Central Lake Research Articles

Tuff, Flint, and Hazelnuts: Final Palaeolithic and Mesolithic Occupation at Netherhall Road, Maryport, Cumbria

Evidence for Final Palaeolithic and Mesolithic occupation at Maryport, Cumbria, was discovered during the excavation of Roman occupation features by CFA Archaeology Ltd. A varied lithic assemblage was recovered including worked flint (55%) and tuff (43%), with the rest consisting of a small amount of chert, chalcedony, and rhyolite. Early occupation, probably dating to the Final Palaeolithic Federmesser-Gruppen, is demonstrated through different technological styles among the lithic assemblage. Three phases of activity were identified from cut features and there was a significant amount of charred hazelnut shell, which gave radiocarbon dates centring around 8200 cal BCE. This site provides the first clear evidence that tuff was exploited directly from sources in the Central Lake District, possibly as early as the Final Palaeolithic. The occupation evidence also demonstrates intensive processing of hazelnuts centring around 8200 cal BCE and lasting for 150–558 years. The dates and occupation span are almost identical to those derived from the Mesolithic structure at Cass ny Hawin 2 on the Isle of Man.

Distribution and provenance of heavy minerals from recent sediments of Green Lake, North Brazil, revisited

Geostatistical and multivariate statistical analyses were applied to heavy mineral data from an Amazonian fluvial-lake system near the Tapajós River mouth to investigate the spatial distribution and source-area of sediments. Twenty-one points were investigated, and the physical characteristics of the Green Lake deepest point were determined. Sand accumulates in the lake margins and mud quantity increases towards the lake center. Heavy mineral assemblage is composed of zircon, tourmaline, kyanite, rutile, staurolite, anatase, sillimanite, garnet, and spinel. Tourmaline, staurolite, and spinel are more abundant in the southeast area of the lake, while kyanite is dominant in the north area and zircon is in the whole lake except in its southeast area. Zircon - tourmaline and zircon - staurolite pairs are negatively correlated (r= -0.947 and -0.775, respectively), while tourmaline - staurolite and sillimanite - anatase pairs have a positive correlation (r= 0.628 and 0.675, respectively) which indicate different source rock types. Geostatistical analysis grouped the heavy minerals in three grups: Group 1 (tourmaline – staurolite – spinel - kyanite) and Group 2 (garnet – rutile – sillimanite - anatase) related to metamorphic source rocks ranging from medium to high grade, and Group 3 (zircon) related to acid igneous source rocks. The heavy mineral assemblage of Green Lake is analogous to the assemblage of the Alter do Chão Formation, indicating that this formation is the source of sediments of Green Lake.

Spatiotemporal analysis of multi-pesticide residues in the largest Central European shallow lake, Lake Balaton, and its sub-catchment area

BackgroundThe present study aimed to gain a comprehensive knowledge of the presence and environmental risks of pesticide and repellent residues in Lake Balaton and its sub-catchment area (Hungary). A unique analysis of 439 active substances and 17 metabolites was carried out on surface waters and one effluent wastewater as the only direct discharge into Lake Balaton from June 2017 until August 2020. Altogether 203 water- and 85 sediment samples were collected and analysed during the 3-year monitoring period. To determine the environmental risks of the detected pesticides to aquatic ecosystems, environmental risk assessment (ERA) was carried out using two approaches (worst- and general-case scenarios).ResultsFifty-two pesticides and one insect repellent were detected, of which 26 belonged to herbicides (24 active substances and two metabolites), 15 to fungicides (15 active substances), and 11 to insecticides (eight active substances and three metabolites), of which only nine of the total analysed compounds are listed to be monitored in surface waters with threshold limit values (TLVs). The most frequently detected compounds were terbuthylazine, diethyltoluamide (DEET), desethyl-atrazine, and metolachlor. Glyphosate, aminomethylphosphonic acid (AMPA), and DEET were found with the highest concentrations of 3.0, 2.0, and 1.57 µg/L, respectively. The pesticide exposures were higher during the summer periods indicating a stable seasonal pattern. According to the performed ERA, the calculated Risk Quotients (RQs) indicated 18 compounds with a high level of risk including nine that had been banned for at least a decade.DiscussionThis study expands knowledge on the spatiotemporal occurrence of pesticides in inland surface waters and highlights the need to consider widening the number of analysed pesticides beyond the European Water Framework Directive (EWFD). According to our results, additional authority and legislation procedures should come into force for pesticides not indexed in the priority European Union Watch List.

