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Sieved Transport and Re-distribution of Bioavailable Phosphorus from Watershed with Complex River Networks to Lake Qitao Yi, Qiuwen Chen, Wenqing Shi, Yuqing Lin, and Liuming Hu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02710 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017
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Sieved Transport and Re-distribution of Bioavailable Phosphorus from
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Watershed with Complex River Networks to Lake
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Qitao Yi 2,3, Qiuwen Chen 1,2*, Wenqing Shi 1, Yuqing Lin 1, Liuming Hu 1
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1
Center for Eco-Environmental Research, Nanjing Hydraulic Research Institute, Nanjing
210098, China 2
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing
100085, China 3
School of Earth and Environment, Anhui University of Science and Technology, Huainan
232001, China *
Corresponding author: Hujuguan 34, Nanjing 210098, China. Tel./Fax: +86 25 85829765, E-
mail:
[email protected] 1
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Table of contents
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Abstract: An innovative approach was developed to reveal phosphorus (P) transport and
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redistribution in large and complex river networks in the Lake Taihu basin by establishing the
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relations between sediment P spatial distribution and P sorption behavior on particles with
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different grain size, sorted by hydrodynamics. The sediment P fractionation composition
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changed greatly across the basin, where 69% consisted of acid-soluble fractions (HCl-P) in
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upstream rivers, while 70% was in fractions associated with reducible metal hydroxides (BD-
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P) and amorphous hydroxides (NaOH25-P) in downstream rivers. Fine particles enriched in
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BD-P and NaOH25-P fractions tended to sorb liberated P during the re-suspension process and
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fine particles were more easily delivered downstream towards the lake, forming a sieved
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transport of P in the river networks. This will lead to a great potential for sediment P release
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when environmental anoxia develops in the sediments, or high pH occurs in the sediment
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surface during intensive algal blooms in the shallow lake. Reduction of external P from point
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sources from urbanized areas is an important requirement at the basin scale; however, long-
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term efforts are needed to restore aquatic macrophytes in the lake, which would decrease P
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recycling rates at the water-sediment interface.
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Keywords: phosphorus; river networks; sediments; Lake Taihu; re-suspension; particle size
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distribution
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INTRODUCTION
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Phosphorus (P) is essential to all life (e.g. plants, animals and bacteria) and is a key ingredient
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in fertilizers to sustain high crop production. Humans dug up geological phosphate reserves to
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produce fertilizers to feed the Green Revolution, creating a large one-way flow of phosphorus
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from rocks to farms to lakes and oceans, dramatically impairing freshwater and coastal marine
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ecosystems.1 The P cycle is largely related to the physical, chemical and biological processes
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activated at the interface between sediment and water; thus, the clarification of these
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processes is vital for the development of effective P management strategies for restoring the
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ecosystem in eutrophic water bodies.
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Phosphorus activity (mobilization or immobilization) is determined by the behaviors of three
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types of compounds, iron (Fe), aluminum (Al) and calcite (Ca). Anoxia can enhance P release
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from sediment Fe-(hydr)oxides.2-5 Elevated concentrations of Al(OH)3 in the non-calcareous
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sediments of low-pH lakes can prevent P release by adsorbing P liberated from Fe hydroxides
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due to the high sorption capacity and stability of Al(OH)3 under both oxic and anoxic
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conditions.6-9 Phosphorus sorption capacity has been linked with calcium carbonate and ferric
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hydroxide in high-pH lakes.7, 10-11 Therefore, changes in environmental conditions, such as
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redox potential, pH values or CO2 concentrations, could result in P mobilization or
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immobilization from the sediment surface by impacting the above-mentioned biogeochemical
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processes.
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Links between P discharge, transport and sedimentation need to be established for P
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management practices at the basin scale. This is challenging in hybrid areas containing both
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urbanization and agriculture, with multiple sources of pollutants.12-13 The problem is
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intensified where complex river networks are concerned, where the complicated flow regime
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makes P transport hard to clarify. Although many studies have addressed the impacts of
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sediment disturbance on P behaviors in shallow lakes,14-18 the understanding of the transport 4
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processes between sources and sinks linked through river networks is not sufficient to support
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P management at the basin scale. In particular, frequent re-suspension of sediments creates a
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changeable redox environment;15, 18-19 this could result in P redistribution on particles with
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size differences or fractional composition, impacting transport towards downstream and lakes.
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The main objectives of this work are to: (1) characterize the spatial distribution and
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fractionation of sediment P in complex river networks with multiple pollution sources at basin
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scale; (2) clarify P transport potential and discuss the related processes in canals under strong
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hydrodynamic disturbance of sediments; (3) re-inspect the effectiveness of P management
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regarding restoration of eutrophic lakes.
