Seasonal dynamics of zooplankton in Lake Huetzalin, Xochimilco (Mexico City, Mexico)

We quantified the seasonal changes in the zooplankton abundances collected from the Huetzalin Lake (Mexico City, Mexico) for two years (February 2003–January 2004 and then March 2005–February 2006). Selected physicochemical variables (Secchi depth, temperature, pH, conductivity, dissolved oxygen, phosphorus, nitrogen, carbon and chlorophyll a concentration) were also measured at the time of zooplankton collection. The data on zooplankton abundances and the physicochemical variables were subjected to multiple correlation analysis and we also derived Shannon–Wiener species diversity index. Secchi depth ranged from 9 to 65 cm. Generally the lake was alkaline (pH 7–12). The conductivity ranged from 500–1000 mS cm⁻¹, while the mean water temperature was 20.5 °C. Dissolved oxygen levels were generally >3 mg L⁻¹ and were higher in the winter than warmer months. Nitrates (90–95 μg L⁻¹) and phosphates (.2–.5 mg L⁻¹) indicated that the water was eutrophic. Chlorophyll a levels ranged from 143 to 696 μg L⁻¹ during the study period. The zooplankton community was dominated by rotifers (46 species), followed by cladocerans (9 species) and there were only two copepod species. The dominant rotifer species were Brachionus angularis, Brachionus calyciflorus, Brachionus havanaensis, Brachionus quadridentatus, Lecane bulla and Polyarthra vulgaris. Rare rotifer species in Lake Huetzalin were Lecane ohioensis, Dicranophorus forcipatus, Lecane pyriformis, Lindia torulosa, Pleurotrocha petromyzon and Brachionus durgae. Highest densities (occasional peaks of 400 ind L⁻¹) of B. quadridentatus occurred between April and December, while B. havanaensis reached peak densities, during June to October. B. calyciflorus reached densities higher than 1240 ind L⁻¹ during May–September. Cladoceran and copepod densities in Lake Huetzalin were much lower than that of the rotifers. This study confirmed the earlier findings that Xochimilco system of canals is dominated by rotifers and the crustacean zooplankton have much lower abundances possibly due to predation from fish.     

Interactions between the rotifer Euchlanis dilatata and the cladocerans Alona glabra and Macrothrix triserialis in relation to diet type

We studied, separately, the effect of the cladocerans Alona glabra and Macrothrix triserialis on the population growth of the rotifer Euchlanis dilatata using three different food types (Chlorella and Scenedesmus, live or dead, offered on a comparable dry weight basis, 3.16 μg C ml⁻¹). Regardless of the presence or absence of competition, E. dilatata cultured using Chlorella (live or dead) showed a higher population growth as compared to Scenedesmus. Compared to heat-killed C. vulgaris, E. dilatata cultured alone and fed on live algae reached higher abundances. Both cladoceran species had an adverse effect on the population growth of Euchlanis, but did not eliminate the rotifers. Of the two cladocerans, M. triserialis had a far more adverse effect on the population densities of E. dilatata as compared to A. glabra. E. dilatata also influenced the population growth of A. glabra and M. triserialis, the former being more adversely affected than the latter. When the egg ratios of E. dilatata were plotted as a function of population density, a significantly inverse relation was obtained only in treatments containing Chlorella but not for other treatments. Statistically the rate of population increase (r) per day of E. dilatata was adversely affected by the diet type and the presence of competing species. The interaction of diet type X competition was also significant. For A. glabra, the presence of E. dilatata and the diet type, but not their interaction, significantly influenced the r. On the other hand for M. triserialis, only the diet type was significant. Our results suggest that the competitive outcome between rotifers and cladocerans was dependent on diet type.     

Late-Glacial/Holocene succession of the chironomid and cladoceran fauna of the Soppensee (Central Switzerland)

Remains of chironomids and cladocerans were studied in a profundal sediment core, which covered a time period since the Oldest Dryas. The aim was to observe long-term variability in the structure of the aquatic fauna. The most dramatic change in the chironomid fauna occurred at the end of the Younger Dryas when the predominating cold-stenothermal Sergentia coracina disappeared due to both climatic amelioration and eutrophic conditions. The chydorids showed a distinct response to the changing temperature at the end of the Oldest Dryas as evidenced by an increase in species diversity, whereas no effect was discernible in the Younger Dryas. A decrease in the proportion of clear water species occurred in the Boreal. In the planktonic Cladocera,Bosmina longispina became extinct in the Atlantic and two Eubosmina morphs of uncertain taxonomic state very recently invaded the lake.     

