Journal of Earth Science, Vol. 25, No. 1, p. 197–206, February 2014 Printed in China DOI: 10.1007/s12583-014-0413-y
ISSN 1674-487X
Spatial and Seasonal Variations of Nutrients in Sediment Profiles and Their Sediment-Water Fluxes in the Pearl River Estuary, Southern China Ling Zhang*1, Lu Wang2, Kedong Yin*3, Ying Lü4, Yongqiang Yang5, Xiaoping Huang1 1. CAS Key Laboratory of Tropical Marine Bio-Resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China 2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China 3. School of Marine Sciences, Sun Yat-Sen University, Guangzhou 510275, China 4. School of Environment, Beijing Normal University, Beijing 100875, China 5. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ABSTRACT: Three cruises were launched in the Pearl River Estuary (PRE) in 2005 to investigate the biogeochemical cycling of nutrients associated with early diagenesis related to degradation of organic matter. Seasonal and spatial variations of pore water nutrient concentrations and profile patterns in sediments were studied. Nutrient fluxes at the sediment-water interface (SWI) were measured by incubation experiments, and we here discussed the accumulation and transformation processes of nutrients at the SWI. The nutrients generally decreased from the Pearl River outlets downstream, indicating anthropogenic influences on the nutrient inputs in the estuary. NO3-N concentration was the highest of the three forms of DIN (dissolved inorganic nitrogen, the sum of NH4-N, NO3-N and NO2-N) in the overlying water, and NH4-N was the main component of DIN in pore water. The gradual increase of NH4-N and the rapid decrease of NO3-N with sediment depth provided the evidence for anaerobic conditions below the SWI. Negative fluxes of NO3-N and positive fluxes of NH4-N were commonly observed, suggesting the denitrification of NO3-N at the SWI. The DIN flux direction suggested that the sediment was the sink of DIN in spring, however, the sediment was generally the source of DIN in summer and winter. PO4-P distribution patterns were distinct while SiO4-Si inconspicuously varied in sediment profiles in different seasons. The flux results indicated that PO4-P mainly diffused from the water column to the sediment while SiO4-Si mainly diffused from the sediment to the water column. Generally, the incubated fluxes were the coupling of diffusion, bioturbation and biochemical reactions, and were relatively accurate in this study. KEY WORDS: nutrient, pore water, sediment-water flux, Pearl River Estuary. 1
INTRODUCTION Estuarine areas are typically environmentally frail zones and are susceptible to anthropogenic activities under the acute interaction of physical, chemical and biological factors. Riverine transport is a principal pathway of nutrients from land to estuaries and oceans, particularly in areas strongly influenced by human activities. Dramatic increases of the delivery of the river-borne nutrients and changes in nutrient ratios owing to anthropogenic activities are known to result in eutrophication and unusual phytoplankton blooms, which changes the plankton community and the biogeochemical cycles (Domingues et al., 2011; Conley et al., 2009). Estuarine and coastal ecosys*Corresponding author:
[email protected];
[email protected]. edu.cn © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2014 Manuscript received February 19, 2013. Manuscript accepted June 2, 2013.
tems are confluence zones between terrestrial, atmospheric and marine areas, and are generally characterized by high productivity, mainly due to freshwater inputs and a tight coupling between pelagic and benthic systems (Denis and Grenz, 2003). Sediments in estuarine and coastal areas play an essential environmental role due to their capacity to store or release different compounds from or to the water column (Brigolin et al., 2011; Wallmann et al., 2008). Phytoplankton produces organic matter (OM) from inorganic nutrients in the euphotic zone through photosynthesis and sinks to the sediment when a nutrient is depleted. Degradation of settled OM and remineralization in the sediment releases inorganic nutrients to pore water, which can flux out into the overlying water and return to the euphotic zone (Van Beusekom et al., 2009; Grandel et al., 2000). Studies on pore water chemistry are widely used in identifying and evaluating biogeochemical changes occurring in aquatic sediments (Beck et al., 2008). The vertical profiles of pore water nutrients reflect the differences in decomposition and remineralization processes of OM with the depth in the sediment (Ehrenhauss et al., 2004). Nutrient exchange at the sediment-water
Zhang, L., Wang, L., Yin, K. D., et al., 2014. Spatial and Seasonal Variations of Nutrients in Sediment Profiles and Their SedimentWater Fluxes in the Pearl River Estuary, Southern China. Journal of Earth Science, 25(1): 197–206, doi:10.1007/s12583-014-0413-y
Ling Zhang, Lu Wang, Kedong Yin, Ying Lü, Yongqiang Yang and Xiaoping Huang
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2.2 Sample Collection and Nutrient Analysis The sediment cores were collected in spring (April), summer (August) and winter (December) in 2005 using a multiple corer (10 cm in diameter and 61 cm long) with a volume of overlying water. Four sediment cores were simultaneously collected at each station without destruction of the sediment-water interface. The
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2 MATERIALS AND METHODS 2.1 Site Description The PRE is composed of multiple estuaries with the Lingdingyang subestuary representing the main estuary, receiving the outflow of freshwater through the four outfalls of Humen, Jiaomen, Hongqimen and Hengmen (Fig. 1). The Lingdingyang subestuary is severely affected by tides and runoff, where the current and material transport obviously vary temporally and spatially. Salinity in Lingdingyang varies in vertical gradients, and is also characterized by horizontal distribution gradient. Lingdingyang is divided into the west shoal, the middle shoal and the east shoal by two channels. Sites 1, 3 and 4 are situated at the middle shoal and sites 2, 5 and 6 at the west shoal. The sampling stations are within the sub-tidal zone with strong freshwater and marine water interactions (Pan et al., 2002), and the estuarine plume of the freshwater-seawater mixture flows along the west part of the estuary due to the Coriolis effect deflection.
