Migrations of haemoglobin-rich Daphnia longispina in a small,steeply stratified,humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.     

Vertical distributions of bacteria and algae in a steeply stratified humic lake under high grazing pressure from Daphnia longispina

The vertical distributions of bacteria and algae in a steeply stratified, highly humic lake were studied during three 24 h periods in summer. The highest bacterial and algal densities and biomasses were recorded in the anoxic hypolimnion. The bacterial biomass in the hypolimnion was composed mainly of photosynthetic green sulphur bacteria (Chlorobium) which occurred at very low light intensity (< 1.5 μmol m⁻² s⁻¹). The numbers and biomasses of bacteria, both in the epilimnion and hypolimnion, were mostly higher at night than during the day, indicating possible asynchrony between the production and loss of bacteria. Because of vertical migration, the diurnal vertical distribution of algae was more variable than that of bacteria. Particularly in July and August, when cryptomonads were abundant, the biomass of algae was much higher in the epilimnion during the day than at night. The flagellated chlorophytes, Chlamydomonas spp. and Scourfieldia cordiformis, stayed mainly in the upper hypolimnion close to the oxic-anoxic boundary zone where only a small proportion of Daphnia longispina was continuously present. Unpalatable Mallomonas chrysophytes with silicified plates and bristles, and small, presumably heterotrophic, flagellates stayed in the oxic epilimnion together with a dense (up to 300 ind l⁻¹) population of D. longispina. The results indicated that, besides the physical and chemical properties of the water column, grazing pressure by Daphnia longispina strongly affected the vertical distribution of microorganisms in this polyhumic lake.     

Planktonic food chains of a highly humic lake

The development and metabolism of the plankton of a highly humic lake were followed over the vernal primary production maximum. The study was made in a mesocosm in which large filter feeders, typical of this lake in summer, were absent. During the rising phase of phytoplankton, the community was predominantly autotrophic. The most important constituents in the algal biomass were a dinoflagellate, Gymnodinium sp. (40–50%), and a prasinophycean, Scourfieldia cordiformis (7%). The biomasses of Chlamydomonas spp. and Chrysococcus spp. reached their maxima a few days later and Cryptomonas sp. became most abundant at the end of the experiment. After the phytoplankton maximum, about one week from the beginning ofthe experiment, grazing of algae by phagotrophic protozoans and phosphate depletion led to a rapid decrease of algal biomass and the community became predominantly heterotrophic. In spite of a large variation in algal biomass and primary production, the biomass of bacteria remained of the same order of magnitude as in algae both before and after the algal maximum. Bacteria were mostly responsible for the plankton respiration, which also showed no dependence on primary production. Since exudation by phytoplankton was also low, the nutrition of bacterioplankton was probably mainly based on allochthonous dissolved organic matter rather than or primary production. Thus the production of bacteria was an additional food source for higher trophic levels along with phytoplankton. Because filter feeding zooplankton was absent in the experiment, protozoans were the only grazers utilizing algae and bacteria. Essentially all growth of bacteria was used by bacterivores.     

Planktonic food chains of a highly humic lake

The development and metabolism of epilimnetic plankton from a highly humic lake was followed in late summer, when the predominant zooplankton species, Daphnia longispina, was very abundant (ca. 200 ind. l^?1). The experiment was made in two tanks: one with an unaltered plankton assemblage and one with larger zooplankton removed. The scarce phytoplankton community was also simple, consisting mainly of one Cryptomonas and two Mallomonas species. The abundance and species composition of smaller plankton was heavily influenced by grazing of Daphnia. In particular, the biomass, of heterotrophic flagellates increased after the removal of Daphnia. The biomass and production of bacterioplankton were not affected, and remained several times higher than that of phytoplankton. Bacterial production and grazing on bacteria were balanced, and when Daphnia was removed its grazing activity was compensated by flagellates. The removal of Daphnia did not affect the respiration or community net production of plankton. Among organisms smaller than zooplankton, bacteria seemed to be responsible for most of the respiration. The community net production was consistently negative even at the water surface, indicating an allochthonous carbon source. The results suggest that phytoplankton primary production was insufficient for the secondary production in the epilimnetic water of the study lake. The food requirements of bacteria and zooplankton, as well as of flagellates, each exceeded that supplied by phytoplankton primary production. The simple food chains in this experiment made it possible to reveal the functioning of the community so completely that dissolved organic matter is certainly comparable to or exceeds the importance of phytoplankton primary production as an energy and carbon source for food webs in this humic lake.     

