scotland, United Kingdom Greenland Climate Research Centre (CO Greenland Institute of National Resources), Kivioq 2, Box 570, 3900 Nuuk, Greenland c Institute of Biology and Nordic Center for Earth Evolution (NordCEE), University of Southern Denmark, Odense M, Denmark b

a r t i c l e

i n f o

Article history: Available online 21 June 2013

a b s t r a c t Benthic carbon mineralization and solute exchange was investigated on a Celtic Sea sandbank during July 2008. The sediment on the top of the bank consisted of consolidated sand, characterized by advective porewater transport and was difficult to sample, this to some extent compromised the investigations at this site. However, intact sediment cores were sampled at 4 stations at the slopes, base of the bank and off the bank (reference site). This sediment was used to assess rates and pathways for benthic diagenesis. Total sediment O2 uptake (TOU) ranged from 5.8 to 9.0 mmol m2 d1 and total sediment release rates of dissolved inorganic carbon (DIC) ranged between 8.60 and 13.8 mmol m2 d1. Microbial denitrification and sulfate reduction accounted for <2% and 12–28% of the total benthic carbon mineralization, respectively. The remaining mineralization is ascribed to O2 and Fe/Mn respiration, respectively. Activity profiles of unsupported 210Pb from all stations indicated deep mixing, presumably caused by intense trawling activity in the area. Calculation based on satellite tracking of fishing vessels suggest that on average 33% of the sediment is affected by trawling activity every year. This is presumed to facilitate high metal respiration rates by continuously oxidizing reduced Fe that would otherwise accumulate in the sediment. Extracted porewater profiles reflected elevated NO 3 levels as compared to microsensor deter mined NO 3 profiles that were measured in parallel. We suggest that this reflects NO3 leakage from meipool in the top 5 cm of the sediment exceeds the porewater pool ofauna – and that the intracellular NO 3 up to a factor of 45. However, this intracellular pool is presumably turned over at very slow rates, compared to the porewater pool subject to microbial denitrification. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The 100–200 m deep Celtic-Sea shelf area develops a strong stratification during the summer (Sharples et al., 2013). In general this leaves the upper 25 m of the water column depleted in nutrients (<1 lM) limiting phytoplankton production in the upper water column. However, recently chlorophyll hotspots have been identified during the summer, and the locally intensified activities have been linked to the presence of sand banks (Sharpels et al., 2013). It is hypothesized that intensified vertical mixing on the slopes of the banks results in an updraw of nutrient-rich water reaching the photic zone, a process that has previously been described for the Celtic Sea shelf edge (Sharples et al., 2007) and for Georges Bank, Gulf of Maine (Horne et al., 1996). The mixing events are mainly tidal driven and the photosynthetic hotspots presumably persist during most of the summer. The locally

⇑ Corresponding author at: Institute of Biology and Nordic Center for Earth Evolution (NordCEE), University of Southern Denmark, Odense M, Denmark. Tel.: +45 6550 2796; fax: +45 6550 2786. E-mail address: [email protected] (M. Larsen). 0079-6611/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pocean.2013.06.010

intensified production results in a high abundance of fish and the bank system is exposed to an intense pelagic and benthic fishery (Pinnegar et al., 2002). Trawling for bottom-dwelling fish is likely to impact the structure and function of the seabed (Duplisea et al., 2001). In the literature the main focus concerning bottom trawling has been on the disturbance that beams and otter trawls inflict on the bottom macro-faunal communities (Rosenberg et al., 2003; Smith et al., 2000). But the disturbance must also redistribute and oxygenate sediment particles that are suspended in large plumes and translocated by near-bottom currents (Riemann and Hoffman, 1991; Durrieu de Madron et al., 2000; Martin et al., 2008). This resuspension of the surface sediment mixes and redistributes particles across redox boundaries, resulting in an overall net sediment oxidation (Fanning et al., 1982; Riemann and Hoffman, 1991; Trimmer et al., 1999; Smith et al., 2000). This study investigates the potential difference in benthic carbon mineralization rates and pathways on and around a natural sand bank in the Celtic Sea during the summer of 2008. We focus on 5 sampling stations on and around the bank, as we hypothesise that the hydrodynamic driven vertical mixing at the bank slopes

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stimulates the pelagic primary production which in turn leads to differences in benthic-pelagic coupling between on and off bank stations. We hypothesise that these differences can lead to changes in the mineralization rates, solute exchange, and carbon preservation among the different stations on the bank. Rates and pathways were investigated in recovered sediment cores using traditional core incubation techniques in combination with labeled tracer incubations and porewater profiles. The findings are discussed in the context of the trawling activity and local hydrodynamic control. 2. Materials and methods 2.1. Study site The investigations were carried out on Jones Bank, which is a tidal bank of the Celtic Sea bank system that consists of more than 30 banks covering an area of 100,000 km2. The banks are in the literature described as elongated, parallel ridges up to 35 m high, 5– 15 km wide, and 40–120 km long, with a regular spacing of 16 km (Reynaud et al., 1999). The banks are oriented at right angles to the shelf break and occur at water depths ranging from 100 m to 170 m. Jones Bank is found at a water depth of 120–130 m, is 40 km long, but higher than what is described by Reynaud et al., 1999, reaching a maximum of 60 m. The study focused on five stations on and around the bank: MS1 (MS; Mooring Station) top of bank slope; MS2, on middle slope of north-east bank; MS3, on North-East base of bank; MS4 flat site South-East of bank (Fig. 1). MS5, to the North-West of the bank

