Impacts of agricultural phosphorus use in catchments on shallow lake water quality: About buffers, time delays and equilibria

Role of Mine Tailings in the Spatio-Temporal Distribution of Phosphorus in River Water: The Case of B1 Dam Break in Brumadinho

Human actions in the drainage network of hydrographic basins interfere with the functioning of ecosystems, causing negative impacts on the environment. Among these impacts, mass loads with a high concentration of phosphorus (P) have a significant potential for point and diffuse pollution of freshwater. The objective of this work was to model P spatially in the Paraopeba River basin, namely in the main water course and 67 sub-basins, and temporally in the years of 2019, 2020, and 2021, after the rupture of B1 tailings dam of Vale, SA company in Brumadinho (Minas Gerais Brazil). The distribution of total phosphorus concentrations (Pt) in relation to environmental attributes (terrain slope, soil class, and land use) and stream flow was assessed with the help of SWAT, the well-known Soil and Water Assessment Tool, coupled with box-plot and cluster analyses. The Pt were obtained from 33 sampling points monitored on a weekly basis. Mean values varied from 0.02 to 1.1 mg/L and maximum from 0.2 to 15.9 mg/L across the basin. The modeling results exposed an impact on the quality of Paraopeba River water in a stretch extending 8.8–155.3 km from the B1 dam, related with the rupture. In this sector, if the contribution from the rupture could be isolated from the other sources, the average Pt would be 0.1 mg/L. The highest Pt (15.9 mg/L) was directly proportional to the urban area of a sub-basin intersecting the limits of Betim town and Belo Horizonte Metropolitan Region. In general, urban sprawl as well as forest-agriculture and forest-mining conversions showed a close relationship with increased Pt, as did sub-basins with a predominance of argisols and an accentuated slope (>20%). There were various moments presenting Pt above legal thresholds (e.g., >0.15 mg/L), mainly in the rainy season.

Timescale of reduction of long-term phosphorus release from sediment in lakes

It is important for lake management and policy to estimate the timescale of recovery from long-term P release from sediment after a reduction in the external load. To provide a scientific basis for this, a condensed model was elaborated, applied and evaluated in four lakes. The model is based on first order kinetics, with an overall rate constant composed of the rate of diagenesis of labile P (kd,2) and rate of burial of P (kb) below an active sediment layer. Using the variation of P fractions in dated sediment cores, kd,2 varied from 0.0155 to 0.383 yr^-1, kb from 0.0184 to 0.073 yr^-1 and the overall rate constant from 0.0230 to 0.446 yr^-1. The active layer depths, 8 to 29 cm, and kd,2 values are within the ranges found by others. The time for a 75% reduction (t₇₅) of labile P in the active layer is 60 years in Lough Melvin, 3 in Ramor, 33 in Sheelin and 41 in Neagh, although P release is only important in Ramor and Neagh. Combining the kd,2 values with other estimates (mean 0.0981 yr^-1, median 0.0426; n=14) produces a t₇₅ value of less than 14 and 33 years. A review of other models indicates a timescale of one to two decades and from lake monitoring also of one to two decades. It is desirable to estimate the timescale directly in all lakes if sediment P release is important, but, generally, it should take between one and three decades.

A case study of factors controlling water quality in two warm monomictic tropical reservoirs located in contrasting agricultural watersheds

