Effects of aerobic respiration and nitrification on dissolved inorganic nitrogen and carbon dioxide in human-perturbed eastern Jiaozhou Bay, China

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul

Effects of aerobic respiration and nitrification on dissolved inorganic nitrogen and carbon dioxide in human-perturbed eastern Jiaozhou Bay, China Ping Hana, Yunxiao Lia, Xufeng Yanga, Liang Xueb, Longjun Zhanga,⁎ a Key Laboratory of Marine Environment and Ecology, Ministry of Education, College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China b Center for Ocean and Climate Research, First Institute of Oceanography, SOA, Qingdao 266061, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Aerobic respiration Nitrification Dissolved inorganic nitrogen composition Carbon dioxide Eastern Jiaozhou Bay

Aerobic respiration and nitrification are important processes for dissolved inorganic nitrogen (DIN) composition change and CO2 production in human-perturbed coastal waters. On-site incubations and field investigations were conducted in the eastern Jiaozhou Bay, a high-urbanization region, from May to August 2014. Results show that aerobic respiration rates reached 15.58 μmol O2 L− 1 d− 1, and NH4+ and NO2− oxidation rates were 0.53 and 0.13 μmol N L− 1 d− 1, respectively, in the human-perturbed northeastern area. The intense aerobic respiration there contributed to high-concentration NH4+, and meanwhile caused a pH decrease of 0.042 units and a pCO2 increase of 166 μatm per day. Moreover, the linear relationship between excess CO2 and apparent oxygen utilization suggested that the excess CO2 in the entire eastern Jiaozhou Bay was mainly from the aerobic respiration. This study may help us better understand the role of aerobic respiration in DIN composition and CO2 sink/source pattern in coastal waters.

1. Introduction Large amounts of terrestrial materials are transported to coastal waters due to rapidly increasing human activities, resulting in ecosystem deterioration and biogeochemical process change (Seitzinger et al., 2005; Dai et al., 2010). Eutrophication has become a major problem in these areas, which promotes phytoplankton blooms and subsequent aerobic respiration (Cai et al., 2011; Rabalais et al., 2014). Intense aerobic respiration not only causes oxygen deficits that threaten ecological systems (Testa and Kemp, 2014; Topcu and Brockmann, 2015), but also produces ammonia (NH4+) (Hsiao et al., 2014). In addition, because of the lack of oxygen, incomplete nitrification leads to the accumulation of nitrite (NO2−) (Dai et al., 2008), threatening the growth and survival of aquatic organisms. Excess carbon dioxide (CO2) produced by aerobic respiration was another important environmental issue in human-perturbed coastal waters. For instance, the southern North Sea acted as a CO2 source in summer due to the intense aerobic respiration, which was fed by significant amounts of terrestrial dissolved organic carbon (DOC) inputs (Thomas et al., 2004, 2005). In the Scheldt estuarine plume, aerobic respiration exceeded primary production because of the decomposition of phytoplankton residues in the late period of algal bloom, resulting in ⁎

net heterotrophy and the supersaturation of CO2 (Borges et al., 2008). Although nitrification does not release CO2 directly like aerobic respiration, it releases H+ and lowers total alkalinity (TAlk) (US EPA, 1993; Hu and Cai, 2011), leading to decreases in pH and increases in pCO2. Zhai et al. (2005) concluded that aerobic respiration was the most important process in maintaining extremely high partial pressure of CO2 (pCO2) levels in the upper Pearl River estuary, inferred from the relationship between excess CO2 and apparent oxygen utilization (AOU). Subsequent summer investigations in the upper Pearl River estuary, however, showed that nitrification rates were as high as 5.4 μmol N L− 1 d− 1 in association with lower oxygen levels. Although aerobic respiration dominated the high pCO2 levels, nitrification was also an important contributor (Dai et al., 2008). The environmental damage of aerobic respiration and nitrification in coastal waters are conspicuous. Quantifying the effects of these processes on dissolved inorganic nitrogen (DIN) composition and CO2 sink/source pattern is beneficial to further understand their roles in coastal environmental evolution. The eastern Jiaozhou Bay is highly affected by urbanization due to its proximity to the city of Qingdao. Large amounts of industrial and domestic sewage were discharged into the bay, which maintained high DIN concentrations in the eastern Jiaozhou Bay (Liu et al., 2005; Lu

Corresponding author. E-mail address: [email protected] (L. Zhang).

http://dx.doi.org/10.1016/j.marpolbul.2017.07.055 Received 3 March 2017; Received in revised form 22 July 2017; Accepted 22 July 2017 0025-326X/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Han, P., Marine Pollution Bulletin (2017), http://dx.doi.org/10.1016/j.marpolbul.2017.07.055

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

followed by three multipronged field investigations in the entire eastern Jiaozhou Bay.

et al., 2016). Additionally, organic matter inputs led to intense aerobic respiration, with the sediment oxygen consumption rates in the eastern Jiaozhou Bay reportedly reaching 20 mmol O2 m− 2 d− 1 (Zhang et al., 2006). Intense aerobic respiration further caused the eastern Jiaozhou Bay to act as a CO2 source in autumn (Zhang et al., 2012). In this study, based on the incubation experiments of aerobic respiration and nitrification conducted in the human-perturbed northeastern area from May to August 2014, the rates and monthly varying patterns of these processes were determined. Combining these data with the results of three surveys carried out in the entire eastern Jiaozhou Bay from June to August, accompanied by a 24-h time series observation in the northeastern area in May, we quantified the contributions of aerobic respiration and nitrification to the high levels of NH4+ and NO2−. Furthermore, we elucidated the effects of these two processes on the CO2 source and acidification in eastern Jiaozhou Bay.

