巢湖微囊藻和浮游甲壳动物昼夜垂直迁移的初步研究

2002年10月进行了巢湖微囊藻和几种优势浮游甲壳动物的昼夜垂直变化的研究,结果表明:微囊藻具有明显的昼夜垂直变化现象。白天上层水中的微囊藻密度显著高于下层水中,夜晚逐渐下沉使得下层水中的密度相对高于上层水。微囊藻与叶绿素a、水温、溶解氧和pH等均呈显著的正相关(p<0.01)。几种优势浮游甲壳动物的昼夜垂直迁移存在较大的差异。短尾秀体溞和角突网纹溞白天在下层水(1.5m和2.5m)中的密度较高,夜晚则倾向于在上层水(0m和0.5m)中活动。相反,卵形盘肠溞白天在上层水中密度较高,象鼻溞则在11:00和15:00时各水层中的密度显著高于夜晚。汤匙华哲水蚤和广布中剑水蚤白天倾向于在下层水中活动,夜晚则逐渐迁移到上层水中。许水蚤在夜晚和凌晨3:00时各水层中的密度显著高于白天。中华窄腹剑水蚤昼夜垂直变化不明显。微囊藻与短尾秀体溞密度呈显著的负相关,而与象鼻溞和卵形盘肠溞呈显著的正相关(p<0.01)。     

Factors affecting abundance and distribution of submerged and floating macrophytes in Lake Naivasha, Kenya

Macrophytes have been shown to perform important ecological roles in Lake Naivasha. Consequently, various studies regarding the impact of biotic factors on the macrophytes have been advanced but related studies on environmental parameters have lagged behind. In an attempt to address this gap, sampling on floating species and submergents was carried out in eight sampling sites in 2003 to investigate how they were influenced by a set of environmental factors. Soil texture (sandy sediments; P  < 0.05, regression coefficient = −0.749) and wind were the most important environmental parameters influencing the distribution and abundance of floating macrophytes. Combination of soil texture and lake-bed slope explained the most (86.3%) variation encountered in the submergents. Continuous translocation of the floating dominant water hyacinth to the western parts by wind has led to displacement of the submergents from those areas. In view of these findings, the maintenance and preservation of the steep Crescent Lake basin whose substratum is dominated by sand thus hosting most submergents remain important, if the whole functional purpose of the macrophytes is to be sustained.     

Durations of development and production estimates by two crustacean zooplankton species Thermocyclops oblongatus Sars (Copepoda) and Diaphanosoma excisum Sars (Cladocera), in Lake Naivasha,Kenya

Durations of embryonic and post-embryonic development of the two dominant zooplankton crustaceans Thermocyclops oblongatus (Copepoda) and Diaphanosoma excisum in Lake Naivasha were determined at various lake water temperatures to provide the time element for the calculation of their production. Developmental times were temperature dependent in as much as food resource was not limiting. Production estimates were calculated. The turnover times of their biomass was 11.4 and 13.7 days, with production rates of 11.0 and 6.0 µg dw m^-3 d^-1 for T. oblongatus and D. excisum, respectively. Annual production was estimated as 3302 mg m^-3 yr^-1 for T. oblongatus and 2176 mg m^-3 yr^-1 for D. excisum. Production was continuous but with several irregular peaks particularly during the wet seasons when plankton biomass was high.     

Changes in fish species composition in Lake Naivasha,Kenya

The fish species composition in Lake Naivasha has been modified during recent years. Out of seven introduced and one indigenous species reported in 1962, only three introduced ones are found today. The elimination of species has been brought about by predation and changing ecological conditions. The most common fish species today is Sarotherodon leucosticuus. The hybridization found by previous workers between S. leucostictus x S. spilurus niger has stopped, as S. s. niger is no longer found in the lake. Most of the hybrid characters have been lost by the few surviving hybrids due to backcrossing between hybrids and between hybrids and S. leucostictus. The hybrids now mostly resemble S. leucostictus.     

