Global food security threatened by potassium neglect

Food security and healthy ecosystems are placed in jeopardy by poor potassium management. Six actions may prevent declines in crop yield due to soil potassium deficiency, safeguard farmers from potash price volatility and address environmental concerns associated with potash mining.

Phosphorus price spikes: A wake-up call for phosphorus resilience

Food systems depend on reliable supplies of phosphorus to fertilize soils. Since 2020, a pandemic, geopolitical disputes, trade wars and escalating fuel prices have driven a >400% increase in phosphorus commodity prices, contributing to the current food crisis. The Russia-Ukraine conflict has disrupted phosphate trade further. Concurrently, phosphorus losses to freshwaters, through insufficient municipal wastewater treatment and inappropriate fertilizer use and land management practices, are a significant threat to water quality globally. Despite precariously balanced food and water security risks, nations are largely unaware of their “phosphorus vulnerability” and phosphorus is markedly absent in national and global policies addressing food and water security. Phosphorus vulnerability can be described as the degree to which people/systems are susceptible to harm due to the physical, geopolitical and socio-economic dimensions of global phosphorus scarcity and pollution. Here, we bring the current price spike into focus, highlighting the drivers, policy responses and their consequences. We highlight the need for an integrated assessment of phosphorus vulnerability that considers environmental, socio-economic and climate change risks across scales. We illustrate how reducing phosphorus waste, increasing phosphorus recycling, and wider system transformation can reduce national reliance on imported phosphorus, whilst enhancing food and water security. The current crisis in fertilizer prices represents a wake-up call for the international community to embrace the global phosphorus challenge.

Making waves. Bridging theory and practice towards multiple stressor management in freshwater ecosystems

Despite advances in conceptual understanding, single-stressor abatement approaches remain common in the management of fresh waters, even though they can produce unexpected ecological responses when multiple stressors interact. Here we identify limitations restricting the development of multiple-stressor management strategies and address these, bridging theory and practice, within a novel empirical framework. Those critical limitations include that (i) monitoring schemes fall short of accounting for theory on relationships between multiple-stressor interactions and ecological responses, (ii) current empirical modelling approaches neglect the prevalence and intensity of multiple-stressor interactions, and (iii) mechanisms of stressor interactions are often poorly understood. We offer practical recommendations for the use of empirical models and experiments to predict the effects of freshwater degradation in response to changes in multiple stressors, demonstrating this approach in a case study. Drawing on our framework, we offer practical recommendations to support the development of effective management strategies in three general multiple-stressor scenarios.

Protecting and restoring Europe's waters: An analysis of the future development needs of the Water Framework Directive

The Water Framework Directive (WFD) is a pioneering piece of legislation that aims to protect and enhance aquatic ecosystems and promote sustainable water use across Europe. There is growing concern that the objective of good status, or higher, in all EU waters by 2027 is a long way from being achieved in many countries. Through questionnaire analysis of almost 100 experts, we provide recommendations to enhance WFD monitoring and assessment systems, improve programmes of measures and further integrate with other sectoral policies. Our analysis highlights that there is great potential to enhance assessment schemes through strategic design of monitoring networks and innovation, such as earth observation. New diagnostic tools that use existing WFD monitoring data, but incorporate novel statistical and trait-based approaches could be used more widely to diagnose the cause of deterioration under conditions of multiple pressures and deliver a hierarchy of solutions for more evidence-driven decisions in river basin management. There is also a growing recognition that measures undertaken in river basin management should deliver multiple benefits across sectors, such as reduced flood risk, and there needs to be robust demonstration studies that evaluate these. Continued efforts in 'mainstreaming' water policy into other policy sectors is clearly needed to deliver wider success with WFD goals, particularly with agricultural policy. Other key policy areas where a need for stronger integration with water policy was recognised included urban planning (waste water treatment), flooding, climate and energy (hydropower). Having a deadline for attaining the policy objective of good status is important, but even more essential is to have a permanent framework for river basin management that addresses the delays in implementation of measures. This requires a long-term perspective, far beyond the current deadline of 2027.

