The occurrence of microcystin-LR in Lake Chivero, Zimbabwe

Lake Chivero is a eutrophic reservoir, initially constructed to supply drinking water to the City of Harare, Zimbabwe. Blooms of blue‐green algae have been a problem in the lake for many years and concern has been expressed about the toxins produced by Microcystis spp. The concentrations of the toxin, microcystin‐LR, produced in cultures of Microcystis aeruginosa from Lake Chivero, were investigated from March–April 2003. Microcystin‐LR was found in algal cells cultured from the lake water in concentrations ranging between 18.02 and 22.48 µg L^?1, with a mean concentration of 19.86 µg L^?1. These concentrations are the highest recorded to date for the lake, raising concerns about the possible effects of the toxin on the health of people who are drinking the water. Based on these study results, there is a need to control eutrophication, reducing algal blooms in order to prevent their potentially detrimental effects from blue‐green algal toxins produced under such conditions.     

Enrichment and geoaccumulation of metals in the superficial sediments of Lake Chivero,Zimbabwe

Sediments act as a sink, being an integrator and an amplifier for metals in rivers, lakes and reservoirs. Thus, sediment quality has been recognized as an important indicator of aquatic pollution. The concentrations of aluminium (Al), calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn) and sodium (Na) were studied in the surface sediments of Lake Chivero, Zimbabwe, to determine metal accumulation, distribution, pollution status and to distinguish natural background metal levels from human pollution sources. Sediment samples were collected from 17 locations during a two‐year (2014–2015) monitoring period during the hot–dry, hot–wet and cool–dry seasons. The overall mean metal concentrations exhibited the following decreasing order: Fe > Al>Mn > Ca>K > Mg>Na. The Mn levels for all sites and seasons were above the severe effect level (SEL). The highest metal and enrichment factor (EF) values were observed for the hot–wet season, indicating the period when the reservoir received the highest metal pollution from its catchment. The Geochemical index (I_geo) values for Mn for all seasons indicated moderately to strong contamination in sediments. The pollution load index (PLI) for all seasons indicated low pollution levels. Sediment contamination was attributed to natural sources for Al, Na and K and anthropogenic sources for Fe, Ca, Mg and Mn. The levels of studied metals in the Lake Chivero sediments are comparable to other eutrophic lentic systems. The results of the present study provide baseline information necessary for developing future metal pollution control strategies for Lake Chivero and its catchment.     

Comparative spatial metal concentrations and partitioning in bottom sediments of two tropical freshwater lake basins,Kenya

Metal concentrations in the water, sediments and biota in alkaline, near neutral water tropical freshwater lake basins were determined and a comparative analysis made on the spatial and accumulated trace metal concentrations, and geochemical partitioning of iron (Fe), manganese (Mn), copper (Cu), chromium (Cr), zinc (Zn), nickel (Ni), cadmium (Cd) and lead (Pb) in bottom lake sediments. Variable accumulated levels were found in the analysed sediments and aquatic biological tissues. Metal partitioning data in the present study from the two areas exhibits similar trends in magnitude. The residual fractions retained relatively higher amounts of metals. Relatively higher contents of Pb, Cr and Cu were found in Lake Victoria sediments. Cd, Ni, Mn and Pb are of concern because a higher percentage of these metals were retained in the more bioavailable fractions in Lake Naivasha. Total levels of sediment mercury were comparable for the two lake basins. Metals were found in significant levels in the tissues of aquatic organisms, highlighting the potential risks to other levels within the aquatic food chain. Although the exposure risk was not evaluated, it is important to continue monitoring these elements in fish, as sediments were found to contain considerable quantities of the metals. Numerous sediment extraction techniques data applied for evaluating metal partitioning in sediments cannot be compared because of the use of different methods. Nevertheless, the information is still useful in differentiating and understanding pollution loads from natural versus anthropogenic sources and their associated risks.     

