Cladocera assemblages from the surface sediments of south-central Ontario (Canada) lakes and their relationships to measured environmental variables

Fossil cladoceran remains preserved in surface sediment samples from 44 oligotrophic lakes in south-central Ontario were examined to evaluate the relationships between species assemblages and measured environmental variables. Differences in cladoceran assemblages were related to physical and chemical variables using multivariate techniques. Redundancy Analysis (RDA) identified five environmental variables as significantly influencing assemblage composition: sulphate (SO₄²⁻), calcium (Ca²⁺), pH, maximum lake depth (Z _max) and dissolved organic carbon (DOC). There was a distinct separation of lakes and taxa along the ion gradient based on SO₄, Ca and pH. Additionally, cladoceran communities in coloured, shallow lakes had relatively higher abundances of littoral chydorid species and the pelagic taxa Holopedium spp., and the Daphnia pulex complex. Deep, clear lakes had relatively higher abundances of other pelagic taxa. Predation by fish (measured as presence–absence) and Chaoborus (measured as density) were less significant than some of the physico-chemical variables in influencing cladoceran assemblage structure. However, this could be due to the limited resolution of the predation data that was available at the time of this study. The distribution of cladocerans in the surface sediment, and their relation to these important environmental variables, suggests that there is considerable potential for the use of sedimentary cladoceran remains as environmental indicators in south-central Ontario lakes. Handling editor: J. Saros     

Patterns in the Distribution of Cladocerans (Crustacea: Branchiopoda) in Lakes Across a North–south Transect in Alaska, USA

The remains of cladocerans were examined from the surface sediments of 51 freshwater sites along a north–south transect spanning Alaska. We identified 27 cladoceran taxa from the sediments, consisting primarily of littoral chydorid species. Variations in cladoceran assemblages were related to measured physical and chemical variables using multivariate techniques. Redundancy analysis (RDA) indicated that lake depth, total phosphorus (TP), and altitude all had a significant influence in determining the composition of cladoceran assemblages. Cladoceran communities in tundra and forest-tundra lakes, which were relatively shallow and nutrient-poor, had relatively low abundances of pelagic Cladocera, and were primarily composed of several littoral chydorid species. Among pelagic cladoceran species, there was a distinct shift in dominance from the Bosminidae in lakes in the southern boreal forest region to Daphniidae in lakes in the northern boreal forest. Daphnia dominated lakes had significantly higher total phosphorus, specific conductivity, and calcium concentrations than lakes dominated by Eubosmina. Overall, the relative importance of physical and chemical factors in structuring cladocerans is similar to other previously studied regions, and suggests the Cladocera may be useful as ecological and paleoenvironmental indicators in this region.     

Effect of Aeolosoma sp. (Aphanoneura: Aeolosomatidae) on the Population Dynamics of Selected Cladoceran Species

Although oligochaete worms naturally coexist with cladocerans in many shallow freshwater ponds and lakes, their influence on the latter is not well established. In this work we studied the effect of Aeolosoma sp. on the population growth of Alona rectangula, Ceriodaphnia dubia, Daphnia pulex, Macrothrix triserialis and Moina macrocopa. Population growth studies were conducted at one algal food density (1 × 10⁶cells ml^–1 of Chlorella vulgaris). The experimental design was similar for all five cladoceran species, where we used 100 ml capacity transparent jars containing 50 ml of EPA medium with the desired algal density and three replicates for each treatment. The test medium was changed daily and fresh algal food was added. The initial density of each of the cladoceran species in the population growth studies was 0.4 ind ml^–1 while that of the worms 1.0 ind ml^–1. Following inoculation, we estimated daily the number of cladocerans and the worms for duration of 21 days. Regardless of the presence of worms, Moina macrocopa and Macrothrix triserialis showed rapid population growth while A. rectangula took more than 2 weeks to reach peak abundances. With the exception of M. triserialis, all the other our cladoceran species declined in the presence of Aeolosoma sp. The lowest peak population density (about 1 ind ml^–1) was observed for M. triserialisin controls. The remaining species had peak densities of about 3–5 ind ml^–1. The rates of population increase per day varied from 0.03 to 0.19 depending on the cladoceran taxa and the treatment. In general we found that pelagic taxa were more adversely affected by the presence of the worms than were the littoral cladocerans.     

