Physico-chemical limnology and plankton dynamics of Mazvikadei,a tropical reservoir in Zimbabwe

The limnology of Mazvikadei Reservoir, northern Zimbabwe, was investigated in 2015 to determine whether it had changed since filling in 1990. The reservoir is characterised by low algal biomass, low nutrients (i.e. N and P) and high water clarity/transparency. Fifty-four species of phytoplankton were recorded, comprising Bacillariophyta, Chlorophyta, Cyanophyta, Desmids, Dinophyta and Euglenophyta. Chlorophyta numerically dominated in the hot dry season, whereas Bacillariophyta, Desmids, Dinophyta and Euglenophyta dominated in the cool dry season. Species richness was highest at the onset of the cool dry season, in response to high nutrient concentrations. Phytoplankton abundance and composition were significantly correlated with temperature, nitrates and total nitrogen. Nineteen zooplankton species were recorded, including Copepoda, Cladocera and Rotifera. Overall, Cladocera were numerically dominant and became most abundant during the cool dry season. Rotifers and copepods dominated during the hot dry season. The zooplankton abundance was correlated with reactive phosphorus and phytoplankton abundance. The trophic state of Mazvikadei Reservoir seems to have stabilised and to have assumed the physico-chemical characteristics and plankton community typical of an oligotrophic lake.     

Temporal dynamics and relationship between climate,limnological variables and zooplankton composition in climate-sensitive Lake Bosumtwi,Ghana

The community composition and the factors affecting seasonal and interannual dynamics of zooplankton in Lake Bosumtwi were studied biweekly at a central index station during 2005 and 2006. The lake zooplankton community was species poor. Mesocyclops bosumtwii was numerically superior seasonally and interannually and was endemic to the lake. Minor constituents included Moina micrura, six rotifer species (except for Hexarthra intermedia) and Chaoborus ceratopogones larvae. Low variance of cyanobacteria-dominated phytoplankton biomass underlined stable zooplankton community structure. Emergence of rare species of rotifers occurred seasonally. The climatic signature on the lake’s stratification and mixing regime was strongly influenced by atmospheric temperature, but weakly by wind strength, because of sheltering of the lake by high crater walls. Increasing mixing depth entrained high TP concentrations from below the thermocline seasonally, but reflected poorly in the phytoplankton biomass behaviour. Total zooplankton abundance did not differ seasonally, but varied markedly from year to year in its timing and magnitude. Herbivores were squeezed between food limitation and high predation pressure from Chaoborus all year round. The low fish planktivory (high fishing pressure) on Chaoborus may create a trophic bottleneck restricting energy transfer efficiency from zooplankton to fish.     

Seasonal change in a saline temporary lake (Fuente de Piedra,southern Spain)

Fuente de Piedra saline lake is located in an endorheic basin in the south of Spain. This lake is very shallow (0.5 m max. depth during 1987–88) and relatively large (± 1350 ha). It is a temporary playa lake, showing irregular cycles, with frequent seasonal drought and a high degree of unpredictability. The lake was sampled monthly during a relatively rainy year (1987–88, 10.5 months permanence). The result of combined analyses for environmental variables (salinity, temperature and soluble inorganic forms of nitrogen and phosphorus), variables related to biological activity (chlorophyll a, sediment organic matter and redox potential) and the direct analysis of the planktonic community, shows the existence of two periods of dominance by autotrophs. The first occurs during winter, exhibits a progressively higher surface to volume ratio for phytoplankton and is followed in the spring by high zooplankton densities (Moina salina, Fabrea salina) and very low phytoplankton densities, suggesting the existence of a period with a detritus-based food web. The summer period coincides with a community better adapted to high salinities that is dominated by Dunaliella salina, D. viridis, diatoms and the ciliate Fabrea salina, and associated with high ammonium concentrations. A new period of organic matter accumulation could be facilitated, in the last moments before the lake dries, by a progressive decrease in zooplankton abundance.     

