• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 53
  • 32
  • 7
  • 4
  • 3
  • 2
  • 2
  • 2
  • 2
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 125
  • 125
  • 40
  • 35
  • 14
  • 9
  • 9
  • 9
  • 9
  • 8
  • 8
  • 7
  • 7
  • 7
  • 6
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Sairaalahoito astmaatikon selviytymisen kuvaajana:rekisteritutkimus sairaalahoidossa olleiden astmaatikkojen ikä- ja sukupuolijakaumasta, uusien hoitojaksojen määrästä, hoitojaksojen vuodenaikavaihtelusta lapsilla, varusmiehillä ja aikuisilla sekä kuolleisuuden vuodenaikavaihtelusta

Harju, T. (Terttu) 25 March 1999 (has links)
Abstract This thesis discusses the age and sex distribution of previously hospitalised asthmatics and the number of new asthma-induced treatment periods as a proportion of all asthma-related hospital admissions. Seasonal fluctuations in treatment periods were analysed with respect to age, and seasonal fluctuations in mortality among previously hospitalised asthmatics by combining the hospital discharge register with the mortality statistics on the basis of the patients social security numbers. A total of 364,871 asthma-induced treatment periods were reported to the hospital discharge register in 1972-1992, of which 192,195 occurred in 1.1.1983-31.12.1992, and 28.6% of these were new cases. Treatment periods relative to population amounted to 2.76/1000 in 1972 and 3.43/1000 in 1992. The number of treatment periods among children was greatest at the age of 1 year, 11.2/1000 for boys and 5.5/1000 for girls, and lowest at 17 years among the boys, and 13 years among the girls. The numbers rose slowly in early adulthood (with the exception of men aged 18-22 years), reaching a new peak at 73 years of age for men, 10.3/1000, and 75 years for women, 9.5/1000, and decreasing gradually thereafter. A half of the treatment periods recorded for patients aged under 1 year or for men aged 20 years represented new cases, whereas otherwise the vast majority of the treatment periods were being readmissions. The seasonal peaks among the total of 59,624 hospital periods involving persons aged under 15 years reported to the discharge register in 1972-1992 occurred in May and October, and the lowest figures in January and July. Conscripts aged 18-22 years experienced 4894 asthma-induced treatment periods in 1982-1992, representing incidence rates of 8.5/1000 in 1982 and 27.7/1000 in 1992. The figure was highest in the month in which the conscripts entered service and the month following that. A total of 81,243 asthma-induced treatment periods were recorded for persons aged over 24 years in 1987-1992, the figures being highest in January-May, 18.2% above the monthly trend in January, and lowest in July, 26.1% below the monthly trend. 7622 of the asthmatics first admitted in 1977-1992 died at an age of over 24 years in 1987-1993, mortality being highest in December and January and lowest in August. Obstructive pulmonary diseases were the primary cause of death for 1283 persons, including 489 who died of asthma itself. Apart from children and men aged 18-22 years, asthma gives rise to a large number of treatment periods among the middle-aged and the elderly. Measures should be taken to diagnose it at an early stage in these groups and to intensify the treatment provided. In addition, the factors provoking asthma during military service should be minimised and preventive measures enhanced in the case of children in May and September. As far as adults are concerned, asthma-related hospitalisation may involve a greater fatality risk in winter, so that aggravations occurring in winter should be treated with particular care.
22

Variation of the Symbiodinium Community Composition in Scleractinian Corals along a Cross-shelf and Depth Gradient

Mejia Restrepo, Alejandro 12 1900 (has links)
Corals form a symbiotic relationship with photosynthetic zooxanthellae from the genus Symbiodinium; the breakdown of this symbiosis results in the phenomenon known as coral bleaching. This relationship is especially vulnerable to high temperature stress, although corals may survive if they have resistant types of symbionts, or switch their community composition towards them. To assess the variation of the symbiont community in different environmental conditions, I recorded the temperature and collected samples from six scleractinian coral species and one calcifying hydrozoan, in two inshore, two mid-shelf, and two offshore reefs at 1, 15, and 30m depth, analyzing Symbiodinium diversity using Next Generation Sequencing with the SymPortal profile typing approach. The temperature was very similar for all points in winter, when coral samples were collected, but variation between points increased until a maximum at summer, with the shallower parts of the inshore reefs showing higher temperatures and the points at 30m depth showing the lowest. The Symbiodinium composition was more similar between samples of the same host species than among samples of the same reefs or depths. Coral species from the Pocilloporidae family and Millepora dichotoma showed specific association with different profile types, specifically, intragenomic variants of Symbiodinium type A1, which appears to be dominant in the Red Sea although it has not been reported for these species in other regions. The other species showed specific associations with types previously reported in other regions, mostly from clade C and D, although also having different types and intragenomic variants. For most cases, certain profile types, which can reflect different species or populations, appeared to be dominant in particular environmental conditions, following a distribution related with depth, reef type, or both. In conclusion, this study showed that the Symbiodinium composition depends more on the host species than on the environmental conditions, and within each species the adaptation to environmental gradients can rely on tolerant symbiont species or populations characteristic of the Red Sea, or association with different types and clades that are common also in other regions.
23