was considered as a reference station unaffected by bank-induced hydrodynamics as the dominating current direction in general and during the investigation period was to the East and Southeast (Sharples et al., 2013). 2.2. Water column characteristics CTD profiles from all stations reflected a surface temperature of 15.2 °C and a bottom temperature of 10.6 °C and showed a distinct thermocline between 24 and 30 m, well above any of the benthic stations (Davidson et al., 2013). Bottom water concentrations of O2 (259–268 lM, 93.0–96.3% air sat.), DIC (2165–2178 lM) and NO 3 (6.1–9.5 lM) did not vary considerably among stations (Table 1). 2.3. Sediment and water sampling At MS1 the sediment was too sandy to be sampled by standard coring techniques; therefore we only recovered sediment with a Day grab (Rees et al., 1999; Sommerfield et al., 1995). Sediment recovered by the Day grab exhibited indications of disturbance, but small sub-cores (I.D. = 2.6 and 5.2 cm) were taken from the grab and used for measurements of sulfate reduction, O2 microprofiles and organic carbon (see below). At MS2, MS3, MS4, and MS5 sediment was sampled using a multiple-corer (MUC) equipped with 8 core liners (I.D. = 10.5 cm) (Barnett et al., 1984). All cores appeared undisturbed with clear overlying water and intact microstructures at the sediment surface. At each station a minimum of 14 cores were used for onboard analysis. Depending on measurements cores were either used intact or subcored. MS2 was sampled twice during the cruise with a 7 day interval; data from the two sampling events are combined and presented as MS2. Six cores were used for measurements of O2 and NO 3 and subsequent determination of total exchange of O2, DIC, and denitrification. Two to four cores were used for microprofiles. One core was used for measuring depth profiles of porosity, organic carbon and one to two cores for concentrations of dissolved inorganic carbon (DIC) and SO2 4 . Three subcores were used to determine sulfate reduction rates (SRR) and one core for measurements of potential anammox rates. Three cores were split and used for porewater extraction of nutrients, metals and for determination of the 210Pb activity, respectively. The number of successfully measured microprofiles varied among stations. 2.4. Basic sediment characteristics

Fig. 1. Bathymetry map of the study area with the position of the 5 sampling stations. A larger geographical map of the Celtic sea area can be found in Sharpels et al. (2013). MS1 (MS; Mooring Station) top of bank slope; MS2, on middle slope of north-east bank; MS3, on North-East base of bank; MS4 flat site South-East of bank. MS5, to the North-West of the bank was considered as a reference station.

Sediment porosity was calculated from the measured density and water content, quantified as water loss after drying at 70 °C for 24 h. In addition to the gravimetrical porosity measurements,

Table 1 Position, water depth, bottom water (BW); temperature, O2 concentration, DIC (dissolved inorganic carbon) and NO 3 concentration, measured on the 5 sampling stations, during the cruise in July 2008. Values in brackets are standard deviation. The measured O2 concentrations represent the mean of multiple measurements in the water column from a single CTD cast, 10 m above the sediment surface. BW Temp (°C)

BW O2 (lM) (% air sat.)

BW DIC (lM)

BW NO 3 (lM)

72

10.7

2170 (±10, n = 5)

6.1 (±1, n = 2)

07°87.40 W

108

10.5

2174 (±5.1, n = 5)

8.5 (±0.4, n = 4)

07°81.90 W

119

10.6

268 (±0.2, n = 21) 96.3 (±0.08, n = 21) 267 (±0.2, n = 21) 96.0 (±0.05, n = 21) 264 (±0.1, n = 21) 94.7 (±0.04, n = 21) 267 (±0.1, n = 21) 96.3 (±0.04, n = 21) 259 (±0.07, n = 21) 93.0 (±0.02, n = 21)

2175 (±8.7, n = 6)

7.9 (±0.3, n = 2)

2165 (±18, n = 6)

6.9 (±0.8, n = 3)

2178 (±11, n = 4)

9.5 (±0.8, n = 2)

Stn. ID

Latitude (N)

Longitude (W)

MS1

49°86.50 N

07°94.90 W

MS2

49°90.70 N

MS3

49°94.30 N 0

0

Water depth (m)

MS4

49°76.0 N

07°67.6 W

123

10.6

MS5

50°00.00 N

08°15.00 W

118

10.5

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porosity near the sediment surface was estimated from O2 microprofiles assuming O2 conservation right across the interface as described in (Glud et al., 1995; Epping et al., 1999):

u¼




dC dz DBL
dC DS dz SED

D0

ð1Þ

where / is the sediment porosity and (dC/dz)DBL and (dC/dz)SED is the O2 concentration gradient in the DBL and sediment, respectively. Sediment organic carbon and nitrogen content was analyzed on a Costech, ECS 4010 Elemental Combustion System (D’Hondt et al., 2009). Activity of 210Pb was measured using a gamma ray spectrometer (EG&G 92X-P, Ortec or InSpector 2000, Canberra) (Turner et al., 2006). Grain size distribution was measured using a laser particle size analyzer (LS230 Coulter Counter, Beckman) (Howe et al., 2007). 2.5. Microprofiles of O2 and NO 3 Sediment cores for microprofiles and subsequent total exchange rates were submerged in a 100 L tank filled with bottom water sampled by a CTD rosette 5 m above the sediment surface. The tank was kept at in situ temperature (10.5 °C) and submerged cores were for all stations pre-incubated at in situ O2 saturation for 24 h prior to any measurements. In situ bottom water O2 concentration was maintained with two mass flow controllers (5850S, Brooks instruments) connected to a control and readout unit (0154, Brooks instrument) using O2-free N2 and compressed atmospheric air. Teflon coated magnets were suspended from the inner wall of each core-liner and their continuous rotation was maintained by an externally rotating magnet; this ensured a well mixed overlying water phase inside each core and a continuous exchange of water between the incubator tank and core-liners (Rasmussen and Jørgensen, 1992). Oxygen profiles were measured by Clark type microelectrodes equipped with an internal reference and a guard cathode (Revsbech, 1989). The sensors had external tip diameters of 10–20 lm, a stirring sensitivity <2% and a 90% response time of <1 s (Gundersen et al., 1998). Microprofiles of NO 3 were measured with newly developed biosensors (Revsbech and Glud, 2009). The tip size of these sensors were 40–50 lm, they had no stirring sensitivity and a 90% response time of 50 s. Both sensor types were mounted on a motor driven micromanipulator and the sensor currents were measured using a picoammeter (PA 2000, Unisense) and transferred via an A/D-converter to a PC. The O2 and NO 3 microprofiles were measured at a vertical resolution of 100 lm and 200 lm, respectively. The vertical profiles were calibrated against the measured analyte concentration of the water in the incubation tank and zero values were recorded at depths in the sediment where analytes were presumed to be depleted. The zero values recorded in the sediment were checked  against signals in O 2 and NO3 -free samples kept at in situ temperature. For O2 microprofiles, the diffusive oxygen uptake (DOU) was calculated using Fick’s first law of diffusion, J = D0(dC/dz), where dC/dz is the O2 concentration gradient at a given layer within the Diffusive Boundary Layer (DBL) and D0 is the molecular diffusion coefficient corrected for salinity and temperature. Volume-specific O2 consumption and NO 3 consumption/production were estimated from the measured microprofiles, using diffusion–reaction models and by assuming zero order reaction kinetics and steady state, using the software package ‘‘Profile’’ (Berg et al., 1998). Microprofiles across the sediment surface were modeled as R = Ds(@ 2C/@z2), with boundary conditions C(z = 0) = C0 stating that solute concentration at the top is held constant (and set equal to the water phase

