Size structure of plankton in a temporary, saline inland lake

The study of the size structure of a planktonic community ina relatively extensive (1350 ha), very shallow (0.46 m maximumdepth), temporary inland saline lake during a relatively rainyyear (1987–88) showed an average biomass-size spectrumwith four clear modes at nano-, micro-, meso- and macroplanktonsize ranges. The size-abundance distribution was dominated bymedium-sized phytoplankton cells (Gymnodinium cf. excavatum,Tetraselmis apiculata, Dunaliella spp. and several diatom species),large ciliates (Fabrea salina) and a few zooplankton speciesadapted to changing temporary environments within a wide sizerange (Cletocamptus retrogressus, Moina salina, Branchinellaspinosa). This polymodal size distribution is compared withresults obtained in other systems, including small physicallycontrolled lakes, large lakes, oceans and marine benthos. Considerationof the size structure within homogeneous groups of organismsshows slopes less negative than the average for the whole sizerange analysed, a feature which is common in small and physicallycontrolled ecosystems. A seasonal study of the size structurerevealed uneven distributions throughout the year, resultingin discontinuities in the size spectra as well as in differencesof total size range. There were noticeable differences amongthe size spectra at each particular time, showing phases ofprimary producer predominance and phases with a marked predominanceof consumers. The time-integrated size spectrum did not exhibitany distinct discontinuity. The influence of extreme shallownessand resuspension on the shape and dynamics of the size spectrumis examined and discussed compared to deeper aquatic systems.     

Food web manipulation in Lake Zwemlust: Positive and negative effects during the first two years

The hypertrophic Lake Zwemlust, a small water body used as a swimming pool, was characterized by algal blooms in summer, reducing the Secchi disk transparency to less than 0.3 m. Since in The Netherlands a Secchi disk transparency of 1 m is obligatory for swimming waters, corrective measures were called for to improve the light climate of the lake. In March, 1987, as an experiment, the lake was drained by pumping out the water to facilitate fish elimination. Planktivorous and benthivorous fish species, which were predominant, were removed by seine- and electro-fishing. After the lake had refilled by seepage it was restocked by a new simple fish community comprising pike (Esox lucius) and rudd (Scardinius erythrophthalmus) only. Stacks of willow twigs (Salix) and macrophytes (roots ofNuphar lutea and seedlings ofChara globularis) were introduced into the lake as spawning grounds and refuges for the pike against cannibalism and as shelter for the zooplankton. The effects of this food web manipulation on the light climate, phytoplankton, zooplankton, fish, macrophytes, macrofauna and on the nutrient concentrations were monitored during 1987 and 1988. In summer 1987, despite of high nutrient concentrations, the phytoplankton density was low, due to control by zooplankton, causing a Secchi disk transparency of 2.5 m, the maximum depth. Chlorophyll-a concentrations were low (<5 g Chl.l^–1), blooms of cyanobacteria did not occur and a shift from rotifers to cladocerans took place. In 1988, however, also some negative effects were noticed. Macrophytes and filamentous green algae reached a much higher biomass (50–60% cover of the lake bottom) than in 1987; some species, growing through the entire water column, interfered with the lake's recreational use. Associated with the macro-vegetation and possibly with the absence of larger cyprinids, the diet of which also comprises snails, a large scale development of the snail population, among themLymnaea peregra var.ovata took place. This species is known to act as an intermediate host of the bird-parasitizing trematodeTrichobilharzia ocellata, the cercariae of which cause an itching sensation at the spot of penetration of the human skin, accompanied by rash (schistosome dermatitis or swimmers' itch); in July, 1988, about 40% of the bathers complained about this itching. A positive effect of the macrophytes and filamentous green algae was the high uptake of nitrogen, resulting in a low nitrogen concentration in the lake and growth limitation of the phytoplankton population by nitrogen in the summer of 1988. In 1988 the cladocerans were abundant in April only; and unlike in 1987, in the summer of 1988 there was a shift from cladocerans to rotifers. Therefore, only in early spring (April) zooplankton grazing controlled phytoplankton growth and in summer nitrogen limitation was the major controlling factor, keeping chlorophyll-a concentrations low.     

