ON THE DISSOLVED SOLIDS OF THE PONGOLO FLOOD PLAIN PANS

SUMMARY

The water retained in the Pongolo flood plain pans differs from that of the Pongolo River not only in having a higher TDS, but also in the composition of the solutes, which approximate to seawater in their equivalent ionic proportions at high concentrations. These solutes are shown to originate from very highly mineralised seepage water characteristic of soils overlying Cretaceous sediments of marine origin found elsewhere in South Africa. This feature of the soils of the flood plain precludes the use of irrigation seepage water to maintain the water levels in the pans, and stresses the need for the release of simulated floods from the Pongolo-poort Dam to flush the system if the present biological characteristics of the pans are to be maintained.     

Reproductive strategies of Arctic charr Salvelinus alpinus (L.) forms in Kiryalta lakes,Transbaikalia, Russia

Lake Abaya and Lake Chamo are the two largest Ethiopian Rift Valley lakes; they are located close to each other, but have a strikingly different water transparency. We explain key differences in the structure and the functioning of the food web with variation in limnological variables and major pelagic food web compartments within and across both lakes. Data from a detailed comparative investigation of physical and chemical variables and zooplankton community characteristics during the wet and dry season from two consecutive years revealed major differences in key limnological variables between Lake Abaya and Lake Chamo. The most pronounced differences were related to water transparency and the amount of suspended solids in the water column. Lake Abaya is much more turbid, has lower phyto- and zooplankton biomass, and has considerably lower primary production than Lake Chamo. Based on our results, we infer that the profound differences in food web structure between both lakes probably result from differences in sediment load. Finally, our results indicate that conservation programs should focus on reducing sediment inflow from the catchments into the lakes.

     

OBSERVATIONS ON THE COEXISTENCE OF FRESH AND SALTWATER INVERTEBRATES IN AN INLAND SALTWORKS

SUMMARY

The fauna of three excavated trenches in the floor of the dry salt-pan in the Brandfort district of the Orange Free State was investigated. Two were found to contain typical saline water invertebrate fauna such as Artemia sp. and the saltwater rotifer Brachionus plicatilis and were without vegetation, while the third supported dense stands of Ruppia maritima, the salt water rotifer Brachionus plicatilis and a variety of freshwater macro-invertebrates. We concluded that certain freshwater macro-invertebrates are able to tolerate saline water, and that this would give them a competitive advantage over stenohaline invertebrates in semi-arid and arid areas where a range of salinities occur within and among the endorheic pans in the region.     

EARLY OBSERVATIONS ON SOME LIMNOLOGICAL CHARACTERISTICS OF THREE ORANGE FREE STATE IMPOUNDMENTS

SUMMARY

The first limnological investigations on three impoundments in the Orange Free State are described. No thermal stratification was found and water temperatures below 8,6°C were not measured. The waters were turbid and the unfavourable underwater light climate was the major factor limiting algal growth. The catchment areas influenced the ionic composition of the waters and large quantities of ions were adsorbed onto the suspended silt.     

Limnology of 46 lakes and ponds on Banks Island, N.W.T., Canadian Arctic Archipelago

The goal of this study was to describe and quantify the physical and chemical limnological properties of 46 lakes and ponds on Banks Island, to explore the effects of ecoclimatic differences on the water chemistry of these study sites, and to establish baseline conditions for this previously unexplored limnological region, which could then be used in subsequent long-term environmental monitoring programs. A key finding was that the study sites on Banks Island represented a large nutrient concentration gradient from ultra-oligotrophic to hypereutrophic waters. In general, the study sites were relatively nutrient rich by Arctic standards (i.e. mean total nitrogen (TN_mean)=504.2μg/l and mean total phosphorus (TPU_mean)=18.0μg/l); concentrations that are amongst the highest of any previous limnological survey from similar latitudes. Dissolved organic carbon (DOC) concentrations were also some of the highest reported to date amongst all other Canadian Arctic island limnological surveys. These values reflect the milder climate, concentrated animal life and lushness of Banks Island, as compared to other Canadian Arctic Archipelago islands. Principal components analysis (PCA) separated sites along a conductivity/ionic and elevation gradient on Axis 1 (λ₁=0.343), and a metal (Fe, Zn, Al) and alkalinity-related (DIC, pH) gradient on the second axis (λ₂=0.187). Canonical variate analysis (CVA) was used to explore the classification of the study sites into Low Arctic, Mid Arctic or High Arctic designations based on their limnological characteristics.     