2.2. Sampling and analytical methods Underway surface temperature, salinity, pCO2 and dissolved oxygen saturation (DO%) were collected continuously during four cruises. Temperature and salinity were measured using a SBE 45 MicroTSG (Sea-Bird Inc., Bellevue, WA, USA). DO% was measured using a YSI 5000 oxygen analyser (YSI Corporation, Yellow Spring, OH, USA) that was calibrated using the Winkler titration method. The precision of DO % is ~ 0.1% (that is, dissolved oxygen (DO) ~0.3 μmolL− 1). pCO2 was measured with an isotopic CO2 Analyzer (Picarro G2131-i, WS-CRDS, USA) based on wavelength-scanned cavity ring-down spectroscopy coupled to an equilibrator. The precision of pCO2 is < 0.2 ppm. pH was measured using an Orion 3-Star Plus pH Benchtop Meter with a Ross pH electrode (Thermo Fisher Scientific Inc., Beverly, MA, USA) that was calibrated according to the National Bureau Standard (NBS Standard). The precision of pH was approximately ± 0.005. The chlorophyll a (Chl a) samples were filtered through glass fibre membrane filters (0.7 μm pore size; Whatman GF/F) at a pressure of < 0.05 MPa. The membranes were then preserved at − 20 °C using saturated magnesium carbonate solution (0.5–1 mL) fixation. After extraction with 90% acetone, Chl a was determined using a fluorescence spectrophotometer (F-4500, Hitachi High-Technologies). Calibrations were performed using Sigma C-5753 chlorophyll (SigmaAldrich). The nutrient samples (NH4+, NO2−, and nitrate (NO3−)) were filtered through pre-treated cellulose acetate membranes (0.45 mm pore size; immersed in 0.1% hydrochloric acid (HCl) solution over 24 h and then flushed with Milli-Q water), poisoned with chloroform (~ 0.2% by volume), and preserved at − 20 °C. After quickly being thawed, the nutrients were analysed using an Auto Analyser (SEAL Analytical (QuAAtro39, Germany)). All analyses of the nutrients were carried out in duplicate and the coefficients of variation were < 0.5%, and the results were expressed as the means. The detection limits for NH4+, NO3− and NO2− were 0.10 μmol L− 1, 0.14 μmol L− 1 and 0.01 μmol L− 1, respectively. The dissolved inorganic carbon (DIC) samples were filtered through cellulose acetate membrane filters (0.45 μm pore size), poisoned with saturated mercury bichloride (0.2‰ by volume, Dickson and Goyet, 1994) and preserved at 4 °C. DIC was measured using a total organic carbon analyser (TOC-VCPN, Shimadzu Corporation, Kyoto, Japan; Zhang et al., 2012) and sample variation between duplicates was < 1%. Calibrations were performed using Certified Reference Material (provided by A. G. Dickson from Scripps Institution of Oceanography). TAlk was calculated based on pH, DIC, temperature and salinity using

2. Materials and methods 2.1. Study area Jiaozhou Bay (35°57′–36°18′N, 120°04′–120°23′E) is a semi-enclosed water body located to the south of Shandong Peninsula in China. The average water depth is ~7 m, and seawater exchange only occurs through the southern bay mouth (Fig. 1). The bay is characterized by regular, semi-diurnal tides with an average tidal range of 2.8 m (Editorial Board of Annals of Bays in China, 1993). Because of strong tidal vertical mixing, the vertical profiles of the seawater temperature and salinity are nearly homogeneous (Liu et al., 2004). The eastern Jiaozhou Bay is adjacent to the city of Qingdao that experienced rapid economic development and increased population. Under the effects of urbanization, the coastal rivers, e.g., Loushan River, Licun River, Haibo River and others, basically deliver no runoff and act merely as channels for industrial and domestic sewage (Gao et al., 2008). Due to the proximity to those estuaries and a long water residence time of > 70 d (Liu et al., 2004), the northeastern area of the bay is characterized by relatively low salinity and high levels of DIN (Liu et al., 2005) and DOC (Zhang et al., 2012). Station 4 is heavily perturbed by human activities, and it can be a representative of the northeastern area (Fig. 1, Zhang et al., 2012). Thus, this station provides us a good opportunity to examine the influences of human activities on DIN composition and CO2 sink/source pattern. On the above basis, on 5 May 2014, we first conducted a 24-h time series observation and incubation experiments of aerobic respiration and nitrification at Station 4. Subsequently, on 13 June, 29 July and 28 August 2014, we continued to carry out incubation experiments of aerobic respiration and nitrification at Station 4,

Fig. 1. Sampling stations in the eastern Jiaozhou Bay. (The red indicates the sample for DIN analysis). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

the program CO2SYS (Lewis et al., 1998; K1, K2 from Mehrbach et al., 1973 refit by Dickson and Millero, 1987). The DOC samples were filtered through pre-treated glass fibre membrane filters (0.7 μm pore size; pre-combusted Whatman GF/F), poisoned with saturated mercury bichloride (0.2‰ by volume), and preserved at 4 °C. DOC was measured using a high-temperature catalytic oxidation technique with a total organic carbon analyser (TOCVCPN, Shimadzu Corporation, Kyoto, Japan).

that happened three days before the cruise, the salinity dropped to 23.62 in the northeastern area and 30.06 in the bay mouth (Fig. 2b). The average surface temperatures in the eastern Jiaozhou Bay were 20.48 °C in June, 24.73 °C in July and 26.09 °C in August (Fig. 2a–c), and gradually increased month by month due to the increasing air temperature.

