3D-modelling of Lake Kivu: Horizontal and vertical flow and temperature structure under spatially variable atmospheric forcing

With the increasing extraction of methane from Lake Kivu, there is a growing need to evaluate the effect of such operations on the lake’s permanent density stratification. This requires understanding of the spatial structure and variability of flow velocities and constituents in Lake Kivu. In this study, we develop a 3D hydrodynamic model of Lake Kivu, set-up within DELFT3D at a 750 m grid spacing and forced by COSMO-CLM atmosphere model results at a 2.8 km grid spacing. Validation shows that the model correctly reproduces the generation and breakdown of the temperature stratification in the upper mixed layer and predicts flow velocity magnitudes and directions similar to measurements both at the surface and at greater depth. Analysis of currents reveals a surface current pattern with two clockwise circulations, one around the whole lake and a smaller one in the northern part, with velocities around 0.1 m/s. This pattern is consistently present over an (ensemble-)averaged day, both in the wet and in the dry season, while day-by-day variations are large. Time-averaged deep currents are found to be a few mm/s at maximum. However, the variations can be substantial, with standard deviations up to 2 cm/s for the currents at 220 m depth, attributed to internal seiches. The temperature stratification, present during the entire wet season, is found to first break down in the dry season in the southern part of the lake. This is explained by the spatial differences in the wind stress and the evaporation heat fluxes during the dry season.     

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。     

Temporal dynamics and drivers of lake ecosystem metabolism using high resolution observations in a shallow,tropical, eutrophic lake (Laguna Lake,Philippines)

The physical, chemical, and biological dynamics under changing atmospheric conditions of Laguna Lake, Philippines were analyzed from intensive observations. Diel measurements were conducted for 48 continuous hours for both dry and wet tropical seasons in addition to fine resolution long-term monitoring. Results revealed significant vertical and diel variations of lake variables in spite of a shallow water depth (2.5 m) caused by the intense surface heating from solar irradiance (~800 W m^?2) and accentuated by the lake's high turbidity (16–32 ftu). Late afternoon land-lake breeze (~5–7 m s^?1) regularly breaks daytime thermal stratification, and convective cooling at night maintains isothermal condition until dawn of the next day. The stratified condition demonstrated a wind-driven, density-induced 2-layer current pattern with a windward moving epilimnion (~4 folds faster) and a compensating hypolimnetic flow in the general lake circulation direction. Laguna Lake was observed to have a dominating diel cycle but also undergoes significant seasonal limnological variations brought primarily by climate, hydrology, and its interaction with the adjacent sea. Significantly correlated variations of pH, chlorophyll-a and DO in the dry season were indicative of the higher biological activity associated with the intrusion of polluted waters from Metro Manila. The non-occurrence of thermal over-turn was observed to be regularly followed by bottom hypoxic conditions (2–4 mg L^?1), indicative of the eutrophic condition of the lake and the importance of diel wind-induced mixing in the bottom supply of DO. Laguna Lake was found to be predominantly net heterotrophic (GPP:R < 1, NEP < 0).     

Diversity and ecology of phytoplankton in Lake Edward (East Africa): Present status and long-term changes

Lake Edward is one of the African Rift Valley lakes draining into the Nile River basin. We conducted three sampling series in Lake Edward in October-November 2016, March-April 2017 and January 2018, in distinct seasonal conditions and in several sites varying by depth and proximity to river outlets, including the Kazinga Channel, which connects the hypertrophic Lake George to Lake Edward. The phytoplankton was examined using microscopy and marker pigment analysis by high performance liquid chromatography (HPLC) and subsequent CHEMTAX processing for estimating abundance of phytoplankton groups. Chlorophyll a concentration in the pelagic and littoral open lake sites barely exceeded 10 µg L⁻¹ whereas, in contrast, in the semi-enclosed Bay of Katwe influenced by the Kazinga Channel chlorophyll a was up to 100 µg L⁻¹. Despite substantial seasonal variations of limnological conditions such as photic and mixed layer depths, cyanoprokaryotes/cyanobacteria represented on average 60% of the phytoplankton biomass, followed by diatoms, which contributed ~25% of chlorophyll a, and by green algae, chrysophytes and cryptophytes. 248 taxa were identified with clear prevalence of cyanobacteria (104 taxa), from the morphological groups of coccal and filamentous species (non-heterocytous and heterocytous). The high proportion of heterocytous cyanobacteria, along with a relatively high particulate organic carbon to nitrogen (C:N) ratio, suggest N limitation as well as light limitation, most pronounced in the pelagic sites. During the rainy season, the most abundant diatoms in the plankton were needle-like Nitzschia. Comparison with previous studies found differences in water transparency, total phosphorus, and phytoplankton composition.     

