A subsurface nitrite maximum was observed along one transection near the Xisha Islands in the South China Sea in early September 2009. Possible causes were examined using in situ observed data and remote sensing data. A cold eddy was identified near the Xisha Islands during the periods of study. The Xisha Islands' water receives copious supplies of nutrients through cold eddies. Accumulation of nitrite about 50 m deep reaches 0.49 μmol l−1, and forms and maintains the primary nitrite maximum. The relationship between NO2− and Chl-a is very significant (R2 = 0.5829, p-value(statistical significance) = 0.00). Nitrification processes would result in apparent oxygen utilization (AOU) ranging from 0.22 to 35.88 μM, around 24.60% of the total biological oxygen demand in the water column of under-saturation of dissolved oxygen. This value is very close to the Redfield stoichiometry (32/138 = 23%) of total oxygen consumption associated with organic matter diagenesis. These results may support the hypothesis that phytoplankton and the nitrification process have an important influence on the PNM or 260 maintenance in the euphotic layer. Our results indicate that physical conditions and biological activities near the Xisha Islands play a significant role in regulating biogeochemistry.
Nitrite (NO2−) is a dynamic component of the marine nitrogen cycle, and is produced and consumed by a variety of processes, such as: nitrification, denitrification and exertion by phytoplankton. Accumulations of nitrite in certain marine strata mark the locations where biologically mediated changes in combined nitrogen occur (Rakestraw, 1936; Vaccaro and Ryther, 1960). The primary nitrite maximum (PNM) located near the base of the euphotic layer or seasonal thermocline is generally observed in oxic oligotrophic seawaters. This maximum peak is often attributed to the accumulation of nitrite produced by biological and physical progresses. Nitrification is the two-step microbial transformation of NH4+ to NO3− via a NO2− intermediary. Nitrification requires the presence of O2 and tends to be inhibited by light, which has important implications for the upper ocean nitrogen cycle. NO2− is mostly controlled by nitrification in the euphotic zone. However, we cannot discount the possibility that excretion from marine phytoplankton engaged in nitrate reduction could be a source of this nitrite (Vaccaro and Ryther, 1960). Nitrite production by non-biological mechanisms is of little importance. Photo-oxidation of ammonia was insignificant in the sea (Hamilton, 1964). Anaerobic processes likely make minimal contributions to NO2− concentrations (e.g. anaerobic microzones in marine snow particles) in the euphotic zones of open-ocean gyres (Kuypers et al., 2003). Nitrite cycling in the ocean is complicated and has many unanswered questions, and biological–physical interactions are also likely important in controlling PNM or 260 maintenance (Lomas and Lipschultz, 2006).
Despite the complexity of the PNM or 260 maintenance, several important characteristics remain apparent. The PNM in the water column is affected by physical, chemical, and biological factors. NO2− reservoirs may have numerous sources and sinks, some of which have yet to be characterized. Previous studies focused on the PNM by using experimental simulation and chemical–biological processes (Al-Qutob et al., 2002; French et al., 1983; Kiefer et al., 1976; Mackey et al., 2011; Wada and Hattori, 1971). Physical processes might not be taken into account. In this study, we seek to improve our understanding of how key physical, chemical, and biological processes contribute to the PNM formation and maintenance. Based on the data obtained during the open cruise of the RV Shiyan 3 (13 August–23 September 2009), we discuss the processes that might regulate the distribution of nitrite in the euphotic zone near the Xisha Islands.
Materials and methods
The South China Sea is one of the largest marginal seas of the western Pacific Ocean. It is characterized by many atolls: The Dongsha Islands on the northern continental slope, the Xisha Islands near the west coast, and the Zhongsha Islands in the deep basin. The Xisha Islands (15°47′–17°08′N, 110°10′–112°55′E) are located in the central South China Sea (Figure 1).
A strong northeastward winter monsoon and a weaker southwestward summer monsoon can lag for up to three months from the south to the north. During the transitional period between winter and summer monsoons, different controlling wind fields may coexist at the sea surface.