Hydrological cycle and interannual variability of the aquatic community in a temporary saline lake (Fuente de Piedra,Southern Spain)

Fuente de Piedra is a shallow, temporary saline lake whoseseasonal behavior is strongly dependent on the annual hydrologicalbudget. In this study, we outline the characteristics of Fuente dePiedra Lake for two years that had different hydrological budgets.The high precipitations in 1989–90 caused the lake not to dry asusual, and decreased both salinity and the amplitude of changes.There were also differences in nutrient dynamics, with generallylower concentrations of soluble reactive phosphorus and ammonium,whereas in the more humid year nitrate showed a distinct maximum inwinter. Winter bloom chlorophyll a concentrations were alsomuch higher in 1989–90 (>600 g l-1) but there wasalso a winter productive phase that was presumably poorly coupledwith consumption processes that predominate in spring. Planktonicassemblages were different between years. Highly halotolerantphytoplankton species (Dunaliella salina and D. viridis) became scarcer, and especially two previouslyunrecorded diatoms (Cyclotella sp. and Chaetoceros sp.)became dominant in the bloom time in the wet year. The speciesrichness of the zooplankton increased in the wet year, with newspecies appearing that were not collected during 1987–88(Branchinectella media, Daphnia mediterranea, Macrothrix sp.,Arctodiaptomus salinus, Cyclops sp., Sigara sp...).There was also a much higher development of macrophytes (Ruppiadrepanensis, Althenia orientalis, Lamprothamnium papulosum)and bird populations, especially flamingoes (Phoenicopterusruber).Important interannual variations in this sort of system pointto the need for long term studies to observe the whole range ofstates that define the lake as an entity.     

Limnological Aspects of Two Tropical Crater Lakes (Lago Biao and Lago Loreto) on the Island of Bioko (Equatorial Guinea)

The limnology of Lago Biao and Lago Loreto, two tropical crater lakes on the island of Bioko, was investigated for the first time in January of 2002. Lago Biao fills a caldera at 1750 m altitude and is 18.5 m deep. This lake was characterized by low conductivity (11.6-13.3 S cm^–1), low pH (5.9-6.2), an oxycline between 6 and 10 m depth (90-27% saturation), and a Secchi depth of 5.5 m. Dinoflagellates, desmids, and green algae dominated the phytoplankton community. The rotifers Anuraeopsis fissa, Polyarthra indica, Trichocerca simoneae, and T. similis were most abundant above or in the thermocline. During the day Tropocyclops prasinus tjeefi and Chaoborus cf. ceratopogones concentrated in the oxycline, whereas Daphnia laevis was most abundant between 0 and 4 m depth. In contrast, L. Loreto is 82 m deep and probably fills an explosion crater (maar) at 1050 m altitude. The lake had a turbid, yellowish appearance and a Secchi depth of only 0.6 m. Oxygen content dropped from 121% at the surface to 6% in 10 m depth. The pH declined from 9.15 at the surface to 7.4 in only 2.5 m depth and became acidic at approximately 15 m depth. One species of cyanobacterium, one dinoflagellate and one green alga dominated the phytoplankton and together reached densities of 12 100 cells ml^–1. Only one rotifer (Brachionus angularis) and one copepod (Tropocyclops prasinus tjeefi) species colonized the open water. Intact algal cells and zooplankton specimens were encountered near the bottom.     

The phytoplankton of Lake Liddell,New South Wales: chlorophyll a concentrations,species seasonal succession,and covariation with nutrients

Samples of the phytoplankton in a freshwater lake, Lake Liddell, New South Wales (Lat: 32° 22 S, Long. 150° 1 E) were collected every 4 weeks between October 1987 and November 1988. Chlorophyll a concentrations ranged from 1.8 g 1^–1 to 9.1 g 1^–1 and were positively correlated with the following nutrient parameters: total and nett mass additions of nitrate/nitrite-N and total-N, total additions of Kjeldahl-N, and nett mass addition N-P ratios. There was no correlation between lake nutrient concentrations and chlorophyll a. Factors other than nutrient concentrations appeared to be effecting chlorophyll a concentrations as summer levels were low despite nutrient concentrations being at a maximum. In spring and summer the phytoplankton was dominated by chlorophytes, with dinoflagellates and diatoms most abundant in autumn. During winter cyanobacteria were the most abundant. The relative abundance of chlorophytes was positively correlated with in lake nitrate/nitrite-N concentrations whereas the relative abundance of cyanobacteria was negatively correlated with this parameter. Based on chlorophyll a concentrations and the phytoplankton flora Lake Liddell can be classified as mesotrophic.     