白马湖退圩还湖方案及效果浅析

根据白马湖圈圩现状和退圩还湖规划标准、目标、建设规模,提出白马湖退圩还湖4个方案,通过各方案水位—库容曲线、入湖流量过程设计、出湖流量过程计算,并进行调洪演算,分析不同方案的防洪滞涝效果,提出了淮安市白马湖近期退圩还湖建议实施方案。     

洞里萨湖水位与面积和容积的关系研究

洞里萨湖是东南亚最大的且颇具国际影响的淡水湖。基于洞里萨湖湖区不同来源的地形资料,构建了不规则三角网TIN,量算得到不同水位下具有0.1m梯度的洞里萨湖面积和容积。对比分析和水文学法复核结果表明本次构建的洞里萨湖水位与面积和容积的关系是合理的,描绘出了洞里萨湖"低水似湖、高水湖相"的自然景观特点。以洞里萨湖甘邦隆站和出湖控制站波雷格丹站为代表,建立了洞里萨湖水位-面积(容积)关系与实测湖水位的响应关系。结果表明洞里萨湖面积、容积与甘邦隆站水位呈非线性一一对应关系,与波雷格丹站水位成绳套关系,绳套两侧分别对应汛期湄公河向洞里萨湖倒灌和汛后洞里萨湖向湄公河补水。基于河湖关系构建了不同时期不同河湖水位差下的洞里萨湖面积、容积与波雷格丹站水位的相关关系,该精细化的洞里萨湖水位-面积(容积)量算成果可为防汛抗旱精准预报提供科技支撑。     

Phytoplankton biomass and primary production dynamics in Lake Kariba

The present study examined spatial, seasonal and depth variations in phytoplankton biomass and primary production (PP), compared with those reported for other tropical African lakes, to determine whether or not measured phytoplankton changes might be linked to climate warming. The biomass of three major phytoplankton classes (Cyanophyceae; Chlorophyceae; Bacillariophyceae) and net PP were measured during the midwinter and midsummer at six different depths at 35 sampling sites distributed across the lake’s five basins. A more rigorous sampling regime was used in the fifth basin, with phytoplankton biomass and PP rates measured every second month over a 24 month period at six different depths at ten sampling sites located in riverine and lacustrine waters. Cyanophyceae, which displayed a gradient of decreasing biomass from Basins 2 to 5, contributed 69% of the total phytoplankton biomass in the lake’s five basins during summer. This percentage was approximately four times greater than that contributed by the Bacillariophyceae and about ten times greater than that contributed by the Chlorophyceae. During winter, Bacillariophyceae biomass was equivalent to that of the Cyanophyceae, but displayed an opposing gradient of increasing biomass from Basins 1 to 3, with a subsequent biomass decline from Basins 3 to 5. Chlorophyceae exhibited no distinct biomass gradient across the five lake basins, being undetectable during winter. The biomass of all three phytoplankton classes and the net PP varied in magnitude and direction monthly between the lacustrine and riverine waters, with increasing water depth and with no distinct seasonal patterns being evident. The monthly variations in biomass were related to the thermal stratification cycle, hydrological gradients and the extent of water mixing, being similar to those reported for other tropical African lakes. It is noteworthy that total phytoplankton biomass and PP in Lake Kariba have declined by about 95% and 50%, respectively, since the 1980s. These declines correspond to an upward shift in the depth of the thermocline, associated with an average temperature increase of 1.9 °C and a 50% reduction in the depth of the euphotic zone, since the 1960s.     

