鄱阳湖丰水期水位波动对浮游动物群落演替的影响

为了解鄱阳湖夏季丰水期水位剧烈波动过程中浮游动物的群落演替特征,2012年夏季鄱阳湖水位剧烈波动期间,于6月24日、7月7日和8月27日当水位下降且接近17.6 m时,在江西鄱阳湖国家级自然保护区内的1个浅水碟形湖泊设置4个采样点进行采样调查。共发现浮游动物65种,其中轮虫52种,枝角类7种,桡足类6种,多为营浮游生活的广温性和嗜温性种类。单因子方差分析(one-way ANOVA)显示3个月之间浮游动物的密度和生物量均具有显著差异(P0.05),7月份浮游动物密度(1030.17±68.18个/L)显著高于6月份(325.16±41.60个/L)和8月份(203.79±24.91个/L);6月份浮游动物的生物量(0.56±0.04 mg/L)显著低于7月份(1.22±0.11 mg/L)和8月份(0.99±0.11 mg/L)。基于浮游动物多度的聚类分析和自组织映射神经网络图均揭示夏季3个月份的浮游动物可区分为明显的3个群落:6月群落、7月群落和8月群落。蒙特卡罗检验发现水温、电导率、浊度和溶氧与浮游动物群落结构变化显著相关(P0.05)。典型相关分析显示,6月份浮游动物群落与叶绿素a含量呈显著正相关关系,7月份浮游动物群落与水体温度呈显著正相关关系,8月份浮游动物群落与水深和电导率、浊度和溶氧呈显著正相关关系(P0.05)。在3个月均为优势物种的盖氏精囊轮虫与叶绿素a含量呈正相关关系(P0.05),与水温、pH、溶氧呈负相关(P0.05)。夏季水位波动过程中浮游动物的群落结构在时间上(月份之间)发生明显演替,呈现轮虫密度逐渐降低,枝角类和桡足类密度逐渐增加的变化规律。水位波动引起环境因子的改变,从而对浮游动物的群落演替产生了重要影响。     

Structure and feeding activities of zooplankton community in Lake Zwemlust,in the two years after biomanipulation

The biomanipulation study in Lake Zwemlust (area 1.5 ha; mean depth 1.5 m) is among the series of such investigations initiated recently in The Netherlands. The effects of the lake's reffilling (after it was first completely drained empty) almost entirely the nutrient-rich seepage water from the River Vecht flowing nearby and of removal of the planktivorous bream (Abramis brama), on zooplankton community structure and feeding activities of crustaceans were monitored for two successive years (1987, 1988). In these years a classical pattern of succession occurred, with the rotifer spring maximum preceding the crustacean maximum by about 3 weeks. Among the fiveDaphnia species, which appeared in quick succession during May–July, two were large-bodied forms (D. magna; D. pulex). OnlyD. pulex persisted and was the important grazer species in the second year, especially in spring. In the first year the crustacean grazing, with several values >100%.d^–1, contributed significantly to the lake's improved water clarity, with Secchi-disc transparencies of 1.5 m and more almost throughout the summer, compared with 0.3 m before the biomanipulation. Even though the water clarity climate in the second year was quite similar to that in the first, the causal factor was high macrophytic vegetation, rather than zooplankton grazing. The lake developed a rich littoral flora and fauna in the second year in response to the optimal light and nutrient conditions. Apparently, the predation by the introduced planktivorous rudd (Scardinius erythrophthalmus) on zooplankton was an important factor in the changes in zooplankton structure, and in the reduced role of zooplankton in seston elimination during the second year. There is some evidence from bioassay work that, simultaneously with the littoral development, nitrogen limitation of the phytoplankton also contributed to the improved light situation in the second year.     

The influence of water level on macrophyte growth and trophic interactions in eutrophic Mediterranean shallow lakes: a mesocosm experiment with and without fish