乌梁素海浅水湖泊沼泽化现状及成因分析

为揭示乌梁素海面临的湖泊沼泽化问题与其驱动因素,针对2013年8月乌梁素海浅水湖泊沼泽化现状及成因,进行了实地勘查和采样,利用基于植物生物量与沉积速率、植物种类和泥深的2种评价方法进行分析。结果表明:乌梁素海沼泽化程度存在空间差异,部分区域属于沼泽化后期,分别分布在总排干入湖口处和湖区的中东部,其中,沼泽化综合指数在近入湖口处各点最大,沼泽化综合指数处于3~4,向湖区中东部逐渐减小,但仍然大于湖区中南部的下游区,该区域的沼泽化综合指标数为2~3,仍属于沼泽化盛期,直至湖区下游的最南端,属湖泊沼泽化不显著区。造成乌梁素海浅水湖泊沼泽化的主要原因很可能是在湖水流速缓慢和富营养化的综合作用下,外源物质淤积和水生植物蔓延,淤积物质和植物残体逐年累积形成底泥泥炭,随着泥炭的增厚和湖水变浅,从而演化为沼泽。     

First estimation of the diffusive methane flux and concentrations from Lake Winnipeg,a large,shallow and eutrophic lake

Freshwater lakes are increasingly recognized as significant sources of atmospheric methane (CH₄), potentially offsetting the terrestrial carbon sink. We present the first study of dissolved CH₄ distributions and lake-air flux from Lake Winnipeg, based on two-years of observations collected during all seasons. Methane concentrations across two years had a median of value of 24.6 nmol L^-1 (mean: 41.6 ± 68.2 nmol L^-1) and ranged between 5.0 and 733.8 nmol L^-1, with a 2018 annual median of 24.4 nmol L^-1 (mean: 46.8 ± 99.3 nmol L^-1) and 25.1 nmol L^-1 (mean: 38.8 ± 45.2 nmol L^-1) in 2019. The median lake-air flux was 1.1 µmol m^?2 h^?1 (range: 0.46–70.1 µmol m^?2h^?1, mean: 2.9 ± 10.2 µmol m^?2 h^?1) in 2018, and 5.5 µmol m^?2h^?1 (range: 0.0–78.4 µmol m^?2 h^?1, mean: 2.7 ± 8.5 µmol m^?2 h^?1) in 2019, for a total diffusive emission of 0.001 Tg of CH₄-C yr^?1. We found evidence of consistent spatial variability, with higher concentrations near river inflows. Significant seasonal trends in CH₄ concentrations were not observed, though fluxes were highest during the fall season due to strong winds. Our findings suggest Lake Winnipeg is a CH₄ source of similar mean magnitude to Lake Erie, with lower concentrations and fluxes per unit area than smaller mid- to high-latitude lakes. Additional work is needed to understand the factors underlying observed spatial variability in dissolved gas concentration, including estimations of production and consumption rates in the water column and sediments.     

Spatio-temporal variability of diatom assemblages in surface sediments of Lake Peipsi

The modern diatom assemblages in surface sediments of large and shallow Lake Peipsi sensu stricto (2611 km²; maximum depth 12.9 m) in north-eastern Europe (Estonia) were studied. Ordination techniques were used to identify physical environmental parameters (grain size of sediment, water depth and distance from the shore) that explain the distribution of diatoms in the lake. Diatom species distribution and concentration in surface sediment showed large variations (concentrations varied from 3.3 × 10⁶ to 149 × 10⁶ g⁻¹ dry weight). The texture of surface sediments and concentration of diatoms are highly correlated (p ≤ 0.05), thus suggesting that the relocation of cohesive particles (silt) due to wind and wave actions affects the spatial variations of diatoms in surface sediments. The diatom data set analysed in the 1970s, and samples taken in 2006 showed some perceptible changes in the composition and concentrations of the diatom assemblages over this time period. However, main dominant diatom taxa have not changed during more than 25 years.     

Spatial variations in sedimentary organic matter in surficial lake sediments of Nyanza Gulf (Lake Victoria,Kenya) after invasion of water hyacinth

No detailed studies have been conducted on the spatial distribution of sediment characteristics within Nyanza Gulf of Lake Victoria, especially after the invasion of water hyacinth. Accordingly, a rapid, inexpensive method was used to determine the spatial sediment organic matter variations, expressed as loss on ignition (LOI). Surficial sediments were collected during different survey periods from about thirty‐two sampling sites between 1994 and 2012, in order to characterize the sediments. Although the deeper (>40 m) depositional areas are located in the open lake, visual observations indicate that most of the offshore sediments consist of muddy deposits, plus significant shell remains, with a more sandy type of sediments located in some areas off the inflowing river mouths. The surficial sediments were characterized by a relatively high water content, with over 89% of the samples containing water contents greater than 75%. The sediment organic matter contents from the 2012 survey were more variable, ranging from 1.90% to 33.47%. The sediment organic carbon contents varied spatially, although there were no significant variations between the different sampling periods. Evaluating the SOM at different heating durations at a constant temperature (550 °C) and a very high temperatures (950 °C) did not identify any significant differences. High primary production and greater settling in the deeper areas contribute to the nature of sediments deposited, also being reflected in the relatively high OC contents. Spatially, it is thought that detritus from water hyacinth and terrestrial external loads are important sources of organic matter deposited on the lake bottom within the gulf and may support its growth because of enrichment of nutrient elements. The importance of sediments in adsorption of contaminant substances and influencing geochemical processes within the lake is also emphasized.     