Genetic assessment of straying rates of wild and hatchery reared lake sturgeon (Acipenser fulvescens) in Lake Superior tributaries

Natal philopatry in lake sturgeon (Acipenser fulvescens) has been hypothesized to be an important factor that has lead to genetically distinct Great Lakes populations. Due to declining abundance, population extirpation, and restricted distribution, hatchery supplementation is being used to augment natural recruitment and to reestablish populations. If hatchery-reared lake sturgeon are more likely to stray than naturally produced individuals, as documented in other well-studied species, outbreeding could potentially jeopardize beneficial site-specific phenotypic and genotypic adaptations. From 1983 to 1994, lake sturgeon propagated using eggs taken from Lake Winnebago adults (Lake Michigan basin) were released in the St. Louis River estuary in western Lake Superior. Our objective was to determine whether these introduced individuals have strayed into annual spawning runs in the Sturgeon River, Michigan. Additionally, we estimated a natural migration rate between the Sturgeon River and Bad River, Wisconsin populations. Presumed primiparous lake sturgeon sampled during Sturgeon River spawning runs from 2003 to 2008 were genotyped at 12 microsatellite loci. Genotypic baselines established for the Sturgeon River (n = 101), Bad River (n = 40), and Lake Winnebago river system (n = 73) revealed a relatively high level of genetic divergence among populations (mean F_ST = 0.103; mean R_ST = 0.124). Likelihood-based assignment tests indicated no straying of stocked Lake Winnebago strain lake sturgeon from the St. Louis River into the Sturgeon River spawning population. One presumed primiparous Sturgeon River individual likely originated from the Bad River population. Four first-generation migrants were detected in the Sturgeon River baseline, indicating an estimated 3.5% natural migration rate for the system.     

Application of a Depletion-Based Stock Reduction Analysis (DB-SRA) to lake sturgeon in Lake Erie

Lake Erie supported the greatest yield of lake sturgeon within the Laurentian Great Lakes near the end of the 19th century with >2000 metric tons caught at the peak of the fishery. The fishery collapsed by the 1920s when <1% of the previous peak catch was removed. Despite closures of the fishery, lake sturgeon remain rare in Lake Erie. We applied a depletion-based stock reduction analysis (DB-SRA) to the catch of lake sturgeon from 1879 to 1929 to gain estimates of sustainable fishery reference points and the historic carrying capacity of Lake Erie for lake sturgeon. We also simulated population growth of lake sturgeon from 1929 to the present with varying assumptions of the current carrying capacity of the lake. The estimated historic carrying capacity of lake sturgeon was 22,652 metric tons. During the height of the fishery, exploitation was as high as 37% which was more than an order of magnitude greater than that required for maximum sustainable yield. Projections of the population from 1929 to 2016 suggest sufficient time has passed since the collapse of the fishery that the population should have recovered to levels that would support a fishery at maximum sustainable yield. However, lake sturgeon remain rare in Lake Erie indicating that other factors such as habitat availability may be limiting their recovery. Our estimates of carrying capacity will be informative when setting recovery targets which consider the amount of habitat loss.     

太湖蓝藻水华和湖泛应急防控能力提升对策研究

本文通过回顾江苏省水利部门2007年以来太湖蓝藻水华及湖泛防控工作进展,总结了太湖蓝藻预警、打捞处置、藻泥利用以及规范化管理等方面的经验做法。同时,分析了现阶段太湖安全度夏面临的蓝藻生物量居高不下、应急防控成本增加、监管能力不足等难点问题,提出了加快骨干引排工程建设、提升蓝藻水华防控能力、加快构建信息化监管平台,以及健全应急防控标准体系等工作建议,为提升太湖应急防控工作水平提供技术支撑。     

