Phosphate oxygen isotopes within aquatic ecosystems:global data synthesis and future research priorities

The oxygen isotope ratio of dissolved inorganic phosphate (δ18Op) represents a novel and potentially powerful stable isotope tracer for biogeochemical research. Analysis of δ18Op may offer new insights into the relative importance of different sources of phosphorus within natural ecosystems. Due to the isotope fractionations that occur alongside the metabolism of phosphorus, δ18Op could also be used to better understand the intracellular and extracellular reaction mechanisms that control phosphorus cycling. In this review focussed on aquatic ecosystems, we examine the theoretical basis to using stable oxygen isotopes within phosphorus research.We consider the methodological challenges involved in accurately determining δ18Op, given aquatic matrices in which potential sources of contaminant oxygen are ubiquitous. Finally,we synthesise the existing global data regarding δ18Op in aquatic ecosystems, concluding by identifying four key areas for future development of δ18Op research. Through this synthesis, we seek to stimulate broader interest in the use of δ18Op to address the significant research and management challenges that continue to surround the stewardship of phosphorus

Long-term Holocene groundwater fluctuations in a chalk catchment: evidence from Rock-Eval pyrolysis of riparian peats

he depositional history of peat-dominated wetlands can be used to understand palaeoclimate and palaeohydrology and also constrain the impacts of future climate change. However, in chalkland valleys, seasonal water table fluctuations and a high alkalinity have diminished key environment indicators such as pollen, and there is a need for alternative investigative techniques. The method of Rock-Eval pyrolysis can track changes in organic matter source and degradation, potentially relating to historic changes in vegetation cover. This is the first Rock-Eval on cores from a groundwater-dependent riparian chalk valley wetland combined with radiocarbon dating. The dating showed that the cores represented approximately 4000 years of depositional history. Changes in hydrocarbon chemistry including normal alkane composition of the peat indicated shifts of around 500 to 1000 years between terrestrial and more aquatic species, relating to periods of climate wetness. These climatic shifts are broadly consistent with other evidence from ombrotrophic peatland and lacustrine sediments across northwest Europe. However, the connection between climate wetness and groundwater dependent chalkland wetlands is complicated by external anthropogenically driven factors relating to land use and vegetation cover changes in the catchment. Nonetheless, this study suggests that Rock-Eval pyrolysis is a useful and cost-effective tool that can provide evidence for long-term Holocene groundwater fluctuation

A combined geochemical and hydrological approach for understanding macronutrient sources

This study employed complementary geochemical techniques and distributed hydrological modelling to investigate multiple sources of catchment macronutrients and characterise their changes in contrasting storm and baseflow conditions. This approach was demonstrated for the Beult catchment in the county of Kent (England), a designated Site of Special Scientific Interest (SSSI) indentified as failing to meet water quality standards for key nutrients under the Water Framework Directive. Significant changes in nutrient stoichiometry and bioavailability are observed for surface waters under contrasting flow regimes. Soluble reactive phosphorus (SRP) concentrations are approximately twice as high during baseflow compared to high flow, while the inverse is true for particulate, colloidal and dissolved hydrolysable phosphorus, dissolved organic carbon and nitrate. Nitrogen (N):phosphorus (P) ratios are lower during baseflow for most surface waters impacted by diffuse sources of pollution. Fluorescence indices of dissolved organic matter (DOM) show that waste water inputs may be locally important sources of more complex low molecular weight DOM, particularly during baseflow. Nitrate N and O isotope signatures, combined with other dissolved chemical tracers, confirm the dominance of wastewater N inputs at sites downsteam of sewerage treatment works during baseflow, with a shift towards the soil N pool in surface waters across the catchment during high flow. Distributed hydrological modelling using the Grid-to-Grid model reveal areas with the greatest runoff also export higher N and P concentrations, and hence deliver a greater flux of macronutrients, while forested areas with low nutrient concentrations reduce runoff and nutrient fluxes. During periods of high runoff, nested sampling indicates that nutrient fluxes scale with catchment area. This combined approach enables a more thorough assessment of the macronutrient sources and dynamics, better informing management options in nutrient impacted catchments

Development and initial application of δ18Op to understand phosphorus cycling in river, lake and groundwater ecosystems.