Assessing the role of bed sediments in the persistence of red mud pollution in a shallow lake (Kinghorn Loch, UK)

Red mud is a by-product of alumina production. Little is known about the long-term fate of red mud constituents in fresh waters or of the processes regulating recovery of fresh waters following pollution control. In 1983, red mud leachate was diverted away from Kinghorn Loch, UK, after many years of polluting this shallow and monomictic lake. We hypothesised that the redox-sensitive constituents of red mud leachate, phosphorus (P), arsenic (As) and vanadium (V), would persist in the Kinghorn Loch for many years following pollution control as a result of cycling between the lake bed sediment and the overlying water column. To test this hypothesis, we conducted a 12-month field campaign in Kinghorn Loch between May 2012 and April 2013 to quantify the seasonal cycling of P, As, and V in relation to environmental conditions (e.g., dissolved oxygen (DO) concentration, pH, redox chemistry and temperature) in the lake surface and bottom waters. To confirm the mechanisms for P, As and V release, a sediment core incubation experiment was conducted using lake sediment sampled in July 2012, in which DO concentrations were manipulated to create either oxic or anoxic conditions similar to the bed conditions found in the lake. The effects on P, As, and V concentrations and species in the water column were measured daily over an eight-day incubation period. Phosphate (PO₄-P) and dissolved As concentrations were significantly higher in the bottom waters (75.9 ± 30.2 μg L^-1 and 23.5 ± 1.83 μg L^-1, respectively) than in the surface waters (12.9 ± 1.50 μg L^-1 and 14.1 ± 2.20 μg L^-1, respectively) in Kinghorn Loch. Sediment release of As and P under anoxic conditions was confirmed by the incubation experiment and by the significant negative correlations between DO and P and As concentrations in the bottom waters of the lake. In contrast, the highest dissolved V concentrations occurred in the bottom waters of Kinghorn Loch under oxic conditions (15.0 ± 3.35 μg L^-1), with the release from the bed sediment apparently being controlled by a combination of competitive ion concentrations, pH and redox conditions.

Macronutrient processing by temperate lakes: A dynamic model for long-term, large-scale application

We developed a model of the biogeochemical and sedimentation behaviour of carbon (C), nitrogen (N) and phosphorus (P) in lakes, designed to be used in long-term (decades to centuries) and large-scale (10⁴-10⁵km²) macronutrient modelling, with a focus on human-induced changes. The model represents settling of inflow suspended particulate matter, production and settling of phytoplankton, decomposition of organic matter in surface sediment, denitrification, and DOM flocculation and decomposition. The model uses 19 parameters, 13 of which are fixed a priori. The remaining 6 were obtained by fitting data from 109 temperate lakes, together with other information from the literature, which between them characterised the stoichiometric incorporation of N and P into phytoplankton via photosynthesis, whole-lake retention of N and P, N removal by denitrification, and the sediment burial of C, N and P. To run the model over the long periods of time necessary to simulate sediment accumulation and properties, simple assumptions were made about increases in inflow concentrations and loads of dissolved N and P and of catchment-derived particulate matter (CPM) during the 20th century. Agreement between observations and calculations is only approximate, but the model is able to capture wide trends in the lakewater and sediment variables, while also making reasonable predictions of net primary production. Modelled results suggest that allochthonous sources of carbon (CPM and dissolved organic matter) contribute more to sediment carbon than the production and settling of algal biomass, but the relative contribution due to algal biomass has increased over time. Simulations for 8 UK lakes with sediment records suggest that during the 20th century average carbon fixation increased 6-fold and carbon burial in sediments by 70%, while the delivery of suspended sediment from the catchments increased by 40% and sediment burial rates of N and P by 131% and 185% respectively.

Assessing the Legacy of Red Mud Pollution in a Shallow Freshwater Lake: Arsenic Accumulation and Speciation in Macrophytes

Little is known about long-term ecological responses in lakes following red mud pollution. Among red mud contaminants, arsenic (As) is of considerable concern. Determination of the species of As accumulated in aquatic organisms provides important information about the biogeochemical cycling of the element and transfer through the aquatic food-web to higher organisms. We used coupled ion chromatography and inductively coupled plasma mass spectrometry (ICP-MS) to assess As speciation in tissues of five macrophyte taxa in Kinghorn Loch, U.K., 30 years following the diversion of red mud pollution from the lake. Toxic inorganic As was the dominant species in the studied macrophytes, with As species concentrations varying with macrophyte taxon and tissue type. The highest As content measured in roots of Persicaria amphibia (L.) Gray (87.2 mg kg(-1)) greatly exceeded the 3-10 mg kg(-1) range suggested as a potential phytotoxic level. Accumulation of toxic As species by plants suggested toxicological risk to higher organisms known to utilize macrophytes as a food source.