鄱阳湖流域河水、湖水及地下水同位素特征分析

利用鄱阳湖流域同位素按月份呈周期性变化的特征,将氘氧同位素关系作为示踪剂,研究降水入渗及地下水、河水与湖水之间的转化关系。结果表明:4月中旬湖水、河水、井水的δD、δ18O的均值分别为(-2.32%,-0.42%)、(-2.86%,-0.48%)和(-3.18%,-0.55%),δ18O~δD关系点都落在全球雨水线GMWL上;湖水最为富集,湖水的补给除河水之外,湖区及周边的降水也占一定比例。7月下旬湖水、河水与井水的δD、δ18O值明显比4月中旬的值贫化,与降水同位素变化趋势相一致,经历干旱高温天气后,湖水、河水与井水的δ18O~δD关系点落在蒸发线(EL2)上。同位素数据表明,井水与河水之间的转化性强,大量降水入渗地下成为潜水,通过地下径流补给河流,降水转化为河水的周期大约经历了1个月。3—6月为雨季,鄱阳湖对长江水的补给较大,影响到长江水中的同位素。     

Evidence of lake trout (Salvelinus namaycush) natural reproduction in Lake Erie

Native lake trout were extirpated from Lake Erie around 1965 and committed restoration efforts began in 1982. In 2021 and 2022, a total of six lake trout (Salvelinus namaycush) in the free embryo or post-embryo life stage were captured in lake trout embryo traps in Lake Erie offshore of Shorehaven Reef, NY. This represents the first conclusive evidence of successful natural reproduction since extirpation. Trapping locations were identified using the results of a fine-scale positioning acoustic telemetry array, visual observations of adult lake trout exhibiting spawning behavior, and underwater cameras to visually identify possible spawning locations. Lake trout utilized a very specific spawning habitat type—the eastern side of shallow offshore humps in 5–8 m of water. These sites were comprised of habitat typically associated with lake trout spawning with slopes of 5–14° and clean rubble-cobble sized rock with visible interstitial spaces. Genetic barcoding was used to identify the post-embryo stage salmonids to species, and microsatellite genotypes assigned strongly to the Seneca strain which comprises the majority of the adult population. These findings represent a significant milestone for lake trout rehabilitation efforts in Lake Erie, confirming that successful reproduction to the post-embryo stage is possible and supporting continued rehabilitation efforts by Lake Erie management agencies.     

DYNHYD与WASP模型在复杂河网区的应用——以湖南省大通湖垸河湖连通为例

通过对湖南大通湖垸进行现场调查并结合河湖连通规划,针对自草尾河引水入东洞庭湖的连通方案,建立了DYNHYD与WASP水质耦合模型。模型参数确定后,模拟了在枯水典型水文年水位条件下,调水后的大通湖及各主要河渠总磷的浓度变化情况。分析模拟结果表明,大通湖达到预期Ⅲ类水标准所需的时间为135d,各河渠中总磷浓度达到预期标准所需的时间明显小于大通湖。分析方法和过程可为DYNHYD与WASP水质耦合模型在南方复杂水网地区河湖连通中的实际应用提供科学参考和实际指导。     

Spatial variations in nutrients and other physicochemical variables in the topographically closed Lake Baringo freshwater basin (Kenya)

Spatial physicochemical parameters were determined from 39 sampling sites distributed throughout Lake Baringo during December 2010. Mean values of temperature, dissolved oxygen concentration and electrical conductivity decreased successively with depth, while the pH remained constant. Only the turbidity values increased marginally with depth. Of the surface water parameters, mean (range) values of dissolved oxygen (DO), pH, electrical conductivity, water transparency and turbidity were 6.9 (4.5–8.4) mg L^?1, 8.3 (7.8–8.5), 573 (556–601)μS cm^?1, 33 (28–37) cm and 43.3 (32.7–54.6) NTU, respectively. Mean and range values of total nitrogen (TN), nitrate‐nitrogen(NO₃‐N), ammonia nitrogen (NH₄‐N), total phosphorus (TP) and soluble reactive phosphorus (SRP) were 788.4 (278–4486) μg L^?1, 4.5 (2.4–10.0) μg L^?1, 42.6 (33.8–56.3) μg L^?1, 102.9 (20.3–585.3) μg L^?1 and 23.5 (15.2–30.5) μg L^?1, respectively. Dissolved silica concentrations ranged from 19.7 to 32.7 mg L^?1, with a mean value of 24.7 mg L^?1. The chlorophyll‐a concentrations were quite low, ranging from 1.4 to 4.9 μg L^?1, with a mean value of 4.2 μg L^?1. In contrast to previous reported values, a key finding in the present study is a relatively high water transparency, indicating a relatively clear water column, due possibly to the fact that the sampling was conducted during the dry period. The nutrient levels remained low, and the chlorophyll‐a concentration also was an almost all time low value. A TP value of 20 μg L^?1 and higher confirms strongly eutrophic conditions prevailing in the lake, with an extremely low potential for fish production and low species diversity, consistent with other studies. The results of the present study, therefore, reinforce the database for future management and monitoring plans for the Lake Baringo ecosystem, which lies adjacent to known geothermally active zones and a saline Lake Bogoria.     