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interface (SWI) can balance the nutrient content in the water column, which is very important to phytoplankton production in many estuaries (Paul et al., 2008; Lü et al., 2006), and so to establish the pore water nutrient profiles and the magnitude of benthic fluxes are important for understanding geochemical budgets and for determining the role of sediment diagenesis in nutrient recycling (Grenz et al., 2010). The Pearl River, the largest subtropical river in southern China, carries on average a water discharge of ~330109 m3·a-1 and a sediment delivery of ~80106 t into the South China Sea (Cai et al., 2004). Freshwater discharge in the wet season (April–September) accounts for 70%–80% of total annual water load, and the remaining 20%–30% is carried out in dry seasons (October–March) (Tian, 1994). With the rapid economic growth and urbanization in the past decades, the Pearl River Delta and coastal zones have experienced increasing human influences and a growing pressure on the local environment. Increasing amounts of untreated or preliminarily treated sewage and other pollutants have discharged into the Pearl River, which has had a significant impact on the aquatic environment (Zhang et al., 2009; Chau, 2006). There have been some studies on the characteristics of the nutrients in the water column in the PRE (Huang et al., 2003; Yin et al., 2001), however, information about the nutrient distribution in sediment profiles and the fluxes at the SWI still remain poorly documented in this area. In this study, the main objectives were (1) to investigate the temporal and spatial distributions of nutrients in sediment pore water, (2) to estimate the seasonal and spatial variations of nutrient fluxes at the SWI, (3) to discuss the nutrient accumulation and transport and probe into how the human activities and early diagenesis affect nutrient cycling, which will provide significant dataset for the environmental research in this area.
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Macau
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Figure 1. Map showing the location of the Pearl River Estuary (PRE) and the sampling sites of sediment cores. overlying water was siphoned and then the sediment cores were sliced at 1 cm interval in a nitrogen-filled glove bag. The sediments were centrifuged to separate pore water on board the ship and then filtered (0.45 μm acetate fiber membrane) and immediately kept in deep frozen conditions (-20 °C) under the protection of HgCl2 until analysis. Concentrations of NH4-N, NO3-N, NO2-N, PO4-P and SiO4-Si in pore water and the overlying water were determined spectrophotometrically with a Skalar Nutrient Analyzer. In detail, nitrate was analyzed by the copper-cadmium reduction method, nitrite using the diazo-azo method and NH4-N using the sodium hypobromite oxidation method. SiO4-Si was analyzed by the silico-molybdenum yellow method and PO4-P by the molybdenum blue method. The precisions of duplication for NH4-N, NO3-N, NO2-N, PO4-P and SiO4-Si were 3.2%, 1.4%, 2.5%, 2.2% and 1.4%, respectively. 2.3 Incubation Experiment and Nutrient Flux The incubation device was structured according to Denis and Grenz (2003). About 30 cm-long sediment cores were sampled using the multiple corers with 15–25 cm thick overlying water. Bottom water samples at each site were taken and then gently transferred to an inflatable reserve tank excluding bubbles and stored under the same conditions as the incubated cores. The undisturbed sediment core was transferred to a water bar where the in situ water temperature and the overlying water thickness was measured before incubation started. The sediment core was covered using a Teflon cap with a small sampling hole on it, and a rotating floating magnet was fixed to the upper cap, which rotated the overlying water at a velocity of 40 r/min. Sampling of the overlying water was done by means of a plastic syringe at 0, 4, 8, 12, 16, 24 and 36 h during the incubation period of 36 h. After the volume was removed from each core tube, the same volume of external bottom water was injected to the tube, which retained the
