Nitrogen sources and cycling revealed by dual ...

Water Research 142 (2018) 459e470

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Nitrogen sources and cycling revealed by dual isotopes of nitrate in a complex urbanized environment Anand Archana a, b, Benoit Thibodeau b, c, Naomi Geeraert a, b, Min Nina Xu d, Shuh-Ji Kao d, David M. Baker a, b, * a

School of Biological Sciences, University of Hong Kong, Pok Fu Lam, Hong Kong, PR China Swire Institute of Marine Science, University of Hong Kong, Cape D'Aguilar, Hong Kong, PR China Department of Earth Sciences, University of Hong Kong, Pok Fu Lam, Hong Kong, PR China d State Key Laboratory of Marine Environmental Science, Xiamen University, PR China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 February 2018 Received in revised form 8 May 2018 Accepted 2 June 2018 Available online 5 June 2018

Elevated nutrient inputs have led to increased eutrophication in coastal marine ecosystems worldwide. An understanding of the relative contribution of different nutrient sources is imperative for effective water quality management. Stable isotope values of nitrate (d15NNO3-, d18ONO3-) can complement conventional water quality monitoring programs to help differentiate natural sources of NO
3 from anthropogenic inputs and estimate the processes involved in N cycling within an ecosystem. We measured nutrient concentrations, d15NNO3-, and d18ONO3- in 76 locations along a salinity gradient from the lower end of the Pearl River Estuary, one of China's largest rivers discharging into the South China Sea, towards the open ocean. NO
3 concentrations decreased with increasing salinity, indicative of conservative mixing of eutrophic freshwater and oligotrophic seawater. However, our data did not follow conservative mixing patterns. At salinities 20 psu, NO3 conisotopes increased, suggesting mixing and/or other transcentrations decreased, while dual NO
3 formation processes. Our analysis yielded mean estimates for isotope enrichment factors (15ε ¼
2.02‰ and 18ε ¼
3.37‰), D(15,18) ¼ -5.5‰ and d15NNO3- - d15NNO2- ¼ 12.3‰. After consideration of potential alternative sources (sewage, atmospheric deposition and groundwater) we concluded that there are three plausible interpretations for deviations from conservative mixing behaviour (1) NO
3 uptake by assimilation (2) in situ NO
3 production (from fixation-derived nitrogen and nitrification of sewagederived effluents) and (3) input of groundwater nitrate carrying a denitrification signal. Through this study, we propose a simple workflow that incorporates a synthesis of numerous isotope-based studies to constrain sources and behaviour of NO
3 in urbanized marine environments. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Nitrate Isotopes Eutrophication Mixing Fractionation Anthropogenic

1. Introduction Since the 1970s, human population has contributed to an unprecedented increase in the creation of reactive nitrogen (N; Galloway et al., 2008). This imbalance has effectively doubled the amount of bioavailable N (particularly in the form of nitrate - NO
3) that enters aquatic ecosystems. These ecosystems play a vital role in

* Corresponding author. School of Biological Sciences, University of Hong Kong, Pok Fu Lam, Hong Kong, PR China. E-mail addresses: [email protected] (A. Archana), [email protected] (B. Thibodeau), [email protected] (N. Geeraert), [email protected] (M.N. Xu), [email protected]. cn (S.-J. Kao), [email protected] (D.M. Baker). https://doi.org/10.1016/j.watres.2018.06.004 0043-1354/© 2018 Elsevier Ltd. All rights reserved.

regulating the fate of N cycling through various processes such as N2 fixation, nitrification, denitrification and phytoplankton assimilation. However, increased coastal urbanization has significantly altered the function of NO
3 removal in rivers, estuaries and oceans €hnke et al., 2008; Wankel et al., 2009; Wong et al., 2014). (Da Consequently, cascading environmental impacts are now the largest threat to aquatic ecosystems known as eutrophication. Eutrophication refers to a condition characterized by elevated inputs of limiting nutrients often caused by direct or indirect external inputs (such as sewage) to the receiving environment. This triggers multiple detrimental ecological and biogeochemical responses that affect primary productivity and oxygen balance, reproduction, growth and survival of organisms and has negative impacts on

