Population dynamic aspects of chironomid larvae of the littoral zone of Lake Maarsseveen I

Population dynamics of chironomid larvae in the littoral zone of Lake Maarsseveen I were investigated to a depth of 6 m. No essential differences in the composition and density between various sampling stations in the shallow sand flats were found. Two species,Cladotanytarsus mancus andStictochironomus sticticus, were found to be the main components of the chironomid population to a depth of 4 m. The density ofCladotanytarsus, which occurred predominantly to a depth of 1 m, increased from June to a maximum in December. In May and June, the entireCladotanytarsus population emerged.Stictochironomus had its peak of abundance in June. From this cohort, a part, differing in size for each of the various years, emerged in August. The remainder of this population, together with the offspring of the August adults, emerged in May. The size of the August midges, and the number of its eggs was smaller than that of the May midges. At depths greater than 4 m,Tanytarsus bathophillus was found to be the dominant chironomid.In summer the abundance of the small (second and third instars)Stictochironomus was primarily affected by water mite predation. In autumn and winter,Cladotanytarsus was the main food source for water mites. In autumn, whenStictochironomus larvae have molted into the fourth instar, predation by bream decreased the density of these larvae. In winter, the impact of this fish is negligible.     

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.     

Effectiveness of the caudal spine as a defense mechanism in Keratella cochlearis

The temporal distribution of spined and unspined forms of Keratella cochlearis in Cubillas reservoir during an annual cycle, and the stomach contents of Asplanchna girodi, were studied. The results suggest that the length of the caudal spine plays a critical role in the defense against predation by Asplanchna girodi. Short-spined forms showed a higher susceptibility to predation than unspined forms, which may have important consequencies for spine length variation in Keratella cochlearis populations.     

Changes in relative abundance of zooplankton in northern Lake Victoria,East Africa

The zooplankton community of Lake Victoria was studied between March 1990 and February 1991 with the aim of identifying its constituent groups/species, their distribution, abundance, and long term changes in community structure. Zooplankton samples were taken from four stations using plankton nets (75–300 m mesh size) hauled vertically through the water column. The plankton comprised mainly Crustacea and to a lesser extent early stages of aquatic insects. Cyclopoid copepods, their nauplii and copepodites constituted the most frequent group at all stations. Calanoid copepods, Cladocera and Caridina nilotica (Roux), though widely distributed, contributed a small proportion. Chaoborus larvae and pupae and mites were occasionally present. A comparison of the relative proportions of the main zooplanktonic groups from three sources revealed remarkable changes in community structure since 1931. These changes are discussed with respect to predation, eutrophication and other changes in the food web structure of the lake.     

In situ predatory behavior ofMysis relicta in Lake Michigan

Selectivity coefficients (W) and predation rates on Lake Michigan zooplankton were determined forMysis relicta during spring through fall using anin situ method. W values indicated the following ranked order of prey preference: Cladocera > copepod copepodites and copepod nauplii > adult diaptomids and cyclopoids. With few exceptions, W values for different prey categories remained fairly constant despite greatly changing relative abundances of prey. Predation rates and prey selectivity were similar in most cases to those determined in laboratory studies. Ingestion rates (percent dry body weight · day^–1) were correlated to total prey biomass (r = 0.38) and to effective prey biomass (r = 0.85), where the weighting factors were overall mean selectivity coefficients for the different prey categories. This result suggested that seasonally varying composition of prey caused much of the variation in ingestion rates among experiments. Feeding trials performed at the same depth with daytime and nighttime assemblages of zooplankton indicated that Cladocera may escape heavyMysis predation at night by migrating from the metalimnetic-hypolimnetic interface into the epilimnion.Contribution 333 from the Great Lakes Research Division, The University of Michigan and contribution 287 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration.Contribution 333 from the Great Lakes Research Division, The University of Michigan and contribution 287 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration.     

Changes in abundance of the icefish Neosalanx pseudotaihuensis Zhang (Salangidae) and the impact on the zooplankton community of Xujiahe Reservoir,central China

The icefish Neosalanx pseudotaihuensis Zhang is a small and commercially exploited zooplanktivorous fish in Xujiahe Reservoir (central China). Data of catch per unit effort (CPUE) from June 1995 to December 1997 show that the abundance of the icefish varied dramatically, which is reflected in the populations of the dominant cladocerans. The densities of Leptodora kindti, Daphnia galeata and Diaphanosoma dubia were the highest in 1997 when the abundance of the icefish was the lowest, and the reverse was the case for Boimina spp. These results document that the population of this annual fish is very variable and high biomass of the fish could exert a pronounced effect on the zooplankton community.     