1. Water‐level fluctuations are typical of lakes located in the semi‐arid Mediterranean region, which is characterised by warm rainy winters and hot dry summers. Ongoing climate change may exacerbate fluctuations and lead to more severe episodes of drought, so information on the effects of water level on the functioning of lake ecosystems in such regions is crucial. 2. In eutrophic Lake Eymir, Turkey, we conducted a 4‐month (summer) field experiment using cylindrical 0.8‐m‐ (low‐water‐level) and 1.6‐m‐deep (high‐water‐level) mesocosms (kept open to the sediment and atmosphere). Fish (tench, Tinca tinca, and bleak, Alburnus escherichii) were added to half of the mesocosms, while the rest were kept fishless. Ten shoots of Potamogeton pectinatus were transplanted to each mesocosm. 3. Sampling for physicochemical variables, chlorophyll a (chl‐a), zooplankton and per cent plant volume inhabited (PVI%) by macrophytes was conducted weekly during the first 5 weeks, and subsequently biweekly. Macrophytes were harvested on the last sampling date. During the course of the experiment, the water level decreased by 0.41 ± 0.06 m. 4. Throughout the experiment, fish affected zooplankton abundance (?), nutrient concentrations (+), chl‐a (+) and water clarity (?) most strongly in the low‐water‐level mesocosms and the zooplankton community shifted towards dominance of small‐sized forms. The fishless mesocosms had a higher zooplankton/phytoplankton ratio, suggesting higher grazing. 5. Greatest macrophyte growth was observed in the low‐water‐level fishless mesocosms. However, despite high nutrient concentrations and low water clarity, macrophytes were also abundant in the fish mesocosms and particularly increased following a water‐level decrease from midsummer onwards. Macrophyte growth was poor in the high‐water‐level mesocosms, even in the fishless ones with high water clarity. This was ascribed to extensive periphyton development reducing light availability for the macrophytes. 6. Our results indicate that a reduction in water level during summer may help maintain the growth of macrophytes in Mediterranean eutrophic shallow lakes, despite a strong negative effect of fish predation on water clarity. It is therefore probable that an expected negative effect of global climate change on water clarity because of eutrophication and enhanced top‐down control of fish may be, at least partly, counteracted by reduced water level, provided that physical disturbance is not severe.     

A summer-winter comparison of zooplankton in the oceanic area Around South Georgia

Summary Zooplankton was sampled with RMT (1+8) gear on a synoptic grid of stations centred on South Georgia during the austral summer (November/December 1981) and winter (July/August 1983). This initial paper compares zooplankton biomass, vertical distribution and species composition from RMT 1 catches in the oceanic portion of the grid (water depth greater than 2000 m) during the two surveys. In the winter survey, mean zooplankton biomass within the top 1000 m of the water column was 68% of its summer level. This drop was largely due to a decrease in abundance of krill (Euphausia superba), although biomass of copepods and remaining zooplankton also decreased. Copepods averaged 48% of total biomass in summer and winter, but outnumbered all other taxa put together by a factor of 10. Antarctic epipelagic species predominated around the island in the summer survey but tended to be replaced by sub-Antarctic or cosmopolitan species during the winter survey. The majority of zooplankton also showed a downwards seasonal migration out of the top 250 m layer in winter. However, several epipelagic species, including E. superba, did not migrate, and these tended to have the largest summer-winter differences in overall abundance. These trends were attributed to variation in the position of the Polar Front, which lay north of the island during the summer survey but lay across the survey area in winter, resulting in a greater influence of sub-Antarctic water and the displacement of Antarctic species.     

An experimental demonstration of trophic interactions affecting water quality of Rice Lake, Ontario (Canada)

Eight cylindrical enclosures (3 m diameter, 2.7 m long, V = 20m³) were installed in eutrophic Rice Lake (Ontario, Canada) in late spring of 1987. Fish (yearling yellow perch (Perca flavescens) and macrophytes (Potamogeton crispus) presence and absence were set at the beginning of the experiment to yield four combinations of duplicate treatments. The purpose of the experiment was to determine if the phytoplankton, zooplankton, macrophytes and fish species resident in the lake interact to influence water quality (major ions, phosphorus, algal densities and water clarity).The presence of fish was associated with: (1) decreased biomass of total zooplankton, (2) decreased number of species in the zooplankton, (3) decreased average size of several zooplankton taxa, (4) higher total phosphorus concentrations, (5) higher phytoplankton and chlorophyll a concentrations, (6) lower water clarity, (7) lower potassium levels during macrophyte die-back, (8) lower pH and higher conductivity in the presence of macrophytes. Biomass of large Daphnia species (but not total zooplankton) was highly correlated with the algal response (r ² = 0.995) and was associated with reduced biomass of several algal taxa including some large forms (Mougeotia, Oedogonium) and several colonial blue-green algae. However, no significant control of late summer growth of the bloom-forming blue-green alga Anabaena planctonica Brun. was achieved by the Daphnia presence-fish absence treatment. Release of phosphorus to the water column during the die-back of P. crispus was not an important phenomenon.     

