Gypsum amendment of agricultural fields to decrease phosphorus losses - Evidence on a catchment scale.

Amending agricultural fields with gypsum has been proposed as a cost-effective measure to reduce P load on coastal waters. We treated 1490 ha of clayey fields with phosphogypsum (4 t ha-1) in Southwest Finland and monitored the recipient river with online sensors and water sampling for the preceding spring and 5 years after the amendment. Gypsum immediately decreased the riverine fluxes, the effect lasting at least 5 years for particulate P (PP), total suspended solids (TSS), and dissolved organic C (DOC) and 1-2 years for dissolved reactive P (DRP). Compared with an upstream control area, the fluxes of PP, TSS, and DOC decreased by 15%, 25%, and 8.9%, respectively, as a 5-year average. Assuming the change in the fluxes occurred only due to gypsum, the amended fields showed 35%, 59%, and 64% lower losses for PP, TSS, and DOC than the unamended ones. More than half of the gypsum remained in the soil even after 5 years; thus, although the efficiency of gypsum lessened over time, its residual effect may be present. However, the difference in the erodibility between the control and treatment areas impacted the validity of the results, especially as the pre-gypsum period was short. In addition, the performance of gypsum showed spatial variation.

Phosphorus and nitrogen fluxes carried by 21 Finnish agricultural rivers in 1985-2006.

The Finnish Agri-Environmental Programme aims to reduce nutrient load to waters. Using national monitoring data, we estimated the agricultural load (incl. natural background) of total phosphorus (TP) and total nitrogen (TN) transported by 21 Finnish rivers to the northern Baltic Sea and analysed the flow-adjusted trends in the loads and concentrations from 1985 to 2006. We also related the loads to spatial and temporal patterns in catchment and agricultural characteristics. Agricultural load of TN increased, especially in the rivers discharging into the Bothnian Bay, while the load of TP decreased in most of the rivers, except those discharging into the Archipelago Sea. The trends may partly be related to a decrease in grassed area (TP, TN) and increased mineralisation (TN), but the available data on catchment and agricultural characteristics did not fully explain the observed pattern. Our study showed that data arising from relatively infrequent monitoring may prove useful for analysing long-term trend. The mutual correlation among the explaining variables hampered the analysis of the load generating factors.

Catchment and lake network modify export of anaerobic oxidation capacity in boreal freshwaters.

Anaerobic terminal electron acceptors (aTEAs, i.e. NO3, Fe, SO4) enable anaerobic respiration, and each has a specific ability to oxidize reduced compounds. However, little is known about how seasonal and lake-specific aTEA fluxes form anaerobic oxidation capacity (AOC) to oxidize organic carbon in boreal systems. We compiled 26 years of data from two interconnected semi-pristine boreal lakes and defined mean daily imports, pools, and exports of aTEAs. In both lakes, the export of NO3 formed 2 %-3 % of the total AOC in summer and autumn, and up to 11 % in winter and spring. In a predominantly monomictic humic lake surrounded by peatlands, Fe was responsible for 15 %-31 % of the seasonal export of AOC, with a large proportion of Fe originating from the lake bottom. A dimictic clear-water lake downstream retained Fe and exported 87 %-95 % of AOC as SO4. In the humic lake, the annual SO4:Fe:NO3 export ratio for AOC was 10:3:1 and in the clear-water lake 15:0.4:1. In the monomictic lake, exports were specifically regulated by stratification; in the dimictic lake, exports were more regulated by spring flooding and the ascending and descending side of the peak flood. These events modified lake dynamics and caused lake-specific NO3, Fe, and SO4 exports which continued for months. We conclude that a catchment and lake network can cause spatial and temporal variation in exports of NO3, Fe, and SO4 affecting AOC export. Such natural variations in exports have significant potential to modify the system's capacity to oxidize C and resist changes in oxidation-reduction reactions coupled to nutrient cycling and the formation of greenhouse gases in downstream water bodies.

Efficient protection of the Baltic Sea needs a revision of phosphorus metric.

