Alonella Nana Research Articles

Structure and heterogeneity of the Cladocera taphocoenoses in lakes of the Sredny Island (Keretsky archipelago, Kandalaksha bay of the White Sea)

Surface sediments, which were selected from various depths in three lakes of the Sredny Island (Karelia, Russia), were analyzed for subfossil Cladocera assemblages. In total at least 30 taxa remains were identified in samples of surface sediments. Predominance of cold-water littoral and cosmopolitan taxa was noted according to biotypic characteristic. Eurybiont Chydorus cf. sphaericus and typical of wetlands, the phytophilous Alonella nana, were most common in Cladocera taphocoenoses. Spatial heterogeneity was noted in subfossil Cladocera assemblages from various depths. Pelagic taxa proportion increased just as littoral taxa proportion decreased towards the center of lake. Several species remains were only found in littoral surface sediments samples.

A hidden species becoming visible: biogeography and ecology of Rhynchotalona latens (Cladocera, Anomopoda, Chydoridae)

A long hidden chydorid (Chydoridae, Cladocera) taxon, first found as fossil specimens and recently redefined as Rhynchotalona latens (Sarmaja-Korjonen et al., Hydrobiologia 436: 165–169, 2000) is investigated for its biogeography and ecology. Late Holocene sediment sequence from Lake Sylvilampi, NE Finnish Lapland, and R. latens spatial distribution in relation to limno-climatic attributes in Finland were examined. Principal component analyses of fossil cladoceran communities showed that R. latens is mostly affiliated with Alonella excisa–Alonopsis elongata–Alonella nana species pool. Generalized linear modeling of R. latens responses to limno-climatic variation indicated that it prefers acidic, mesotrophic, humic and shallow lakes with organic sediments in NE Lapland and has a north boreal-subarctic climatic affiliation. At the northern end of its geographical distribution (NE Lapland), it reproduces with abundant gamogenesis under environmental stress. The specialized taxon is a benthic detritivore and scraper and has a Holarctic northern–alpine distribution. It is a glacial relict associated with modern analogs of periglacial aquatic environments, and it occurs in semi-aquatic wetlands, lush lake littorals and clear and cold waters. Examination of chydorids as bioindicators, especially those with restricted niches, allow us to understand biodiversity responses of lake littorals under changing limno-climatic regimes.

Response of Cladocera Fauna to Heavy Metal Pollution, Based on Sediments from Subsidence Ponds Downstream of a Mine Discharge (S. Poland)

Mining is recognized to deeply influence invertebrate assemblages in aquatic systems, but different invertebrates respond in different ways to mining cessation. Here, we document the response of the cladoceran assemblage of the Chechło river, S. Poland (southern Poland) to the cessation of Pb-Zn ore mining. The aquatic system includes the river and associated subsidence ponds in the valley. Some ponds were contaminated during the period of mining, which ceased in 2009, while one of the ponds only appeared after mining had stopped. We used Cladocera to reveal how the cessation of mine water discharge reflected on the structure and density of organisms. A total of 20 Cladocera taxa were identified in the sediment of subsidence ponds. Their density ranged from 0 to 109 ind./1 cm3. The concentrations of Zn, Cd, Cu and Pb were much higher in sediments of the ponds formed during peak mining than in the ponds formed after the closure of the mine. Statistical analysis (CCA) showed that Alonella nana, Alona affinis, Alona sp. and Pleuroxus sp. strongly correlated with pond age and did not tolerate high concentrations of heavy metals (Cu and Cd). This analysis indicated that the rate of water exchange by the river flow and the presence of aquatic plants, affect species composition more than pond age itself.