Are the controls of species composition similar for contemporary and sub-fossil cladoceran assemblages? A study of 39 shallow lakes of contrasting trophic status

To assess the similarity, not only in community structure, but also in the factors that shape cladoceran assemblages, we analysed the contemporary zooplankton populations and their sub-fossil remains in 39 shallow UK and Danish lakes. Contemporary zooplankton populations sampled from both the lake edge and the open water in August were compared with surficial sediment assemblages. The sedimentary assemblage data combined counts of both ephippial and chitinous remains in order to provide some representation of Daphnia and Ceriodaphnia. A relatively large volume of sediment (>5 cm³) was analysed for ephippial remains so as to include those species best represented by the larger ephippia. Ephippia were identified to species level in the case of Daphnia magna, and to species aggregates for other groups such as Daphnia hyalina agg., Daphnia pulex agg. and Ceriodaphnia spp. In accordance with previous work several species found in contemporary samples (copepods and the cladocerans Polyphemus pediculus, Scapholeberis mucronata and Diaphanosoma brachyurum) were absent from the surface sediments. There were extensive supporting environmental data sets for the 39 sites. It was therefore possible to determine the factors which influenced assemblage composition for the two datasets by a combination of constrained ordination, in this case redundancy analysis (RDA), partial RDA and Procrustes rotation. The same two factors, zooplanktivorous fish density and submerged macrophyte abundance, were not only the main structuring forces for both data sets, but also explained very similar amounts of the variation in the different assemblages. Thus, we conclude that the living communities and their sedimentary remains reflect the environment they are shaped by in broadly similar ways. Calibration of sub-fossil cladoceran assemblages against modern environmental data to reconstruct environmental change can, therefore, validly employ principles elucidated by contemporary studies to determine the most appropriate modelling technique.     

滇西北地区高山湖泊沃迪错近两百年来环境变化及枝角类群落响应

区域增温和大气氮沉降作用已成为高山湖泊面临的重要环境胁迫,已有高山湖泊生物群落响应的长期模式研究主要集中于藻类而缺乏更高营养级生物(如浮游动物)的系统调查。本研究选择滇西北地区深水型的高山湖泊沃迪错开展沉积物调查,通过多指标分析(总氮、总磷、叶绿素a、氮稳定同位素等)并结合区域气候重建记录,识别近两百年来该湖泊及流域环境的变化历史,进一步利用枝角类群落指标(物种组成、生物量等)定量评价了湖泊生物群落的响应模式与驱动因子。结果表明,湖泊营养水平(如总氮浓度)和初级生产力(叶绿素a浓度等)在过去近两百年总体呈上升趋势。相关分析显示,大气氮沉降和流域外源输入是影响总氮上升的主要因素,同时区域增温和营养盐富集促进了湖泊初级生产力的不断上升。自1960s以来区域升温明显,湖泊营养水平和叶绿素a浓度呈现加速上升的趋势。钻孔中枝角类群落以浮游属种(Daphnia longispina等)为优势种,在1900AD以前D.longispina相对丰度较为稳定(40.83%±8.02%),之后出现下降趋势且在1948—1965年间明显下降,之后再次明显上升并成为主要优势种。排序分析显示,气温、叶绿素a和总...     

Changes of water level in the Eemian palaeolake at Imbramowice (SW Poland) based on isotopic and cladoceran data

Isotopic and cladoceran investigations of Eemian (MIS 5e) lake sediments from Imbramowice, SW Poland, allow us to reconstruct the environmental conditions, especially changes of water level and trophic status, during the early and middle Eemian Interglaciation. We analyzed the sediments from 6.5 to 11.0 m depth in a core provided by Mamakowa (1989). The upper 6.5 m had insufficient carbonates and cladoceran contents for analyses. The analyzed section consists of sandy and organic silts at the bottom, followed by gyttja characterized by increasing CaCO₃ content. Measured δ¹⁸O values oscillate from ca. - 9 to - 4‰ and δ¹³C from - 3.5 to above + 6‰. Based on stable isotope analyses of carbonates, we define and characterize eight isotopic horizons (Is). We identify 26 taxa of subfossil Cladocera and seven zones (CLZ) of faunal development. Probably the greatest depth of the lake occurred with pollen zone E2; shallowing then took place. During pollen zones E2-E3, gradual warming is observed and expressed through a positive trend in both δ¹⁸O and δ¹³C values. Pollen zone E4 is characterized by frequent changes of water level. During the Eemian Interglaciation, excluding the initial phase of lake evolution, the lake was meso-eutrophic and eutrophic with high phytoplankton productivity.     