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A. Archana et al. / Water Research 142 (2018) 459e470

biodiversity and ecosystem function (Baker et al., 2013; Wear and Vega Thurber, 2015). Korth et al. (2014) reported that nutrient inputs in the Baltic Sea are 400% higher than what they were in the 1950s when human activity was much lower. More recently, Sinha et al. (2017) reported hypoxia events in the Mississippi river delta that are linked to increasing terrestrial nutrient inputs. Nitrate (NO
3 ) is an important component of the total N pool and is a major cause of concern owing to its role in coastal eutrophication. Measuring the concentration of NO
3 can help identify regions of NO
3 pollution necessitating follow-up remedial action. However, NO
3 can originate from multiple sources (such as sewage, atmospheric deposition, groundwater, nitrification and river runoff), and recycling and dispersal processes (such as assimilation, denitrification, nitrification and anammox). Therefore, additional tools are required to identify putative sources of contamination, and monitor their fate and natural attenuation processes for better management of water resources (Granger and Wankel, 2016). Stable isotopes of nitrate (d15NNO3-) have been used since the 1970s to characterize the source and fate of NO
3 in coastal marine environments (Mariotti et al., 1981). More recently, stable isotopes of oxygen in nitrate (d18ONO3-) when used in combination with d15NNO3- have provided further resolution to identify NO
3 source mixing and NO
3 removal/production processes (Thibodeau et al., 2013; Wankel et al., 2007). In fact, studies have successfully used dual NO
3 isotopes to quantify sources and internal turnover processes of NO
3 in several estuaries worldwide (some examples are Werribee River Estuary - Wong et al., 2014; River Beult - Goody et al., 2016; Elbe Estuary - D€ ahnke et al., 2008; Elkhorn Slough Wankel et al., 2009; Pearl River Estuary - Ye et al., 2015). Previous estuarine studies have utilized the dual isotope technique to identify sources (e.g. e N2 fixation, nitrification, external sources) and sinks (e.g. - denitrification and phytoplankton assim€hnke et al., 2008). However, the complexity of the N cycle ilation; Da and the discrimination of N isotopes associated with biogeochemical transformations makes their interpretation more challenging than what was previously understood. To this end, different studies use different environmental parameters (dissolved oxygen, chlorophyll-a, “Redfield Ratio” - deviation in the nitrate-to3

phosphate (NO
3 to PO4 ) relationship, N* ¼ NO3 - 16  3
15 18 PO4 þ 2.9), isotope enrichment factors ( ε and ε; isotope discrimination associated with transformation), other parameters (Deviations of dual isotopes from expected values D(15,18) ¼ (d15Nd15Nm) - 15ε/18ε x (d18O-d18Om), Dd15N ¼ d15NNO3- - d15NNO2-; refer to Sigman et al., 2005) and mixing models (with dual isotopes and multiple end members) with varying degrees of success to constrain N sources and transformations. Moreover, new evidence has emerged regarding the isotope effects associated with nitrified NO
3 (Granger and Wankel, 2016) that necessitates the consideration of several isotope fractionation processes for an accurate interpretation of nitrate sources and dynamics. Therefore, this study proposes a simple workflow that synthesizes approaches from previous isotope-based studies for improved investigation of the source and fate of NO
3 in Hong Kong, a highly urbanized megacity with a long established eutrophication problem.

PRE creates a marked gradient in seawater characteristics in Hong Kong's marine environment that ranges from estuarine (low salinity) in the west and south to oceanic (high salinity) in the east. The marine environment is naturally stratified due to the high discharge from the PRE (Wilson, 1994) that results in a natural stratification with cold and saline bottom water separated from the fresh and warm surface water. The stratification is especially strong during the wet season when the Pearl River discharge is high (summer). Hong Kong is one of the most urbanized environments with the second highest population density on Earth. The marine environment is ~1050 km2 in area and is home to a diversity of habitats and marine life. In the 1980s and 1990s, rapid development in the form of reclamation increased the city's pollution footprint, while investment in wastewater treatment technology trailed. Consequently, eutrophication has contributed to the loss of foundational species like hard corals and thus reduced the complexity, diversity and function of benthic ecosystems since at least 1975, giving rise to environmental economic losses. Today there is speculation that eutrophication in Hong Kong is driven by regional sources (owing to inorganic nitrogen inputs in the Pearl River; Ye et al., 2015) and local sources (e.g. wastewater discharge, groundwater discharge, and leaking sewers; putative end members e Table 1). Yet, to our knowledge, there is no study that has suc
cessfully quantified the sources of NO
3 pollution and NO3 transformation dynamics in the surface waters. 2.2. Data collection 2.2.1. Seawater sampling Surface seawater samples were collected from 76 long-term water quality monitoring stations in July 2016. Samples were collected by the Environmental Protection Department of the Hong Kong Special Administrative Region (EPD) using a computercontrolled rosette water sampler and stored at 4  C before nutrient analysis.