Characteristics of Zooplankton in Syamozero Lake (South Karelia) Based on the Results of Long-Term Monitoring

The results of long-term studies on zooplankton from Syamozero Lake, South Karelia, have been analyzed. The relationship between the quantitative indices of the zooplankton community and the value and dynamics of the biogenic load on the lake is established. The redistribution of the ratio of the major groups and dominant zooplankton species, the decrease in the Shannon diversity index, the increase in the Mäemets trophic state index, and with the increase in the abundance of indicator species of higher nutrient supply have been determined. The effect of anthropogenic factors on the population characteristics of the zooplankton community and changes in the trophic status of the lake with a high resource potential have been found.     

Zooplankton contribution to the particulate N and P in Lake Kinneret,Israel, under changing water levels

The association of water level changes and the relative (%) contributions of crustacean zooplankton to particulate N (PNz) and particulate P (PPz) in Lake Kinneret, Israel were studied. The PNz and PPz were assessed for a period of 10 years (1999–2008) in relation to water level (WL) changes which occurred during that period. We estimated PNz and PPz, based on crustacean N and P content measured seasonally over 2 years, and a 10-year record of zooplankton densities. Mean cladoceran N and P contents were 8.7 and 1.2% of dry weight, respectively, while for copepods they were 9.5 and 1.5% of dry weight, respectively. Zooplankton density, and hence PNz and PPz, changed dramatically during the 10 years, concurrent with extreme variations in the lake’s WL. The lowest mean values of PNz and PPz occurred during high WL years and the highest PNz and PPz were during low WL years. PNz and PPz were negatively correlated with the total PN and PP concentrations, respectively, in the lake. The reduction in zooplankton contribution to the particulate N and P during high WL is probably due to higher loading of particulate matter in wet years, causing an increase of PN and PP concentration in the lake, as well as lower densities of zooplankton, caused by higher fish predation pressure, both are a by-product of the large water influx during extreme wet winters.     

The impact of planktivorous flsh on the structure of a plankton community

SUMMARY. 1. The abundance of pianktivorous juvenile yellow perch, Perca flavescens , was manipulated in three 750 m³ enclosures in a eutrophic lake.
2. There was a significant negative relationship between fish and zoopiankton biomasses. At high fish densities the zooplankton community was dominated by small filter-feeding cladocera. primarily bosmi- nids. At low fish densities the zooplankton community was dominated by large filter-feeding cladocera, primarily daphnids.
3. There was no significant relationship between zooplankton and phytoplankton biomasses when considered over the whole experiment but there was a trend towards lower phytoplankton biomass in the enclosure dominated by daphnids during mid-summer.
4. We conclude that although planktivorous fish have a strong negative impact on zooplankton community biomass and size structure, the relationship at the next lower trophic level, zooplankton and phytoplankton, is much weaker. Therefore, the biomanipulation of planktivorous fish populations as a management technique to control phytoplankton abundance is largely ineffective.     

Field estimates of zooplankton community grazing in the limed Lago d'Orta (Italy) by the use of a radioisotope-free technique

Grazing experiments were carried out in a limed lake (Lago d'Orta) during the periods, 23–24 March and 28–29 June 1990. A simple in situ technique based on cell counts before and after incubation was used in order to evaluate the impact of the natural zooplankton community on the lake phytoplankton. An estimation of the daily consumption by the natural zooplankton community showed that about 4.6% and 1.04% of the standing algal biomass was removed during March and June respectively. The differences in grazing activities were likely due to the difference in zooplankton community structure and to the surrounding environmental conditions, mainly influenced by the liming intervention, started in the lake water since 1989.     