Phytoplankton community responses in a shallow lake following lanthanum-bentonite application

The release of phosphorus (P) from bed sediments to the overlying water can delay the recovery of lakes for decades following reductions in catchment contributions, preventing water quality targets being met within timeframes set out by environmental legislation (e.g. EU Water Framework Directive: WFD). Therefore supplementary solutions for restoring lakes have been explored, including the capping of sediment P sources using a lanthanum (La)-modified bentonite clay to reduce internal P loading and enhance the recovery process. Here we present results from Loch Flemington where the first long-term field trial documenting responses of phytoplankton community structure and abundance, and the UK WFD phytoplankton metric to a La-bentonite application was performed. A Before-After-Control-Impact (BACI) analysis was used to distinguish natural variability from treatment effect and confirmed significant reductions in the magnitude of summer cyanobacterial blooms in Loch Flemington, relative to the control site, following La-bentonite application. However this initial cyanobacterial response was not sustained beyond two years after application, which implied that the reduction in internal P loading was short-lived; several possible explanations for this are discussed. One reason is that this ecological quality indicator is sensitive to inter-annual variability in weather patterns, particularly summer rainfall and water temperature. Over the monitoring period, the phytoplankton community structure of Loch Flemington became less dominated by cyanobacteria and more functionally diverse. This resulted in continual improvements in the phytoplankton compositional and abundance metrics, which were not observed at the control site, and may suggest an ecological response to the sustained reduction in filterable reactive phosphorus (FRP) concentration following La-bentonite application. Overall, phytoplankton classification indicated that the lake moved from poor to moderate ecological status but did not reach the proxy water quality target (i.e. WFD Good Ecological Status) within four years of the application. As for many other shallow lakes, the effective control of internal P loading in Loch Flemington will require further implementation of both in-lake and catchment-based measures. Our work emphasizes the need for appropriate experimental design and long-term monitoring programmes, to ascertain the efficacy of intervention measures in delivering environmental improvements at the field scale.

Eutrophication management in surface waters using lanthanum modified bentonite: A review

This paper reviews the scientific knowledge on the use of a lanthanum modified bentonite (LMB) to manage eutrophication in surface water. The LMB has been applied in around 200 environments worldwide and it has undergone extensive testing at laboratory, mesocosm, and whole lake scales. The available data underline a high efficiency for phosphorus binding. This efficiency can be limited by the presence of humic substances and competing oxyanions. Lanthanum concentrations detected during a LMB application are generally below acute toxicological threshold of different organisms, except in low alkalinity waters. To date there are no indications for long-term negative effects on LMB treated ecosystems, but issues related to La accumulation, increase of suspended solids and drastic resources depletion still need to be explored, in particular for sediment dwelling organisms. Application of LMB in saline waters need a careful risk evaluation due to potential lanthanum release.

A meta-analysis of water quality and aquatic macrophyte responses in 18 lakes treated with lanthanum modified bentonite (Phoslock(®))

