The Seasonal Dynamics and Trophic Relations of the Plankton Components in Lake Peipsi (Peipus)

The seasonal dynamics of the biomass and production of phyto-, zoo- and bacterioplankton was investigated during the vegetation periods (from May to November) in 1985 and 1986 in the pelagial of the large eutrophic lake Peipsi (Estonia). The average values of productions per vegetation period for the investigation years were as follows: phytoplanktion − 203.5 gC · m⁻²; bacterioplankton − 37.9 gC · m⁻²; filter-feeding zooplankton − 20.6 gC · m⁻² and predatory zooplankton − 1.5 gC · m⁻². The herbivorous zooplankton production constituted 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain — filtrators are feeding mostly on living algae and the detrital food chain seems of little importance. The dominance of large forms (Melosira sp., Aphanothece saxicola), in the phytoplankton during the major part of the vegetation period is assumed to be a result of high grazing pressure on small algae. Zooplankton grazing was investigated in situ in a specially constructed twin bathometer. Experimental measurements revealed, that zooplanktion presence in the experimental vessel actually stimulated the phytoplankton growth in many cases — the negative grazing values have been registered. That could be caused by the stimulation effect of nutrients (N, P), excreted by the concentrated zooplankton in the grazing chamber, which led to an increase of the nongrazed phytoplankton production. Bacteria have satisfied the zooplankton food requirements on average by 11%. Grazing on bacteria increased, when grazing on phytoplankton was somehow disturbed.     

Taxonomic composition of rotifers in Lake Peipsi

161 rotifer taxa 46 of which are new records for Estonia were identified in the north-western part of Lake Peipsi. The greatest number of taxa occurred in August (93), followed by September (79). In all, 19 families (Monogononta) are represented with 41 genera. The dominant families are Brachionidae (32 taxa) and Synchaetidae (16). The species of Anuraeopsis, Conochilus, Kellicottia, Keratella, Notholca, Polyarthra and Synchaeta are dominating. The taxonomic composition is analysed in the ecological aspect. Some species of zoogeographic interest, including a supposedly endemic Ploesoma peipsiense, are reported.     

The effect of Cyanophyta upon zooplankton in a eutrophic tropical lake (Lake Valencia,Venezuela)

The effect which Cyanophyta have upon the zooplankton varies according to the form of the alga (mucilaginous colonies or filaments) and its abundance. Periodical blooms of Microcystis aeruginosa were not detrimental for the zooplankton, in spite of the fact that copepods, cladocerans and rotifers consume small colonies. High concentrations of Lyngbya limnetica and Oscillatoria limnetica in Lake Valencia, Venezuela, proved to be inhibitory for cladocerans. A total absence of cladocerans was detected when filaments increased.     

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos in hypertrophic Lake Teganuma

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos were intensively surveyed in Lake Teganuma during May 1983–April 1984. The annual mean chlorophyll a concentrations were as high as 304 µg · l^–1–383 µg · l^–1. The daily gross primary production of phytoplankton was high throughout the year. The peak production rate was recorded in August and September, when blue-green algae bloomed. The annual gross primary production was estimated as 1450 g C · m^–2 · y^–1, extremely high as compared with other temperate eutrophic lakes. Zooplankton was predominantly composed of rotifers. The annual mean standing crop of zooplankton was 0.182 g C · m^–2 around the middle between the inlets and the outlet and was lower than in most other temperate eutrophic lakes. Zoobenthos was mostly composed of Oligochaeta and chironomids. The annual mean standing crop of zoobenthos ranged from 0.052 g C · m^–2 to 0.265 g C · m^–2, the lowest values among temperate eutrophic lakes, which is in contrast to the high primary production.     

