Changes in benthic community composition in response to reservoir aging

The effects of reservoir aging on the benthic macroinvertebrate community in Pawnee Reservoir were documented by comparing species composition and biomass of samples collected from October 1991 through September 1992, to a similar survey conducted in 1968–70 by Hergenrader & Lessig (1980). Filling of the basin with sediment and associated material and the subsequent change in the benthic environment, has resulted in a relatively homogenous bottom substrate at each of the three sampling transects (dam, middle, and inflow). Sediment enrichment has limited the benthic fauna to species tolerant of brief periods of bottom anoxia and increased levels of organic matter, which has resulted in the disappearance of many taxa and a decrease in the abundance of remaining invertebrates. Significant differences in total biomass were found at each transect, as well as for the whole lake, between study periods. The dam, middle, inflow and total biomasses for the 1968–70 study period were 2.4, 1.5, 2.3 and 2.0 g m^–2, respectively, compared to 0.2, 0.2, 0.3, and 0.2 g m^–2 for the 1991–92 study period. The total disappearance of eight invertebrate taxa, in particular two sphaerid clam species, and significant declines in other dominant taxa such as Chaoborus punctipennis and Chironomus sp. accounted for these major differences in biomass between study periods. Reductions in the number of taxa present has resulted in an increase in benthic faunal similarity at each transect, with tubificid oligochaetes, Coelotanypus sp., C. punctipennis, and Chironomus sp., comprising 90% of both the total density and biomass of benthic invertebrates in Pawnee Reservoir.     

Relationship between feeding of blueback herring and the zooplankton community of a Texas reservoir

Following the introduction of blueback herring (Alosa aestivalis), we examined herring food habits and the crustacean zooplankton community in Lake Theo, a 30-hectare reservoir located on a tributary of the Red River in north Texas. Prior to the introduction in spring, 1982, the reservoir contained an established fish community dominated by centrarchids. Blueback herring stomachs and zooplankton were sampled quarterly from summer, 1982 through winter, 1985. Cladocerans accounted for 89.4% of the zooplankton consumed by herring. Median lengths of cladocerans and copepods in the herring diet were significantly greater than those in the reservoir. No appreciable changes in lengths of cladocerans and copepods in the reservoir were observed after blueback herring introduction, but the zooplankton community shifted from cladoceran to copepod domination.     

Impact of Holoshestes heterodon Eigenmann (Pisces: Characidae) on the plankton community of a subtropical reservoir: the importance of predation by Chaoborus larvae

We analyzed the effects of planktivorous Holeshestes heterodon Eigenmann (Characidae) predation on the plankton community of a small subtropical reservoir, using four enclosures (volume about 17.5 m³), open to the sediment, established in the littoral zone. Two enclosures were stocked with fish (mean TL 5.7 cm), at a density of about 4–5 fish m^–3 (approx. 8 g m^–3), whereas two remained fishless. The experiment lasted a little longer than one month. In the fish enclosures, most Crustacea and Chaoborus larvae remained scarce, probably as a result of visually selective fish predation. In both fishless enclosures, Chaoborus larvae became abundant. However, in only one of these did large individuals become relatively numerous; this discrepancy in the demographic structure of the Chaoborus populations between the two fishless enclosures is unexplained. Only in the fishless enclosure without appreciable numbers of large Chaoborus did densities of Crustacea increase greatly. It is suggested that in the enclosure containing large Chaoborus individuals, crustacean populations were prevented from developing due to predation pressure, while the small Chaoborus larvae of the other enclosure could not readily consume these prey. Rotifers were low in abundance in the absence of fish, probably as a consequence of Chaoborus predation. Phytoplankton density increased in all four enclosures, due probably to the lack of water flow. Only in the fishless enclosure with high densities of crustaceans did phytoplankton abundance decrease markedly at the end of the experiment, perhaps because of grazing losses.     

