Soluble Reactive Phosphorus Fluxes Research Articles

Fine-scale measurements unravel the side effects of biochar capping on the bioavailability and mobility of phosphorus in sediments

Biochar is widely used for sediment remediation owing to its excellent adsorption properties and low carbon footprint. However, the impacts of biochar capping on phosphorus (P) bioavailability and mobility in the sediment are little known. In this study, the P mobilization processes in sediments capped with biochar were investigated by combining advanced high-resolution sampling techniques and microbiome analysis. The results showed that biochar is a double-edged sword for the sediment P release, depending on the application dosage and the capping time. In the short term (30 days), 2-cm biochar capping decreased the release flux of soluble reactive phosphorus (SRP) by 73.1%, whereas the 1-cm biochar capping significantly increased the release flux of SRP by 51.0%. After aging of biochar (80 days), the resupply capacity of sediment P was improved, resulting in increases of more than 33.7% and 121.5% in the release fluxes of SRP in the 1-cm and 2-cm capping groups, respectively, compared to the control group. Chemisorption played a pivotal role in regulating the levels of SRP, particularly during the short-term capping period. And more biochar can provide more adsorption sites on P. The P mobilization increase could be attributed to P desorption from biochar after biochar aging. Furthermore, biochar capping intensified the microbial-mediated iron reduction and organic matter decomposition, which enhanced P mobility. Our study highlights the importance of biochar application dosage and the capping time in sediment remediation, providing a scientific basis for the optimization of biochar capping techniques.Graphical

Hydrodynamics and phosphorus loading in an urbanized river channel influences response to future managed change: Insights from advection-dispersion modelling

There is a need to understand what makes certain targeted measures for in-river phosphorus load reduction more effective than others. Therefore, this paper investigates multiple development scenarios in a small lowland polluted river draining an urban area (The Cut, Bracknell, UK), using an advection-dispersion model (ADModel-P). A comparative analysis is presented whereby changes in concentrations and fluxes of soluble reactive phosphorus (SRP) and organic phosphorus (OP) have been attributed to specific transformations (mineralization, sedimentation, resuspension, adsorption-desorption, and algal uptake) and correlated to controlling factors. Under present day conditions the river stretch is a net source of SRP (10.4 % increase in mean concentration) implying a release of previously accumulated material. Scenarios with the greatest impact are those based on managed reduction of phosphorus load in sources (e.g., 20 % increase in afforestation causes an in-river SRP and OP reduction of 1.3 % to 12.6 %) followed by scenarios involving changes in water temperature (e.g., 1 °C decrease leads to in-river SRP reduction around 3.1 %). Measures involving increased river residence time show the lowest effects (e.g., 16 % decrease in velocity results in under 0.02 % in-river SRP and OP reduction). For better understanding downstream persistence of phosphorus pollution and the effectiveness of mitigation measures the research demonstrates the importance of establishing when and where reaches are net adsorbers or desorbers, and whether sedimentation or resuspension is important.

Seasonal variation and release of soluble reactive phosphorus in an agricultural upland headwater in central Germany

Abstract. Soluble reactive phosphorus (SRP) concentrations in agricultural headwaters can display pronounced seasonal variability at low flow, often with the highest concentrations occurring in summer. These SRP concentrations often exceed eutrophication levels, but their main sources, spatial distribution, and temporal dynamics are often unknown. The purpose of this study is therefore to differentiate between potential SRP losses and releases from soil drainage, anoxic riparian wetlands, and stream sediments in an agricultural headwater catchment. To identify the dominant SRP sources, we carried out three longitudinal stream sampling campaigns for SRP concentrations and fluxes. We used salt dilution tests and natural 222Rn to determine water fluxes in different sections of the stream, and we sampled for SRP, Fe, and 14C dissolved organic carbon (DOC) to examine possible redox-mediated mobilization from riparian wetlands and stream sediments. The results indicate that a single short section in the upper headwater reach was responsible for most of the SRP fluxes to the stream. Analysis of samples taken under summer low-flow conditions revealed that the stream water SRP concentrations, the fraction of SRP within total dissolved P (TDP), and DOC radiocarbon ages matched those in the groundwater entering the gaining section. Pore water from the stream sediment showed evidence of reductive mobilization of SRP, but the exchange fluxes were probably too small to contribute substantially to SRP stream concentrations. We also found no evidence that shallow flow paths from riparian wetlands contributed to the observed SRP loads in the stream. Combined, the results of this campaign and previous monitoring suggest that groundwater is the main long-term contributor of SRP at low flow, and agricultural phosphorus is largely buffered in the soil zone. We argue that the seasonal variation of SRP concentrations was mainly caused by variations in the proportion of groundwater present in the streamflow, which was highest during summer low-flow periods. Accurate knowledge of the various input pathways is important for choosing effective management measures in a given catchment, as it is also possible that observations of seasonal SRP dilution patterns stem from increased mobilization in riparian zones or from point sources.