Spatial and Seasonal Variations of Nutrients in Sediment Profiles and Their Sediment-Water Fluxes physical and chemical properties and total volume in the overlying water. The differences between concentration changes in the overlying water of each core and bottom water allowed the calculation of sediment-water fluxes. Water samples were then filtered (0.45 μm acetate fiber membranes) and immediately kept in deep frozen conditions (-20 °C) under the protection of HgCl2 until later analysis for nutrients. The concentrations measured in the chamber were corrected for the refilled volume. Flux (F) was calculated by fitting a linear regression to the changes in concentration versus time. Based on the concentration measurements of each nutrient, flux of a nutrient was calculated according to (Zhang, 2005) F=h·dC/dt (mmol·m-2·d-1) (1) where h is the height (m) of the enclosed overlying water column in the flux chamber and dC/dt (mmol·m-3·d-1) represents the increasing/decreasing rate of a nutrient in the flux chamber. 3 RESULTS AND DISCUSSION 3.1 Nutrient Concentrations and Distribution in the Overlying Water Nitrogenous nutrient concentrations in the overlying water and pore water were illustrated in Fig. 2. Concentrations of NH4-N in the overlying water were 0.97−9.98 (avg. 5.14), 15.78−65.70 (avg. 28.99) and 3.17−26.08 (avg. 16.48) μmol·L-1, respectively, in spring, summer and winter. NO3-N concentrations were 9.10−46.42 (avg. 27.52), 21.42−92.28 (avg. 61.81), 21.17−41.60 (avg. 30.42) μmol·L-1, and NO2-N concentrations were 2.49−11.86 (avg. 6.78), 4.20−10.2 (avg. 6.34), 2.99−8.19 (avg. 5.37) μmol·L-1, respectively, in spring, summer and winter. Concentrations of DIN were 10.76−68.26 (avg. 30.27), 73.68−119.22 (avg. 97.14) and 34.05−68.72 (avg. 49.55) μmol·L-1, respectively. DIN concentrations in this study were much higher than in the Changjiang Estuary (17.99 μmol·L-1, Wang et al., 2008), and the value in summer was two times higher than in the study of Pan et al. (2002) (41.63 μmol·L-1) in the PRE. The average DIN was 57.19 μmol·L-1, which was a little higher than those from previous studies in the PRE (38−43 μmol·L-1, Yin et al., 2011; Huang et al., 2003). DIN showed obvious variations between sites and the distribution patterns were distinct between seasons, because the nutrient material and organic matter in the PRE mainly originated from the inputs from the rivers (Zhang et al., 2009), which induced the high DIN concentration in the overlying water, especially in summer with high runoff. The concentrations in spring and summer generally decreased downstream, which may be attributed to the decreasing trend of Pearl River runoff downstream. NO3-N concentration was the highest of the three forms of DIN, in addition, NO3-N and NO2-N concentrations were both distinctly higher in the overlying water than those in pore water (Fig. 2). Concentrations of PO4-P were 0.36−1.03 (avg. 0.74), 1.92−2.16 (avg. 2.04) and 0.40−2.45 (avg. 1.67) μmol·L-1, respectively, in spring, summer and winter with lower values in spring relative to summer and winter (Fig. 3). The average value (1.30 μmol·L-1) was much higher than 0.61 μmol·L-1 in previous study in the PRE (Pan et al., 2002) and the value in the Changjiang Estuary (0.42 μmol·L-1, Wang et al., 2008), but was consistent with the value from Ma et al. (2009) (1.30 μmol·L-1) and from Yin et al. (2011) (~1.00 μmol·L-1) in the PRE. The SiO4-Si concentrations
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were 6.82−42.06 (avg. 28.72), 29.70−79.62 (avg. 55.74) and 13.07−42.08 (avg. 26.33) μmol·L-1, respectively, in spring, summer and winter (Fig. 3). The values in winter were generally the lowest, which was lower than 72 μmol·L-1 from Ma et al. (2009), and SiO4-Si in the PRE was much higher than in the Changjiang Estuary (16.78 μmol·L-1, Wang et al., 2008). DIN/P molar ratios were 12.96−66.27, 37.92−61.09, 16.31−119.48, and DIN/Si ratios were 18.94−66.78, 2.33−37.91, 8.60−53.48, respectively, in spring, summer and winter. DIN/P ratios were generally much higher than the classical Redfield ratios of 16 : 1 and changed seasonally, and the values on the whole showed a decreasing trend downstream in spring and summer, but generally increased downstream in winter. Most of DIN/Si ratios were a little higher than the Redfield ratios of 1 : 1, and the values generally showed a decreasing trend due to the decrease of DIN concentrations downstream. Study showed that P in the PRE was too low to allow phytoplankton to utilize the entire ambient N and hence, low P resulted in excess N in summer (Yin et al., 2004). In the study of Yuan et al. (2011), P availability limited bacterial growth in the Pearl River estuarine coastal upwelled waters and near the Pearl River Estuary. P limitation has been found in many other coastal waters such as in the Yangtze River Estuary in the East China Sea (Chai et al., 2009). There were also some reports of P limitation in the Chesapeake Bay (Fisher et al., 1999) and in the Mississippi River plume (Sylvan and Ammerman, 2013). Additionally, potential Si limitation occurred in the PRE due to the contribution of sewage effluent with high N and P enrichment all year (Xu et al., 2008). 