Sampling and laboratory analysis
The open cruise was conducted from 31 August to 23 September 2009 aboard the RV Shiyan 3. The transect near Xisha was conducted on 6–7 September 2009. Temperature and salinity profiles were obtained using a Sea-Bird 911 plus CTD. Seven layers of water samples were obtained; surface waters (0.5 m beneath the sea surface), 25 m, 50 m, 75 m, 100 m, 150 m and 200 m. In one deep station, up to ten layers (300 m, 500 m, 800 m and 1000 m) were sampled. Seawater samples were collected with Niskin bottles. Immediately after sample collection, DO (Winkler titration method) was measured. Seawater for nutrient analysis (PO43−, NH4+, NO3−, NO2− and SiO3−) by spectrophotometry (GB–17378.4-2007, China) was filtered through acid-cleaned acetate cellulose filters (pore size: 0.45 μm) with detection limits of 0.02 μmol l−1, 0.03 μmol l−1, 0.05 μmol l−1, 0.02 μmol l−1 and 0.45 μmol l−1, respectively. Filtrates were then poisoned with HgCl2 and stored in the dark at 4°C until analysis. Apparent oxygen utilisation (AOU) is the difference between the measured dissolved oxygen concentration and its equilibrium saturation concentration in water with the same physical and chemical properties.
The daily diffuse attenuation coefficient, Kd(490), PAR, Chl-a concentration, and sea surface temperature (SST) data Level 3, with a spatial resolution of 4 km, from the ocean color sensor Moderate Resolution Imaging Spectroradiometer (MODIS) were obtained from the Distributed Active Archive Center (DAAC) of the US National Aeronautics and Space Administration (NASA) (http://oceancolor.gsfc.nasa.gov). Sea level anomaly (SLA) data gathered from the TOPEX/Poseidon and JASON altimeters (http://www.aviso.oceanobs.com/) were used to highlight mesoscale oceanic features such as eddies and surface circulations. The oceanic mixed layer is a layer in which active turbulence has homogenized some range of depths. Time series data of mixed layer depth and total surface chlorophyll were obtained from the NASA Ocean Biogeochemical Model (http://gdata1.sci.gsfc.nasa.gov/daac-bin/G3/).
Temperature and salinity
There was a clear temperature gradient pattern in the study area, decreasing with the depth. Temperatures ranged from 15.8 to 29.3°C, while salinity ranged from 33.2 to 34.8 psu. The water column was, in general, stratified because of the high surface temperature and low salinity observed. The vertical distribution of temperature and salinity is relatively uniform. The temperature and salinity at the same depth above 150 m decreased and increased from Y01 to Y07, respectively (Figure 2). Sea surface temperatures decreased 2°C more at the station (Y07) than at other stations (Figure 2). This is consistent with the presence of a cold eddy occurring at the observed stations (Y06 and Y07; Figure 2). More specifically, while stations Y06 and Y07 consistently exhibited surface temperatures cooler than other stations (Figure 2), station Y07 exhibited the lowest surface water temperature because of the influence of the cold eddy.
Nitrate was abundant with increasing depth in the upper euphotic layer (Figure 3a). Nitrite concentration ranged from under the detectable limit to 0.49 μmol l−1. Nitrite concentration was relatively scarce throughout the water column, but increased above a depth of 50 m. A notable exception to this occurred when NO2− concentrations with large peaks were presented in the upper 50 m of the water column (Figure 3b). Within the euphotic zone, biological activities break the balance among the various production and consumption processes where NO2− accumulation occurs, and the primary nitrite maximum (PNM) is formed.
Elevated dissolved oxygen levels were obs-erved in surface waters at the observed stations (Y01-Y05) compared to these stations (Y07 and Y07). Dissolved oxygen was abundant with increasing depth in the upper euphotic layer (Figure 4a). Chlorophyll a concentration was low in the surface water. The highest chlorophyll a concentration was observed at a depth of 50 m (Figure 4b).
The time series of SST represents the daily variation of SST, which was taken from 3–8 September (Figure 5). SST decreased by about 1°C (from 29.53°C to 28.61°C) in the three days prior to the sampling.