The Bosmina kessleri-like morphotype of Eubosmina in Lake Muskoka,Ontario, Canada,as putative interspecific hybrids

Like Bythotrephes, Bosmina (Eubosmina) coregoni coregoni Baird appears to have invaded North America from Eurasia, since there are no records from before 1965. All previously known North American populations have a well-rounded ventrocaudal corner of the carapace, suggesting that the founding race is one of the rotunda type, the most common eubosminid in European lakes. In Lake Muskoka, immediately to the east of Georgian Bay, Ontario, eubosminids were found that shared features of B. (E.) longicornis kessleri Ulianine. In contrast to other North America populations of B.(E. )c. coregoni, these specimens have a short, broadly based mucro. Morphological features suggest that these are interspecific F₁-hybrids between B. (E.) longispina Leydig and B. (E.) coregoni, without any signs of introgression.This paper is dedicated to the memory of Dr. John C.H. Carter.     

Zooplankton dynamics in the upstream part of Stratos reservoir (Greece)

The investigation of the zooplankton community in the upstream part of Stratos reservoir during a 24 months survey (September 2004–August 2006) revealed 26 invertebrate species (14 rotifers, 6 cladocerans, 5 copepods and one mollusk larva). The mean abundance of the total zooplankton was higher in the first sampling period (2004–2005) and ranged between 8.81 and 47.74 ind. L⁻¹, than the second period (2005–2006) when fluctuated between 1.91 and 43.09 ind. L⁻¹. The seasonal variation was strongly influenced by the presence of rotifers, which accounting on average for 68.4% in total. Among them Keratella cochlearis and the order Bdelloidea were numerically the most important, while Macrocyclops albidus prevailed among the copepods and Bosmina longirostris among the cladocerans. Dreissena polymorpha was the only mollusk found in the zooplankton community. Rotifers, copepods and cladocerans showed a seasonal succession with the former preceding in the abundance having their first maximum in spring, while copepods and cladocerans followed, having peaks of abundance in early summer and in autumn, respectively. No seasonal succession among the cladoceran species was observed. The intense water flow in the upstream part of the reservoir, as well as temperature, conductivity, DO, pH, phosphates and silicates, were significant parameters controlling abiotic and biotic elements of the ecosystem and consequently influencing the seasonal variation and the dynamics of the zooplankton community.     

Zooplankton densities in a Hydrilla infested lake

The number of individuals and species of zooplankton were sampled concurrently with Hydrilla biomass and water quality for one year in a small, eutrophic central Florida lake. Throughout the study, rotifer species and individuals dominated the zooplankton. The abundance of the zooplankton tended to remain high when Hydrilla biomass was at its seasonal low during late winter and early spring. When hydrilla growth increased in the late spring and summer months causing a decrease in total alkalinity, specific conductivity, water color, turbidity, orthophosphate and chlorophyll a concentrations; the abundance of the zooplankton declined. During this time, there was a shift from limnetic to littoral species, principally rotifers. Hydrilla growth did not affect the mean number of cladoceran or copepod species, but may have led to an increase in rotifer species.     

Zooplankton response to a warmer northern Wadden Sea

Weekly measurements of mesozooplankton (>76 μm) and hydrographic parameters have been carried out since 1984 in the List Tidal Basin (northern Wadden Sea). Monthly water temperature significantly increased by 0.04°C year⁻¹. The largest increase by 3°C in 22 years occurred in September, implying an extension of the warm summer period. Mean annual copepod abundance and length of copepod season correlated significantly with mean temperature from January to May. Except for an increasing Acartia sp. abundance during spring (April–May), no long-term trends in copepod abundance were observed. The percentage of carnivorous zooplankton increased significantly since 1984 mainly due to a sudden increase in the cyclopoid copepod Oithona similis in 1997. We expect that global warming will lead to a longer copepod season and higher copepod abundances in the northern Wadden Sea.     