Internal nitrogen and phosphorus loading in a seasonally stratified reservoir: Implications for eutrophication management of deep-water ecosystems.

Water eutrophication is a serious global issue because of excess external and internal nutrient inputs. Understanding the intensity and contribution of internal nitrogen (N) and phosphorus (P) loading in deep-water ecosystems is of great significance for water body eutrophication management. In this study, we combined intact sediment core incubation, high-resolution peeper (HR-Peeper) sampling, and analysis of N and P forms and other environmental factors in the water column and sediments to evaluate the contributions of internal N and P loading to water eutrophication by N and P fluxes across the sediment-water interface (SWI) of the Panjiakou Reservoir (PJKR), a deep-water ecosystem where eutrophication threatens the security of the local drinking water supply in North China. The results indicated that the PJKR showed obvious thermal and dissolved oxygen (DO) stratification in the warm seasons and full mixing in the cold seasons. The mean DO concentration was 9.9 and 3.55mg/L in the epilimnion and hypolimnion, respectively, in warm seasons and 10.7mg/L in cold seasons. The sediment acted as a source of soluble reactive phosphorus (SRP), NH4+-N, and NO2--N and a sink of NO3--N. The SRP fluxes were 5.28±4.34 and 2.30±1.93mgm-2·d-1 in warm and cold seasons, respectively, and the dissolved inorganic nitrogen (DIN) fluxes were -0.66±47.84 and 44.04±84.05mgm-2·d-1. Seasonal hypoxia accelerated the release of P rather than N from the sediments in warm seasons, which came mainly from Fe-P and Org-P under anoxic conditions. The strong negative NO3--N flux (diffusion from the water column to the sediment) implied an intensive denitrification process at the SWI, which can counteract the release flux of NH4+-N and NO2--N, resulting in the sediment acting as a weak dissolved inorganic nitrogen (DIN) source for the overlying water. We also found that internal N loading accounted for only ∼9% of the total N loading, while internal P loading accounted for 43% of the total P loading of the reservoir. Our results highlight that efforts to manage the internal loading of deep-water ecosystems should focus on P and seasonal hypoxia.

Patterns of internal nitrogen and phosphorus loadings in a cascade reservoir with a large water level gradient: Effects of reservoir operation and water depth

Internal nutrient loadings pose a high risk of being an additional N and P source, exacerbating eutrophication and deteriorating water quality. In this study, we selected the Daheiting Reservoir (DHTR) in North China, with a pronounced water level gradient, to investigate internal N and P loadings, estimate N and P fluxes across the sediment‒water interface based on the pore water profiles, and reveal the potential effects of water discharge from an upstream reservoir and high-intensity cage aquaculture on the risks of internal N and P release. The results indicated that DHTR presented with severe internal nutrient loadings, and the N and P fluxes showed significant spatiotemporal variations. NH4+−N and soluble reactive phosphorus (SRP) fluxes were higher in deep areas (averages of 26.14 and 9.9 mgm−2d−1, respectively) than in shallow areas near inflows (averages of 5.0 and 1.24 mgm−2d−1, respectively). Unexpectedly, the estimated NH4+−N and SRP fluxes were the lowest in summer (averages of 3.94 and 0.33 mgm−2d−1, respectively), which may have been influenced by seasonal thermal stratification and copious discharge from the hypolimnion of the upstream reservoir (Panjiakou Reservoir). Comparison of annual internal and external N and P loadings revealed that water discharge from the upstream Panjiakou Reservoir was the dominant source of N and P to the reservoir, contributing up to 83.6% of N input and 55.4% of P input. The internal P loading also contributed to water eutrophication to a great extent, accounting for 34.7% of the total P input. Our results highlight the impact of upstream reservoir discharge operation on downstream reservoir water quality and the importance of controlling the internal nutrient loading in cascade reservoirs, and further provide theoretical and practical foundations for the development of policies and strategies to conserve reservoir ecosystems.