2.3. Incubation experiments Community respiration incubation was carried out using a dark bottle method and nitrification incubation was conducted using an inhibitor technique (Feliatra and Bianchi, 1993; Bianchi et al., 1997). Water samples were first homogenized in a 25 L pre-cleaned polyethylene container, and then dispensed into three low-density polyethylene (LDPE) dark incubation bottles with a volume of 4 L. One sample was incubated with allylthiourea (ATU, at 100 mg L− 1) to inhibit the oxidation of NH4+ to NO2−. Another sample was incubated with sodium chlorate (NaClO3, at 10 mg L− 1) to inhibit the oxidation of NO2− to NO3−. A third sample was subjected to no disposal to study community respiration and served as a control for the nitrification incubation. The bottles were all kept in a dark environment and the temperature was maintained with continuously flowing surface water during the incubation. Sub-samples were taken out at 6–10 h intervals for analysing DO (only in the control sample) and NO2−. Community respiration rates were estimated based on the decrease in DO concentrations in the control samples. NH4+ and NO2− oxidation rates were estimated based on the increase or decrease in NO2− concentrations in the nitrification incubation samples (Bianchi et al., 1992). Plankton community respiration is the process of organic carbon and nitrogen decomposing to CO2 and NO3− under aerobic conditions (Eq. (1), Redfield, 1963). It includes the aerobic respiration process in which organic matter is decomposed to CO2 and NH4+ (Eq. (2)), the ammonia oxidation process in which NH4+ is oxidized to NO2− (Eq. (3), US EPA, 1993), and the nitrite oxidation process in which NO2− is oxidized to NO3− (Eq. (4), US EPA, 1993). According to Eqs. (3) and (4), the nitrification oxygen consumption rate was calculated using the stoichiometric coefficients between nitrogen and oxygen. Then, the aerobic respiration rate can be calculated by subtracting the nitrification induced oxygen consumption rate from the community respiration rate.

The surface water of the eastern Jiaozhou Bay had low DO saturation (Fig. 2d–f). In June and July, nearly two-thirds and one-thirds of the investigation area was in an undersaturated state, with the lowest values of < 90% all occurring in the northeastern area of the bay. Even in the bay mouth, the DO% was only ~100%. But in the bay centre in July, those values exceeded 120% (Fig. 2e). In August, DO was undersaturated across the entire investigation area, with a DO% range of 75.6–85.0%, and the lowest DO% value still presented in the northeastern area. As a whole, the DO% increased from the northeastern area to the mouth of the bay during the investigation. However, the spatial distribution patterns of Chl a were different from those of the DO% (Fig. 2g–i). The Chl a concentrations varied from 0.27 to 4.75 μg L− 1 in June and 0.09 to 2.01 μg L− 1 in August, and peaked during both months in the northeastern area where the DO% values were the lowest. In July, the Chl a concentrations varied from 1.91 to 16.16 μg L− 1. Although the maximum Chl a value coincided with the maximum DO% value in the bay centre, a secondary Chl a peak was identified in the northeastern area where DO was undersaturated (Fig. 2h). During this study, the concentrations of NH4+, NO2− and NO3− had ranges of 0.60–21.12 μmol L− 1, 0.22–8.97 μmol L− 1 and 0.75–22.24 μmol L− 1 (Fig. 3). The regional differences in the various parameters were considerable, with the concentrations of NH4+, NO2− and NO3− in the hypohaline northeastern area of the bay being approximately 5 times higher than in the bay mouth. In addition, NH4+ and NO2− accounted for as much as 44% and 14% of DIN, respectively, indicating high levels of NH4+ and NO2− in the northeastern area. With respect to the monthly variation, the NH4+ concentrations in June were close to those in August (Fig. 3a), while NO2− concentrations in June were lower than in August (Fig. 3b). In July, a rainstorm occurred, and the concentrations of NH4+ and NO2− increased to nearly two times the values in June and August (Fig. 3a, b).

(CH2 O)106 (NH3 )16 (H3 PO4 ) + 138O2

3.3. Carbonate system parameters

→ 106CO2 + 16NO3− + H2 PO4− + 122H2 O + 17H+

3.2. DO%, Chl a and DIN

(1) The surface pCO2 over the eastern Jiaozhou Bay was at a high level (Fig. 2j–l). During the investigation, the maximum values in the northeastern area of the bay were all above 1000 μatm, and even in the bay mouth adjacent to the Yellow Sea, these values still exceeded 500 μatm. pCO2 values of < 400 μatm occurred only in July in the centre of the bay (Fig. 2k) where DO% values were higher than 120% (Fig. 2e). According to the atmospheric CO2 levels of 402 ppm collected by NOAA on the Tae-ahn Peninsula (126.13°E, 36.73°N) adjacent to the southern Yellow Sea, the air-sea pCO2 differences were up to 55–822 μatm (except for the bay centre in July), indicating that the eastern Jiaozhou Bay was a strong CO2 source. The distributions of pH agreed well with those of pCO2 (Fig. 2m–o). In the northeastern area where the pCO2 was 1000 μatm or more, the pH was lower than 7.8. Even in the mouth of the bay, the pH was only ~8.0. Thus, the eastern Jiaozhou Bay had low pH values. Only in July, pH values higher than 8.1 (Fig. 2n) appeared in the bay centre where pCO2 values were lower than 400 μatm. The DIC values ranged from 2010 to 2270 μmol kg− 1 in June 1882 to 2065 μmol kg− 1 in July and 1904 to 2118 μmol kg− 1 in August (Fig. 2p–r), showing great regional differences. The DIC from the northeastern area of the bay with the lowest salinity were 252 μmol kg− 1, 127 μmol kg− 1 and 214 μmol kg− 1 higher than in the

(CH2 O)106 (NH3 )16 (H3 PO4) + 106O2 + 15H+ → 106CO2 + 16NH 4+ + H2 PO4− + 106H2 O

(2)

NH 4+ + 1.44O2 + 0.05CO2 → 0.01C5 H7 O2 N + 0.99NO2− + 1.99H+ + 0.97H2 O

(3)

NO2− + 0.01NH 4+ + 0.03CO2 + 0.50O2 + 0.01H2 O → 0.01C5 H7 O2 N + NO3− + 0.01H+

(4)

3. Results 3.1. Hydrological data The salinity in the eastern Jiaozhou Bay varied little, ranging from 29.27 to 30.80 in June and 28.23 to 30.22 in August, and all increased from the northeastern area of the bay to the mouth (Fig. 2a, c). The lowest salinity value from the northeastern area was only 1.53 lower than the highest salinity value from the bay mouth in June and 1.99 in August. Due to the lack of natural river runoff and rainstorm, no obvious signal of freshwater input was observed. In July, after a rainstorm 3

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

Fig. 2. Distributions of (a–c) salinity, temperature, (d–f) DO%, (g–i) Chl a, (j–l) pCO2, (m–o) pH and (p–r) DIC in June, July and August.

enhanced photosynthesis in this area.

bay mouth from June to August. Furthermore, in July, DIC values lower than 1900 μmol kg− 1 appeared in the bay centre (Fig. 2q). We note that the bay centre was also characterized by high values of salinity (Fig. 2b), DO% (Fig. 2e), Chl a (Fig. 2h) and pH (Fig. 2n), as well as low values of pCO2 (Fig. 2k) in July, suggesting that there might be

3.4. Diurnal variations of various parameters Station 4 is located in the northeastern area and heavily perturbed 4

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

Fig. 3. Concentrations of (a) NH4+, (b) NO2− and (c) NO3− in June, July and August. Salinity indicated the average value in these three months.