Sezonní proměnlivost v teritoriální aktivitě bobra evorpského (Castor fiber L.) / Seasonal variation of the territorial behaviour of the European beaver (Castor fiber L.).

Kadlecová, Hana January 2010 (has links)
The presented thesis deals with the temporal and spatial changes in activity of the European beaver (Castor fiber L.) during different seasons. Basic dataset was obtained by continuous telemetric monitoring of selected individuals in years 2008 and 2009 in two different geographical areas, namely in the Bohemian forest and in southern Moravia. The scope of the thesis was to record, evaluate and interpret the behavior of European beaver during the year. I focused myself especially on the beginning and end of activity in relation to incidence of sunrise and sunset, duration of stay in the field, changes in the size of actively-used area and time-duration and length of the movement. In cases where there were more active individuals monitored within the studied area, I also watched for their mutual interaction. In statistical analysis of gained data I also took into account the sex and age of the individuals. Beaver's behavior varies during the year. They are most active in spring and autumn, while in winter their activity decreases significantly. This observation is valid not only for the time duration and length of movement trajectories, but also for the size of actively-used area. The beginning of activity is strongly correlated with the sunset, while the end does not show any correlation with...
24

Außentemperatur, Jahreszeit und individuelle Trajektorien beeinflussen den Blutdruck von Kindern, Jugendlichen und jungen Erwachsenen.

Miersch, André 19 May 2014 (has links) (PDF)
Background: Seasonal fluctuations in outdoor temperature have been shown to affect blood pressure in adults. We have asked whether or not there is either a seasonal variation or an influence of outdoor temperature on blood pressure measurements in children and adolescents in middle Europe. Methods: Blood pressure was routinely measured in healthy and sick children and adolescents in outpatient clinics and during hospitalisation in a total of 6714 subjects (3,497 boys/3,237 girls), age 3 to 21 years, with a median of 10.6 years. Results: Cross-sectional analysis showed a significant seasonal variation in blood pressure measurements. The mean increase of systolic and diastolic blood pressure was 4.45/2.42 mmHg during winter. A significant correlation between average outdoor temperatures and systolic blood pressure was found (ρ=-0.074 p<0.001). However, the effect was only detectable at an average temperature below 0° C/32° F and above 10° C/50° F. For each degree Celsius increase in average outdoor temperature, the systolic blood pressure fell by 0.12 mmHg. Conclusion: Blood pressure measurements in children and adolescents, even in a temperate climate, are influenced by temperature and seasonal variation. Considering seasonal variations in blood pressure could be of clinical interest. / Background: High blood pressure is a major risk factor for cardiovascular disease. Blood pressure tracking could help to identify individuals with potential hypertension. Therefore, we have asked whether or not tracking was of predictive value for the development of hypertension in early life. Methods: Blood pressure was routinely measured in 13,261children and adolescents in outpatient clinics as well as during hospitalization. In a sub-analysis 568 individuals with normotensive and elevated blood pressure were compared after three follow-up periods (two, four and six years) and 2,157 normotensive individuals were compared in a paired t-test. Results: The follow-up analysis showed a significant tracking effect. However, the Pearson correlation coefficients of the systolic and diastolic blood pressure SDS decreased over time. Upon the follow-up after six years 35.6% of the children and adolescents with elevated blood pressure values remained in the elevated range group. Of the children within the normotensive blood pressure range 80.4% remained normotensive after six years. Children with normotensive blood pressure showed a stronger tracking than those who had had one hypertensive blood pressure reading. Children with high body weight gain left their SDS track to higher blood pressure values. Conclusion: Blood pressure tracking in children and adolescents is moderate. We conclude that the predictive power of a single hypertensive blood pressure measurement during a single visit is rather small, and thus repetitive measurements across several consecutive visits are necessary.
25

