Chydorus arcticus n.sp., a new cladoceran crustacean (Chydoridae: Chydorinae) from the North Atlantic arctic and Subarctic areas

Chydorus arcticus n.sp. (Cladocera: Chydoridae: Chydorinae) is described, figured, and differentiated from the closely relatedC. sphaericus (O.F. Müller, 1785). The known distribution of the species is given, and some aspects of speciation of arctic crustaceans are pointed out.     

Growth and reproduction of three cladoceran species from a small wetland in the south-eastern U.S.A.

SUMMARY 1. Growth, reproduction and life‐history parameters were measured for three cladoceran species from a small south‐eastern wetland, U.S.A. Simocephalus serrulatus, Diaphanosoma brachyurum and Scapholeberis mucronata juveniles were reared at temperatures between 10 and 25 °C on natural food resources. 2. Growth rate increased with temperature and decreased with individual size for all three species. Maximum somatic growth rate was higher for Simocephalus (49–72% day^?1) and Diaphanosoma (21–91% day^?1) than for Scapholeberis (11–45% day^?1). Multiple regression equations were developed which predict temperature‐ and mass‐specific growth rates for each species. 3. Scapholeberis egg production was positively related to temperature; however, maximum egg production occurred at intermediate temperatures for Simocephalus and Diaphanosoma. Mean cumulative egg production was higher for Scapholeberis (28–92 eggs per female) than for Simocephalus (18–25 eggs per female) and Diaphanosoma (1–41 eggs per female), and was related to differences in reproductive strategy and survival. 4. Survival was inversely related to temperature in most cases. For all three cladocerans, the intrinsic rate of increase (r) and net reproductive rate (R₀) increased with temperature, whereas generation time (G) decreased. Greater egg production by Scapholeberis compared with the other two cladocerans was consistent with higher R₀ values for Scapholeberis at any given temperature. Although r was very similar among species, G was typically longer for Scapholeberis than for Simocephalus and Diaphanosoma. 5. This analysis provides basic information about the population parameters of these coexisting wetland species, and the growth rate models can be applied to field data to determine production dynamics.     

Comparison of five cladoceran species in short- and long-term copper exposure

The study was carried out to assess the effect of both short-and long-term copper exposure on five cladoceran species differing in body size and habitat, i.e. Daphnia magna, D. pulex, D. galeata, Bosmina longirostris, and Chydorus sphaericus. The species-specific 48 h EC₅₀ values for fed neonates were used to determine the chronic exposure levels (52% and 65% of the EC₅₀ values). The experiment was run at two food levels. Long-term copper exposure retarded growth in each of the species studied. However, the copper treatments did not affect the clutch sizes of the experimental animals. At a low food level, copper exposure increased mortality in every species studied. The intrinsic rate of increase, r, was reduced in the copper-exposed populations. The sensitivity to copper was higher in the small lake species, compared to D. magna and D. pulex that are commonly used in standard toxicity tests. Of the species studied, B. longirostris had the highest sensitivity to both acute and chronic copper stress. The different sensitivities of the species studied are discussed in an ecological context.     

Empirical relationships between cladoceran fauna and trophic state in thirteen northern German lakes: analysis of surficial sediments

The cladoceran faunas from 13 northern German lakes (the Plön district) of different trophic levels were analysed using the bosminid and chydorid remains in the surficial sediment as an integrated sample of the total lake fauna. In the dimictic lakes, a shift occurred in the composition of planktonic Cladocera from Bosmina coregoni, including morphs with a mucro, to predominance of Chydorus sphaericus and Bosmina c. thersites as trophic state increased. In the benthic Cladocera (Chydoridae), there was a decrease in species diversity, in the percentages of phytophilous species and in the portion of species that are typical of clear water lakes, but there was an increase both in the number of mud dwellers and species that are typical of polluted lakes. No positive correlation was found between mean depth of the lakes and the planktonic/littoral ratio.     

Effects of competitors and Chaoborus predation on the cladocerans of a eutrophic lake: an enclosure study

The role of large laboratory grown food competitors ofthe genus Daphnia as well as the predationimpact of Chaoborus on the cladoceran communityof an eutrophic lake was assessed in five insitu enclosure experiments. The hypothesis tested wasthat the outcome of competition and gape-limitedpredation on cladocerans is size dependent. Accordingto the generally accepted assumptions on competitionand invertebrate predation, large-bodied cladocerantaxa were expected to be less affected by competingcongeners and by Chaoborus than weresmall-bodied taxa. Effects of the predator upon anassemblage of differently sized cladoceran taxa weremuch more pronounced than effects of competition.There was a tendency of predation and competitionimpact to decrease with cladoceran size, but predationpressure was also low for some small cladocerans andhigh for some large cladocerans. The general trendswere further obscured by factors not or indirectlylinked to body size.     