Fig. 4. Diurnal variations in various parameters at Station 4. (The star indicates the sample for incubation).

by human activities. A 24-h time series observation was conducted there in May to examine the diurnal variations of various parameters (Fig. 4). During one day and night, the DO% was lower than 86%, the pCO2 was higher than 1000 μatm, and there was a strong positive correlation between them (R2 = 0.71, n = 12, p < 0.001). Moreover, the pH was lower than 7.7, while the concentrations of NH4+, NO2− and DIC remained high levels throughout the 24 h. During this period, the water quality at Station 4 was bad. In addition, the high values of pCO2, NH4+, NO2− and DIC, and the low values of DO% and pH appeared to occur at low tide and at night. In contrast, during the daytime and at high tide, the opposite conditions were observed. 3.5. Community respiration rates, nitrification rates and aerobic respiration rates Fig. 5 shows the evolution of DO values in the course of community respiration incubation experiments at Station 4 from May to August. In the absence of light, DO decreased linearly in all experiments, indicating that the oxygen consumption was in order. Based on the slopes of the best fit line between the incubation time and DO, the community respiration rates were calculated, ranging from 8.70 to 16.38 μmol O2 L− 1 d− 1 (Table 1). The maximum rate appeared in May, while the lowest rate appeared in June. Furthermore, community respiration rates in June showed little difference between bottom and surface samples, indicating the well-mixed nature of the water column. Fig. 6 shows the evolution of NO2− over the course of the

Fig. 5. Evolution of DO concentrations during the community respiration incubation at Station 4.

nitrification incubation experiments at Station 4. When NH4+ oxidation was inhibited by the addition of ATU, a linear decrease in NO2− occurred. In contrast, there was a linear increase in NO2− when NaClO3 was added to inhibit NO2− oxidation. The NO2− in the control sample either linearly increased or stayed constant, since it reflected the 5

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

Table 1 Community respiration rates, ammonia and nitrite oxidation rates, aerobic respiration rates and corresponding concentrations of various parameters at Station 4. Month

May June-surface June-bottom July August

Community respiration rates

Ammonia oxidation rates

(μmol O2 L− 1 d− 1)

(μmol N L− 1 d− 1)

16.38 9.08 8.70 13.65 15.00

0.53 0.13 0.12 0.21 0.38

Nitrite oxidation rates

0.06 0.12 0.13 0.11 0.05

combined effects of NH4+ and NO2− oxidation. The nitrifying activities at Station 4 can be roughly divided into two categories. The first was observed in May, July and August, when NH4+ oxidation was more intense than NO2− oxidation, as can be seen from the slopes, and highconcentration NO2− appeared due to incomplete nitrification (Fig. 6a, d, e). The second category was observed in June, when NH4+ oxidation was equal to NO2− oxidation, indicating complete nitrification, as evidenced by low NO2− concentrations (Fig. 6b, c). Based on a comparison with the monthly variation in DO% (Table 1), high levels of NO2− were apt to appear in waters with low DO% values. NH4+ and NO2− oxidation rates at Station 4 were calculated using the slopes of the linear decreases or increases in NO2− concentrations (Fig. 6), which ranged from 0.12 to 0.53 μmol O2 L− 1 d− 1 and 0.06 to 0.13 μmol N L− 1 d− 1, respectively (Table 1). Similar to community respiration, NH4+ and NO2− oxidation rates showed little difference between surface and bottom samples. After subtracting the nitrification induced oxygen consumption rates from the community respiration rates, aerobic respiration rates were calculated, with values ranging from 8.46 to 15.58 μmol O2 L− 1 d− 1 (Table 1). The concentrations of DOC at Station 4 in this study are also listed in Table 1.

NO2−

Aerobic respiration rates

DO

NH4+

(μmol O2 L− 1 d− 1)

%

(μmol L− 1)

(mg L− 1)

15.58 8.83 8.46 13.29 14.43

75.0 91.4 92.1 90.4 77.6

16.04 11.35 11.30 21.12 12.63

2.88 2.32 2.29 3.15 2.47

9.06 1.89 1.69 7.37 4.67

DOC

of organic matter may be the most important controlling factors for the aerobic respiration. Surface water temperatures at Station 4 were all above 20 °C (Fig. 7a), which were suitable for aerobic respiration (Thamdrup et al., 1998). Moreover, a strong positive correlation was observed between the temperatures and aerobic respiration rates (except for May, R2 = 1.0, Fig. 7a), revealing the strong influence of temperature on aerobic respiration. In addition to temperature, DOC was another important factor for the monthly variations in aerobic respiration. In June, both DOC and temperature values were the lowest, resulting in a lowest aerobic respiration rate. Similarly, in August, a relative high aerobic respiration rate appeared due to its relative high temperature and DOC values. However, the highest DOC concentration in July failed to promote intense aerobic respiration. This pattern was likely related to the inhibitory bacterial activity induced by rapid changes in salinity (Fig. 7a, Martin et al., 2009), which caused by a rainstorm that happened three days before the cruise. With respect to May, although its temperature was lowest in the four months, the aerobic respiration rate was highest. We note that the DOC concentration in May was the second highest (Fig. 7a), thus the aerobic respiration rate appeared to be controlled by organic matter supply. Because high content of oxygen had a negative effect on the growth of nitrifying bacteria, intense nitrification reactions were very likely to happen in a low DO range of 15–78 μmol L− 1 (Abeliovich, 1987; Dai et al., 2008). At Station 4, the DO% values were all above 75% (that is, DO > 162 μmol L− 1), indicating low NH4+ and NO2− oxidation rates (Table 1). Furthermore, the NH4+ oxidation rates and aerobic respiration rates at Station 4 varied in a similar pattern with the fluctuations in DO and NH4+, as can be seen in Fig. 7b and c. This pattern was likely due to the fact that NH4+ oxidation was controlled by the DO environment modified by aerobic respiration. Aerobic respiration