Effects of nitrogen enrichment on zooplankton biomass and N:P recycling ratios across a DOC gradient in northern-latitude lakes

We used data from whole-lake studies to assess how changes in food quantity (phytoplankton biomass) and quality (phytoplankton community composition, seston C:P and N:P) with N fertilization affect zooplankton biomass, community composition and C:N:P stoichiometry, and their N:P recycling ratio along a gradient in lake DOC concentrations. We found that despite major differences in phytoplankton biomass with DOC (unimodal distributions, especially with N fertilization), no major differences in zooplankton biomass were detectable. Instead, phytoplankton to zooplankton biomass ratios were high, especially at intermediate DOC and after N fertilization, implying low trophic transfer efficiencies. An explanation for the observed low phytoplankton resource use, and biomass responses in zooplankton, was dominance of colony forming chlorophytes of reduced edibility at intermediate lake DOC, combined with reduced phytoplankton mineral quality (enhanced seston N:P) with N fertilization. N fertilization, however, increased zooplankton N:P recycling ratios, with largest impact at low DOC where phytoplankton benefitted from light sufficiently to cause enhanced seston N:P. Our results suggest that although N enrichment and increased phytoplankton biomass do not necessarily increase zooplankton biomass, bottom-up effects may still impact zooplankton and their N:P recycling ratio through promotion of phytoplankton species of low edibility and altered mineral quality.

     

The zooplankton communities of an artificially divided subtropical coastal estuarine–lake system in South Africa

Nhlabane coastal lake and estuary system in South Africa is divided by a permanent barrage constructed at the outlet of the lake. This study compared the zooplankton assemblages of the lake and estuary systems after 15 years of separation. Before sampling commenced the mouth of the estuary closed and remained so throughout the duration of the study. The lake was completely fresh and the estuary had a low salinity not exceeding 4‰. Both the estuary and lake are shallow systems (less than 2 m) and were well mixed. Turbidity levels were generally below 10 NTUs. Temperature fluctuated seasonally between 18.5 and 30°C. Oxygen levels ranged between 4.3 and 9.9 mg/l but were mostly above 5 mg/l for both systems. The estuarine and lake plankton was dominated by copepod instars, especially of the calanoid Pseudodiaptomus hessei. During the entire study period the lake also supported high densities of rotifers. Rotifers only appeared in the estuarine plankton at the end of the sampling period. During the first sampling sessions zooplankton diversity in the estuary was higher but declined thereafter to levels within the range calculated for the lake. Estuarine relict species recorded reflect the estuarine history of the lake. Multivariate analyses indicated a change in the composition of the zooplankton community in the estuary during the duration of the study, while that of the lake remained comparatively stable. Species composition analyses showed that this change in the estuary was due to a shift from an estuarine to a more freshwater zooplankton assemblage. During this change some estuarine species declined in abundance or were not recorded anymore. These changes in the zooplankton community of the estuary may well illustrate the changes that occurred in the lower (south) section of the lake after completion of the barrage which isolated it from the estuary. This revised version was published online in July 2006 with corrections to the Cover Date.     

Limnology of a desertic terminal lake,Walker Lake,Nevada, U.S.A.

Walker Lake, a large terminal lake in west-central Nevada, is rapidly desiccating and becoming more saline. From 1937 to 1977, total dissolved solids increased from 5 650 to 10 650 mg l^–1. Ionically, the lake is a chlorocarbonate one; however, as desiccation continues it should eventually have about equal amounts of chloride, sulfate and carbonate. With regard to algal nutrients, the lake appears to be nitrogen limited. The phytoplankton is currently dominated by Nodularia spumigena; at times this produces noxious blooms. The zooplankton community is depauperate and composed of three species. The fish fauna is limited to three endemic fishes, with tui chub, Gila bicolor, dominant. Lahontan cutthroat trout, Salmo clarki henshawi, is restricted during summer to a narrow mid-water zone low enough in temperature and high enough in dissolved oxygen to meet physiological requirements.Dr. Koch passed away in October 1983.     

