Ciliate communities in shallow groundwater: seasonal and spatial characteristics

1. Ciliated protozoans (Phylum Ciliophora) were collected from five sites in a shallow groundwater system in southern Ontario, Canada over a 13‐month period: one at the spring source, two along the channel banks, and two in the stream channel. Ciliates and environmental data were collected from surface water and at five depths into the sediment, located at 20, 40, 60, 80 and 100 cm. 2. Species richness was high (170 ciliate species belonging to 89 genera were identified) and variable, both spatially and temporally. Highest species richness (86) occurred between 20 and 60 cm, and typically decreased below 60 cm. 3. Ciliate densities were also seasonally and spatially variable. Densities peaked in March between 40 cm (as high as 69 900 cells L⁻¹) and 60 cm, and again in May and June at 80 and 100 cm. Densities were lowest in winter. The surface‐water ciliate community had a different species composition and lower population densities. 4. At all depths, small (<50 μm) bacterivorous ciliates typically dominated, but omnivorous and predatory species were also present (combined, up to 30% of the average density). 5. Several ciliate genera, traditionally considered planktonic, occurred at low densities from 40 cm down to 100 cm. 6. Ordination analysis indicated that the main factors influencing the shallow groundwater ciliate communities were depth and temperature. 7. Dissolved oxygen also appeared to influence these communities in that they typically comprised genera that preferred either low‐oxygen or anaerobic conditions.     

Phenology of Acacia albida trees in Zambezi riverine woodlands

The phenology of Acacia albida trees was studied over eight years in Mana Pools National Park, Zimbabwe. Leaf flush occurred towards the end of the rains. It was followed by flowering, with ripe fruit-fall in the late dry season. Leaf-fall occurred early in the following wet season. After seasons of low rainfall, trees were severely defoliated by caterpillars soon after leaf flush and this prompted a second flush of leaves. In these years there were also two periods of flowering and fruit production. During dry years, the proportion of trees which failed to set fruits after flowering increased, fruit production by those trees which did bear fruit was reduced and fruit-fall started earlier in the year.     

Cyclic Variations in Nitrogen Uptake Rate in Soybean Plants: Uptake During Reproductive Growth

Net uptake of by non-nodulated soybean plants [Glycme max(L ) Merr cv Ransom] growing in flowing hydroponicculture was measured daily during a 63 d period of reproductivedevelopment between the first florally inductive photopenodand late seed growth Removal of from a replenished solution containing 10 mol m^– was determined by ion chromatography Uptake of continued throughout reproductive development The net uptakerate of cycled between maxima and minima with a periodicity of oscillation of 3 to 7 d during the floralstage and about 6 d during the fruiting stage. Coupled withincreasing concentrations of carbon and C:N ratios in tissues,the oscillations in net uptake rates of are evidence that the demand for carbohydrate by reproductiveorgans is contingent on the availability of nitrogen in theshoot pool rather than that the demand for nitrogen followsthe flux of carbohydrate into reproductive tissues. Key words: Nitrate uptake rate, carbon-nitrogen partitioning, Glycme max (L ) Merrill     

Influence of nymphal size, sex and morphotype on microdistribution of Deleatidium (Ephemeroptera: Leptophlebiidae) in a New Zealand river

• 1 The interaction of water depth and velocity with size, sex and morphotype of nymphs belonging to the leptophlebiid mayfly genus Deleatidium was investigated in a New Zealand river.
• 2 Velocity had a significant effect on distribution such that larger nymphs tended to be found more commonly in faster water. Depth also influenced size distribution, but in different ways at different sites.
• 3 Water velocity or depth alone did not significantly influence the distribution of sexes (1.0 mm head width), but the combined effect of these two factors was significant. Final instar males appeared to be relatively more abundant in slower water than final instar females.
• 4 Nymphs (1.0 mm head width) belonging to the informal lillii group (apically pointed gills) were present over a wide range of depth and velocity conditions, but those belonging to the myzobranchia group (round-tipped gills) were largely restricted to fast water (>0.9ms^?1).
• 5 Several mechanisms thought to cause variations in microdistribution are discussed. These include differential effects of oxygen availability, food requirements and drag forces, nymphal behaviour, and selection of oviposition sites by adults.

     

Population Interactions and the Determinants of Population Size

Abstract The flux of individuals within populations is dependent upon six controlling processes: the intrinsic rate of increase of the plant, intraspecific competition for resources, interspecific competition, natural enemies, mutualisms and refuge effects such as the immigration of seeds from other populations. Although population interactions are generally believed to play a major role in determining the flux of individuals within populations, little attempt has been made to quantify the strength of these interactions and their role in the dynamics of populations. In this paper we examine the role of competition, herbivory and mutualistic interactions in determining the dynamics of a range of annual plant species. Firstly, it is shown that the dynamics of three weed species ( Bromus sterilis, Galium aparine, Papaver rhoeas ) in an experimental community in an arable cropping system of winter wheat are determined primarily by the rapid population growth of B. sterilis . Interactions between the species play a minor part in the dynamics of the system. Secondly, it is shown that current levels of grazing by overwintering populations of brent geese have a minor impact on the abundance of Salicornia europaea , but that increased grazing has the potential to reduce abundance and increase the instability of S. europaea populations. This is a consequence of the aggregative response of the geese, which results in an increasing proportion of the seeds of S. europaea being eaten as plant density increases. Thirdly, it is shown that there is a complex interaction between root pathogenic and arbuscular mycorrhizal fungi in natural field populations of Vulpia ciliata and that the benefit of mycorrhizal fungi to the plant is in providing protection against pathogens.     

Biomass dynamics of herbaceous vegetation in Zambezi riverine woodlands

Seasonal changes in biomass, productivity and large mammal utilization of annual grasslands on Zambezi alluvium in Mana Pools National Park, Zimbabwe, were recorded over four years and compared with perennial grasslands. Annual grasses grew only in the wet season, but forb productivity peaked at the start of the dry season. Large mammals and termites ate dead annual grasses in the dry season. Production was usually greater in perennial grasslands, than in annual ones, and perennial grasses often grew throughout the year. Large mammal utilization of grasslands varied between 53 and 99% of annual production.

Résumé

L'on rapporte les changements saisonniers de biomasse, productivité et utilisation par les grands mammifères des herbages annuels dans les plaines alluviales du Zambèze au Parc National de Mana Pools, Zimbabwe, pendant 4 ans et on les compare avec ceux des herbages pérennes. Les herbes annuelles ne croissent que pendant la saison des pluies mais la productivité du fourrage était au maximum en début de pérennes que dans les annuels et les herbes pérennes poussaient souvent tout au long de I'année. L'utilisation par les grands mammifères variait de 53 à 99% de la production annuelle.     

Fungi from decaying intertidal fronds of Nypa fruticans, including three new genera and four new species

HYDE, K. D., 1992. Fungi from decaying fronds of Nypa fruticans, including three new genera and four new species. Decaying fronds and exposed stems of the mangrove palm Nypa fruticans were collected from beaches and intertidal regions of mangroves in Brunei and examined for the presence of higher marine fungi. Forty-three species were identified, four of which are new to science. The fungi included 32 Ascomycotina, one Basidiomycotina and ten Deuteromycotina. Three new genera and four new species are described and illustrated.