Large Intercontinental Differentiation of Moina Micrura (Crustacea: Anomopoda): One Less Cosmopolitan Cladoceran?

Moina micrura Kurz, 1874 (Crustacea: Anomopoda), commonly regarded as a cosmopolitan cladoceran species, can be found almost all over the world except for arctic and cold-temperate regions. M. micrura has been recorded from virtually all types of limnetic habitats, including temperate permanent ponds, lakes, ephemeral desert pools, and tropical brackish fishponds. Its high morphological and ecological variability has so far been considered a result of intraspecific diversity. We tested the hypothesis that genetically isolated groups exist within M. micrura with crossing experiment and DNA sequence analysis. We compared two clones originating from populations from Central Europe (area of the type locality) and Australia. As there was extremely low production of hybrid eggs, the results of the crossing experiment strongly suggest the existence of reproductive isolation. The sequence divergence of mitochondrial gene for 12S rRNA (7.9%) was comparable to intraspecific differences within Daphnia pulex group. The sequence divergence of the clones of Moina macrocopa from Europe and Central Africa was much lower (1.4%). Our results suggest that at least two sibling species exist within M. micrura complex, which is in agreement with other data questioning the cosmopolitanism of various cladoceran species.     

Intra-lake heterogeneity of sedimentary cladoceran (Crustacea) assemblages forced by local hydrology

Intra-lake variation of fossil Cladocera (Crustacea) assemblages in 31 surface sediment samples in Lake Pieni-Kauro and River Saavanjoki, eastern Finland, was examined with an objective to identify habitat specificity of Cladocera in relation to local hydrology-related environmental factors. The surface sediment assemblages showed high levels of heterogeneity, mainly as to water depth and lentic–lotic gradients in the lake–river complex. This was evident from the principal component analysis which indicated a major trend from shallow to deep samples and a secondary trend from lentic to lotic samples, and from redundancy analysis (RDA), which recognized water depth and river flow as the most important environmental variables in explaining cladoceran variability within the dataset. According to the RDA and generalized linear models, Daphnia spp., Bosmina (Eubosmina), and Alona quadrangularis showed association with deep water localities, whereas Bosmina longirostris and Alona affinis preferred littoral habitats. Acroperus harpae and Chydorus sphaericus s.l. appeared to favor lotic habitats. The results propose that littoral taxa are primarily deposited postmortem or after molting close to their shallow water habitats, while planktonic cladocerans accumulate principally in deepwater locations. Accordingly, it appears that in heterogeneous basins intra-lake surface sediment samples integrate locally living fauna that is driven by local hydrology-related factors, such as water depth, sediment properties, macrophytes, and river flow and coupling biotic interactions.     

Chydorid assemblages in the sedimentary sequence of Lake La Cruz (Spain) subject to water level changes

Changes among chydorid assemblages in Lake La Cruzsediments, from a core of 178 cm length, have beenstudied using community structure indices andmultivariate methods (cluster analysis, principalcomponents analysis).The results revealed the existence of two main sourcesof variation in these assemblages. One (axis I ofPCA1) is associated with the trophic level of thelake, which is hypothetised to be greater in thebottom and upper part of the core. Both zones arecharacterised by an increase in the relative frequencyof Chydorus sphaericus, accompanied by a markedreduction in the relative frequency of the mostabundant species throughout the history of the lake,Acroperus neglectus.The other source of variation (axis II of PCA1) may beinterpreted as the alternation of periods of dryer and wetter weather. Ouranalyses show the separation of two assemblages whichalternatively prevail in the sedimentary sequence, oneconstituted by Pleuroxus laevis, Alona guttata,Graptoleberis testudinaria and, sometimes, Alonarectangula, and another dominated by Chydorussphaericus, Alona quadrangularis, Alona affinis andsometimes Leydigia species. According to theecological preferences of these species, the dominanceof the first chydorid assemblage can be related toepisodes of higher temperature and with very low andfluctuating water level, or with a high water levelbut a very reduced extent of shallow water in thelake. Prevalence of the second group corresponds toepisodes with a colder environment and permanentwaters, the level of which could be variable but insuch a way that a benthic sublittoral zone maydevelop. The structure of the community is stronglyaffected by changes in lake level. High indices offluctuation in community structure and small mean bodysize of the chydorid assemblage marked the period withmore extreme dry conditions, characterised by Alona rectangula maxima. The peaks of C. sphaericuscoincided with peaks of the index offluctuation D₀ and prevalence of smaller speciesof the chydorid assemblage.     