Short Time-Response of Psammic Communities of Rotifera to Abiotic Changes in their Habitat

Sandy microhabitats and their fauna are usually characterised as extremely unstable and unpredictable. The aim of the paper is to evaluate the scale and the character of these phenomena. Rotifer composition and density and abiotic variables were investigated every half a day in the hygroarenal of the eutrophic Mikołajskie Lake.In total, 62 species were found with only two species occurring permanently throughout the study period. Densities in general were extremely variable and strong fluctuations were observed even at 12 h intervals. On average, psammophilic rotifers dominated the community (74%) while psammoxenes and psammobionts constituted only 18 and 8%, respectively. The most stable group were the psammophiles. The results support the presumption that fluctuating environments are ideal for organisms that reproduce quickly, may colonize vacant habitats rapidly and have wide niches, i.e. eurybionts.     

Typhloplanid flatworms (Mesostoma and related genera): Mechanisms of predation and evidence that they structure aquatic invertebrate communities

Mesostoma species and other typhloplanid flatworms are predators found in a wide variety of habitats. Laboratory observations of some Mesostoma spp found in shallow aquatic habitats (referred to as littoral species) have revealed a wide variety of prey killing mechanisms: 1) mucus trapping, 2) sit-and-wait predation, 3) releasing a toxin into the water, and 4) active searching. We review the existing literature on these mechanisms. We also describe for the first time the predatory behavior of a pelagic Mesostoma sp. found in Brazilian lakes.The existing literature is also reviewed to assess the potential impact of Mesostoma species and related genera on the aquatic invertebrate community. Mesostoma has a high potential for population increase and has been found in high densities in some shallow aquatic habitats. Single prey experiments show that a number of Mesostoma species feed heavily on mosquito larvae, some chironomid larvae and some daphnids but considerably less on most copepods and ostracods. Prey preference experiments reflect the same trends. Hence, these predation studies suggest that the flatworms, at high densities, should reduce populations of certain prey speces and, consequently, alter community structure. Field studies support this prediction. Mesostoma species, at high densities, appear to be important predators of mosquito larvae in shallow aquatic habitats even under conditions where high densities of planktivorous fishes had little impact. They also appear to alter zooplankton community structure. However, much of the field evidence is correlational and not experimental. No studies have assessed the impact of pelagic typhloplanids on zooplankton community structure.It is concluded that many more manipulative field experiments are needed to assess the impact of typhloplanids on aquatic invertebrate communities.     

Estimating predation risk in zooplankton communities: the importance of vertical overlap

In order to estimate predation risk in nature, two basic components of predation need to be quantified: prey vulnerability, and density risk. Prey vulnerability can be estimated from clearance rates obtained from enclosure experiments with and without predators. Density risk is a function of predator density, and the spatial and temporal overlap of the predator and prey populations. In the current study we examine the importance of the vertical component of overlap in making accurate estimates of predation risk from the invertebrate predator Mesocyclops edax on rotifer versus crustacean prey. The results indicate that assumptions of uniform predator and prey densities cause a significant underestimation of predation risk for many crustacean prey due to the coincident vertical migration of these prey with the predator. The assumption of uniformity is more reasonable for estimating predation risk for most rotifer prey.     

Long-term dynamics of Asplanchna priodonta in Lake Windsborn with comments on the diet

We evaluate trophic interactions between the rotifer Asplanchna priodonta and the plankton community of Lake Windsborn (Eifel District, Germany). The long-term population dynamics of this species and of the two most abundant zooplankton species, Keratella cochlearis and Bosmina longirostris, were analyzed in relation to hydrochemical and climatic variates. The maximum abundance of A. priodonta was limited to the summer months with temperature being the only significant predictor of abundance. Intrinsic rate of growth and abundance of the three species were independent from each other suggesting the absence of trophic interactions. In Lake Windsborn, A. priodonta was a grazer rather than a predator. Its diet included high proportions of colonial algae and cyanobacteria which cannot be ingested by the other abundant zooplankton species, whereas zooplanktonic food were rarely found. Thus, our study supports the hypothesis that A. priodonta is an opportunistic feeder whose trophic position entirely depends on the structure of the plankton community.     