Lanthanum (La) modified bentonite is being increasingly used as a geo-engineering tool for the control of phosphorus (P) release from lake bed sediments to overlying waters. However, little is known about its effectiveness in controlling P across a wide range of lake conditions or of its potential to promote rapid ecological recovery. We combined data from 18 treated lakes to examine the lake population responses in the 24 months following La-bentonite application (range of La-bentonite loads: 1.4-6.7 tonnes ha(-1)) in concentrations of surface water total phosphorus (TP; data available from 15 lakes), soluble reactive phosphorus (SRP; 14 lakes), and chlorophyll a (15 lakes), and in Secchi disk depths (15 lakes), aquatic macrophyte species numbers (6 lakes) and aquatic macrophyte maximum colonisation depths (4 lakes) across the treated lakes. Data availability varied across the lakes and variables, and in general monitoring was more frequent closer to the application dates. Median annual TP concentrations decreased significantly across the lakes, following the La-bentonite applications (from 0.08 mg L(-1) in the 24 months pre-application to 0.03 mg L(-1) in the 24 months post-application), particularly in autumn (0.08 mg L(-1) to 0.03 mg L(-1)) and winter (0.08 mg L(-1) to 0.02 mg L(-1)). Significant decreases in SRP concentrations over annual (0.019 mg L(-1) to 0.005 mg L(-1)), summer (0.018 mg L(-1) to 0.004 mg L(-1)), autumn (0.019 mg L(-1) to 0.005 mg L(-1)) and winter (0.033 mg L(-1) to 0.005 mg L(-1)) periods were also reported. P concentrations following La-bentonite application varied across the lakes and were correlated positively with dissolved organic carbon concentrations. Relatively weak, but significant responses were reported for summer chlorophyll a concentrations and Secchi disk depths following La-bentonite applications, the 75th percentile values decreasing from 119 μg L(-1) to 74 μg L(-1) and increasing from 398 cm to 506 cm, respectively. Aquatic macrophyte species numbers and maximum colonisation depths increased following La-bentonite application from a median of 5.5 species to 7.0 species and a median of 1.8 m to 2.5 m, respectively. The aquatic macrophyte responses varied significantly between lakes. La-bentonite application resulted in a general improvement in water quality leading to an improvement in the aquatic macrophyte community within 24 months. However, because, the responses were highly site-specific, we stress the need for comprehensive pre- and post-application assessments of processes driving ecological structure and function in candidate lakes to inform future use of this and similar products.

Assessment of changes in potential nutrient limitation in an impounded river after application of lanthanum-modified bentonite

With the advent of phosphorus (P)-adsorbent materials and techniques to address eutrophication in aquatic systems, there is a need to develop interpretive techniques to rapidly assess changes in potential nutrient limitation. In a trial application of the P-adsorbent, lanthanum-modified bentonite (LMB) to an impounded section of the Canning River, Western Australia, a combination of potential P, nitrogen (N) and silicon (Si) nutrient limitation diagrams based on dissolved molar nutrient ratios and actual dissolved nutrient concentrations have been used to interpret trial outcomes. Application of LMB resulted in rapid and effective removal of filterable reactive P (FRP) from the water column and also effectively intercepted FRP released from bottom sediments until the advent of a major unseasonal flood event. A shift from potential N-limitation to potential P-limitation also occurred in surface waters. In the absence of other factors, the reduction in FRP was likely to be sufficient to induce actual nutrient limitation of phytoplankton growth. The outcomes of this experiment underpins the concept that, where possible in the short-term, in managing eutrophication the focus should not be on the limiting nutrient under eutrophic conditions (here N), but the one that can be made limiting most rapidly and cost-effectively (P).

Responses in sediment phosphorus and lanthanum concentrations and composition across 10 lakes following applications of lanthanum modified bentonite

A combined field and laboratory scale study of 10 European lakes treated between 2006 and 2013 with a lanthanum (La) modified bentonite (LMB) to control sediment phosphorus (P) release was conducted. The study followed the responses in sediment characteristics including La and P fractions and binding forms, P adsorption capacity of discrete sediment layers, and pore water P concentrations. Lanthanum phosphate mineral phases were confirmed by solid state (31)P MAS NMR and LIII EXAFS spectroscopy. Rhabdophane (LaPO4 · nH2O) was the major phase although indications of monazite (LaPO4) formation were also reported, in the earliest treated lake. Molar ratios between La and P in the sediments were generally above 1, demonstrating excess La relative to P. Lanthanum was vertically mixed in the sediment down to a depth of 10 cm for eight of the ten lakes, and recovery of La in excess of 100% of the theoretical aerial load indicated translocation of the LMB towards the deepest areas of the lakes. Lanthanum was generally recovered from bed sediment samples following sequential chemical extraction from the HCl fraction. Soluble reactive P (SRP) release experiments on intact sediment cores indicated conditions of P retention (with the exception of two lakes) by sediments, indicating effective control of sediment P release, i.e. between two and nine years after treatment.