concentration) and no flux at the bottom of the profile. Use of these boundary conditions allowed an independent comparison between the depth-integrated turn-over rates and the diffusive exchange calculated from the DBL gradient. The diffusion coefficient of O2 and NO 3 at in situ temperature and salinity were estimated according to Li and Gregory (1974). Sediment diffusivity was determined as Ds = D0/2 where D0 is the free solution diffusion coefficient and / is sediment porosity (Ullman and Aller, 1982). 2.6. Determination of total solute exchange and NO 3 reduction rates After pre-incubation and microprofiling of O2 and NO 3 (as described above) the 6 cores were capped with acrylic lids and incubated. For every 4–5 h one core was de-capped and water samples for DIC and nutrients were collected. The O2 concentration of the enclosed water was monitored continuously with an optode (Fibox 3, PreSens GmbH) until de-capping of the individual cores. In none of the incubations did the O2 concentration drop by more than 20% of the initial concentration. DIC samples were taken in triplicates with a gravity feed overflow and stored in 12 mL glass vials (Exetainers, Labco) spiked with 50 lL saturated HgCl2 until later analysis. DIC concentrations were analyzed using a CO2 Analyzer (Coulometer CM5014, UIC) (Rysgaard et al., 1998) after the sample had been acidified using an acidification module (CM5130, UIC). 3 þ Samples for nutrients (NO 3 , NH4 , PO4 and silicate) were filtered, frozen at 20 °C and stored until further analysis – only NO 3 results are reported here, but the remaining data can be supplied by contacting the authors. Nutrient concentrations were analyzed colourimetrically on a flow injection auto-analyzer (Quichchem 8500, Lachat) (Carvalho and Eyre, 2011). In order to determine total sediment NO 3 reduction rates by denitrification and anammox we applied a modified version of the isotope pairing techniques (IPT) (Risgaard-Petersen et al., 2003). After preincubation for total exchange, the de-capped cores were allowed to re-equilibrate for at least 4 h and subsequently the water level of the tank was lowered below the edge of the cores. Hereafter, 15 NO 3 was added to each core reaching a final concentration of 8.2–12.8 lM. Given the natural NO 3 concentration in the bottom water this resulted in a specific labeling of 0.27–0.68. After addition of 15 NO 3 , the cores were incubated for 12 h to ensure sufficient time for the added 15 NO 3 to diffuse through the oxic zone. During this period the O2 concentration in the individual cores was kept at in situ concentrations by aeration of the gas mixer. As pre-incubation time was relative short, only very little labeled N2 was lost during this procedure and a start sample was taken before re-capping the cores and incubated for a maximum of 32 h. Samples for 29/30N2 from the water phase were collected from each core as they were successively de-capped at a 4–6 h interval. After sampling the clear water phase, 2 mL ZnCl2 (50% w/w) was added to the remaining water, before it was gently slurried with the top 4 cm of the sediment. After coarser sediment particles had settled, water samples for 29/30N2 determination were collected. All samples were taken using a gravity-fed overflow. Samples for 29/30N2 determination in the water phase and slurry were taken in duplicates in 12-mL glass vials (Exetainers, Labco) and 200 lL ZnCl2 (50% w/v) was added. The production of 14N15N and 15N15N from the incubations with 15 NO 3 enrichment were determined using continuous-flow isotope ratio mass spectrometry in line with a gas chromatograph (Delta V Plus, Thermo). Denitrification rates (r14) were calculated as described in Risgaard-Petersen et al. (2003). This calculation requires a separate determination of the relative importance of anammox for total N2 production (see below):

r14 ¼

ð1  raÞ  R29  ra ð2  raÞ

ð2Þ

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M. Larsen et al. / Progress in Oceanography 117 (2013) 64–75

where R29 is the ratio between 29N2 (14N15N) and 30N2 (15N15N) production and ra is the relative contribution of N2 through anammox. Substituting Eq. (2) in (3) gives the genuine N2 production, p14:

p14 ¼ 2  r 14  ½p29 N2 þ p30 N2  ð1  r 14 Þ

ð3Þ

where p29N2 and p30N2 are total production of p29N2 and p30N2. From Eq. (2) the production of N2 via denitrification D14 can be calculated:

D14 ¼ p14  ð1  raÞ

ð4Þ

2.7. Anammox potential  Rates of anammox, i.e. NHþ 4 oxidation by NO2 to N2 (Thamdrup and Dalsgaard, 2002; Dalsgaard and Thamdrup, 2002), were investigated in four different parallel anoxic time-series incubations 15 14 spiked with different combinations of 15 NH NO NH 4, 4 , and 3, 14 NO3 (Rysgaard et al., 2004). Each series consisted of six, 12 mL glass vials (Exetainers, Labco) filled with 2 mL homogenized sediment sampled 3–4 mm below the determined O2 penetration depth. The Exetainers were topped up with anoxic NO 3 -free seawater. The four series were spiked with the following tracer com15 14 14 15 bination; A: 15 NHþ NHþ NO NHþ NO 4 , B: 4 + 4 + 3 , C: 3 and D:  15 NO3 , all individually added to a final concentration of 40 lM, corresponding to a specific labeling of 0.83–0.97 and 0.90–0.92 of þ the indigenous NO 3 and NH4 pool, respectively. Incubations were stopped by adding 200 ll 50% ZnCl2 (50% w/v) to the glass vials from each time series approximately every 4 h. The first sample was taken immediately after tracer addition and was used to confirm the specific labeling. The accumulation of 29/30N2 in the time series A and B was used to determine potential denitrification (Eq. (5)), and accumulation in C and D to determine potential anammox rates (Eq. (6)) (Thamdrup and Dalsgaard, 2002):

Denitrification ¼ p30 N2  F 2 N

ð5Þ

1 29 30 Anammox ¼ F 1 N  ½p N2  2  ð1  FN Þ  FN  p N2  29

14

15

ð6Þ

30

where p N2 is the production of N N, p N2 is the production of N15N, and FN is the fraction of 15N in the NO 3 pool.