Effect of dissolved oxygen on nitrogen and phosphorus fluxes from lake sediments and their thresholds based on incubation using a simple and stable dissolved oxygen control method

AbstractLaboratory incubation of intact sediments is a popular and useful method for obtaining nutrient fluxes from lake sediments to the overlying water. However, one of the main problems facing this method is that the lake sediment core itself consumes oxygen from the overlying water due to the decomposition of organic matter. In this study, addition of headspace with controlled oxygen concentration and circulation of overlying water to the traditional intact sediment incubation method allows for incubation with a stable dissolved oxygen (DO) concentration in the overlying water. In 24 columns incubated for 179–269 h, the mean DO concentration in the overlying water ranged from 0.35 to 9.95 mgO2 L−1. In most cases, stability was high, with the standard deviation of the fluctuation in DO concentration less than 0.1 mgO2 L−1. The DO concentration of the overlying water could be adjusted by changing the partial pressure of oxygen in the headspace. This method supported a detailed description of the relationship between DO concentration and nutrient fluxes from sediments. The soluble reactive phosphorus fluxes and first‐order reaction constants of NO2− + NO3− fluxes increased rapidly when the DO concentration decreased below the thresholds of 1.5 and 3.0 mgO2 L−1, respectively, in Lake Biwa. The thresholds and precise relationships between DO concentration and nutrient fluxes provide useful information for predicting the effect of DO concentration decreases in the lake bottom on nutrient release from sediments. Our results indicate that this incubation method is a powerful tool for clarifying those relationships.

Dynamics of sediment phosphorus affected by mobile aeration: Pilot-scale simulation study in a hypereutrophic pond

Controlling the release of phosphorus (P) in sediments is important to prevent eutrophication and harmful algal blooms in water bodies. Here we explored the effect of mobile aerators on the control of P release from sediments in a eutrophic pond. The dissolved oxygen in the water body recovered significantly after six months of aeration, becoming 4.2–5.8 times higher than in the control. The pH and Eh values at the sediment–water interface considerably increased, while the concentration of soluble reactive phosphorus (SRP) in pore water significantly decreased, resulting in the alteration of SRP fluxes from 1.69 mg/m2 d to −53.49 mg/m2 d. Moreover, the inert P in sediments increased by 5.2% of the total P at the end of the study compared with the initial state, and the calcium-bound phosphorus (HCl–P) increased by 96.6%. However, although aeration reduced the concentration of SRP in the water column, the total P concentration was 2.45 times higher than that of the control, and the content of redox-sensitive P (BD-P) in the sediment also increased by 200%. Overall, although mobile aeration can maintain the microenvironment of the sediment interface and increase the inert P content in the sediment to reduce the P flux, it cannot reduce the risk of release of mobile P.

Controls on Nutrient Cycling in Estuarine Mangrove Lake Sediments

We estimated the net exchange of nitrogen and phosphorus species using core incubations under light and dark conditions in estuarine lakes that are the aquatic interface between the freshwater Everglades and marine Florida Bay. These lakes and adjacent shallow water Florida Bay environments are sites where the restoration of hydrological flows will likely have the largest impact on salinity. Sediment respiration, measured by oxygen uptake, averaged (±S.D.) −2400 ± 1300, −300 ± 1000, and 1900 ± 1400 μmol m−2 h−1 for dark incubations, light incubations, and gross photosynthesis estimates, respectively, with dark incubations consistent with oxygen uptake measured by microelectrode profiles. Although most fluxes of soluble reactive phosphorus, nitrate, and N2–N were low under both light and dark incubation conditions, we observed a number of very high efflux events of NH4+ during dark incubations. A significant decrease in NH4+flux was observed in the light. The largest differences between light and dark effluxes of NH4+ occurred in lakes during periods of low coverage of the aquatic macrophyte Chara hornemannii Wallman, with NH4+ effluxes > 200 μmol m−2 h−1. Increasing freshwater flow from the Everglades is expected to expand lower salinity environments suitable for Chara, and therefore, diminish the sediment NH4+ effluxes that may fuel algal blooms.