Interactions between Phytoplankton,Zooplankton, and Fish in the Nutrient Rich Shallow Lake Hjarbæk Fjord,Denmark

In the shallow, nutrient-rich dammed-up lake Hjarbæk Fjord the balance between phytoplankton, zooplankton, and fish was completely disturbed during the summer period. Extensive growth of small blue-green algae (Oscillatoria cfr. trichoides, Pseudanabaena galeata) and chlorococcal green algae (Scenedesmus spp., Monoraphidium contortum, Actinastrum hantzschii) induced a pH rise to 10.5 in July. Fish were driven into the rivers, and many that did not escape died. After the fish had disappeared daphniae multiplied explosively and grazed away the major part of the phytoplankton. Ammonia concentrations rose and oxygen concentrations dropped to noxious levels for highly sensitive fish in August–September. Larger phytoplankton species (Aphanizomenon flos-aquae, Pediastrum boryanum, Pandorina morum, a. o.) were not grazed by the daphniae and gave rise to a maximum of copepods in September. Fish returned to the lake in October, when pH, ammonia, and oxygen levels had normalized.     

Some ecological aspects of Lake Qarun,Fayoum, Egypt. Part II. Production of plankton and benthic organisms

Distribution and abundance of phyto-, zooplankton and benthic organisms in Lake Qarun were investigated during the period from January 1974 to December 1977.Average number of phytoplankton cells was 152,300 cells/L and its biomass was 0.365 g/C/m³; average number of zooplankton was 31.44 × 10³/m³ and its biomass was 194.19 mg/m³. The average number of benthic fauna was 19889/m² and its biomass was 400.22 g/m² (dry wt.). Therefore, Lake Qarun may be considered as a highly eutrophic body of water.Freshwater planktonic species, that used to inhabit the lake, such as Diaptomus salinus and the cladoceran Moina salinarum, disappeared completely when the salinity of the lake water reached 30–34 However, some Rotatoria were able to withstand the high salinity. The new composition of the zooplankton community shows that the marine zooplankton species include not only Acartia latisetosa and Cirripedia nauplii, but also other species such as Polychaeta, Obelia medusae, etc.The benthos of Lake Qarun is characterised by an intensive growth of few species. The major part (i.e. 93.54% by weight) of bottom fauna in the lake is Mollusca, mainly Cerastoderma glaucum (69·84% by weight).     

Molecular Cloning and Stress-Dependent Expression of a Gene Encoding ω3-Fatty Acid Desaturase in the Microalga Dunaliella salina

A fragment of the gene des3-1 encoding 3 fatty acid desaturase was cloned from a cDNA library of the unicellular green galophilic alga Dunaliella salina. The comparative phylogenetic analysis of 3-desaturase amino acid sequences from diverse organisms placed the desaturase of D. salina between cyanobacteria and higher plants in the evolutionary range of desaturases. The expression of des3-1 was studied in D. salina cells exposed to low temperatures, high irradiance, and high CO₂ concentrations. Lowering the external temperature from 32 to 22°C produced a transient increase in the level of specific mRNA. Considerable accumulation of mRNA for 3-desaturase was also observed when CO₂ concentration in gas–air mixture was raised from 2 to 10%. An irradiation increase from 70 to 500 mol/(m² s) did not affect the level of specific mRNA. The latter evidence presumes that in Dunaliella cells, this desaturase is probably located in the endoplasmic reticulum, rather than in the chloroplast.     

