Bottom hypoxia and consequential hydrogen sulfide (H2S) release from sediment in eutrophic estuaries is a major global environmental issue. We investigated dissolved oxygen, pH and H2S concentration profiles with microsensors and by sectioning sediment cores followed by colorimetric analysis. The results of these analyses were then compared with the physicochemical properties of the bottom water and sediment samples to determine their relationships with H2S production in sediment. High organic matter and fine particle composition of the sediment reduced the oxidation-reduction potential, stimulating H2S production. Use of a microsensor enabled measurement of H2S concentration profiles with submillimetre resolution, whereas the conventional sediment-sectioning method gave H2S measurements with a spatial resolution of 10 mm. Furthermore, microsensor measurements revealed H2S consumption occurring at the sediment surface in both the microbial mat and the sediment anoxic layer, which were not observed with sectioning. This H2S consumption prevented H2S release into the overlying water. However, the microsensor measurements had the potential to underestimate H2S concentrations. We propose that a combination of several techniques to measure microbial activity and determine its relationships with physicochemical properties of the sediment is essential to understanding the sulfur cycle under hypoxic conditions in eutrophic sediments.

INTRODUCTION

Eutrophication caused by anthropogenic activity is a major environmental problem in estuaries. In particular, urbanized and industrialized watersheds can result in large inputs of organic matter and nutrients into estuaries. Degradation of organic matter in an estuary results in reduction or complete depletion of dissolved oxygen (DO) in the bottom water, which are termed hypoxia and anoxia, respectively (Kodama et al. 2010; Sato et al. 2012). Low DO can lead to increased mortality of macrobenthic fauna and fish, as well as to accelerated production of hydrogen sulfide (H2S) in hypoxic bottom sediments (Valdemarsen et al. 2009). Because H2S is both toxic to benthic biota and consumes DO, H2S production is harmful to estuarine ecosystems and should be prevented (Kodama et al. 2010). However, the mechanism of H2S production is complicated and depends on sediment physicochemical properties including DO, oxidation-reduction potential (ORP), pH, and nitrate, sulfate and organic matter concentrations, as well as sediment grain size and porosity (Valdemarsen et al. 2009); therefore, it is difficult to control.

To understand the mechanism of H2S production, H2S concentration profiles in the sediment have been examined using several different techniques. Traditionally, H2S profiles have been determined by sectioning sediment cores and performing colorimetric H2S determination of interstitial water samples obtained by squeezing or in situ equilibration of dialysis cells (Koretsky et al. 2007). However, the difficulties associated with fine scale sediment sectioning result in a spatial resolution of only a few centimetres. At the sediment–water interface, steep concentration gradients can exist at a submillimetre scale, making the sectioning technique inadequate (Nakamura et al. 2004). Therefore, other analytical methods with higher spatial resolution (e.g., diffusive equilibration (DET) and diffusive gradients in thin-films (DGT), planar optodes and microsensors) have been developed and used for the measurement of H2S concentration profiles. DET and DGT techniques are used to investigate the two-dimensional distribution of H2S in the sediment. In these methods, H2S diffusing into thin hydrogel is trapped by an immobilized binding agent, then subjected to colorimetric determination (Pages et al. 2014). These techniques typically have a spatial resolution in the millimetre scale (Gregusova & Docekal 2013); however, they require labor-intensive and time-consuming steps such as gel preparation and several days of gel deployment for equilibration and H2S determination. Planar optodes allow two-dimensional measurements of dissolved matter at higher spatial (micrometre) and temporal (minutes) resolution than either the DET or DGT technique. Planar optodes have been used to determine the two-dimensional distribution and dynamics of DO and pH (Larsen et al. 2011) and CO2 (Zhu & Aller 2010) in both sediment and soil. However, the types of fluorescent indicators available for planar optodes are limited; therefore, they can only be used for a few analytes (Glud 2008).