Submarine Groundwater Discharge on the Western Shelf of the Northern South China Sea Influenced by the Pearl River Plume and Upwelling

AbstractSubmarine groundwater discharge (SGD) is a crucial material transport pathway across the continent–ocean margins. To evaluate the geochemical impacts of SGD on the western shelf of the northern South China Sea (NSCS), salinity, temperature, and radium were investigated in the summer of 2012. With Ra box models, the flux of SGD was estimated to be (1.0 ± 0.44) × 108 m3 day−1 (3.0 ± 1.3 cm day−1) in the upwelling‐influenced area and (5.4 ± 2.3) × 108 m3 day−1 (1.8 ± 0.8 cm day−1) in the no‐upwelling area. The flux of SGD in the no‐upwelling area estimated using a three end‐member mixing model was (2.1 ± 1.7) × 108 m3 day−1 (0.7 ± 0.5 cm day−1), consistent with the flux estimates above. In the upwelling‐influenced area, the average net fluxes of dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP), dissolved silicate (DSi), dissolved carbon (including dissolved inorganic carbon, DIC, and organic carbon), and total alkalinity (TA) via SGD were 1–27 times greater than those from the upwelling. In the no‐upwelling area, the average SGD‐associated DIN, SRP, DSi, TA, and dissolved carbon fluxes were 22%–42% equivalent to the Pearl River estuarine exports. The net SRP flux from SGD could support 11%–13% of the new production on the shelf. The concentration of DIC on the shelf could increase by about 50 μmol L−1 due to the net DIC fluxes via SGD. These results indicate that SGD is a significant nutrient and carbon source more important than the upwelling and second to the Pearl River on the western shelf of the NSCS and needs to be considered in the shelf biogeochemistry.

Seasonal evaluation of biotic and abiotic factors suggests phosphorus retention in constructed floodplains in three agricultural streams

Floodplain restoration constructed via the two-stage ditch in agricultural streams has the potential to enhance nutrient retention and prevent the eutrophication of downstream ecosystems. Identifying the role of biotic and abiotic factors influencing soluble reactive phosphorus (SRP) retention in floodplains is of interest given that changing redox conditions associated with floodplain inundation can result in a release of geochemically sorbed SRP to the water column. In three agricultural waterways (Indiana, USA), we conducted seasonal measurements of a suite of biogeochemical pools (total P, bioavailable P and Fe) and processes (SRP flux and microbial respiration) from multiple floodplain transects, along with their adjacent stream sediments, to determine the role of biotic and abiotic processes on floodplain SRP retention or release. Across floodplain soils, organic matter explained a significant amount of variation in soil respiration, and SRP flux from the water column to the floodplain soils was driven by the molar ratio of Fe: P, with values >6 indicating potential SRP sorption due to increased available sorption sites. We developed a mass balance model at a single site to relate seasonal floodplain processes with water column SRP export, above and below the study reach, using measurements in this study combined with data from the literature. Grab sample data suggest that the reach retained 26% of incoming SRP, which the mass balance model attributed to seasonal synergy between plant assimilation in spring and summer (removing P from floodplain soils) and abiotic P sorption during winter and spring inundation (adding SRP to the floodplain). Retention of SRP was higher in floodplain soils compared to stream sediments based on the modeled SRP budget. Thus, we suggest that these constructed floodplains will maximize SRP retention from the water column if they inundate regularly, have floodplain soils with Fe:P > 3–6, and that promote sustained plant life.

Phosphorus, nitrogen and dissolved organic carbon fluxes from sediments in freshwater rivermouths entering Green Bay (Lake Michigan; USA)