15

2.8. Porewater profiles Sediment cores were sectioned in 1–0.5 cm depth intervals in a N2-flushed glove bag. Porewater for NO3 and metals (Fe/Mn) was subsequently extracted by centrifugation for 10 min at 5000 rpm. Samples for DIC determination were filtered and transferred into 3 mL glass vials (Exetainers, Labco) spiked with 20 lL saturated HgCl2 and kept at 5 °C until analysis (see above). Samples for  3 þ SO2 4 NO3 , NH4 , PO4 and silicate determination were transferred to 2 mL Eppendorf tubes and frozen (18 °C) until analysis. SO2 4 was analyzed on an ion chromatograph (ICS-2000, Dionex) (Mor3 þ ales et al., 2000) and samples for NO 3 , NH4 , PO4 and silicate were analyzed as described above. Porewater samples for trace metals were acidified with 100 lL HNO3 and stored at 5 °C until analysis on an ICP-MS (VG PQ3 Quadrupole S+) (Dean et al., 2007; Howe et al., 2007). 2.9. Sulfate reduction rates For determination of sulfate reduction (SRR) 5 lL-aliquots of SO2 (80 kBq lL1), were injected horizontally through silicone 4 ports at 1 cm intervals from the sediment surface and down to 15 cm (Jørgensen, 1978). After 35 SO2 injection cores were dark4 incubated at in situ temperature for 24 h, and subsequently sliced into 1 cm slices that were transferred to 50 mL centrifuge tubes

35

containing 5.7 mL ZnCl2 (20% w/v) and shaken vigorously. Samples were then frozen at 18 °C until further analysis. The production of reduced 35S was quantified by the single-step chromium reduction method (Fossing and Jorgensen, 1989) and SRR was calculated as follows:

SRR ¼

ðSO2 4 Þ  a  24  1:06 ðA þ aÞ  h

ð7Þ

where a is the total radioactivity of ZnS, A is the total radioactivity 2 of SO2 4 after incubation, h is the incubation time in hours, (SO4 ) is 3 the sulfate concentration in nmol cm sediment, and 1.06 is a correction factor for the assumed isotope fractionation. 2.10. Fishing vessel tracking Fishing frequency was calculated using data from European Community Satellite Vessel Monitoring System (VMS). VMS monitors all bigger fishing vessels (>15 m) and records vessel identification, date, time, course and speed every 2 h. Trawling activity was assumed if vessel speed was less than 9.3 km h1 (Hiddink et al., 2007). Fishing vessel data from the last 6 years were used to determine the fishing activity in the area. To estimate the area impacted by the trawling activity it is assumed that the width of the total fishing gear is 24 m (two beam trawls each 12 m wide or one otter trawl of 24 m wide) (Hiddink et al., 2007). Therefore, at a VMS record frequency of once every 2 h, one satellite monitoring record in one grid cell represents a trawled area of 0.449 km2. The grid cell size for the Jones Bank area is 1290 km2. 3. Results 3.1. Sediment characteristic The sediment surface at the top of the bank (MS1) consisted of medium grained bioclastic sand with an average grain size of 513 lm (SD ± 130) and an organic carbon content of 0.76% dry wt/wt. No sediment porosity could be calculated for this station using the gravimetrical approach. For MS1 the organic carbon content showed no depth trend and varied between 0.75 and 0.90% of sediment dry weight to the maximum measuring depth of 10 cm. Sediment at stations MS2 to MS5 consisted of hard packed and poorly sorted cohesive muddy/fine sand. Mollusk shells, especially from Scaphopoda, were found at depths of 15 cm, but only little living macrofauna was observed at any of the stations. The amount of organic carbon did not show any obvious trends with depth in the top 10 cm of the sediment, and values remained between 3.8% and 4.2% of sediment dry weight for MS2-5 (Fig. 2). For all stations (except MS1) sediment porosity decreased from 0.63 to 0.70 near the sediment surface down to 4–5 cm where the values reached a plateau of 0.59–0.53 (Fig. 2). Porosity at the very sediment surface as estimated from Eq. (1) amounted to a minimum of 0.85 (SD ± 3.3, n = 4) for MS1 and a maximum for 0.90 (SD ± 6.6, n = 5) for MS 4. This is considerably higher than quantified by the standard gravimetrical procedure. However, as the microsensor derived values represent conditions at the very surface we used them for all microprofile calculations (see below), while calculations on deeper sediment layers were based on the gravimetrical procedure. Permeability can be calculated from the porosity and the grain diameter using the empirical Kozeny–Carman equation, assuming a fairly uniform sediment grain diameter (Bear, 1972; Boudreau, 1997): 2

K¼

ð5:6  103  u3  ds Þ 2

ð1  uÞ

ð8Þ

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M. Larsen et al. / Progress in Oceanography 117 (2013) 64–75

0

Depth (cm)

2

4

6

8

10 0

2

4

6

8 50

Organic Carbon (wt %)

60

70

0

10

Porosity (vol %)

20

C/N (wt/wt)

Fig. 2. Depth profiles of organic carbon, porosity and C/N ratio for MS2 (d) MS3(s) MS4(.) and MS5(D). Horizontal line represents the sediment surface.

where / is the porosity and ds is the sediment grain diameter. The calculated permeability for MS1 was 1227 D, which clearly indicates permeable conditions with extensive advection. However, the calculated permeability for the remaining stations only ranged from 59 to 144 D (average for top 5 cm), lowest at MS2 and highest MS5. According to Huettel and Gust (1992) sediments with permeability coefficients as found on MS2 to MS5 can potentially be affected by advective porewater transport at high flow velocities, but at the given setting they can be regarded as cohesive. Sedimentation rates were evaluated based on activity profiles of unsupported 210Pb from MS2 to MS4 which showed activities ranging from 2.3 to 43.7 Bq kg1. None of the profiles showed the expected decrease with increasing depth (Fig 3). Thus, all profiles indicated mixing down to the bottom of the recovered sediment

cores and no sediment accumulation rates could be inferred. The only station with a potential depth trend was station MS5, but values were very noisy and the activity decline could not be quantified with any certainty. 3.2. Microprofiles of O2 and NO 3 O2 microelectrode profiles showed that O2 penetration ranged from 6.1 mm (SD ± 0.31, n = 5) at MS3 to a maximum of 8.3 mm (SD ± 1.2, n = 8) at MS5. The diffusive O2 uptake (DOU) calculated from the concentration gradient within the DBL, ranged from 5.6 (SD ± 0.84, n = 6) at MS1 to 8.1 (SD ± 1.9, n = 8) mmol m2 d1 at MS3. The modeled depth-integrated O2 uptake was on average, for all stations, 4.6% higher (SD ± 0.25, n = 18) than DOU calculated on the basis of the DBL-gradient (Table 2).