4. Discussion 4.1. Interrelationship between aerobic respiration, nitrification and environmental conditions Aerobic respiration rates at Station 4, which was located in the human-perturbed northeastern area, varied significantly from May to August, with the highest rate (May) being nearly twice the lowest (June) (Table 1). Variations in the monthly temperature and the supply

Fig. 6. Evolution of NO2− concentrations over the course of NH4+ and NO2− oxidation incubations at Station 4.

6

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

Fig 7. Interrelationship between aerobic respiration rates, NH4+ oxidation rates, NO2− oxidation rates and environmental variables at Station 4. (a) Aerobic respiration rates vs. environmental variables; (b) DO% vs. ammonia oxidation rates and aerobic respiration rates; (c) ammonia concentrations vs. ammonia oxidation rates and aerobic respiration rates; (d) nitrite oxidation rates vs. DO% and nitrite concentrations. Fig. 8. (a) Relationship between the accumulation rates and on-site concentrations of NH4+ at Station 4; (b) relationship between the accumulation rates and on-site concentrations of NO2− at Station 4.

consumed DO and produced NH4+, thus, higher aerobic respiration rates were associated with lower DO% values (Fig. 7b) and higher NH4+ concentrations (Fig. 7c, except in July). Also, low DO and high NH4+ concentrations promoted NH4+ oxidation, thus, intense NH4+ oxidation was associated with low DO% values (Fig. 7b) and high levels of NH4+ (Fig. 7c, except in July). In July, NH4+ concentrations were highest during the investigation, while the ammonia oxidation was weak. The inhibitory nitrifying activity induced by rapid changes in salinity (Fig. 7a, Bernhard et al., 2007) after a rainstorm should be responsible for this phenomenon. In general, NO2− rarely accumulates in oxygenated seawater environments (Capone et al., 2008). However, in oxygen-depleted seawater environments, the accumulation of NO2− primarily depends on the relative intensity of NH4+ and NO2− oxidation. At Station 4, the DO % value and NO2− oxidation rate reached the highest levels in June, whereas the NH4+ oxidation rate was the lowest, resulting in a lowest NO2− concentration (Fig. 7d). In contrast, the DO% values were

considerably low in May and August, and the NH4+ oxidation rates were 8 and 7 times higher than the NO2− oxidation rates in these two months, respectively; thus, NO2− accumulation occurred (Fig. 7d). In July, the NH4+ oxidation rate was only slightly higher than the NO2− oxidation rate, but NO2− concentration was quite high (Fig. 7d). Given the low salinity in July (Fig. 7a), this pattern may be related to direct terrestrial inputs of NO2− caused by the rainstorm. 4.2. The contribution of aerobic respiration and nitrification to NH4+ and NO2− The NH4+ and NO2− concentrations were quite high in the northeastern area of the bay, and were even higher than 10 μmol L− 1 and 3 μmol L− 1 at Station 4 (Fig. 3a, b). Although terrestrial inputs may directly introduce considerable amounts of NH4+ and NO2− to the northeastern area (Liu et al., 2005), the locally generated NH4+ and NO2− should not be neglected, given that the aerobic respiration rates 7

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

were much higher than the NH4+ oxidation rates and that the NH4+ oxidation rates were much higher than the NO2− oxidation rates (Table 1). In the present study, we assumed that aerobic respiration produced 16 mol NH4+ when consuming 106 mol O2 (Eq. (2)), that NH4+ oxidation produced 0.99 mol NO2− when consuming 1 mol NH4+ (Eq. (3)) and that NO2− oxidation produced 1 mol NO3− when consuming 1 mol NO2− (Eq. (4)). Then the accumulation rates of NH4+ and NO2− derived from aerobic respiration and nitrification can be calculated (Fig. 8). During the four months, the accumulation rates of NH4+ and NO2− at Station 4 were highest in May, reaching 1.97 μmol N L− 1 d− 1 and 0.47 μmol N L− 1 d− 1, respectively (Fig. 8). Even in June, when the accumulation rates were the lowest, these rates reached 1.20 μmol N L− 1 d− 1 and 0.01 μmol N L− 1 d− 1. Additionally, the daily accumulation rates of NH4+ and NO2− showed a good correspondence with their on-site concentrations (except for that in July), suggesting that aerobic respiration and NH4+ oxidation were important contributors of high levels of NH4+ and NO2− in the northeastern area. The considerably high levels of NH4+ and NO2− in July might be related to the rainstorm-induced terrestrial inputs instead of aerobic respiration and NH4+ oxidation, since its salinity was quite low (Fig. 2b).

with thermodynamic constants of Mehrbach et al. (1973) refit by Dickson and Millero. (1987). These results are listed in Table 2. The RDIC caused by aerobic respiration at Station 4 ranged from 13.29 to 15.58 μmol C kg− 1 d− 1 in May, July and August. Even in June, when the aerobic respiration was the lowest, this rate reached 8.83 μmol C kg− 1 d− 1 (Table 2). Moreover, the DIC (CO2*) production during one month would reach 265–467 μmol C kg− 1, under the assumption of current aerobic respiration rates lasting for one month due to a long water residence time of > 70 d (Liu et al., 2004). Therefore, aerobic respiration was an important process for the higher DIC concentrations at Station 4. CO2* produced by aerobic respiration can lower the pH and increase the pCO2 by the dynamic equilibrium of carbonate system. In May, for instance, the estimated decrease in pH and increase in pCO2 at Station 4 reached 0.042 units and 166 μatm per day (calculated using the program CO2SYS). However, the influence on pH and pCO2 from NH4+ oxidation was small as compared to aerobic respiration (Table 1). The contribution of aerobic respiration to the decrease in pH and increase in pCO2 from May to August was 93%, 97%, 97%, and 95%, respectively, while NH4+ oxidation accounted for 7% or less. 4.4. Excess CO2 and AOU in the eastern Jiaozhou Bay