Environmental risk assessment of floating gardens in Inle Lake, Myanmar

Inle Lake is a valuable wetland for local ecosystems and one of Myanmar's freshwater biodiversity hotspot. In this lake, poor management of floating gardens cultivation by some farmers has resulted in leaching of excess nutrients, and then water and sediment were contaminated by chemical residues. This study aims to assess the environmental risk to Lake Ecosystem by investigating the nutrient contamination of water, sediment and monitoring pesticide residues from fish, sediment along with water. Sediment and Water samples have been gathered from non-GAP and GAP floating gardens, including the central lake as a reference point, during the wet season of 2019 and dry season of 2020. Health risk linked with intake of pesticide residues in fish from floating gardens and central lake are assessed. The result showed that the pesticide and fertilizer applications were increased in non-GAP when compared with GAP method. The nutrients (nitrogen, phosphorus) contamination was higher in non-GAP than in GAP for both water and sediment. The presence of pesticide residues in surface sediment shows that the non-GAP farms had higher concentrations than the GAP farms. For the water pollution assessment, the water quality index value was higher in non-GAP than in GAP. Nutrient pollution assessment indicated that nutrient contamination threatens in floating garden sites but non-GAP has potentially influence on Lake Ecosystem. The health risk assessment model resulted that current used pesticide residues are not caused detrimental effects on human health but in long run, low doses exposure has been linked to human health. This study concludes that poor management of agricultural practices by more agrochemicals usage has had a negative effect on the surface water, ecosystem and sediment of the lake.

Multiproxy constraints on Central Paratethys Sea and Lake Pannon paleoclimate and paleoenvironment transitions during the Middle-Late Miocene (Danube Basin, Slovakia)

The Danube Basin represents a northwestern depocenter of the Middle Miocene Central Paratethys Sea, which was succeed by the Late Miocene Lake Pannon. Although this is an extensively examined area, the application of multidisciplinary studies has proven capable of drawing attention to novel information concerning the depositional environment. Thus, this study aims to reveal climatic and paleoenvironmental changes by using both archive and fresh data drawn from biomarkers, palynology, sedimentology, and geochemistry. The article also addresses the quality, quantity, and thermal maturity of organic matter in relation to hydrocarbon potential. In general, the beginning of Serravallian stage relates to a regression forced by the development of the Antarctic icesheet followed by a pronounced transgression. The results presented here show that the Central Paratethys Sea reacted to the late Badenian (Serravallian) flooding, by an event which triggered a dysoxic, but not euxinic, bottom waters. The climate remained warm and humid, with paratropical to subtropical forests on the continent. Several new depocenters developed during the Sarmatian and divided the depositional environments into a shelf-brake slope in the central part of the Danube Basin, and into deltaic and swamp environments on the basin margins. The climate changed to temperate, leading to the disappearance of subtropical taxa. A shallow lake and swamp environment developed at the beginning of the Pannonian (Tortonian), forcing a salinity decrease connected to the increase in humidity. The degree of hydrocarbon richness of the mudstones is generally fair to good, and the kerogen is of mixed marine-terrestrial origin (II and III). Nonetheless, sediments at the basin margin are clearly immature due to insufficient burial.

Pollen–vegetation relationship based on lake surface sediments of arid and semi-arid northern China; considerations and limitations

In this paper, we investigated pollen assemblages of 92 lake center surface sediments of arid and semi-arid northern China to statistically disclose the pollen–vegetation relationships at the regional scale and explore considerations and limitations for lacustrine pollen-based reconstructions. As a result, the Principal component analysis (PCA), the Indicator species analysis, the Discriminate analysis (DA), the Random forest (RF), and the Supportive vector machine (SVM) generally reveal positive correlations between pollen assemblages and vegetation types or ecotones and suggests capabilities of pollen to represent regional vegetation and climate. However, the DA, RF, and SVM reveal that the samples of desert steppe, typical steppe, and meadow steppe cannot be differentiated by pollen assemblages, suggesting weaknesses to represent regional vegetation and climate. Mental test analyses of pollen assemblages and azonal vegetation of the dune desert and the Gobi desert yield better correlations than that of regional vegetation, suggesting that the pollen of azonal vegetation distort the regional pollen–vegetation relationships and thus weaken the capabilities to represent regional vegetation and climate. Our results present considerations and limitations for pollen-based past vegetation and climate reconstructions and provide references for interpretations of lacustrine fossil pollen records in arid and semi-arid northern China.