Biomass and photosynthesis of size-fractionated phytoplankton in Canadian Shield lakes

Both in situ primary production and biomass (chlorophyll ) of fractionated phytoplankton (<64,µ, <25 µm and < 10 µm) were studied in 10 Canadian Shield lakes to elucidate the spatial and temporal variability of the contribution of size fractions to the biomass and primary production of the phytoplankton community. Mean summer biomass and production of each size fraction varied significantly between lakes. Within lakes, temporal variation was low for biomass but great for production. However, temporal variation can be considered of minor importance during the sampling period, as compared to the spatial variation between lakes. Algae from the < 10 µm size fraction were the most important in biomass (41–65 %) and production (23–69%). The temporal trends for both phytoplankton variables thus generally followed closely that of the < 10 µm size fraction. Among the physical, chemical and morphometric variables of the studied lakes, water transparency (Secchi disk), total phosphorus, lake volume, lake area, and mean depth gave the best correlations with phytoplankton variables.Contribution number 354 from the Groupe de recherches en Ecologie des Eaux douces, Limnological Research Group, Université de Montréal.     

Algal picoplankton production and contribution to food-webs in oligotrophic British Columbia lakes

The pelagic communities of two contrasting oligotrophic lakes in British Columbia were studied to determine why an interior, dimictic lake (Quesnel) supports a greater biomass of zooplankton and produces larger planktivorous sockeye salmon (Oncorhynchus nerka) than a coastal warm-monomictic lake (Sproat). The ultra-oligotrophic status and differing planktivore densities in Sproat Lake increased the relative importance of algal picoplankton, diminished the abundance of large zooplankton, and increased the significance of rotifers and other small-bodied zooplankton. These picoplankton based food webs result in longer, indirect and less efficient pathways of carbon flow from phytoplankton to fish. In contrast, Quesnel Lake is a more productive oligotrophic lake and its pelagic food webs are based more on nanoplankton and small microphytoplankton that support larger-bodied zooplankton (Daphnia, Diaptomus), and a more direct and efficient two-step transfer to fish. The greater variability of the annual recruitment of sockeye fry in interior lakes may keep zooplankton communities in a non-steady state, this in turn may perpetuate the occurrence of quadrennial cyclic dominance in adult salmon returning to these systems.     

Zooplankton, phytoplankton and the microbial food web in two turbid and two clearwater shallow lakes in Belgium

Components of the pelagic food web in four eutrophic shallow lakes in two wetland reserves in Belgium (Blankaart and De Maten) were monitored during the course of 1998–1999. In each wetland reserve, a clearwater and a turbid lake were sampled. The two lakes in each wetland reserve had similar nutrient loadings and occurred in close proximity of each other. In accordance with the alternative stable states theory, food web structure differed strongly between the clearwater and turbid lakes. Phytoplankton biomass was higher in the turbid than the clearwater lakes. Whereas chlorophytes dominated the phytoplankton in the turbid lakes, cryptophytes were the most important phytoplankton group in the clearwater lakes. The biomass of microheterotrophs (bacteria, heterotrophic nanoflagellates and ciliates) was higher in the turbid than the clearwater lakes. Biomass and community composition of micro- and macrozooplankton was not clearly related to water clarity. The ratio of macrozooplankton to phytoplankton biomass – an indicator of zooplankton grazing pressure on phytoplankton – was higher in the clearwater when compared to the turbid lakes. The factors potentially regulating water clarity, phytoplankton, microheterotrophs and macrozooplankton are discussed. Implications for the management of these lakes are discussed.     

The structuring role of free-floating versus submerged plants in a subtropical shallow lake