The biogeography of littoral Lecane Rotifera

Little is known on the biogeography of Rotifera, particularly regarding the littoral taxa. Here, the biogeography of the most specious group of littoral Rotifera, Lecane, is discussed based on a recent revision of the group, and considering only verified records.Only 41.3% of all 167 Lecane morphospecies are widely distributed. Of these, 21 (12.6%) are cosmopolitan (sensu strictu), 26 (15.6%) are Tropicopolitan. There are 6 (3.6%) Arctic-temperate and Pantropical taxa. These categories of widely distributed taxa are not clear-cut, as differences in latitudinal distribution are largely gradual. Ten taxa have odd distributions. Most of these are insufficiently known and some may be recent introductions.All the major zoogeographical regions have their share of endemic taxa, with endemicity rates varying from 6.5% to 21.8%. Thirteen Holarctic, one widespread Eastern hemisphere, eleven Palaearctic, six Palaeotropical, one Australasian and five widespread Western hemisphere taxa have circumscribed ranges encompassing more than one region. The Palaeotropical and Holarctic components are particularly noteworthy, and may result from relatively recent faunal exchange between the Palaearctic and Nearctic, and between the African, Oriental and Australian regions. The different orientation of the major mountain chains of the Eastern and Western hemisphere, and the presence of extensive arid regions in Northern Africa, Arabia and Asia may explain the difference in faunal similarity between the tropical and temperate faunas of the Eastern and Western hemispheres. Ornithochoric dispersal and human introductions may have played a role in the exchange of faunas.The analysis of pairs or groups of closely related taxa only rarely reveals a causal relation between tectonical events and phylogeny. Most vicariant distributions are readily explained by climatological rather than by tectonical considerations. The ranges of some closely related taxa indicate that they have different capacities of dispersal.When compared to pelagic Rotifera, littoral Lecane exhibit a relatively wide variety of distribution patterns, and relatively many have restricted distributions. This may be a consequence of, either or both, their lower abundance, or be related to the littoral habitat which, being less predictable, implies more specific adaptations. Lecane is tropic-centred, which may be a consequence of avoidance of competition and/or predation by Cladocera and, eventually, Ostracoda.The most striking feature in rotifer chorology is the large range of many morphospecies. As such, the group's biogeography is more similar to that of freshwater algae than to that of other freshwater zooplankton. The likely explanations for this are the group's great ability for passive dispersal, combined with a short life-cycle and high fecundity, which are consequences of their parthenogenetic reproduction. So, relatively recent long-distance dispersal defines the ranges of the morphospecies. Vicariance plays a role in the distribution of Rotifera, but its importance is generally subordinate to that of dispersal. However, insufficient taxonomic resolution may be responsible for the apparently large ranges of some morphospecies.     

Temporal and spatial distribution of rotifera in a Chilean reservoir: A possible effect of impoundment hydrodynamics

Rotifers were sampled at monthly intervals for a year at four monitoring stations in Rapel Reservoir (a hydroelectric impoundment) in Central Chile (34°02S; 71°35W). Fifteen species were identified, but only Keratella cochlearis was consistently found at each station, it usually was the most abundant (> 50% of total rotifers). Marked differences were found among sites; stations I and 2, those nearest to the dam, showed greatest rotifer densities in spring and autumn. Station 4, that nearest to the inflowing rivers, had its highest rotifer abundance in summer. Station 8 had the highest mean density but the least marked seasonal changes in rotifer numbers.The impoundment water dynamics and the relative locations within the lake (distance from the dam or from the rivers) are suggested as determinants for different rotifer assemblages and, probably, for most other planktonic organisms.     