Relationships between environmental variables and zooplankton subfossils in the surface sediments of 36 shallow coastal brackish lakes with special emphasis on the role of fish

Subfossil zooplankton assemblages (Cladocera 22 taxa, Rotifera 1 taxon) were identified from the surface sediments of 36 shallow (median depth = 0.7 m) Danish coastal brackish lakes differing in epilimnic salinity (SAL, range 0.2–17.4), summer-mean total phosphorus (TP, 27–327 g l^–1) and total nitrogen (TN, 0.850–2.629 mg l^–1), as well as in submerged macrophyte coverage and planktivorous fish density (PL-CPUE). Cladoceran species richness declined significantly with increasing SAL, TP and TN, while no significant correlation was found to either PL-CPUE, macrophyte coverage or lake surface area. Bonferroni-adjusted forward selection within canonical correspondence analysis (CCA) showed that 22.1% of the variation in zooplankton data was explained by PL-CPUE, SAL and TP uniquely; each variable explaining an almost equally significant amount of variation in the zooplankton data. Predictive models to infer PL-CPUE, SAL and TP were developed using variance weighted-averaging (WA) procedures. Almost similar values of boot-strapped coefficient of determination (r²_{boot-strapped} 0.22–0.38) were produced by the WA inference models of PL-CPUE, SAL and TP, while the inference models of TP produced the lowest boot-strapped root-mean-squared-error of prediction (RMSEP_{boot-strapped} 0.29–0.36 log(TP + 1), g l^–1). Yet, zooplankton TP and SAL optima (WA) were strongly correlated (r² = 0.46), while PL-CPUE optima (WA) were independent of both TP and SAL optima, indicating that only the PL-CPUE inference models are suitable for making reconstructions.     

The long-term succession of high-altitude cladoceran assemblages: a 9000-year record from Sägistalsee (Swiss Alps)

Cladoceran remains were analysed in a 1344 cm long sediment core from Sägistalsee (Swiss Alps, 1935 m asl) which covered the last 9000 years. Planktonic Cladocera were almost exclusively represented by Daphnia species, which occurred throughout the core. The chydorid fauna consisted of four species: Alona quadrangularis, Alona affinis, Acroperus harpae and Chydorus sphaericus of which the former was by far the most frequent species. The chydorid succession was characterised by disappearance and re-appearance of Acroperus harpae and Chydorus sphaericus at about 8400 and 3340 cal. BP, respectively. As a result, there was a long period of about 5000 years in which only two chydorid species were present with strong predominance (88.9%) of Alona quadrangularis. There was also a long-term trend of an increase of Alona affinis at the expense of Alona quadrangularis throughout the core.     

Temporal variations of fossil Cladocera in the sediments of Lake Orta (N. Italy) over the last 400 years

We use Cladocera remains in the sediments to reconstruct the longterm history of Lake Orta, a lake which experienced severe pollution from copper and ammonium sulphate, and started to recover during the last 15 years. Both human and natural impacts were detected over almost 400 years. Pollution was manifested by a dramatic decrease in the number of remains, in the planktonic/littoral (P/L) ratio, and in chydorid species diversity. Most species, even those most tolerant to stress, disappeared. The recolonization of biota was initially sustained by one species of chydorid,Chydorus sphaericus, which had a three-fold increase at the beginning of the 1980s, when studies on plankton recorded the development of a pelagic population. This fact, and the appearance in the sediments ofAlona quadrangularis, attest to the development of filamentous algae which was reported in studies on the plankton. The colonization of the lake by pelagic species came only in the last twelve years and was manifested also as a stabilization of the P/L ratio. Two other periods of disturbance were detected in the sediments: the first, at the end of the 19th century, was related to the introduction of exotic fish species; the second occurred during the second half of the 17th century. This event probably cannot be ascribed to human impact, but may be related to a decrease in temperature in that period (the Maunder Minimum).