In shallow temperate lakes many ecological processes depend on submerged macrophytes. In subtropical and tropical lakes, free-floating macrophytes may be equally or more important. We tested the hypothesis that different macrophyte growth forms would be linked with different bottom-up and top-down mechanisms in out-competing phytoplankton. We compared experimentally the effects of submerged and free-floating plants on water chemistry, phytoplankton biomass, zooplankton and fish community structure in a shallow hypertrophic lake (Lake Rodó, 34°55S 56°10W, Uruguay). Except for the retention of suspended solids, we found no other significant bottom-up process connected with either Eichhornia crassipes or Potamogeton pectinatus. Free-floating plants had a lower abundance of medium-sized zooplankton than any other microhabitat and submerged plants were apparently preferred by microcrustaceans. Fish showed a differential habitat use according to species, size-class and feeding habits. Dominant omnivore-planktivores, particularly the smallest size classes, preferred submerged plants. In contrast, omnivore-piscivores were significantly associated with free-floating plants. The density of omnivorous-planktivorous fish, by size class, significantly explained the distribution of medium-sized zooplankton, the high number of size 0 fish being the main factor. The abiotic environment and the structure of the zooplankton community explained little of the fish distribution pattern. Our results suggest that bottom-up effects of free-floating plants are weak when cover is low or intermediate. Top-down effects are complex, as effects on zooplankton and fish communities seem contradictory. The low piscivores:planktivores ratio in all microhabitats suggests, however, that cascading effects on phytoplankton through free-floating plant impacts on piscivorous fish are unlikely to be strong.     

Tolerance to a whitefly-transmitted virus causing muskmelon yellows disease in Spain

Summary Muskmelon yellowing disease was one of the most serious problems affecting muskmelon crops along the south-east coast of Spain throughout the 1980s. The causal agent of this disease is a virus that we call muskmelon yellows virus (MYV); MYV is transmitted by the greenhouse whitefly Trialeurodes vaporariorum Westwood. It has proven impossible to find sources of resistance to MYV within a wide collection of Spanish muskmelon landraces and exotic varieties. However, Nagata Kin Makuwa and PI 161375, lines of Asiatic origin, show tolerance to this disease. These two lines were studied together with two others (Galia and Piel de Sapo type) that are very susceptible to MYV. The crosses between them (susceptible x tolerant) and the segregant generations derived from these crosses were also investigated. The studies were carried out in two different places and years. The expression of tolerance is influenced by the environment. A parabolic type relationship exists between the average value of percentage of tolerant plants and their variation. This allowed us to quantify the expected response in the segregant generations. The results observed in these generations agreed with a simple genetic control of tolerance. This tolerance, combined with protective measures which delay the infection, can contribute notably to mitigating the effects of MYV.     

Response of a shallow boreal lake to recent nutrient enrichment – implications for diatom-based phosphorus reconstructions

Water quality of the shallow, mesotrophic, and macrophyte-dominated Lake Kaljasjärvi has been monitored at three to four year intervals since 1978. During the monitoring period, surface-water total phosphorus (TP) concentrations have typically varied between 20 and 25 g P l^–1. However, elevated total phosphorus concentrations were measured in 1987, 1991, and 1999. Diatom-based reconstruction of the historical lake-water TP concentrations was therefore employed to study the recent development of the lake. However, the diatom-TP model did not predict the high measured phosphorus concentrations despite the changes observable in diatom assemblages. In addition, the ratio of sedimentary diatom remains to chrysophycean stomatocysts declined towards the top of the sediment core, indicating decreasing trophy rather than eutrophication. Analysis of sedimentary pigments and phosphorus fractions, used to examine further the changes, also produced results that contradicted the simple eutrophication hypothesis. In particular, the proportion of chlorophyll derivatives instead of carotenoids increased and there was a rise in the concentration of refractory instead of NaOH-extracted phosphorus. These features appear to be related to the extensive littoral areas of the lake since enhanced littoral production can explain both the observed changes in sediment chemistry and the low diatom-inferred TP (DI-TP). Littoral primary producers are suggested to have benefited from the increased phosphorus inputs to the lake, transferring some of the phosphorus to the detrital pool and contributing to the increased pigment concentrations of sedimentary organic matter. High proportions of non-planktonic diatoms in the samples lower DI-TP because periphytic taxa are assigned low TP optima in the inference models used. Abundant aquatic macrophytes may also have made the lake resistant to eutrophication by assimilating nutrients, providing refuge for zooplankton, and having an allelopathic effect on phytoplankton. Since 1980, however, the sedimentary diatom assemblages also indicate increasingly eutrophic conditions. Additional loading from numerous cottages during the last 20 years seems to have caused observable changes in the phytoplankton communities.     