Seasonal and Spatial Variation in Leaf Area Index, Litter Production, and Light Levels in Myrica Cerifera,Shrub Thickets Across a Barrier Island Chronsequence

Brantley, Steven T. 01 January 2005 (has links)
Leaf area index (LAI), litter production and understory light levels of Myrica cergera shrub thickets were assessed on Hog Island, Virginia to quantify spatial and seasonal variations in leaf area and light attenuation among four thickets representing a successional chronosequence and compare methods of estimating LA1 in shrub- dominated systems. Seasonal LA1 estimates were made seven times throughout the year with a portable integrating radiometer (Li-Cor LAI-2000) and three times by measurement of photosynthetically active radiation (PAR) and use of the Beer-Larnbert law. Leaf area index was also estimated through leaf litter collection and use of allometric relationships between stem diameter and leaf area. The oldest of the four thickets had the lowest LA1 and litter production throughout the year. Peak LA1 measurements with the portable integrating radiometer ranged from 2.0 ± 0.2 to 4.0 ± 0.3 for the oldest and youngest thickets, respectively which appear to be substantial underestimates of the actual values. According to annual litterfall, LA1 ranged from 4.1 ± 0.5 for the oldest thicket to 10.2 ± 0.4 for a 20 year old thicket. Leaf area index varied from 6.4 ± 0.2 for the oldest thicket to 1 1.4 ± 2.1 for the 40 year old thicket according to the allometric relationships, which may overestimate LA1 in stands with many large stems. The high light attenuation by the shrub canopy and heavy litterfall may contribute to the low diversity within thicket canopies by inhibiting germination and growth of other species. The study demonstrates that these low diversity shrub thickets have a higher LA1 and litter production, and thus higher potential for primary productivity, than many temperate forested systems and that stand age may be the major determinant of productivity.
26

Estimation of Carbon Dioxide emissions from forest soils based on CO2 concentrations

Dennis Wilson, Dennis January 2017 (has links)
Forest soil is an important source of atmospheric CO2. Emission of CO2 from soil is the result of respiration of plant roots and soil organisms (Autotrophic and Heterotrophic respiration). This soil CO2 emission has a variation throughout the year with maximum emissions being in the summer. However, the seasonal variation affected by the external factors is not fully known. The aim of this thesis is to analyze a relationship between concentration of CO2 in the soil-atmosphere and CO2 emissions to the aboveground atmosphere. When knowing the relationship between CO2 concentration in the soil-atmosphere and the emission of CO2 from the soil atmosphere, a function (equation) can be established. Usually, the best fit is considered to establish the relationship. With the equations obtained, it is possible to calculate CO2 emissions using data different projects, where only soil-atmosphere CO2 concentrations were determined. Using the relationships, emissions rates in different soil types and in forest transect have been analyzed for a large number of samples. The effect of nitrogen deposition on CO2 emissions and seasonal variation of CO2 emission has also been studied. The sampled sites chosen for this study were located in different parts of Southern Scandinavia and Germany. A closed chamber was used to measure CO2 emission from soil. Soil CO2 concentrations were measured at every station and the equations were established. Finally, these relationships were used for analyses and comparison of the sites. An equation (best fit) obtained was used to calculate the emission values of CO2. The soil texture had a great influence on the CO2 from the soil besides the atmospheric pressure and temperature variations during the seasons. It is concluded that, therefore the soil texture and had a great influence on the CO2 emission from the soil besides the atmospheric pressure and temperature variations during the season. When knowing the equation between CO2 concentration and emission for a special type of soil, it is possible to estimate emissions based on CO2 concentrations. Therefore large scale sampling of CO2 concentrations could be done and this will facilitate the inventories carried out in e.g. global change studies.
27

Estudos das variações sazonais e intrapopulacional dos metabólitos secundários majoritários das folhas de \'Eremanthus glomerulatus Less\' (Asteraceae) / Study of the seasonal and intrapopulational variations of the secondary metabolites from the leaves of Eremanthus glomerulatus Less (Asteraceae).