0

Depth (cm)

2

4

6

8

10

0

5

10

C0 /Cz

15

20

0

5

10

C0 /Cz

15

20

0

5

10

C0 /Cz

15

20

0

5

10

15

20

C0 /Cz

Fig. 3. Depth profiles of unsupported 210Pb from all stations, expressed as C0/Cz. C0 values are 39.6, 30.6, 31.4 and 240 Bq kg1 for MS2, 3, 4 and 5 respectively. Measurements represent the mean values from 3 cores.

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M. Larsen et al. / Progress in Oceanography 117 (2013) 64–75

Table 2 Summary of O2 and NO 3 microprofiles data. Mean ± standard deviation (in brackets) at each station. OPD: Oxygen penetration depth. DOUDBL: Diffusive oxygen uptake calculated from the O2 gradient in the Diffusive Boundary layer. DOUMODL: Model depth integrated O2 uptake. Rvol max: Maximum volume specific O2 uptake. NPD: NO 3 penetration depth.  NO 3 flux: Modelled net NO3 flux across the sediment surface, negative values represent uptake. N2 prod.: Modelled N2 production. ND., No data. O2 uptakes rates are show as positive values. Stn. ID

MS1 MS2 MS3 MS4 MS5

NO 3 profiles

O2 profiles OPD (mm)

DOUDBL (mmol m2 d1)

DOUMODL (mmol m2 d1)

Rvol max (nmol cm3 d1)

NPD (mm)

NO 3 flux. (mmol m2 d1)

N2 prod. (mmol m2 d1)

6.9 7.8 6.1 7.7 8.3

5.6 5.0 8.1 7.9 6.2

7.3 5.9 7.6 6.4 6.6

2518 1993 2592 1395 1850

7.9 (1.7, n = 5) 8.2 (0.14, n = 2) 8.6 (1.6, n = 5) 10.4 (1.7, n = 5) ND

0.04 (0.02, n = 5) 0.07 (n = 1) 0.1 (0.09, n = 4) 0.05 (0.08, n = 3) ND

0.10 0.14 0.14 0.24 ND

(1.2, n = 7) (1.0, n = 7) (0.31, n = 5) (0.70, n = 8) (1.2, n = 8)

(0.84, n = 6) (1.5, n = 6) (1.9, n = 5) (1.9, n = 3) (1.6, n = 4)

A

0.0

(1.8, n = 6) (1.8, n = 6) (1.4, n = 5) (0.88, n = 3) (0.62, n = 4)

A

MS3

(1185, n = 6) (1039, n = 6) (1082, n = 5) (443, n = 3) (998, n = 4)

B

MS5

B

MS3

(0.02, n = 5) (n = 1) (0.03, n = 3) (0.1, n = 3)

MS4

Depth (cm)

0.2 DOU

DOU

6.7 mmol m-2 d-1

6.5 mmol m-2 d-1

Depth integrated comsumption

Depth integrated comsumption

6.6 mmol m-2 d-1

5.5 mmol m-2 d-1

0.4

0.6

0.8

1.0 0

150

300

0

[O2 ] (µM) 0

500

150

300

0

1500 0

O2 cons. (nmol cm-3 d-1)

500

10

15

20

0

[NO3 ] (µM)

1000

1500

-150

NO 3

O2 cons. (nmol cm-3 d-1)

-75

0

75

prod. (nmol

cm-3

5

10

15

20

[NO3 - ] (µM)

-

[O2 ] (µM)

1000

5

150 -150

d-1 )

NO3-

-75

0

prod. (nmol

75

cm -3

150

d -1)

Fig. 4. (A) O2 microprofiles from Jones bank sediment, obtained from bank (MS3) and reference station (MS5), respectively. Rates of DOU and depth integrated consumption O2 are shown. Dots represent measured O2 concentration and the line fitted by model software profile. Fig. 4B. NO 3 microprofiles from Jones bank sediment, obtained from bank (MS3) and off-bank station (MS4), respectively. Dots represent measured NO 3 concentration and the line fitted by model software ‘‘profile’’. Consumption rates are shown with bars. Horizontal line represents the sediment surface.

Table 3 Mean flux and process, rates ± standard deviation (in brackets) at each station. DOU (diffusive oxygen uptake) and TOU (total oxygen uptake) rates are show as positive values. DIC: Dissolved inorganic carbon. CRQ: Community respiration quotient. SRR: Sulfate reduction rates. Uptake rates are shown as positive values. Stn. ID

Fluxes (mmol m2 d1) DOU

MS2 MS3 MS4 MS5

5.8 8.3 7.1 5.8

(2.4, (1.7, (2.8, (1.3,

Rate processes (mmol m2 d1)

TOU n = 7) n = 6) n = 8) n = 7)

7.3 7.5 9.0 5.8

(1.7, (2.3, (3.0, (1.0,

n = 10) n = 6) n = 7) n = 6)

DIC

CRQ

SRR

Denitrification

% Anammox

Total N2 prod.

10.7 (2.0, n = 3) 8.9 (2.3, n = 4) 13.8 (4.1, n = 4) 8.6 (1.8, n = 4)

1.47 1.20 1.53 1.49

0.93 (0.12, n = 3) 1.1 (0.17, n = 3) 0.76 (0.05, n = 3) 0.68 (0.89, n = 3)

0.127 (0.093, n = 12) N.D 0.079 (0.044, n = 6) 0.039 (0.046, n = 6)

14.8 N.D 8.8 22.0

0,154 N.D. 0.087 0.051

The highest volume-specific consumption rates were generally found in the middle and bottom of the oxic zone on all stations and profiles, with a mean rate of 2005 nmol cm3 d1 (SD ± 968, n = 18), for MS2-MS5. The location of maximal O2 consumption suggests a zone with reoxidation of reduced chemical species diffusing up into the oxic zone, in contrast to the oxidation of newly settled fresh labile carbon at the very surface (Fig. 4A). NO 3 microprofiles showed penetration depths of 7.9–10.4 mm, and values were not statistically different among stations (Table 2)

(P = 0.13). Profiles from MS3 and MS4 showed a sub surface NO 3 maximum, indicating active nitrification (Fig. 4B). Some profiles from MS1 showed more or less constant NO 3 concentrations (bottom water concentrations) down to 1.6–3.9 mm. Rates of NO 3 consumption modeled from the porewater microprofiles ranged from 0.07 mmol m2 d1 (n = 1) at MS2 to 0.10 (SD ± 0.09, n = 4) at MS3. Rates of NO 3 consumption modeled for the prevailing anoxic zone ranged from 0.10 mmol N2 m2 d1 (SD ± 0.02, n = 5) at MS 1 to 0.24 mmol N2 m2 d1 (SD ± 0.1, n = 3) at MS5 (Table 2).