大亚湾海域浮游动物生态特征

2004年3月(春季)、5月(夏季)、9月(秋季)和12月(冬季)4个航次对大亚湾海域浮游动物进行了调查。共计鉴定浮游动物128个种类,浮游幼体14个类群。浮游动物出现的种类数依次是:夏季(90种),秋季(81种),冬季(71种),春季(47种)。浮游动物的丰度以夏季最高,平均1013.38 ind.m-3;其次是秋季,平均913.30 ind.m-3;春季最低,平均162.37 ind.m-3。浮游动物生物量的季节变化:秋季(773.89 mg.m-3),夏季(472.82 mg.m-3),冬季(286.44 mg.m-3),春季(164.11 mg.m-3),生物量高低与种类组成关系密切,秋季优势类群为水母类和毛颚类等大型浮游动物,所以生物量较高。4个季节的Shannon-Wiener多样性指数H′值、物种均匀度指数J值和物种丰富度指数D值的变化趋势十分相似:春季的3项指数值均最低,冬季最高,夏、秋季节相差不大。此外,分析结果表明:毛颚类作为优势种且4个季节均有出现,是对大亚湾海域水体较20a前水温上升的响应;浮游动物近岸生物量高于湾中部;从丰度的平面分布看,大亚湾海域浮游动物栖息环境已经不同程度受到大型建设工程和人类活动的影响。     

引水和疏浚工程支配下杭州西湖浮游动物的群落变化

研究了引水和疏浚工程支配下浅水、富营养化杭州西湖浮游动物群落的长期变化,包括浮游动物的优势种类组成、密度及生物量与水环境因子的相关分析.疏浚后的2003年调查中,西湖3个采样站的定量样品中共发现69种浮游动物,其中原生动物26种,轮虫27种,枝角类和桡足类各8种.Ⅰ站浮游动物年平均生物量从1990年的0.186mg/L上升到2003年的0.705mg/L,Ⅱ站和Ⅲ站分别从0.665mg/L和0.740mg/L上升到1.399mg/L和1.195mg/L.浮游动物数量组成中原生动物和轮虫平均占99%,并占78%的生物量.在1980～2003年期间,一些优势种类如砂壳纤毛虫（Tintinnoinea）、针簇多肢轮虫（Polyarthra trigla）和长额象鼻溞（Bosmina longirostris）等显著增加了它们的丰度和优势度;暗小异尾轮虫（Trichocerca pusilla）的优势度在引水后的1990～1995年增加了,但在疏浚后的2003年下降了;而20世纪80年代的一些优势种如毛板壳虫（Coleps hirtus）、螺形龟甲轮虫（Keratella cochelearis）和短尾秀体溞（Diaphanosoma brachyurum）等在3个采样站中失去优势种地位或消失.原生动物和轮虫生物量在营养水平较高的Ⅱ～Ⅲ站明显高于营养水平较低的Ⅰ站;长肢秀体溞（Diaphanosoma leuchtenbergianum）、长额象鼻溞、颈沟基合溞（Bosminopsis deitersi）和汤匙华哲水蚤（Sinocalanus dorrii）在营养水平较低的Ⅰ站具有较大的密度和生物量,而微型裸腹溞（Moina micrura）和粗壮温剑水蚤（Thermocyclops dybowskii）则在营养水平较高的Ⅱ～Ⅲ站具有较大的密度和生物量.西湖各类浮游动物在不同湖区形成不同的分布格局主要由引水水流和水体营养状态差异造成.1990～2003年期间,在采样站变异下,浮游动物中轮虫年平均生物量与水体年平均pH值和叶绿素a含量之间分别有极显著和显著的正相关关系,与水体透明度之间有极显著的负相关关系.引水后的1995年,与轮虫生物量最密切的生态因子是叶绿素a含量,而疏浚后水体碱性环境是影响轮虫生物量最密切的生态因子.     

Zooplankton of Lake Peipsi-Pihkva in 1909–1987

164 taxa were identified in the net zooplankton of the pelagial of L. Peipsi-Pihkva in 1909–1987, including 3 species of protozoans, 74 species of rotifers, 58 species of cladocerans, 28 species of copepods and 1 mollusc. One rotifer species, Ploesoma peipsiense Mäemets et Kutikova, has been described as new for science here. The zooplankton of L. Peipsi-Pihkva is remarkably rich in species including rarities in Estonia: Limnosida frontosa, Drepanothrix dentata, Bythotrephes longimanus, B. cederstroemi etc. Due to its large surface area, L. Peipsi-Pihkva provides a large scale of biotopes of a diverse trophic state and humic content, which support species with different ecological requirements. Most of the aquatory of the lake has lately been mesotrophic, favouring the coexistence of indicators of oligo- and mesotrophic state and species preferring a higher trophic state. The occurrece of 10 species of the genus Bosmina including B. berolinensis, B. gibbera, B. lilljeborgi, B. thersites and B. crassicornis, sparse in Estonian lakes, is the most noteworthy feature of the zooplankton of L. Peipsi-Pihkva. The coexistence of B. coregoni and B. berolinensis, B. gibbera, B. lilljeborgi etc. which were earlier regarded as subspecies of B. c. coregoni proves that they are different species producing usually no hybrids. The species composition was subjected to certain changes during the years under consideration. Larvae of Dreissena were first found in zooplankton in 1962. The oligo-mesotrophic indicator Holopedium gibberum occurred in the lake in 1909–1964, but was lacking in later samples.     