Variation in the stable isotope composition of oxygen within dissolved phosphate (δ18Op) represents a novel and potentially powerful environmental tracer. In freshwater, marine and terrestrial ecosystems, δ18Op can act as an inherent label for the sources of phosphorus and the extent to which phosphorus from different sources is metabolised. This paper focuses on the methodological development and initial application of δ18Op across a range of freshwater ecosystems. Initially, we report modifications to the analytical protocol for δ18Op that are designed to minimise incorporation of contaminant oxygen in the final silver phosphate precipitate prior to pyrolysis. This is critical given the range of possible sources of contaminant oxygen within freshwater matrices. Subsequently, we consider the potential utility of δ18Op through application of the technique within a range of freshwater ecosystems in England, UK. Firstly, we characterise δ18Op in river water and effluents from Sewage Treatment Works (STW), and examine the opportunity to use the 18Op of STW effluents to trace the entry and downstream fate of phosphorus from these point sources in rivers. Secondly, we analyse δ18Op to gain insights into variations in the sources and biological cycling of phosphorus in a seasonally stratified lake ecosystem. Thirdly, we characterise δ18Op in shallow and deep groundwater samples, considering whether δ18Op might provide evidence for variation in source and extent of metabolism for phosphorus in groundwater ecosystems. Taken together, these data extend the catalogue of δ18Op in freshwater ecosystems, and further the scope of δ18Op as a tool to better understand phosphorus biogeochemistry

Determining the Impact of Riparian Wetlands on Nutrient Cycling, Storage and Export in Permeable Agricultural Catchments

The impact of riparian wetlands on the cycling, retention and export of nutrients from land to water varies according to local environmental conditions and is poorly resolved in catchment management approaches. To determine the role a specific wetland might play in a catchment mitigation strategy, an alternative approach is needed to the high-frequency and spatially detailed monitoring programme that would otherwise be needed. Here, we present a new approach using a combination of novel and well-established geochemical, geophysical and isotope ratio methods. This combined approach was developed and tested against a 2-year high-resolution sampling programme in a lowland permeable wetland in the Lambourn catchment, UK. The monitoring programme identified multiple pathways and water sources feeding into the wetland, generating large spatial and temporal variations in nutrient cycling, retention and export behaviours within the wetland. This complexity of contributing source areas and biogeochemical functions within the wetland were effectively identified using the new toolkit approach. We propose that this technique could be used to determine the likely net source/sink function of riparian wetlands prior to their incorporation into any catchment management plan, with relatively low resource implications when compared to a full high-frequency nutrient speciation and isotope geochemistry-based monitoring approach

Isotopic fingerprint for phosphorus in drinking water supplies

Phosphate dosing of drinking water supplies, coupled with leakage from distribution networks, represents a significant input of phosphorus to the environment. The oxygen isotope composition of phosphate (δ18OPO4), a novel stable isotope tracer for phosphorus, offers new opportunities to understand the importance of phosphorus derived from sources such as drinking water. We report the first assessment of δ18OPO4 within drinking water supplies. A total of 40 samples from phosphate-dosed distribution networks were analyzed from across England and Wales. In addition, samples of the source orthophosphoric acid used for dosing were also analyzed. Two distinct isotopic signatures for drinking water were identified (average = +13.2 or +19.7‰), primarily determined by δ18OPO4 of the source acid (average = +12.4 or +19.7‰). Dependent upon the source acid used, drinking water δ18OPO4 appears isotopically distinct from a number of other phosphorus sources. Isotopic offsets from the source acid ranging from −0.9 to +2.8‰ were observed. There was little evidence that equilibrium isotope fractionation dominated within the networks, with offsets from temperature-dependent equilibrium ranging from −4.8 to +4.2‰. While partial equilibrium fractionation may have occurred, kinetic effects associated with microbial uptake of phosphorus or abiotic sorption and dissolution reactions may also contribute to δ18OPO4 within drinking water supplies

Opening opportunities for high-resolution isotope analysis - Quantification of δ15NNO3 and δ18ONO3 in diffusive equilibrium in thin–film passive samplers

The fate of nitrate transported across groundwater-surface water interfaces has been intensively studied in recent decades. The interfaces between aquifers and rivers or lakes have been identified as biogeochemical hotspots with steep redox gradients. However, a detailed understanding of the spatial heterogeneity and potential temporal variability of these hotspots, and the consequences for nitrogen processing, is still hindered by a paucity of adequate measurement techniques. A novel methodology is presented here, using Diffusive Equilibrium in Thin-film (DET) gels as high-spatial-resolution passive-samplers of δ15NNO3 and δ18ONO3 to investigate nitrogen cycling. Fractionation of δ15NNO3 and δ18ONO3 during diffusion of nitrate through the DET gel was determined using varying equilibrium times and nitrate concentrations. This demonstrated that nitrate isotopes of δ15NNO3 and δ18ONO3 do not fractionate when sampled with a DET gel. δ15NNO3 values from the DET gels ranged between 2.3 ± 0.2 and 2.7 ± 0.3‰ for a NO3– stock solution value of 2.7 ± 0.4‰, and δ18ONO3 values ranged between 18.3 ± 1.0 and 21.5 ± 0.8‰ for a NO3– stock solution of 19.7 ± 0.9‰. Nitrate recovery and isotope values were independent of equilibrium time and nitrate concentration. Additionally, an in situ study showed that nitrate concentration and isotopes provide unique, high-resolution data that enable improved understanding of nitrogen cycling in freshwater sediments

