The dilemma of controlling cultural eutrophication of lakes

The management of eutrophication has been impeded by reliance on short-term experimental additions of nutrients to bottles and mesocosms. These measures of proximate nutrient limitation fail to account for the gradual changes in biogeochemical nutrient cycles and nutrient fluxes from sediments, and succession of communities that are important components of whole-ecosystem responses. Erroneous assumptions about ecosystem processes and lack of accounting for hysteresis during lake recovery have further confused management of eutrophication. I conclude that long-term, whole-ecosystem experiments and case histories of lake recovery provide the only reliable evidence for policies to reduce eutrophication. The only method that has had proven success in reducing the eutrophication of lakes is reducing input of phosphorus. There are no case histories or long-term ecosystem-scale experiments to support recent claims that to reduce eutrophication of lakes, nitrogen must be controlled instead of or in addition to phosphorus. Before expensive policies to reduce nitrogen input are implemented, they require ecosystem-scale verification. The recent claim that the ‘phosphorus paradigm’ for recovering lakes from eutrophication has been ‘eroded’ has no basis. Instead, the case for phosphorus control has been strengthened by numerous case histories and large-scale experiments spanning several decades.     

基于SPEI的北京低频干旱与气候指数关系

干旱是北京地区发生最频繁、波及面积最大、持续时间最长的一种自然灾害。基于1868—2010年每月的降水和平均气温数据,应用综合了降水和气温变化共同效应的新的干旱指标标准化降水蒸散指数(SPEI)定量描述北京地区的干湿状况,并利用历史旱灾记录对其进行验证;采用连续小波转换(CWT)分析近150 a来的干旱振荡特征,并利用交叉小波变换(XWT)探论了干旱与大尺度气候因子之间的关系。结果表明:1)SPEI揭示的干旱与历史记录比较吻合,证明该指数可以在多时间尺度上有效地反映北京地区旱涝程度及其持续时间;2)北京地区干旱具有80—120个月年际尺度和250个月、480个月年代际尺度的周期振荡,呈现了同大尺度气候因子相似的变化特征;3)北京干旱变化与四大气候因子存在着多时间尺度的显著相关性,SPEI和北大西洋涛动(NAO)、北极涛动(AO)、太平洋涛动(PDO)都具有100—120个月和250个月的年代际主共振周期,而SPEI和厄尔尼诺-南方涛动(ENSO)在整个研究期内都表现出极显著的32—64个月年际主共振周期,同时SPEI与4个气候因子在共振周期上均体现出比较明确的时滞特征(2—6月不等)。因此,可以基于大尺度气象因子结合SPEI预测北京地区未来的干旱变化。     

Distribution of Sediment Phosphorus Fractions in Hypertrophic Strongly Stratified Lake Verevi

Lake Verevi is a hypertrophic and strongly stratified (partly meromictic) small temperate lake. Vertical distribution of sediment phosphorus fractions as well as iron, manganese, organic matter and calcium carbonate of the deep bottom sediment was determined. The study focused on the ecologically important layer of the sediment [<20(45) cm]. In the uppermost layers of the sediment, NaOH-NRP (organic P) dominated while HCl-RP (apatite-P) became dominant in some deeper layers below 7 cm. Extremely high concentrations of labile phosphorus fraction (NH₄Cl-RP) indicated the low binding capacity of phosphorus by lake sediment. Due to sediment and hypolimnion anoxia, the internal load of phosphorus in this lake is most likely. Potentially mobile phosphorus fractions (NH₄Cl-RP, BD-RP, NaOH-NRP) formed 301 kg in upper 10 cm thick sediment layer of hypolimnetic bottom sediment (40% of lake bottom area).     