3.2 Nutrient Concentration and Distribution in Pore Water 3.2.1 DIN NH4-N concentrations in pore water were 1.08−282.78 (avg. 90.08), 24.48−266.58 (avg. 131.74), 3.81−73.15 (avg. 57.60) μmol·L-1, respectively, in spring, summer, winter and the values elevated downcore at all sites in spring and the sites near the outfalls in summer (Fig. 2). NH4-N increased up to a depth of about 4 cm and then oscillated at site 4 and decreased at sites 5, 6 with more variable values in summer. In winter, NH4-N increased downcore from the SWI to about 4 cm depth and then kept constant, except that the values were more variable at Site 3. NO3-N concentrations were 0.84−79.02 (avg. 6.08), 0.42−29.76 (avg. 4.46), and 0.21−25.07 (avg. 3.55) μmol·L-1, respectively, in spring, summer and winter. Generally, NO3-N concentrations were relatively high in the overlying water and then diffused to the sediment from the SWI, furthermore, NO3-N decreased to a value and remained constant in sediment at the depth more than 3 cm at most sites due to the denitrification. Pore water NO3-N profiles were generally similar in the three seasons, nevertheless, there were some characteristics about the pattern in winter, namely, the values were more variable at the sites near the river outfalls (sites 1, 2, 3 and 4) compared with sites 5, 6 downstream. NO2-N concentrations were 0.12−3.84 (avg. 0.63), 0.06−3.36 (avg. 0.54) and 0.21−1.55 (avg. 0.43) μmol·L-1, respectively, in spring, summer and winter. As the same to NO3-N, there was an obvious concentration decrease of NO2-N from the interface to the surface sediment, and then the values remained relatively constant with the depth at most sites. Variations in temperature, in deposition of organic material, and in macrobenthos activities led to sea-
Ling Zhang, Lu Wang, Kedong Yin, Ying Lü, Yongqiang Yang and Xiaoping Huang
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sonal pattern in nutrient pore water concentrations (Magni and Montani, 2006). The impact of seasonal temperature changes depended on the supply of organic matter into the sediments, and organic compounds was hydrolyzed, fermented, and terminally oxidised by micro organisms, which mediated organic matter remineralisation in anoxic sediments (Alperin et al., 1994). In the study area, pore water temperatures showed significant changes in the sediment according to the seasons. Hydrolysis rates and activities of fermentative micro organisms were enhanced at higher temperatures resulting in higher concentrations of metabolisable organic matter in summer (Jahnke et al., 2005). NH4-N concentrations were comparable to those at many other regions listed in Table 1 (He et al., 2008; Hu et al., 2006; Denis and Grenz, 2003; Hopkinson Jr. et al., 2001). There were some features about NH4-N patterns in this study, i.e., NH4-N generally showed an increasing trend with the depth in summer and spring in the upper reach of the PRE, which was similar to some others in Table 1. Whereas NH4-N showed the decreasing trend in the lower reach of the RRE, which was maybe attributed to the NH4-N removal through the anaerobic ammonium oxidation process. Pore water (NO3-N)+(NO2-N) concentrations in the PRE system (~5.23 μmol·L-1) were comparable to NH 4- N (µmol·L -1) 5
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that in the Yalujiang Estuary (3 μmol·L-1, Ye et al., 2002), whereas were considerably lower than in the equatorial northeastern Pacific (25.8−38.5 μmol·L-1, Ni et al., 2005) and the Southern Ocean (3.0−58.6 μmol·L-1, Hu et al., 2006), Daya Bay (30−230 μmol·L-1, He et al., 2008) etc.. Pore water (NO3-N)+(NO2-N) distribution patterns were similar to those from Hu et al. (2006) and Ye et al. (2002) etc., i.e., NO3-N concentrations strongly decreased at the SWI, then changed slightly with the depth. Nutrient profiles indicated their accumulation at different sediment depths. NO3-N and NO2-N diffused from the water column to the sediment across the SWI driven by the concentration gradient, and then decreased severely under the interface (about 3 cm) due to the denitrification. To the contrary, NH4-N was the main form of inorganic nitrogen in pore water and generally increased downcore with elevated reducibility in the sediment. NH4-N mainly derived from the mineralization of sedimentary OM, namely, organic nitrogen changed to NH4-N with the work of bacteria, and then NH4-N entered into the pore water under an anaerobic situation. On the other hand, the distribution of NH4-N in the sediment was mainly affected by the disturbance of zoobenthos. The irrigation of zoobenthos impelled NH4-N in upper layers sediment to diffuse preferentially
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Figure 2. Nitrogenous nutrient concentrations (µmol·L-1) in the overlying water and pore water in spring, summer and winter in the PRE.