The changes in the composition of Cladocera community in bottom sediments of Lake Maloye Shibrozero (Zaonezhsky Peninsula) as a consequence of shifts of environmental and climatic conditions

The study aims to explore the evolution of lakes of the boreal zone during the late- and postglacial time on the south-eastern periphery of the Fennoscandian crystalline shield since the last deglaciation. In order to reconstruct the past for virgin territories of the Zaonezhsky Peninsula current investigation on bottom sediments of Lake Maloye Shibrozero was conducted. Analyzes were performed using the new paleoindicator – subfossil remains of Cladocera (Cladocera, Branchiopoda). The 28 samples of bottom sediments were analyzed. It has been determined that discovered Cladocera remains belong to representatives of 6 families and 38 taxa. Species inhabiting Palaearctic zone are predominant in lake deposits; most of the identified subfossil remains are related to the pelagic species inhabiting the open part of the lake. According to the Lubarsky scale the dominant of Cladocera community is Bosmina (Eubosmina) cf. longispina. Secondary taxa are Chydorus sphaericus, Bosmina coregoni, Alonella nana, Alona guadrangularis, A. affinis, Chydorus gibbus. At a depth of 650-653 cm, a partial replacement of Bosmina (Eubosmina) cf. longispina by Bosmina coregoni takes place with a simultaneous increase in the significance of Chydorus sphaericus, which is used to be an indicator of eutrophication and increasing trophic status of the reservoir. Changes in Cladocera community could be attributed to decreasing the level of periglacial lake, as a result of which the Lake Maloye Shibrozero became a small isolated lake with the trend to trophic status increasing. Cold-water species were replaced by thermophilic ones with a further return to a cold-water fauna. In the upper layers of the column an increase of the number of phytophilous species is noted.

Paleoecological change of cladoceran and the response to climatic environmental change from an alpine lake-Jiren Co

高山湖泊吉仁错位于川西高原树线以上，受现代冰川融水补给影响.通过吉仁错沉积钻孔²¹⁰Pb/¹³⁷Cs定年和沉积枝角类和多环境代用指标分析，结合主成分分析和冗余分析等方法，高分辨率重建了近200 a来吉仁错枝角类组合和环境变化的过程.结果表明，尽管过去200 a来吉仁错枝角类组合一直以沿岸种为主，但仍经历了3个明显的变化阶段.1850 AD后，枝角类组合中Alonella nana、Chydorus sphaericus 和Pleuroxus sp.的增加、以及Alona rustica和Alona guttata丰度的下降指示了水温和水体pH值的上升.1900 AD后枝角类通量的增高并不同步于组合的变化，指示了湖泊营养开始增加，这种不一致的变化分别代表了大气氮沉降和区域气候变暖的影响结果.多指标综合分析得出，过去150 a来，气候变暖和大气污染沉降通过直接和间接作用（流域冰川融水过程和植被土壤过程），较深刻地影响了湖泊物理过程（水温升高和无冰期加长）、营养过程（氮、磷营养升高）和酸碱平衡过程（碱性增强），改变了湖泊生物的生长季节，并通过促进藻类发育，最终引起了湖泊枝角类群落组合的变化和生物量的增加.1945 AD前后吉仁错枝角类与藻类群落结构和湖泊环境的同步变化，响应于持续增温背景下多环境过程的相互作用.;Jiren Co is a small oligotrophic lake in West Sichuan plateau, located above the present tree-line and influenced by modern glacier meltwater input. Based on ²¹⁰Pb/¹³⁷Cs dating and multi-proxies (i.e., cladoceran, diatom and geochemical) from Jiren Co sediment core and using principal component analysis and redundancy analysis, this study analyzed the history of cladoceran assemblage change during the past 200 years, and reconstructed the environmental change of Jiren Co. Although cladoceran remains in Jiren Co was dominated by littoral species with significant changes over time, the core assemblages can divided into three distinct zones. After 1850 AD, the increase in abundance of Alonella nana, Chydorus sphaericus and Pleuroxus sp. and the decrease in Alona rustica and Alona guttata all indicate rising temperature and pH. After 1900 AD, cladoceran flux increased earlier than the change in abundance, which indicates the beginning of lake nutrition increases. This inconformity reflects the influence of climate warming and atmospheric nitrogen deposition, respectively. Multi-index analysis suggests that, over the past 150 years, climate warming and atmospheric deposition in the directly and indirectly effects (ice melt, vegetation and soil development) have affected the physical processes on the water temperature rise and prolonged ice-free period, nutrition processes on nitrogen and phosphorus increased, acid-base equilibrium processes on the increased alkalin. It changed the growing season and promoted the growth of algae, and finally affected on the assemblage and production of cladoceran. The synchronization change of cladoceran assemblage, algae and geochemistry proxies indicates a response of alpine lake to multi-environmental forcing under the background of continuous climate warming.