Editorial - A critical perspective on geo-engineering for eutrophication management in lakes

Eutrophication is the primary worldwide water quality issue. Reducing excessive external nutrient loading is the most straightforward action in mitigating eutrophication, but lakes, ponds and reservoirs often show little, if any, signs of recovery in the years following external load reduction. This is due to internal cycling of phosphorus (P). Geo-engineering, which we can here define as activities intervening with biogeochemical cycles to control eutrophication in inland waters, represents a promising approach, under appropriate conditions, to reduce P release from bed sediments and cyanobacteria accumulation in surface waters, thereby speeding up recovery. In this overview, we draw on evidence from this special issue Geoengineering in Lakes, and on supporting literature to provide a critical perspective on the approach. We demonstrate that many of the strong P sorbents in the literature will not be applicable in the field because of costs and other constraints. Aluminium and lanthanum modified compounds are among the most effective compounds for targeting P. Flocculants and ballast compounds can be used to sink cyanobacteria, in the short term. We emphasize that the first step in managing eutrophication is a system analysis that will reveal the main water and P flows and the biological structure of the waterbody. These site specific traits can be significant confounding factors dictating successful eutrophication management. Geo-engineering techniques, considered collectively, as part of a tool kit, may ensure successful management of eutrophication through a range of target effects. In addition, novel developments in modified zeolites offer simultaneous P and nitrogen control. To facilitate research and reduce the delay from concept to market a multi-national centre of excellence is required.

Lake responses following lanthanum-modified bentonite clay (Phoslock®) application: an analysis of water column lanthanum data from 16 case study lakes

Phoslock(®) is a lanthanum (La) modified bentonite clay that is being increasingly used as a geo-engineering tool for the control of legacy phosphorus (P) release from lake bed sediments to overlying waters. This study investigates the potential for negative ecological impacts from elevated La concentrations associated with the use of Phoslock(®) across 16 case study lakes. Impact-recovery trajectories associated with total lanthanum (TLa) and filterable La (FLa) concentrations in surface and bottom waters were quantified over a period of up to 60 months following Phoslock(®) application. Both surface and bottom water TLa and FLa concentrations were <0.001 mg L(-1) in all lakes prior to the application of Phoslock(®). The effects of Phoslock(®) application were evident in the post-application maximum TLa and FLa concentrations reported for surface waters between 0.026 mg L(-1)-2.30 mg L(-1) and 0.002 mg L(-1) to 0.14 mg L(-1), respectively. Results of generalised additive modelling indicated that recovery trajectories for TLa and FLa in surface and bottom waters in lakes were represented by 2nd order decay relationships, with time, and that recovery reached an end-point between 3 and 12 months post-application. Recovery in bottom water was slower (11-12 months) than surface waters (3-8 months), most probably as a result of variation in physicochemical conditions of the receiving waters and associated effects on product settling rates and processes relating to the disturbance of bed sediments. CHEAQS PRO modelling was also undertaken on 11 of the treated lakes in order to predict concentrations of La(3+) ions and the potential for negative ecological impacts. This modelling indicated that the concentrations of La(3+) ions will be very low (<0.0004 mg L(-1)) in lakes of moderately low to high alkalinity (>0.8 mEq L(-1)), but higher (up to 0.12 mg L(-1)) in lakes characterised by very low alkalinity. The effects of elevated La(3+) concentrations following Phoslock(®) applications in lakes of very low alkalinity requires further evaluation. The implications for the use of Phoslock(®) in eutrophication management are discussed.

Assessing the mode of action of Phoslock® in the control of phosphorus release from the bed sediments in a shallow lake (Loch Flemington, UK)