4.3. The contribution of aerobic respiration to high pCO2 and low pH Under the effects of human activities, the undersaturated DO area covered nearly two-thirds, one-thirds and all of the eastern Jiaozhou Bay in June, July and August, respectively (Fig. 2d–f). Corresponding to the undersaturated DO, the entire eastern Jiaozhou Bay was a CO2 source (Fig. 2j–l). The undersaturation of DO and supersaturation of pCO2 suggested that this region experienced intense aerobic respiration. The relationship between excess CO2 and AOU is a useful tool for discussing the mechanisms responsible for high pCO2 and low DO conditions in coastal waters (Zhai et al., 2005; Gupta et al., 2009). In the eastern Jiaozhou Bay, excess CO2 had a strong positive linear correlation with AOU during all the cruises (Fig. 9, except for the area of DO% > 120% in the centre of the Bay in July). The slopes of these linear regression equations from May to August were 0.4239, 0.4041, 0.3749 and 0.4868, respectively, with several very nearshore stations characterized by high pCO2 values in August (indicated by hollow stars) being excluded. Because of the weak nitrification shown by the incubation experiments (Table 1), aerobic respiration was the main source of the excess CO2 in the eastern Jiaozhou Bay. When all the organic carbon decomposes to CO2*, the ratio of excess CO2 and AOU should be 0.77 (according to Eq. (1)) or 1 (according to Eq. (2)). However, the slopes of the linear regression equations between excess CO2 and AOU in the eastern Jiaozhou Bay were considerably low, only reaching a value of ~0.4. This difference was likely due to the equilibrium of the CO2‑carbonate system in the eastern Jiaozhou Bay. As shown in Section 4.3, aerobic respiration resulted in CO2* production rates of 8.83–15.58 μmol C kg− 1 d− 1, however, the increase rates of pCO2 were only 2.09–5.39 μmol C kg− 1 d− 1 after the dynamic

Aerobic respiration and nitrification not only cause the accumulation of NH4+ and NO2−, but also lead to increases in pCO2 and decreases in pH. During the investigation periods, pH remained low and pCO2 was high in the northeastern area of the bay (Fig. 2j–o), which is likely associated with aerobic respiration and nitrification. In this study, we assumed that aerobic respiration produced 106 mol CO2* and 15 mol TAlk when consuming 106 mol O2 (Eq. (2)); hence, the increase in CO2* was dominant. Similarly, according to Eq. (3), we assumed that NH4+ oxidation consumed 1.99 mol TAlk and 0.05 mol CO2* when oxidizing 1 mol NH4+, which resulted primarily in the consumption of TAlk. In contrast, NO2− oxidation only consumed 0.03 mol CO2* and 0.01 mol TAlk when oxidizing 1 mol NO2− (Eq. (4)), along with the low NO2− oxidation rates at Station 4 (Table 1), the effect of NO2− oxidation on the carbonate system can be ignored. The increase or decrease in DIC and TAlk derived from aerobic respiration and NH4+ oxidation changed the original carbonate equilibrium. Aerobic respiration resulted in a decrease in the ratio of TAlk to DIC (TAlk/DIC) through a greater increase in DIC than in TAlk, whereas NH4+ oxidation made TAlk/DIC decrease through a greater decrease in TAlk than in DIC; those decreases in TAlk/DIC further resulted in higher pCO2 and lower pH levels (Broecker et al., 1982). On the above basis, the rates of increase or decrease in DIC and TAlk (RDIC and RTAlk) caused by aerobic respiration and NH4+ oxidation can be obtained. Then, based on the original data of DIC, TAlk, pCO2 and pH, as well as those changes in DIC and TAlk, the rates of decrease or increase in pH and pCO2 (RpH and RpCO2) were calculated using the program CO2SYS Table 2 The effects of aerobic respiration and ammonia oxidation on the carbonate system. Month

pH

DIC μmol kg

May

7.70

2360

June

7.80

2215

July

7.78

2065

August

7.76

2098

RDIC −1

μmol C kg Aerobic respiration NH4+ oxidation Aerobic respiration NH4+ oxidation Aerobic respiration NH4+ oxidation Aerobic respiration NH4+ oxidation

RTAlk −1

15.58 − 0.026 8.83 − 0.006 13.29 − 0.010 14.43 − 0.019

−1

d

μmol kg 2.90 − 1.05 1.25 − 0.26 1.88 − 0.42 2.04 − 0.75

RpH −1

−1

RpCO2 μatm d− 1 (μmol C kg− 1 d− 1)

d

− 0.042 − 0.0031 − 0.023 − 0.0007 − 0.039 − 0.0013 − 0.039 − 0.0022

166 (5.39) 11 (0.36) 68 (2.09) 2 (0.06) 127 (3.66) 4 (0.11) 127 (3.54) 6 (0.18)

Note: pH and DIC represent the on-site concentrations recorded at Station 4; RDIC, RTAlk, RpH, and RpCO2 represent the productive (positive values) and consumption (negative values) rates of DIC, TAlk, pH, and pCO2.

8

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

NO2− corresponded well to their on-site concentrations, indicating that the aerobic respiration and nitrification were important contributors of high levels of NH4+ and NO2− in the northeastern area. 3. After the dynamic equilibrium of carbonate system, aerobic respiration caused a pH decrease of up to 0.023–0.042 units and a pCO2 increase of up to 68–166 μatm per day in the northeastern area. Moreover, the linear relationship between excess CO2 and AOU suggested that the excess CO2 in the entire eastern Jiaozhou Bay was mainly from the aerobic respiration. Acknowledgements This work was supported by the National Natural Science Foundation of China - Shandong Joint Fund for Marine Science Research Centres (NSFC) (Grant No. U1606404), the National Natural Science Foundation of China (NSFC) (Grant No. 41376123), the FIO Basic Science and Research Programs (Grant No. 2016Q01) and the National Natural Science Foundation for Creative Research Groups (Grant No. 41521064). We thank Ming Xue for his useful suggestions. We also thank Yaoru Li, Qianqian Jiang for the sampling and measuring work.