Reconstruction of flood events in an oxbow lake (Marótzugi-Holt-Tisza, NE Hungary) by using subfossil cladocerans remains and sediments

<p>Oxbow lakes are important components of the floodplain systems of lowland rivers. During flood events, oxbows are connected with the main river channel, and behave as lotic systems, while during inter-flood periods, these lakes can be considered as lentic ecosystems. Rivers are generally poor in planktonic organisms and their sediments contain scarce biological remains in comparison to lentic water ecosystems. However, due to their alternating running and standing water regime, sedimentary biological remains of oxbow lakes can be used as proxies for tracking changes of past hydrological regimes. In this study we investigated how cladoceran communities respond to flood events, and whether flood events can be recognized by community analysis of cladoceran remains. A sediment core from Marótzugi-Holt-Tisza oxbow lake was analyzed for identification of past flood events based on changes in the subfossil Cladocera community. Floods were defined based on the proportion of fine sand (50 µm grain size) in the oxbow sediments. If the fine sand portion was <3%, the water regime of the oxbow was considered as lentic, otherwise it was lotic. Both organic and pigment contents were significantly higher in the core sections deposited during lentic stages. Thirty-four Cladocera species were determined in this core, all common to littoral habitats of eutrophic shallow lakes in Hungary. One planktonic (<em>Bosmina longirostris</em>) and four chydorid species (<em>Alona rectangula</em>, <em>Acroperus harpae</em>, <em>Alonella nana</em> and <em>Chydorus sphaericus</em>) were dominant throughout the core and contributed >90% of total remains. Discriminant analysis on cladoceran data confirmed that lotic and lentic hydrological stages were characterized by different Cladocera species associations. <em>Bosmina longirostris, Chydorus sphaericus, Alona rectangula, Acroperus harpae, Leydigia leydigi, A. quadrangularis </em>and<em> A. nana</em> were mainly responsible for the differences between lotic and lentic species assemblages. Our results revealed that Cladocera remains can be used to track changes in the hydrological regime of oxbow lakes. </p><p> </p>

Niche separation by littoral-benthic Chydoridae (Cladocera, Crustacea) in a deep lake - potential drivers of their distribution and role in littoral-pelagic coupling

Niche separation of 22 species of benthic Chydoridae was examined in a depth gradient in the littoral zone of a deep Lake Piaseczno (eastern Poland), by using a trap sampler. Individual species showed high segregation across the littoral zone according to depth, macrophyte biomass, total organic carbon (TOC), conductivity and pH, of which the depth turned out to be the most important. The species Acroperus harpae, Alona affinis, Alonella nana, Leydigia acanthocercoides, Pseudochydorus globosus, Pleuroxus trigonellus and Pleuroxus aduncus were found in the whole littoral zone, but the remaining Chydoridae species displayed clear spatial distribution, e.g. Coronatella rectangula, Acroperus elongatus, Alona guttata, Alonella excisa, Alonella exigua, Rhychotalona falcata, Monospilus dispar and Pleuroxus truncatus settled the shallower parts of the littoral, whereas Eurycercus lamellatus, Camptocercus rectirostris, Pleuroxus laevis, Graptoleberis testudinaria, Alona costata, Pleuroxus uncinatus and Alona intermedia were found mainly in areas located 3-6 m deep. To find out the role of Chydoridae in littoral-pelagic food web coupling, the contribution of chydorids to total number of Cladocera in the guts of fish was estimated showing that they are an important intervening link in nutrient movement from the littoral to the pelagic zone, as their contribution to the total cladoceran number found in guts of fish caught in the pelagic zone was even as high as 98%. This study provides more insight into the nature of constraints associated with littoral-benthic cladocerans’ habitat preferences and their importance in littoral-pelagic coupling.