Creating conditions for changes in prey community structure by Chaoborus spp. in a lake in Sweden

In Lake G»rdsjön (Southwest Sweden), liming as an experimental improvement of living conditions for pelagic algae, resulted in a significant increase of algal biomass and a reduction of mean cell size. The algal development was beneficial for small sized filter feeding zooplankton, particularly rotifers, which showed a significant increase. The increase in abundance of small sized zooplankton created better food conditions for the smaller instars, and thus a much better overall survival of Chaoborus larvae. The resulting, 6–7 times larger population of Chaoborus larvae significantly changed the structure of the crustacean zooplankton community. Bosmina coregoni, the fastest swimmer of the crustacean species suffered most and was strongly reduced by the increased predation from Chaoborus. The share of cladocerans decreased, while copepods increased in importance.     

Morphological Response of Microcystis aeruginosa to Grazing by Different Sorts of Zooplankton

In the experiment we investigated the effect of grazing by different sorts of zooplankton on the induction of defensive morphology in the cyanobacterium Microcystis aeruginosa. The results showed that protozoan flagellate Ochromonas sp. grazing could induce colony formation in M. aeruginosa, whereas M. aeruginosa populations in the control and the grazing treatments of copepod Eudiaptomus graciloides, cladoceran Daphnia magna, and rotifer Brachionus calyciflorus were still strongly dominated by unicells and paired cells and no colony forma occurred. In the protozoan grazing treatment, the proportion of unicells reduced from 83.2% to 15.7%, while the proportion of cells in colonial form increased from 0% to 68.7% of the population at the end of the experiment. The occurrence of a majority of colonial M. aeruginosa being in the treatment with flagellates, indicated that flagellate grazing on solitary cells could induce colony formation in M. aeruginosa. The colonies could effectively deter flagellate from further grazing and thus increase the survival of M. aeruginosa. The colony formation in M. aeruginosa may be considered as an inducible defense against flagellate grazing under the conditions that toxin cannot deter flagellate from grazing effectively.     

Zooplankton and their relationships with water quality and fisheries

Zooplankton population structure in Rutland Water changed over the first five years of the reservoir's existence. Daphnia pulex, initially dominant was replaced by D. hyalina in late 1975 and since 1979 this latter cladoceran has coexisted for part of each year with Bosmina longirostris. Population fluctuations of all the main crustacean zooplankton — D. hyalina, Cyclops spp. and Diaptomus gracilis — were erratic in the first three years (1975–1977) but have since become more regular. The relationships of the zooplankton with their potential algal food supply and with their potential predators, the fish population, are discussed.     

Impact of selective predation by Mesocyclops pehpeiensis on a zooplankton community: experimental analysis using mesocosms

Predation by cyclopoid copepods is an important factor affecting zooplankton communities in freshwater habitats. Experiments provide strong evidence of the role of selective predation by cyclopoid copepods in structuring zooplankton communities. To assess the predation impact of a cyclopoid copepod, Mesocyclops pehpeiensis, we conducted a mesocosm experiment using 20-l polyethylene tanks in which the density of the predator and the food available to herbivorous zooplankton varied. M. pehpeiensis had a notable but selective effect on the zooplankton community. The population of a small cladoceran, Bosmina fatalis was affected negatively, but M. pehpeiensis did not have any apparent impact on the population dynamics of another Bosmina species, B. longirostris. On the other hand, the population of small rotifers responded positively to the presence of M. pehpeiensis, and their densities increased in mesocosms with a high density of M. pehpeiensis. It seems that suppression of B. fatalis by M. pehpeiensis predation indirectly affected rotifers by releasing them from competition with B. fatalis. The results suggest that copepod predation is a powerful factor regulating zooplankton communities directly and indirectly.     