Fig. 9. Relationship between excess CO2 and apparent oxygen utilization in the eastern Jiaozhou Bay from June to August and continuous observations at Station 4 in May. Note: excess CO2 = [CO2*] − KHCO2 × pCO2 (in air), where [CO2*] is the concentration of total free CO2, (i.e., [CO2*] = [CO2] + [H2CO3] = KHCO2 × pCO2 (in water)); KHCO2 is the solubility coefficient of CO2; AOU = ΔO2 = [O2] − [O2]eq, where [O2]eq is the DO concentration at equilibrium with the atmosphere and [O2] is the in situ DO concentration. The regression lines are as follows: Continuous observations at Station 4 in May y = 0.4239x + 6.304, R2 = 0.7420, n = 12, p < 0.001; June - y = 0.4041x + 6.556, R2 = 0.8390, n = 22, p < 0.0001; July - y = 0.3749x + 9.546, R2 = 0.4538, n = 16, p < 0.001; August - y = 0.4141x − 1.727, R2 = 0.8202, n = 16, p < 0.0001.

References Abeliovich, A., 1987. Nitrifying bacteria in wastewater reservoirs. Appl. Environ. Microbiol. 53 (4), 754–760. Bernhard, A.E., Tucker, J., Giblin, A.E., Stahl, D.A., 2007. Functionally distinct communities of ammonia-oxidizing bacteria along an estuarine salinity gradient. Environ. Microbiol. 9 (6), 1439–1447. Bianchi, M., Marty, D., Teyssie, J.L., Fowler, S.W., 1992. Strictly aerobic and anaerobicbacteria associated with sinking particulate matter and zooplankton fecal pellets. Mar. Ecol. Prog. Ser. 88 (1), 55–60. Bianchi, M., Feliatra, F., Tréguer, P., Vincendeau, M., Morvan, J., 1997. Nitrification rates, ammonium and nitrate distribution in upper layers of the water column and in sediments of the Indian sector of the Southern Ocean. Deep-Sea Res. II Top. Stud. Oceanogr. 44 (5), 1017–1032. Borges, A.V., Ruddick, K., Delille, L.S., 2008. Net ecosystem production and carbon dioxide fluxes in the Scheldt estuarine plume. BMC Ecol. 8 (1), 15. Broecker, W.S., Peng, T., Beng, Z., 1982. Tracers in the Sea. Lamont-Doherty Geological Observatory, Columbia University. Cai, W.-J., Guo, X., Chen, C.-T., Dai, M., Zhang, L., Zhai, W., Lohrenz, S.E., Yin, K., Harrison, P.J., Wang, Y., 2008. A comparative overview of weathering intensity and HCO3− flux in the world's major rivers with emphasis on the Changjiang, Huanghe, Zhujiang (Pearl) and Mississippi rivers. Cont. Shelf Res. 28 (12), 1538–1549. Cai, W.-J., Hu, X., Huang, W., Murrell, M.C., Lehrter, J.C., Lohrenz, S.E., Chou, w., Zhai W., Hollibaugh, J.T., Wang, Y., Zhao, P., Guo, X., Gundersen, K., Dai, M. and Gong, G., 2011. Acidification of subsurface coastal waters enhanced by eutrophication. Nat. Geosci. 4 (11), 766–770. Capone, D.G., Bronk, D.A., Mulholland, M.R., Carpenter, E.J., 2008. Nitrogen in the Marine Environment. Academic Press. Dai, M., Wang, L., Guo, X., Zhai, W., Li, Q., He, B., Kao, S., 2008. Nitrification and inorganic nitrogen distribution in a large perturbed river/estuarine system: the Pearl River Estuary, China. Biogeosci. Discuss. 5 (2), 1545–1585. Dai, Z., Du, J., Zhang, X., Su, N., Li, J., 2010. Variation of riverine material loads and environmental consequences on the Changjiang (Yangtze) estuary in recent decades (1955–2008)†. Environ. Sci. Technol. 45 (1), 223–227. Dickson, A.G., Goyet, C., 1994. Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water. (publisher not identified). Dickson, A.G., Millero, F.J., 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep Sea Res. Part A 34 (10), 1733–1743. Editorial Board of Annals of Bays in China, 1993. Annals of Bays in China. 4 Ocean Press, Beijing (448 pp. in Chinese). Feliatra, F., Bianchi, M., 1993. Rates of nitrification and carbon uptake in the Rhone River plume (northwestern Mediterranean Sea). Microb. Ecol. 26 (1), 21–28. Gao, Z., Yang, D., Qin, J., Xiang, L., Zhang, K., 2008. The land-sourced pollution in the Jiaozhou Bay. Chin. J. Oceanol. Limnol. 26, 229–232. Gupta, G., Thottathil, S.D., Balachandran, K.K., Madhu, N.V., Madeswaran, P., Nair, S., 2009. CO2 supersaturation and net heterotrophy in a tropical estuary (Cochin, India): influence of anthropogenic effect. Ecosystems 12 (7), 1145–1157. Hsiao, S., Hsu, T., Liu, J., Xie, X., Zhang, Y., Lin, J., Wang, H., Yang, J., Hsu, S., Dai, M., 2014. Nitrification and its oxygen consumption along the turbid Chang Jiang River plume. Biogeosciences 11 (7), 2083–2098. Hu, X., Cai, W.-J., 2011. An assessment of ocean margin anaerobic processes on oceanic alkalinity budget. Glob. Biogeochem. Cycles 25 (3). Lewis, E., Wallace, D., Allison, L.J., 1998. Program Developed for CO2 System Calculations. Carbon Dioxide Information Analysis Center, managed by Lockheed Martin Energy Research Corporation for the US Department of Energy Tennessee.