The rise of potentially toxin producing cyanobacteria in Lake Naivasha,Great African Rift Valley,Kenya

Lake Naivasha, an important inland water ecosystem and a crucial freshwater resource in the Great African Rift Valley, has displayed clear signals of degradation in recent decades. We studied the phytoplankton composition and biomass levels in the period 2001–2013 and noted a progressive increase in the occurrence of potentially toxic cyanobacteria. Analyses for the presence of cyanotoxins such as microcystins (MC), cylindrospermopsin (CYN) and anatoxin-a (ATX-a) were carried out on samples collected in 2008–2013. Among the cyanotoxins tested, low concentrations of MC were detected in the lake. This is the first record of the occurrence of MC in Lake Naivasha. For the first time, molecular phylogenetic investigations of field clones of cyanobacteria from Lake Naivasha were carried out to establish the taxa of the dominant species. Amplification of the aminotrasferase (AMT) domain responsible for cyanotoxin production confirmed the presence of the mcyE gene belonging to the microcystin synthesis gene cluster in field samples containing Microcystis and Planktothrix species. These findings suggest that toxin producing cyanobacteria could become a threat to users of this over-exploited tropical lake in the near future.     

The diet of largemouth bass, Micropterus salmoides, in Lake Naivasha, Kenya

Lake Naivasha is a freshwater lake situated in the eastern rift valley of Kenya. Only five species of fish are present, all of which have been introduced. Of these, Oreochromis leucostictus, Tilapia zillii and Micropterus salmoides (largemouth black bass) support an important gillnet fishery with bass also being taken for sport. Until bass reached 260 mm f.l. they depended upon invertebrate food organisms. Thereafter crayfish, fish and frogs became increasingly important the larger the size of the bass. The most important invertebrate prey species was the water boatman, Micronecta scutellaris , followed by chironomid and culicid pupae. Zooplank-ton was consumed but only in large quantity by fish smaller than 80 mm. For bass over 260 mm the crayfish, Procambarus darkii , was the principal food. The largemouth bass in Lake Naivasha are generalized macro-predators, feeding principally on free-living animals of a kind most likely to be found in the littoral zones.     

The possibility of predator avoidance by Lake Michigan zooplankton

Low densities of Diaptomus ashlandi, Diacyclops thomasi, and Daphnia galeata mendotae were measured at depths where Mysis relicta formed nighttime aggregations. Calculations suggest that mysid predation can not account for the rarity of prey animals at these depths, which further suggests that the prey avoided the mysids. Unlike D. galeata mendotae, Daphnia pulicaria was common in mysid aggregations. The somewhat larger size of D. pulicaria may reduce its vulnerability to mysid predation, and consequently may explain the vertical distribution differences between the two congeners. Vertical distributions of Limnocalanus macrurus and copepod nauplii showed no obvious relationships to the mysid distributions. These were the only two taxa with distributions that were correlated with chlorophyll a concentrations. All crustacean taxa were rare in the epilimnion at night when sonar recorded a dense fish school.     

Analysis of trophic relationships in two shallow equatorial lakes Lake Naivasha (Kenya) and Lake Ihema (Rwanda) using a multispecifies trophic model

A multispecifies trophic model called ECOPATH II, which can be used to describe the trophic relationships in aquatic ecosystems on a quantitative basis, is briefly presented. When properly used, it can help to explain the trophic relationships in ecosystems and possible evolution of fishstocks after modifications of the environment (e.g. eutrophication, introduction of a new population and/or a significant increase of the fishing effort), and to compare the trophic structure of several ecosystems. Examples are provided on two shallow lakes: Lake Ihema and Lake Naivasha. They are compared with Lake George which was previously documented.     

Diel migration and vertical distribution of Cladocera in Lake D. Helvecio (Minas Gerais,Brazil)

Six genera of Clad ocera (Diaphanosoma, Daphnia, Ceriodaphnia, Moina, Bosmina, Bosminopsis), each of them usually with only one species were found in Lake D. Helvecio, a natural valley lake located in the eastern part of Brazil. Diurnal migratory movements of the organisms observed in this lake showed a different pattern in different species. Closely related species, which explore the same food source, live in different layers, thus avoiding interspecific competition. The migratory behaviour of the species was studied mainly in relation to temperature and oxygen distribution in the lake. Thus, analyses were made in the summer (January, 1978) when a strong stratification occurs with the establishment of a thermocline and an oxycline. Comparisons were made also with the data obtained in winter (July, 1978), when a complete mixing of water occurs.     