Factors controlling production of phytoplankton and bacteria under ice in a humic, boreal lake

The factors controlling pelagic primary and bacterial productionof a humic, boreal lake in winter were investigated, combininglaboratory and field experiments where some of the predictedconsequences of the climate change, i.e.the increased load ofphosphate-phosphorus and dissolved organic matter (DOM), weresimulated. In situ incubations were performed in eight acrylictubes lowered underneath the ice cover for 1–4 months.In the lake, production of phytoplankton (0.03–0.33 µgC l^–1 day^–1) was lower than that of bacteria (0.2–2.3µg C l^–1 day^–1) from the end of January tomid-April. Later in spring, the light conditions improved dueto the disappearance of snow and finally ice itself, and primaryproduction was revived. The importance of light as the factorcontrolling primary production in winter was confirmed in laboratoryexperiments where additions of phosphorus and DOM did not enhancethe primary production. These same enrichments resulted in highergrowth rates and production of bacteria. However, bacterioplanktonwas simultaneously controlled by heterotrophic flagellates,as in the laboratory experiments the exclusion of flagellatesalways resulted in higher yields of bacteria of increased cellsize. There was also some evidence that the quality of substratesis of importance for microbial activities. The importance oftemperature was highlighted with the significant correlation(r² = 0.59) between bacterial production and temperature withinthe range 0.6–2.1C. Thus, on the boreal zone the possiblewarming of climate may lead to enhanced activities in winter,but under unchanged light conditions no effects on primary productionwill be expected.     

The relative importance of algae and bacteria as food for Daphnia longispina (Cladocera) in a polyhumic lake

SUMMARY. 1. Grazing on algae and bacteria by the planktonic cladoceran, Daphnia longispina , was studied in a small polyhumic lake with low phytoplankton primary production in southern Finland.
2. D. longispina filtered algae at average rates of 0.09-0.82 ml ind⁻¹ h⁻¹. The filtering rates on bacteria were 26-92% of those on algae in parallel experiments.
3. From June to August algae, including mixotrophic and heterotrophic forms, comprised 56-93% of the food ingested by D. longispina. In mid-September and early October, when the Daphnia population was declining and the algal biomass was low, bacteria comprised 73% and 55% respectively of the food of Daphnia.
4. For D. longispina , an energy pathway via bacteria and bactivorous flagellates is probably a more important link to allochthonous organic matter than direct utilization of epilimnetic bacteria.     

Growth of Daphnia longispina L. in a polyhumic lake under various availabilities of algal,bacterial and detrital food

The availability and importance of food sources for growth of Daphnia longispina L. from a highly coloured fishless lake with anoxic hypolimnion were assessed by combining in situ and laboratory experiments. In in situ experiments populations were enclosed in tubes with natural temperature stratification and with or without anoxic hypolimnion. In the laboratory experiments the importance of food source (littoral zone vs pelagic epilimnion) was assessed by enclosing moss thalli and a natural zooplankton population in a large-scale flow-through system supplying food for experimental Daphnia. Growth of juveniles of Daphnia in epilimnetic water was determined in batch culture experiments and the importance of increasing concentrations of bacteria and algae for their growth and development was investigated with a small-scale flow-through system. Access to the anoxic hypolimnion enhanced the growth of Daphnia by 23–24%. Growth rates in the tubes with anoxic hypolimnion were 0.36 and 0.16 d^–1 in July and August respectively. In tubes without anoxia the corresponding values were 0.29 and 0.13. In batch-cultures the highest growth rate determined was 0.16 and the overall rates were lower than in in situ experiments. In batch culture Daphnia was able to grow in darkness for 10 days with a rate of 0.16. In the large-scale flow-through system Daphnia population fed with littoral water reproduced well despite the low concentration of algae and increased its number by a factor of c. 32 in 10 days. However, the animals were small and net production of Daphnia population thus lower under the littoral influence than in the control treatment. Population could survive and grew slowly on pelagial water processed by a natural zooplankton community and with very little algae left. It is thus possible that bacteria serve as a life-support system enabling the population survival over periods of algal shortage. Small-scale flow-through experiments revealed that Daphnia longispina is able to mature and reproduce on a bacterial diet if the food concentration is high enough and Daphnia on bacterial food could achieve growth rates similar to those on an algal diet. The threshold food level for Daphnia longispina was estimated to be c. 18.5 g C 1^–1. Detrital material is of limited value in nutrition of Daphnia even in a lake where more than 75% of carbon is bound in particulate detritus.     