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3.3. Sediment–water O2 uptake and nutrient fluxes across the interface Total Oxygen Uptake rates (TOU) ranged from 5.8 mmol m2 d (SD ± 1.0, n = 6) at MS5 to 9.0 mmol m2 d1 (SD ± 3.0, n = 7) at MS4, but the rates were not significantly different (P = 0.17) between stations (Table 3). DIC fluxes were slightly higher than the TOU fluxes at all stations, ranging between 8.6 mmol m2 d1 (SD ± 1.8, n = 4) at MS3 and 13.8 mmol m2 d1 at MS4 (SD ± 4.1, n = 3). The apparent community respiratory quotient (CRQ = DIC/ TOU) thereby varied between 1.20 and 1.53, highest at MS4 and lowest at MS3. DOU was generally slightly lower than the measured TOU from the same station, although not statistically significantly (P = 0.38) different (Table 3). This reflects a low contribution of fauna mediated O2 uptake consistent with the observed low macro faunal abundance in the recovered sediment cores. Some of the observed discrepancy between DOU and TOU could potentially be ascribed to the sediment relief which in reality describe a 3D landscape, not accounted for by the 1D microprofile derived diffusive O2 exchange. In general this, however, only accounts for 10% of the TOU in typical coastal sediments (Røy et al., 2002; Glud et al., 2003).

N2 production rates accounted for 91% and 41%of the N2 production estimated from the NO profiles, for MS2 and MS4, 3 respectively.

1

3.5. Porewater NO 3 , DIC and Fe Nitrate profiles measured on porewater extracted by centrifugation showed no obvious trends with depth but rather a constant elevated level of 35 – 161 lM as compared to the microprofiles. Porewater DIC from station MS2 to MS5 increased from 2172 lM (SD ± 5.0, n = 5) in the bottom water to a broad maximum concentration of 2534–2975 lM, 2–3 cm below the surface. Below the maximum the concentration gradually declined with the sediment depth. Overall, profiles showed sign of disturbance (Fig. 5). Porewater profiles of Fe showed relative large variations between the different stations and no clear trend with depth. MS2 and MS4 had maximum Fe concentrations of 5.4 and 3.0 lM whereas MS3 and MS5 had maximum concentrations of 25 and 14 lM. MS2, 3 and 5 showed a relative broad peak in Fe concentration 3–9 cm below the sediment surface (Fig. 6), while at MS4 no broad peak was observed. For all stations the concentration decreased to less than 2 lM at 9–11 cm depth.

3.4. Denitrification and anammox 3.6. Sulfate reduction rates Rates of denitrification were low and showed a high degree of variation, both among cores at the respective stations and between stations. MS2 showed the highest denitrification rates of 0.12 mmol N2 m2 d1 (SD ± 0.09, n = 12), while lowest rates of 0.04 mmol m2 d1 (SD ± 0.05, n = 6) were found at MS5 (Table 3). Based on the anoxic slurry incubations (see above), anammox accounted for 8.8–22% of the total N2 production rates for all stations (Table 2). Total N2 production from denitrification and anammox was calculated to be: 0.154, 0.087 and 0.051 mmol N2 m2 d1, for MS2, MS4 and MS5 (Table 3). NO 3 consumption from the profiles was converted to N2 production using the equation: N2 production = NO 3 consumption  (1 + ra), which takes into account that half of the N2 from anammox is derived from NH4+. IPT based

Depth-integrated sulfate reduction rates (0–15 cm) varied between 0.03 mmol m2 d1 and 0.92 mmol m2 d1. Rates were lowest on the sandy top of the bank, MS1 and highest on MS3. Rates from the other cohesive stations were lower but comparable to those of MS3 (Table 3). The peak activity was found at depths ranging between 6 and 10 cm (7.9–8.2 nmol cm3 d1). Sulfate reduction rates declined gradually below peak activity, but at all stations activity still occurred at the deepest measuring point (15 cm). Linear extrapolation of the sulfate reduction rate depth profiles suggested that sulfate reduction was active down to 21.0 cm (excluding MS1), (Fig. 7).

MS3

MS5

0

Depth (cm)

2

4

6

8

10 2000

2500

[DIC] (µM)

3000 2000

2500

3000

[DIC] (µM)

Fig. 5. Porewater concentrations of DIC, from MS3 (bank) and MS5 (reference station). Error bars are standard deviation, n = 1–3 from multiple measurements from one core. Horizontal line represents the sediment surface.

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MS5

MS3 0

Depth (cm)

2

4

6

8

10 0

25

50

0

25

[Fe] (µM)

50

[Fe] (µM)

Fig. 6. Porewater concentrations of Fe, from MS3 (bank) and MS5 (reference station). Error bars are standard deviation (n = 3) form 3 individual cores. Horizontal line represents the sediment surface.

MS3

MS5

0

Depth (cm)

5

10

15

20

25 0

2

4

6

SRR (nmol

8

10 12 14

cm-3 s-1)

0

2

4

6

SRR (nmol

8

10 12 14

cm-3 s-1)

Fig. 7. Depth profiles of volume specific sulfate reduction (SRR) rates from MS3 (bank) and MS5 (reference station). Error bars are standard deviation (n = 3) from 3 individual cores. Solid lines represent the line used for extrapolating the total depth integrated SRR (see text for explanation). Horizontal line represents the sediment surface.