The role of light for fish–zooplankton–phytoplankton interactions during winter in shallow lakes – a climate change perspective

1. Variations in the light regime can affect the availability and quality of food for zooplankton grazers as well as their exposure to fish predation. In northern lakes light is particularly low in winter and, with increasing warming, the northern limit of some present-day plankton communities may move further north and the plankton will thus receive less winter light.
2. We followed the changes in the biomass and community structure of zooplankton and phytoplankton in a clear and a turbid shallow lake during winter (November–March) in enclosures both with and without fish and with four different light treatments (100%, 55%, 7% and <1% of incoming light).
3. In both lakes total zooplankton biomass and chlorophyll- a were influenced by light availability and the presence of fish. Presence of fish irrespective of the light level led to low crustacean biomass, high rotifer biomass and changes in the life history of copepods. The strength of the fish effect on zooplankton biomass diminished with declining light and the effect of light was strongest in the presence of fish.
4. When fish were present, reduced light led to a shift from rotifers to calanoid copepods in the clear lake and from rotifers to cyclopoid copepods in the turbid lake. Light affected the phytoplankton biomass and, to a lesser extent, the phytoplankton community composition and size. However, the fish effect on phytoplankton was overall weak.
5. Our results from typical Danish shallow eutrophic lakes suggest that major changes in winter light conditions are needed in order to have a significant effect on the plankton community. The change in light occurring when such plankton communities move northwards in response to global warming will mostly be of modest importance for this lake type, at least for the rest of this century in an IPCC A2 scenario, while stronger effects may be observed in deep lakes.     

鄱阳湖不同水文期浮游生物群落结构特征和影响因素及水质评价

为阐明鄱阳湖不同水文期浮游生物群落结构特征及其影响因素,研究于2017年8月(丰水期)和12月(枯水期)在鄱阳湖湖区典型水域设置5个采样点进行浮游生物采样调查。研究期间共鉴定浮游植物8门75属186种,丰水期与枯水期均以硅藻门和绿藻门为主。共鉴定浮游动物4类76种,丰水期与枯水期均以原生动物和轮虫为主。方差分析显示:浮游植物密度与生物量在不同水文期之间的差异均为极显著(P<0.01),浮游动物丰水期密度高于枯水期,但无显著差异(P>0.05),浮游动物生物量(P<0.05)在不同水文期差异显著。冗余分析(RDA)显示:丰水期透明度和浮游生物呈显著负相关关系,电导率和浮游生物呈显著正相关。透明度、电导率与营养盐是影响丰水期浮游生物群落结构的主要环境因素,枯水期水温和溶解氧是驱动鄱阳湖浮游生物群落生态分布的主要环境因素。基于Shannon-Wiener(H′)、Margalef(d)和Pielou(J)等生物多样性指数的水质评价结果表明:鄱阳湖研究区域水质状态处于寡污-中污之间。研究揭示了2个水文期对通江湖泊浮游生物的影响:季节变化不改变湖泊浮游生物的物种组成及优势种,但...     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Zooplankton and environmental factors of a recovering eutrophic lake (Lysimachia Lake, Western Greece)

The present study investigates the zooplankton community dynamics and the abiotic environment in the eutrophic Lake Lysimachia (western Greece). The lake is considered to be recovering from eutrophication after the termination of an urban sewage inflow in 2000, and its waters are replenished constantly from the nearby oligotrophic Lake Trichonis. The results show that, although a decrease in nutrient concentrations was observed compared to the past, the lake still has eutrophic characteristics. This was reflected in the zooplankton community which is typical of those found in eutrophic lakes where rotifers prevail. Similarities among this lake and other nearby lakes were found considering the zooplankton community composition and seasonal variation. However, Lake Lysimachia is inhabited also by a number of different and even unique species (e.g., Moina micrura), suggesting that this ecosystem may be an important biodiversity refuge. Most of the zooplankton species were correlated with water temperature and, to a lesser extent, eutrophication key-water quality variables. Although there are few available data on the zooplankton of the lake, the abundance and composition of the community presenting characteristics indicative of intermediate trophic conditions and suggesting that the lake is probably under a kind of “biological” recovery.     