呼伦湖沉积物中环境敏感粒度组分提取

2015年3月在中国北方呼伦湖采集100 cm湖泊沉积物岩芯并进行粒度分析,揭示了呼伦湖岩芯粒度参数的垂向分布序列。研究区柱状沉积物粒度组成以18 cm、60 cm和86 cm为界限分4个主要的阶段,并且小于16μm组分含量和大于32μm组分含量基本呈反向变化趋势,中值粒径的变化趋势和砂质组分含量的变化趋势一致。通过粒径-标准偏差法,获得了呼伦湖沉积物岩芯中对沉积时期环境变化较为敏感粒度组分范围约为大于30μm。利用主成分分析法,得到两个主控因子F1和F2,它们贡献了湖泊沉积物近88.12%的粒度变化特征。这两种方法相互结合,可以有效的探讨呼伦湖沉积物沉积时的古气候(降水,季风等)信息,同时其它一些环境要素也有较好地体现。     

烟台套子湾海域表层沉积物粒度分布特征及其动力响应

根据2010年10月采集的山东烟台套子湾大范围表层沉积物样品以及收集到的实测水文资料,分析研究套子湾海域表层沉积物的粒度分布特征,同时在应用GSTA模型对沉积物进行输运趋势分析的基础上,探讨该区域泥沙运移趋势.结果表明:整个套子湾内主要以细砂或极细砂和粉砂质砂为主,沉积物总体呈条带状分布向外逐渐变细;海区大部分区域的表层沉积物分选性都很好,相对与较小的潮流动力而言,波浪动力因素对泥沙的长期分选起到了主要作用;套子湾海域沉积物内细颗粒含量很少,平均值仅在3.49％,该海域细颗粒泥沙来源少,揭示了套子湾海域以波浪作用为主的沉积环境.应用GSTA模型对表层沉积物运移趋势分析结果显示,套子湾海域以垂直岸线横向输沙为主,近岸存在一定的纵向输沙,但强度不大,在优势浪垂岸入射和横向输沙作用控制下,纵向输沙强度较弱.     

Distribution and carbon isotope composition of lipid biomarkers in Lake Erhai and Lake Gahai sediments on the Tibetan Plateau

Lipid biomarkers and carbon isotopes of n-alkanes, n-fatty acids, and n-alkan-2-ones were used to determine organic carbon sources to surface sediments in lakes Erhai and Gahai on the Tibetan Plateau. All sediment samples contained n-fatty acids with lower concentrations of n-alkanes and n-alkan-2-ones. Long chain n-alkanes in lake sediments were characteristic of a source mixture of epicuticular waxes of higher plants and submerged littoral zone plants while n-fatty acids sources were bacteria and floating and/or submergent macrophytes; the n-alkan-2-ones had a possible source in epicuticular waxes of higher plants and/or grass from catchment which entered the lake and were reworked by the microorganisms. Sediment samples examined from Lake Erhai had heavier δ¹³C values of bulk organic carbon and specific carbon compounds than samples from Lake Gahai and meadow soil. This heavier isotopic composition can be best interpreted by the enhanced productivity occurring in Lake Erhai due to its lower salinity and possibly to limited CO₂ concentrations; for the latter, this may have enriched ¹³C in the dissolved inorganic carbon pool of the lake water.     

Phytoplankton primary production, chlorophyll-a and nutrient concentrations in the water column of mountainous Lake Phewa, Nepal

The seasonal patterns of phytoplankton primary production, chlorophyll‐a concentration, cell number and several other limnological variables in Lake Phewa, located in the active monsoon zone in Central Himalaya, Nepal, were studied for a year beginning in April 2001. During the study period, the gross primary production and chlorophyll‐a concentrations were relatively low during the monsoon season. The phytoplankton cell number, represented by 24 genera, also fluctuated seasonally, but tended to increase in the pre‐ and post‐monsoon period. These results suggest that the monsoon plays a crucial role in the primary production and phytoplankton dynamics for Lake Phewa. Among the phytoplankton species, Microcystis aeruginosa, a representative species for eutrophic lakes, was the dominant phytoplankton. At the same time, however, it is clear that the lake is not yet heavily eutrophic. The present study suggests that the exchange of lake water during the monsoon season contributes to maintaining the health of the lake against further degradation. Nevertheless, the silt carried in the monsoon rain run‐off from the lake's catchment area suggests increasingly serious degradation problems for this small mountainous lake.     