Seasonal variability of sediment controls of nitrogen cycling in an agricultural stream

Agricultural streams receive large inputs of nutrients, such as nitrate (NO3−) and ammonium (NH4+), which impact water quality and stream health. Streambed sediments are hotspots of biogeochemical reactivity, characterised by high rates of nutrient attenuation and denitrification. High concentrations of nitrous oxide (N2O) previously observed in stream sediments point to incomplete denitrification, with sediments acting as a potentially significant source of global N2O. We investigated the effect of sediment type and seasonal variation on denitrification and N2O production in the streambed of an agricultural UK stream. Denitrification was strongly controlled by sediment type, with sand-dominated sediments exhibiting potential rates of denitrification almost 10 times higher than those observed in gravel-dominated sediments (0.026 ± 0.004 N2O–N μg g−1 h−1 for sand-dominated and 0.003 ± 0.003 N2O–N μg g−1 h−1 for gravel-dominated). In-situ measurements supported this finding, with higher concentrations of NO3−, nitrite (NO2−) and N2O observed in the porewaters of gravel-dominated sediments. Denitrification varied substantially between seasons, with denitrification increasing from winter to autumn. Our results indicate highest NO3− reduction occurred in sand-dominated sediments whilst highest N2O concentrations occurred in gravel-dominated sediments. This suggests that finer-grained streambeds could play an important role in removing excess nitrogen from agricultural catchments without producing excess N2O

Reply to ‘Pseudoreplication and greenhouse-gas emissions from rivers'

Tiegs et al.1 highlight the significance and relevance of the findings of Comer-Warner et al.2 on greenhouse-gas emissions from streambed sediments but raise questions about some aspects of the experimental design. We support their call for more detailed field and laboratory-based studies on this subject. However, we believe that their concerns relate to uncertainties and limitations in the experimental design that were discussed explicitly in the original paper (and accompanying transparent peer review process—available online), or represent criticisms related to highly improbable minor anomalies that may unnecessarily dismiss experimental results as discussed below

A multi-stable isotope framework to understand eutrophication in aquatic ecosystems

Eutrophication is a globally significant challenge facing aquatic ecosystems, associated with human induced enrichment of these ecosystems with nitrogen (N) and phosphorus (P). However, the limited availability of inherent labels for P and N has constrained understanding of the triggers for eutrophication in natural ecosystems and appropriate targeting of management responses. This paper proposes and evaluates a new multi-stable isotope framework that offers inherent labels to track biogeochemical reactions governing both P and N in natural ecosystems. The framework couples highly novel analysis of the oxygen isotope composition of phosphate (δ18OPO4) with dual isotope analysis of oxygen and N within nitrate (δ15NNO3, δ18ONO3) and with stable N isotope analysis in ammonium (δ15NNH4). The River Beult in England is used as an exemplar system for initial evaluation of this framework. Our data demonstrate the potential to use stable isotope labels to track the input and downstream fate of nutrients from point sources, on the basis of isotopic differentiation for both P and N between river water and waste water treatment work effluent (mean difference = +1.7‰ for δ18OPO4; +15.5‰ for δ15NNH4 (under high flow); +7.3‰ for δ18ONO3 and +4.4‰ for δ15NNO3). Stable isotope data reveal nutrient inputs to the river upstream of the waste water treatment works that are consistent with partially denitrified sewage or livestock sources of nitrate (δ15NNO3 range = +11.5 to +13.1‰) and with agricultural sources of phosphate (δ18OPO4 range = +16.6 to +19.0‰). The importance of abiotic and metabolic processes for the in-river fate of N and P are also explored through the stable isotope framework. Microbial uptake of ammonium to meet metabolic demand for N is suggested by substantial enrichment of δ15NNH4 (by 10.2‰ over a 100 m reach) under summer low flow conditions. Whilst the concentration of both nitrate and phosphate decreased substantially along the same reach, the stable isotope composition of these ions did not vary significantly, indicating that concentration changes are likely driven by abiotic processes of dilution or sorption. The in-river stable isotope composition and the concentration of P and N were also largely constant downstream of the waste water treatment works, indicating that effluent-derived nutrients were not strongly coupled to metabolism along this in-river transect. Combined with in-situ and laboratory hydrochemical data, we believe that a multi-stable isotope framework represents a powerful approach for understanding and managing eutrophication in natural aquatic ecosystems