Characteristics of declining dissolved oxygen concentrations in Lakes Kasumigaura and Kitaura,two shallow polymictic eutrophic lakes in Japan

To elucidate the patterns and characteristics of dissolved oxygen (DO) decline near bottom in shallow polymictic eutrophic lakes, we conducted a statistical analysis of monthly measurements for 12 years at 14 stations and hourly observation for 8 years at two stations in Lakes Kasumigaura and Kitaura. Results indicated that DO decline (hereafter, <2 mg/L) was negligible at stations with a depth of <5 m. Thus, we consider this depth to be critical depth for studying DO decline conditions in these lakes. The collected data revealed that DO decline events lasting more than 2 days occurred a few times a year. During these events, wind blew weakly (usually, <2 m/s), and diurnal stratification was formed by solar radiation. The averaged DO declining rate leading up to these events was 4.3 (±2.7) mg L^?1 d^?1, suggesting that within a few days, DO can fall into a declined state. The occurrence probability of DO declined events in the respective summer increased significantly with the proportion of weak wind hours (<2 m/s). Significant correlations between wind velocity and DO change rate (1 hr difference in DO) supported the importance of wind on DO changes in polymictic lakes. The influence of mean air temperature on DO decline events was insignificant.     

Bacterial abundance on the skin,gills and intestines of Cyprinus carpio in Lake Naivasha,Kenya: Implications for public health and fish quality

Cyprinus carpio is the most important fish species in the Lake Naivasha fishery, comprising 51% of the total catch in the lake. Microflora, especially enteric bacteria of human or animal origin, are the causative agent for fish contamination and spoilage. Poor sanitation standards and poor sewage treatment and disposal methods within Lake Naivasha and its catchment pose a great threat for degrading the quality of C. carpio. The potential impact is rejection of the fish in the local, regional and international markets, risking the collapse of the Lake Naivasha fishery. This study determined the bacterial quality of water and C. carpio from three different sites within Lake Naivasha, namely Malewa River mouth, sewage discharge point and a mid‐lake site, based on plate count techniques. Physicochemical parameters characterizing the lake water also were also measured in situ. This study results indicated that both the fish and water in Lake Naivasha exhibited poor bacterial quality. All the physicochemical parameters were within the recommended range for fish culture, although they also were conducive to the proliferation of bacteria. Most of the sampling sites exhibited significant spatial variation in their bacterial abundance (P < 0.05). The sewage discharge sampling site exhibited the highest mean density values for bacterial densities and clearly degrade the quality of the fish in the lake. Proper sewage treatment, and the installation of modern sanitation facilities, is recommended to improve the bacterial quality of the fish.     

Regime shifts observed in Lake Kasumigaura,a large shallow lake in Japan: Analysis of a 40‐year limnological record

Lake Kasumigaura, which is composed of the two basins (Nishiura and Kitaura), is a large, shallow, hypereutrophic lake. Phytoplankton and water quality records from the past forty years were analysed to elucidate whether or not, when, and what type of certain regime shifts may have occurred, based on using inferential regime shift detectors. Characteristics of the phytoplankton and water quality changes were similar at 6 sampling sites in the two basins, with 20 water quality parameters being classified into four groups, based on cluster analysis. Shifts in dominant plankton groups (DPGs) and water quality occurring almost concomitantly, concentration on the period from 1987 to 1992 (Shift A) and from 1997 to 2001 (Shift B), with those observed for the two basins usually being similar with small differences. Two types of inferential regime shift detectors (sequential t‐test type; Rodionov's RSD and sequential F‐test type: package strucchange in R) yielded similar timings and significances of the shifts. Furthermore, changes in skewness and conditional heteroskedasticity (package early warnings in R) usually represented early warning signals before the shifts. Correlation analysis and ratios of dissolved inorganic nitrogen (DIN) vs. total phosphorus (TP) supported the hypothesis that phosphorus was the phytoplankton biomass limiting nutrient, except for one period for the Nishiura basin. Neither the nitrogen:phosphorus (N/P) ratio hypothesis and ammonia–nitrate (NH₄‐N/NO₃‐N) hypothesis satisfactorily explain the DPGs before and after Shift A (Microcystis spp. and Planktothrix spp., respectively), although it may be possible that these ratios triggered the DPG change in this shift. A considerable increase in silicon was observed for Shift B when the DPGs changed from cyanobacteria to diatoms. Further studies on the accurate types and triggers of the regime shifts are necessary to better understand the interactions between ecosystem and water quality for this and similar lakes elsewhere.     