Switchgrass cropping systems affect soil carbon and nitrogen and microbial diversity and activity on marginal lands

AbstractSwitchgrass (Panicum virgatum L.), as a dedicated bioenergy crop, can provide cellulosic feedstock for biofuel production while improving or maintaining soil quality. However, comprehensive evaluations of how switchgrass cultivation and nitrogen (N) management impact soil and plant parameters remain incomplete. We conducted field trials in three years (2016–2018) at six locations in the North Central Great Lakes Region to evaluate the effects of cropping systems (switchgrass, restored prairie, undisturbed control) and N rates (0, 56 kg N ha−1 year−1) on biomass yield and soil physicochemical, microbial, and enzymatic parameters. Switchgrass cropping system yielded an aboveground biomass 2.9–3.3 times higher than the other two systems (Jayawardena et al., unpublished data) but our study found that this biomass accumulation did not reduce soil dissolved organic C, total dissolved N (TDN), or bacterial diversity. The annual aboveground biomass removal for bioenergy feedstock, however, reduced soil microbial biomass C (MBC) and microbial biomass N (MBN) and bacterial richness in the second and third years; despite this, continuous monocropping of switchgrass improved soil TDN, inorganic N, bacterial diversity, and shoot biomass in the second and/or third years compared with the first year. N fertilization increased aboveground biomass yield by 1.2 times and significantly increased soil TDN, MBN, and the shoot biomass of switchgrass compared with the unfertilized control. Locations with higher C and N contents and lower C:N ratio had higher aboveground biomass, MBC, MBN, and the activity of BG, CBH, and UREA enzymes; by contrast, locations with higher pH had higher soil TDN and activity of NAG and LAP enzymes. Our research demonstrates that switchgrass cultivation could improve or maintain soil N content and N fertilization can increase plant biomass yield. The comprehensive data also can inform future biogeochemical models to successfully implement switchgrass for bioenergy production.

Solar circulator to restore dissolved oxygen in a hypoxic ice-covered lake

Hypoxia is common to shallow ice-covered lakes during the winter season, and restorative actions to prevent impacts to aquatic ecosystems are desired yet untested in remote settings. The use of a solar photovoltaic circulator was investigated for reoxygenation in a shallow hypoxic lake in the northern Rocky Mountains. During the fall of 2019, a solar powered lake circulator (SolarBee SB10000LH; hereinafter circulator) was installed near the center of Upper Red Rock Lake, Montana USA (latitude 44° 36’N) and dissolved oxygen (DO), temperature, turbidity, and changes to ice formation were monitored until ice-out the following spring of 2020 using an array of real-time and data logging sondes. Observations indicate the circulator formed a polynya that lasted until late November, did not increase lake turbidity, and facilitated oxygen exchange through the circulator-created-polynya for at least 3 weeks after an adjacent lake became ice covered. Thereafter, operation of the solar circulator failed from accumulation of snow and ice on the solar panels such that the lake froze completely over during a period of low light in December. From that point on throughout the winter, DO subsequently declined from supersaturation to hypoxia over a 41-day period and remained that way for nearly four months until ice-out in April. Based on this outcome, additional work is required to improve the solar-powered circulator design before attempting comparable applications elsewhere as a means of reducing the severity of hypoxia in shallow-lake systems during winter.