Environmental effects of introducing whitefish,Coregonus lavaretus (L.), in Lake Ring

The impact of whitefish (Coregonus lavaretus (L.)) on the trophic structure of eutrophic lakes was studied in Lake Ring, a small eutrophic Danish lake (22.5 ha, mean depth 2.9 m) in which the natural fish fauna is dominated by pike (Esox lucius L.), perch (Perca fluviatilis L.), and eel (Anguilla anguilla (L.)), roach (Rutilus rutilus (L.)) and burbot (Lota lota (L.)) being the only other fish species present. A total of 10993 0+ whitefish were stocked in the lake from October 1989 to July 1990 and the structure of the fish, zooplankton and benthic invertebrate communities studied during the period 1989–1991. Stomach contents analysis revealed that the whitefish mainly ate Daphnia and copepods in 1990–1991, the proportion of copepods decreasing with increasing size of the fish and Daphnia being the overall most important food source. The density of Daphnia in the lake decreased from 72 ind. 1^-1 in 1989 to 9 ind. 1^-1 in 1991; concomitantly the large species Daphnia magna and D. pulex almost disappeared and the density of cyclopoid copepods increased from 72 to 101 ind. 1^-1, presumably because of improved food conditions, while that of calanoid copepods remained virtually unchanged. As a result chl-a increased from 19 to 47 µg 1^-1 and Secchi depth decreased from 2.4 m to 1.7 m, despite there being no change in total P and total N (0.6 mg P 1^-1 and 1.3 mg N 1^-1, respectively). Changes were also observed in the benthic invertebrates; Chaoborus, oligochaetes, and chironomids all decreased, whereas Pisidium increased. It is concluded that the stocking of whitefish in eutrophic lakes for commercial purposes may delay their recovery, or even lead to enhanced eutrophication.     

Different Predation Impacts of Two Cyclopoid Species on a Small-sized Zooplankton Community: An Experimental Analysis with Mesocosms

We used mesocosms to analyze predation impacts on the prey populations and prey community structures by two cyclopoid copepod species, the larger Mesocyclops pehpeiensis and the smaller Thermocyclops taihokuensis, who coexist with small-sized herbivorous zooplankton species in a fish-abundant lake. The overall predation impact on the prey populations was stronger for Mesocyclops than for Thermocyclops. Mesocyclops had a strong and less selective impact on the rotifer community but a selective impact on the crustaceans. In contrast, Thermocyclops had a selective predation impact on rotifers but a weak and less selective impact on the crustacean community. As a result, the former predator reduced the diversity of the crustacean community but not the rotifer community, while the latter had an opposite impact on the diversities of the two communities. It has been suggested that fish induce development of a zooplankton community dominated by the small-sized zooplankton species in fish-abundant lakes. Our results demonstrated that cyclopoid copepods altered species composition and diversity of the small-sized zooplankton community in such lakes. Thus, the results have given an important suggestion on the role of the invertebrate predator cyclopoid copepods, which often coexist with fish, that they determine population dynamics and community structures of small-sized zooplankton in fish-abundant lakes.     

Cyclops scutifer SARS in Lake Latnjajaure,Swedish Lapland

In Latnjajaure, an ultra-oligotrophic mountain lake in Swedish Lapland, Cyclops scutifer is always present in considerable numbers. During a developing time of 3 years from egg to adult, losses remain low. High survival rates may be explained by lack of predators, even if there is selfcontrol by cannibalism. Temperature, not food supply, appears to be the most important growth steering factor. During more than 9 months of winter, the population is accumulated in water layers below 15 m depth. After the ice-break (July), the adults display diurnal vertical migration, and the distribution pattern in the lake appears extremely patchy. Food search and mating advantages are possible reasons for this behaviour.     

Population dynamics of Bosmina longirostris (Crustacea: Cladocera) in Lake Kariba,Zimbabwe

The population dynamics of Bosmina longirostris in a large tropical reservoir, Lake Kariba, Zimbabwe, are described. Population density showed two peaks associated with periods of high nutrient fluxes. Fecundity was always low. Two eggs per female was the maximum ever recorded. Instantaneous rates of birth, death and population growth calculated from field data using the egg ratio method are discussed. The rate of biomass change fluctuated in a regular pattern, probably linked to food availability or predation by a clupeid, Limnothrissa miodon.     