Succession and fluctuation in a wet dune slack in relation to hydrological changes

Vegetation changes in a wet dune slack complex have been studied over a period of 23 yr. The vegetation was recorded in 40 permanent plots: all plots in 1964, 1977, 1983 and 1987 and some also in five other years. There were large fluctuations in annual precipitation during this period, including some extremely dry (1976) and wet (1985) years. Many species of wet calcareous and pioneer habitats declined in cover between 1964 and 1977, when there were prolonged dry summer periods, though few species were lost from the sampled quadrats. Practically all calciphilous and pioneer species disappeared between 1977 and 1983, when there were relatively many wet summers. It is suggested that extremely dry conditions temporarily obscure the impacts of hydrological changes in local hydrological systems, by retarding the vegetation succession. After a period of excessive precipitation the (acidifying) effects of ground-water withdrawal rapidly became evident in the species composition of the dune slack.     

Observations on the limnology and primary production of a small man-made lake in the West African savanna

1. The results of limnological studies on a small, 35-ha, triangular man-made lake and a small adjacent fish pond in the coastal savanna of Ghana are described. 2. The lake, which has a renewal time of approximately 0.96 years, is subject to large, predictable seasonal changes. Water volume and surface area rapidly reach maximum values in the wet season from April to June despite the low runoff values which average only 1.2% of the precipitation. Minimum values occur in the dry season when up to 76% of the lake area may be exposed. Most of the water is lost by evaporation which may amount to 193 cm annually. 3. The annual cycle in water temperature follows that in solar radiation with peaks towards the end of the dry season in March to April and deep troughs in the wet season from April to June. During the dry season the water column is characterized by greater stability, a higher degree of stratification, and a much higher amplitude of diurnal temperature change (4-5°C) than in the wet season (1°C). Isothermal conditions tend to be approached nocturnally and even during the day in the wet season. 4. The ionic content of the water is low and dominated by bicarbonate, sodium, silica and iron. During the wet season flood water results in a decline in conductivity from 214 to 30 μmhos/cm², a decrease in pH and transparency, and an increase in oxidizable organic material. Except for the conductivity these values return to normal in about 6 weeks as a result of rapid decomposition of allochthonous materials. 5. The oxygen pulse is strong and photosynthetic activity by phytoplankton high during the dry season whereas in the wet season the converse is true. As a result of the low concentration of organic carbon (0.10–0.35%) in the sediments their oxygen demands are low. Consequently, dissolved oxygen values near the sediment are relatively high and oxygen stratification poorly-developed. Most of the organic carbon appears to be recycled in the water column rather than in the sediments as is generally the case in tropical ecosystems. 6. Rooted vegetation became permanently established in the fish pond but not i n the lake itself. Emergent and floating vegetation which grows profusely in the draw-down area in the wet season is for the most part exported from the system by wind and biological agencies and, therefore, contributes little to the energy and mineral budgets of the lake. Primary production in the water column is mainly by Myxophyceae and Bacillariophyceae and is low during the dry season (1.35 gC/m²/day) and very low in the wet season. Possible reasons for the low level of primary production are discussed.     