Eutrophication of the Baltic Sea is driven by phosphorus and nitrogen. While the anthropogenic point source loads of both nutrients have decreased markedly, further reductions are needed. This is true particularly for phosphorus, as highlighted by its stringent abatement targets in HELCOM's Baltic Sea Action Plan. To meet the targets, more results need to be achieved in non-point source abatement, specifically from agricultural sources. The growing pressure for phosphorus abatement from agriculture may lead to environmentally and economically inefficient outcomes unless we account for the variability in how different forms of phosphorus respond to abatement measures, and how these forms contribute to eutrophication. The precautionary and efficiency improving way to advance policies is to either replace or supplement the Total Phosphorus metric with a metric more accurate in reflecting the biologically available phosphorus. This policy fix becomes more important as the relative share of agricultural emissions of total pollution increases.

Does control of soil erosion inhibit aquatic eutrophication?

Much of the phosphorus (P) from erosive soils is transported to water bodies together with eroded soil. Studies clarifying the impact of soil erosion on eutrophication have sought largely to quantify the reserves of P in soil particles that can be desorbed in different types of receiving waters. Aquatic microbiology has revealed that the cycling of P is coupled to the availability of common electron acceptors, Fe oxides and SO4, through anaerobic mineralization in sediments. Eroded soil is also rich in Fe oxides, and their effect on the coupled cycling of C, Fe, S, and P has been neglected in eutrophication research. Soil erosion, and its control, should therefore be studied by considering not only the processes occurring in the water phase but also those taking place after the soil particles have settled to the bottom. We propose that in SO4-rich systems, Fe oxides transported by eroded soil may promote Fe cycling, inhibit microbial SO4 reduction and maintain the ability of sediment to retain P. We discuss the mechanisms through which eroded soil may affect benthic mineralization processes and the manner in which soil erosion may contribute to or counteract eutrophication.

Risk Assessment of Gypsum Amendment on Agricultural Fields: Effects of Sulfate on Riverine Biota.

Gypsum (CaSO4 ∙2H2 O) amendment is a promising way of decreasing the phosphorus loading of arable lands, and thus preventing aquatic eutrophication. However, in freshwaters with low sulfate concentrations, gypsum-released sulfate may pose a threat to the biota. To assess such risks, we performed a series of sulfate toxicity tests in the laboratory and conducted field surveys. These field surveys were associated with a large-scale pilot exercise involving spreading gypsum on agricultural fields covering 18% of the Savijoki River (Finland) catchment area. The gypsum amendment in such fields resulted in approximately a four-fold increase in the mean sulfate concentration for a 2-month period, and a transient, early peak reaching approximately 220 mg/L. The sulfate concentration gradually decreased almost to the pregypsum level after 3 years. Laboratory experiments with Unio crassus mussels and gypsum-spiked river water showed significant effects on foot movement activity, which was more intense with the highest sulfate concentration (1100 mg/L) than with the control. Survival of the glochidia after 24 and 48 h of exposure was not significantly affected by sulfate concentrations up to 1000 mg/L, nor was the length growth of the moss Fontinalis antipyretica affected. The field studies on benthic algal biomass accrual, mussel and fish density, and Salmo trutta embryo survival did not show gypsum amendment effects. Gypsum treatment did not raise the sulfate concentrations even to a level just close to critical for the biota studied. However, because the effects of sulfate are dependent on both the spatial and the temporal contexts, we advocate water quality and biota monitoring with proper temporal and spatial control in rivers within gypsum treatment areas. Environ Toxicol Chem 2022;41:108-121. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Diffuse sources dominate the sulfate load into Finnish surface waters.

Sulfate (SO42-) affects the cycling of ecologically important substances, such as carbon, nutrients and metals, but the contribution of anthropogenic activities in sulfate load entering aquatic systems is poorly known. We calculate specific sulfate loads for land cover types, atmospheric deposition and point sources, and then estimate the source-specific flux of sulfate to Finnish surface waters. The largest sulfate flux, entering mostly the Baltic Sea, originates from agricultural fields on acid sulfate soil (24% of total flux). Forests on mineral soil, which cover 67% of the country's surface area, form the second largest source (21%). Additionally, agricultural fields on non-acid soil cause noticeable sulfate flux (16%). Pulp and paper mills were the key point sources (20%) for sulfate. We find that anthropogenic activities contribute to elevated sulfate levels in waters potentially affecting the cycling of nutrients, metals and formation of greenhouse gases in naturally sulfate-poor freshwaters. Based on these findings, sulfate should be systematically included in monitoring and pollution control measures.