Temporal and spatial variability of PCB concentrations in lake trout (Salvelinus namaycush) in Lake Superior from 1995 to 2016

Production of polychlorinated biphenyl compounds (PCBs) was banned over 40 years ago in the United States and Canada, but they remain a dominant contaminant in Great Lakes fish. Although PCB concentrations in Great Lakes fish have declined since the 1980s, there is concern that the rate of decline has slowed in recent years. Canadian and U.S. national agencies monitor trends in fish contaminants at only a few sites in each lake, while states and provinces monitor fish at more locations. In this study, we compare fish PCB measurements made by five agencies at multiple locations in order to evaluate spatial as well as temporal patterns in Lake Superior. For several monitoring locations, rapid increases and decreases in concentrations were observed. The wide range of concentrations (up to 1000 ng/g) reported among all stations in any single year is unlikely to result solely from differences in fish preparation or analytical techniques. Recent measurements indicate that spatial variation in fish PCB concentrations exists with peak concentrations 30-fold higher than lowest concentrations. After 1995, statistically significant changes, all declines, in PCB concentrations were observed at only three of seven locations; half-lives in these locations ranged from 9 to 14 years. Differences in diet and food web structure likely contribute both to the spatial variability of concentrations within the lake as well as to the rapid, short-term changes in concentrations at single sites that make long-term trends difficult to discern and to predict.     

层次分析法在衡水湖环境功能区划方案遴选中的应用

通过对环境、社会和技术经济的综合分析,利用层次分析法对衡水湖环境功能区划备选方案进行遴选,确定了最佳方案为:东洼蓄当地水,西洼蓄引江水,不建中隔堤,提高了评价结果的直观性和可操作性。     

Differences in pyloric caeca counts between lean and siscowet lake trout (Salvelinus namaycush) in southern Lake Superior

There are four documented morphotypes of lake trout (Salvelinus namaycush) in Lake Superior, with the two dominant forms being the shallow water lean and the deep water siscowet. These morphotypes are differentiated externally by morphometrics and meristics. Pyloric caeca counts have been used to distinguish closely related fish species including the Salvelinus genus. From samples collected in 2009, 2011–2013, and 2017, we counted and measured pyloric caeca from 116 lean and 119 siscowet lake trout from southern Lake Superior. For a subset of 22 leans and 19 siscowets between 575 and 625 mm, we measured individual pyloric caecum basal diameter, length, and estimated individual caecum and total caeca surface areas. Siscowets had significantly fewer and thicker pyloric caeca than leans, but caecum length did not differ between the morphotypes. Mean pyloric caeca count for siscowets and leans was 131 and 153, respectively. Mean individual caecum surface area was 31% higher in siscowets than in leans. When adjusted for mean total number of pyloric caeca, total caecum surface area for siscowets was 12% greater than leans. We postulate that greater pyloric caecum surface area in siscowets may be an adaption for greater lipid uptake because they have substantially higher lipid content than leans. Based on our findings, pyloric caeca counts can be used in addition to other meristic and morphometric characteristics to help distinguish lean and siscowet lake trout.     