Sex ratio and reproduction of invasive red devil, (Amphilophus labiatus: Cichlidae) in Lake Sentani,Indonesia

The Red Devil Cichlid Amphilophus labiatus (Günther 1864), native to Lake Nicaragua and Lake Managua in Central America, is widely cultivated as an attractive, yet aggressive, aquarium fish that has been introduced to several freshwater locations around the world. It was recently introduced into Lake Sentani in Indonesian New Guinea, a naturally rich aquatic habitat home exhibiting a diverse fauna with high endemism. To better understand the invasive potential and impacts of A. labiatus, the present study was undertaken to describe the sex ratio and reproductive potential of A. labiatus in Lake Sentani through determination of its gonad maturation stages, gonado‐somatic index and fecundity. The fish were collected using floating gill nets at six locations around the lake between June and August 2019. Sex identification, gonads type, gonad stage and weight, fecundity were observed in the laboratory. Of the total 542 of A. labiatus collected during the survey, 345 individuals were sampled for sex ratio and reproduction. The overall male:female ratio was skewed towards males in a ratio of 3.28:1. The males were generally found to be in the gonad maturation stage I, while the females were commonly found in gonad maturation stages III and IV for all sampling locations during the study period. The highest male mean GSI value was found for the Yakonde site (0.24 ± 0.21) and at the Ayapo site (1.11 ± 1.01) for the females, with a fecundity of 191–2,720 eggs. The fish have spread to most parts of the lake, being conditioned to potentially spawn at any location, thereby providing an explanation for the observed rapid increase of the A. labiatus population over the last ten years. The results of the present study suggest a high invasive potential for this species in this and other similar lake habitats, warning that their careful management should be undertaken to restrict their further spread in Indonesia and elsewhere.     

鄱阳湖湖区三站点水质评价及其变化特征研究

应用模糊综合定量评价法对鄱阳湖星子站、都昌站、棠荫站区域的水质进行了评价,以3个指标(氨氮、高锰酸盐指数、总磷)作为评价因子,建立评价矩阵,计算出因子权重,得出特征指数,最后评价出水质级别,该方法能够得到其它方法难以达到的客观性和综合性。水质监测数据和评价结果表明:2003-2008年鄱阳湖3站点水质状况尚属良好,大部分时期在Ⅱ、Ⅲ类水平,但也有部分时段超标;在劣于Ⅱ类水的情况下,3个站点所在区域枯水期水质均比丰水期和平水期差。     

Growth,mortality and recruitment of silver cyprinid (Rastrineobola argentea) in the open waters of Lake Victoria,Kenya

This study investigated the growth, mortality and recruitment of Rastrineobola argentea in Lake Victoria on the basis of length–frequency data collected during the period 2014–2015. The asymptotic length (L _∞ ) had a mean (±SE ) value of 53.50 ± 0.50 mm SL , growth curvature (K ) of 0.81 ± 0.06 year^?1, total mortality (Z ) of 2.96 ± 0.12 year^?1, a natural mortality (M ) of 1.23 ± 0.06 year^?1, a fishing mortality (F ) of 1.74 ± 0.07 year^?1, an exploitation rate (E ) of 0.59 ± 0.01, a growth performance index (?′ ) of 3.36 ± 0.02 and a length at 50% capture (L ₅₀) of 28.25 ± 0.43 mm SL . The fish exhibited a peak breeding during the months of May and August. The Beverton and Holt relative yield‐per‐recruit model indicated mean (±SE ) indices as 0.37 ± 0.01 for optimum sustainable yield (E _0.5), 0.76 ± 0.01 for maximum sustainable yield (E _max) and 0.66 ± 0.01 for economic yield (E _0.1). Compared with previous studies, there is a great decline in the sizes of R. argentea stocks in Lake Victoria. Thus, management measures should include restriction on illegal seine nets <10 mm mesh size and re‐enforcement of a fishing ban or a closed season.     