Spatial and Seasonal Variations of Nutrients in Sediment Profiles and Their Sediment-Water Fluxes
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Figure 3. PO4-P and SiO4-Si concentrations (µmol·L-1) in the overlying water and pore water in spring, summer and winter in the PRE. Table 1
Pore water nutrient concentrations at some other regions in the world (umol·L-1)
Gulf of St. Lawrence Seine Estuary
NO2+3-N 2.3–27.5 −
NH4-N 0–50 0–1 364
PO4-P 2–20 210.5–631.6
SiO4-Si 30–250 −
Source Thibodeau et al. (2010) Bally et al. (2004)
Equatorial northeastern Pacific
25.8–38.5
−
1.2–2.5
120–450
Ni et al. (2005)
Massachusetts/Cape Cod Bay Gulf of Lions Southern Ocean Yalujiang Estuary
−
4–300
0–100
100–600
Hopkinson et al. (2001)
5.6–17.0 3.0–58.6
70–175 11–624
0.15–4 −
− 229.0–759.6
Denis and Grenz (2003) Hu et al. (2006)
3.0
−
0.80–70.4
111.6–1 054.3
Ye et al. (2002)
30–230
10–200
23–80
440–940
He et al. (2008)
PRE
0–34
501.5
−
−
Pan et al. (2002)
PRE
5.2
93.1
1.9
150.5
This study
Daya Bay
to the overlying water (Sun and Song, 2002), and the NH4-N that entered into the overlying water was the direct nitrogen source of aquatic organisms and the initial nitrogen source for nitrification. Moreover, NH4-N distributions showed different spatial patterns as the result of distinct biogeochemical processes and the hydrodynamics from the sites near outfalls to downstream sites in different seasons. The study area was influenced by the interaction of river runoff and the tide, additionally, the water flow and the material
transfer obviously changed spatially, which caused the discrepancy in nutrient concentration in pore water at different sites and the decreasing trend downstream. In anoxic situations, the removal of NH4-N was attributed to three reasons, namely, chemical oxidation (reacting with MnO2), anaerobic ammonium oxidation (anammox) and oxygen-limited autotrophic nitrification and denitrification reactions (Dalsgaard et al., 2005). Traditionally, N2 in air mainly came from nitrification and denitrification. NH4-N can be oxidized to NO2-N or NO3-N
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Ling Zhang, Lu Wang, Kedong Yin, Ying Lü, Yongqiang Yang and Xiaoping Huang
in aerobic situations, and then NO2-N or NO3-N was reduced to N2. Recently, scientists found that in anaerobic situations, NH4-N can be oxidized to N2 with the action of anammox bacteria taking NO2-N or NO3-N as electron acceptor (Kawagoshi et al., 2009; Dalsgaard et al., 2003). In this study, we observed that the concentrations of NO3-N and NO2-N decrease downcore from the depth of about 4 cm at sites 5, 6 in summer, indicating that anaerobic ammonium oxidation may be the main process to NH4-N removal there. 3.2.2 PO4-P Pore water PO4-P concentrations were 0.45−2.98 (avg. 1.52), 0.90−13.32 (avg. 2.62) and 0.03−5.23 (avg. 1.54) μmol·L-1, respectively, in spring, summer and winter. Figure 3 showed that PO4-P distinctly changed in different seasons at the sites near the outfalls (sites 1, 2 and 3), which suggested that runoff inputs influenced more to PO4-P in sediment in different season. PO4-P concentrations were the highest in summer, perhaps the high water temperature in summer elevated the activities of microbio organisms, and thus stimulated the degradation of OM by microbiological agents, which enhanced the further release of PO4-P. The concentration of PO4-P in pore water was also related to the adsorption/desorption by iron oxide to dissolved phosphate (Slomp et al., 1998). Sediments played an important role in the regeneration of phosphate in the estuary and shelf shallow water environments. Phosphorus in pore water was mainly derived from the degradation of OM, and sediment can receive dissolved/ indiscerptible organic/inorganic phosphorus from the overlying water and ambience, and then phosphorus released to the pore water by the form of phosphate along with some dissolved/ decomposed compounds (Clavero et al., 2000). During this diagenesis, iron oxide (hydroxide) was deoxidated under a relatively reductive situation, which made phosphate combined with hydroxide dissolve and release (Santschi et al., 1990). That may also partly explain the high phosphorus concentration at estuarine sites in summer generally companied with lower dissolved oxygen contents (Yin et al., 2004). On the contrary, PO4-P concentrations showed slight variability between seasons at sites downstream, which may be attributed to the process that phosphate was released preferentially to water during the settlement of particulate OM, and the releasing process was almost complete before phosphate reached to the sediment. PO4-P patterns displayed odd trends and the values were more variable with severe fluctuations at some sites, and this local fluctuation may be related to the local hydrology and microbioturbation. PO4-P concentrations in this study were comparable to those in the equatorial northeastern Pacific (1.2−2.5 μmol·L-1, Ni et al., 2005) and the Gulf of Lions (0.15−4 μmol·L-1, Denis and Grenz, 2003). It was much lower than in the Seine Estuary (210.5−631.6 μmol·L-1, Bally et al., 2004), Cape Cod Bay (0−100 μmol·L-1, Hopkinson Jr. et al., 2001), Daya Bay (23−80 μmol·L-1, He et al., 2008) and Yalujiang Estuary (0.80−70.4 μmol·L-1, Ye et al., 2002) (Table 1). 3.2.3