Recolonization of lake whitefish river spawning ecotypes and estimates of riverine larval production in Green Bay,Lake Michigan

Lake whitefish (Coregonus clupeaformis) within the waters of Green Bay, Lake Michigan have recently shown a substantial increase in abundance. Furthermore, after over 100 years of extirpation, adult lake whitefish are found spawning within major Wisconsin tributaries to Green Bay. Many knowledge gaps still exist with respect to the chronology of adult river migrations, including the physical characteristics of upstream habitats selected for reproduction and the extent of larval production by these riverine ecotypes. Here, we use hydroacoustic imaging along with egg and larval surveys to evaluate this novel riverine spawning in 2017 and 2018. Highest abundance of adults was observed in the month of November as temperatures declined below 8 °C. Spawning areas consisted of cobble substrates, and site-specific fish densities were primarily correlated with river flows between 0.3 and 1.0 m/s, with specific values varying by tributary and year. Locations of egg deposition mirrored areas of high observed fish densities. Larval production was documented on each tributary using active trawl ichthyoplankton sampling, and larvae were observed outmigrating to open water environments. We estimated tributaries produced 452,000 larvae in 2017 and 721,000 larvae in 2018. To our knowledge, this represents the first documentation of successful lake whitefish larval production from Green Bay tributaries and suggests tributary spawning populations contribute to the greater abundance of lake whitefish observed in recent years.     

Sediment geochemistry and contributions to carbon and nutrient cycling in a deep meromictic tropical lake: Lake Malawi (East Africa)

The biogeochemical functioning of large tropical lakes differs substantially from temperate lakes, yet remains poorly understood. We characterized the carbon, nitrogen, and phosphorus cycling in the water column and sediments of a deep meromictic tropical Lake Malawi (East Africa) by measuring geochemical distributions and compiling whole-lake geochemical budgets. Four locations (100 to 650 m water depth) were characterized. The results reveal that sediments contribute significantly to lake-wide biogeochemical budgets. Sedimentation rates have significantly increased in recent decades. While the export efficiency of organic matter from photic zone to deep sediments is low (14%), organic carbon is buried in the anoxic sediments with high efficiency (27–46%). Area-specific rates of carbon mineralization (4.1 mmol m^? 2 d^? 1) are similar to those in temperate well-oxygenated large lakes and marine sediments in similar water depths. Ammonium effluxes from sediments (0.44 mmol m^? 2 d^? 1) contribute 29% to the total nitrogen inputs into the water column, while sediment denitrification (0.035 mmol m^? 2 d^? 1) and burial of organic nitrogen (0.27 mmol m^? 2 d^? 1) remove 28% of total inputs in the lake. The recycling efficiency of phosphorus in anoxic sediments is high (73%). P effluxes average 0.037 mmol m^? 2 d^? 1, suggesting a large and previously unquantified contribution (42%) to water column P inputs. The results underscore the importance of sediments in the geochemical budgets of even large lakes and suggest trends in lacustrine carbon cycling that hold across a wide range of environments.     

Seasonal patterns of thermal stratification and primary production in the northern parts of Lake Tanganyika

Thermal stratification in meromictic Lake Tanganyika weakens during the cool, dry and windy season, allowing intrusions of deeper nutrient-rich waters into the upper mixed layer enhancing primary productivity. The current study examined the seasonal influence of thermal structure on the patterns of primary production in Lake Tanganyika. Two sites (Kigoma Bay and Mahale) were sampled on a monthly basis for 1?year. Water temperature and chlorophyll a fluorescence profiles (0–100?m) were measured using a multisonde. Water samples were taken every 20?m (0–100?m) to measure soluble reactive phosphorus (SRP), chlorophyll a and primary production. Pulse Amplitude Modulated Fluorometry was used for the measurements of primary production. The results show that the dry season coincided with higher wind speeds, elevated SRP and some peaks of chlorophyll a and primary production at both sites. During the wet season, high levels of chlorophyll a and primary production coincided with an increase in the euphotic depth, a deep chlorophyll maximum (DCM) and a contribution from metalimnetic areal production at both sites. Our results suggest that the vertical distribution of primary production in Lake Tanganyika is affected by the lake thermal structure, nutrient availability and the extent of the euphotic zone. In Lake Tanganyika, estimates of areal primary production were significantly affected by DCM and required estimation throughout the entire upper mixed layer and the metalimnion which extended to ~89?m.