Seasonal dynamics and composition of cladoceran and copepod assemblages in ponds of a Hungarian cutaway peatland

Seasonal dynamics of cladoceran and copepod zooplankton were studied over a one‐year period in five permanent ponds of a cutaway peatland, situated in the Danube−Tisza Interfluve, Central Hungary. 17 cladoceran, 11 cyclopoid copepod and 6 harpacticoid copepod species were identified and most of them were typical of small lowland ponds. Nevertheless, some taxa like Cyclops insignis, Ceriodaphnia setosa and Macrocyclops distinctus are considered to be rare in Hungary. The microcrustacean assemblages exhibited apparent seasonal succession with typical seasonal species. There appears to be at least two main successional patterns in the five ponds. After general cyclopoid copepod dominance in winter (Cyclops strenuus and Cyclops insignis), at sites with higher proportion of open water and submerged vegetation, spring was characterized by the dominance of the large cladoceran Daphnia curvirostris, which declined during summer, when microcrustacean assemblages composed mainly of smaller, littoral cladocerans. At these sites, species richness and diversity reached their maximum in autumn. In the case of duckweed covered ponds, succession led to less diverse autumn assemblages with fewer species, dominated by Simocephalus exspinosus. Our results draw the attention to the importance of long‐term investigations and the often neglected winter sampling in the accurate evaluation of species richness (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Long-term change of the littoral Cladocera in the Tatra Mountain lakes through a major acidification event

In our study, we focused on littoral Cladocera living and feeding in shallow shore parts of 46 mountain lakes in the Tatra Mountains (Slovakia and Poland). The studied lakes underwent a major acidification event in the 1980s and are now in the process of recovery. Lakes were divided into three categories based on their sensitivity to acidification: 5 extremely sensitive (ES), 11 acid sensitive (AS), and 30 non-sensitive (NS) lakes. In our study, we included historical data from the literature, and data from sediment core and littoral samples, which together represent the evolution of the littoral communities from a pre-industrial period up to the present. In total, 11 littoral species were found belonging to three cladoceran families. Most of the species were members of the family Chydoridae: Alona affinis, A. quadrangularis, A. rectangula, A. guttata, Acroperus harpae, Alonella excisa, A. nana, Chydorus sphaericus, and Eurycercus lamellatus. One species belonged to each family Daphniidae (Ceriodaphnia quadrangula) and Polyphemidae (Polyphemus pediculus). The most numerous littoral taxa were Alona affinis, Acroperus harpae, and Chydorus sphaericus. All species reacted to decreased pH levels during peak acidification in the 1980s by disappearing from most of the lakes of all categories; the only persisting species was Chydorus sphaericus. Most species returned to the lakes when pH started to increase in the 1990s, although their return was noticeably slower in AS lakes. Alona quadrangularis decreased its distribution range over the studied period; Polyphemus pediculus was mostly detected in the 1910s only. The number of species was highest in all lake categories when dwarf pine was present in the lake catchment. On the whole, the littoral community was richest in NS lakes.     