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MATERIALS AND METHODS
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Study Area. Lake Taihu is the third largest freshwater lake in China, with an area of 2338
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km2, an average depth of 1.9 meters and a corresponding volume of 4.4 billion m3. The Lake
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Taihu basin has an area of approximately 36,895 km2 and is located in the Yangtze River
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delta, the most developed region in China (Figure 1). The main rivers of the networks have
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been shaped to form man-made canals serving navigation, flood control and ecological
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functions, with numerous hydraulic works regulating water exchange between Lake Taihu
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and the Yangtze River. More details on climate, topography, hydrology can be found in a
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related reference.20
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The Lake Taihu basin is divided into eight parts (sub-watersheds) in terms of hydrological and
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hydraulic characteristics (Figure 1a), featured by complex plain river networks. This study
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focuses on the Huxi part, the main drainage areas located in the northwest of the basin,
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occupying an area of 7791 km2 and discharging 70% (74.3108 out of 105.6108 m3 in
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2014) of the total inflow to Lake Taihu (Table S1 in SI). The flow regime of the river
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networks is complex, mainly controlled by hydrological conditions and a large number of
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water gates. Hydrological and flow regimes during the research period of 2014 and 2015-
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2016 are provided in Figure S1 and Figure S2 in SI, and the reference.20 Hydrodynamic
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disturbance in the main rivers is strong and frequent due to busy canal transportation between
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cities, causing sediment resuspension with turbid water, as shown in a typical scene in Photo
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S1 in SI. Considering the topography, flow direction and land use together, we defined
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upstream, midstream and downstream with the Danjinlicaohe River (D1~D4), the Mengjinhe
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River (M1~M4) and the concentrated inflowing rivers (I1~I13) in the north-south direction as
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the boundaries (Figure 1b, Figure 1c and Table S2 in SI). The Huxi part is the most heavily
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polluted area, which contributed the most to water quality deterioration and eutrophication in
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Lake Taihu. The details of land use related to water quality in this area can be found in the
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related references.13,20
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Figure 1. Study areas (a), sampling sites in the inflowing river networks (b) and thirteen concentrated
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inflowing river mouths and sediment core sites of Lake Taihu (c). (Note: the river network is simplified
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from more complex rivers).
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Sampling Design. Sampling campaigns covered the scale from basin to lake, including 37
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sites in the main rivers, 13 sites in the concentrated inflowing river mouths surrounding Lake
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Taihu, and 2 sites in the lake. Specific information concerning sampling sites is listed in Table
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S1 in the Supporting Information. Four sampling campaigns were conducted across the
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seasons, with collections occurring in January (winter), April (spring), June (summer) and
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October (fall) of 2014. Surface water samples were collected in all seasons, but surface 6
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sediments were collected only in January of 2014. In addition, monthly sampling campaigns
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from June 2015 to April 2016 were conducted in the 13 concentrated inflowing river mouths
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to obtain details on P loading patterns towards the lake. In April of 2015, one more sampling
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campaign was conducted to obtain sediment cores at two sites, located in the west and east
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parts of the lake respectively (Figure 1c), for analyzing P accumulation in different lake areas
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and its links with river transport processes. The west part is the concentrated inflow area of
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Lake Taihu, dominated by microalgae with sporadic aquatic macrophytes. The east part is the
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concentrated outflow area of the lake, dominated by aquatic vegetation. The west part and
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east part represent two typical states in the eutrophic lake.
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Water Sampling and Analysis. Water samples were taken at 0.5–1.0 m depth under the
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surface using a 5 L Plexiglas water sampler in each site. Temperatures, pH, dissolved oxygen
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and electrical conductivity were read on-site using portable YSI electrodes (Xylem Company,
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New York, USA). The water samples were stored in an icebox, and brought to the laboratory
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and analyzed immediately. Water parameters, including total suspended solids, total
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phosphorus, soluble reactive phosphate, and total nitrogen, were analyzed according to
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Standard Methods.21 Detailed description of specific methods is presented in SI. In addition,
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the total suspended solids in water samples collected in the winter of 2014 were settled and
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concentrated for analysis of the particle sizes using a laser particle size analyzer (Mastersizer
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2000, Malvern Co. United, Kingdom).
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Sediment Sampling, Separation and Analysis. Surface sediment samples in the river
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networks were collected using a Peterson gravity sampler. Undisturbed sediment cores, at the
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west site and east site of the lake, were sampled using a gravity corer, and were vertically
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sliced into 20 sub-samples on-site, breaking each core into 1-cm increments from top (at the
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sediment-water interface) to bottom (at depth). All sediment samples were directly stored in
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air-sealed plastic bags and transported to the lab in a portable freezer. Site J2 in the Beijing-
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Hangzhou Great Canal, where sediment experienced repetitive suspension and sedimentation
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due to strong hydrodynamic disturbance by ship propellers, was selected as a representative
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site to analyse P fractionation and adsorption behavior as a function of particle size
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distribution. Details about the particle separation column are given in Figure S3 in SI. Five
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size-groups of particles were obtained in this research, in ranges of “