Microsensors, which have been developed for a variety of analytes in the last three decades, may provide a solution to the shortfalls of the previous methods. Microsensors are biochemical sensors with a tip having a diameter at the micrometre scale. Because of their small size, microsensors have been used to determine the concentration profiles of dissolved matter with minimal disturbance of the sediment sample. Revsbech et al. (1980), who were the first to use an oxygen (O2) microsensor for sediment analysis, revealed the steep concentration gradients of DO and heterogeneity of its distribution. Since then, various types of microelectrodes have been developed and introduced to determine concentration profiles of H2S (Kühl et al. 1998), N2O (Binnerup et al. 1992) and metals (Luther et al. 1998) in lake, estuary and continental shelf sediments. However, no studies have investigated the relationship between the measured H2S concentration profile and the physicochemical properties of hypoxic and/or anoxic sediment in eutrophic estuaries. An understanding of the relationship between H2S production/consumption and the physicochemical properties of the water–sediment interface is essential for the prevention of H2S release and subsequent hypoxia in the sediment. In this study, DO, pH and H2S concentration profiles at the water–sediment interface of hypoxic sediment were monitored by microsensors. H2S concentration profiles in the sediment samples were determined using the H2S microsensor and by sectioning sediment cores and then subjecting the sections to colorimetric analysis. The sediment samples were obtained from Tokyo Bay because previous studies have demonstrated that eutrophication causes hypoxia from spring to autumn in this system (Sato et al. 2012). H2S profiles were compared with physicochemical properties of the sediment samples to better understand the role of sediment physicochemistry in the sulfur cycle.

MATERIALS AND METHODS

Samples

Samples of overlying water and sediments were collected from three sites in Tokyo Bay between May 2010 and August 2011: the Chiba Light Beacon vicinity (34°34′05″N, 140°2′45″E, 12–13 m water depth), Sanmaizu (35°36′53″N, 139°51′34″E, 6–7 m water depth) and the Tokyo Light Beacon (35°34′9″N, 139°50′51″E, 15–16 m water depth), hereafter referred to as Sites 1, 2 and 3, respectively (Figure 1). Vertical profiles of the physicochemical parameters (DO, temperature and salinity) in the bottom water were obtained in situ using a sounder (Hydrolab DataSonde 4a; Hach Environmental, Loveland, CO, USA). ORP was measured with a glass electrode (9300-10D, Horiba, Kyoto, Japan).
Figure 1

Map of Tokyo Bay and the location of the study sites.

Figure 1

Map of Tokyo Bay and the location of the study sites.

Sediment cores were sampled in situ using acrylic tubes (4 cm inner diameter, 40 cm in length) inserted directly into the sediment by divers. After plugging both ends of the acrylic tubes with rubber lids, the tubes were transported onto boats and stored in cooling boxes until laboratory analysis. Sediment grain size and composition were determined using the sieving and precipitation method following the test method for particle size distribution of soil (Japanese Industrial Standards A 1204:2009 and Japanese Geotechnical Society (JGS) 0131-2009) described in the Laboratory Testing Standards of Geomaterials published by the JGS. Sediments were dried to constant weight at 105 °C (approximately 2 h) to determine water content (%). Subsamples (approximately 1 g) were burned for 2 h at 650 °C to determine loss on ignition (LOI). Acid volatile sulfide (AVS) content in the sediment samples was assayed using HYDROTEC-S (GASTEC, Japan). Briefly, approximately 1 g of wet sediment was transferred into a glass vessel, which acted as a H2S gas generation tube, to which 2 mL of 18 N sulfuric acid was added. The H2S gas generated was then drawn into a H2S detecting tube by suction via a sealed glass cap and a gas sampling pump. The ORP and temperature were measured directly by inserting a glass electrode into the sediment samples.