The Chydorid Crustacean Alona diaphana (King), Limnetic in Lakes Awasa and Zwai,Ethiopia, with a Discussion of Littoral and Limnetic Zooplankton

Alona diaphana, a common littoral tropicopolitan cladoceran, occurs in the limnetic of two Ethiopian lakes in considerable numbers and over long periods of time. Our results are presented and the composition of limnetic and littoral zooplankton, in terms of previous records of such incursions, are discussed. The high persistent densities of A. diaphana are due to the extensive littoral areas in the lakes, where it originates, the high intrinsic rates of increase of this species, the low diversity of zooplankton, reducing competition and the low level of predation.     

Ecological studies of Cladocera in the ricefields of Tanjung Karang,Malaysia, subjected to pesticide treatment

Temporal changes in the population densities of four dominant Cladocera (Moina micrura, Diaphanosoma excisum, Alona guttata and Macrothrix spinosa) were studied in untreated and pesticide treated rice plots over a growing season. M. micrura was the first dominant species to occur in all the plots which were initially devoid of vegetation except for the newly transplanted rice seedlings. With the growth of the rice seedlings and the appearance of aquatic macrophytes, open water ricefields were transformed into vegetated littoral conditions precipitating the disappearance of M. micrura and the appearance of the littoral A. guttata and M. spinosa, and the eurytopic D. excisum. The cladoceran community was affected by spraying of the pesticide FMC35001, an analogue of Furadan®. The response of the four dominant species to the pesticide treatment is discussed.     

Modern and pre-Industrial Age distributions of Cladocera in Italian and Swiss Alpine lakes

We compared contemporary and pre-Industrial Age (before 1850 AD) fossil assemblages of Cladocera in sediment cores from 25 lakes in the Italian and Swiss Alps to investigate the impact of mounting anthropogenic stresses over the last 150 years on community composition. In addition, we sought relationships between specific species and their environments by measuring nutrients, major ions, pH, alkalinity, conductivity, chlorophyll, and lake and catchment morphological features at the time of core collection for comparison with surface sediment fossil assemblages. The modern (surface sediment) communities of the study lakes consisted mainly of benthic chydorids, primarily Alona affinis, A. quadrangularis, Acroperus harpae, and Chydorus sphaericus-type, with Daphnia as the sole planktonic genus. Principal component analysis (PCA), relating the modern Cladocera assemblages to environmental variables at the time of sampling, indicated that A. affinis and A. quadrangularis are influenced by altitude and dissolved inorganic nitrogen (PCA axis 1) whereas Acroperus harpae and C. sphaericus-type are more influenced by potassium (PCA axis 2). Redundancy analysis, however, identified lake water pH and potassium as the measured variables most impacting modern cladoceran assemblages, and especially the distribution of C. sphaericus-type and A. harpae. Pre-Industrial Age samples contained more Daphnia (longispina-type primarily) than modern samples, and some harbored Eurycercus lamellatus, which has since been extirpated from these lakes. The directional shifts in Cladocera assemblages from the pre-industrial period to the present, illustrated as changes in the sample scores along the PCA axes 1 and 2, were associated with the secondary PCA gradient in almost all lakes, and thus were probably the indirect responses to a cascade of limnological alterations, perhaps initiated by large anthropogenic forcing factors such as atmospheric pollution or fish introductions.     

Contrasts between dystrophic and clearwater lakes in the long‐term effects of acidification on cladoceran assemblages