The integration of internal (e.g., stratification) and external (e.g., pollution) factors on a comprehensive assessment of reservoir water quality determines the success of ecosystem restoration initiatives and aids watershed management. However, integrated analyses are scarcer than studies addressing factors separately. Integration is likely more efficient in studies of small well-characterized (experimental) reservoir watersheds, because the isolation of factor contributions is presumably clearer. But those studies are uncommon. This work describes the water quality of two small 5.5 m-deep reservoirs (MD-Main and VD-Voçoroca dams) located in Pindorama Experimental Center, state of São Paulo, Brazil, considering the interplay between reservoir dimension, seasonal thermal stratification, chemical gradients, erosive rainfall events, presence of natural biofilters, and land uses and landscape patterns around the reservoirs and within the contributing watersheds. The monitoring of agricultural activities and water quality parameters occurred in October 2018-July 2019. A 4 °C thermal stratification occurred in October (difference between surface and bottom water temperature), which decreased until disappearance in January (VD) or April (MD). The longer stratification period of MD was justified by its larger area relative to VD (≈10×). Thermal stratification triggered hypoxia at the bottom of both reservoirs (DO ≈ 1 mg/L), more prolonged and severe in MD. Hypoxia activated Ec and TDS peaks in January likely explained by bottom-sediment nutrient releases, presumably phosphorus. The Ec peak reached 560 μS/cm in MD and 290 μS/cm in VD. The smaller VD peak was probably explained by the action of macrophytes. In March, a 240 NTU turbidity peak occurred in MD, caused by precedent erosive rainfall and the lack of vegetation protection alongside the south border. As expected, the study accomplished clear isolation of factor contributions, verified by Factor and Cluster analyses. Our results can subsidize studies on larger reservoir watersheds requiring restoration, where the isolation of factors is more challenging.

Point sources and agricultural practices control spatial-temporal patterns of orthophosphate in tributaries to Chesapeake Bay

Orthophosphate (PO₄) is the most bioavailable form of phosphorus (P). Excess PO₄ may cause harmful algal blooms in aquatic ecosystems. A major restoration effort is underway for Chesapeake Bay (CB) to reduce P, nitrogen, and sediment loading to CB. Although PO₄ cycling and delivery to streams has been characterized in small-scale studies, regional drivers of PO₄ patterns remain poorly understood because most water quality trend assessment focus on total P. Moreover, these trend assessments are usually at an annual timestep. To address this research gap, we analyzed PO₄ patterns over a 9-year period at 53 monitoring stations across the CB watershed to: 1) characterize the role of PO₄ in total P fluxes and trends; 2) describe spatial and temporal patterns of PO₄ concentrations across seasons and streamflow; and 3) explore factors explaining these patterns. Agricultural watersheds exported the most total P compared with watersheds under different land uses (e.g., urban or forest), with PO₄ comprising up to 50% of those exports. Although PO₄ exports are declining at many sites, some agricultural regions are experiencing increasing trends at a rate sufficient to drive total P trends. Regression modeling results suggest that point source load reductions are likely responsible for decreasing PO₄ concentrations observed at many sites. Watersheds with more Conservation Reserve Program enrollment had lower summer PO₄ concentrations, highlighting the effectiveness of this practice. Manure inputs strongly predicted PO₄ concentrations at high flows across all seasons. Both manure applications and conservation tillage were correlated with changes in PO₄ concentrations at high flow, suggesting these activities could contribute to increasing PO₄ concentrations. This study highlights the effectiveness of point source control for reducing PO₄ exports and underscores the need for management strategies to target sources, practices, and landscape factors determining PO₄ loss from soils where manure inputs remain high.

Long-term efficiency of lake restoration by chemical phosphorus precipitation: Scenario analysis with a phosphorus balance model

An artificial increase of phosphorus (P) retention in lakes with a long residence time and/or a large mobile sediment P pool by adding P binding chemicals can drastically shorten the time these lakes require to reach water quality targets. Suitable tools to optimize timing and extent of external and internal measures are lacking. The one-box model, a mass balance tool for predicting the P trend in the water under different management options was applied to highly eutrophic Lake Arendsee (a = 5.14 km(2), zmax = 49 m), Germany. Mass developments of blue green algae and increasing hypolimnetic oxygen deficiencies are urgent reasons for restoring Lake Arendsee. Detailed studies of P cycling and scenario analyses with the one-box model led to the following conclusions: i) immediate improvement of the trophic state is only possible by in-lake P inactivation because of the long water residence time (56 years); ii) a gradual external P load reduction, even if the effect is delayed, will assure the sustainability of the scheduled Al application beyond one decade; iii) a twofold precipitation reduces the risk of failure compared to a singular application with an overdose related to the relevant internal P pools.