Zooplankton-trophic state relationships in some north and central Florida lakes

In a survey of eight lake systems located in north-central Florida, total zooplankton abundance showed a strong positive correlation (r²=0.87, a=0.01) with trophic state. Zooplankton abundance averaged 1.0 × 10⁵ organisms · m^–2 in oligotrophic systems and up to 8.2 × 10⁵ organisms · m^–2 in the eutrophic systems. Seasonal variations in total abundance were greatest in the eutrophic lakes where rotifers dominated and periodically produced sharp population peaks (approaching 2.0 × 10⁶· m^–2). In contrast, the more oligotrophic systems had relatively stable levels of total abundance and were dominated by copepods. Diversities of the major taxa in the lakes were variable with one to three species of copepods, zero to four species of cladocera, and two to seven species of rotifers dominant at any one time. Planktonic cladoceran communities were often composed of only one or two species. Low cladocera diversity in these subtropical systems was suggestive of increased predation pressure on this group of crustaceans. A comparison of the total crustacean abundance in the Florida systems to those of some of the Great Lakes indicated that lower standing crops of crustacean zooplankton in the Florida lakes may be a response to both predation and temperature.Contribution Number 043, Marine Science Programs Laboratory, Dauphin Island, Alabama, U.S.A.Contribution Number 043, Marine Science Programs Laboratory, Dauphin Island, Alabama, U.S.A.     

Production and Ecology of Benthic Chironomid Larvae (Diptera) in Lake Hayes,New Zealand,a Warm-monomictic Eutrophic Lake

The distribution and abundance of larval chironomids in Lake Hayes were studied from December 1973 to March 1975. The mean annual production of the two dominant species, Chironomus zealandicus and Chironomus sp. a, was 29.2 g m⁻² dry weight which is approximately 4.3 % of the average annual phytoplankton production in the lake. A high annual P/B ratio of 18.5 is consistent with the multivoltine life cycle of C. zealandicus. Larval chironomid production in the second summer when Anabaena blooms were absent was only one quarter of that in the first summer and is consistent with the hypothesis that the production of benthic chironomids in Lake Hayes is closely linked to that of the phytoplankton through the sedimentation of autochthonous organic matter.     

乐清湾浮游动物的季节变动及摄食率

2002年8月、11月、2003年2月和5月,在乐清湾进行了4个航次生物、化学和水文等专业综合调查。根据采集的浮游动物样品的分析鉴定及海上现场实验结果,对浮游动物的群落组成、生物量、丰度、多样性指数的分布和季节变动及其浮游动物对浮游植物的摄食率进行研究。结果表明,乐清湾已鉴定的浮游动物有56属,75种,17类浮游幼体,主要可划分为4个生态类群,以近岸低盐类群为主,其优势种为真刺唇角水蚤Labidoceraeuchaeta、太平洋纺锤水蚤Acartiapacifica、驼背隆哲水蚤Acrocalanusgibber、中华假磷虾Pseudeuphausiasinica和百陶箭虫Sagittabedoti等,半咸水河口类群、暖水性外海种和广布种的种数相对较少。浮游动物生物量和丰度的平面分布趋势基本一致,有明显季节变化。2月份和5月份,浮游动物生物量和丰度,从湾顶向湾口呈逐渐增加趋势;8月份,生物量和丰度的分布与2月份、5月份的分布趋势不同,从湾顶向湾口,生物量和丰度逐渐降低;11月份,生物量和丰度的平面分布相对均匀。浮游动物种类多样性指数有明显的季节变化,其动态变化与浮游动物种数和丰度的变化一致。微型浮游动物对浮游植物存在摄食压力,且摄食率有季节变化,摄食率的变化在0.15~0.48d-1。     

