The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm3 l–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads). 相似文献
Accumulation of high levels of salts in the soil is characteristic of arid and semi-arid regions. Although different curative and management measures are being used to render salt-affected soils fit for agriculture, they are extremely expensive and do not provide permanent solutions to overcome the salinity problem. In contrast, a biotic approach for overcoming salinity stress has gained considerable recognition within the past few decades in view of the vast experimental evidence from what has happened in nature concerning the evolution of highly salt-tolerant ecotypes of different plant species, and also from the remarkable achievements that have been made in improveing different agronomic traits through artificial selection.
Considerable improvements in salt tolerance of important crop species have been achieved in the past 2 decades using barley, rice, pearl millet, maize, sorghum, alfalfa, and many grass species. Such achievements relied solely on assessment of the phenotypic expression of the features involved. Knowledge of the underlying physiological mechanisms producing those salt-tolerant individuals was not clearly known. The present review highlights the relationships between different physiological/biochemical variables being recommended as selection criteria, and salt tolerance of different plant species. This paper also lists different sources of genetic variation for salt tolerance since it is evident that for successful improvement in a character there must be a great amount of genetic variation present in the gene pool of a species. 相似文献
Abstract Grazing on transplants of a grass, a forb and a tree was examined in low-diversity grassland and more diverse heath in Australia's Snowy Mountains. Transplants were surrounded by 2 mm mesh netting. In one grassland plot, grazers (probably soil invertebrates) attacked 40–90% of tree and forb seedlings but no grass seedlings. In heath, which had about half the grass cover of grassland, grazers consumed grasses but not trees or forbs. The results suggest that grazers can depress diversity in grassland by attacking species other than grass. In heath, they may promote diversity by attacking only grass and releasing other species from competition. 相似文献
Specific growth rate of Cryptomonas ovata var. palustris Pringsheim was measured in batch culture at 14 light-temperature combinations. Both the maximum growth rate (μm) and optimum light intensity (Iopt) fit an empirical function that increases exponentially with temperature up to an optimum (Topt), then declines rapidly as temperature exceeds Topt. Incorporation of these functions into Steele's growth equation gives a good estimate of specific growth rate over a wide range of temperature and light intensity. Rates of phosphate, ammonium and nitrate uptake were measured separately at 16 combinations of irradiance and temperature and following a spike addition of all starved cells initially took up nutrient at a rapid rate. This transitory surge was followed by a period of steady, substrate-saturated uptake that persisted until external nutrient concentration fell. Substrate-saturated NO3?-uptake proceeded at very slow rates in the dark and was stimulated by both increased temperature and irradiance; NH4+-uptake apparently proceeded at a basal rate at 8 and l4 C and was also stimulated by increased temperature and irradiance. Rates of NH4?-uptake were much higher than NO3?-uptake at all light-temperature combinations. Below 20 C, PO4?3-uptake was more rapid in dark than in light, but was light enhanced at 26 C. 相似文献
Nine larval types within the genera Anisakis, Terranova, Thynnascaris and Contracaecum were recovered from south-eastern Queensland marine fishes. Data on (i) incidence, (ii) intensity of infection, (iii) host diet and (iv) habitat for each type suggest Anisakis is an open water type, Contracaecum an inshore, shallow water type and both Terranova and Thynnascaris have intermediate distributions. Host diets indicate Anisakis and Terranova are found in predators of nekton, Contracaecum and Thynnascaris in benthic feeders. 相似文献
The epipelic and epilithic algal communities in a small eutrophic stream situated in southern England expanded rapidly during March of both 1973 and 1974 primarily in response to changing light conditions. Although numbers varied greatly during the summer, these fluctuations were probably not due to nutrient, temperature or light conditions. High rates of disappearance of algae from the substrate were correlated with flooding, a deterioration of attachment characteristics and high metabolic rates. The episammic algal community consisted of only a few species, all of which showed maximum development during the summer. Although the well developed attachment mechanism and small size of the species undoubtedly aid in their ability to colonize sand grains, each species must be able to withstand frequent burial in the bottom deposits. Although temperature was probably an important factor controlling the number of epiphytes associated with Cladophora glomerata, light seemed to be of relatively less importance. Large numbers of isopods, amphipods and copepods occurred in the stream but their grazing seemed to have had little effect of the standing crop of the algae. 相似文献
Die Grünalge Oedogonium cardiacum Wittrock wurde bei verschiedenen Intensitätsstufen von Weißlicht (8, 40 und 200 Mikroeinstein m—2 sec—1 = 500; 2500 und 12500 Lux; Kombination aus Leuchtstoffröhren mit warmweißem und weißem Licht) kultiviert, um die Ausbildung von ?Sonnen- und Schattenpflanzen” zu provozieren. Die Struktur des netzförmigen Cloroplasten und seine Abwandlungen unter Lichteinfluß wurden qualitativ und quantitative untersucht und im Zusammenhang mit dem Pigmentgehalt beschrieben. Die Größe des Chloroplasten ist bei verschiedenen Lichtbedingungen nicht modifiziert. Dagegen zeigt die Struktur, insbesondere des Lamellarsystems, erhebliche Unterschiede. Erstaunlicherweise ist es bei einer mittleren Lichtintensität von 40 μE am besten ausgebildet. Bei diesen Bedingungen sind auch die meisten zytoplasmatischen Invaginationen in den Pyrenoiden sichtbar. Die Neigung zur Partitionbildung der Thylakoide wächst dagegen erheblich bei hoher Lichtintensität. Gleichzeitig steigt der Anteil des Chlorophyll b am Gesamtpigmentgehalt. Hier wird eine allgemeine Struktur-Funktion-Beziehung sichtbar, welche auch für Höhere Pflanzen gilt. Insgesamt zeigen die Ergebnisse, daß auch Algen ontogenetisch ?Sonnenund Schattenindividuen” zu bilden vermögen und daß sie sich damit dem lokal herrschenden Lichtklima anpassen. 相似文献
Summary Lowland rice (RD 3) was cultivated in containers of clay soil submerged with 5 cm water under controlled conditions in the phytotron. Deep placement of urea supergranules 5 cm in the soil significantly enhanced both plant growth and fertilizer efficiency when the plants were cultivated under high light intensity (70 Wm–2). At the highest urea level grain yield increased 119% above the control level, while growth and fertilizer efficiency was not as high when deep placement of calcium nitrate was used.The application of urea prills and calcium nitrate (18.4g Nm–2) in two split doses on the soil surface increased grain yield as much as 91% above the control level. At the lower nitrogen concentration (9.2 g N m–2), the urea prills were more efficient than calcium nitrate as indicated by the grain yield. The height of those plants fertilized by surface application was affected by the concentration and not the type of fertilizer. The number of tillers, however, was significantly higher on urea fertilized plants.When the rice plants were cultivated under low light intensity 930 Wm–2), neither the nitrogen fertilizers nor the method of application had a significant effect on growth and yield. 相似文献