equilibrium of carbonate system (Table 2). Nearly 65–76% of the CO2* produced by aerobic respiration was converted to HCO3−, resulting in a relatively low excess CO2 as compared to AOU. Moreover, excess CO2 in the eastern Jiaozhou Bay could also partly come from pCO2 derived from terrestrial DIC. The lowest salinity value from the northeastern area was 1.53 and 1.99 lower than those in the bay mouth in June and August mainly due to sewage discharge from the city of Qingdao, and 6.44 in July after a rainstorm that happened three days before the cruise. However, the TA/DIC values of sewage and surface runoff are high compared to aerobic respiration, e.g. ~ 1 in rivers (Cai et al., 2008). Their effects on pCO2 equals to the aerobic respiration only when the terrestrial DIC was 7 times the aerobic respiration, as the TA/ DIC is 0.14 (15/106) for the aerobic respiration. Based on the salinity difference between the northeastern area and the bay mouth, terrestrial inputs had a slight influence on excess CO2. Aerobic respiration was still the main source of excess CO2 in the eastern Jiaozhou Bay. With the development of urbanization, the current conditions in the eastern Jiaozhou Bay, involving the production of large amounts of NH4+, the decrease in seawater pH and the release of CO2 associated with the aerobic respiration, will remain if no feasible measures were taken to change the pollution status. Therefore, controlling the organic matter flux into the eastern Jiaozhou Bay and weakening the aerobic respiration intensity are important processes for the environment quality improvement and CO2 sink/source pattern conversion. 5. Conclusions 1. Aerobic respiration was intense in the human-perturbed northeastern area from May to August, and was controlled by both temperature and DOC. The NH4+ oxidation was considerably low and related to the DO and NH4+ environment modified by aerobic respiration. 2. The calculated accumulation rates of NH4+ and NO2− associated with aerobic respiration and nitrification in the northeastern area ranged from 1.20–1.82 μmol N L− 1 d− 1 and 0.01–0.47 μmol N L− 1 d− 1, respectively. Additionally, the monthly accumulation rates of NH4+ and 9

Marine Pollution Bulletin xxx (xxxx) xxx–xxx

P. Han et al.

Testa, J.M., Kemp, W.M., 2014. Spatial and temporal patterns of winter–spring oxygen depletion in Chesapeake Bay bottom water. Estuar. Coasts 37 (6), 1432–1448. Thamdrup, B., Hansen, J.W., Jørgensen, B.B., 1998. Temperature dependence of aerobic respiration in a coastal sediment. FEMS Microbiol. Ecol. 25 (2), 189–200. Thomas, H., Bozec, Y., Elkalay, K., De Baar, H.J., 2004. Enhanced open ocean storage of CO2 from shelf sea pumping. Science 304 (5673), 1005–1008. Thomas, H., Bozec, Y., de Baar, H.J., Elkalay, K., Frankignoulle, M., Schiettecatte, L.S., Kattner, G., Borges, A.V., 2005. The carbon budget of the North Sea. Biogeosciences 2 (1), 81–96. Topcu, H.D., Brockmann, U.H., 2015. Seasonal oxygen depletion in the North Sea, a review. Mar. Pollut. Bull. 99 (1), 5–27. US EPA, 1993. Process Design Manual for Nitrogen Removal. EPA/625/R-93/010. U.S. Environmental Protection Agency, Washington, DC (88 pp.). Zhai, W., Dai, M., Cai, W., Wang, Y., Wang, Z., 2005. High partial pressure of CO2 and its maintaining mechanism in a subtropical estuary: the Pearl River estuary, China. Mar. Chem. 93 (1), 21–32. http://dx.doi.org/10.1016/j.marchem.2004.07.003. Zhang, X., Zhu, M., Chen, S., Grant, J., Martin, J., 2006. Study on sediment oxygen consumption rate in the Sanggou Bay and Jiaozhou Bay. Adv. Mar. Sci. 24 (1), 91–96 (in Chinese with English Abstract). Zhang, L., Xue, M., Liu, Q., 2012. Distribution and seasonal variation in the partial pressure of CO2 during autumn and winter in Jiaozhou Bay, a region of high urbanization. Mar. Pollut. Bull. 64 (1), 56–65.

Liu, Z., Wei, H., Liu, G., Zhang, J., 2004. Simulation of water exchange in Jiaozhou Bay by average residence time approach. Estuar. Coast. Shelf Sci. 61 (1), 25–35. Liu, S., Zhang, J., Chen, H., Zhang, G., 2005. Factors influencing nutrient dynamics in the eutrophic Jiaozhou Bay, North China. Prog. Oceanogr. 66 (1), 66–85. Lu, D., Yang, N., Liang, S., Li, K., Wang, X., 2016. Comparison of land-based sources with ambient estuarine concentrations of total dissolved nitrogen in Jiaozhou Bay (China). Estuar. Coast. Shelf Sci. 180, 82–90. Martin, A., Hall, J., Ryan, K., 2009. Low salinity and high-level UV-B radiation reduce single-cell activity in Antarctic sea ice bacteria. Appl. Environ. Microbiol. 75 (23), 7570–7573. Mehrbach, C., Culberson, C.H., Hawley, J.E., Pytkowicx, R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnol. Oceanogr. 18 (6), 897–907. Rabalais, N.N., Cai, W., Carstensen, J., Conley, D.J., Fry, B., Hu, X., Quinones-Rivera, Z., Rosenberg, R., Slomp, C.P., Turner, R.E., 2014. Eutrophication-driven deoxygenation in the coastal ocean. Oceanography 27 (1), 172–183. Redfield, A.C., 1963. The influence of organisms on the composition of sea-water. In: The Sea, pp. 26–77. Seitzinger, S.P., Harrison, J.A., Dumont, E., Beusen, A.H., Bouwman, A.F., 2005. Sources and delivery of carbon, nitrogen, and phosphorus to the coastal zone: an overview of Global Nutrient Export from Watersheds (NEWS) models and their application. Glob. Biogeochem. Cycles 19 (4).

10