Colorimetric determination of H2S in interstitial water of the sediment samples

Colorimetric determination of dissolved sulfide, which was defined as the sum of H2S, HS and S2–, was accomplished according to the modified methods of Cline (1969) and Sugahara et al. (2010). The sulfide concentration measured using this method was referred to as Σ(H2S). Each sediment core was horizontally sliced 1, 2 and 3 cm from the top of the core by stepwise extrusion from the corer using a plunger. Each sediment slice was then placed in a plastic cup, and a sample was immediately taken using a 30 mL glass syringe. The samples were volumetrically adjusted to 2.5 mL, after which 20 mL of N2-gas-purged, DO-free deionized water was added directly to each syringe via a three-way stopcock. The sediment samples were suspended in the deionized water by manual shaking after ejecting the air from the syringes. Each suspended sediment sample was then filtered with a cellulose acetate membrane filter cartridge (25 mm diameter, 0.45 μm pore size, DISMIC-25CS, Advantec, Japan), and 10 mL of filtrate was introduced into a new syringe via a three-way stopcock. One millilitre of 0.23 M zinc acetate solution was then added directly to the filtrate in each syringe to form zinc sulfide, after which 2 mL of 6 N hydrochloric acid solution was added to the zinc sulfide, and the Σ(H2S) concentration was determined using methylene blue. This process involves the addition of 0.5 mL of coloring reagent (a mixture of N,N-dimethyl p-phenylene diamine sulfate (DMPDS) and ferric chloride (FeCl3)) to the acidified samples, which are then rested for 15 minutes to allow the formation of methylene blue as a chromophore. The methylene blue concentration was then determined by spectrophotometry at a wavelength of 667 nm, 2 hours after the addition of DMPDS solution. Two types of coloring reagent solutions were used according to the Σ(H2S) concentration: (1) 0.4 g of DMPDS and 0.6 g of FeCl3•6H2O containing 100 mL of 6 N HCl for low Σ(H2S) concentration samples (<2 mg Σ(H2S) L–1) and (2) 4.0 g of DMPDS and 6.0 g of FeCl3•6H2O containing 100 mL of 6 N HCl for high Σ(H2S) concentration samples (<30 mg Σ(H2S) L–1). Determination of high Σ(H2S) concentration samples required dilution of the methylene blue solution with deionized water to adjust the concentration to within the quantitative range of absorbance values measurable by the spectrophotometer.

Microsensor measurements

Steady-state concentration profiles of DO, T-H2S (Σ(H2S) measured with a H2S microsensor) and pH were measured according to the method described by Okabe et al. (1999). Clark-type O2 and H2S microsensors and a pH microsensor were purchased from Unisense (Aarhus, Denmark) and calibrated according to the manufacturer instructions. DO, T-H2S and pH concentration profiles in the sediment samples were obtained using a micromanipulator at intervals of 200 to 500 μm. The microsensor tip was positioned with a dissecting microscope (model Stemi 2000; Carl Zeiss). Three concentration profiles for each species were measured at different points in each sediment sample.

The net volumetric T-H2S production rate (R(T-H2S)) in each sediment sample was calculated from the mean steady-state concentration profiles of T-H2S using Fick's second law of diffusion as described by Okabe et al. (1999): 
formula
where C is the concentration of the chemical species at depth x and time t, Ds is the sediment diffusion coefficient of the chemical species, and R is the production rate of the chemical species at depth x. Positive and negative values of R(T-H2S) indicate T-H2S production (i.e., sulfate reduction) and consumption, respectively. We used the Ds values previously determined in sediments: Ds(O2) = 1.31 × 10−5 cm–2 s–1 (Nakamura et al. 2004) and Ds(H2S) = 1.39 × 10−5 cm–2 s–1 (Okabe et al. 1999). The rates were calculated as described previously by Okabe et al. (1999). The local diffusive fluxes of T-H2S (J(T-H2S)) and DO (J(O2)) were calculated using Fick's first law of diffusion (Okabe et al. 1999): 
formula
where Φ is the sediment porosity measured in this study.

To examine the effectiveness of each method of sulfide measurement, the ratio of sulfide determined by the microsensor (quantity of T-H2S) to that determined by colorimetric determination (quantity of Σ(H2S)) was calculated. The quantity of T-H2S was estimated by integrating the T-H2S concentration profile over a depth of 10 mm, while that of Σ(H2S) was estimated as Σ(H2S) multiplied by 10 mm of depth.

RESULTS

Physicochemical properties of the sediment and bottom water

The average percentage of sand, silt and clay in the sediment samples taken from three sites in the inner Tokyo Bay between May 2010 and August 2011 (n = 9) are shown in Table 1. The samples were taken at a sediment depth of 0.1 m. In Sites 1 and 3, the sediment was mainly composed of silt and clay, whereas sand comprised 52% of the sediment in Site 2.