Zooplankton community changes in Lake Kinneret (Israel) during 1969–1985

Long-term records (1969–1985) of zooplankton density in Lake Kinneret indicated significant reduction of biomass (Copepods, Cladocera, Rotifera) and production (Copepods, Cladocera). Nauplius and adult copepod densities decreased but those of copepodites did not change. Mesocyclops was suppressed more than the smaller Thermocyclops and males of both genera became more abundant relative to the larger females. Ratios of small/large Cladocera densities became higher. Numbers of total cladocerans were stable, and therefore reduction of Cladocera grazing capacity is assumed. The abundant Keratella spp. were reduced. It is likely that intensification of fish visual-attack-predation pressure shifted the size-class structure towards smaller adult copepods and cladocerans. Reduction of Keratella spp. and copepod nauplii was probably affected by increasing pressure of fish filter-feeders. Data on fish food consumption, feeding behaviour and fisheries management suggested their direct impact on long-term changes of zooplankton in Lake Kinneret.     

Effectiveness of phytoplankton control by large-bodied and small-bodied zooplankton

Employingin situ enclosures containing inocula of the lake zooplankton (mainlyDaphnia galeata, Daphnia cucullata andBosmina spp.) from a moderately eutrophic Lake Ros (Northern Poland) or large-bodiedDaphina magna, the following observations on succession of phytoplankton were made: 1) whereasD. magna could control the density of all the photoplankton size classes, the lake zooplankton could not suppress the large-sized phytoplankters or net phytoplankton; 2) the lake zooplankton was able to control the density of small algae (< 50μm), but its effect on large algae may be opposite: a promotion of net phytoplankton growth by removing small-sized algae which can out-compete net phytoplankton for limited PO₄-P resources (<5μg P l⁻¹). Since efficiency of phytoplankton density control byD. magna decreased with an increase in net phytoplankton abundance, biomanipulation could not be successful without introducing or maintaining a high population of large-bodied cladoceran species before high densities of large algae would make the control of phytoplankton inefficient.     

Availability of suitable phytoplanktonic food for zooplankton in an ice-covered lake

Edible phytoplankters were found throughout the winter in a shallow ice-covered lake in Pennsylvania, U.S.A., in conjunction with a diverse population of zooplankton. Environmental conditions during the winter seemed to eliminate large and non-motile types of algae in favor of the smaller motile forms which may be the best foods for zooplankters. The phytoplankton were classified for study both by systematic position and by size group. The total volume of phytoplankton comprising the group Chrysophyta and algae 80 µ declined rapidly during the period of ice-cover, while algae 15 µ and Chlorophyta maintained more stable volumes. During the latter weeks of ice-cover phytoplankton densities were low (50–200/ml), and bacteria, detritus, or µ-algae (2 µ) not detected by the counting methods used, may have been important foods for the zooplankters. The lack of suitable phytoplanktonic foods may have contributed to the decline of calonoid copepods during the late winter.Based on a thesis submitted to the Department of Biology, East Stroudsburg State College, as a partial requirement for the degree of M.S. in Biology.     

The mass culture of Dunaliella viridis (Volvocales,Chlorophyta) for oxygenated carotenoids : laboratory and pilot plant studies

The biology, and hence the mass culture, of Dunaliella viridis closely follows that of Dunaliella salina, which is successfully mass cultured for the production of -carotene. Both algae can grow at extremely high salinities and light intensities. They co-exist in the coastal salt lake, Hutt Lagoon, Western Australia. In contrast to D. salina, D. viridis does not accumulate large amounts of -carotene, producing only up to 0.7% of mixed carotenoids (lutein, zeaxathin, other oxygenated carotenoids and -carotene), compared to D. salina's ca 10% dry wt of mainly -carotene. However, in laboratory experiments, D. viridisgrew much faster and to much higher cell densities than D. salina, and attained levels of mixed carotenoids similar to those of D. salina (ca 13 mg L^–1 carotenoid). Preliminary experiments in outdoor ponds were much less promising. Harvesting by chemical flocculation was as effective as with D. salina, but extraction of carotenoids directly into vegetable oil proved inefficient. When incorporated into feed, caretonoids derived from D. viridis pigmented hen eggs acceptably. Extrapolating from laboratory results, and using costing derived from D. salina technology, the cost of production of mixed oxygenated carotenoids from D. viridis was similar to that for the production of -carotene from D. salina, at ca $A500 kg^–1.     