Breeding biology of the Louisiana red swamp crayfish Procambarus clarkii Girard in Lake Naivasha,Kenya

This paper describes the breeding biology of the Louisiana red swamp crayfish (Procambarus clarkii Girard) that has been introduced in a tropical lake, Naivasha. Notable differences in crayfish biology are found between the place of origin in Louisiana and lake Naivasha.In Naivasha sexual maturity in females is reached at a larger size (42 mm carapace length (CL), 76 mm full length (FL)) than in males (40 mm CL, 71 mm FL). The corresponding figures for Chalmette area of Louisiana are 31–32 mm CL for both sexes and at Ben Hur Research Centre, Louisiana, 33 mm CL. The females weigh between 15 and 20 g at the onset of egg deposition. The corresponding Louisiana values are from 5 to 10 g. Crayfish in Naivasha as in Louisiana breed both inside burrows which are 10–70 cm deep, and on the sediment in shallow water at a depth of between 0.5 and 4 m. These observations suggest that reproduction is very much a lake edge activity. The number of breeding individuals in Naivasha varies with the trend of the water surface level. Sexually active females (form I) as judged from secondary sexual characters (appearance of basal hooks at the base of 3rd and 4th walking legs) persist throughout the year at no less than 60% of the adult male population. This is confirmed by the appearance of recently hatched juveniles (3.4 to 6.5 mm CL) in the water throughout the year. Similarly berried females in Naivasha are found throughout the year, though with peaks at irregular intervals. In Louisiana breeding takes place only from August through November with a single peak in October. The fecundity of crayfish in Naivasha is 433 (SE ± 21, N = 133) and there is a positive correlation between body length and egg number.     

Ecology and role of zooplankton in the fishery of Lake Naivasha

Taxonomic composition, distribution, community structure and seasonality of Lake Naivasha zooplankton between 1978 and 1980 are described. The ecological status of the zooplankton in relation to prevailing ecological factors and the lakes fishery are discussed. Species composition of the zooplankton community in Lake Naivasha has been remarkably constant since first observations between 1929 and 1931. The community structure and distribution of the zooplankton in the lake varied little from July 1978 to July 1980, however, there was evidence of species succession among the larger zooplankters, especially the cladocerans. In the littoral area zooplankton contribute significantly to the food and production of juvenile fish. There is an absence of fish zooplanktivores in the limnetic area of the lake. Consequently the limnetic zooplankton is not utilised by higher trophic levels.     

A Study of a population of Micronecta scutellaris Stal (Hemiptera : Corixidae) in Lake Naivasha,Kenya

Micronecta scutellaris is one of the most abundant macro-invertebrates found in Lake Naivasha, Kenya and forms an important component of the diet of juvenile large mouthed bass.Its distribution in the lake and breeding biology are described and considered in relation to the lake's food chains.     

Geochemical and physical characteristics of river and lake [2pt] sediments at Naivasha,Kenya

Prior studies on Lake Naivasha relevant to understanding sediment dynamics include a bathymetric map, a paleolimnological study of fossil invertebrate assemblages in lake sediment, an overview of lake level fluctuations throughout the 20th century, and identification of a dynamic assemblage of macrophyte zones that has responded both to these changes in lake level and to more recent, alien species. Sediment samples collected from the rivers systems and the lake were examined physically and chemically. River sediment characteristics reflect geology and geomorphological processes in the catchment, whereas lake sediment stratigraphy has responded to past lake level changes. Such changes have caused significant changes in aquatic vegetation assemblages. Present day sediment dynamics in the lake are governed by the presence of river point sources in the north and wave-induced re-suspension, such that sediments introduced by rivers are transported in easterly and southerly directions, and are eventually deposited in the eastern, central and southern parts of the lake. Sedimentary deposition is also occurring in northern areas that once were protected by papyrus swamp vegetation but now only have a narrow fringe, highlighting the important role of swamp vegetation in filtering out suspended particulates and thereby controlling water quality in the lake. Geochemical analyses of river and lake sediments indicate that they represent fairly undisturbed background conditions. Higher-than-expected concentrations of cadmium, iron, nickel and zinc found in both river and lake sediment are likely to derive from volcanic rocks and/or lateritic soils found in the lake catchment.     