1. Most studies on zooplankton responses to acidification have focused on clearwater lakes with a dramatic acidification history. The role of dissolved organic carbon (DOC) in moderating zooplankton responses to acidification in naturally acidic, dystrophic lakes is less well understood and is partially impeded by a lack of baseline data. 2. Cladocera leave identifiable remains preserved in lake sediments that can be used to provide information on pre‐industrial species assemblages and their responses to environmental stressors such as acidification. Therefore, we used palaeolimnological approaches to track cladoceran assemblage responses to acidification since c.1850 (inferred from sedimentary diatom assemblages) in three acidified lakes in Kejimkujik National Park (Nova Scotia, Canada) that differ markedly in DOC content. These include two highly dystrophic lakes (Kejimkujik and Pebbleogittch lakes), and one clearwater lake (Beaverskin Lake). 3. In dystrophic Pebbleogittch Lake, an increase in the acid‐tolerant, jelly‐clad, pelagic taxon Holopedium glacialis occurred coincident with diatom‐inferred pH (DI‐pH) declines, but no other notable cladoceran assemblage shifts occurred. Similarly, Cladocera assemblages did not appear to respond to lakewater acidification in dystrophic Kejimkujik Lake. 4. In contrast, in the clearwater Beaverskin Lake, several observed shifts in cladoceran assemblage corresponded to DI‐pH declines, including an increase in the proportion of littoral taxa and an increase in Hill’s N2 species diversity. This may indicate increased water clarity as a result of acidification‐related decreases in DOC, which may have enhanced growth of emergent aquatic macrophytes and improved visibility for planktivorous fish, leading to increased predation on pelagic taxa. Species shifts within the littoral assemblage of Beaverskin Lake may reflect the differing tolerances of littoral taxa to low pH and aluminium toxicity. 5. Overall, our results suggest that cladoceran assemblages in naturally acidic, dystrophic lakes may be resilient against additional pH declines related to industrial emissions of acidifying agents, as dystrophic lakes are less vulnerable to increased aluminium toxicity and acidification‐induced increases in water clarity and often have a pre‐industrial cladoceran assemblage already adapted to acidic conditions.     

Are cladoceran diversity and community structure linked to spatial heterogeneity in urban landscapes and pond environments?

Biodiversity patterns in cladoceran communities were investigated in urban waterbodies in relation with residential land use, pond management, and waterbody environments. We evaluated species richness in the pelagic and littoral zones of eighteen waterbodies of a large Canadian city. Gamma diversity (26 species) observed at a small scale in the urban survey was important comparatively to large-scale surveys of lakes. Beta diversity ranged from 1 to 8 species among waterbodies. We tested if littoral species greatly contributed to regional diversity in urban waterbodies. Littoral species (Chydoridae, Ilyocryptidae, Macrothricidae, Polyphemidae) accounted for 58% of the total species pool. We distinguished five cladoceran assemblages associated to different waterbodies (temporary ponds, permanent lakes, and wetlands). Cladoceran communities were more diverse and variable in permanent lakes than in temporary ponds. Changes in cladoceran species assemblages among waterbodies were driven by variations in waterbody size and phosphorus enrichment, macrophyte and algal biomass, urban density, pond management practices, and the presence of potential predators as fish and macroinvertebrates. Our study indicates that both artificial ponds and lakes and natural wetlands are valuable habitats for the conservation of cladoceran biodiversity and rare endemic species in urban regions. Further research on pond management strategies promoting urban aquatic biodiversity should be undertaken.     

A LIMNOLOGICAL PROFILE OF THE UPPER OKAVANGO DELTA AT LOW WATER LEVEL

Summary

Selected limnological attributes of the Okavango Delta panhandle were measured during a brief summer survey of “open-water” habitats extending from the permanent mainstream channel, through contiguous off-channel lagoons and still backwaters, to seasonally isolated floodplain lagoon and temporary pool biotopes on the left-bank of the Okavango River at Seronga.

Wide ranges in most determinants were evident along this profile:- temperature (27–34°C); conductivity(4–12.7mS m^?1); pH(5.7–9.2); transparency(0.2–>2.5 m Secchi depth); dissolved oxygen (20–220% saturation); PO₄-P (61–110 μg ?^?1): SiO₂—Si (6.9–14.0 mg ?^?1): NH₄-N (30–44 μg ?^?1): chlorophyll (1.3–183 μ.g ?^?1). Zooplankton was variably diverse (species richness from ≥ 3 to 20), comprising both euplanktonic (Bosmina, Ceriodaphnia, Daphnia, Diaphanosoma, Moina, Mesocyclops, Thermocyclops, Tropodiaptomus) and more typical epiphytic crustacean taxa (Alona, Macrothrix, Pleuroxus), along with various rotifers (Brachionus, Hexarthra, Keratella, Trichocera) and other taxa (Arcella, Ostracoda. Chaoborus). Abundance varied widely between habitats. The littoral macrozoobenthos showed surprisingly low diversity, and was dominated by freshwater shrimps (Caridina).