Amaral, Juliano Geraldo 22 February 2008 (has links)
O trabalho buscou investigar as variações sazonais de metabólitos secundários em Eremanthus glomerulatus Less (Asteraceae). Embora não ser utilizada para fins medicinais esta espécie contem substâncias biologicamente ativas. Para este estudo, foram selecionados dez indivíduos de uma mesma população localizada na região do município de Ibiraci - MG, que durante um período de aproximadamente dois anos (06/2000 a 04/2002) tiveram um de seus ramos coletados mensalmente. Visando a análise das folhas destes ramos uma metodologia para extração e análise de seus metabólitos secundários majoritários por CLAE-UV foi desenvolvida e utilizada para determinação de suas variações sazonais. A identificação destes metabólitos se deu principalmente pelas técnicas hifenadas CLAE-DAD-EM e CLAE-DAD-EM/EM. Os metabólitos que não puderam ser identificados por estas ferramentas foram isolados do extrato etanólico foliar desta espécie e identificados por RMN de 1H e 13C e EM. Durante este estudo foi verificado que o envelhecimento das folhas resulta em uma significativa redução no conteúdo de todos os seus metabólitos secundários. Também foi possível identificar nove metabólitos secundários pertencentes as classes dos ácidos cafeoilquínicos, derivados fenólicos e flavonóides, dos quais foram isolados três, sendo dois ainda não relatados na literatura. Quanto a variação sazonal pode-se verificar que os indivíduos apresentam perfis metabólicos bem semelhante com diferenças quantitativas mas praticamente não apresentando variações qualitativas entre os indivíduos. Pode-se observar também que há um metabolismo bem diversificado no qual nem sempre todos os indivíduos apresentam aumento de produção de um mesmo metabólito em determinada estação do ano. Apesar das variáveis é possível definir o outono e a primavera como as estações que durante o estudo apresentaram os melhores índices de produção de praticamente todos os metabólitos analisados. Por fim pode-se concluir que as variações sazonais em E. glomerutalus são significativas e que são determinantes para definir a época de futuras coletas para posteriores estudos químicos ou farmacológicos. / This study was performed to investigate the seasonal intrapopulational variations of the secondary metabolites in Eremanthus glomerulatus Less (Asteraceae). Although this species is not used for medicinal purposes, it contains biologically active substances. For this study, ten individuals from the same population located in the region of Ibirici city, Minas Gerais state, had one of their lbranches collected monthly for about two years (from 06/2000 to 04/2002). Aiming the analysis of the leaves from the branches, a methodology on UV-HPLC was developed and used for the extraction and analysis of major secondary metabolites to determine the seasonal variations. The identification of the metabolites has occurred mainly by the HPLC-DAD-MS and HPLC-DAD-MS/MS hyphenated techniques. The metabolites that could not be identified by these methods were isolated from ethanolic extract of leaves of these species and identified by NMR 1H and 13C and MS. During this study it was noticed that the aging of the leaves results in a significant reduction on the rate of all the secondary metabolites. It was also possible to identify nine secondary metabolites belonging to caffeoylquinic acids, phenolical derivatives and flavonoids, from which, three compounds were isolated, however two compounds wasnt described in the literature. Relating to the seasonal variation, it was possible to verify that the individuals present similar metabolic profiles with quantitative differences but practically without qualitative difference. It was also possible to observe that there is a diversified metabolism in which not all individuals present an increase in the production of the same metabolite on a specific season. Despite these variations, it is possible to define autumn and spring as the seasons that presented the best rates of production on practically all the analyzed metabolites. Finally, it is possible to conclude that the seasonal variation of E. glomerutalus is significant and decisive to define the right time for future collection for further chemical and pharmacological studies.
28

Seasonal changes and serum 25-hydroxyvitamin D levels among community-dwelling elders who live in Boston, Massachusetts and Stockholm, Sweden

Chang, Kuang-Wei 01 November 2017 (has links)
BACKGROUND: The prevalence of Vitamin D deficiency is roughly 40% in the world and is increasing every year. Populations 65 years and older show a higher prevalence of vitamin D deficiency, because the aging process decreases the capacity of the skin to produce vitamin D. Some studies have reported that the prevalence of vitamin D deficiency is higher in the winter, however the effect of seasonal change on serum vitamin D level remains controversial in some specific populations. Moreover, this association remains uncertain in the elderly population because there is no study that specifically targets individuals over the age of 65. This study investigated the effect of seasonal changes and serum 25-hydroxyvitamin D among individuals 65 years and older residing in the Boston, Massachusetts and Stockholm, Sweden. METHODS: Cross-sectional and longitudinal cohort designs were both adapted to examine an existing data from VIVE2 parent study; the data was collected from 2012 to 2014. Data from the subjects who had finished this 6-month trial were analyzed for this study. Serum 25(OH)D levels, BMI, sex, study sites and age were collected and analyzed by univariate regression analysis and t-test. Serum 25(OH)D and confounders were included in multivariate analysis. Study sites were analyzed by effect modification model. RESULTS: In total, the prevalence of vitamin D deficiency (serum 25(OH)D levels less than 20 ng/ml) was 70%, while the mean serum 25(OH)D level was 20 ng/ml in summer and 16.4 ng/ml in winter. The average of seasonal serum 25(OH)D level changes were 6 ng/ml and 3 ng/ml in Stockholm, Sweden and Boston, MA, respectively. In addition, the prevalence of vitamin D deficiency increased 80% during winter (95CI: 1.1 – 2.9). There was no significant different in serum 25(OH)D levels among elderly populations between low latitude study site Boston, MA and high latitude site Stockholm, Sweden. There was no significant relation found in BMI, age and sex with serum 25(OH)D levels in the study. The seasonal serum 25(OH)D level changes was significantly different in the cross-sectional study design but not in the longitudinal study. CONCLUSION: Serum 25(OH)D levels were higher in the summer than in the winter among the elderly population resided in Boston, MA and Stockholm, Sweden.
29