2. Materials and methods

2.2.2. Water quality and stable isotope measurements Seawater data for physical and aggregate properties (salinity, dissolved oxygen - DO), nutrients and inorganic constituents (nitrate-NO-3, nitrite-NO-2, ammonium-NHþ 4 , total inorganic nitrogen, total Kjeldahl nitrogen, total nitrogen, orthophosphate phosphorous, total phosphorous) were obtained from EPD's water quality database (Refer to details of methods in EPD, 2016). Stable isotopes of nitrate (d15NNO3- and d18ONO3-) were determined using the bacterial denitrifier method (Casciotti et al., 2002; Sigman et al., 2001) after NO
2 removal with sulfamic acid (Granger and Sigman, 2009). The bacterial denitrifier method is based on the measurement of the isotopic composition of N2O after NO
3 gets converted to N2O by bacteria that lack N2O reductase. The isotopic composition of nitrite (d15NNO2-) was measured following the protocol of Mcllvin and Altabet (2005). This involved quantitatively converting NO
2 to N2O by a helium-purged 1:1 (v:v) solution of 2 molL
1 sodium azide and 20% acetic acid. Isotope measurements are reported in the delta notation in permil (‰), relative to atmospheric N2 (AIR) for d15N and VSMOW for d18O. Variations in isotope ratios are expressed using the delta notation as follows:

2.1. Study area

d15N [(15N/14Nsample)/(15N/14Nstandard)
1] x 1000

Hong Kong is situated in the Pearl River Delta (Fig. 1) - the fastest developing region in Southern China populated by ~120 million people. The region is also well known for the Pearl River Estuary (PRE; 3.26  1011 m3 yr
1) through which one of China's largest river systems (the Pearl River) drains into the northern South China Sea (SCS). The enormous volume of freshwater discharge from the

d18O [(18O/16Osample)/(18O/16Ostandard)
1] x 1000 Two international reference materials, USGS34: d15N ¼
1.8‰, d O ¼
27.9‰ and IAEA-N3: d15N ¼ þ4.7‰, d18O ¼ þ25.6‰ € hlke et al., 2003), and two in-house laboratory working stan(Bo dards, KNO3 (d15N ¼ þ5.1‰, d18O ¼ þ26‰) and KNO3 18

A. Archana et al. / Water Research 142 (2018) 459e470

461

Fig. 1. Study site e HKSAR with locations of 76 seawater sampling stations. The Pearl River Estuary is responsible for an estuarine-influenced environment in the West, which transitions into oceanic in the East with flushing from the South China Sea.

Table 1 Characteristics of putative end members of NO
3 to estimate the contribution of sources in the study area. End members

d15NNO3-ð‰Þ

d18NNO3-ð‰Þ


1 NO
3 (umol L )

Flux (106m3d
1)

Source

Atmospheric deposition Marine River Sewage Groundwater


2.6 4 5.7 12.2 10.5

58.8 2.5 1.6 3.8 34.9

19.5 5.0 86.6 270.0 15.5

34.6 1047.6 2000.0 2.6 8.6

Ye et al. (2015) Ye et al. (2015) Ye et al. (2015) Archana et al. (2016) Unpublished results Luo et al. (2014)

(d15N ¼ þ13.8‰) were used. Blanks consisted of vials that had no injected water samples or microbes. Stable isotope measurements were made at the State Key Laboratory of Marine Environmental Science, Xiamen University by a GasBench II coupled to a continuous flow isotope ratio mass spectrometer (IRMS, Thermo Delta V Advantage). The pooled standard deviation was