The seasonality of zooplanktivorous fish in an African reservoir (Gebel Aulia Reservoir,White Nile,Sudan)

The fish fauna of Gebel Aulia reservoir (White Nile) consists of 52 species withAlestes baremoze Joannis andBrycinus nurse Rüppell dominating in numbers and biomass. The seasonality and condition ofA. baremoze, A. dentex (L.) andB. nurse are described in relation to various limnological factors in the reservoir, close to the dam. The zooplankton assemblage is small-sized, consists mainly of small Cladocera and shows striking seasonality. The hydrological regime, temperature and the grazing effect of fish are the major factors influencing its development.The three species belong to a eurytopic but basically riverine fish guild. They show a seasonal cycle of abundance with high numbers at periods of water storage (October–April) and low numbers during riverine conditions. This seasonal pattern of occurrence, the size structure of the population and the maturity indices give strong evidence that no resident population is present at the study site, but upstream migrations of the immature fish occur at low water level (May–September). Return migrations of juvenile fish into the area at the dam proper take place from October until April.

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Zusammenfassung In der Fischfauna des Gebel Aulia Stausees am Weißen Nil im Sudan dominieren Arten der Gatung Alestes. Die quantitative Bedeutung und das jahreszeitliche Auftreten vonA. baremoze Joannis,A. dentex Cuvier andB. nurse Rüppell wurde im Zusammenhang mit dem saisonalen Zyklus der hydrologischen und limnologischen Bedingungen untersucht. Die Häufigkeit der Fische im Staubereich ist in der Stauphase (Oktober–April) hoch und in den Monaten, in denen kein Rückstau erfolgt (Mai–September), gering.Das saisonale Muster des Auftretens, die größenstruktur der Fischpopulation and die Maturitätsfaktoren im eigentlichen Staubereich keine seßhafte Population vorhanden ist. Die Fische unternehmen kurz vor der Geschlechtsreife anadrome Wanderungen, die Jungfische wandern katadrom.

     

Zooplankton community structure driven by vertebrate and invertebrate predators

Summary A zooplankton community was established in outdoor experimental ponds, into which a vertebrate predator (topmouth gudgeon: Pseudorasbora parva) and/or an invertebrate predator (phantom midge larva: Chaoborus flavicans) were introduced and their predation effects on the zooplankton community structure were evaluated. In the ponds which had Chaoborus but not fish, small- and medium-sized cladocerans and calanoid copepods were eliminated while rotifers became abundant. A large-sized cladoceran Daphnia longispina, whose juveniles had high helmets and long tailspines as anti-predator devices, escaped from Chaoborus predation and increased. In the ponds which had fish but not Chaoborus, the large-sized Daphnia was selectively predated by the fish while small-and medium-sized cladocerans and calanoid copepods predominated. In the ponds containing both Chaoborus and fish, the fish reduced the late instar larvae (III and IV) of Chaoborus but increased the early instar larvae (I and II). Small- and large-sized cladocerans were scarcely found. The former might have been eliminated by predation of the early instar larvae of Chaoborus, while the latter was probably predated by fish. Consequently, the medium-sized cladocerans, which may have succeeded in escaping from both types of predator, appeared abundantly. The results suggest that various combinations of vertebrate and invertebrate predators are able to drive various kinds of zooplankton community structure.     

Low fish predation pressure in London reservoirs: II. Consequences to zooplankton community structure

The 1992 survey of zooplankton structure in fourteen London supply reservoirs showed the overall dominance of large-bodied zooplankton, mainly species of Daphnia. These reservoirs can be considered as ‘anti-fish’ by virtue of their steeply sloping concrete or brick sides. The average biomass of large Daphnia spp (retained on a 710 μm sieve) in the total zooplankton biomass was higher than 20% for twelve out of fourteen reservoirs. The cladoceran-copepod ratio was inversely correlated with both dominance of large-bodied Daphnia magna and cladoceran body-size structure. Parallelly, there were tendency of more efficient utilization of lowered algal crops in reservoirs dominated by large-bodied Daphnia spp. A graphical model is presented which relates daphnid species composition and zooplankton size structure to a presumed gradient of fish biomass in these reservoirs.     