Multi-Elements in Waters and Sediments of Shallow Lakes: Relationships with Water, Sediment, and Watershed Characteristics

We measured concentrations of multiple elements, including rare earth elements, in waters and sediments of 38 shallow lakes of varying turbidity and macrophyte cover in the Prairie Parkland (PP) and Laurentian Mixed Forest (LMF) provinces of Minnesota. PP shallow lakes had higher element concentrations in waters and sediments compared to LMF sites. Redundancy analysis indicated that a combination of site- and watershed-scale features explained a large proportion of among-lake variability in element concentrations in lake water and sediments. Percent woodland cover in watersheds, turbidity, open water area, and macrophyte cover collectively explained 65.2 % of variation in element concentrations in lake waters. Sediment fraction smaller than 63 µm, percent woodland in watersheds, open water area, and sediment organic matter collectively explained 64.2 % of variation in element concentrations in lake sediments. In contrast to earlier work on shallow lakes, our results showed the extent to which multiple elements in shallow lake waters and sediments were influenced by a combination of variables including sediment characteristics, lake morphology, and percent land cover in watersheds. These results are informative because they help illustrate the extent of functional connectivity between shallow lakes and adjacent lands within these lake watersheds.

A palaeolimnological investigation into nutrient impact and recovery in an agricultural catchment

Widespread deterioration in water quality as a result of anthropogenic activity has led to the development and implementation of measures aimed at the protection of water resources in the EU. To date, however, relatively little attention has been paid to the effectiveness of these measures. Evidence from an enrichment-sensitive lake permitted reconstructions of changes in ecological and chemical water quality over the last c. 150-200 years, a period that includes a mid to late 20th century intensification of agriculture that was widely experienced across the European Union and the subsequent implementation of measures aimed at protecting water resources against pollution from farming. The data show the development of a more nutrient-tolerant diatom community from early in the 20th century, while the main trophic changes occurred from the 1950s, with the site becoming eutrophic by the 1960s. Heightened enrichment is thought to be linked to enhanced levels of phosphorus (P) transfers from the surrounding grassland catchment owing to an intensification of agricultural activities locally. Most recently, since the late 1990s and particularly post-2007, evidence suggests a decrease in aquatic enrichment, despite continued increases in agricultural intensification. This decoupling is likely to mark a successful implementation in 2006 of measures aimed at decreasing diffuse nutrient transfers from catchments linked to agri-environmental policies in Europe. The research highlights the importance of enrichment-sensitive water bodies as sentinel sites in the monitoring of both external and internal nutrient loadings as agricultural activities and other pressures change within the context of implementing regulatory responses to earlier declines in water quality.

Rapid ecosystem recovery from diffuse pollution after the Great Irish Famine

Remarkably little is known about the effectiveness or rates of recovery of aquatic ecosystems from reductions in human-associated pressures at landscape scales. The retention of anthropogenic contaminants within ecosystems can retard rates of recovery considerably, while the trajectories of recovery processes vary with the extent of disturbance and the resilience of biotic assemblages. The Great Irish Famine of 1845-1850 comprised one of the most significant human disasters of the 19th century, causing the death of approximately one million people and the emigration of a further two million from the country between 1845 and 1855. We found, through analysis of detailed historical census data combined with paleolimnological investigation of sedimentary nutrient concentrations, stable isotope ratios, and diatom assemblages, that the trophic level of Lough Carra, a largely shallow calcareous lake in the west of Ireland with no urban areas or point sources of any significance in its catchment, reduced considerably during and immediately after the Great Famine, shifting to new equilibria within just 2-10 years. Our results demonstrate that the reduction of human pressures from diffuse sources at landscape scales can result in the rapid and monotonic recovery of aquatic ecosystems. Moreover, the recovery of ecosystems from diffuse pollution need not necessarily take longer than recovery from pollution from point sources.