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The sulfate reduction rates from sediment deeper than 15 cm estimated from the extrapolation, varied from 0.09 to 0.16 mmol m2 d1 equivalent to 14% of the total depth integrated sulfate reduction rate (Table 3). These values should be regarded as minimum estimates (see Section 4). 3.7. Fishing vessel tracking VMS data from the last 6 years showed that the Jones Bank area experiences higher trawling intensity than any adjacent areas. The mean intensity throughout all months of the last 6 years indicates that on average sediment around Jones Bank is disturbed by trawling gear for 72 h per month. Assuming that all trawling activity is carried out at a constant speed of 9.3 km h1 with a width of the trawling gear of 24 m, 32 km2 sediment would be affected by trawling per month per grid cell (1290 km2). This translates into that on average 30% of the sediment surface is affected by trawling every year. Taking into account the quality of the VMS data this must be seen as a low estimate due to low spatial resolution and the problems in determining ‘‘trawling’’ from the discrete measurements of vessel speed and the fact that not all fishing vessels are monitored. Although there are some obvious limitations in the VMS data; it is evident that Jones Bank sediment is heavily affected by trawling gear. 4. Discussion 4.1. Sandy station on top of bank (MS1) The sediment at the top of the Bank (MS1) clearly differed from that of the other stations. High current velocities at the station (>40 cm s1) (Ellis et al., 2013) must erode fine silt/clay particles leaving coarser material behind and the sediment was permeable and in situ biogeochemical processing must have been affected by advective porewater transport. The porewater advection at this site compromises any interpretation of data derived by onboard procedures as it was impossible to maintain advection under any realistic regime. This is not only a shortcoming of the present study, but a general problem investigating permeable sediments (Huettel and Webster, 2001; Cook et al., 2007). The problem is especially explicit for porewater profiles while process rate measurements might be less affected. However, as the OPD presumably is markedly reduced during the incubations as compared to in situ conditions the relative contribution of anaerobic mineralization is most likely overestimated (Cook et al., 2007). Unfortunately MS1 is the least sampled station, making comparison between stations difficult. As the top bank makes up 30% of the total bank area, sediment biogeochemical processes on Jones Bank may to a relatively large extent be affected by advective transport processes. Thus, one of the dominating sediment types for Jones Bank has not been represented fully in this study. This is a challenge that would require a different sampling strategy that presented here, applying specialized benthic landers (e.g. Janssen et al., 2005), or repacked sediment in percolating cores, advective chambers or flumes (Cook et al., 2006; Rao et al., 2007; Kessler et al., 2012). 4.2. Benthic O2 uptake and nutrient fluxes Total O2 uptake rates measured on stations MS2, MS3, MS4 and MS5 were similar but in the lower range of comparable shelf areas; Southern North Sea 5.3–27.8 mmol m2 d1 (Upton et al., 1993), Irish Sea 2.29–20 mmol m2 d1 (Van Raaphorst et al., 1992; Trimmer et al., 1999; Trimmer et al., 2003).

Sediment CRQ-values ranged between 1.20 and 1.53 (Table 2). These are typical for coastal sediments values but might likely be slightly higher than the annual average due to intensified anaerobic activity and accumulating O2 debt (i.e. Fe(II), Mn(II/III), H2S) in the sediment during the summer (Therkildsen and Lomstein, 1993). Fluxes of DIC are likely to be affected by CaCO3 dissolution; which was not quantified during this study, but a reasonable estimate for the contribution of CaCO3 dissolution to the DIC release for this area is 10% (Stahl et al., 2004; Anderson et al., 1986; Green and Aller, 1998). Indeed, mollusk shells were observed throughout the recovered sediment cores. In the following calculations we have applied the CaCO3 corrected DIC fluxes for assessing the relative importance of the respective mineralization pathways. 4.3. Porewater profiles of NO 3 Porewater profiles of NO 3 measured with microelectrodes were very different from the porewater profiles measured on porewater extracted by centrifuging. Integrating over the top 5 cm of the sediment, the traditional method yielded concentrations up to 45 times higher than what was measured with the microelectrodes (Fig. 8). The results suggest that NO 3 is released during porewater extraction. Foraminifera can accumulate large internal pools of NO 3 for subsequent respiration and this pool can make up 80% of the total sediment NO 3 pool (Risgaard-Petersen et al., 2006; Høgslund et al., 2008; Glud et al., 2009). Several species known to accumulate NO 3 internally (Piña-Ochoa et al., 2010a) were found in the sediment from Jones Bank (Ammonia beccarii, Bolivina subspinescens, Bullimina aculeate, Bullimina marginata, Cassidulina laevigata, Nonionella turgida, Stainforthia fusiformis), accounting for 33% of the total number of species. Thus, it is likely that release from NO 3 -concentrating foraminifera contributed to the NO 3 of extracted pore 1 water. Assuming an average NO 3 concentration of 101 pmol cell for species found on Jones Bank (mean of values from individual species from Piña-Ochoa et al., 2010a), densities need to reach 495 individuals cm3 to account for the observed concentrations in extracted pore water. This is higher than the average density observed on Jones Bank (98.2, SD ± 103, n = 10) and suggests additional sources. Piña-Ochoa et al. (2010a,b) also noted the ability of species of the Gromiida, a sister group of the foraminifera in the Rhizaria, to accumulate and respire NO 3 through denitrification. Species of Gromiida from the North Sea and Skagerrak on average contained NO 3 pools of 35 nmol pr cell (SE ± 17, n = 12). The internal NO pool from less than two individuals per cm3 3 would lead to an equivalent porewater concentrations high enough to explain the discrepancy between the concentration measured by microprofiling and porewater extraction. The presence of Gromiida species was not investigated during the present study, but given their presence in the North sea, Skagerrak and Bay of Biscay, they are most likely also present in the Celtic Sea and at Jones Bank. Intracellular accumulation of NO 3 has previously also been found for large sulfur bacteria (Fossing et al., 1995; Schulz and Jørgensen, 2001) and microalgae (Garcia-Robledo et al., 2010; Kamp et al., 2011). However as sediments appeared well oxidized with no indications of free H2S and the average chlorophyll a concentration in the top 5 cm was low (<0.7 lg g DW1), we find these alternatives to be less likely sources for the observed high NO 3 porewater concentration.  Simultaneous measurements of porewater NO3 concentration with both techniques have not previously been presented and the current study clearly demonstrates the potential of overestimating porewater NO 3 pools using traditional extraction methods. The turnover time of the internal NO 3 pool in foraminifera is estimated to 0.5–3 months (Risgaard-Petersen et al., 2006; Glud et al., 2009; Piña-Ochoa et al., 2010b), which is much slower that the

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MS3

MS4

0

Depth (cm)

1

2

3

4

5

0

10

20

30

40

50

0

10

[NO3 -] µM 0

20

40

60

20

30

40

50

80

100

[NO3 -] µM 80

100

0

20

40

60

NO 3

Fig. 8. Porewater profiles in Jones bank sediment, measured with microsensor (s) and from extracted porewater, see text for explanation (j). Microsensor profiles represent measurements from a single profile. Extracted porewater profiles represent the average of 3 individual cores, error bars are standard deviation. Notice different xscales, top scale for microsensor measurements and bottom scale for porewater extraction. Horizontal line represents the sediment surface.

interstitial turnover time of 0.5 day calculated from our NO 3 microprofiles. Existing estimates of denitrification rates for various foraminifera range from 46 to 248 pmol pr cell d1 (Risgaard-Petersen et al., 2006; Piña-Ochoa et al., 2010a). At these rates, the densities of NO 3 respiring meiofauna found on Jones Bank would lead to denitrification rates equivalent to 0.4% and 10.5% of the measured IPT rates and are therefore only of minor importance as compared to the microbial driven denitrification. This is consistent with the few other studies comparing meiofauna and bacteria driven denitrification rates (Glud et al., 2009; Schumacher et al., 2007; Prokopenko et al., 2011).