Algal-available phosphorus entering the Gulf of Finland as estimated by algal assays and chemical analyses.

Algal-available phosphorus (Paa) in river water and wastewater entering the Gulf of Finland (a Baltic Sea sub-basin) was estimated by a fresh-water and a brackish-water modification of the dual-culture algal assay. The assay results were further related to those obtained by routine chemical analyses. According to the brackish-water assay, an average of 44% (range, 9-88%) of total phosphorus (TP) in water samples from the Neva, Kymijoki, and Narva rivers consisted of Paa, whereas the mean value given by the fresh-water assay was 22% (range, 0-48%). Phaeodactylum tricornutum Bohlin, which was used as the test alga in the brackish-water assay, had higher phosphoesterase activity and P affinity than did Pseudokirchneriella subcapitata Korschikov, which was used in the fresh-water assay. This difference may explain the higher values of Paa shown by the brackish-water assay. Of the analytical P forms, total dissolved P best approximated, yet underestimated, the Paa in river water samples. As for the biologically purified wastewaters of the city of St. Petersburg, both assays suggested that about 80% of TP (range, 59-103%) was available. That the assays gave similar results was probably due to the fact that most of the P in the wastewater samples was in the form of readily available dissolved reactive P. In untreated urban wastewaters, the mean proportion of Paa in TP was 46% (range, 19-76%). Although the true Paa may not be obtained by any assay, our findings corroborate the view that severe underestimation may occur if the test conditions are suboptimal for the release and uptake of P.

Coastal eutrophication thresholds: a matter of sediment microbial processes.

In marine sediments, the major anaerobic mineralization processes are Fe(III) oxide reduction and sulfate reduction. In this article, we propose that the two alternative microbial mineralization pathways in sediments exert decisively different impacts on aquatic ecosystems. In systems where iron reduction dominates in the recently deposited sediment layers, the fraction of Fe(III) oxides that is dissolved to Fe(II) upon reduction will ultimately be transported to the oxic layer, where it will be reoxidized. Phosphorus, which is released from Fe(III) oxides and decomposing organic matter from the sediment, will be largely trapped by this newly formed Fe(III) oxide layer. Consequently, there are low concentrations of phosphorus in near-bottom and productive water layers and primary production tends to be limited by phosphorus (State 1). By contrast, in systems where sulfate reduction dominates, Fe(III) oxides are reduced by sulfides. This chemical reduction leads to the formation and permanent burial of iron as solid iron sulfides that are unable to capture phosphorus. In addition, the cycling of iron is blocked, and phosphorus is released to overlying water. Owing to the enrichment of phosphorus in water, the nitrogen : phosphorus ratio is lowered and nitrogen tends to limit algal growth, giving an advantage to nitrogen-fixing blue-green algae (State 2). A major factor causing a shift from State 1 to State 2 is an increase in the flux of labile organic carbon to the bottom sediments; upon accelerating eutrophication a critical point will be reached when the availability of Fe(III) oxides in sediments will be exhausted and sulfate reduction will become dominant. Because the reserves of Fe(III) oxides are replenished only slowly, reversal to State 1 may markedly exceed the time needed to reduce the flux of organic carbon to the sediment. A key factor affecting the sensitivity of a coastal system to such a regime shift is formed by the hydrodynamic alterations that decrease the transport of O2 to the near-bottom water, e.g., due to variations in salinity and temperature stratification.

Integrating ecological and economic modeling of eutrophication: toward optimal solutions for a coastal area suffering from sediment release of phosphorus.