南四湖表层底泥重金属空间分布及污染程度评价

测试南四湖湖区29个表层底泥中重金属Hg、Cd、Cr、Pb、Ni、Cu、Zn和类金属As的质量比,得到南四湖湖区重金属污染的空间分布特征。结合地质积累指数法和潜在生态风险指数法,对南四湖重金属污染程度进行评价。结果表明,南四湖已受到重金属元素的污染,属于中等程度污染湖泊,其中以As、Hg、Cd、Pb元素污染严重。除了As、Cd元素在下级湖表层底泥中的质量比略高外,其他重金属元素在上级湖的污染程度普遍比在下级湖的高;二级坝以北的昭阳湖湖区潜在生态危害最为严重,其次是南阳湖、微山湖、独山湖和二级坝以南的昭阳湖湖区。     

Effects of habitat and fishing on fish assemblages in a tropical reservoir: Lake Kariba,Zimbabwe

This study investigated the effects of habitat and fishing on fish communities along inshore areas of the Sanyati Basin, Lake Kariba. A one‐year experimental gillnet survey was carried out in 2007 from eight sampling sites located in two different habitats (river mouths and sheltered bays), and in areas open or closed to fishing. A total of 16 species were captured, although the numbers caught at each site varied widely, with only four species recorded at each of the eight sampling sites. The numbers of fish were higher in the fished areas, mainly because of large numbers of Synodontis zambezensis, although the differences were not significant. In contrast, there were significant differences in the abundance of all species between habitats, with the exception of Cyphomyrus discorhynchus and Marcusenius macrolepidotus. There were no significant differences in the diversity of fish in these areas, although mean diversity and evenness were slightly higher in unfished areas and in sheltered bays. Overall, 90% of the fish specimens caught were within a 10–25 cm size range, although there were no systematic differences in the mean length in relation to fishing. In contrast, most fish were generally larger in river mouth areas than in sheltered bays. These data suggest that fishing had little effect on fish communities. This is in contrast to general habitat, which had a significant effect, a finding consistent with earlier studies showing that vegetation, in particular, had a significant effect on fish stocks.     

玄武湖沉积物磷形态的垂向变化和生物有效性

采用不同的化学提取法,分析玄武湖西北湖区沉积物中各形态磷、生物可利用磷(BAP)的垂向变化,探讨了各层沉积物中磷的生物有效性。结果表明,沉积物中总磷以无机磷为主,BAP占总磷的20%~52.6%,玄武湖底泥的磷有较好的生物可利用性。不同地点沉积物各磷形态和BAP的垂向变化有较大的差异。1号采样点的总磷、无机磷、铁磷和BAP质量比在沉积0~20 cm处较高,20~40 cm逐渐减小,说明0~20 cm内释磷潜力较大;2号采样点的总磷和无机磷质量比先减小,20~40 cm逐渐增加,而BAP和铁磷质量比随深度增加呈上升趋势,说明2点深层具有较大的释磷潜力。铁磷与BAP相关性较好,这部分磷具有较大的潜在释放风险。     

Long-term dynamics of Lake Erie benthos: One lake,three distinct communities

Lake Erie has experienced multiple anthropogenic-driven changes in the past century, including cultural eutrophication, phosphorus abatement initiatives, and the introduction of invasive species. The benthos of Lake Erie has been studied infrequently over nine decades and can provide not only insights into the impact of environmental changes but can also be used to examine ecosystem recovery through time. We used multivariate analyses to examine temporal changes in community composition and to assess the major drivers of long-term changes in benthos. Eutrophication, water quality improvement, and dreissenid introduction were the major drivers of changes in benthos in the western basin, while hypoxia was a major factor in the central basin, and dreissenid introduction was most important in the eastern basin. Non-dreissenid community composition of the western basin has changed dramatically over 90 years from benthic species indicative of good water quality in the 1930s, with a diverse community dominated by Hexagenia, to one of low diversity dominated by oligochaetes and other pollution-tolerant species in the 1960s, followed by recovery in the early 2000s to a state similar to that reported in 1930. In contrast, the non-dreissenid benthic community of the central basin over 60 years was consistently dominated by low oxygen-tolerant taxa, signifying the persistence of hypoxia, the major community driver in this basin. The eastern basin community also changed dramatically, including the disappearance of Diporeia after the introduction of Dreissena in the 1990s and more recent declines in oligochaetes, amphipods, gastropods, sphaeriid clams, and leeches.     