Zooplankton feeding in the fish Trichogaster pectoralis Regan

The traditional culture method of Trichogaster pectoralis, using zooplankton produced from fermenting aquatic weeds as a source of food for the fry, leads to productions of ca 1 000 kg ha^–1 a^–1. If chicken manure is applied at a rate of ca 450 kg ha^–1 month^–1 instead, much more zooplankton is produced more rapidly, and fish production increases to ca 2 000 kg ha^–1 a^–1. This results from a higher survival rate of the fry, which feeds on rotifers, ciliates, copepods and cladocera until a size of ca 15.0 mm, when plant material also becomes important in the diet.     

Chaoborus predation and zooplankton community structure in a rotifer-dominated lake

During summer, Chaoborus punctipennis larval densities in the water column of fishless, eutrophic Triangle Lake become very high, and coincidently, the spined loricate rotifer Kelfcottia bostoniensis becomes the dominant zooplankter. Research was done to test the hypothesis that selective predation by Chaoborus on soft-bodied rotifers controls species dominance in the mid-summer zooplankton of this lake. In situ predation experiments showed positive selection by Chaoborus for the soft-bodied Synchaeta oblonga, negative selection for K. bostoniensis, and intermediate selection for Polyarthra vulgaris, a species with rapid escape tactics. However, during a 21 day in situ mesocosm experiment, zooplankton dominance and succession in Chaoborus-free enclosures was identical to that in enclosures with Chaoborus at lake density. Despite the selective predation, Chaoborus larvae may not exert significant top-down control on rotifers, whose intense reproductive output during mid-summer in temperate eutrophic lakes results in new individuals at rates that exceed predatory losses.     

Zooplankton grazing methods using radioactive tracers: Technical problems

Some of the technical problems specific to the application of radiotracer techniques for measuring grazing of zooplankton are highlighted and, in some cases, the corrective measures suggested.The major drawbacks of measuring zooplankton grazing in the laboratory, for studying the role of zooplankton in nature, namely, changes in water temperature and light and heavy mortality of some cladocerans, are overcome by the use ofin situ grazing chambers. However, at present there are no suitable techniques available to measure short-term assimilation rates which usually suffer from the respiratory loss during the experiment of the assimilated radiotracer.In the ecosystem research in which zooplankton grazing is an integral part of the studies, the radiotracer food manufactured fromin situ lake seston is preferred to labelled monoculture of algae, bacteria or yeast. The specific activities of the different tracer components and size fractions of the seston comprising the tracer may considerably differ in short-term labelling. On the other hand, long labelling periods,i.e. exceeding a day for example, may lead to shifts in size spectra of and composition of the different phytoplankton species. Changes during the incubation in the size spectra of the tracerfood are probably of greater importance than those in species composition, especially because a size-selective feeding may often exist.Simultaneous use of different radiotracers for labelling phytoplankton and bacteria or for mixtures of algae that differ in sizes offers some promising applications. More work in this field is needed, particularly because of the overlapping energy spectra of the radiotracers, and differential loss rates of these tracers from the preserved animals before the radioactivity counting. Such losses of the tracer from the experimental animals may lead to significant errors, especially in the short-term grazing measurements. This problem can be, nevertheless, prevented to a large extent by a rapid freeze-drying of the animals immediately on concluding the experiment.Self absorption may be a problem in radioactivity counting, more so if the animals are large. Use of sample oxidizers offers a good solution if the tracer can be trapped in its oxidized, gaseous form (e.g. ¹⁴CO₂) before further processing for counting. Modern solubilizing agents, like Soluene 350, with their relatively high water-holding capacity are quite effective in dissolving the different tissues, except the exoskeletal material.dedicated to Prof. H.F. Linskens, chairman of the Managing Committee of the Limnological Institute, in honour of his 65th birthday.     