Population dynamics and nutrient fluxes in an aquatic microcosm

An aquatic microcosm, consisting of three spatially separated yet mutually dependent trophic levels, was established in the laboratory and monitored for 310 days. A three-fold research approach evaluates the experimental potential of this large, multicompartmental microecosystem. Realistic biological and chemical features and nutrient fluxes parallel identifiable patterns observed in natural aquatic ecosystems as well as in published laboratory observations. Two successional patterns developed in the autotrophic community: a sequential change in species composition and a progression from a one-compartment planktonic situation to a two-phased planktonic-attached system. Although the microcosm was initially seeded with an axenic culture of Cryptomonas ovata var. palustris Ehr, contamination by Chlorella, Scenedesmus, Closterium, and Anabaena occurred within 41 days. The appearance of attached algae, noted on day 5, marked the transition from a planktonically-based ecosystem to a heterogeneous system. Crashes in the cladoceran population occurred on days 103 and 202. The second collapse was final. Repeated attempts to reestablish Daphnia middendorffiana failed. Mineralization and nutrient cycling are recognizable properties of the microcosm. Ammonification, nitrification, and nitrogen assimilation occurred predominantly in the decomposer tank as did the regeneration of inorganic phosphorus. A peak on day 205 in the ammonia input to the algal tank drawn from beneath the bacterial filter bed followed a peak in total Kjeldahl nitrogen (TKN) (day 135) and preceded peaks in nitrate (day 219) and TKN (day 233). Although levels in the algal tank were undetectable after three weeks, dissolved orthophosphate was actively regenerated in the decomposer bed, recycled to the autotroph unit, and rapidly assimilated by the algae. Characteristic patterns of radiotracer circulation also were evident. Sequential movement of ³²P from the dissolved compartment to phytoplankton to attached algae was proposed. Conversely, ¹⁴C was steadily incorporated into the phytoplankton compartment; filtrate activities fluctuated. Tracer behaviors in the cladoceran compartment were superficially cyclic. Carbon turnover times in the algal and zooplankton compartments were 17 and 11.11 hours, respectively. Indicative of the greater biological mobility of phosphorus, respective turnover times of 2.50 and 2.44 hours were similarly calculated for phosphorus. Unlike dissolved carbon which had a turnover time of 625 hours, dissolved phosphorus was rapidly cycled into the algae (turnover time = 0.58 h).     

Ecology and management of moorland pools: balancing acidification and eutrophication

Moorland pools originally are shallow, often hydrologically isolated, soft-water bodies, with a low productivity. Some thousands of moorland pools originated from the late Pleistocene onwards in the heathland landscape in The Netherlands and adjacent areas, where soils have a poor buffering capacity. As the pools are largely fed by atmospheric precipitation, they are very vulnerable to changes in the environment, e.g. eutrophication and acidification. Moorland pools are exposed to very high rates of wet atmospheric deposition: 44–50 mmol m⁻² yr⁻¹ sulphate and 84–103 mmol m ⁻² yr⁻¹ ammonium. Mass budgets indicate that 20–70% of the input of sulphate by precipitation is reduced, 40–100% of the ammonium input escapes to the air or sediments, apparently due to nitrification, and 90–100% of incoming nitrate disappears. The production of alkalinity ranges from 12 to 52 meq m⁻² yr⁻¹. The efficiency of these processes augments with pH-values of surface water increasing from 4.1 to 5.4. The accumulation of reduced sulphur compounds in the sediments is a threat in extremely dry summers, when desiccation causes oxydation of these compounds, resulting in very low pH-values (≤ 3.7). Acidification by acid atmospheric deposition and eutrophication by agricultural acidification are the main threats to the moorland pool ecosystems and affect the species composition of assemblages of aquatic macrophytes, desmids, diatoms, macrofauna, fishes and amphibians, as has been shown by comparison of old and recent records on their distribution and paleolimnological methods. Afforestation exacerbates acidification and also reduces wind dynamics. Particularly the decrease of isoetids and desmids by both processes indicate the biological impoverishment of the pools. Reductions of (potential) acid atmospheric deposition to less than 40 mmol m⁻² yr and of ammonia to less than 30 mmol m⁻² yr are necessary for recovery of the moorland pools. Methods for the addition of buffering material to a number of moorland pools, to counteract acidification until these deposition rates have lowered sufficiently, are given, as well as other methods for restoring the biological quality of moorland pools.     

Species richness of the Cladocera (Branchiopoda: Anomopoda and Ctenopoda) in southern Thailand, and its complementarity with neighboring regions

Seventy-two cladoceran species from southern Thailand include eleven first records. Species accumulation curves were used to estimate the total number of species present, and Chaos estimator was used to extrapolate the species number observed in 212 samples to the total number present. This S*max amounted to 76.06 species, with a low ratio of variance/estimator. Cladoceran faunas were compared by a complementarity index at three levels: between habitat types, between different zones of Thailand, and between Thailand and other countries. The geographical gradient was quite strong, but because not all areas have been studied to the same degree and with the same taxonomic accuracy, some comparisons are robust while others are not.     

Geographic and phylogenetic evidence for dispersed nuclear introgression in a daphniid with sexual propagules

The role of among-species gene flow in eukaryotic evolution remains controversial. Putative hybrid lineages are common in water fleas, but their ecological success is often associated with polyploidy and the production of asexual propagules. Advanced hybrid lineages with sexual propagules are expected to be geographically restricted because their successful dispersal is contingent on overcoming fertility complications, assimilation by parent taxa, and competition with parent taxa. Here we provide evidence that a diploid lineage of Daphnia has been formed by introgression between distantly related species and attained a broad distribution (Nearctic) despite its requirement for sexual propagules. The evidence is based on geographical discordance, phylogenetic discordance, recombinant genotypes and additive genotypes of the nuclear internal transcribed spacer regions (ITS) and mitochondrial DNA. Additive genotypes also provided evidence of hybridization between introduced European Daphnia and North American Daphnia. We argue that the unique biology of Holarctic lacustrine water fleas and the spatial separation of lineages during Pleistocene glaciation have promoted hybridization and its evolutionary consequences.