The impact of gear will be very local as passing otter doors will create furrows up to 20 cm deep (Løkkeborg, 2005), whereas the remaining trawling gear predominantly has an impact on the surface sediments. Following a given disturbance the porewater profiles will gradually reestablish, but steady state mediated by diffusion will first be reached after several weeks or months upon a heavy impact. Sediments cores within a relatively confined area may therefore reflect transiently recovering sediment profiles after independent trawling events. 4.5. Relative importance of different electron acceptors on carbon mineralization

4.4. Possible effects of trawling and sediment mixing The sediments at Jones Bank are trawled intensively but the VMS data are not detailed enough to verify any difference in trawling intensity among the stations. The porewater data did not suggest any clear difference in trawling impact among the stations. The 210Pb profiles actually suggest that all stations were heavily affected by trawling.

Using CaCO3 adjusted DIC fluxes as a measure of the total carbon mineralization rate within the sediment, the relative contribution of sulfate reduction can be estimated. Assuming a reaction stoichiometry of SO42- to organic C of 1:2 (i.e. SO2 4 + 2CH2O + 2H+ ? 2CO 2 + H2 S + 2H2 O) carbon mineralization ascribed to sulfate reduction is equivalent to 1.4–2.2 mmol C m2 d1 equal to 12–29% of the total carbon mineralization. As in most studies,

Table 4 Carbon mineralization on Jones Bank. Total carbon mineralization estimated from DIC fluxes. Absolute and relative rates of total carbon mineralization through sulfate reduction (SRR) and denitrification. Oxic + Fe/Mn respiration is the part of the total carbon mineralization unaccounted for by sulfate reduction and denitrification. Values in brackets are % of the total carbon mineralization. St ID

Carbon mineralization (mmol C m2 d1) Total C. mineralization

SRR

Denitrification

Oxic + Fe/Mn respiration

MS2 MS3 MS4 MS5

9.6 8.0 12.4 7.7

1.9 2.2 1.5 1.4

0.102 (1.0%) N.D. 0.063 (0.5%) 0.031 (0.4%)

7.6 (79%) 7.7 (72%) 10.9 (88%) 6.4 (82%)

(19%) (28%) (12%) (17%)

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we only assessed the sulfate reduction rate for the top 15 cm of the sediment. Measurements by Jørgensen et al. (1990) from the North Sea, demonstrated that sulfate reduction can occur to depths greater than 3.5 m and that on average >50% of the total sediment sulfate reduction, occurred in sediments deeper that 15 cm. This indicates that the integrated sulfate reduction rates from Jones Bank are likely to be underestimated. Rates of denitrification found in this study were low and thus carbon mineralization due to denitrification was negligible. Assuming a reaction stoichiometry of NO 3 to organic C of 4:5 + (i.e. 4NO + 5CH O + 4H ? 5CO + N + 2H 2 2 2 2O) and ignoring the 3 minute contribution from foraminifera, carbon mineralization due to denitrification amounted to 0.03–0.15 mmol C m2 d1 equal to 0.4–1.1% of the total carbon mineralization for all stations (Table 4). Therefore 79–88% of the carbon mineralization is not accounted for by sulfate reduction and denitrification, and must be due to mineralization with O2, Fe(III) or Mn(III, IV) as terminal electron acceptor. We did not directly quantify the Fe(III) or the Mn(III,IV) mineralization rates at Jones Bank, but metal respiration can contribute considerably to organic carbon mineralization (Canfield et al., 1993a; Rysgaard et al., 1998). A data compilation indicates that Fe(II) can account for 1–50% of the total carbon mineralization with an average of 17% while Mn(III,IV) generally contributes by less than 10% (Thamdrup, 2000). The broad maximum of porewater Fe observed in the current study indicates a very active Fe cycle that could be coupled to carbon mineralization. Importantly, the repeated mixing of the sediment by trawling would give favorable conditions for Fe(III) reduction, as reduced Fe(II) is continuously re-oxidized back to the oxide maintaining a high Fe(III) respiration (Canfield et al., 1993a). Carbon mineralization with Mn(III, IV) would likewise be stimulated by repeated sediment mixing. Even though the contribution of metal reduction to the total carbon mineralization is not quantified, it is plausible that both Fe(III) and Mn(III, IV) reduction contributed to the total benthic carbon mineralization around Jones Bank. As mentioned the mineralization not accounted for by sulfate respiration and denitrification must be ascribed to metal or O2 respiration. There is no direct way to quantify the importance of O2 mineralization and we have no means to discriminate between metal and O2 respiration, to determine their relative importance in this study. But we argue that both of them most likely are quantitatively important at Jones bank. A data compilation for coastal sediments concludes that between 45% and 90% of the total carbon mineralization can be ascribed to O2 and metal respiration (Canfield et al., 1993b; Thamdrup and Canfield, 1996; Rysgaard et al., 1998). We argue that metal and O2 respiration contributions to total carbon mineralization on Jones Bank, presumably are among the highest, facilitated by the intense trawling activity. The 5 stations at Jones Bank expressed very similar carbon degradation rates and pathways. It was expected that the topography of the bank and hence different hydrodynamic regimes would be reflected in benthic mineralization rates, but this could not be confirmed. Rather trawling disturbances seem to be a characteristic feature of the investigated sediments. Trawling induced heterogeneity may even overcast any potential systematic variation between stations. Apart from the sediment on top of the bank, there is little evidence that bank topography has impacts on benthic carbon mineralization. Acknowledgments We thank the captain and the crew of James Cook for their assistance during the cruise. We also thank Susan McKinley for excellent technical assistance during the cruise, while Anni Glud is thanked for manufacturing the microelectrodes used in this study. We acknowledge the constructive and valuable suggestions

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