Cladocera record from Budzewo (Skaliska Basin, north-eastern Poland)

ABSTRACT The sediment sequence from Budzewo (Skaliska Basin, north-eastern Poland) contains cladoceran records beginning from the early Holocene. A total of 33 Cladocera taxa were identified in the entire sequence. The cladoceran fauna of the initial stage of palaeolake history in the Preboreal shows high similarity to early Holocene Estonian and Scandinavian records. Benthic Alonella nana was dominant at that time. After that, probably during the Boreal and early Atlantic periods, Cladocera species diversity increased and planktonic forms (bosminas) became dominant, pointing to a rise of water level. The species composition indicates that the lake was meso- to eutrophic. The lake began to shallow during the middle Atlantic. The process was completed in the Subboreal and the lake transformed into a bog. The fall in water level and finally the terrestrialization of the lake is correlated with similar processes recorded in other sediment sequences in northern Poland, suggesting that this event may have been driven by regional factors such as lower precipitation.

Response of subfossil Cladocera in Gerzensee (Swiss Plateau) to early Late Glacial environmental change

Sub-fossil Cladocera were studied in a core from Gerzensee (Swiss Plateau) for the late-glacial periods of Oldest Dryas, Bølling, and Allerød. Cladocera assemblages were dominated by cold-tolerant littoral taxa Chydorus sphaericus, Acroperus harpae, Alonella nana, Alona affinis, and Alonella excisa. The rapid warming at the beginning of the Bølling (GI-1e) ca. 14,650yr before present (BP: before AD 1950) was indicated by an abrupt 2‰ shift in carbonate δ18O and a clear change in pollen assemblages. Cladocera assemblages, in contrast, changed more gradually. C. sphaericus and A. harpae are the most cold-tolerant, and their abundance was highest in the earliest part of the record. Only 150–200years after the beginning of the Bølling warming we observed an increase in less cold-tolerant A. excisa and A. affinis. The establishment of Alona guttata, A. guttata var. tuberculata, and Pleuroxus unicatus was delayed by ca. 350, 770, and 800years respectively after the onset of the Bølling. The development of the Cladocera assemblages suggests increasing water temperatures during the Bølling/Allerød, which agrees with the interpretation by von Grafenstein et al. (2013-this issue) that decreasing δ18O values in carbonates in this period reflect increasing summer water temperatures at the sediment–water interface. Other processes also affected the Cladocera community, including the development and diversification of aquatic vegetation favourable for Cladocera. The record is clearly dominated by Chydoridae, as expected for a littoral core. Yet, the planktonic Eubosmina-group occurred throughout the core, with the exception of a period at ca. 13,760–13,420yr BP. Lake levels reconstructed for this period are relatively low, indicating that the littoral location might have become too shallow for Eubosmina in that period.

The impact of conifer plantation forestry on the Chydoridae (Cladocera) communities of peatland lakes