The role of macroinvertebrates and fish in regulating the provision by macrophytes of refugia for zooplankton in a warm temperate shallow lake

1. The zooplankton often undergoes diel horizontal migration (DHM) from the open water to the littoral of shallow lakes, thus avoiding predators in the former. This behaviour has functional impacts within the lake, as it enhances zooplankton survival, increases their control of phytoplankton and tends to stabilise the clear water state. However, most of the evidence supporting this migration pattern comes from cold north temperate lakes, and more evidence from tropical and subtropical areas, as well as from southern temperate areas, is needed. 2. We conducted a field study of the diel horizontal and vertical migration of zooplankton, and the horizontal distribution of potential predatory macroinvertebrates and fish, over two consecutive days in the summer in a temperate lake in the southern hemisphere. We took zooplankton samples at two depths, at three sampling stations (inside beds of aquatic macrophytes, at their edge and in open water) along three transects running from the centre of a bed of Ceratophyllum demersum to open water. At each sampling station, we also took samples of macroinvertebrates and fish and measured physical and chemical environmental variables. 3. Zooplankton (pelagic cladocerans, calanoid copepods and rotifers) avoided the shore, probably because of the greater risk from predators there. Larger and more vulnerable cladocerans, such as Diaphanosoma brachyurum and Moina micrura, were two to four times more abundant in open water than at the edge of or inside beds of macrophytes, respectively, by both day and night. Less vulnerable zooplankton [i.e. of medium body size (Ceriodaphnia dubia) or with the ability to swim fast (calanoid copepods)] were distributed evenly between open water and the edge of the plant beds. Small zooplankton, Bosmina huaronensis and pelagic rotifers, showed an even distribution among the three sampling stations. Accordingly, no DHM of zooplankton occurred, although larger organisms migrated vertically inside C. demersum stands. 4. Macrophytes contained high densities of predatory macroinvertebrates and fish. The predator assemblage, composed of large‐bodied macroinvertebrates (including odonates and shrimps) and small littoral fish, was permanently associated with submerged macrophytes. None of these groups moved outside the plant beds or changed their population structure (fish) over the diel cycle. 5. Submerged macrophyte beds do not represent a refuge for zooplankton in lakes where predators are numerous among the plants, implying a weaker top‐down control of phytoplankton biomass by zooplankton and, consequently, a more turbid lake. The effectiveness of macrophytes as a refuge for zooplankton depends on the associated assemblage of predatory macroinvertebrates and fish among the plants.     

Comparative limnology of Sambhar and Didwana lakes (Rajasthan,NW India)

Two alkaline saline inland lakes of Indian arid region were studied during 1984 and 1985, to assess functioning and interaction of various environmental and biological factors. Changes in physical and chemical variables, planktonic composition, chlorophyll content and phytoplankton primary productivity were examined.Salinity in both lakes fluctuated from almost fresh water (1.80), to hypersaline (300) and acted as the main controlling factor for almost all the biotic parameters. Maximum total alkalinities were 2162 mg l^–1 and 2090 mg l^–1, respectively in Sambhar and Didwana lakes. Dissolved oxygen ranged from completely anoxic conditions to maxima of 11.68 and 7.29 mg 1^–1, respectively in Sambhar and Didwana lakes. Nutrient enrichment in the lakes was low.The phytoplankton species composition of Sambhar lake was reduced from an earlier reported 20 genera to only 11 (Nostoc, Microcystis, Spirulina, Aphanocapsa, Oscillatoria, Merismopedia, Nitzschia, Navicula, Synedra, Cosmarium and Closterium). Phytoplankton of Didwana was composed of only 9 genera including Anabaena and Nodularia. Sambhar lake, which once contained Artemia, is now totally devoid of them. On the other hand, Artemia was the most dominant zooplankter in Didwana lake at a salinity range of 15–288. Other zooplankters such as Moina, Cyclops and Brachionus flourished at lower salinity levels in Didwana lake. The seasonal quantitative and qualitative phyto- and zooplankton changes in relation to salinity are documented.