In situ hatching of Artemia monica cysts in hypersaline Mono Lake,California

Two emergence trap designs were tested in Mono Lake, California, to measure in situ hatching of Artemia monica cysts on the lake bottom. One design incorporated a removable sample bottle; the other had a catch tube which was pumped from the surface. Both traps rested on the bottom and had a narrow gap between the collecting funnel and bottom flange to allow the chemical conditions within the trap to be similar to those outside. This gap was open during April and May but, because some animals entered from outside the area enclosed by the trap, the gap was covered with 400 µm or 800 µm screen during June and July. The two trap types without screens sampled a station in oxic water 7 m deep similarly in April and May 1985. Mean daily hatching rates from April to May 1985 ranged from 720 to 25 340 shrimp m^-2 day^-1. In contrast, mean daily hatching rates during the same period at a station in anoxic water 21 m deep were from 3 to 138 shrimp m^-2 day^-1. June and July hatching rates in the shallow station were lower than in the spring, usually less than 1000 shrimp m^-2 day^-1.     

Analysis of the change in dominant phytoplankton species in unstratified Lake Oshima-Ohnuma estimated by a bottle incubation experiment

The densities and growth rates of dominant phytoplankton in Lake Oshima-Ohnuma were determined during May to July 1996 to clarify the mechanism of change from spring-dominant to summer-dominant phytoplankton species, examining the influence of zooplankton grazing and nutrient limitation. The dominant phytoplankton in April and May were Asterionella gracillima and Nitzschia spp. Their growth rates were low in May and their densities fell thereafter. The bottle experiment suggests that the main reason for decreases in dominant species has been nutrient deficiency, and zooplankton (sized from 30 to 160 μm) have supported the growth of A. gracillima to recycle nutrients in May. The maximum growth rate was shown by Melosira (Aulacoseira) ambigua living at 2 m in May and June. Although the growth rate of the summer-dominant species Melosira (Aulacoseira) granulata was not as high as that of M. ambigua in May and June, the ambient density increased from late June. Survival in the lower zone of the unstratified euphotic layer might have contributed to the abundance of M. granulata, since the growth rate of this species was not so low as that of M. ambigua at 6 m in June and July. The bottle experiment suggests that the growth of M. granulata was also nutrient limited in June and July and that zooplankton grazing was partially responsible for repressing the biomass of M. granulata before it began growing rapidly in late June. Received: January 31, 2000 / Accepted: September 6, 2000     

Factors influencing the spatial distribution of zooplankton and fish in Loch Ness, UK

• 1 The vertical and horizontal distribution of phytoplankton, zooplankton and fish in Loch Ness, Scotland, were monitored during one day‐time and one night‐time survey in July 1992. The vertical samples were collected at a site located at the northern end of the loch and the horizontal samples along a longitudinal transect.
• 2 The vertical distribution surveys demonstrated that the phytoplankton, the zooplankton and the fish were concentrated in the top 30 m of water above the seasonal thermocline. Within this layer, Cyclops stayed much closer to the surface than Eudiaptomus but both species moved towards the surface at night.
• 3 The most important factor influencing the horizontal distribution of the phytoplankton was the north‐ south gradient in productivity. The sub‐catchments surrounding the north basin contain a greater proportion of arable land than those to the south and the concentrations of nitrate‐nitrogen and phytoplankton chlorophyll increased systematically from south to north.
• 4 Zooplankton distribution patterns were influenced by wind‐induced water movements and the dispersion of allochthonous material from the main inflows. The highest concentrations of Cyclops were recorded in the north, where there was more phytoplankton, and the highest concentrations of Eudiaptomus in the south, where there were higher concentrations of non‐algal particulates.
• 5 There was no spatial correlation between total zooplankton and total fish abundance but the highest concentrations of small (1–5 cm) fish were recorded in the south where there was a large patch of Eudiaptomus. The number of Eudiaptomus at specific locations within this patch were, however, negatively correlated with the numbers of small fish. These results suggest that the fish were actively foraging within the patch and were depleting their zooplankton prey in the areas where they were most abundant.