This paper puts forward a model for managing eutrophication that integrates the salient ecological and economic characteristics of a coastal area suffering from severe nutrient enrichment. The model links the development of phosphorus concentration over time to nutrient emissions from agriculture and habitation. It accounts for differences in agricultural and municipal abatement options and their costs, as well as the need to undertake irreversible investments to set up wastewater treatment facilities. Furthermore, it considers sediment release of phosphorus as a function of annual nutrient loads. The model is parameterized for a 30-km-wide area off the Finnish coast of the Gulf of Finland. The socially optimal policy, which minimizes the sum of monetary damage caused by eutrophication and the costs of nutrient abatement over time, is determined using a dynamic programming approach. The results suggest that considerable investments are warranted to bring wastewater treatment facilities up to date. Continued efforts to reduce agricultural nutrient loading are nevertheless also called for. The analysis provided is a first step toward an integrated analysis of eutrophication that accounts for complexities inherent in the problem, such as sediment release of phosphorus and irreversible investments in abatement technology. The results are sensitive in particular to ecological assumptions and parameterization, and further research is needed in these areas.

Showing similarity of results given by two methods: a commentary.

There is a frequent need in the environmental sciences to show the similarity of the results given by two analytical methods. This cannot, however, be done within the conventional 'there is a difference' statistical hypothesis setting of, among others, Student's t-test. We demonstrate here a more appropriate approach that originates from drug testing and that can be applied with standard statistical software. It is a challenging approach, as it requires quantification of the similarity limit. If no pre-determined value is given for similarity, a potential data-supported similarity limit can be explored from the data. The approach has numerous other potential application areas, e.g. parallelism of regression slopes, homogeneity of variances and lack of interaction.

Determining algal-available phosphorus in pulp and paper mill effluents: algal assays vs routine phosphorus analyses.

We determined the potentially algal-available P (P(aa)) in pulp and paper mill effluents by two bioassay techniques (a dual-culture assay and a batch approach) and compared it with chemically analysed P (total P, total dissolved P, reactive P, dissolved reactive P and dissolved hydrolysable P). The mean P(aa) given by the dual-culture assay was within a 40% range of that given by the batch approach. The P(aa) obtained by both bioassays differed from total P. Dissolved reactive P appeared to be the most readily available chemical P fraction, but other P fractions also contributed to P(aa). The analysis of reactive P, which involves direct staining of an unfiltered sample, best approximated the P(aa) given by the dual-culture assay (relative error 9%). The results suggest that assessment of eutrophying effluent P can be improved by analysing simple chemical P fractions in wastewater. The fractions to be analysed, however, may be site specific.

Runoff changes have a land cover specific effect on the seasonal fluxes of terminal electron acceptors in the boreal catchments.

Climate change influences the volume and seasonal distribution of runoff in the northern regions. Here, we study how the seasonal variation in the runoff affects the concentrations and export of terminal electron acceptors (i.e. TEAs: NO3, Mn, Fe and SO4) in different boreal land-cover classes. Also, we make a prediction how the anticipated climate change induced increase in runoff will alter the export of TEAs in boreal catchments. Our results show that there is a strong positive relationship between runoff and the concentration of NO3-N, Mn and Fe in agricultural catchments. In peaty catchments, the relationship is poorer and the concentrations of TEAs tend to decrease with increasing runoff. In forested catchments, the correlation between runoff and TEA concentrations was weak. In most catchments, the concentrations of SO4 decrease with an increase in runoff regardless of the land cover or season. The wet years export much higher amounts of TEAs than the dry years. In southern agricultural catchments, the wet years increased the TEA export for both spring (January-May) and autumn (September-December) periods, while in the peaty and forested catchments in eastern and northern Finland the export only increased in the autumn. Our predictions for the year 2099 indicate that the export of TEAs will increase especially from agricultural but also from forested catchments. Additionally, the predictions show an increase in the export of Fe and SO4 for all the catchments for the autumn. Thus, the climate induced change in the runoff regime is likely to alter the exported amount of TEAs and the timing of the export downstream. The changes in the amounts and timing in the export of TEAs have a potential to modify the mineralization pathways in the receiving water bodies, with feedbacks in the cycling of C, nutrients and metals in aquatic ecosystems.

Labile organic carbon regulates phosphorus release from eroded soil transported into anaerobic coastal systems.