Relationships between heterotrophic nanoflagellates and the demographic response of Daphnia longispina in a eutrophic lake with poor food quality conditions

1. Based on both field data and laboratory studies, the summer population of Daphnia longispina living in a stratified eutrophic lake was examined in relation to the abundance of algae, nanoflagellates and picocyanobacteria.
2. In early July, the Daphnia population replaced Bosmina and remained the dominant zooplankter during summer 1994. Its development in July was concomitant with an increase of edible algae but, despite the apparent abundance in available food, the Daphnia population decreased throughout August suggesting poor food conditions.
3. From mid-August to the beginning of September, the biomass of inedible phytoplankton was greater than that of the smaller, edible fraction. Consequently the average rate of increase, birth rates and fecundity of Daphnia remained low. Although the biomass of heterotrophic nanoflagellates was consistently low, the demographic parameters of Daphnia were correlated throughout this period with these protozoans.
4. Life table experiments run in the laboratory showed that epilimnetic food supported both the growth in length of individual Daphnia and an increase in fecundity, but metalimnetic food supported only individual growth. D. longispina probably failed to reproduce because of the abundance of detritus mixed with the heterotrophic nanoflagellates in the metalimnetic water at that period of the year. The vertical migration of Daphnia into these deeper layers could be caused by a predator avoidance mechanism.     

Epizooic ciliates (Vorticella sp.) compete for food with their host Daphnia longispina in a small polyhumic lake

Summary Clearance rates of epizooic ciliates (Vorticella sp.) were measured together with their host, a planktonic cladoceran Daphnia longispina by using fluorescent latex beads as tracers of food. Vorticellans and their host graze on food of same size range (nanoplanktonic algae and bacteria). Individual clearance rates of Vorticella averaged 6.9 and 7.0 l ind^-1 h^-1 and those of Daphnia 463 and 708 l ind^-1 h^-1 for beads with diameter 2.00 and 3.92 m. On the average, epizooic vorticellans together on the carapace of Daphnia cleared particles with rates representing 25–33% of that the host cleared, the maximum rates being 50–80%. In a steeply stratified polyhumic lake vorticellans take advantage of following Daphnia to food patches and they can severely compete for food with their host.     

Size distribution of Daphnia longispina in the vertical profile

D. longispina of the meromictic lake El Tobar is a round-headed form. It never has a helmet, but in summer a small proportion of immature individuals (0.9–1.2 mm females and males) have one or two neck teeth. The size structure of this Daphnia population, as well as the vertical distribution and migration of different size-classes, were studied in September and November of 1991 and April of 1992. The large variation in mean size and size at first reproduction, as well as the occurrence of different patterns of vertical migration are interpreted as responses to different predator situations. At the end of April, when Daphnia mortality by visually hunting predators is dominating, a typical nocturnal migration is adopted and size distribution is biased to smaller size classes. In November, when mortality is mainly attributed to the nocturnally migrating Chaoborus, Daphnia shows a reversed migration pattern. In September, when the population of Daphnia is responding to both visual (fish) and non-visual predators (Chaoborus), it adopts a pattern of twilight migration. The presence of neck teeth in vulnerable size classes in September might be an additional adaptative response to Chaoborus predation. In September, the size structure of the Daphnia population is shifted to larger classes and the vertical distribution of size classes shows a pronounced segregation between juveniles and adults. Juveniles are found closer to the surface, while adults dwell predominantly in the rich, deep waters near the oxicline. This suggests that an additional advantage of the ascent of the adult Daphnia exploiting those deep resources is the release of young in more oxygenated and warmer waters. The Daphnia population of lake El Tobar is known to be clonally diverse, and the changing frequency of genotypes could play an important part in the observed seasonal differences in behaviour and size.     