Transitional areas between ecosystem types are often active biogeochemically due to resource limitation changes. Lotic-to-lentic transitions in freshwaters appear active biogeochemically, but few studies have directly measured nutrient processing rates to assess whether processing within the rivermouth is important for load estimates or the local communities. We measured oxic fluxes of inorganic nitrogen and phosphorus and dissolved organic carbon (DOC) from sediments in two rivermouths of Green Bay (Lake Michigan, USA). Soluble reactive phosphorus (SRP) flux was positive in most cases (overall mean 1.74 mg SRP m− 2 day− 1), as was ammonium (NH4) flux (40.6 mg NH4 m− 2 day− 1). Partial least square regression (PLSR) indicated a latent variable associated with both sediment [loosely bound phosphorus (P), iron bound P, organic content] and water column properties [temperature, DOC:dissolved inorganic nitrogen (DIN) and DOC:SRP ratios (negatively)] that was moderately associated with variation in SRP flux. PLSR analysis also indicated several sediment characteristics were moderately related to NH4 flux, especially organic content, density (negative), and porosity. Flux of nitrates/nitrites (NOX) and DOC were positively associated with the water column concentrations of NOX and DOC and qualitative estimates of the labile, non-humic types of DOC. In early summer, water column NOX and DOC concentrations were high and labile DOC may have fueled denitrification, resulting in net flux into sediments of both NOX and DOC. By late summer, water column NOX and DOC were very low and both these constituents were fluxing out of sediments into the water column. Based on our estimates for the entire period from April through September, rivermouth sediments were a net source of SRP and DIN, with a DIN:SRP ratio of ~ 44 and a NH4:NOX > 1. We estimated that the sediments in the Fox rivermouth probably contributed a small proportion of the total Fox River load during the growing season 2016 (< 5%), but at times may have contributed as much as 14% of the daily load. Despite the small size of the Fox rivermouth (< 0.5% of the watershed area), these results indicate that at times sediments can contribute substantially to the overall delivery of nitrogen and phosphorus to the nearshore zone.

Carbon and nutrient fluxes from seagrass and mangrove wrack are mediated by soil interactions

Shoreline accumulation of vegetative wrack may contribute nutrient subsidies for primary or secondary production, detritivore food-web support, habitat provision, and sediment stabilization. This study addresses a knowledge gap in the literature by going beyond quantification of carbon (C) and nutrient flux potentials to investigate the biogeochemical interaction between wrack and shoreline soil. Results from a year-long shoreline survey of wrack type and abundance in Mosquito Lagoon, FL, were combined with a two-month intact core experiment observing the interaction of mangrove and seagrass leaves with organic and mineral shoreline soils. Experimental treatments were subjected to tidal cycles; at low tide, measurements were made of soil respiration and drainage water nutrient content. A measure called Soil Interaction with Litter Effect (SoILE) is introduced to quantify the suppression or enhancement of C and nutrient fluxes from a combination of wrack with shoreline soils compared to the potential fluxes of wrack and soil separately.Monthly mean (±1 S.E.) accumulation of wrack dry mass in Mosquito Lagoon was 37.65 ± 2.99 g m−2. Unusually high values (72.16 ± 13.74 g m−2) occurred in September 2017 following Hurricane Irma. In a comparison of seagrass and mangrove leaves and organic and mineral soils, seagrass leaves were the greatest source of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) by factors of 24 and 13, respectively. Seagrass also had the highest fluxes of dissolved organic carbon (DOC) and CO2 by factors of 2 and 2.7, respectively. The most common effect of combining wrack and soil was to suppress the magnitude of DOC and CO2 fluxes. There were few differences between combined and additive fluxes of DIN and SRP; the DIN flux was suppressed from the combination of seagrass with organic soil, and the SRP flux was enhanced from the combination of mangrove leaves with mineral soil. Suppression and enhancement of SoILE values were likely both attributable to a “smothering” effect caused by the physical interaction of vegetation with the soil surface that altered redox conditions. Wrack accumulation on restored living shorelines is likely to have a two-fold positive effect of subsidizing nutrients for primary production by shoreline plants and contributing to an increase in soil carbon by interacting with soils to result in a net suppression of DOC and CO2 effluxes.

The role of turbulence in internal phosphorus release: Turbulence intensity matters