SiO4-Si Silicon was an important element in marine ecosystem and the regeneration of silicate in sediment was essential to nutrient cycling and the maintainance of primary production. SiO4-Si concentrations were 49.92−402.06 (avg. 145.76), 27.42−386.94
(avg. 157.87) and 74.64−306.03 (avg. 177.63) μmol·L-1 in pore water, respectively, in spring, summer and winter. Generally, SiO4-Si concentrations increased with the depth, and the values were more variable in spring and summer compared with in winter. Nutrient concentrations in the sediment were controlled by transport (sediment-water exchange), nutrient release (by fauna, bacteria and dissolution) and uptake (by plants, bacteria and adsorption) (Wallmann et al., 2008). Diatoms and some other organisms in marine systems absorbed hydrated silica to form their siliceous cellwall, and the biogenic silica in their body dissolved when they died. When the remains reached the sediment, some biogenic silica continued to dissolve, and others formed aluminosilicate mineral, and the observed increasing SiO4 concentrations with depth in the study were the result of biogenic silica dissolution in sediment (Grandel et al., 2000). Namely, the reducibility became strong with the depth in sediment, and then the decomposition of OM became elevated, which made plenty of Si dissolve into the pore water. It was displayed in Fig. 3 that concentrations of SiO4-Si varied inconspicuously between seasons. The dissolved Si was consumed in the season of phytoplankton blooms, but plenty of material combining Si entered into marine bypass wind and runoff, which replenished the Si in water and sediment, so the concentration of SiO4-Si could keep a nearly constant level throughout the year. SiO4-Si concentration was comparable to those in some regions (Thibodeau et al., 2010; Ni et al., 2005), but was lower than those in the southern Ocean (Hu et al., 2006), Daya Bay (He et al., 2008) and Yalujiang Estuary (Ye et al., 2002) (Table 1). SiO4-Si increased along the sediment core due to enhanced dissolution of biogenic Si, which was similar to the patterns in these regions. 3.3 Flux of Nutrient at the Sediment-Water Interface (SWI) 3.3.1 DIN Incubated nutrient fluxes were displayed in Table 2. The results showed that the fluxes of NH4-N were -1.318 4 to 0.985 4, -1.978 6 to 5.345 6 and -2.600 1 to 7.119 0 mmol·m-2·d-1 in spring, summer and winter, respectively. The negative values showed that NH4-N generally diffused from the water column to the sediment (except site 5) in spring. Positive results showed that NH4-N generally diffused from the sediment to the water column and sediment was the source of NH4-N in summer and winter. Additional, observations during incubation experiments indicated that NH4-N concentrations in the overlying water decreased with time at some sites (Zhang, 2005), suggesting that there was strong nitrification at the SWI, which could offset some NO3-N and NO2-N to the overlying water. On the other hand, when the NH4-N entering the overlying water from sediment exceeded the sum of NH4-N needed for nitrification, the fluxes showed positive values, otherwise, NH4-N fluxes showed negative values. NO3-N fluxes were -0.558 3 to 0.469 2, -3.553 3 to 9.793 4, -4.788 6 to 3.340 3 mmol·m-2·d-1 in spring, summer and winter, respectively, and NO2-N fluxes were -1.518 8 to 0.143 8, -5.008 4 to 0.771 5 and -1.297 8 to -0.138 5 mmol·m-2·d-1, respectively. There was the contrary trend about the transport direction of NO3-N and NO2-N relative to NH4-N. NO3-N and NO2-N mainly diffused