世界古老果树榅桲在中国的归化及其种群动态——以湖南省褒忠山种群为例

外来植物归化因其对本地生态系统有潜在的入侵风险而受到生态学家的广泛关注。群落和种群结构反映植物的环境适应性、种群地位、稳定性和发展趋势,可为判定外来植物归化提供可靠依据。榅桲(Cydonia oblonga)为在全球广泛引种栽培达4000多年的古老果树,引入中国已有2500多年,但关于其野外逸生种群的生存状态未见报道。以目前中国唯一逸生地——湖南省褒忠山的榅桲种群为对象,在其所在群落物种组成和群落结构分析的基础上,采用时空替代法,运用静态生命表、生存曲线和时间序列预测模型对榅桲种群动态进行了定量分析。结果表明：(1)群落内共有维管束植物20科28属31种,其中木本植物16种,草本植物15种;区系以温带成分为主,占群落内非世界分布总属数的75%;(2)群落具有明显层次结构,可分灌木层和草本层,灌木层个体集中于0—1.5 m高度级,占总个体数的81.81%;(3)种群龄级结构呈近倒“J”型,存活曲线趋近于Deevey-Ⅱ型,具有稳定的更新潜力;(4)生存分析表明榅桲种群具有前期优势、中期减少、后期稳定的特点;时间序列预测表明该种群个体数量未来能够持续增长,但幼苗及小树个体基本保持稳定,其种...     

Evidence for predatory flatworms as organizers of zooplankton and mosquito community structure in rice fields

I present evidence from two field studies and one laboratory experiment that the predatory flatworm, tentatively indentified as Mesostoma nr. lingua, can be important in organizing invertebrate communities in rice fields. In the first field study, I assessed the importance of various predators including two fishes, in explaining among-plot variation in mosquito abundance during a five week interval. Mosquitofish (Gambusia affinis) abundance was not significantly associated with densities of either mosquito, Culex tarsalis or Anopheles freeborni, except for the final week. Associations between green sunfish (Lepomis cyanellus) juveniles and mosquitoes were demonstrated for only A. freeborni — a positive association. Densities of Mesostoma were negatively associated with C. tarsalis and A. freeborni densities. C. tarsalis, protected from flatworms in small-mesh cages, had a much greater survival rate than those that were exposed in large-mesh cages. C. tarsalis survival, when left unprotected in large-mesh cages, was negatively correlated with field densities of flatworms and with the number of flatworms found inside the cages. Survival was also positively correlated with field densities of mosquitoes.In the second field study, Mesostoma densities were negatively correlated with C. tarsalis, A. freeborni, Cladocera and Ostracoda in rice field enclosures early in the season. Cyclopoid copepod densities were not significantly associated with this predator.In a laboratory experiment, predation by Mesostoma on C. tarsalis larvae was reduced when presented with an alternative prey, Moina micrura (a cladoceran).These results suggest that flatworms, via predation, can have large effects on the structure of mosquito and zooplankton communities in rice fields.     

Measuringin situ predation byMysis relicta and observations on underdispersed microdistributions of zooplankton

Described are a method and apparatus that allowin situ measurement of predation on zooplankton byMysis relicta. The method, which can be generalized to other predators, involves lowering paired large-volume (30-1) plankton traps to the depth of interest, with subsequent trapping of the ambient zooplankton assemblage in each trap and release of predators into one of the traps. The statistical adequacy of the method was shown by error propagation theory to depend on the percentage of available prey consumed, on the number of prey captured by the traps, and on the distribution of zooplankton within the volume of water captured by the traps. Repeated casts of the apparatus showed that, in contrast to other studies of zooplankton distribution, various zooplankton categories were statistically underdispersed (evenly dispersed in space) or at least not more statistically dispersed (clumped) than was a random distribution at a space scale of 1 m. An error analysis of many replicated feeding experiments showed that the errors obtained were reasonably small and that they conformed with or were less than those predicted by error propagation theory that assumed random distribution of zooplankton. Thus, these results supported the practical application of the method and corroborated the conclusion of random dispersion or underdispersion drawn from the experiment of repeated casts of the apparatus.     

Population dynamics of phototrophic bacteria in three basins of Lake Banyoles (Spain)

The annual dynamics of the phototrophic bacterial populations developing in the anoxic layers has been monitored in three basins of the northern area of Lake Banyoles (Spain). Although two of the studied basins are meromictic and one is holomictic, chemical properties of the water are almost identical. Therefore, differences in both the spatial and temporal distribution, as well as in the composition of phototrophic bacterial communities, dominated by Chlorobium phaeobacteroides and Chromatium minus, are discussed on the basis of the structural and morphometric characteristics of each basin. Both species showed the same physiological adaptations to light intensity changes by modifying the carotenoid/bacteriochlorophyll ratio. Light reaching the oxic-anoxic boundary appears to be the most important factor controlling the growth of phototrophic bacteria in Lake Banyoles. The oxic-anoxic boundary becomes shallower as summer advances, until enough light is available for bacterial photosynthesis.