Hydrodynamic fluctuations can trigger sediment suspension concomitantly with internal phosphorus release, while the interactive effect of turbulence mixing and sediment suspension on the regulation of phosphorus dynamics is in need of deep understanding. This study addressed the changes in total phosphorus (TP), phosphate (PO43--P) and suspended sediment (SS) in the overlying water, and measured the profile of dissolved oxygen (DO), Fe(II) and soluble reactive phosphorus (SRP) across the sediment-water interface in the simulated environmental turbulence scenario, For a turbulence intensity (ε) of 3.6 × 10−3 m2/s3, the SRP flux increased hence PO43--P showed a 36.36% increase relative to its initial level. Although ε of 1.3 × 10−2 m2/s3 benefited the delivery of oxygen from the bulk aqueous phase to the upper sediment which can trigger the formation of Fe oxides and hydroxides, the turbulence-induced phosphorus diffusion from the sediment exceeded its inactivation and resulted in a large SRP flux. However, a protion of the released PO43--P can be immobilized through SS adsorption and biotic (likely cyanobacteria) assimilation. Higher turbulence intensities (ε of 3.3 × 10−2 and 7.4 × 10−2 m2/s3) led to an approximately 40-fold increase in TP concentration and a significant increase in sediment suspension, which contributed to the immobilization of a majority of the phosphate through adsorption; thus, the PO43--P concentrations in the overlying water displayed 47.75% and 41.67% decline, respectively. This study also confirmed the sequential phosphorus buffer mechanisms associated with increasing turbulence intensities. With an ε of 3.6 × 10−3 m2/s3, bounding to Fe ion had a significant impact on phosphorus inactivation but with an ε of 7.4 × 10−2 m2/s3, the main immobilization mechanism is switched to phosphorus adsorption from the large quantity of suspended sediment.

Stoichiometry of regenerated nutrients differs between native and invasive freshwater mussels with implications for algal growth

Abstract Filter‐feeding mussels couple benthic and pelagic environments and create biogeochemical hot spots. Mussels may exert either top‐down control (via filtration) or bottom‐up stimulation (via biodeposition and excretion) of primary producers. Mussel metabolism may be species‐specific and the disappearance of native species or their replacement by invasive species may affect ecosystem functioning, notably gas exchange, nutrient fluxes, and stoichiometry at the sediment–water interface. In this study, we tested experimentally how native (unionids) and invasive (dreissenids) mussels, singly and in association, affect benthic fluxes of dissolved gas and nutrients in the light and in the dark, and indirectly phytoplankton growth from a freshwater estuary, the Curonian Lagoon, where the two species of mussel coexist. We show that native and invasive mussels stimulated O2 consumption and the production of total dissolved inorganic carbon and N2 increasing sediment heterotrophy and nutrient regeneration, with species‐specific effects on nutrient stoichiometry and algal growth. Ammonium and SiO2 exchanges were similar for both species, while soluble reactive phosphorus fluxes and excretion rates were significantly higher in sediments with dreissenids. Phytoplankton growth was also significantly higher in the presence of dreissenids as compared to unionids. Dreissenid mussels decreased the dissolved inorganic N to P ratio of regenerated nutrients and may favour the growth of cyanobacteria. Hence, the replacement of native with invasive mussels may produce large changes in benthic nutrient cycling and in phytoplankton growth and community composition.

Tracking changes in nutrient delivery to western Lake Erie: Approaches to compensate for variability and trends in streamflow

Abstract Tracking changes in stream nutrient inputs to Lake Erie over multidecadal time scales depends on the use of statistical methods that can remove the influence of year-to-year variability of streamflow but also explicitly consider the influence of long-term trends in streamflow. The methods introduced in this paper include an extended version of Weighted Regressions on Time, Discharge, and Season (WRTDS) modeling that explicitly considers nonstationary streamflow by incorporating information on changes in the frequency distribution of daily measured streamflow (discharge) over time. Soluble reactive phosphorus (SRP) trends in annual flow-normalized fluxes (loads) at five long-term monitoring sites in the western Lake Erie drainage basin show increases of 109 to 322% over the period 1995 to 2015. About one-third of the increase appears attributable to increasing discharge trends, while the remaining two-thirds appears to be driven by changes in concentration versus discharge relationships reflecting higher concentrations for any given discharge during recent years. Trends in total phosphorus and three nitrogen parameters (total nitrogen, nitrate-nitrite, and total Kjeldahl nitrogen) at the 10 sites analyzed were much less pronounced, and commonly show decreases in concentration-discharge relationships accompanied by increases in discharge, resulting in little net change in total flux. Trends in monthly SRP fluxes and discharge, dissolved versus particulate fractions of nutrients, and N:P flux ratios were also evaluated. The methods described here provide tools to more clearly discern the effectiveness of nutrient-control strategies and can serve as ongoing measures of progress, or lack of progress, towards nutrient-reduction goals.