披山岛海区浮游动物种类组成及数量分布特点

根据2007年披山岛海区春季和夏季2个季度浮游动物调查资料,分析和研究了浙江省玉环县披山岛海区浮游动物种类组成和数量分布特点.结果表明:浮游动物共有18种,属3门12属,其中桡足类的种类最多,占总种数的72.22%;主要优势种为中华哲水蚤(Calanus sinicus)、瘦拟哲水蚤(Paracalanus gracilis)、针刺拟哲水蚤(Paracalanus aculeatus)和红小毛猛水蚤(Microsetella rosea).浮游动物的平均丰度及生物量均为春季比夏季高,春季的平均丰度为6.55×103个/m3,夏季为6.13×103个/m3;春季的平均生物量为216.49 mg/m3,夏季为98.26 mg/m3.桡足类的生物量占浮游动物总量的75.97%～97.55%,是决定披山岛浮游动物数量分布和变动的主要成分.与历史资料和邻近海域相比较,披山岛海区生物量与其接近或大于东海各海域.春、夏两季生物多样性(H′)有显著性差异(P<0.05),春季具有较高的生物多样性,且远海的生物多样性大于近海.     

The Structure of a Macroinvertebrate Community in a Northern German Lake Outlet (Lake Belau,Schleswig-Holstein) with Special Emphasis on Abundance,Biomass and Secondary Production

The structure of macroinvertebrate communities was studied at I I sampling sites of the outlet of Lake Belau in the lowlands of northern Germany. To describe the structures of macrobenthic animal communities three different units were examined: abundance, biomass, and secondary production. 112 taxa were collected from the entire stream. The numbers of species ranged from 31 (fine sand) to 70 (submerged macrophytes). For the stream, average macroinvertebrate density was 18.400 ind. M⁻¹. Density was highest at the macrophytes amounting to 35,630 individuals per m², and lowest in the pure sand with only 3,900 ind. M⁻². Average biomass (dry mass) was 194 g DM m⁻² varying from 9.8 (peat) to 381 g DM m⁻² (gravel with mollusk shells near the upstream lake). For the stream, average annual production was 129 g DM m⁻² varying from 15 (peat) to 286 g DM m⁻² (macrophytes). The highest values for each unit were found in stream sections with gravel and submerged macrophytes. Lower values occured in sections that contained peat and sand. Usually, a single structure of the macroinvertebrate community was dominated by less than ten taxa, which varied at each sampling site depending on the units observed.     

三门湾浮游动物的季节变动及微型浮游动物摄食影响

2002年8月、11月、2003年2月和5月,在三门湾进行了4个航次生物、化学和水文等专业综合调查。根据采集的浮游动物样品的分析鉴定及海上现场实验结果,对浮游动物的群落组成、生物量、丰度、多样性指数的分布和季节变动及其浮游动物对浮游植物的摄食影响进行研究。结果表明,三门湾浮游动物有67属,89种,16类浮游幼体,主要可划分为4个生态类群：以近岸低盐类群为主,其优势种为中华哲水蚤Calanus sinicus、真刺唇角水蚤Labidocera etwhaeta、捷氏歪水蚤Tortanus derjugini、太平洋纺锤水蚤Acartiapacifica、中华假磷虾Pseudeuphausia sinica和百陶箭虫Sagitta bedoti等。半咸水河口类群、暖水性外海类群和广布种相对较少。浮游动物生物量和丰度的平面分布趋势除了夏季有所差异外,其它季节基本一致。2月份和5月份,浮游动物生物量和丰度,从湾顶向湾口呈逐渐增加趋势;8月份,湾口区生物量最高,而丰度高值区出现在湾顶部;11月份,生物量和丰度的平面分布相对均匀。浮游动物种类多样性指数有明显的季节变化,其动态变化与浮游动物种数和丰度的变化一致。微型浮游动物对浮游植物存在摄食压力,且有季节变化,摄食率的变化在0．18．0．68d^-1,微型浮游动物的摄食率低于相同季节的浮游植物生长率。微型浮游动物对浮游植物摄食压力的变化范围为16．1％-49．1％d^-1,对初级生产力摄食压力的变化在58．3％-83．6％d^-1。11月份,微型浮游动物对浮游植物和初级生产力的摄食压力均出现最高值。     

Seasonal dynamics of zooplankton and grazing impact of microzooplankton on phytoplankton in Sanmen Bay, China