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。     

Optical and biological properties of Lake Ülemiste, a water reservoir of the city of Tallinn I: Water transparency and optically active substances in the water

Optical and biological measurements were performed in Lake Ülemiste in the summer of 1997 (four measurement days), and from May to October in 1998 (12 measurement days). This kind of data, describing the type and amount of optically active substances in the water, phytoplankton characteristics, the underwater light field, and temperature–oxygen situation in the lake are necessary when estimating the ecological state of the lake. Lake Ülemiste is the main drinking water reservoir of Tallinn, the capital of Estonia. Phytoplankton abundance and biomass, chlorophyll a and suspended matter were determined from collected Lake Ülemiste water samples in the laboratory. Spectrophotometrical processing of the filtered and unfiltered water was carried out to describe the beam attenuation coefficient spectra and optical influence of yellow substance in the water. Vertical profiles of downwelling irradiance of the PAR (400–700 m) region of the spectrum (and from these data the averaged over depth diffuse attenuation coefficient) were determined. The relative transparency of the water was estimated by using a Secchi disk. Passive optical remote measurements were episodically made from aboard a boat. Results obtained confirmed that Lake Ülemiste is turbid (almost hypertrophic), comparable with most turbid lakes in Estonia (e.g. Lake Võrtsjärv) and Finland (Lake Tuusulanjärvi). Its chlorophyll a content varied within the range 13–121 mg m^–3, phytoplankton biomass 3–107 mg L^–1, phytoplankton abundance 65 000–999 000 mL^–1, suspended matter 8–34 mg L^–1, effective concentration of the yellow substance 6–30 mg L^–1, diffuse attenuation coefficient of light in the PAR region 1.0–3.3 m^–1 and a Secchi disk depth of 0.5–1.75 m. The temporal variation of the spatial averages of these parameters during 1998 was analysed. Almost all characteristics showed an increase from May to midsummer with a maximum in late July or in August (correspondingly the Secchi depth values were minimal in late summer). The amount of yellow substance was an exception, which was nearly constant during the observation period. The maximum level of chlorophyll a content in July and August 1998 markedly exceeded that in 1997, despite the fact that the summer of 1997 was sunny, but the summer of 1998 was cold and rainy. The correlative relationships between the different parameters were investigated together with the respective data for other lakes. They show that the data of Lake Ülemiste supplemented the correlation graph in the region of turbid lakes, whereby in all cases the correlation coefficient increased following the addition of Lake Ülemiste data. The highest correlation coefficient was obtained when light attenuation coefficient values were correlated with a sum of weighted concentrations of chlorophyll a, yellow substance and suspended matter (multiple regression analysis).     

Mean hydroclimatic and hydrological conditions during two climatic modes in the Devils Lake Basin,North Dakota (USA)

Proxy variables from palaeolimnological studies of lakes in the Prairie Pothole Region of North America have been used to infer large oscillations during the late Holocene between longer periods of high‐salinity–dry conditions and shorter periods of low‐salinity–wet conditions producing a normative pattern marked by the absence of hydrological stability. Studies of the historical rise in lake level at Devils Lake have identified 1980 as a transition point between two such hydroclimatic modes. This study uses multiple datasets to characterize the mean hydroclimatological and hydrological conditions of these two climatic modes. Mode 1 is a cool and dry phase, and mode 2 is a warmer and wetter phase. Precipitation onto the lake increased by 24% from mode 1 to mode 2. This small, but sustained, increase produced significant changes in the mean hydroclimatic and hydrological states for the basin, including a 383% increase in surface run‐off to the lake, and a 282% increase in the basin run‐off ratio. Devils Lake Basin is located along a hydrotone (region of strong hydroclimatic gradients) where small changes in hydrological drivers are amplified into large changes in regional moisture. The effects of the fluctuating climatic modes and strong hydroclimatic gradients are probably further amplified by the unique fill–spill hydrology of the northern glaciated plains, which can result in nonlinear precipitation–run‐off relationships. This natural pattern of extreme hydrological variations for Devils Lake produces enormous challenges for lake management.     

Assessment of catfish (Clarias gariepinus) stock in Lake Baringo,Kenya

The management measures used for sustainable utilization of Clarias gariepinus in Lake Baringo do not have a stock assessment reference, attributable mainly to a lack of information on biological limits and target reference points. Assessment of Clarias gariepinus stock in Lake Baringo was carried out between August 2013 and July 2014. A total of 2772 fish were sampled from 25 boats (40%) for 5 days each week for length and weight measurements. Fish Stock Assessment Tools and yield model were used to estimate population parameters, exploitation rate and optimal fishing scenarios. Annual C. gariepinus standing biomass was estimated at 21 383 kg, L_∞ = 114.30, K = 0.37 year^?1, W = 0.0147L^2.81, Z = 1.14 year^?1, M = 0.61 year^?1, F = 0.53 year^?1, and exploitation rate = 0.46 year^?1. The relative yield‐per‐recruit (Y′/R) and biomass‐per‐recruit (B′/R) resulted in E_max = 0.44 and F_MSY = 0.50 year^?1. The yield‐per‐recruit ratio at maximum sustainable yield was 29.12%, and the SSB_MSY per recruit ratio = 56.10%. The steady‐state biomass, exploitation rate and optimal fishing scenario indicated a pristine fishery for the lake, suggesting the current fishing efforts should not be exceeded to enable sustainable economic utilization of C. gariepinus.     