Estrutura da comunidade de formigas poneromorfas (Hymenoptera:Formicidae) em uma ?rea da Floresta Amaz?nica

PEREIRA, Luana Priscila de Carvalho 26 June 2012 (has links)
Submitted by Jorge Silva (jorgelmsilva@ufrrj.br) on 2017-04-05T19:15:35Z No. of bitstreams: 1 2012 - Luana Priscila de Carvalho Pereira.pdf: 1129568 bytes, checksum: 6b12f4f6e3ee122c2cacabbf7752a126 (MD5) / Made available in DSpace on 2017-04-05T19:15:35Z (GMT). No. of bitstreams: 1 2012 - Luana Priscila de Carvalho Pereira.pdf: 1129568 bytes, checksum: 6b12f4f6e3ee122c2cacabbf7752a126 (MD5) Previous issue date: 2012-06-26 / CAPES / Tropical forests have been recognized as the biome of the planet that has the highest species richness and high endemism. The Amazon forest is part of this scenario and its high diversity has not been fully identified, as well as the patterns that govern it. Although research on the biodiversity of this biome have advanced in recent years, certain groups still have limited knowledge about their geographical and seasonal distribution or even ignored in studies about monitoring, conservation and management. In this sense, the main objective of this study was to determine the composition, richness and rarity of poneromorph ants using different sampling techniques, as well as investigate the effect of seasonality and certain environmental factors in the community. The ants were collected in the National Forest Tapirap?-Aquir? in July 2009, January and July 2010 and January 2011 in three different areas (A1, A2 and A3). In areas A1 and A2 were determined four sampling points inside the forest where they were made two transects of 100 meters and installed every 10 meters a pitfall trap and attractive sardine baits on the undergrowth vegetation and soil. The pitfall was active for 48 hours and baits for an hour. In the A3 was determined only one distance into the woods where it was made the same sampling procedures. We found 46 species of poneromorph ants, belonging to eight genera. The sampling technique that was more effective against the richness of the ant fauna found was the pitfall, which recorded 44 species (95.7% of total) and included all the genera found. Then the bait on the soil showed the richness of 19 species (41.3% of total) and the bait on vegetation presented species richness of six (13% of total). All ants found belong to two sub-families, which were Ponerinae and Ectatominae. The species Gnamptogenys striatula Mayr, and Pachycondyla unidentata 1883 (Mayr, 1862) were found only in the bait on vegetation, they are absent on the ground baits and pitfall throughout sampling. The results for the composition of the poneromorph ant fauna and its seasonal distribution do not show a clear stratification in the community, and the ant species that occurred in the dry and rainy seasons do not form distinct groups in the three sampled areas. Statistical tests to evaluate the influence of environmental variables on the distribution of poneromorfas ants species revealed no association patterns. The identification of the species found has special relevance because contribute to the consolidation of the knowledge of the biodiversity of the Amazon rainforest poneromorfas ants and allow comparative studies with other tropical forest regions. / As florestas tropicais tem sido reconhecidas como o bioma do planeta que possui a maior riqueza de esp?cies, al?m de elevado endemismo. A floresta amaz?nica faz parte desse cen?rio e sua alta diversidade ainda n?o foi totalmente identificada, assim como os padr?es que a regulam. Embora as pesquisas sobre a biodiversidade desse bioma tenham avan?ado nos ?ltimos anos, determinados grupos ainda apresentam conhecimento restrito sobre sua distribui??o geogr?fica e sazonal ou at? mesmo s?o ignorados em estudos de monitoramento, conserva??o e manejo. Nesse sentido, o objetivo principal desse estudo foi conhecer a composi??o, riqueza e raridade das esp?cies de formigas poneromorfas utilizando diferentes t?cnicas de amostragem, assim como investigar o efeito da sazonalidade e de determinados fatores ambientais na comunidade. As formigas foram coletadas na Floresta Nacional Tapirap?-Aquir? em julho de 2009, janeiro e julho de 2010, e janeiro de 2011 em tr?s ?reas distintas (A1, A2 e A3). Nas ?reas A1 e A2 foram determinados quatro pontos de amostragem no interior da mata onde foram feitos dois transectos de 100 metros e instalada a cada 10 metros uma armadilha tipo pitfall e iscas atrativas de sardinha sobre a vegeta??o subarbustiva e no solo. O pitfall permaneceu ativo por 48 horas e as iscas durante uma hora. Na ?rea A3 foi determinada apenas uma dist?ncia dentro da mata onde foram realizados os mesmos procedimentos de amostragem. Foram encontradas 46 esp?cies de formigas poneromorfas, pertencentes a oito g?neros. A t?cnica de amostragem que teve maior efic?cia em rela??o a riqueza da mirmecofauna encontrada foi o pitfall, que registrou 44 esp?cies (95,7% do total) e incluiu todos os g?neros encontrados. Em seguida, a isca no solo apresentou riqueza de 19 esp?cies (41,3% do total) e a isca sobre a vegeta??o apresentou riqueza de seis esp?cies (13% do total). Todas as formigas encontradas pertencem a duas sub-fam?lias, sendo elas Ponerinae e Ectatomminae. As esp?cies Gnamptogenys striatula Mayr, 1883 e Pachycondyla unidentata (Mayr, 1862) foram encontradas apenas nas iscas sobre a vegeta??o, estando ausentes nas iscas no solo e no pitfall em toda amostragem. Os resultados para composi??o da fauna de formigas poneromorfas e sua distribui??o sazonal indicam n?o haver uma clara estratifica??o na comunidade, sendo que as esp?cies de formigas que ocorreram na esta??o seca e na esta??o chuvosa n?o formam grupos distintos nas tr?s ?reas amostradas. Os testes estat?sticos aplicados para avaliar a influ?ncia das vari?veis ambientais medidas sobre a distribui??o das esp?cies de formigas poneromorfas n?o revelaram padr?es de associa??o. A identifica??o das esp?cies encontradas tem especial relev?ncia pelo fato de contribuir para a consolida??o do conhecimento da biodiversidade de formigas poneromorfas na Floresta Amaz?nica e permitir estudos comparativos com outras regi?es de floresta tropical.
30