     

Zooplankton community structure, micro-zooplankton grazing impact, and seston energy content in the St. Johns river system, Florida as influenced by the toxic cyanobacterium Cylindrospermopsis raciborskii

Zooplankton can influence the phytoplankton community through preferential grazing. In turn, nuisance cyanobacteria may affect zooplankton community structure by allowing certain species to out-compete others. We examined zooplankton-phytoplankton interactions, micro-zooplankton (< 200 m) grazing, and biochemical components of the seston in the St. Johns River System (SJR), Florida in the presence and absence of the toxin-producing cyanobacterium Cylindrospermopsis raciborskii. We tested whether this cyanobacterium would cause a decrease in the size structure of the zooplankton community and postulated a resultant decline in the metabolic energy and carbon available to higher consumers (i.e. fish). When numbers of C. raciborskii were low or undetectable, zooplankton were more diverse and were comprised of larger species. Rotifers were the dominant zooplankton, and their numbers relative to other zooplankton increased as C. raciborskii concentrations increased. Micro-zooplankton grazing was higher in times of C. raciborskii abundance, suggesting competitive and predatory exclusion by larger zooplankton in times of higher phytoplankton diversity. Total caloric content of the seston was higher in times of C. raciborskii abundance. However, essential fatty acids and phosphorus may be lacking in the seston, or nutrients may potentially be sequestered by the cyanobacteria and remain as organic matter in the water column. In such cases, higher trophic levels would not be able to obtain optimal energy requirements. Overall, there was a greater impact of micro-grazers on phytoplankton in the presence of C. raciborskii and apparent negative effects on the larger zooplankton species, suggesting a potential for changes in zooplankton and higher trophic level community structure.     

Diel Vertical Migration of zooplankton in a eutrophic bay of Lake Victoria

We studied the Diel Vertical Migration (DVM) of several zooplankton taxa and an important zooplanktivore Rastrineobola argentea in a eutrophic bay of Lake Victoria for a total of 3 months during wet and dry seasons. Zooplankton were sampled twice a month at full moon and new moon. The zooplankton community of this lake was numerically dominated by cyclopoid copepods (>80%) of which 80% were juveniles. The composition of zooplankton was not significantly different (P > 0.05) for the different sampling months (November 2000, March 2001 and July 2001), but total zooplankton abundance was significantly lower (P < 0.05) in July (dry season) than during March or November (wet seasons). DVM behaviour was observed in some of the zooplankton groups including Tropocyclops spp., Thermocyclops spp., Thermodiaptomus galeboides and Diaphanosoma excisum. In these groups, we also observed that they were significantly (P < 0.05) more abundant at full moon than at new moon in some months, a pattern not seen in non-DVM groups. The amplitude of migration tended to be higher at new moon than full moon for all DVM zooplankton but not significantly so (P > 0.05). The zooplanktivore R. argentea exhibits typical DVM behaviour, perhaps to avoid visual predation itself, but it also increases the predation pressure on zooplankton in deeper waters. Feeding times for R. argentea peak just after dusk and dawn. For the copepod calanoid T. galeboides, there was a strong relationship (P < 0.05) between the amplitude of migration and midday (12:00) water transparency, suggesting that these zooplankton may have descended deeper on days with higher water transparency. In the absence of temperature and food gradients but with predation pressure from a migrating zooplanktivore, the depth of descent of zooplankton may be well associated with water transparency.     

Growth of Daphnia longispina L. in a polyhumic lake under various availabilities of algal,bacterial and detrital food

The availability and importance of food sources for growth of Daphnia longispina L. from a highly coloured fishless lake with anoxic hypolimnion were assessed by combining in situ and laboratory experiments. In in situ experiments populations were enclosed in tubes with natural temperature stratification and with or without anoxic hypolimnion. In the laboratory experiments the importance of food source (littoral zone vs pelagic epilimnion) was assessed by enclosing moss thalli and a natural zooplankton population in a large-scale flow-through system supplying food for experimental Daphnia. Growth of juveniles of Daphnia in epilimnetic water was determined in batch culture experiments and the importance of increasing concentrations of bacteria and algae for their growth and development was investigated with a small-scale flow-through system. Access to the anoxic hypolimnion enhanced the growth of Daphnia by 23–24%. Growth rates in the tubes with anoxic hypolimnion were 0.36 and 0.16 d^–1 in July and August respectively. In tubes without anoxia the corresponding values were 0.29 and 0.13. In batch-cultures the highest growth rate determined was 0.16 and the overall rates were lower than in in situ experiments. In batch culture Daphnia was able to grow in darkness for 10 days with a rate of 0.16. In the large-scale flow-through system Daphnia population fed with littoral water reproduced well despite the low concentration of algae and increased its number by a factor of c. 32 in 10 days. However, the animals were small and net production of Daphnia population thus lower under the littoral influence than in the control treatment. Population could survive and grew slowly on pelagial water processed by a natural zooplankton community and with very little algae left. It is thus possible that bacteria serve as a life-support system enabling the population survival over periods of algal shortage. Small-scale flow-through experiments revealed that Daphnia longispina is able to mature and reproduce on a bacterial diet if the food concentration is high enough and Daphnia on bacterial food could achieve growth rates similar to those on an algal diet. The threshold food level for Daphnia longispina was estimated to be c. 18.5 g C 1^–1. Detrital material is of limited value in nutrition of Daphnia even in a lake where more than 75% of carbon is bound in particulate detritus.     