Coastal eutrophication is expected to increase due to expanding and intensifying agriculture which causes a large amount of soil-associated P to be transported into aquatic systems. We performed anaerobic long-term incubations on field soil to mimic the conditions that eroded soil encounters in brackish sediments. The release of P from soil increased with the amount of labile organic C (acetate) addition and decreased with the soil/solution ratio. We deduce that in less-productive brackish systems, microbial Fe reduction allows for the maintenance of the coupled cycling of Fe and P and restricts the amount of P entering the oxic water. In more eutrophic systems, the formation of Fe sulfides as a result of SO4 reduction inactivates Fe, and leads to a higher release of P, thus generating an adverse feedback effect. The dependence of the fate of soil-bound Fe and P on the trophic status of the receiving water should be recognized in eutrophication management.

Lakes in the Finnish Eurowaternet: status and trends.

The Eurowaternet is a monitoring network designed to cover all European countries, with the overall objective of obtaining timely, quantitative and comparative information on the status of inland waters so that their key environmental problems can be defined, quantified and monitored. Finland launched the Eurowaternet for lakes in 2000. The network was set up according to guidelines of the European Environment Agency (EEA) and was based on national, regional and local monitoring programmes. It includes 253 lake sites, divided into the following groups by, for example, their loading background and size: (i) reference sites, (ii) point-source loading (impact) sites, (iii) agricultural sites, (iv) representative sites, i.e. lakes <100 km(2) with typical water quality in the catchment and (v) lake deep sites in large lakes (>100 km(2)). We examined the water quality and its development during 1976-2001 in these groups, using data from surface and near-bottom water layers at summer and winter stagnation. In addition, the representativeness of the network was evaluated. Summer mean concentrations of total P (TP) during 1990-2001 in surface water were (i) 6.8+/-1.8 microg 1(-1) in reference lakes, (ii) 24+/-5 microg 1(-1) in lakes affected by point loading, (iii) 58+/-32 microg l(-1) in agricultural lakes, (iv) 14+/-2.2 microg l(-1) in representative lakes and (v) 11+/-2.1 microg l(-1) in large lakes. Comparison with reference lakes revealed major impact of human activities on TP and total N (TN)-and consequently on chlorophyll a (a-chl) and transparency-in all other groups, especially agricultural lakes. Decreasing TP and increasing O(2) trends were found at impact sites, reflecting water protection measures taken in pulp and paper mills and municipalities. More surprisingly, we found increasing alkalinity trends in all but agricultural sites and decreasing NO(2)-N+NO(3)-N (NO(x)-N) trends particularly in pelagic areas of large lakes. Comparison of water quality in Eurowaternet lakes with those in randomly sampled lakes (Finnish Lake Survey) revealed that the Eurowaternet covers the entire range of concentrations of TP, TN and conductivity in Finnish lakes, but <1-km(2), eutrophic and/or brown water lakes are underrepresented. Only 0.2% of a total of 13114 small lakes (0.1-1 km(2)) are included in the annual monitoring program of the Eurowaternet.

Phosphorus in runoff assessed by anion exchange resin extraction and an algal assay.

Eutrophication of surface waters can be accelerated by agricultural inputs of phosphorus (P), provided that P is in a form that can be utilized by aquatic algae. We studied anion exchange resin (AER) extraction and a dual culture algal assay (DCAA) for the determination of potentially algal-available P in water samples without sediment preconcentration. Our material consisted of agricultural and forest runoff and wastewaters. The results obtained by the two methods were essentially equal when the samples contained only small amounts of particulate phosphorus (PP) in relation to dissolved molybdate-reactive phosphorus (DRP). However, in turbid agricultural runoff, P extracted with AER averaged 72% (n = 17) of the P yield of the 3-wk DCAA (R2 = 0.94). When the runoff samples were diluted for the AER extraction in the same manner as for the DCAA, the AER-P yield increased to 85% (n = 5) of DCAA-P. The minimum detectable value was greater for the AER test (41 microg L(-1) AER-extractable P) than for the DCAA (7 microg L(-1) DCAA-P). At concentrations greater than about 50 microg L(-1) AER-P or DCAA-P, the accuracy of the methods was satisfactory, with the coefficient of variation in replicated analyses being less than 10% for the AER test and less than 20% for the DCAA. Other anions competing for the exchange sites of the AER decreased P recovery by 15 to 20% when their equivalent concentration exceeded about 4 mmol, L(-1), and this effect was relatively constant over a large concentration range. We consider that AER extraction is a suitable low-cost method to estimate the algal availability of P in runoff samples.