Phoslock(®) is increasingly used worldwide to control sediment phosphorus (P) release and cyanobacterial blooms despite the fact that little is known about its mode of action in lake bed sediments. This study quantified the effects of Phoslock(®) on sediment elemental composition and P fractionation (one year pre- and post-application of 170 g Phoslock(®) m(-2)) in an attempt to address these knowledge gaps. Post-application, sediment La content was significantly higher in the top 10 cm of the sediment compared to pre-application conditions. Mass balance calculations indicated that the applied mass of La had the potential to bind 25% of potentially release-sensitive P (Pmobile; sum 'labile P', 'reductant-soluble P' and 'organic P' fraction) present in the top 4 cm or 10% of Pmobile present in the top 10 cm of the sediment. Assessing variation in sediment P partitioning indicated that the application caused a significant increase in the mass of P present in the more refractory 'apatite bound P' fraction between post-application month 4 and 7 compared to Pmobile. This suggests that Phoslock(®) controls sediment P release by increasing the mass of P permanently bound in the sediment. To address uncertainty in estimating product dose required to control sediment P release we conducted laboratory assays using intact sediment cores to which we added serial additions of Phoslock(®) under either aerobic or anaerobic conditions. The laboratory experiment indicated that the original dose to Loch Flemington was sufficient to control sediment P release under aerobic conditions but that significant P release will occur should prolonged anaerobic conditions persist. However, Phoslock(®) may be a viable option to control sediment P-release under anaerobic conditions which would require an estimated additional application of up to 510 g Phoslock(®) m(-2). A conceptual model is proposed for the use of P-capping agents in lake remediation projects which is likely to increase cost-effectiveness and reduce non-target effects by applying multiple smaller doses compared to a single high dose.

Community history affects the predictability of microbial ecosystem development

Microbial communities mediate crucial biogeochemical, biomedical and biotechnological processes, yet our understanding of their assembly, and our ability to control its outcome, remain poor. Existing evidence presents conflicting views on whether microbial ecosystem assembly is predictable, or inherently unpredictable. We address this issue using a well-controlled laboratory model system, in which source microbial communities colonize a pristine environment to form complex, nutrient-cycling ecosystems. When the source communities colonize a novel environment, final community composition and function (as measured by redox potential) are unpredictable, although a signature of the community's previous history is maintained. However, when the source communities are pre-conditioned to their new habitat, community development is more reproducible. This situation contrasts with some studies of communities of macro-organisms, where strong selection under novel environmental conditions leads to reproducible community structure, whereas communities under weaker selection show more variability. Our results suggest that the microbial rare biosphere may have an important role in the predictability of microbial community development, and that pre-conditioning may help to reduce unpredictability in the design of microbial communities for biotechnological applications.

Variation in chlorophyll a to total phosphorus ratio across 94 UK and Irish lakes: implications for lake management

Eutrophication is the most widespread pressure impacting on lakes worldwide and, in general, its control is underpinned by the premise that algal biomass is regulated by phosphorus availability. This paper aims to demonstrate that not all lakes will conform to the underlying principle of the Chl:TP relationships using variables collected widely in lake monitoring programmes across the world (i.e. total phosphorus (TP) and chlorophyll a (Chl) concentrations). The ratio of annual mean Chl and TP concentrations in 94 lakes (2007 and 2008) was used as a measure of the efficiency with which TP is transferred into algal biomass (i.e. as an indicator of P limitation) to investigate the validity of the phosphorus reduction approach. Chl:TP ratios ranged from 0.02 to 0.84 in 2007 and from 0.03 to 0.96 in 2008. Chl and TP values were positively correlated (p < 0.05) with alkalinity, and negatively correlated with depth and surface area, in both years. In general, mean annual Chl and TP concentrations increased as alkalinity increased and depth decreased. However, Chl:TP ratio was highest in high alkalinity lakes in 2007, and moderate alkalinity lakes in 2008. Our results indicate that the use of TP water quality targets alone is insufficient to accurately manage eutrophication pressures at the lake specific scale, and that a wider range of ecological and ecosystem service evaluation targets would provide a more comprehensive assessment of management needs. The wide range of Chl:TP ratio values reported in this study suggests that, although reducing TP concentrations in lakes is undoubtedly a sensible approach to eutrophication management in many cases, TP reductions alone may not result in the expected reductions in phytoplankton biomass in all lakes.