The role of zooplankton grazing in the formation of `clear water phase' in a shallow charophyte-dominated lake

In Chara-dominated shallow eutrophic Lake Prossa (Estonia), the collapse of spring phytoplankton community occurred in late May after which both primary production (PP) and phytoplankton biomass (B&pinf;) stayed at a very low level. By mid-June the Secchi depth had increased up to 2.6 m indicating the achievement of the `clear water phase', which persisted thoughout the rest of the vegetation period. The biomass of `edible' phytoplankton formed on average 53% of the total phytoplankton biomass, and the share of herbivorous zooplankton was on average 61% of the total zooplankton biomass. In spring zooplankton removed daily 27% of the total B&pinf; and 29% of PP by grazing while in summer these values rarely exceeded 5%. Zooplankton grazing was responsible for the decrease of `edible' (<31 μm) phytoplankton after its spring peak as well as for maintaining its biomass at a very low level during the whole vegetation period. Depletion of mineral forms of nitrogen and phosphorus that occurred most probably because of the development of charophytes by the end of May supported the collapse of the whole phytoplankton community and kept the water clear throughout the summer and autumn.

     

Zooplankton in Rainbow Bay, a Carolina bay pond: population dynamics in a temporary habitat

SUMMARY.

• 1 Rainbow Bay is a temporary freshwater pond in South Carolina, U.S.A., that typically fills in winter and dries in spring or summer. We studied population dynamics of zooplankton in 1984 and 1985 when it held water for 282 and 57 days, respectively. The zooplankton were diverse, including one species of calanoid copepod, nine or more cyclopoid copepods, nineteen or more cladocerans, and twelve or more rotifers. The community was initially dominated by the copepods Diaptomus stagnalis and Acanthocyclops vernalis or Diacyclops haueri, later by cyclopoids and cladocerans including Daphnia laevis and Simocephalus spp.
• 2 Laboratory experiments with sediments from the dry pond support the inference from field data that time of emergence from resting stages was a proximate cause of the initial succession of species. The experiments also showed that eggs of the calanoid copepod, Diaptomus stagnalis, were inhibited from hatching in early November, but became ready to hatch by late November. A similar inhibition was not observed for cladocerans or cyclopoid copepods.
• 3 Food limitation appeared to influence population dynamics following emergence. For the cladocerans Daphnia laevis and Simocephalus spp., great reductions in size-specific fecundity occurred 1 month (1985) or 3 months (1984) after the pond filled and were associated with disappearance of the rotifer Conochilus hippocrepis. The cladocerans produced ephippia when their fecundities declined. Reproduction of Diaptomus occurred before cladoceran fecundities decreased in 1984, but during their decline in 1985. Broods of Diaptomus were smaller in 1985.
• 4 The impact of invertebrate predators was probably greater than that of vertebrate predators, but neither was sufficient to prevent over-exploitation of food resources. Prey consumption rates by salamander larvae were estimated from diets and population densities of the larvae in 1984 (Taylor et al., 1988). Their impact on the zooplankton was low in February, increased in March and April as the larvae grew, and then

     

Feeding ecology of the lantern fish Benthosema glaciale in a subarctic region

Summary The myctophid, Benthosema glaciale, was found to occur in the region of Davis Strait, but not in northern Baffin Bay. The factors that limited its northern range were believed to be the cold temperature and the long periods of day light in Baffin Bay. During the day B. glaciale was found at a depth of between 300 and 900 m with the population concentrating at about 500 m. At night about 46% of the population migrated to the upper 65 m to feed on copepods, primarily Calanus finmarchicus stage V and C. hyperboreus stage IV for which they showed a high degree of selectivity. The weight of stomach contents of fish with 100% full stomachs was equal to about 3% of their body weight. Feeding occurred mainly at night in the upper layers above the coldest layer of water with a temperature of about 0.5 °C. The biomass of zooplankton and myctophids m^-2 were similar to those for these groups on the Nova Scotia slope during June. The fish fed on the same size range of prey in Davis Strait as on the Nova Scotia slope even though the zooplankton species composition was different in the two regions.     