The myths and realities in the management of the Kapenta fishery at Lake Kariba (Zimbabwe)

Biological considerations have dominated the management of the Kapenta fishery since its inception. State fisheries' managers employed the traditional scientific models that proved useful in managing temperate fisheries. This study uses both secondary and primary data to discuss the problems of using scientific models to manage tropical river‐fed lakes. Where fish stock size fluctuates in response to environmental factors such as river flows and lake level (which influence the level of nutrients in the water body), the concepts of maximum sustainable yield (MSY) and maximum economic yield (MEY) become problematic. This study demonstrated that, in an unstable environment, and where the target species is resilient to high levels of fishing effort, biological models lose their predictive power, allowing the state to determine the proportion of the resource to be distributed among its nationals. The redistribution of access rights or fishing licences (as a way of redressing a legacy of an unequal distribution of resources between emerging local entrepreneurs and companies established during the colonial era) is a major management issue in Zimbabwe. This study uses historical appraisal to illustrate how biological growth models failed to determine the optimal level of fishery exploitation upon which informed decisions could be made regarding whether to redistribute fish access rights or to broaden participation (or just recruit more fishers into the industry). This study also investigates tensions between the industry (advocating for broadening of access) and the state (seeking to redistribute existing resource use rights). Each stakeholder group has evoked institutions and discourses supporting their positions in influencing the strategies to eliminate the existing imbalances.     

Distribution and abundance of freshwater polychaetes, Manayunkia speciosa (Polychaeta), in the Great Lakes with a 70-year case history for western Lake Erie

Manayunkia speciosa has been a taxonomic curiosity for 150 years with little interest until 1977 when it was identified as an intermediate host of a fish parasite (Ceratomyxa shasta) responsible for fish mortalities (e.g., chinook salmon). Manayunkia was first reported in the Great Lakes in 1929. Since its discovery, the taxon has been reported in 50% (20 of 40 studies) of benthos studies published between 1960 and 2007. When found, Manayunkia comprised < 1% of benthos in 70% of examined studies. In one extensive study, Manayunkia occurred in only 26% of 378 sampled events (1991–2009). The taxon was found at higher densities in one area of Lake Erie (mean = 3658/m²) and Georgian Bay (1790/m²) than in five other areas (mean = 60 to 553/m²) of the lakes. A 70-year history of Manayunkia in western Lake Erie indicates it was not found in 1930, was most abundant in 1961 (mean = 8039, maximum = 67,748/m²), and decreased in successive periods of 1982 (3529, 49,639/m²), 1993 (1876, 25,332/m²), and 2003 (79, 2583/m²). It occurred at 48% of stations in 1961, 58% in 1982, 52% in 1993, and 6% of stations in 2003. In all years, Manayunkia was distributed primarily near the mouth of the Detroit River. Causes for declines in distribution and abundance are unknown, but may be related to pollution-abatement programs that began in the 1970s, and invasion of dreissenid mussels in the late-1980s which contributed to de-eutrophication of western Lake Erie. At present, importance of the long-term decline of Manayunkia in Lake Erie is unknown.     

Distribution of nuisance Cladophora in the lower Great Lakes: Patterns with land use, near shore water quality and dreissenid abundance

Selected shorelines and offshore shoals in Lakes Erie, Huron and Ontario were surveyed with a high frequency hydroacoustic system to investigate current spatial patterns of nuisance benthic filamentous algal (e.g., Cladophora) cover and stand height. Cladophora reached nuisance levels at all sites in Lakes Erie and Ontario, but not in Lake Huron or Georgian Bay. Despite clear gradients in coastal land cover, near shore water quality gradients were generally weak, and for Lakes Erie and Ontario, measures of near shore water quality were similar to that at offshore shoals. Hierarchical partitioning analysis suggested that while dreissenid mussel abundance appeared to be important in determining the magnitude of Cladophora standing crop, the joint contribution of catchment land cover, near shore water quality (nutrient levels and suspended matter) and dreissenid mussel abundance explained nearly 95% of the total variance in nuisance Cladophora standing crop observed in this study. Although the results from this study are necessarily correlative in nature and definition of causal relationships is not possible, these results provide corroborating evidence from sites across a gradient within and across the lower Great Lakes that is consistent with the operation of the near shore shunt model.