Seasonal variations in the diel vertical distribution of phytoplankton and zooplankton in a shallow pond

The seasonal succession of phytoplankton diversity, and the variations in the diel vertical distribution of phyto‐ and zooplankton were investigated in a small shallow pond (1.7 m water depth) in 2003. It was inferred that the water tended to stratify weakly in the daytime from February to June. In February and April, the green alga Golenkinia radiata Chodat dominated the phytoplankton assemblage. The cell density of G. radiata greatly decreased in April, when rotifers increased near the bottom. The vertical mixing was attenuated in June, large populations of the euglenoids (Lepocinclis salina Fritsch, Phacus acuminatus Stokes, Trachelomonas hispida (Perty) Stein et Deflandre) developed, and the cyanobacterium Aphanizomenon flos‐aquae var. klebahnii Elenk. appeared at low density. Euglenoids and A. flos‐aquae were mostly distributed in the bottom layer. In late September, when the water was mixed throughout the day, euglenoids and A. flos‐aquae were distributed evenly throughout the water column. The zooplankton (cyclopoid copepods and rotifers) densities in September were the lowest throughout the year. The vertical mixing increased in November, and the phytoplankton community was composed of A. flos‐aquae, P. acuminatus, T. hispida and the green alga Ankistrodesmus falcatus (Corda) Ralfs. In November, at the final stage of water bloom of A. flos‐aquae, its population density decreased with depth. The two euglenoids exhibited similar cell distributions at 0.8 m and 1.6 m during 1–3 November. A. falcatus was distributed evenly throughout the water column; however, when the vertical mixing lessened, the cells at the surface started to sink. Copepod nauplii and rotifers appeared at high densities in November. Seasonal variation in the phytoplankton community structure in the pond seemed to be related to the vertical mixing of the water. In addition, zooplankton, especially rotifers, might play an important role in initiating a spring clear‐water phase and in the bloom collapse of A. flos‐aquae.     

夏季南黄海浮游动物的垂直分布与昼夜垂直移动

对 2 0 0 1年 8月南黄海青岛外海至济州岛断面 ,浮游动物的日垂直变化、断面垂直分布及其与海区水文特征的关系进行了研究。共鉴定浮游动物 40种。断面浮游动物的平均丰度为 13 62 ind/ m 3 ;种类丰富度与暖水种数呈正相关 ,分布趋势为测区东部高、西部低 ;优势种为中华哲水蚤 (Calanus sinicus)、小拟哲水蚤 (Paracalanus parvus)、拟长腹剑水蚤 (Oithona similis)、强壮箭虫 (Sagitta crassa)、近缘大眼剑水蚤 (Corycaeus affinis)。在不同水层 ,浮游动物丰度分布不同 ,随水深增加而减少 ,表层丰度最高 (3 2 2 1ind/ m3) ,温跃层和底层分别为 743 ind/ m3、43 8ind/ m3;种类组成也不相同 ,表层数量最多的种类为鸟喙尖头(Penilia avirostris)、温跃层及其以下水体为中华哲水蚤 ,其它主要种类亦表现不同水层取向。在连续观测站 ,根据浮游动物主要种类的昼夜垂直移动与温跃层关系 ,将之分为表层分布 ,近温跃层分布 ,底层分布及全水层分布 4种类型     