Conifer plantation forestry is recognised as a potential source of diffuse pollution to surface waters and represents a risk to their ecological status. In this study, the water chemistry and Chydoridae (Cladocera) communities of 26 small blanket bog lakes were investigated to assess the impact of plantation forestry. The study was conducted over a 12-month period in 2009–2010 by comparing lakes with three distinct catchment land uses: (i) unplanted blanket bog only present in the catchment, (ii) mature conifer plantation forests only present in the catchment and (iii) catchments containing mature conifer plantation forests with recently clearfelled areas. All three catchment land uses were replicated across two geologies: sandstone and granite. Lakes with afforested catchments had very high concentrations of plant nutrients (P and N), total dissolved organic carbon (TDOC) and heavy metals (Al and Fe), the highest concentrations being recorded from the clearfelled lakes. Similarly, the chydorid communities differed between lakes of contrasting catchment land use. The dominance of Alonopsis elongata in the unplanted blanket bog lakes shifted to dominance by the smaller bodied Chydorus sphaericus, along with Alonella nana, Alonella excisa and Alonella exigua, in the plantation forestry-effected lakes, consistent with a shift in lake trophy. Our findings have shown that plantation forestry can have a profound impact on the water quality of small peatland lakes, especially at the clearfell stage. The response of the chydorid communities is consistent with plantation forestry exerting a trophic, rather than an acidic or toxic, effect on lake ecosystems.

The Afromontane Cladocera (Crustacea: Branchiopoda) of the Rwenzori (Uganda–D. R. Congo): taxonomy, ecology and biogeography

The timely characterization of high-altitude freshwater habitats allows an assessment of the diversity of its biota and provides the basis for monitoring community change. In this study, we investigate the Cladocera fauna of 29 water bodies (pools, freshwater lakes, and surrounding swamps sampled at various occasions between 2005 and 2009) in the Rwenzori Mountains (Uganda, D. R. Congo), which are part of the East African Sky Island Complex. All sites except one are located above 3700 m altitude. We include notes on the morphology, taxonomy, distribution, and ecology of each recorded taxon and describe a new species of the Alona rustica-group (Alona sphagnophila n.sp.; Chydoridae). We found 11 species of which seven are restricted to Lake Mahoma, the lowest lake in our study area (2990 m) (Alona affinis barbata, A. intermedia, Alonella exisa, Alonella nana, Daphnia cf. obtusa, Pleuroxus aduncus) and/or Lake Bujuku (Daphnia cf. curvirostris, P. aduncus) (3900 m). Two taxa (Ilyocryptus cf. gouldeni, A. sphagnophila n.sp.) are restricted to Carex/Sphagnum bogs surrounding lakes in the afroalpine zone. Pigmented populations of Chydorus cf. sphaericus occur in all the sites. It is the only cladoceran species surviving the extreme alpine and nival conditions in the Rwenzori. The species is joined by A. guttata at locations at lower altitudes (ca. 3000–4000 m), present in about half of the sites. The Rwenzori Cladocera fauna is characterized by a strong extratropical temperate component and a low level of speciation/endemism. Harboring an impoverished boreal cladoceran community, Lake Mahoma is given closer attention. At 2990 m, the lake is a cold-temperate aquatic island in the tropics and may function as a stepping stone for Palaearctic taxa. We introduce a new term for high-altitude, cold-water habitats in the tropics, which act as climatic islands for extratropical freshwater faunas, Loffler Islands, in honor of Dr Heinz Loffler. In comparison to surveys in 1961, we list five new records in Lake Mahoma, which could indicate cladoceran community changes over the past few decades at ca. 3000 m in the Rwenzori. Since the species distributions correlate to temperature and catchment properties of the lakes, the Rwenzori cladoceran fauna can be expected as sensitive indicators for local changes.

Paleolimnological evidence for increased sexual reproduction in chydorids (Chydoridae, Cladocera) under environmental stress

To investigate the extent to which anthropogenic perturbations such as eutrophication and trace metal pollution (i.e., environmental stress sensu Odum 1985) influence the reproductive modes of cladoceran populations, we analyzed the abundance of subfossils of the chydorids Alonella nana (Baird, 1850) and Alona affinis (Leydig, 1860) in sediment cores from three Finnish lakes. Reconstruction of lakes' pollutant history and the biological response of chydorids indicate that in two of the lakes the proportion of individuals reproducing sexually increased with environmental stressors. More specifically, A. nana responded to eutrophication in Lake Hampträsk with greater production of ephippia, while A. affinis responded to aluminum pollution or acidification in Lake Pieni Majaslampi. In contrast, the reference lake, Lake Iso Lehmälampi, showed no radical changes in sexual reproduction over the twomillennium long sediment record. We conclude that chydorids may use sexual reproduction as a strategy for overcoming unexpected environmental stresses.