Large Hardy-Weinberg equilibrium deviations in the Daphnia longispina of Lake El Tobar

The population structure of Daphnia longispina in Lake El Tobar, Spain was studied by measuring variation at the aldehyde oxidase (AO), phosphoglucose isomerase (PGI) and phosphoglucose mutase (PGM) loci in each of 1337 individuals from four collections. In 9 of the 12 comparisons between observed allele frequencies and those expected by Hardy-Weinberg equilibrium there was an excess of heterozygotes. We found 27 of the potential number of 54 composite electromorphs (clones) based on the three allozymes. Clone diversities were rather high in all collections. Three clones reached frequencies of over 25% and different clones were dominant in each of the four collections. Strong temporal variation was found in the genetic structure of this Daphnia population. This variation was driven by changes in the relative frequencies of the component clones in the lake rather than by a recruitment of novel clones into the population.We conclude with a consideration of the role of models relating allele and genotype frequencies in populations of cyclical parthenogens. Because the breeding system of these populations infrequently involves recombination between clones, models such as the Hardy-Weinberg have limited value in providing meaningful measures of population structure.     

Late Holocene changes in the humic state of a boreal lake and their associations with organic matter transport and climate dynamics

An important factor in the ontogeny of boreal lakes is the development of their humic state through terrigenous input of organic matter (carbon) that affects strongly the functioning and structure of these ecosystems. The long-term dynamics and role of humic substances for these systems in relation to climate are not clear. In this study, a boreal lake from southern Finland was investigated using paleolimnological methods, including diatom, chironomid and geochemical analyses, for Late Holocene changes in the humic state. The aim was to examine the relationship between sediment biogeochemistry and climate variation. Consistent trends were found in diatom-inferred total lake-water organic carbon (TOC) and in the ratio of humic/oligohumic chironomids. Sediment geochemistry provided further evidence for the limnological development of the lake and related long-term climate trends in the region. The results indicated three distinct phases with differing humic state; the beginning of the record at ca. 4,500 cal year BP was characterized by extremely humic conditions coinciding with warm and dry climatic conditions, a meso-oligohumic period between ca. 3,000–500 cal year BP with increasing allochthonous organic matter transport and cooler and wetter climate, and recent period with polyhumic (TOC >10 mg L^?1) lake status and warming climate. This study shows that instead of straightforward linear development, boreal lakes evolve through dynamic humic stages related to climate and lake-catchment coupling processes. As the changes in the humic state are ultimately climate-driven, the ongoing climate change probably has a major influence on boreal lakes through direct and indirect effects on organic carbon transport, utilization and accumulation.     

Seasonal succession in the plankton of a naturally acidic, highly humic lake in Northeastern Ohio, USA

A 13 month study of the plankton in a naturally acidic bog lakeshowed high algal biovohune, year-round dominance by flagellatesand mid-summer dominance by Gonyostomum semen. Zooplankton abundanceand species richness was very low. Rotifers were the year-rounddominants.     

Empty loricas and the dynamics of Kellicottia longispina in a subalpine,oligotrophic lake

The seasonal dynamics of Kellicottia longispina in Lake Tahoe and an isolated embayment of Lake Tahoe, Emerald Bay, were investigated for an 18-month period in 1977–79. Population birth and death rates were similar in the two systems, although productivity and Mysis relicta densities were higher in Emerald Bay. The timing of population changes was also similar. A major population increase in late winter 1978 was preceded by an increase in egg ratio; the subsequent spring decline of K. longispina was concurrent with decreased birth rates and increased death rates. Empty loricas of K. longispina were occasionally abundant in the plankton samples and seemed to result from K. longispina deaths when densities were high and when egg ratios were declining; it is possible that population senescence was responsible for the high densities of empty loricas observed. A potentially important predator, M. relicta, defecates K. longispina remains in compact fecal pellets; however, it is unlikely that the observed empty loricas resulted from Mysis-related deaths.     

Tolerance to Copper and to Salinity in Daphnia longispina: Implications within a Climate Change Scenario

Considering IPPC climate change scenarios, it is pertinent to predict situations where coastal ecosystems already impacted with chemical contamination became exposed to an additional stressor under a future scenario of seawater intrusion. Accordingly, the present study aimed at evaluating if a negative association between tolerance to a metal and to saltwater exists among genotypes of a freshwater organism. For this, five clonal lineages of the cladoceran Daphnia longispina O.F. Müller, exhibiting a differential tolerance to lethal levels of copper, were selected. Each clonal lineage was exposed to lethal and sublethal concentrations of sodium chloride (assumed as a protective surrogate to evaluate the toxicity of increased salinity to freshwater organisms). Mortality, time to release the first brood and total number of neonates per female were monitored and the somatic growth rate and intrinsic rate of natural increase were computed for each clonal lineage. Data here obtained were compared with their lethal responses to copper and significant negative correlations were found. These results suggest that genetically eroded populations of D. longispina, due to copper or salinity, may be particularly susceptible to a later exposure to the other contaminant supporting the multiple stressors differential tolerance.     