An investigation of the effects of capping on internal phosphorus release from sediments under rooted macrophytes (Phragmites australis) revegetation

In eutrophic lake restorations, in situ capping is an often considered method to control sediment internal phosphorus (P) pollution for mitigating eutrophication status. Subsequent aquatic macrophyte revegetation can directly derive P from the sediment for growth. However, the effects of capping with clean soils on internal P release from sediments under rooted aquatic macrophyte revegetation are still unclear. In the present study, the influences of sediment P remobilization by P. australis revegetation on P inactivation by capping were investigated based on an entire growth simulation study. Our findings showed during the growth of P. australis, tests conducted on total phosphorous (TP), calcium-bound P (Ca-P), loosely bound P (loose-P), organic P (Org-P), and iron-adsorbed P (Fe-P) found significant changes (p < 0.001). Specifically, the mean contents of TP and Ca-P decreased by 291.1 and 224.2mgkg-1, respectively, while those of Fe-P increased from 26.4 to 124.8mgkg-1. In addition, sediment mobile-P contents increased coincidentally with the growth of P. australis during the whole course of experiment. Further analysis indicated calculated diffusion fluxes of soluble reactive phosphorus (SRP) generally increased with incubation time, although capping effectively induced the reduction of SRP concentration in pore water and its release to waters. Therefore, sediment P remobilization by P. australis revegetation was able to enhance P lability in lake sediments, with intermediate activation ability compared to other correlated water bodies. This phenomenon was most likely attributed to solubilization of sediment P by organic acids secreted from P. australis rhizosphere. Overall, sediment P remobilization by rooted macrophytes is unfavorable for capping to control internal P release to water column during eutrophic lake restorations.

Phosphorus transformations at the sediment–water interface in shallow freshwater ecosystems caused by decomposition of plant debris

We studied the processes and mechanisms that drove phosphorus (P) release and transformations at the sediment-water interface (SWI) because of the decomposition of plant debris. The results showed that, as the simulation time increased, the pH, dissolved oxygen (DO), and oxidation reduction potential (ORP) in Duckweed+Sediment+Water (DWS) and Duckweed+Water (DW) initially decreased and then increased before stabilizing. Changes in the physicochemical characteristics affect the microhabitat and the release and transformations of P at the SWI. The initial flux of total P (TP), total dissolved phosphorus (TDP), and soluble reactive phosphorus (SRP) was 886, 515, and 441 mg m−2 d−1 in DWS and 626, 376, and 330 mg m−2 d−1 in DW, respectively. As the plant debris decomposed, the fluxes of TP, TDP, and SRP decreased, and after 11 days, the fluxes remained at around 0 mg m−2 d−1. The dissolved organic phosphorus (DOP) flux followed different trends in DWS and DW, and increased first to a maximum of 285 and 109 mg m−2 d−1, respectively, by day 6. The results of this study indicate that plant debris decomposition drive P transformations at the SWI in shallow freshwater ecosystems. Therefore, to control internal sources and transformations of P, plant debris should be removed and harvested. This study also indicates that intervention is needed to ensure the health of freshwater ecosystems, and we cannot hope to get satisfactory results from only improving the national wastewater discharge standards.

Effects of Salt Pond Restoration on Benthic Flux: Sediment as a Source of Nutrients to the Water Column

Understanding nutrient flux between the benthos and the overlying water (benthic flux) is critical to restoration of water quality and biological resources because it can represent a major source of nutrients to the water column. Extensive water management commenced in the San Francisco Bay, Beginning around 1850, San Francisco Bay wetlands were converted to salt ponds and mined extensively for more than a century. Long-term (decadal) salt pond restoration efforts began in 2003. A patented device for sampling porewater at varying depths, to calculate the gradient, was employed between 2010 and 2012. Within the former ponds, the benthic flux of soluble reactive phosphorus and that of dissolved ammonia were consistently positive (i.e., moving out of the sediment into the water column). The lack of measurable nitrate or nitrite concentration gradients across the sediment-water interface suggested negligible fluxes for dissolved nitrate and nitrite. The dominance of ammonia in the porewater indicated anoxic sediment conditions, even at only 1 cm depth, which is consistent with the observed, elevated sediment oxygen demand. Nearby open-estuary sediments showed much lower benthic flux values for nutrients than the salt ponds under resortation. Allochthonous solute transport provides a nutrient advective flux for comparison to benthic flux. For ammonia, averaged for all sites and dates, benthic flux was about 80,000 kg/year, well above the advective flux range of -50 to 1500 kg/year, with much of the variability depending on the tidal cycle. By contrast, the average benthic flux of soluble reactive phosphorus was about 12,000 kg/year, of significant magnitude, but less than the advective flux range of 21,500 to 30,000 kg/year. These benthic flux estimates, based on solute diffusion across the sediment-water interface, reveal a significant nutrient source to the water column of the pond which stimulates algal blooms (often autotrophic). This benthic source may be augmented further by bioturbation, bioirrigation and episodic sediment resuspension events.