Does oxygen availability regulate sexual reproduction in local populations of the littoral cladoceran Alonella nana?

We investigated sexual reproduction patterns of a ubiquitous littoral cladoceran Alonella nana (Anomopoda, Chydoridae), using ecological and paleoecological approaches to clarify the forcing mechanisms behind its sexual reproduction. Contemporary sampling of A. nana populations in two environmentally different lakes showed abundant sexual reproduction in the lake with eutrophic, oxygen-deficient conditions and scarce gamogenesis in the oligotrophic, well-oxygenated lake. Sediment core studies from the same lakes indicated that comparable sexual reproduction patterns had prevailed for many centuries. However, in eutrophic Lake Hamptrask, A. nana increased its sexual reproduction from 1700 ad onward, consistently with decreasing trend in chironomid-inferred oxygen. The core samples, together with a surface sediment dataset of 25 lakes from southern and central Finland, showed a negative correlation between ephippia of A. nana and winter oxygen, although the surface sediment data per se did not show any significant correlation. When the dataset was scaled locally to include lakes in close proximity in southern Finland, the correlation became clearer. The results imply that the spatial and temporal variations in sexual reproduction of A. nana populations may partly be explained by differences in oxygen levels.

Matching thirty years of ecosystem monitoring with a high resolution microfossil record

Summary 1. Monitoring of the ecosystem of Lake Mývatn, Iceland, since 1975 has revealed extreme fluctuations in important food web components, such as chironomids and cladocerans, with amplitudes of several orders of magnitude and a period of 5–8 years. This study uses sediment cores from the lake to examine if the food web fluctuations appear in the microfossil record of the sediment. 2. Dating was achieved by means of a combination of 137Cs and volcanic tephra and was fine-tuned by wiggle-matching of chironomid microfossil and monitoring data. 3. Cladocera exuviae and chironomid egg capsules in the uppermost 34 cm of sediment were compared with the monitoring record that consisted of 30 years of window trap catches of flying chironomids and a 16-year record of chydorid Cladocera caught in activity traps. 4. The observed chironomid and cladoceran population fluctuations were reflected in the sediment record of chironomid eggs and of the exuviae of three of seven cladocerans: Alonella nana, Alona rectangula and Eurycercus lamellatus, which also had the most extreme fluctuations in the monitoring data (3–4 orders of magnitude). Chydorus sphaericus, and to some extent Alona quadrangularis and Acroperus harpae, showed regular fluctuations in the core that the monitoring did not reveal. Density of subfossil chironomid eggs correlated positively with that of larval head capsules but not with other microfossils. 5. This study shows a reasonably good correspondence between the fossil records of chironomids and cladocerans on the one hand and biomonitoring data on the other. The results pave the way for an extension of the food web history to much earlier time intervals of the ecosystem, allowing the study of long-term variation in the food web dynamics, including the impact of climatic variation and other external forcing. The results also indicate the usefulness of chironomid egg capsules in palaeolimnological studies.

Temperature sensitivity of gamogenesis in littoral cladocerans and its ecological implications

Temperature sensitivity of chydorid (Cladocera, Chydoridae) gamogenesis in southern Finland was examined. Chydorid sweep net samples and littoral surface water temperature measurements of seven lakes were taken weekly. Percentage abundances of gamogenetic females were used to estimate the temperature optima and tolerances for gamogenesis of individual chydorid species. The estimations were carried out using a weighted averaging (WA) technique. The results indicated that the optimum temperature of gamogenesis ranged between 7.0 and 9.7 °C and the total tolerances were between 2.0 and 4.7 °C suggesting a species-specific response for gamogenesis. Alonella nana and Chydorus sphaericus s.l. had the lowest optima, which was possibly due to their ubiquitous ecological preference and wide geographical distribution. Camptocercus rectirostris, Alonella excisa and Grabtoleberis testudinaria had high optima for their gamogenesis. These estimations provide first numerical autecological data on gamogenesis of natural and contemporary chydorid populations. Furthermore, the results provide species-specific estimations of the effect of the present climate warming on chydorid reproduction patterns. This information can be useful background data in global change assessments regarding freshwater lakes.