Sea level anomaly (SLA)
Meanwhile, satellite derived near-real-time daily altimeter images showed a low sea level anomaly (SLA) forming, which might be related to the cold eddy (Figure 6). During the studying period (1 August to 6 September 2009), the lower SLA value (112°E, 15°N) moved eastward. Before sampling, studying stations (Y06 and Y07) located in the vicinity of the upwelling area had low SLA values (−5 ∼0 cm).
The surface mixed layer in the study area (110°E∼113°E, 15.5°N∼18°N) deepens to more than 50 m during winter and becomes as shallow as 20 m in summer (Figure 7a). The strong northeast winds during winter cause convective mixing in the upper ocean. The deepening of the mixed layer in the wintertime is associated with a strong increase in surface chlorophyll a (Figure 7b).
Euphotic layer depth
We used the estimated Kd(490) data from MODIS to compute the euphotic depth of the South China Sea. The relationship between the euphotic depth and the diffuse attenuation coefficient Kd(490) of the South China Sea based on in situ measurement data was 2.784/Kd(490) (Tang et al., 2007). The depth of the euphotic (1%PAR) was about 77 m based MODIS data, the depth of 10%PAR was about 38 m. In summertime, the euphotic depth in the offshore waters can reach more than 80 m in the northwest South China Sea (Song et al., 2012).
In the well-oxygenated water column near the Xisha Islands, the dissimilatory reduction of nitrate to nitrite by denitrifying bacteria is likely to be negligible. In addition, the contributions of photochemical processes might be neglected except for those in a very thin layer of the sea surface. The distribution of nitrite might be regulated by biochemical processes and physical oceanographic factors, such as diffusion, advection, or some combination of the two. Whether these processes co-occur in the study area and, if so, how physical factors influence which process dominates and at what depth in the water column is not clear. Thus, we consider the following processes potentially responsible for the generation and consumption of ambient nitrite near the Xisha Islands.
The surface mixed layer in the study area was as shallow as 10 to 20 m during the studying period. Because the mixed layer in summer (∼20 m) is significantly shallower than it is in winter (∼80 m), high concentrations of nitrate may not be entrained into the euphotic zone from the deeper water. As a result, nitrite was homogeneously distributed throughout the mixed layer in summer. The depth of the euphotic (1%PAR) is deeper than the mixed layer depth in summer. High phytoplankton biomass can be maintained near the bottom of the mixed layer fueled by the nutrient supply deeper in northern South China Sea (Song et al., 2012). Nutrients were introduced into the euphotic layer due to the strong upward movement of the nutrient-rich deeper upwelling waters at the study stations. This physical dynamic might help form and/or maintain PNM.
Phytoplankton excretion is probably important for the formation and sustainability of the PNM in the euphotic layer (Rakestraw, 1936). Our attempts to demonstrate phytoplankton excretion in the model without nitrification considered for formation and/or maintenance of the PNM proved successful; therefore, we suggest phytoplankton excretion in the sea is important (Kiefer et al., 1976). A physical disturbance (lateral advection) was used to estimate rates of biological NO2− production and eddy diffusive losses in the Gulf of Mexico (French et al., 1983). They found that NO2− production by phytoplankton during the day exceeded the amount lost fromeddy diffusive and any other processes. Eddy diffusion was maximal (equivalent to ∼50 nmol l−1 h−1) during the day, but negligible at night. These studies found that light plays an important role on NO2− production by phytoplankton. Other studies also found that PNM appeared at the base of the euphotic zone (∼1% PAR). However, that is inconsistent with the results that showincreasing NO2− release in light up to 10% of incident irradiance. In addition, NO2− uptake exceeds release in the upper euphotic zone so there is no PNM (Olson, 1981). Nitrite only accumulated in remarkable concentrations in light samples, not in dark samples (Al-Qutob et al., 2002). Nitrite is an intermediate in the nitrate assimilation of photosynthetic organisms and that process is stimulated by illumination. Much more nitrite was produced in the light by pure cultures of Phaeodactylumtricornutum and of Chaetoceros sp. (Wada and Hattori, 1971). Nitrite production from photochemical change can be neglected (Wada and Hattori, 1971). The production of nitrite might originate form phytoplankton excretion in the upper layer of the euphotic zone. Phytoplankton growth plays the key role in nitrite accumulation within the mixed water column (Al-Qutob et al., 2002). The rates of nitrite production at different depths in the upper layer of the euphotic zone are affected by light intensity. The 1%PAR depth shoals following a shallower PNM, and deepens following a deeper PNM (Lomas and Lipschultz, 2006). The relationship between NO2− and Chl-a (Figure 8) is significant (R2 = 0.5829, p-value(statistical significance) = 0.00). These results could support the hypothesis that phytoplankton are a controlling influence on the PNM, and predominantly important in PNM formation and maintenance.