Prey utilization by Chaoborus punctipennis Say in a small,eutrophic reservoir

Prey selection by 2nd-, 3rd-, and 4th-instar larvae of Chaoborus punctipennis was evaluated by comparing larval crop contents to the species structure of the zooplankton community in a small, eutrophic lake. The rotifers, Brachionus caudatus and Trichocerca similis, were positively selected by and comprised most of the diet of 2nd- and 3rd-instar larvae. Rotifers comprised 67% of all prey items identified in the crops of even 4th-instar larvae. Whereas 2nd-and 3rd-instar larvae selected the two rotifers with equalfrequency, 4th-instar larvae selected Trichocerca significantly more frequently than they did Brachionus. Although copepods comprised only 8% of all prey identified in 3rd-instar crops, they comprised 33% of the crop contents of 4th-instar larvae. Both 3rd and 4th instars exhibited a positive electivity for Cyclops vernalis significantly more frequently than for Diaptomus pallidus. We conclude that the larvae of Chaoborus punctipennis can, and do, select between Trichocerca and Brachionus and between Cyclops and Diaptomus.The paper by Brooks and Dodson (1965) triggered a flurry of studies on aquatic predators and their ability to discriminate between available prey species. Although the original work dealt with vertebrate predators, it has become increasingly apparent that invertebrate predators may also exhibit considerable selectivity in choosing between prey categories. The larval stages of the various species of Chaoborus are ubiquitous lentic predators, especially in mesotrophic or eutrophic systems. Although there have been a number of recent publications which deal with the food habits and preferences of several species of Chaoborus, only a few studies have included analyses of larval crop contents and none of these have included Chaoborus punctipennis. This species is of particular interest because it is one of the smaller species in the genus, and perhaps for this reason, because it is frequently present in lakes which support large populations of planktivorous fish (von Ende 1979). The purpose of our research was to characterize the food habits and preferences of Chaoborus punctipennis in a small, eutrophic reservoir which supports a rather depauperate number of species of zooplankters.     

Zooplankton distribution in four ponds of different salinity: a seasonal study in the solar salterns of Sfax (Tunisia)

The seasonal distribution of metazooplankton and large-sized ciliates was studied in four ponds of different salinity in the solar salterns of Sfax (Tunisia). Total zooplankton abundance varied from 1 × 10³ to 4.7 × 10⁶ ind m^–3. Salinity had a negative effect on the abundance of copepods and rotifers which were absent in the pond with the highest salt concentration (180) in which the number of taxa was low and Artemia or the ciliate Fabrea largely dominated the zooplankton community. Temperature and the presence of Dunaliella salina as prey appeared as key factors in controlling the abundance of Artemia, while organic detritus appeared as important in the diet of Fabrea. Change in zooplankton species composition along the hypersaline gradient (40–90) was primarily related to salinity. However, our data suggest the importance of both the abundance and composition of food in the spatial and temporal variations of some zooplankton species.     

Bacterial Activity in a Reservoir Determined by Autoradiography and its Relationships to Phyto- and Zooplankton

In the drinking water reservoir Římov (Southern Bohemia) bacterioplankton was studied during 1983. Special attention was given to the relationships between parameters of bacterial abundance, total and individual activity. Bacterial counts and biomass was assessed and autoradiographic determinations of the proportion of active bacteria incorporating thymidine (Th) and mixture of amino acids (AA) and total uptake rate of AA were made over a year in the surface layer and during summer stratification from the thermocline and 15 m depth. Specific activity of metabolically active bacteria (SAMAB) and specific activity per unit of biomass (SAUB) were negatively correlated with counts of metabolizing cells and with bacterial biomass, respectively. Total and individual heterotrophic activity and counts of bacteria coincided with the changes of phytoplankton biomass, whereas bacteria incorporating Th were more tightly correlated with primary production. The most significant relation of metabolically active bacteria was found to cladoceran biomass. Thus, this part of heterotrophic bacterial activity seems to be stimulated by leakage of dissolved organic matter from phytoplankton being disrupted and incompletely digested by cladocerans rather than from healthy photosynthetizing cells.