The diurnal distribution of the zooplankton community of the littoral zone of Lake Wielkowiejskie (Wielkopolski National Park, Poland)

Abstract The main aim of this study was to analyze the diel distribution of zooplankton communities within the zones of Nymphaea albae, Myriophyllum verticillatum, and Typha angustifolia, and, compared to that, in the open water area of a lake. It also aimed to analyze the relationship between zooplankton densities and the concentration of physicochemical factors and chlorophy11 a content. A total of 149 zooplankton taxa were noted, and, despite the narrow range of the research in the analyzed lake, some rare species were noted, among which two - Colurella sinistra and Lecane inopinata - were recorded for the first time among Polish fauna. The rush zone was taxonomically the richest, while the open water area was the poorest. The highest number of individuals was observed in the Myriophyllum bed, while the lowest was noted in the zone dominated by Nymphaea alba. The highest densities of zooplankton occurred at dusk, while the lowest occurred during the day, irrespective of the sampling station. Because Lake Wielkowiejskie is a shallow and macrophyte-dominated water body, the differences in the diurnal distribution of zooplankton communities may suggest horizontal migrations between the pelagic zone and the phytolittoral, as well as between particular macrophyte stands. A positive correlation between the densities of some zooplankters and the concentration of chlorophyll a was found, while a negative relationship with the concentration of nitrites was recorded for Lecane quadridentata and Alonella nana.

Intensity of autumnal gamogenesis in chydorid (Cladocera, Chydoridae) communities in southern Finland, with a focus on Alonella nana (Baird)

The aim of this study was to examine whether there was variation in the intensity of gamogenesis (sexual reproduction) in communities of chydorid cladocerans during the autumnal sexual reproduction period. The proportions of gamogenetic individuals (i.e., intensity) in the chydorid communities of seven lakes in southern Finland were determined in weekly samples throughout the autumn of 2005. The period of gamogenetic reproduction began very synchronously in the lakes as a response to climatic forcing and proportions of gamogenetic individuals progressively increased towards winter. However, wide variation was found in intensity among the communities. The high intensity probably was a response to some environmental stressors (e.g., invertebrate predation, crowding, competition, or changes in water chemistry) to ensure genetic variability and future populations. One common species, Alonella nana showed exceptional, dualistic, gamogenetic behavior, since in some communities it reproduced with high and in others with extremely low gamogenetic intensity. It is possible that in the former it responded to environmental stressors by exhibiting high intensity of gamogenesis, thus renewing its genotypes, while in the latter it succeeded primarily by parthenogenetic (asexual) reproduction, and was possibly perennial. The high gamogenetic intensity in A. nana was related to dystrophic and mesotrophic conditions, but it correlated positively only with water conductivity.