贵州百花湖夏季浮游植物昼夜垂直分布特征

为了探讨浮游植物在水体中的昼夜垂直分布格局,于2012年7月31日至8月1日对百花湖(水库)浮游植物进行昼夜24 h定点分层研究。研究结果表明:蓝藻、绿藻、硅藻种类数在水体中具有明显的分层现象,蓝藻种类数在0.5—2 m居多,绿藻种类数在0.5—6 m明显多于7—14 m,硅藻种类数主要分布在6 m及以深的水层中。湖泊假鱼腥藻(Pseudanabaena limnetica)为绝对优势种,相对丰度为82.69%。8:00时湖泊假鱼腥藻在2—3 m处聚集程度最高,10:00时聚集程度最高的水层上升至0.5 m,10:00—12:00湖泊假鱼腥藻的细胞丰度由0.5 m向2 m扩增,12:00时在2 m处达到全天峰值,此时0.5 m处的细胞丰度是除6:00外的最小值,12:00—14:00湖泊假鱼腥藻的细胞丰度由2 m向0.5 m扩增,16:00时又大量聚集于1 m处,说明湖泊假鱼腥藻在白天具有明显的垂直迁移现象;湖泊假鱼腥藻丰度的MI指数白天在1.45—2.07之间,夜间在1.40—1.46之间,变化趋势与时间深度等值图结果相符,说明湖泊假鱼腥藻在水体中昼夜均呈聚集分布,且白天的聚集程度及变化幅度大于夜间;百花湖浮游植物总丰度的昼夜垂直分布格局与湖泊假鱼腥藻一致;水体中浮游植物总丰度和湖泊假鱼腥藻丰度夜间低于白天。光照的昼夜交替和水柱温差的昼夜变化是影响浮游植物总丰度和湖泊假鱼腥藻垂直分布格局昼夜变化的重要环境因素。     

The use of the Shannon-Wiener diversity index to delineate the horizontal distribution of crustacean zooplankton communities in Lake Superior, 1973

The Shannon-Wiener diversity index was calculated for the total zooplankton community found in Lake Superior in 1973, as well as for its components (ie. Calanoida, Cyclopoida, Cladocera and nauplii). These values were then subjected to a model to develop statistically homogeneous zones, and to determine the temporal and spatial variability of the diversity values. A region roughly following the loo meter depth profile and having an average yearly temperature of 9.8°C was found to exist along the south and southwestern shores of Lake Superior. In this region significantly (P 0.10) higher diversity indices were found.     

Changes of phytoplankton communities in Lakes Naivasha and Oloidien, examples of degradation and salinization of lakes in the Kenyan Rift Valley

Increasing degradation of the water quality, caused by overuse and salinization, leads to considerable changes of the phytoplankton composition in Kenyan Rift Valley lakes. Exemplarily, the phytoplankton communities and biomasses of deteriorating freshwater Lake Naivasha and salinizing Lake Oloidien were studied between 2001 and 2005, accompanied by physico-chemical measurements (pH, total phosphorus and nitrogen, alkalinity, conductivity). Over the last three decades, the ecology of these two water basins has been subjected to dramatic changes, caused by excessive use of water and catchment area by man. In L. Naivasha a shift in the dominance of coccoid cyanobacteria towards dominance of Chlorophyceae (Botryococcus terribilis) was observed. Lake Oloidien exhibited a shift in the dominance of coccoid Chlorophyceae towards dominance of cyanobacteria (Arthrospira fusiformis, Anabaenopsis elenkinii). Phytoplankton findings and chemical data demonstrate that L. Naivasha has developed towards a eutrophic freshwater lake while L. Oloidien has progressed towards a hypereutrophic alkaline-saline lake. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. Padisak     

五大连池浮游动物群落结构的时空分布特征

基于2011—2013年五大连池的调查,对浮游动物群落结构时空分布特征进行分析,探讨重点放养鱼类之一鳙Aristichthys nobilis的合理放养量。结果表明,五大连池浮游动物4类82种(含未定种);主要优势种集中于原生动物和轮虫,有恩氏筒壳虫Tintinnidium entzii、纵长异尾轮虫Trichocerca elongate等,大型浮游动物中仅剑水蚤Cyclops sp.、无节幼体Nauplii形成优势;密度、生物量总计为3794.98ind/L和2.4558mg/L;多样性指数H'、J、d均值分别为2.15、0.7和0.98;浮游动物群落时空差异显著,五池密度、生物量、多样性、均匀度较高,二池(P2)、三池(P3)相似且次于五池;另外,二池(P2)、三池(P3)浮游动物的密度、生物量、多样性均呈现夏季高于春秋季;根据结果获得五大连池鳙的合理放养量为5.71×105尾。研究可为五大连池生境保护和渔业资源的可持续利用提供科学依据。