Lake level changes and paleo-precipitation estimations based on colluvial stratigraphy of Holocene sediments in West Anatolia (Simav Graben)

Time-controlled quantitative climate data are crucial for reconstructing past environmental contexts of human history. In west Anatolia, the Simav Graben used to be occupied by a lake. However, today it is drained and associated with Holocene sediments that record lake level changes. An initial drill-core at the lake's centre reveals an arid phase before ca. 14 ka (i.e. prior to the Late Glacial warming) followed by a wet phase (marshes and lake) throughout the Holocene. Along the lake's margin, two additional sediment sequences contain a detailed Holocene record of changes in humidity throughout the Simav Graben's watershed. Classification of the shoreline facies from these sequences records drought events marked by colluvium prograding towards the lake. Using 13 radiocarbon dates, sedimentary facies suggest rapid lake contraction phases grossly around 8.0, 3.9, 2.9, 2.4 and 2.0 ka. After the drier periods, recorded by phases of terrestrial progradation, lake level increase is marked by retrogradational lake muds and peats onlapping the colluvium, consistent with increases in total lake volume. Based on modern lake morphology, climate and runoff data, and the elevations recorded by the shoreline facies identified within the marginal Holocene sedimentary sequence, we constructed a paleohydrologic balance model and a precipitation curve. According to the model, annual precipitation may have been fallen below 460 mm during the 8.2 ka event, and 468–478 mm during the 4.2 ka and 3.2 ka events – demonstrating an extreme decrease in precipitation compared to the earlier wet phases. In turn, Holocene Archaeology in West Anatolia outlines (i) a widespread abandonment and/or notable fire events at the end of the Early Bronze Age III (ca. 4.2 ka), and (ii) a high level of social instability at the beginning of the Early Iron Age (ca. 3.2 ka). The correlation between the timing of lake level falls at Simav (water volume depletion caused by drying trends) at 8.0 ka, 3.9 ka and 2.8 ka and cultural fluctuations suggest that drought events may have played a major role in these socio-political changes in western Anatolia. Furthermore, the high lake-level stages (associated with precipitation >500 mm) coincide with the Late Chalcolithic to Early Bronze Age I transition as well as the Hellenistic and Roman periods, both times of economic stability and growth in the region.

Sub-Bottom Profiler and Side Scan Sonar investigations, with the assistance of hydrochemical and isotopic analysis of Sawa Lake, Al-Muthana Governorate, Southern Iraq

A marine geophysical survey are including Sub Bottom Profilers and Side Scan Sonar integrating with under-water imagery, as well as hydrochemical and isotopic investigations have been carried out for Sawa Lake, one of the unique, shallow and closed water body in Iraq. The results showed that the depth of the Lake is shallow, ranged between 1 m in areas near the edges of the Lake and 2 m in the Lake center. The Recharge site has been detected exactly (this is a first recording (Nov. 2012)), as a hole. It has a longitudinal an axis of 50 m with NW-SE tren, which resembles the trending of the longitudinal axis of the Lake and Abu-Jir fault trending. The maximum width of this hole ranging from 23.0-29.5 m. The hole depth about 20 m, but may be greater then, because the emergence of a high proportion of noise in the geophysical sections. The Lake bottom is homogeneity and does not has any diacritical features. Also, the results showed discontinuous layers on either side of the hole due to the existance a number of caves under the bottom of the Lake extends into several meters with different dimensions and forms. Hydrochemical analysis of the water samples collected from different depths inside the hole by scientific diving with specefic equibments and the result showed that the water is NaCl type, with domination of Mg and Cl over Ca and SO4. The Stiff diagram, detected that very high TDS arising mainly from sodium and chloride, it is mostly typical marine water. Values of δ18O and δ2H in water samples of Sawa Lake, when plotted along the global meteoric water line shows that the three collected water samples from the lake, which have a similar isotopic composition, indicating that all of them are receiving water from the same flow zones, with diffuse recharge type. Also, the isotopic parameter showed that the Sawa lake water suffering from strong evaporation.

Rainfall-Induced or Lake-Water-Level-Controlled Landslide? An Example from the Steep Slopes of Lake Balaton, Hungary

The present paper provides a case study and an overview of the stability of high banks bordering the eastern basin of the largest central European lake, Lake Balaton, in Hungary. External factors such as rain, lake-water level, or pore water pressure are considered in the stability analysis. The slope stability was assessed using two software with different approximations and calculation methods. The results of the models using Plaxis and Rocscience Slide2 are compared. One of the major identified failure scenarios is the deep-seating rotational landslides that are triggered by rain and elevated groundwater table. The modeled slip surfaces are in good agreement with the inclinometer monitoring data of the site; meanwhile, the top of the slope failures is not influenced by water level.