Seasonal Variation in the Species Composition, Abundance, and Size-Frequency Distribution of Zooplankton in Bear Lake, Utah-Idaho

Moreno, Edmundo G. 01 May 1989 (has links)
Bear Lake, a large oligotrophic lake (282 km2), was studied from October 1986 to December 1987 to determine the temporal changes in the zooplankton assemblage at one site in the pelagic zone and one in the littoral area. In this study, species composition, abundance, biomass, and size frequency distribution were determined. Additionally, chlorophyll a, water transparency, and temperature and oxygen profiles were measured to help interpret zooplankton changes during the study. The zooplankton species assemblage comprised eight species of cladocera, seven species of copepods, and five species of rotifers. The abundance and biomass of the zooplankton assemblage, particularly that of crustaceans, were very low and comparable with those of oligotrophic systems. Mean densities of crustaceans in the pelagic zone, excluding copepod nauplii, varied from 250 to 1,700 organisms/m3. The analysis of the size structure of the zooplankton indicated the dominance of small organisms and the scarcity of large organisms, particularly cladocera. The zooplankton assemblage in the littoral zone was similar in species composition, abundance, and size structure to that in the pelagic zone. Many results suggest the littoral zooplankton assemblage is an extension of the pelagic assemblage. Low zooplankton food resources and interference of calcium carbonate particles in the feeding behavior of crustaceans are suggested as the primary factors controlling the low abundance and biomass of zooplankton in Bear Lake. Size-selective fish predation probably causes the assemblage to be dominated by small species. The analysis of Epischura nevadensis, the dominant species in the system, indicated that this species is bivoltine in Bear Lake. In the spring and summer, adult E. nevadensis were more abundant in the littoral zone, whereas copepodites were more abundant offshore. The low density, biomass, and small size structure of the zooplankton in Bear Lake limits its importance as a source of food for fishes.

Page generated in 0.5195 seconds