Substantial allochthonous inputs to the Okavango swamps were evident from the significant concentrations of total suspended solids (7.6–12.6 mg ?^?1 , organic content of 33–41%) carried by the mainstream Okavango River during the survey.     

Contributions to the Cladocera fauna from Papua New Guinea

Twenty-eight taxa of the Cladocera are identified in collections from Papua New Guinea, 17 being new records for New Guinea, bringing the total number of Cladocera taxa reported for this region to 39. Most of the taxa are circumtropical. One species (Sarsilatona papuana) is endemic to Papua New Guinea and northern Australia. The species list includes two species that are normally listed as Holarctic:Alonella nana andAlona rustica. Widespread genera such asDaphnia, Pleuroxus, Disparalona, Acroperus were strikingly absent from the Papua New Guinean material.     

Effect of prey type and inorganic turbidity on littoral predator–prey interactions in a shallow lake: an experimental approach

Predation often represents the prevailing process shaping aquatic ecosystems. As foraging and antipredatory behaviour frequently relate to vision, turbidity may often impair the interactions between the predator and its prey, depending on prey type and source and level of turbidity. We studied the effect of inorganic turbidity (0–30 NTU) on the effectiveness of fish feeding on two types of prey in different habitats: free-swimming cladoceran (Daphnia pulex) in open water and plant-associated cladoceran (Sida crystallina) attached to Nuphar lutea leaves. For the planktivore, we used vision-oriented perch (Perca fluviatilis) common in the littoral zone of temperate lakes. In our study, increasing inorganic turbidity did not appear to initiate any significant change in the feeding efficiency of perch on free-swimming Daphnia pulex. However, we saw a markedly different feeding efficiency when perch targeted plant-attached Sida crystallina. Our results substantiate that floating-leaved macrophytes in turbid lakes may provide a favourable habitat for plant-attached cladocerans.     

A laboratory study on the demography and competition of three species of littoral cladocerans from Lake Huetzalin,Xochimilco, Mexico

Lake Huetzalin (Xochimilco) is a shallow waterbody in Mexico City with a dominance of the littoral cladocerans Simocephalus vetulus, Alona rectangula, and Pleuroxus aduncus. Here, we present data on the life table and population dynamics of these three cladoceran species on two types of diets (alga Scenedesmus acutus and detritus). We also analyzed the effect of competition on population growth using Scenedesmus, which proved to be the better diet than detritus. We hypothesize that feces of the larger cladoceran species (S. vetulus) could be a food source for the smaller taxa (A. rectangula and P. aduncus). Experiments were conducted in 100 ml beakers with 50 ml of medium in each at a temperature of 23 ± 1°C. We studied the demography of three test species using two food types with four replicates for each treatment in the life table experiments. Based on the data thus obtained we further studied the competition between the cladocerans on a diet of Scenedesmus. The maximal growth rates obtained for P. aduncus, S. vetulus, and A. rectangula fed Scenedesmus acutus were: 0.1, 0.3, and 0.5 d⁻¹, respectively. Among the test species, S. vetulus had the highest survivorship of 77 days when cultured on detritus and P. aduncus the lowest of 20 days on Scenedesmus. The gross reproductive rate (offspring female⁻¹ lifespan⁻¹) ranged from 6 neonates in Pleuroxus to 160 in Simocephalus per female. From the competition experiments conducted using alga as the diet, we observed that A. rectangula and, especially P. aduncus, had higher growth rates in the presence of S. vetulus than in the controls. This facilitation could be one of the reasons for higher species diversity in the littoral of water bodies.     