Salinization alters fluxes of bioreactive elements from stream ecosystems across land use

Abstract. There has been increased salinization of fresh water over decades due to the use of road salt deicers, wastewater discharges, saltwater intrusion, human-accelerated weathering, and groundwater irrigation. Salinization can mobilize bioreactive elements (carbon, nitrogen, phosphorus, sulfur) chemically via ion exchange and/or biologically via influencing of microbial activity. However, the effects of salinization on coupled biogeochemical cycles are still not well understood. We investigated potential impacts of increased salinization on fluxes of bioreactive elements from stream ecosystems (sediments and riparian soils) to overlying stream water and evaluated the implications of percent urban land use on salinization effects. Two-day incubations of sediments and soils with stream and deionized water across three salt levels were conducted at eight routine monitoring stations across a land-use gradient at the Baltimore Ecosystem Study Long-Term Ecological Research (LTER) site in the Chesapeake Bay watershed. Results indicated (1) salinization typically increased sediment releases of labile dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved Kjeldahl nitrogen (TKN) (ammonium + ammonia + dissolved organic nitrogen), and sediment transformations of nitrate; (2) salinization generally decreased DOC aromaticity and fluxes of soluble reactive phosphorus from both sediments and soils; (3) the effects of increased salinization on sediment releases of DOC and TKN and DOC quality increased with percentage watershed urbanization. Biogeochemical responses to salinization varied between sediments and riparian soils in releases of DOC and DIC, and nitrate transformations. The differential responses of riparian soils and sediments to increased salinization were likely due to differences in organic matter sources and composition. Our results suggest that short-term increases in salinization can cause releases of significant amounts of labile organic carbon and nitrogen from stream substrates and organic transformations of nitrogen and phosphorus in urban watersheds. Given that salinization of fresh water will increase in the future due to human activities, significant impacts on carbon and nutrient mobilization and water quality should be anticipated.

Sediment Denitrification and Nutrient Fluxes in the San José Lagoon, a Tropical Lagoon in the Highly Urbanized San Juan Bay Estuary, Puerto Rico

Direct measurements of net N2 production, oxygen, and inorganic nutrient fluxes at the sediment–water interface were conducted in the San Jose Lagoon (SJL), a tropical coastal lagoon located at the San Juan Bay Estuary, Puerto Rico, that is heavily impacted by anthropogenic activities. Sediment of the SJL had high organic matter (average = 16.7 %; SD = 9.0), pore water ammonium (541 μM ± 422), and pore water soluble reactive phosphorus (SRP) (57 μM ± 48) concentrations. High fluxes of oxygen (−1999 μmol m−2 h−1 ± 2180) and N2–N (192 μmol m−2 h−1 ± 315) fluxes were observed. Similarly, high fluxes of ammonium (770 μmol m−2 h−1 ± 701) and SRP fluxes (167 μmol m−2 h−1 ± 138) were measured, with moderate nitrate fluxes (−40 μmol m−2 h−1 ± 53) mostly directed into the sediment. The dissolved inorganic nitrogen and soluble reactive phosphorus ratio (DIN/SRP) was low relative to the Redfield ratio and suggested that the sediment would support SJL as a nitrogen-limited system. An experiment with higher nitrate concentration resulted in a significant response (p < 0.05) of denitrifying microbes and ammonium fluxes to a 100-μM KNO3 addition. Net N2 production was an important sink in the nitrogen balance of the SJL, though most remineralized N was released as ammonium. Efforts to mitigate eutrophication in the SJL require consideration of the substantial role that sediments play in biogeochemical cycles.