The species composition, biomass, abundance and species diversity of zooplankton were determined for samples collected from 12 stations in Sanmen Bay, China, in four cruises from August 2002 to May 2003. Growth of phytoplankton and grazing rates of microzooplankton were measured using the dilution technique. The spatial and temporal variation of zooplankton and its relationship with environmental factors were also analyzed. The results showed that a total of 89 species of zooplankton belonging to 67 genera and 16 groups of pelagic larvae were found in Sanmen Bay. The coastal low-saline species was the dominant ecotype in the study area, and the dominant species were Calanus sinicus, Labidocera euchaeta, Tortanus derjugini, Acartia pacifica, Pseudeuphausia sinica and Sagitta bedoti. Maximum biomass was recorded in August, followed by November and May, and the lowest biomass was recorded in February. Similarly, the highest abundance of zooplankton was observed in August, followed by May, November, and February. Grazing pressure of microzooplankton on phytoplankton in Sanmen Bay existed throughout the year, although the grazing rate of microzooplankton on phytoplankton varied with the season. Estimates for growth rate of phytoplankton ranged from 0.25 d⁻¹ to 0.89 d⁻¹, whereas grazing rate of microzooplankton ranged between 0.18 d⁻¹ and 0.68 d⁻¹ in different seasons. The growth rate of phytoplankton exceeded the grazing rate of microzooplankton in all the seasons. Grazing pressure of microzooplankton on phytoplankton ranged from 16.1% d⁻¹ to 49.1% d⁻¹, and the grazing pressure of microzooplankton on primary production of phytoplankton ranged from 58.3% d⁻¹ to 83.6% d⁻¹ in different seasons.     

Seasonal dynamics of zooplankton and grazing impact of microzooplankton on phytoplankton in Sanmen Bay, China

The species composition, biomass, abundance and species diversity of zooplankton were determined for samples collected from 12 stations in Sanmen Bay, China, in four cruises from August 2002 to May 2003. Growth of phytoplankton and grazing rates of microzooplankton were measured using the dilution technique. The spatial and temporal variation of zooplankton and its relationship with environmental factors were also analyzed. The results showed that a total of 89 species of zooplankton belonging to 67 genera and 16 groups of pelagic larvae were found in Sanmen Bay. The coastal low-saline species was the dominant ecotype in the study area, and the dominant species were Calanus sinicus, Labidocera euchaeta, Tortanus derjugini, Acartia pacifica, Pseudeuphausia sinica and Sagitta bedoti. Maximum biomass was recorded in August, followed by November and May, and the lowest biomass was recorded in February. Similarly, the highest abundance of zooplankton was observed in August, followed by May, November, and February. Grazing pressure of microzooplankton on phytoplankton in Sanmen Bay existed throughout the year, although the grazing rate of microzooplankton on phytoplankton varied with the season. Estimates for growth rate of phytoplankton ranged from 0.25 d^?1 to 0.89 d^?1, whereas grazing rate of microzooplankton ranged between 0.18 d^?1 and 0.68 d^?1 in different seasons. The growth rate of phytoplankton exceeded the grazing rate of microzooplankton in all the seasons. Grazing pressure of microzooplankton on phytoplankton ranged from 16.1% d^?1 to 49.1% d^?1, and the grazing pressure of microzooplankton on primary production of phytoplankton ranged from 58.3% d^?1 to 83.6% d^?1 in different seasons.     