Long-term changes in benthic Cladocera populations in Lake Myvatn, Iceland

Benthic Cladocera were monitored at five sites in Lake Myvatn, Iceland, over a decade (1990–1999), as part of a programme documenting the population fluctuations of animals at different trophic levels in the lake. The species composition remained relatively stable over the first seven years, but in 1997 the population of Eurycercus lamellatus was greatly reduced at all sites. The following year saw a mass occurrence of Alona rectangula and Alonella nana that were previously abundant only locally and rare at most sites. Alona affinis, A. quadrangularis, Acroperus harpae and Chydorus sphaericus were not affected. In 1999 the Cladocera assemblages returned to the pre-1997 situation. The shift was from large-bodied epibenthic and planktonic species (Eurycercus, Daphnia) to small infaunal (Alona rectangula) and ubiquitous (Alonella nana) species. Medium sized (Alona, Acroperus) and some small cladocerans (Chydorus) were not affected. The course of events was reminiscent of a trophic cascade caused by a change in size-selective predation pressure. If so, the impact of a predatory fish population (three-spined stickleback, Gasterosteus aculeatus) depended on whether cyclic chironomid populations were in a high or a low phase. The change in the Cladocera coincided with profound changes in the sediment characteristics associated with low chironomid abundance. We suggest that the relative competitive ability of the Cladocera species is reversed when the sediment has become homogeneous and nutrient-poor after overexploitation by the dominant, tube building and detritivorous chironomid Tanytarsus gracilentus.

Spatial and temporal variation of benthic Cladocera (Crustacea) studied with activity traps in Lake Myvatn, Iceland

Benthic Cladocera were studied with a modified type of an activity trap in Lake Myvatn in 1990–1992. After feasibility experiments, the operation time and the distance of the traps from the benthic substrate were adjusted in order to minimize the effects of diurnal variation and the trapping of planktonic organisms. The trap catches of Eurycercus lamellatus were positively correlated with their abundance at the bottom as estimated by grab sampling (r = 0.910, P < 0.001). The usefulness of the activity trap was demonstrated by: (1) a lake-wide survey of the benthic Cladocera; (2) a study of the seasonal variation in the size distribution, abundance and sex ratio of E. lamellatus; and (3) a study of the seasonal succession of Chydorus sphaericus and Alona affinis. The variation of benthic Cladocera among 21 trap sites distributed on a 1–2-km scale across the lake exceeded the within-site variation. The sampling sites could be divided into five main groups based on cluster analysis. Eurycercus lamellatus was the most common species in the mat of filamentous green algae (Cladophorales). Alonella nana dominated the area of spring water inflow in the north basin and Macrothrix hirsuticornis the area of spring water inflow in the southeastern part of the lake. In other parts of the lake either Chydorus sphaericus or Alona quadrangularis tended to dominate. The size distribution and sex ratio of E. lamellatus was followed at two sites through one summer. In early summer most individuals were females less than 1.45 mm long. Around mid-summer they had grown to 0.69–3.1 mm. By the end of August the size distribution had become bimodal, with a large number of small males and a smaller number of much larger females. The seasonal succession in the abundance of E. lamellatus, A. affinis and C. sphaericus was followed at four sites over two seasons. With some exceptions the abundance of a species followed a similar seasonal trajectory on the different stations in any one year. There was, however, a marked difference between the two years (1991 and 1992), probably related to different temperatures.

Sedimentary Cladoceran remains and their relationship to nutrients and other limnological variables in 53 lakes from British Columbia, Canada

To develop models to predict past lake-water nutrient levels, the sedimentary remains of Cladocera were sampled from 53 lakes in central British Columbia, Canada. At the same time, the lakes were sampled for a suite of chemical variables. In addition, a host of physical and spatial explanatory variables were collected from each site. Canonical correspondence analysis showed that total phosphorus (TP), which ranged from 5 to 146 µg·L–1, was the measured environmental variable that best described the differences in species composition among the lakes. Additionally, lake depth and surface water temperature were also important in explaining the distribution of cladoceran taxa. Chydorus brevilabris, Daphnia ambigua, Daphnia cf. pulex, and Graptoleberis testudinaria had a preference for eutrophic lakes, whereas Acroperus harpae, Alonella nana, Alonella excisa, Chydorus piger, Daphnia cf. dentifera, and Eubosmina spp. were found in the less productive lakes. Predictive models to estimate TP from species abundance data were developed using weighted averaging techniques. This research has produced strong and significant inference models, which can now be used to reconstruct past changes in lake trophic status from remains of Cladocera in sediment cores.