For example, provided that all of the nitrate and nitrite were derived from the nitrification process, and regardless of the contribution from any other sources (e.g. Equation (1), and/or terrestrial input), 1.0 μM nitrite and nitrate would then mean consuming 1.5 μM and 2.0 μM DO, respectively (Equations (2) and (3)). Considering the nitrate and nitrite concentrations, such processes would result in AOU ranging from 0.22 to 35.88 μM. Redfield stoichiometry implies that nitrification is responsible for 23% of the total oxygen consumption associated with organic matter diagenesis. Nitrification accounts for around 24.60% of the total biological oxygen demand in the water column under-saturated with DO (AOU > 0). Nitrification should play an important role in the water column. In the tropical and subtropical ocean, oligotrophic and permanent stratification might recycle organic matter and result in the preferential retention of nitrogen within the euphotic zone. Under such conditions even low rates of nitrification become significant, as they ultimately represent a major source of recycled NO3− that can bias new production estimates(Clark et al., 2008; Yool et al., 2007).
Nitrite production rates
A mean rate of nitrite and nitrate production or consumption of 10−9μmol l−1·s−1 and 10−8μmol l−1·s−1 at a depth of 50 m was obtained, respectively. In this study, the box which contained a source for the primary nitrite peak also contained a sink for nitrate, although the ratio was / = −0.55. This value lies between the range value (−0.2∼−0.7). Thus nitrite is produced at a depth of 50 m and at about 55% of the rate at which nitrate is consumed. This result is higher than other studies (Mackey et al., 2011). They found that the fraction of NO2− generated relative to NO3− consumed about 15% in the mixed layer of the Gulf of Aqaba Red Sea. We infer that the biochemical activity leading to nitrite production in and near the nitrite maximum layer is high enough to maintain the high nitrite concentration, despite lossesfrom diffusion processes.
Primary nitrite maximums have received much attention in the past few decades in the open ocean, while receiving very little in the SCS. Many studies focus on the forming and accumulating mechanism of nitrite at the base of euphotic layers, concentrating on biological activities and chemical translation. In this study, an emphasis was put on the biogeochemical cycling of nitrogen along one transect near the Xisha Islands in the South China Sea (SCS) by the physical conditions that were observed. The accumulation of nitrite at about 50 m reaches 0.49 μmol l−1 and forms the primary nitrite maximum (PNM). The PNM layer is consistent with the chlorophyll maximum layer in the euphotic layer. The relationship between NO2− and Chl-a is very significant (R2 = 0.5829, p-value(statistical significance) = 0.00). Nitrification accounts for around 24.60% of the total biological oxygen demand (close to the organic matter nitrification of Redfield stoichiometry) in the water column of under saturation of DO. Nitrite is produced within the euphotic layer at about 55% of the rate at which nitrate is consumed. Thus, phytoplankton and the nitrification process had important influences on the PNM formation and maintenance in euphotic layer. Our results indicate that physical conditions and biological activities near the Xisha Islands play a significant role in regulating biogeochemistry. The schematic for biogeochemical processing of primary nitrite maximum in euphotic layer is shown in Figure 10.
We thank all the reviewers who provided insightful comments on earlier drafts, as well as the Captain and crew of the RV Shiyan 3 for help at sea.
This research was supported by the National Natural Science Foundation of China (Nos. 31270528 and 41206082), the project of Knowledge Innovation Program of Chinese Academy of Sciences (No. SQ200913), Key Laboratory of Marine Ecology and Environmental Science and Engineering, SOA (MESE-2013-02) and the Key Laboratory for Ecological Environment in Coastal Areas, State Oceanic Administration (No. 201211).