Seasonal dynamics and grazing rate of zooplankton in Yueqing Bay,China

The species composition,biomass,abundance,and species diversity of zooplankton were determined for samples collected from August 2002 to May 2003 from 14 stations in Yueqing Bay,China.Phytoplankton growth rate and microzooplankton grazing rate were obtained by using the dilution method developed by Landry and Hassett.The spatial and temporal variations of zooplankton and its relationship with environmental factors were also analyzed.The results showed that the zooplankton in the Yueqing Bay could be divided into four ecotypes,namely coastal low saline species,estuary brackish water species,offshore warm water species,and eurytopic species.A total of 75 species of zooplankton belonging to 56 genera and 17 groups of pelagic larva were identified in the Yueqing Bay.The coastal low saline species was the dominant ecotype in the study area,and the dominant species were Labidocera euchaeta,Acartia pacifica,Acrocalanus gibber,Pseudeuphausia sinica,and Sagitta bedoti among others.There was considerable seasonal variation in zooplankton biomass and abundance in the surveyed areas.The peak biomass appeared in August,descending in November and in May,and the lowest biomass appeared in February.Similarly,the highest abundance of zooplankton was observed in August,with the abundance descending in the following months:May,November,and February.There were similar horizontal distribution patterns for the biomass and the abundance of zooplankton.They both increased from the upper to the lower bay in February and May,but decreased from the upper to the lower bay in August.Biomass and abundance were evenly distributed in the Yueqing Bay in November.Moreover,there was marked seasonal variation in the species diversity of zooplankton,which conformed to the abundance of zooplankton.Results of the dilution experiments indicated that there was grazing pressure of microzooplankton on phytoplankton in the Yueqing Bay throughout the year though the rate of microzooplankton grazing on phytoplankton varied seasonally.Phytoplanktons were growing at 0.26-2.07/d and grazed by microzooplankton at a rate of 0.15--0.48/d in different seasons.     

Seasonal dynamics and grazing rate of zooplankton in Yueqing Bay, China

The species composition, biomass, abundance, and species diversity of zooplankton were determined for samples collected from August 2002 to May 2003 from 14 stations in Yueqing Bay, China. Phytoplankton growth rate and microzooplankton grazing rate were obtained by using the dilution method developed by Landry and Hassett. The spatial and temporal variations of zooplankton and its relationship with environmental factors were also analyzed. The results showed that the zooplankton in the Yueqing Bay could be divided into four ecotypes, namely coastal low saline species, estuary brackish water species, offshore warm water species, and eurytopic species. A total of 75 species of zooplankton belonging to 56 genera and 17 groups of pelagic larva were identified in the Yueqing Bay. The coastal low saline species was the dominant ecotype in the study area, and the dominant species were Labidocera euchaeta, Acartia pacifica, Acrocalanus gibber, Pseudeuphausia sinica, and Sagitta bedoti among others. There was considerable seasonal variation in zooplankton biomass and abundance in the surveyed areas. The peak biomass appeared in August, descending in November and in May, and the lowest biomass appeared in February. Similarly, the highest abundance of zooplankton was observed in August, with the abundance descending in the following months: May, November, and February. There were similar horizontal distribution patterns for the biomass and the abundance of zooplankton. They both increased from the upper to the lower bay in February and May, but decreased from the upper to the lower bay in August. Biomass and abundance were evenly distributed in the Yueqing Bay in November. Moreover, there was marked seasonal variation in the species diversity of zooplankton, which conformed to the abundance of zooplankton. Results of the dilution experiments indicated that there was grazing pressure of microzooplankton on phytoplankton in the Yueqing Bay throughout the year though the rate of microzooplankton grazing on phytoplankton varied seasonally. Phytoplanktons were growing at 0.26–2.07/d and grazed by microzooplankton at a rate of 0.15–0.48/d in different seasons. __________ Translated from Acta Ecologica Sinica, 2005, 25(8): 1853–1862 [译自: 生态学报, 2005, 25(8): 1853–1862]     

Observations on the crustaceous zooplankton in some freshwater bodies of the sub-antarctic island marion

The Entomostraca Pseudoboeckella volucris and Daphniopsis studeri dominate the freshwater zooplankton of the sub-Antarctic island Marion. The largest numbers of zooplankton are found in biotically fertilized water bodies. Seasonal and diurnal variations were recorded and a correlation between zooplankton biomass and primary production could be established.     

Long-term changes in zooplankton abundance and water transparency in Lake Geneva

The water quality of Lake Geneva has declined steadily since the 1960s, due to a continuous increase of external phosphorus loading. Average P level in the lake increased steadily to a peak in 1979, and even 1981 in the case of P content in the trophogenic layer. Since then, reduced external inputs related to the delayed effects of phosphorus removal from waste waters initiated many years previously has led to a decrease in P level, and resulted in present stabilization and even improvement in water quality. Long-term changes in zooplankton abundance correspond quite closely to eutrophication level changes. After increasing since the 1960s, maximum zooplankton biomass was recorded for the first time in 1971; a second main peak appeared in 1981 together with the highest eutrophication level. Over the last seven years, zooplankton abundance has decreased continuously, while water transparency has decreased and phytoplankton production has remained at a high level.