Assessing the inundation dynamics and its impacts on habitat suitability in Poyang Lake based on integrating Landsat and MODIS observations

Poyang Lake, the largest freshwater lake in China, serves critical ecosystem function for water regulation and biodiversity conservation. However, it experienced dramatic changes in lake inundation due to recent climate change and human activities, causing ecological and economic problems. Here, we applied a multiple-index water detection rule to integrated Landsat and MODIS products to reconstruct surface water series at 30-m and 8-day resolutions and quantified the spatio-temporal inundation dynamics in Poyang Lake over the past 20 years (2000–2019). Furthermore, their influences on habitat suitability for herbivorous birds were also assessed from the perspectives of hydroperiod and vegetation growth threshold. The significant declining trend (−26.66 km2 yr−1, p < 0.001) for the annual minimum water areas throughout the 20 years implied that Poyang Lake was undergoing a continuous shrinkage in the low-water season. On the monthly scale, inundation frequency (IF) decreases were more significant in September–January, most of which occurred in the alluvial delta zones near the lake center. The altered water regime after the Three Gorges Dam (TGD) might be the dominant contributor responsible for the continuous lake shrinkage during the recent low-water period. The sub-lakes suffered from spring drought rather than winter drought, triggering vegetation successions regarding the reversed trend of the well-documented xerophilization in Poyang Lake. The recent earlier and prolonged dry seasons caused an increase of suitable habitat for herbivorous birds (13.92 km2 year−1, p < 0.1), but triggered a potential risk of food quality degradation when the migratory waterbirds peaked in December. These results provide a clear reference for optimizing the hydrologic management and biodiversity conservation of Poyang Lake.

Geoacoustic inversion for gassy sediment parameters using reflection and scattering of acoustic signals

Results of experimental study and theoretical modeling of acoustic propagation and reverberation in Lake Kinneret (Israel) with gassy sediments are presented. The presence of methane bubbles in sediments significantly influences reflection and scattering of sound signals from the bottom, which in turn allows estimating properties of sediment using acoustic sensing. Experiments were carried out using R/V Hermona with sound source located at a 7 m depth radiating 1 s-long 0.3–7 kHz LFM sweeps with intervals from 1 to 20 s. Acoustic pressure time series were received on a single hydrophone at ∼1 m from the source and two vertical arrays fixed in the lake center at either 40 m or 10 m from each other. Analysis of both monostatic and bistatic experimental data (including long range propagation) was aimed to estimate acoustic characteristics of the bottom and then to infer the related gas content and its spatial and temporal variability. The sound speed in sediments at different locations (with different depth of water layer, maximal depth 40 m) was estimated to be ∼170–250 m/s that corresponds to gas volume concentrations ∼1%, which is in accordance with direct measurements made using non-acoustic probes. [Work was supported by BSF grant 2018150.]

Review: Life-Destroying Diagrams, by Eugenie Brinkema

Review: <i>Life-Destroying Diagrams</i>, by Eugenie Brinkema

Central Mongolian lake sediments reveal new insights on climate change and equestrian empires in the Eastern Steppes

The repeated expansion of East Asian steppe cultures was a key driver of Eurasian history, forging new social, economic, and biological links across the continent. Climate has been suggested as important driver of these poorly understood cultural expansions, but paleoclimate records from the Mongolian Plateau often suffer from poor age control or ambiguous proxy interpretation. Here, we use a combination of geochemical analyses and comprehensive radiocarbon dating to establish the first robust and detailed record of paleohydrological conditions for Lake Telmen, Mongolia, covering the past ~ 4000 years. Our record shows that humid conditions coincided with solar minima, and hydrological modeling confirms the high sensitivity of the lake to paleoclimate changes. Careful comparisons with archaeological and historical records suggest that in the vast semi-arid grasslands of eastern Eurasia, solar minima led to reduced temperatures, less evaporation, and high biomass production, expanding the power base for pastoral economies and horse cavalry. Our findings suggest a crucial link between temperature dynamics in the Eastern Steppe and key social developments, such as the emergence of pastoral empires, and fuel concerns that global warming enhances water scarcity in the semi-arid regions of interior Eurasia.