1963-2020年蒙古锡林河流域极端气候及其对水文干旱的影响

全球变暖导致区域气候变化加剧、极端气候事件频发,改变流域水文过程,并进一步加剧径流的不确定性。阐明长时间尺度极端气候与径流变化的相互关系,对于生态环境脆弱的半干旱区具有重要意义。基于1963-2020年间的高分辨率气候和径流实测数据,采用Mann-kendall检验、Pettitt检验和小波分析等方法,探究了锡林河流域极端气候变化特征及其对水文干旱的影响。研究结果表明:(1)降水量呈现-0.2mm/10a的波动下降趋势,而气温和潜在蒸散发则分别以0.4℃/10a和8.2mm/10a的速率显著上升。(2)标准化径流指数(SRI)以-0.2/10a的速率呈显著下降趋势,且在2001年发生突变,水文干旱趋势增强。(3)极端气温强度和频率指数均显著上升,月最高气温极大值、年最大日最低气温、夏季日数、热夜日数的气候倾向率分别为0.4℃/10a、0.5℃/10a、4.4d/10a、1.0d/10a;而极端降水指数则呈不显著下降趋势,强降水总量、普通日降水强度、持续湿润指数、强降水日数(R10mm)的气候倾向率分别为-0.2mm/10a、-0.1mm/10a、-0.2d/10a、-0.1d/10a。(4)SRI与降水、极端降水的强度和频率之间呈正相关,与温度、潜在蒸散发及极端温度的强度、频率呈负相关。(5)SRI与降水、气温和潜在蒸散发之间主要存在6-10a的显著共振周期,而与极端气候之间的显著共振周期则为2-3a。研究发现,锡林河流域的气候变化呈现暖干化趋势,极端气温事件显著增多而极端降水事件未能同步增加导致水文干旱风险加剧,其中降水和R10mm是影响径流变化的关键因素。研究结果可为干旱半干旱地区在极端气候影响下的流域水资源管理提供参考。     

Oligotrophication outweighs effects of global warming in a large,deep, stratified lake ecosystem

Between 1951 and 1979, total phosphorous concentrations in Lake Constance increased from 7 to 87 μg L^?1. Following wastewater treatment, phosphorus levels were brought under control, returning to 7.6 μg L^?1 by spring 2007. The biological and chemical data from 1980 to 2004 were first modelled by seasonal time series analyses and then used to create a general model. Excluding collinear variables allowed the data set to be condensed to six variables that could be fitted into a general linear model that explained ～75% of the observed annual variation in chlorophyll a. A clear seasonal influence was apparent, with chlorophyll a tracking trends in temperature and the progress of spring. A nonseasonal influence was also observed in the interaction of two biological components, the proportion of phytoplankton biomass available to Daphnia (i.e. the percentage of ingestible size <30 μm) and the grazing intensity. In combination, these biotic variables had a negative impact on chlorophyll a levels. In contrast, the concentration of soluble reactive phosphorus (SRP) correlated positively with chlorophyll a. The effect of SRP showed a significant seasonal component, as it was more abundant in spring than at other times of year. In general, the model predicts a negative exponential response of chlorophyll a to further depletion of SRP in Lake Constance, while the temperature trends predicted by current global warming scenarios will result in a moderate increase in productivity. Data from 2005 to 2007 were used to verify the model. The modelled chlorophyll a values were nonbiased and showed a close match to the measured values (r²: 75%). Thus the applicability, reliability, and informative value of the model for pelagic Lake Constance was confirmed. The approach might easily be applied to other waters.     

Effects of different macrophyte growth forms on sediment and P resuspension in a shallow lake

The effects of floating-leaved, submerged and emergent macrophytes on sediment resuspension and internal phosphorus loading were studied in the shallow Kirkkojärvi basin by placing sedimentation traps among different plant beds and adjacent open water and by sediment and water samples. All the three life forms considerably reduced sediment resuspension compared with non-vegetated areas. Both among submerged (Ceratophyllum demersum, Potamogeton obtusifolius, Ranunculus circinatus) and emergent (Typha angustifolia) plants, resuspension rate was on average 43% of that in the adjacent open water, while within floating-leaved plants (Nuphar lutea) the corresponding value was 87%. The effects of submerged and emergent vegetation increased in the course of the growing season together with increasing plant density. Among floating-leaved vegetation, such seasonal trend in resuspension effects was not observed. Compared with the non-vegetated area, floating-leaved, submerged and emergent plants reduced internal phosphorus loading on average by 21, 12 and 26 mg m⁻² d⁻¹, respectively. The effects of floating-leaved plants on resuspension-mediated internal phosphosrus loading were thus comparable to the effects of the other two life forms.     

气候变化与人类活动双重驱动的冷水湖泊富营养化

富营养化对水生生态系统造成的负面影响已在世界范围内广泛发生,尤其对淡水水源地湖泊的水环境质量影响深远,进而引起当地居民的饮用水安全与健康隐患。在人类活动和气候变化的双重驱动下,富营养化辐射的范围不断扩大,从过去主要集中于温带大型浅水湖泊已经扩展到寒冷地区的冷水湖泊。分析了近年来世界范围内高寒地区冷水湖泊富营养化的趋势特征与研究进展,探讨了气候变化、人类干扰(农业活动、畜牧业生产、管理措施不当等)在不同地区冷水湖泊富营养化进程中的作用。在未来的研究中,应进一步加强对冷水湖泊富营养化机制的探讨,并对已有富营养化症状的湖泊进行生态修复,以确保冷水湖泊生态系统健康并改善饮用水源地的环境质量。     