Table 1

Average (maximum–minimum) percentage of sand, silt and clay by dry weight in sediment samples taken from three sites in Tokyo Bay between May 2010 and August 2011 (n = 9)

  Site 1 Site 2 Site 3 
Sediment (particle size) 
 Medium sand (0.425–0.850 mm) 3 (13–0) 6 (16–1) 0 (0–0) 
 Fine sand (0.106–0.425 mm) 5 (17–2) 46 (74–10) 1 (2–0) 
 Silt 71 (88–46) 32 (63–4) 82 (90–72) 
 Clay 20 (28–10) 15 (28–6) 17 (27–9) 
  Site 1 Site 2 Site 3 
Sediment (particle size) 
 Medium sand (0.425–0.850 mm) 3 (13–0) 6 (16–1) 0 (0–0) 
 Fine sand (0.106–0.425 mm) 5 (17–2) 46 (74–10) 1 (2–0) 
 Silt 71 (88–46) 32 (63–4) 82 (90–72) 
 Clay 20 (28–10) 15 (28–6) 17 (27–9) 

The physicochemical properties of the bottom water and of the sediment samples at a sediment depth of 3 cm for each of the three study sites are shown in Table 2. Sediment AVS concentrations were higher and ORP values were lower at Site 3 than at Sites 1 or 2. LOI, which is an indicator of organic matter content, was higher at Site 3 than at the other sites. Sediment water content was high at Sites 1 and 3, but low at Site 2, indicating more extensive dewatering and compaction of the sediment at Site 2. DO in the bottom water was below 15 μM (0.5 mg L–1) at all sites on all three sampling dates, except for Site 2 in July 2010 (37 μM), when ORP in the bottom water was also much higher (+165 mV) than at other sites or other times. Overall, sediment AVS concentrations were relatively high at Site 3, which was characterized by fine sediments, lower ORP and higher LOI.

Table 2

Physical and chemical properties of the surface sediment and bottom water at three sites in Tokyo Bay

  Site 1
 
Site 2
 
Site 3
 
 2010 July 2010 August 2011 August 2010 July 2010 August 2011 August 2010 July 2010 August 2011 August 
Sediment (0–3 cm) 
 AVS (mg g-dry–10.8 1.3 1.8 1.2 1.1 3.3 2.3 2.6 
 LOI (%) 12 12 13 10 15 15 16 
 ORP (mV) –329 –399 –387 –311 –299 –309 –411 –410 –388 
 Water content (%) 80 79 82 62 70 59 90 90 90 
 Temperature (°C) 26 24 24 26 28 25 22 19 23 
Bottom water 
 DO (μM) 37 14 
 ORP (mV) –124 –92 N.M. 165 –18 N.M. –125 –38 N.M. 
 Temperature (°C) 24 23 23 25 28 24 22 21 21 
 Salinity 27 32 34 26 27 33 29 33 35 
  Site 1
 
Site 2
 
Site 3
 
 2010 July 2010 August 2011 August 2010 July 2010 August 2011 August 2010 July 2010 August 2011 August 
Sediment (0–3 cm) 
 AVS (mg g-dry–10.8 1.3 1.8 1.2 1.1 3.3 2.3 2.6 
 LOI (%) 12 12 13 10 15 15 16 
 ORP (mV) –329 –399 –387 –311 –299 –309 –411 –410 –388 
 Water content (%) 80 79 82 62 70 59 90 90 90 
 Temperature (°C) 26 24 24 26 28 25 22 19 23 
Bottom water 
 DO (μM) 37 14 
 ORP (mV) –124 –92 N.M. 165 –18 N.M. –125 –38 N.M. 
 Temperature (°C) 24 23 23 25 28 24 22 21 21 
 Salinity 27 32 34 26 27 33 29 33 35 

N.M.: not measured; AVS: acid volatile sulfide; LOI: loss on ignition; ORP: oxidation-reduction potential; DO: dissolved oxygen.