Assessing Spatial and Temporal Distribution of Algal Blooms Using Gini Coefficient and Lorenz Asymmetry Coefficient

Algal bloom in an inland lake is characterized by significant spatial and temporal dynamics. Accurate assessment of algal bloom distribution and dynamics is highly required for tracing the causes of and creating countermeasures for algal bloom. Satellite remote sensing provides a fast and efficient way to capture algal bloom distribution at a large scale, but it is difficult to directly derive accurate and quantitative assessment based on satellite images. In this study, the Gini coefficient and Lorenz asymmetry coefficient were introduced to examine the spatio-temporal algal bloom distribution of Chaohu Lake, the fifth largest inland lake in China. A total of 61 remote sensing images from three satellite sensors, Landsat, Gaofen, and Sentinel were selected to obtain algal bloom distributions. By dividing remote sensing images into 0.01°*0.01° grid cells, the normalized difference vegetation index (NDVI) for each grid cell was derived, forming a spatial and time series database for quantitative analysis. Two coefficients, Gini coefficient and Lorenz asymmetry coefficient, were used to evaluate the overall intensity, unevenness, and attribution of algal bloom in Chaohu Lake from 2011 to 2020. The Gini coefficient results show a large variety of algal bloom in the spatial and temporal scales of Chaohu lake. The lake edge and northwestern part had longer lasting and more severe algal bloom than the lake center, which was mainly due to nutrient import, especially from three northwestern tributaries that flow through the upstream city. The Lorenz asymmetry coefficient revealed the exact source of the unevenness. Spatial uncertainties were mostly caused by the tiny areas with high NDVI values, accounting for 53 cases out of 61 cases. Temporal unevenness in northwestern and northeastern parts of the lake was due to the most severe breakout occurrences, while unevenness in the lake center was mainly due to the large number of light occurrences. Finally, the advantage of Gini coefficient and Lorenz asymmetry coefficient are discussed by comparison with traditional statistical coefficients. By incorporating the two coefficients, this paper provides a quantitative and comprehensive assessment method for the spatial and temporal distribution of algal bloom.

Environmental stressors alter multiple determinants of individual reproductive output in the acid‐tolerant mayfly Leptophlebia vespertina

Abstract1. Environmental stressors such as acidification modify community composition in freshwater habitats through their direct and indirect effects on individuals. However, the effect of acidification on various aspects of individual fitness is poorly known in aquatic insects, a key group in many freshwater habitats.2. Here, the influence of acidification (acidity and Al toxicity), food quality and habitat properties on the reproduction of an acid‐tolerant mayfly Leptophlebia vespertina (Linnaeus, 1758) in a group of Central European lakes is explored. The focus is on last‐instar larvae as the short‐lived, semelparous adults cannot mitigate stress experienced by the larvae.3. It is shown that the environment affects multiple determinants of individual reproductive output both directly by affecting body size and fecundity in last‐instar larvae, and indirectly by releasing the larvae from density dependence. Populations tended to be denser in more acidic sites and individuals in denser populations were substantially smaller and had lower size‐dependent fecundity and reproductive effort. All else being equal, larvae from colder sites were larger, suggesting an important role of the temperature‐size rule in this species. Size‐dependent fecundity and reproductive effort increased with better food conditions as expected, but neither reproductive measure was affected by acidity or temperature. Finally, a weak egg size‐number trade‐off with slightly smaller eggs in more fecund females was detected.4. These results imply that indirect ecological feedbacks and food quality, rather than the direct effects of stressful environment, may dominate the effects of environmental stressors on the reproductive output of acid‐tolerant species such as L. vespertina.