Natural distribution and co-infection patterns of microsporidia parasites in the Daphnia longispina complex

Microsporidia are intracellular parasites, frequently infecting the planktonic crustacean Daphnia. Questioning the ability to detect and identify microsporidia with conventional microscopic techniques, we applied molecular methods in order to investigate the distribution and co-infection patterns of this parasite among 8 communities of the Daphnia longispina hybrid complex. Eight microsporidian taxa were detected, including 3 that previously had not been characterized genetically. Microsporidian communities from nearby lakes were found to be more similar to each other, apparently due to short distance dispersal via secondary hosts. Moreover, we detected seasonal (but not interannual) changes in microsporidian community structure. With some microsporidia being host-specific, these changes might have resulted from seasonal changes in host taxon and clonal composition. The 2 dominant and closely related parasite species were found mainly in single infections, whereas another pair of related microsporidians was found predominantly in co-infections; suggesting species-level differences in the ability to colonize infected hosts. By applying molecular methods, we were not only able to unambiguously identify parasite taxa but also to reveal multiple infections that otherwise would have remained undetected. Given the increased level of accuracy and sensitivity, we highly recommend molecular approaches in future parasite surveys of Daphnia infections.     

Structural variations of the eye linked to daily vertical migrations of Daphnia longispina

The role of the eyes, and more precisely that of the ommatidian pigments, in the control of daily rhythms of movement of Daphnia longispina is investigated. In the laboratory, under permanent light (LL), the pigmentary modifications observed are globally similar to those observed in situ, except in their timing, around dusk, LL cycles do not coincide with the ascent of daphnids but precede it. This is the expression of an endogenous free-running rhythm. Therefore, in D. longispina, an internal oscillator controls DVMs, which are circadian and not nycthemeral, and, in situ, illumination at dawn suffices to synchronize migrations on dark/light alternation.     

Seasonal dynamics,typhoons and the regulation of lake metabolism in a subtropical humic lake

1. We used high‐frequency in situ dissolved oxygen measurements to investigate the seasonal variability and factors regulating metabolism in a subtropical alpine lake in Taiwan between May 2004 and October 2005, specifically exploring how the typhoon season (from June or July to October) affects lake metabolism. 2. Gross primary production (GPP) and ecosystem respiration (R) both peaked in early summer and mid‐autumn but dropped during the typhoon season and winter. Yuan‐Yang Lake is a net heterotrophic ecosystem (annual mean net ecosystem production ?39.6 μmole O₂ m^?3). 3. Compared to the summer peaks, seasonal averages of GPP and R decreased by approximately 50% and 25%, respectively, during the typhoon season. Ecosystem respiration was more resistant to external disturbances than GPP and showed strong daily variation during typhoon seasons. 4. Changes in the quality and quantity of dissolved organic carbon controlled the temporal dynamics and metabolic regulation. External disturbances (typhoons) caused increased allochthony, increasing DOC and water colour and influencing lake metabolism. 5. Seasonal winter mixing and typhoon‐induced water mixing in summer and autumn play a key role in determining the extent to which the lake is a seasonal carbon sink or source to the atmosphere.     

Ecological genetics of Norwegian Daphnia. II. Distribution of Daphnia longispina genotypes in relation to short-wave radiation and water colour

In the mountain range of South Norway, transparent and melanic members of the Daphnia longispina complex occur. Melanic populations inhabit clearwater lakes and ponds, while transparent populations are found in ponds with slightly humic water. Mixed populations have not been detected. The distribution patterns of the two morphs are related to ambient levels of short-wave radiation, and the light absorptive properties of the inhabited waters. The pigment present is probably melanin, which is deposited in the dorsally directed parts of the carapace, head shield and antennae. Allozyme studies indicate that these distinct morphs are only remotely related, the alpine transparent group being closer to lowland, likewise transparent, populations. A melanic population found at Svalbard is closely related to the melanic mainland populations. Clonal diversity in ponds and shallow lakes is very low, in contrast to populations of deeper lakes.Deceased.     

Phenotypic plasticity and developmental noise in hybrid and parental clones of Daphnia longispina complex

Aquatic Ecology - According to the “temporal hybrid superiority hypothesis”, seasonal variability in environmental factors in temperate lakes gives hybrid clones within the D....