Vertical distribution of H2S concentrations and rates of sulfate reduction and sulfide oxidation at the sediment surface

The vertical distribution of bulk Σ(H2S) at the sediment surface of each site was determined by the colorimetric method (Figure 2). Bulk Σ(H2S) concentrations increased with depth in all samples. Bulk Σ(H2S) concentrations in the deepest layer (i.e., 20–30 mm depth) of the sediment were highest at Site 3 (2,000–12,000 μM) and lowest at Site 2 (<250 μM). AVS concentrations followed a similar trend among sites (Table 2).
Figure 2

Vertical distribution of bulk Σ(H2S) (boxes) and concentration profiles of DO (filled circle), T-H2S (filled triangle) and pH (cross) in the surface sediment of three study sites: Site 1 (a), Site 2 (b) and Site 3 (c) in July 2010; Site 1 (d), Site 2 (e) and Site 3 (f) in August 2010; and Site 1 (g), Site 2 (h) and Site 3 (i) in August 2011. Note that the maximal value of the horizontal axis is different between the study sites. The concentration profiles of DO, T-H2S and pH are average values (n = 3) and error bars represent the standard deviations of triplicate measurements. Zero on the vertical axis corresponds to the surface of the sediment.

Figure 2

Vertical distribution of bulk Σ(H2S) (boxes) and concentration profiles of DO (filled circle), T-H2S (filled triangle) and pH (cross) in the surface sediment of three study sites: Site 1 (a), Site 2 (b) and Site 3 (c) in July 2010; Site 1 (d), Site 2 (e) and Site 3 (f) in August 2010; and Site 1 (g), Site 2 (h) and Site 3 (i) in August 2011. Note that the maximal value of the horizontal axis is different between the study sites. The concentration profiles of DO, T-H2S and pH are average values (n = 3) and error bars represent the standard deviations of triplicate measurements. Zero on the vertical axis corresponds to the surface of the sediment.

The steady-state concentration profiles of DO, T-H2S and pH measured with microsensors are depicted in Figure 2. DO was depleted at the sediment surface in most samples; thus, DO profiles are only shown for Sites 1 and 2 in July 2010 (Figure 2(a) and (b)). DO penetrated only 0.4 mm into the sediment at Site 2 in July 2010 (Figure 2(a)), even when the DO of the bottom water was 100 μM. DO depletion was likely caused by a combination of high microbial respiration during oxidation of organic matter, low DO in the bottom water, low liquid flow rate at the sediment surface during measurement, and/or fine silt or clay particles inhibiting DO diffusion into the sediment. J(O2) fluxes, which were calculated from the linear DO concentration profiles at the sediment surface, were 0.049 and 0.045 μmol cm–2 h–1 at Sites 1 and 2, respectively, in July 2010. Because the samples were incubated in the dark, DO showed no net increase, indicating insignificant photosynthesis in all samples. These reducing conditions resulted in sulfate reduction in the sediment. The T-H2S concentrations increased with depth at all three sites, although they were higher at Sites 1 and 3 (∼2,700 μM) than at Site 2 (<600 μM) on each sampling date. The T-H2S concentrations were highest in August 2010 for all samples.

The standard deviation of the T-H2S concentration at each sediment depth was lower than 5%, except for Site 2 in August 2010 (<20%) and August 2011 (<60%). These findings indicate that the vertical distribution of sulfate reduction and sulfide oxidation was relatively homogeneous. Therefore, the values of R(T-H2S) were calculated from the average steady-state concentration profiles of T-H2S by diffusion reaction modeling (Figure 3). In addition, J(T-H2S) values were calculated from the linear T-H2S concentration profiles of the measured sediment depth range (Table 3). R(T-H2S) in Sites 1 and 3 in August 2010 were relatively high (<0.26 μmol cm–3 h–1), resulting in high J(T-H2S) in these samples (Table 3). Thus, significant concentrations of H2S were detected in the bottom waters at Site 1 (0.02 mg S L–1) and Site 3 (0.74 mg S L–1). In some sediment samples, rates of T-H2S oxidation were so low that T-H2S reached the sediment surface (Table 3) and diffused into the overlying water (Figure 2). In contrast, at Sites 1 and 3 in July 2010 and Site 2 in August 2010, T-H2S was completely oxidized at the surface sediment (Table 3). H2S oxidation at the surface sediment is vital for preventing toxic H2S release from the sediment into the overlying water.
Table 3