The ecology of ruffe,Gymnocephalus cernuus (Pisces: Percidae) introduced to Mildevatn,western Norway

Synopsis Diet, habitat use, diel and seasonal activity and a number of population parameters were studied on ruffe,Gymnocephalus cernuus, introduced to Mildevatn, western Norway. This lake is sited outside the natural range of the ruffe and has a lower fish diversity and a different fish species composition than within its native range. From June through September the ruffe was planktivorous and mainly caught at 4 to 6 m depth in the benthic zone. At other times of year ruffe was feeding on zoobenthos and caught deeper in the benthic zone. Ruffe was mainly day active. Zooplankton feeding during summer is the clearest difference compared to ruffe populations living within its natural range. Presence of large zooplankton organisms available for ruffe is suggested as the main reason for the difference found in food choice. The availability of large zooplankton is probably due to community structure caused by a predator and lack of interspecific competition for zooplankton in the deeper parts of the lake. Piscivorous brown trout.Salmo trutta, restrict the habitat of threespined stickleback,Gasterosteus aculeatus, to the zone of littoral vegetation, allowing high densities of larger zooplankton species likeBythotrephes longimanus to be present in the lake. Brown trout is present only in the upper light and well oxygenated parts of the lake, leaving a refuge for the ruffe, where they can feed on the rich zooplankton community.     

西藏达则错盐湖沉积背景与有机沉积结构

以西藏拟溞(Daphniopsis tibetana Sars)为优势浮游动物物种的低盐度盐湖是西藏湖泊的一个重要类型,以达则错为代表,分析了其沉积背景及沉积物组成。结果如下:(1)湖泊敞水区无机沉积以内生化学沉积为主,可代表深水盐湖无机沉积物的自然沉积过程。(2)达则错盐湖浮游植物以蓝藻、硅藻、裸藻、绿藻为主,总生物量11.35 mg/L;浮游动物生物量为4.92 mg/L,其中西藏拟溞占 82.30%;浮游植物残体受盐梯度影响在盐梯度层之上聚集,而浮游动物残体及粪粒(Fecal pellets)因外表有碳酸盐附着可穿过盐梯度层沉积湖底,生物残体与浮游动物代谢产物构成了沉积有机物的物质基础。(3)表层沉积物平均含水量为66.70%,粒径0.004-0.02 mm范围内的颗粒物含量最大,占20.42%,其次为<0.004 mm的粘土,占4.53%。(4)表层沉积物总有机碳(TOC)平均含量为27.99 mg/g(干重),其中颗粒有机碳(POC)约为18.11 mg/g,占TOC的64.70%;在POC中,西藏拟溞粪粒贡献最大,约占POC的60.48%,占TOC的39.06%,占沉积物总量的1.12%,其次为西藏拟溞残体,占POC的38.85%。分析结果表明盐湖因其独特的水化学和生物学特征具有较强的沉积能力,以化学沉积为主的无机沉积及以西藏拟溞粪粒和残肢碎屑为主的有机沉积构成了该类型盐湖颗粒物沉降及沉积的主要过程。     

Fluctuations in water level and the dynamics of zooplankton: a data‐driven modelling approach

1. The zooplankton in Lake Kinneret (Israel) have undergone large fluctuations in recent decades, which have been linked to both biotic and abiotic processes. 2. By applying a data‐driven modelling approach to a long‐term database, and focusing on key abiotic (lake‐level change) and biotic (prey abundance) variables, we attempted to identify the possible factors impacting the lake’s zooplankton community. 3. We hypothesised that changes in the predatory zooplankton (adult cyclopoids) assemblage are driven by changes in lake level during years of large changes. We further postulated that lake‐level changes would have a similar impact on the herbivorous zooplankton (cladocerans and cyclopoid copepodites) but an opposite effect on the microzooplankton. In the years of moderate changes to lake level, however, the abundance of predatory zooplankton would determine the size of the herbivore and microzooplankton populations rather than their food sources, that is, top‐down control. 4. The resulting decision trees supported the hypotheses stressing the importance of the annual rate of lake‐level change in shaping the zooplankton community in the lake. In addition, and in contrast to expectations, bottom‐up processes seem to play a role in determining zooplankton abundance.     