Spatial and Seasonal Variations of Nutrients in Sediment Profiles and Their Sediment-Water Fluxes from the water column to the sediment and showed the highest fluxes in summer, which suggested that the sediment was the sink of NO3-N and NO2-N. Fluxes showed that DIN mainly diffused from the water column to the sediment and the overlying water was the source of DIN in spring (except site 5). However, DIN generally diffused from the sediment to the water column at most sites and indicated that sediment was the source of DIN in summer and winter. This indicated that the estuarine sediment was a net source of DIN in summer and winter and a sink in spring. In summer and winter, NH4-N was the main form of released DIN from sediment, and NH4-N can constitute a substantial portion of benthic nitrogen flux in estuarine and coastal environments (Blackburn and Henriksen, 1983). The strong regeneration of nutrients in sediments in the PRE will accelerate the cycling of nutrients in the system, which may affect the balance of N and the primary productivity in waters. The different flux direction at some sites maybe related to the reactions at the SWI influenced by nitrification, denitrification and bioturbation collectively (Nizzoli et al., 2007). Additionally, dissolved oxygen and temperature are the most potential factors influencing the fluxes of NH4-N and NO3-N at the SWI (Seiki et al., 1989). Seen from Tables 2 and 3, NH4-N fluxes in summer and winter were similar to those at other regions with the direction from the sediment to the water column, but the values were higher than those listed in Table 3. NO3-N fluxes generally directed from the water column to the sediment in our study, which was consistent with those in the Marano and Grado Lagoon (De Vittor et al., 2012) and the East China Sea (Shi et al., 2004). In contrast, in the Gulf of Lions (Denis and Grenz, 2003) and Bohai (Liu et al., 1999), positive fluxes indicated that NO3-N diffused from the sediment to the water column and sediment was the source of NO3-N in these regions. Table 2
203
3.3.2 PO4-P The fluxes of PO4-P were -0.246 4 to 0.093 9 and -0.466 8 to 0.159 0 mmol·m-2·d-1, respectively, in spring and summer. Results generally indicated that PO4-P mainly diffused from the water column to the sediment, and sediment was the sink of PO4-P. Phosphorus in pore water came from the degradation of OM in sediment, and the diffusion of PO4-P was controlled by the concentration gradient in pore water and the overlying water. PO4-P fluxes at the SWI may be attributed to the integrative result of dissolution, diffusion and adsorption/desorption, moreover, was related to the form of P and the redox situation in sediment (Slomp et al., 1998). At the same time, iron hydroxide adsorption/desorption to phosphorus would alter the concentration of PO4-P in pore water, thus would influence PO4-P exchange at the SWI (Slomp et al., 1998). Our PO4-P fluxes showed that the sediment was generally a sink of PO4-P in the PRE, which was consistent with the results in the Sanggou Estuary (Zhang et al., 2004), the East China Sea (Shi et al., 2004) and the Marano and Grado Lagoon (De Vittor et al., 2012). On the contrary, in the Gulf of Lions (Denis and Grenz, 2003), Seine Estuary (Bally et al., 2004), Jiaozhou Bay (Zhang et al., 2004) and Bohai (Song et al., 2000), PO4-P directed from the sediment to the water column and sediment was its source. 3.3.3 SiO4-Si Incubated results showed that the fluxes of SiO4-Si were -1.967 3 to 3.883 1, -36.755 9 to 39.534 9, -0.056 7 to 9.347 0 mmol·m-2·d-1 respectively, in spring, summer and winter. Flux direction generally showed that SiO4-Si mainly diffused from the sediment to the water column, indicating that sediment was the source of SiO4-Si in this area. The incubated fluxes reflected the combined effects of biological driving forces and diffusion across the SWI, and it has been traditionally attributed to a bioturbation
Incubated fluxes of DIN, PO4-P and SiO4-Si (mmol m-2·d-1) across the SWI in the PRE
Site Spring
Summer
Winter
Spring
Flux Summer
Winter
Spring
Summer
Winter
NH4-N 4.090 7 -1.978 6
0.870 5 5.413 0
0.469 2 -0.186 2
NO3-N -0.527 1 −
3.3403 -0.7065
0.143 8 -1.518 8
NO2-N -0.323 6 -5.008 4
-1.290 6 -0.138 5 -1.297 8
1 2
-1.138 8 -0.033 8
3
-1.318 4
5.345 6
5.346 4
0.138 3
-1.775 5
-4.7886
-0.760 9
-0.434 0
4
−
-1.110 5
1.470 4
−
1.259 7
-0.4803
−
0.574 0
-0.605 9
5
0.985 4
4.409 9
7.119 0
0.351 9
-3.553 3
-0.3918
-0.832 9
-1.839 9
-0.778 0
6
-0.318 3
2.349 3
-2.600 1
-0.558 3
9.793 4
-4.3078
-0.167 0
0.771 5
-0.552 4
Site
Spring
Summer DIN
Winter
Spring
Summer PO4-P
Winter
Spring
Summer SiO4-Si
Winter
1 2
-0.525 817 27 -1.738 8
3.240 0 -6.987
2.920 2 4.568 0
-0.034 1 -0.097 3
-0.122 5 -0.466 8
− −
2.519 8 -1.967 3
1.288 5 -36.755 9
9.347 0 1.146 8
3
-1.941 015 4
3.136 1
-0.740 0
0.044 8
-0.152 7
−
3.576 7
5.405 3
-0.056 7
4
−
0.723 2
0.384 2
−
0.159 0
−
−
15.447 8
4.586 2
5
0.504 4
-0.983 3
5.949 2
-0.246 4
-0.191 2
−
3.883 1
-9.016 1
0.427 3
6
-1.043 6
12.914 2
-7.460 3
0.093 9
-0.047 8
−
2.285 2
39.534 9
2.913 4
Positive numbers indicated fluxes out of the sediment and negative numbers indicated fluxes into the sediment. “−” means the fluxes could not be calculated in this study.