Expression of the oligopeptide transporter, PepT1, in larval Atlantic cod (Gadus morhua)

The intestinal absorption of di- and tri-peptides generally occurs via the oligopeptide transporter, PepT1. This study evaluates the expression of PepT1 in larval Atlantic cod (Gadus morhua) during the three weeks following the onset of exogenous feeding. Larval Atlantic cod were fed either wild captured zooplankton or enriched rotifers. cDNA was prepared from whole cod larvae preceding first feeding and at 1000 each Tuesday and Thursday for the following three weeks. Spatial and temporal expression patterns of PepT1 mRNA were compared between fish consuming the two prey types using in situ hybridization and quantitative real-time PCR. Results indicated that PepT1 mRNA was expressed prior to the onset of exogenous feeding. In addition, PepT1 was expressed throughout the digestive system except the esophagus and sphincter regions. Expression slightly increased following first-feeding and continued to increase throughout the study for larvae feeding on both prey types. When comparing PepT1 expression in larvae larger than 0.15-mg dry mass with expression levels in larvae prior to feeding, no differences were detected for larvae fed rotifers, but the larvae fed zooplankton had significantly greater PepT1 expression at the larger size. In addition, PepT1 expression in the zooplankton fed larvae larger than 0.15-mg dry mass had significantly greater expression than rotifer fed larvae of a similar weight. Switching prey types did not affect PepT1 expression. These results indicate that Atlantic cod PepT1 expression was slightly different relative to dietary treatment during the three weeks following first-feeding. In addition, PepT1 may play an important role in the larval nutrition since it is widely expressed in the digestive tract.     

Zooplankton biomass gradient off south-western Nova Scotia: nearshore ctenophore predation or hydrographic separation?

A persistent large-scale cross-shelf gradient in zooplanktonbiomass >1050 µm was evident off south-western NovaScotia during annual spring surveys between 1985 and 1987, withrelatively low levels inshore and higher levels offshore. Conversely,the abundance of the tentaculate ctenophore Pleurobrachia pileuswas the greatest inshore, and distributed reciprocally to zooplankton>1050 µm. The principle prey of both adult ctenophoresand post-larval cod is zooplankton >1050 µm (primarilycalanoid copepods), and cod growth rates are strongly influencedby prey biomass. Ctenophore predation appears to have been responsiblefor the low nearshore zooplankton biomass, whereas the influenceof hydrographic factors on the zooplankton gradient was minimal.On a smaller scale, persistent, abrupt changes in zooplanktonbiomass >1050 µm and ctenophore density existed 3–30km from shore, in contrast to linear gradients in water density(₁) during a 5 week sampling period in spring 1987. Ctenophoreswere confined to depths <55 m and zooplankton >1050 µmpredominantly occurred at depths >55 m. High concentrationsof chlorophyll and phaeopigment were evident at depths <55m also suggesting intense predation by ctenophores on largeherbivores. The relatively high proportion of smaller zooplankton(153–308 –m) in the nearshore is also consistentwith the predation hypothesis. The reduced growth experiencedby post-larval cod inshore appears generated by ctenophore predationof a common prey resource.     

The consequences of a drastic fish stock reduction in the large and shallow Lake Wolderwijd,The Netherlands. Can we understand what happened?