Impacts of a proposed water control project on the inundation regime in China's largest freshwater lake (Poyang Lake): Quantification and ecological implications

Study regionPoyang Lake, the largest freshwater lake in China, is connected to the Yangtze River and has been undergoing shrinkage over the past decade. Study focusThe Poyang Lake water control project (PLWCP) was proposed to protect the lake against rapid shrinkage. However, the quantitative impact of the project on the floodplain inundation regime is not yet clear. This study used a hydrodynamic model to quantify the influence of the PLWCP on the inundation regime of upstream floodplains in Poyang Lake. New hydrological insights for the regionPoyang Lake was characterised by high spatiotemporal heterogeneity in inundation patterns and that the northern lake areas were most affected, followed by the central and eastern lake areas. The western and southern lake areas were less affected. The water levels of the northern channels increased by a maximum of 4–5 m during the first recession period (16 Sep–31 Oct) and increased by 1–2 m during the impounding period (1 Sep–15 Sep). Most of the lake areas except the northern channels showed a water level increase of less than 0.5 m from November to March. The increased inundation areas were mostly 100–400 km2 (<20%) and reached greater than 800 km2 (~30%) from September to October, of which over 300 km2 (10%) lasted approximately 2 months. Potential suitable habitats in the northern, central and eastern lakes might be impacted.

Response of planktonic diatoms to eutrophication in Nyanza Gulf of Lake Victoria, Kenya

In recent decades, the planktonic diatom community in central Lake Victoria changed from dominance of Aulacoseira species (mainly African endemic A. agassizii, A. nyassensis) to dominance of long needle shaped Nitzschia (often referred to as N. acicularis). This shift was referred to be caused by eutrophication, resulting in increased growth of diatom taxa with reduced requirement of silica in open water. In contrast, in the large shallow Nyanza Gulf A. granulata and A. granulata var. angustissima remained abundant, probably because of increased turbidity and nutrient availability. In this study, from July 2017 to July 2018, the planktonic diatom community composition was investigated monthly at three sampling stations in Nyanza Gulf, i.e. West Gulf (WG, located close to the open basin via Mbita channel), Mid Gulf (MG, central part of the Gulf) and East Gulf (EG, located most closely to Kisumu bay). A significant gradient in water quality conditions was observed along distance of 60 km from the main basin, i.e. the water transparency decreased from 0.8–1.2 m to 0.2–0.5 m while suspended solids increased from 3.3–9.8 mg/L to 12–63.8 mg/L. In total 19 common diatom morphospecies were documented and related to the gradient in trophic state. At WG, A. granulata and A. granulata var. angustissima formed diatom blooms or co-occurred with long needle shaped Nitzschia species such as N. kavirondoensis, N. lacustris, N. nyassensis, N. rusingae, Synedra cunningtonii, as well as Cyclostephanos malawiensis, C. damasii and Urosolenia victoriae indicating more oligotrophic conditions closer to the main basin. In the inner Gulf A. granulata (var. angustissima) and N. cf. fonticola var. pelagica possibly living epiphytically on the colony-forming cyanobacterium Microcystis aeruginosa dominated. In contrast to N. cf. fonticola var. pelagica, however, Aulacoseira spp. biovolume was found negatively related to Microcystis biovolume implying its suppression under Microcystis bloom conditions.

A modelling framework to track phosphorus sources of the drinking water intakes in a large eutrophic lake

Excess phosphorus (P) in freshwater lakes is a global challenge due to its potential cause of harmful algal blooms threating drinking water safety. However, quantifying the P sources for a specific site in a large lake is extremely challenging due to the complex interaction between internal and external P loading to surface water. To address this challenge, this study developed a modelling framework to track P sources of two drinking water intakes in a large shallow lake (Lake Taihu) in China. The framework proposed a new-developed index (PSCI, P source contribution index) to quantify the contributions of various P sources for the drinking water intakes. PSCI was derived by a three-dimensional hydrodynamic and water quality model that describes the complex processes of P sources, sinks and transportation in both horizontal and vertical directions within the lake. Application of the framework to these two drinking water intakes (Jinshu and Shangshan) achieved a surprising finding that internal P loading from sediment was a significant P source with a contribution as high as 47.1% and 30.4%, respectively. Central Lake Taihu had a large contribution of 49.5% and 68.3% for Jinshu and Shangshan, and inflow river (Wangyu River) had a contribution of < 5%. The high contribution of internal P loading was mainly due to the sediment P accumulation for years and large external P loading. Our study highlighted the important role of internal P loading affecting the P concentration of drinking water intake in a large eutrophic lake, and demonstrated the high value of the modelling framework in quantifying the P sources for a specific site (e.g., drinking water intake) in a lake.