Local diffusive fluxes of T-H2S (J(T-H2S)), zone in which J(T-H2S) was calculated, T-H2S concentration at the sediment surface (Conc.) and ratio of the quantities of T-H2S and Σ(H2S)

  Site 1
 
Site 2
 
Site 3
 
 J(T-H2S) (μmol cm–2 h−1Zone (mm) Conc. (μM) Ratio J(T-H2S) (μmol cm–2 h−1Zone (mm) Conc. (μM) Ratio J(T-H2S) (μmol cm–2 h−1Zone (mm) Conc. (μM) Ratio 
July 2010 0.011 0 − 9.8 0.5  0.246 9.4 − 10.4 0.01 
August 2010 0.121 0 − 2.0 201 0.5 0.011 3.0 – 30 1.21 0.146  0 − 2.0 109 0.26 
August 2011 0.018 0 − 3.9 142 0.29 0.021  0 − 1.5 69 0.42 0.041  0 − 5.1 170 9.7 
  Site 1
 
Site 2
 
Site 3
 
 J(T-H2S) (μmol cm–2 h−1Zone (mm) Conc. (μM) Ratio J(T-H2S) (μmol cm–2 h−1Zone (mm) Conc. (μM) Ratio J(T-H2S) (μmol cm–2 h−1Zone (mm) Conc. (μM) Ratio 
July 2010 0.011 0 − 9.8 0.5  0.246 9.4 − 10.4 0.01 
August 2010 0.121 0 − 2.0 201 0.5 0.011 3.0 – 30 1.21 0.146  0 − 2.0 109 0.26 
August 2011 0.018 0 − 3.9 142 0.29 0.021  0 − 1.5 69 0.42 0.041  0 − 5.1 170 9.7 
Figure 3

Vertical distribution and magnitude of the net volumetric T-H2S production rates (R(T-H2S)) in the sediment samples at Site 1 (a), Site 2 (b) and Site 3 (c). The rates were calculated based on the corresponding concentration profiles shown in Figure 2. Negative values indicate consumption. Zero on the vertical axis corresponds to the surface of the sediment.

Figure 3

Vertical distribution and magnitude of the net volumetric T-H2S production rates (R(T-H2S)) in the sediment samples at Site 1 (a), Site 2 (b) and Site 3 (c). The rates were calculated based on the corresponding concentration profiles shown in Figure 2. Negative values indicate consumption. Zero on the vertical axis corresponds to the surface of the sediment.

DISCUSSION

Among the study sites in Tokyo Bay, sulfate reduction, which was represented by AVS, bulk Σ(H2S) and T-H2S accumulation, was highest at Site 3 on each sampling date. Moreover, Site 3 was characterized by higher LOI (organic matter) than Sites 1 or 2 (Table 2). High organic matter in the sediment can stimulate sulfate reduction via consumption of DO followed by a decrease in ORP in sediment or by providing increased quantities of electron donors for sulfate-reducing bacteria (Holmer & Storkholm 2001). Rates of sulfate reduction (R(T-H2S)) in this study (<0.26 μmol cm–3 h–1) were substantially higher than those of natural sediments in acidic lakes (Kühl et al. 1998) and sandy intertidal sediments (de Beer et al. 2005) (<0.01 μmol cm–3 h–1), while they were comparable to those of anaerobic sewer biofilm (Mohanakrishnan et al. 2009) and organic-rich sludge in a marine aquaculture system (Schwermer et al. 2010) (<1.0 μmol cm–3 h–1). Thus, these results indicate a massive input of organic matter to Tokyo Bay sediments is occurring. Moreover, the fine particles constituting the sediment at Site 3 may reduce the diffusive transport of oxygenated species (e.g., DO, nitrate and ferric oxide) and eventually stimulate sulfate reduction. Kodama et al. (2010) also reported severe bottom hypoxia (<45 μM of DO) in a northern area of Tokyo Bay, including all sites in this study in August 2005, and found H2S concentrations were higher in silt and clay sediments than in sand.