In 1990 an experiment started in the large and shallow lake Wolderwijd (2700 ha, mean depth 1.5 m) to improve the water quality. About 75% of the fish stock was removed (425 000 kg fish). The fish was mainly composed of bream and roach. In May 600000 young pikes (3–4 cm) were introduced.In May 1991 the water became very clear (Secchi depth 1.8 m) during a spring bloom of large Daphnia. Then the grazing by zooplankton was eight times higher than the primary production of algae and the total suspended matter concentration became very low. Compared to the situation before the fish reduction, the grazing had increased only slightly, while the primary production had decreased significantly in early spring. The fish stock reduction might have contributed to the reduction in primary production by a reduced internal nutrient load. The clear water period lasted six weeks. Daphnia disappeared in July due to food limitation, the algal biomass increased and the Secchi depth became 50 cm. Daphnia did not recover during summer, due to predation that was not caused by 0 + fish but by the mysid shrimp Neomysis integer. Neomysis could develop abundantly, because of the reduced biomass of the predator perch. The production of young fish had been low because of the cold spring weather. The cold weather was probably also responsible for the slow increase in density of macrophytes. After 1991, perch probably can control Neomysis. Due to lack of spawning places and shelter for 0 + pike, pike was probably not able to control the production of 0 + fish. In a lake of this scale, it will not be easy to get more than 50% coverage of macrophytes, which seems necessary to keep the algal biomass low by nutrient competition. Therefore, we expect also in the future a decrease in transparency in the summer. Locally, especially near Characeae, the water might stay clear.     

Seasonality and abundance of some dominant crustacean zooplankton in Lake Awasa,a tropical rift valley lake in Ethiopia

The zooplankton of a Rift Valley lake in Ethiopia, Awasa, was sampled at 3 stations for 2 years (1986 and 1987) concurrently with various meteorological and limnological measurements. The spatial and temporal variation in abundance of some numerically dominant crustaceans, Mesocyclops aequatorialis similis (Copepoda), Thermocyclops consimilis (Copepoda) and Diaphanosoma excisum (Cladocera) is discussed. Temporal (months, sampling dates) rather than spatial (station) variability accounts for more than 50% of the total variance in zooplankton abundance but horizontal patchiness exists during periods of high zooplankton density. Sampling errors were generally low, except for counts of cyclopoid nauplii (subsampling) and Diaphanosoma (inter-replicate variance). Zooplankton showed distinct seasonality associated with the mixing cycle of the lake. Total numbers increased to more than 200 000 m⁻³ during the unstratified period (July to September). Low numbers were evident during stratification (February to May) when zooplankton numbers did not exceed 15 000 m⁻³. Individual zooplankton species and age classes showed variable seasonal amplitudes, ranging from 6.4 (nauplius 3) to 44.8 (copepodite 3 of Mesocyclops). We discuss some possible causes for zooplankton seasonality in Lake Awasa, and also review zooplankton seasonal cycles in other tropical lakes, especially African ones.     

Seasonal distribution,composition and abundance of the planktonic Ciliata and Testacea of Cayuga Lake

The planktonic protozoans (Ciliata and Testacea) of Cayuga Lake were studied from September 1974 through July 1975. Monthly collections were taken at four depths (surface, 15,31 and 92 m) at each of seven stations along the 125 m depth contour. All samples were collected with a 6-liter Van Dorn bottle sampler. Densities and biomass were determined for total protozoans and total zooplankton at each depth on each sampling date. Temperature profiles at one-meter intervals were also determined on or within a few days of the zooplankton sampling.The vertical distribution of the total protozoans was directly related to temperature; maximum densities corresponded to high summer surface temperatures. Dominant protozoan taxa included Didinium nasutum, Stokesia vernalis, Codonella cratera, Strobilidium gyrans, Strobilidium sp., Strombidium viride, Paradileptus canellai, Difflugia limnetica, and unidentified ciliates belonging to the families Halteriidae, Holophryidae, Epistylidae, and Vorticellidae. Protozoans composed 69% or more of the total zooplankton density on nine of eleven dates. Protozoan dominance was most evident during winter and spring, corresponding to the near absence of rotifers and microcrustaceans. Protozoans accounted for 30% or more of the zooplankton biomass from April through June, reaching 47% in May.     

Vertical distributions of primary production and grazing by Calanus glacialis jaschnov and C. hyperboreus krøyer in Arctic waters (Barents Sea)

Summary Simultaneous measurements of phytoplankton primary production, and zooplankton grazing by Calanus glacialis and C. hyperboreus were carried out over a 24h period at two stations northeast of Spitzbergen in July and August 1985. Phytoplankton biomass was concentrated in a sub-surface maximum, and our results suggested that 65% to 90% of the carbon produced was grazed by these copepods. At both stations maximum grazing rates by these zooplankton were observed within and below the depth of maximum primary production. At least one third of regenerated primary production could be sustained by ammonia excreted by the large zooplankton species Calanus glacialis and C. hyperboreus.