Responses of tropical cladocerans to a gradient of resource quality

1. The response of three tropical cladocerans to a gradient of resource quality was compared in a series of growth bioassays using seston collected from five lakes of different depth and trophic structure in Michigan, U.S.A. To assess the food quality in terms of digestibility, assimilation experiments were performed with ³²P‐labelled seston from the same lakes. Animals were also analysed for P‐content in their tissues at the end of these assays. 2. In general, assimilation efficiency was higher when animals fed on seston from shallow compared to deep lakes, and was significantly correlated with growth rates, suggesting that shallow lakes have the best food resources in terms of digestibility and P availability. 3. Results also showed that all cladoceran species responded similarly to the resource gradient, with lower growth rates in deep lakes and higher growth rates in shallow lakes, although the strength of response (sensitivity) was different among the species tested. 4. The cladoceran Moina micrura was the most sensitive species, and also displayed the highest P‐content and maximal growth rate, a pattern consistent with the growth rate hypothesis. 5. However, seston C : P ratio and growth rates in the different resources did not correlate with the animals’ P‐contents, showing an uncoupling between RNA‐phosphorus demands for growth and seston food quality. 6. In conclusion, our results support the idea that digestion resistance in algae is a major constraint to cladocerans in natural plankton communities.     

Climate change and Saharan dust drive recent cladoceran and primary production changes in remote alpine lakes of Sierra Nevada,Spain

Recent anthropogenic climate change and the exponential increase over the past few decades of Saharan dust deposition, containing ecologically important inputs of phosphorus (P) and calcium (Ca), are potentially affecting remote aquatic ecosystems. In this study, we examine changes in cladoceran assemblage composition and chlorophyll‐a concentrations over the past ~150 years from high‐resolution, well‐dated sediment cores retrieved from six remote high mountain lakes in the Sierra Nevada Mountains of Southern Spain, a region affected by Saharan dust deposition. In each lake, marked shifts in cladoceran assemblages and chlorophyll‐a concentrations in recent decades indicate a regional‐scale response to climate and Saharan dust deposition. Chlorophyll‐a concentrations have increased since the 1970s, consistent with a response to rising air temperatures and the intensification of atmospheric deposition of Saharan P. Similar shifts in cladoceran taxa across lakes began over a century ago, but have intensified over the past ~50 years, concurrent with trends in regional air temperature, precipitation, and increased Saharan dust deposition. An abrupt increase in the relative abundance of the benthic cladoceran Alona quadrangularis at the expense of Chydorus sphaericus, and a significant increase in Daphnia pulex gr. was a common trend in these softwater lakes. Differences in the magnitude and timing of these changes are likely due to catchment and lake‐specific differences. In contrast with other alpine lakes that are often affected by acid deposition, atmospheric Ca deposition appears to be a significant explanatory factor, among others, for the changes in the lake biota of Sierra Nevada that has not been previously considered. The effects observed in Sierra Nevada are likely occurring in other Mediterranean lake districts, especially in softwater, oligotrophic lakes. The predicted increases in global temperature and Saharan dust deposition in the future will further impact the ecological condition of these ecosystems.     

Cladocerans respond to differences in trophic state in deeper nutrient poor lakes from Southern Norway

We investigated the role of trophic state in structuring the cladoceran assemblages in 35 relatively deep lakes of low nutrient levels in Southern Norway. The lakes cover gradients of altitude, latitude, longitude, area and total phosphorus concentration. The environmental control of the cladoceran assemblages was analyzed using canonical correspondence analysis. Total phosphorus was the most important variable in the minimal adequate model. Hence, the cladoceran assemblage changed with trophic state. Typical clear water species were displaced by indicators of eutrofication with increasing trophic levels. Littoral species constituted the majority of the species recorded. Littoral and pelagic species richness showed a unimodal relationship with trophic state. Along with the change in trophic state and shift in cladoceran assemblage, we observed an increase in the ratio of pelagic to littoral species as well as in the ratio of pelagic efficient bacterial feeders to total pelagic filter feeders. The study indicates that zooplankton could provide a valuable indicator of ecosystem’s structure and function in deeper nutrient poor lakes if included in water quality assessments according to the EU Water Framework Directive. Including sampling of the littoral zooplankton would improve diversity estimates and incorporate the response of the littoral zone to eutrophication.