Distribution and release of sedimentary phosphorus in Lake Ladoga

Sedimentary phosphorus fractions and phosphorus release from the sediments were studied in Lake Ladoga at altogether 46 sampling sites, representing the full range of sediment types encountered in the lake. Determination of P fractions and physico-chemical analyses were made of surface sediment cores (10–20 cm long, each sampled at 3–4 levels) and in the overlying water. The range of total phosphorus per dry weight of sediment was 0.2–3.3 mg g^–1, and that of inorganic P 0.1–2.5 mg g^–1. The levels of interstitial soluble phosphorus, range 2–613 µg 1^–1 for total P and 1–315 µg 1^–1 for inorganic P, were higher than those of dissolved P concentrations in the overlying water. Diffusive fluxes of phosphate from sediment to the overlying water were estimated using three independent methods. The estimated range was 4–914 µg P m^–2 d^–1; the mean value for the whole bottom area, 0.1 mg P m^–2 d^–1, is lower than previously published estimates. The estimated annual contribution of sedimentary inorganic P flux to Lake Ladoga water is equal to 620 tons of P per year, which amounts to more than 10% of the estimated external P load into the lake. 68% of the total diffusive flux emanates from deep water sediments, which are not exposed to seasonal variation of conditions. In deep lakes, such as Lake Ladoga, phosphorus release from the sediments is controlled primarily by diffusive mechanisms. Wave action and currents as well as bioturbation are probably of importance mainly in shallow near-shore areas. Phosphorus release by gas ebullition and macrophytes is considered negligible.     

Phosphorus sorption characteristics of the Bronx River bed sediments

Abstract

Phosphorus (P) is a major nutrient for plant growth, and it is often the primary limiting nutrient in freshwater ecosystems controlling algal blooms. The Bronx River of New York City, New York, USA includes freshwater and coastal water systems. The water quality of both fresh and saline water is lower than the standard levels designated by New York State, and classified as Class B and Class I waters, respectively. Algal blooms and oxygen depletion within the river have degraded the water quality, endangered fishing, and limited recreational use. The internal loading of P, an important bioavailability indicator in the Bronx River, is determined by the sorption processes, i.e., cycling of P between solid and liquid phases. The objectives of this study were to understand how P sorption characteristics affect the internal loading of P and the conditions that might give rise to a flux of P from sediment to the water column, and to estimate the effects of physicochemical properties of the sediments on P sorption parameters. Bed sediments were collected from 15 sites along the Bronx River, from the origin in Westchester Davis Brook, Kensico Dam through the Bronx to the Sound View Park estuary. Phosphorus sorption maximum (S_max) were significantly correlated with oxalate–extractable iron (Ox–Fe) and aluminum (Ox–Al), acid-extractable calcium (HCl–Ca) and magnesium (HCl–Mg), and total organic matter (OM), suggesting that not only metal ions affected P sorption characteristics, but OM also influenced the P sorption processes. This study also showed that originally sorbed P (S₀) was significantly correlated with Ox–Fe, Ox–Al, HCl–Mg, and OM. The extremely high values of the percentage of sorbed P retained in sediments (>98% for all sites except the two estuary sites: site 13 of 88% and site 14 of 92%) suggest that a large flux of P to the water column from the sediments could potentially occur under changing hydro-climatic conditions, such as the changes in pH, ionic strength and redox conditions, which may, in turn, exacerbate eutrophic conditions and subsequent algal blooms.     

Recent histories of six productive lakes in the Irish Ecoregion based on multiproxy palaeolimnological evidence

Palaeolimnological data from six mesotrophic, eutrophic and hypertrophic lakes in the Irish Ecoregion, in the form of microfossil (cladocera, diatoms and pollen) and sediment chemistry data from radiometrically dated sediment cores, were used to reconstruct past variations in lake water quality and catchment conditions. Basal sediments from sediment cores from the six sites ranged in age from the late 18th century to the early 20th century. A weighted averaging partial least squares regression model was developed to reconstruct past epilimnetic total phosphorus concentrations. The results indicate that all but one of the study sites currently are in a far more productive state compared with the beginning of the sediment core record and that those same five lakes have experienced accelerated enrichment post c. 1980. Two of the sites demonstrated long-term enrichment, in one case beginning in the late 19th century, while both eutrophication and oligotrophication have occurred at three sites. The results highlight the difficulties in applying a general temporal end-point for reference conditions and demonstrate that productive lakes in the Irish Ecoregion have complex, locally specific and often long histories of enrichment. These may not be responsive to reduced external loadings of phosphorus and, as a result, restoration could prove particularly challenging. The results also provide evidence of the ways in which palaeolimnological techniques can assist implementation of the EU Water Framework Directive. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.