Assessment of the impacts of a new artificial lake mouth on the hydrobiology and fisheries of Chilika Lake, India

The degraded state of the fragile ecosystem of Chilika Lake, located on the east coast of India, was undergoing restoration through an effective hydrological intervention during the year 2000. Studies on the lake’s hydrobiology and fisheries for the period of 7 years before and 7 years after the hydrological intervention indicated a rapid recovery of the lake fishery immediately after opening of the new lake mouth, with a sixfold increase in the average annual fish landings. During 2000–2001 to 2006–2007, the average fisheries output (11 051.3 t), catch per unit effort (6.2 kg boat^?1 day^?1), the economic valuation of the average annual catch (637 million rupees) and productivity (11.97 t km^?2) exhibited dramatic increases of 498%, 464%, 1177% and 498.5%, respectively, compared with the 7‐year pre‐intervention data. Multivariate statistical analysis inferred that most of the lake’s environmental variables are strongly associated with salinity factor, which seems to have governed the lake ecology. The salinity dynamics of the lake are governed by both freshwater inflows and seawater ingress through the new artificial lake mouth. Correlation analysis indicated that salinity was positively correlated with prawn catch (R² = 0.542; d.f. = 25; P < 0.01), crab catch (R² = 0.628; d.f. = 25; P < 0.001) and fish catch (R² = 0.476; d.f. = 25, P < 0.05). The average increase in the salinity regime (43.8%) for the lake during the post‐hydrological intervention period, compared with the pre‐hydrological intervention period, appears to have positively impacted the fish, prawn and mud crab catches. A gradual decrease in total fisheries output since 2005–2006, however, was attributed mainly to a continuing increase in destructive fishing practices in the absence of any conservation and regulatory measures for fishing, and large‐scale collection of shrimp juveniles from the outer channel for shrimp aquaculture. Thus, carefully planned conservation and regulation measures must be ensured, with active participation of local communities during this early phase of lake restoration. In the absence of such measures, the present scenario of fisheries enhancement might not be sustained over the long term.     

Larval lake whitefish distribution in the open waters of Green Bay,Lake Michigan

Most lake whitefish populations within Lake Michigan are experiencing widespread population declines due to low levels of recruitment. However, the stock within Green Bay is flourishing, and various assessment tools suggest consistent recruitment to the juvenile stage each year. The distribution of offshore and drifting lake whitefish larvae were last described in the 1970s, prior to the resurgence of the southern Green Bay stock and prior to increasing knowledge of adult spawning behavior. We conducted a Green Bay wide survey of lake whitefish larvae over three years to provide an update of larval abundance and distribution and to discern influential biotic and abiotic factors that correlate to larval densities. In contrast with historical studies, our sampling illustrated high densities of larvae in both offshore and nearshore environments. Offshore expansion may be a factor in the population increase, but long term survival effects are still unknown. This study also provides the first documentation of larvae within the southern end of the bay below Chambers Island, WI and near re-colonized rivers. Average overall densities were relatively consistent each year, ranging between 29.5 and 49.6 per 1000 m³. Lake whitefish larvae were patchy in distribution and were associated with warmer and more productive waters at small spatial scales. Larval lake whitefish appear to be distributed widely throughout the entirety of Green Bay during the drift life stage, and we discuss our results in light of re-established river spawning and the overall increase in abundance.     

Baseline assessment of metals and hydrocarbons in the sediments of Lake Mulwala, Australia

An assessment of hydrocarbon and metal/metalloids (arsenic, cadmium, copper, lead, mercury, selenium, zinc) contamination in sediments from Lake Mulwala, Australia, was undertaken. The objectives of the study were: (i) to determine the extent of contamination in the lake sediments, compared to Australian and international sediment quality guidelines, and (ii) to attempt to identify the contaminant sources to the lake. With the exception of a few samples containing elevated levels of arsenic and/or mercury, the levels of all contaminants in the sediment samples taken from the lake were below the ‘lower trigger’ of the Australian Sediment Quality Guidelines that would warrant further investigation. High molecular weight hydrocarbons (up to 700 mg kg⁻¹) were found in most sediment samples. Non‐metric statistical analysis indicated that the contaminant distribution was different in different parts of the lake, with the lowest concentrations generally found at the influent to the lake. No definitive source(s) of contamination could be identified for either metalloids or hydrocarbons.