极端水文干旱下鄱阳湖浮游动物群落结构特征及其影响因素

2022年鄱阳湖流域发生了特大干旱事件,对鄱阳湖生态环境产生了严重影响。为揭示极端水文干旱年的鄱阳湖浮游动物群落结构特征及其主要影响因素,于2022年1月(冬季)、4月(春季)、7月(夏季)和10月(秋季)对鄱阳湖浮游动物进行了综合调查。本次调查共鉴定出浮游动物70种(轮虫40种、桡足类17种和枝角类13种),丰度和生物量范围分别为0—152.67个/L和0—1.52 mg/L。浮游动物群落结构具有较大的时空差异：在季节上,物种数夏季最多,丰度和生物量呈现夏季最高、秋季最低的特征,干旱季节的Shannon-Wiener多样性指数和优势种组成明显不同于非干旱季节;在空间上,南部湖区的物种数、丰度、生物量高于北部湖区,多样性指数在中部湖区最高、北部湖区最低。极端水文干旱年的物种数、丰度和生物量均明显低于往年同期,但空间上的差异较小。相关性分析和冗余分析结果表明,浮游动物群落结构在干旱季节和非干旱季节的主要影响因素存在明显差异,其中干旱季节浮游动物群落结构主要受水温、水位、硝态氮、氨氮等的共同影响,非干旱季节受化学需氧量和水位的影响较大。总体上,极端水文干旱使得鄱阳湖浮游动物群落结构稳定性较...     

基于宏基因组的保安湖浮游动物多样性研究

研究于2019年春、夏、秋、冬四季对保安湖进行了水样采集, 基于宏基因组测序, 在优化物种鉴定和丰度计算方法的基础上, 考察了保安湖浮游动物的多样性、群落结构及其影响因素。共鉴定到浮游动物OTU 374种, 其中原生动物282个; 枝角类45个; 桡足类26个; 轮虫21个。从季节来看, 夏、秋季保安湖的浮游动物多样性高; 从湖区来看, 肖四海和主湖区浮游动物多样性高。季节因素对保安湖浮游动物群落结构的影响高于湖区影响。保安湖营养状态为中营养型, 水体温度、叶绿素a是影响保安湖浮游动物群落结构的主要环境因子, 不同类群与环境因子相关性不同, 总体可分为5类。其中原生动物优势类群为混合营养的纤毛虫和丝足虫, 同硝氮、化学需氧量、温度有明显的相关性, 而枝角类和桡足类同环境因子的关系较为相似, 与溶氧、叶绿素a、正磷酸盐存在明显相关。研究利用宏基因组方法对保安湖浮游动物多样性开展了研究, 为从浮游动物这一角度来理解保安湖这一江湖阻隔型湖泊的生物多样性的变化提供了支撑。     

Fluctuation of the zooplankton community in Lake Biwa during the 20th century: a paleolimnological analysis

Detailed zooplankton records from a 26-cm sediment core with a time resolution of approximately 3–10 years were obtained from Lake Biwa, Japan, to examine the historical variations in the zooplankton community during the 20th century. In the sediments, selected zooplankton remains have fluctuated over the years. Daphnia – large zooplankton herbivores – did not occur from 1900 to 1920, and formed a very minor component of the zooplankton community in the following 30 years, while Bosmina – small zooplankton herbivores – were common during this period. In the mid-1960s, however, when eutrophication was noticeable in this lake, Daphnia numbers increased dramatically and became the dominant zooplankton thereafter. In contrast, Difflugia brevicolla and D. biwae, two amoeboid protozoans that live in connection with the lake bottom environment, occurred abundantly until the late 1950s, but gradually decreased after the mid-1960s. In particular, D. biwae, a species peculiar to this lake, was not found in sediment dated after 1980, suggesting its extinction. These results indicate that the zooplankton community structure changed greatly in the 1960s, and suggest that the eutrophication occurring at this time altered the relative strength of top-down and bottom-up forces on the zooplankton community in Lake Biwa.