Ling Zhang, Lu Wang, Kedong Yin, Ying Lü, Yongqiang Yang and Xiaoping Huang
204 Table 3
Incubated nutrient fluxes across the SWI at some other regions in the world (mmol·m-2·d-1) NO3-N
NH4-N
PO4-P
SiO4-Si
Source
0.139−0.166 -0.39 to -7.94
-0.034 to 0.101 -1.4 to 0.01
0−0.036 -0.03
0.290−1.800 -3.18 to -11.43
Denis and Grenz (2003) De Vittor et al. (2012)
Seine Estuary
−
0.1−0.3
0.3−0.8
−
Bally et al. (2004)
Sanggou Bay
−
0.76
-1.17
−
Zhang et al. (2004) Zhang et al. (2004)
Gulf of Lions Marano and Grado Lagoon
Jiaozhou Bay
−
0.67
0.01
−
Bohai
0.038−3.65
0.96−2.52
−
−
Liu et al. (1999)
Bohai
−
−
0.019 2
0.396
Song et al. (2000)
East China Sea PRE
-0.07
0.48
-0.01
4.12
Shi et al. (2004)
-4.31 to 9.79
-2.60 to 2.35
-0.05 to 0.09
2.28−39.53
This study
Positive numbers indicated fluxes out of the sediment and negative numbers indicated fluxes into the sediment. process by the benthic macrofauna and irrigation in sediment. SiO4-Si fluxes showed seasonal variability, which agreed with the seasonal variability of water temperature. Temperature was the major factor controlling the diffusion and solution process, so increase of the temperature in summer strengthened the solution and diffusion and elevated SiO4-Si exchange velocity and the exchange flux (Bode et al., 2005). Negative SiO4-Si fluxes at some sites may be attributed to the adsorption of iron oxide and clay minerals to SiO4-Si at the SWI (Mayer et al., 1991). In addition, the generation of autogenetic aluminosilicate minerals maybe also enhanced SiO4-Si to enter sediment from the overlying water (Mortimer et al., 1999). Generally, the sediment was the source of SiO4-Si in the PRE system, which agreed with the Gulf of Lions (Denis and Grenz, 2003), Bohai (Song et al., 2000) and the East China Sea (Shi et al., 2004). In fact, nutrient behavior was controlled by physical and chemical parameters, such as the quantity and quality of OM inputs, the hydrodynamics, Eh and pH conditions, which differentiated nutrient concentrations and fluxes in different marine systems. To sum up, in this study, the PRE sediment was generally the sink of PO4 and the source of DIN and SiO4. The efflux ratios of DIN: SiO4 were 0.13−0.88, 0.05−2.51 and 0.08−13.05, respectively, in spring, summer and winter. The values were generally less than the Redfield ratios of 1 : 1, which maybe can buffer the Si limitation in the PRE waters and stimulate the phytoplankton growth, which will reduce the excessive N in the overlying water. Furthermore, Such a departure from the expected Redfield value can be attributed to several geochemical processes which distorted the release ratio such as adsorption of NH4 onto clay minerals (Rosenfeld, 1979), coupled nitrification and denitrification (Jenkins and Kemp, 1984) in the sediments. 4
CONCLUSIONS (1) High DIN concentration occurred in the overlying water with distinct patterns between seasons, and DIN generally decreased downstream in spring and summer, which may be attributed to the decreasing trend of Pearl River runoff downstream in the PRE and the uptake in the estuary. (2) NO3-N was primary in the overlying water but NH4-N was dominant in the pore water. NH4-N concentrations varied seasonally and elevated downcore due to the increase of re-
ducibility in sediment. NO3-N and NO2-N were highest in surface and subsurface sediments and then quickly decreased with the depth. (3) Fluxes of NH4-N and NO3-N displayed high values, indicating the strong early diagenesis of OM there. DIN flux direction suggested that the sediment was the sink of DIN in spring and winter, however, the sediment was the source of DIN in summer. (4) Pore water PO4-P changed distinctly in different seasons at the sites near the outfalls with higher values, which suggested that runoff inputs influenced more to PO4-P in sediment. The fluxes indicated that PO4-P mainly diffused from the water column to the sediment and sediment was the sink of PO4-P. (5) Pore water SiO4-Si increased with the depth due to enhanced dissolution of biogenic Si. The increasing reducibility with the depth in sediment elevated the decomposition of OM and so plenty of Si dissolved into the pore water. Fluxes generally showed that SiO4-Si mainly diffused from the sediment to the water column and sediment was the source of SiO4-Si. ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Nos. 91328203 and 41306110). We gratefully acknowledge the editors and anonymous reviewers, and their work significantly improved this article. And we appreciate Dr. Z Y Wang (SCSIO) for the valuable advice and Dr. Z Shi (SCSIO) for the pictures’ process. REFERENCES CITED Alperin, M. J., Albert, D. B., Martens, C. S., 1994. Seasonal Variations in Production and Consumption Rates of Dissolved Organic Carbon in An Organic-Rich Coastal Sediment. Geochimica et Cosmochimica Acta, 58(22): 4909–4930 Bally, G., Mesnage, V., Deloffre, J., et al., 2004. Chemical Characterization of Porewaters in an Intertidal Mudflat of the Seine Estuary: Relationship to Erosion-Deposition Cycles. Marine Pollution Bulletin, 49(3): 163–173 Beck, M., Dellwig, O., Liebezeit, G., et al., 2008. Spatial and Seasonal Variations of Sulphate, Dissolved Organic Carbon, and Nutrients in Deep Pore Waters of Intertidal Flat Sedi-
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