Comparison of the T-H2S concentration profiles measured by a microsensor with the vertical distribution of bulk Σ(H2S) demonstrated that T-H2S could be determined at depth intervals of 0.2 mm, whereas bulk Σ(H2S) concentration intervals were 10 mm. The microsensor measurements showed that T-H2S concentrations were changing at the submillimetre scale. It should be noted that the T-H2S was completely oxidized at a depth of 5.5 mm in the Site 3 sediment in July 2010, and that no T-H2S was detected in the surface layer of the sediment, but a microbial mat of white filamentous bacteria, approximately 10 mm thick, was found on the sediment surface (Figure 4). This was likely to consist of Beggiatoa spp., which can oxidize H2S with DO and/or nitrate (Kamp et al. 2006). It is well known that mats of Beggiatoa at the sediment surface can prevent the diffusion of toxic H2S into overlying waters because biological H2S oxidation is much more rapid and efficient than chemical H2S oxidation (Kamp et al. 2006). DO was depleted at the sediment surface because of low DO in the bottom water layer, and the sum of nitrate and nitrite concentrations in the bottom water was only 6 μM at Site 3 in July 2010. In addition, electric currents may not have directly connected oxygen reduction at the sediment surface to sulfide oxidation at deeper sediment depths in this sample, as pH was observed to increase between the oxygen reduction and sulfide oxidation zones (Pfeffer et al. 2012). Therefore, an electron acceptor for Beggiatoa-like bacteria has not been identified. Furthermore, a decrease in T-H2S concentrations was observed in anoxic sediments deeper than 3 mm at Site 2 in August 2011. Decreased T-H2S may have been caused by bioturbation providing oxygen and/or nitrate (Rabouille et al. 2009) or by upward diffusion of dissolved Fe2+ reacting with H2S to form FeS under reducing conditions in the sediment (Dittrich et al. 2009). Without the use of a microsensor, these unusual phenomena would have been overlooked.
Figure 4

Photograph of the surface sediment sampled in Site 3 in July 2010. Inner diameter of the acrylic tube is 4 cm.

Figure 4

Photograph of the surface sediment sampled in Site 3 in July 2010. Inner diameter of the acrylic tube is 4 cm.

T-H2S and bulk Σ(H2S) concentration profiles showed some similarities; specifically, both increased with sediment depth and were lower in sediments at Site 2 than at Sites 1 or 3. However, the ratio of the quantity of T-H2S to bulk Σ(H2S) in the top 10 mm of sediment ranged from 0.01 to 10 (Table 3), indicating that microsensor measurements may frequently underestimate H2S concentrations. Although the exact reason remains unknown, one explanation may be that T-H2S concentration profiles were measured in the laboratory under different conditions than those in situ. Alternatively, the T-H2S concentration profiles may not necessarily represent an averaged concentration profile because of inadequate replication (only three points per sediment sample). Nevertheless, our results indicate that microsensor measurement is necessary to properly analyze the sulfur cycle in sediment at the submillimetre scale.

CONCLUSIONS

The H2S concentration profiles and rates of sulfate reduction and sulfide oxidation in the sediment of Tokyo Bay were determined at a submillimetre scale using a microsensor, whereas the spatial resolution of the sediment core sectioning method was 10 mm. The calculated T-H2S production rates in some eutrophic sediment samples were much higher than those found in natural sediments in other studies. Among the sediment properties measured, high organic matter content and fine particle composition (i.e., silt and clay) may primarily be responsible for the inhibition of DO diffusion into the sediment, the low ORP and the stimulation of sulfate reduction at the surface sediment in Tokyo Bay. Sulfate reduction and H2S oxidation in the sediment showed dynamic variation with depth at the submillimetre scale and differed among sites. Without the use of a microsensor, these spatial dynamics with depth would have been overlooked. Overall, microsensor measurements provided critical insight into the sulfur cycle in the hypoxic surface sediment of a eutrophic estuary.

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