Comparison of phosphorus (P) removal properties of materials proposed for the control of sediment P release in UK lakes

Of growing interest in the control of sediment phosphorus (P) release in lakes is the use of solid phase P-sorbing products (PSPs) including industrial by-products and naturally occurring or modified mineral complexes. However, there is a need to report on novel PSPs proposed by suppliers for use in lake remediation projects at the national scale. We comparatively assessed the elemental composition and P sorption properties of six industrial waste-products (waste-products from treatment of abandoned mine waters - 'red ochre' and 'black ochre'; waste products from building practices: 'gypsum', 'sander dust', 'mag dust' and 'vermiculite') and one commercially available modified lanthanum (La) bentonite product (Phoslock®), all of which have been proposed for use in remediation projects in UK lakes. P sorption was well described (r(2)>0.70) by Langmuir isotherms for all products with the exception of 'gypsum' for which no significant P sorption was reported. P sorption capacities ranged from 4 mgPg(-1) dry weight (d.w.) PSP ('black ochre') to 63 mgg(-1) d.w. ('sander dust'), with products composed mainly of calcium oxide (CaO) and silicon dioxide (SiO(2)) (i.e. 'vermiculite', 'mag dust' and 'sander dust' PSPs) having significantly higher sorption capacities than all other PSPs. Estimates of the equilibrium P concentration (EPC(0)) from Langmuir isotherms indicated that all PSPs, with the exception of 'gypsum', were capable of reducing soluble reactive P (SRP) concentrations to <5.1 μgL(-1). Further research and development required to underpin regulatory policy decisions regarding the use of PSPs is discussed. Dose estimates for each PSP required to treat a eutrophic shallow loch (40.6 ha) with persistent internal loading issues are reported.

Sediment amendment with Phoslock® in Clatto Reservoir (Dundee, UK): Investigating changes in sediment elemental composition and phosphorus fractionation

Lanthanum-modified bentonite clay (Phoslock(®) is a lake remediation tool designed to strip dissolved phosphorus (P) from the water column and increase the sediment P-sorption capacity. This study investigated short term alterations in sediment elemental composition and sediment P-fractions based on sediment cores taken 2 days before and 28 days following the application of 24 t of Phoslock® to a 9 ha, man-made reservoir. Following the application, sediment lanthanum (La) content increased significantly (p < 0.05; n = 4) in the top 8 cm of the sediment, thereby theoretically increasing sediment P-binding capacity on the whole reservoir scale by 250 kg. Mass balance calculations were used to estimate the theoretical binding of release-sensitive P (P(mobile); sum of 'labile P', 'reductant-soluble P' and 'organic P' fraction) by La across the top 4 cm and 10 cm depth of sediment. The amended mass of La in the sediment had the potential to bind 42% of P(mobile) present in the top 4 cm or 17% of P(mobile) present in the top 10 cm. However, with the exception of a significant increase (p<0.05; n=4) in the 'residual P' fraction in the top 2 cm, sediment P-fractions, including P(mobile,) did not differ significantly following the Phoslock® application. Experimental P-adsorption studies indicated P-saturation values for Phoslock® of 21,670 mg P kg⁻¹ Phoslock®. Sequential extraction of P from saturated Phoslock® under laboratory conditions indicated that around 21% of P bound by Phoslock® was release-sensitive, while around 79% of bound P was unlikely to be released under reducing or common pH (5-9) conditions in shallow lakes. Applying Phoslock® is, therefore, likely to increase the P-sorption capacity of sediments under reducing conditions.

Spatial and historical variation in sediment phosphorus fractions and mobility in a shallow lake

Temporal and spatial variation in sediment P composition and mobility were investigated in Loch Leven. Little change was observed in total sediment P (surface sediment at 4m depth), in comparison to a previous study (1990), despite significant reduction of external point sources of P. Labile P and residual P have both increased (0.007-0.039 mg PO(4)-P and 0.121-0.420 mg PO(4)-P per gram dry weight of sediment, respectively) since 1990. An analysis of P fractions, along a depth transect, indicated elevated labile P concentrations in shallow water sediment (<12 m overlying water depth). Regression analysis showed that spatial variability in reductant-adsorbed P was significantly related to sediment chlorophyll a concentration (R(2)=0.733, p<0.05). This may be linked to the production of oxygen, by benthic algae, resulting in the maintenance of an oxygenated layer at the sediment surface. Variation in labile P was best explained by overlying water temperature and equilibrium phosphate concentration (EPC0).