Succession and herbivory in monsoonal wetlands

This paper considers the applicability of the Gleasonian succession model to monsoonal wetlands, particularly those in Australia and India. The similarities in climate and taxa shared among these wetlands produce a setting in which successional cycles may be replicated. The Gleasonian succession model was first designed to describe cyclic changes in prairie pothole wetlands in response to water flux and herbivore activity over 5–25 year intervals. However, monsoonal wetlands change over an annual cycle. The Gleasonian succession may therefore be faster in these wetlands than in temperate ones. Herbivores create openings in flooded vegetation during the wet season in India by killing emergent species. During the dry season, Paspalum distichum revegetates these openings via the sprouting of vegetative fragments (VS-I; establishment by vegetative means in drawndown conditions). In Australia, in those monsoonal wetlands dominated by the annual Oryza meridionalis (AS-II; establishment by seed germination in flooded conditions), revegetation is from the seed bank during the wet season. The Gleasonian succession model can help to explain the invasion of exotics in monsoonal wetlands; in Australia, sites dominated by Oryza meridionalis (wet season annual) can be invaded during the dry season by Mimosa pigra (PS-I; perennial, establishment by seed germination in drawndown conditions). Thus the role of herbivores in succession is more complicated than current predictive models of vegetation change encompass. Herbivores select specific habitats over plant species, and also can form symbiotic relationships with plants as seed dispersers. Nonetheless, if appropriately modified with respect to the life history requirements of these plant species, the Gleasonian succession model is useful in describing annual and long-term changes in the vegetation of monsoonal wetlands.     

Weather effects on birds of different size are mediated by long‐term climate and vegetation type in endangered temperate woodlands

Species occurrence is influenced by a range of factors including habitat attributes, climate, weather, and human landscape modification. These drivers are likely to interact, but their effects are frequently quantified independently. Here, we report the results of a 13‐year study of temperate woodland birds in south‐eastern Australia to quantify how different‐sized birds respond to the interacting effects of: (a) short‐term weather (rainfall and temperature in the 12 months preceding our surveys), (b) long‐term climate (average rainfall and maximum and minimum temperatures over the period 1970–2014), and (c) broad structural forms of vegetation (old‐growth woodland, regrowth woodland, and restoration plantings). We uncovered significant interactions between bird body size, vegetation type, climate, and weather. High short‐term rainfall was associated with decreased occurrence of large birds in old‐growth and regrowth woodland, but not in restoration plantings. Conversely, small bird occurrence peaked in wet years, but this effect was most pronounced in locations with a history of high rainfall, and was actually reversed (peak occurrence in dry years) in restoration plantings in dry climates. The occurrence of small birds was depressed—and large birds elevated—in hot years, except in restoration plantings which supported few large birds under these circumstances. Our investigation suggests that different mechanisms may underpin contrasting responses of small and large birds to the interacting effects of climate, weather, and vegetation type. A diversity of vegetation cover is needed across a landscape to promote the occurrence of different‐sized bird species in agriculture‐dominated landscapes, particularly under variable weather conditions. Climate change is predicted to lead to widespread drying of our study region, and restoration plantings—especially currently climatically wet areas—may become critically important for conserving bird species, particularly small‐bodied taxa.     

THE FRACTIONATION OF CLAY SIZED PARTICLES FROM THE WURAS DAM WATER PHASE

SUMMARY

The Coulter Counter has been employed with success to estimate the number and sizes of suspended particles in seawater. In the present study this method was applied to Wuras Dam, a turbid man made lake south of Bloemfontein.

High concentrations of larger clay sized particles occur in the inflow water as well as in the water of the dam proper. Fractionation of particles between ca 1 and 2 pm in diameter have been achieved, and a higher concentration of small sized particles (ca 1 μm) occurred in the dam water while the concentration of the larger particles (1,69 to 2,03 μm) was lower.

Surface area estimations were made for each size fraction. A considerable surface area (ca 163 m² m^?3) was associated with the investigated suspended particles in Wuras Dam. This might be of extreme importance for the physical, chemical and biological characteristics of the dam. The limnological importance of the suspended particles is briefly discussed.     

A study on annual changes in surface cover of floating-leaved plants in a lake using aerial photography

A simple method of aerial photography was used to investigate annual and between-year changes in surface cover of the floating-leaved plants, Nymphoides peltata, Nelumbo nucifera and Trapa natans, in natural stands in Lake Kasumigaura. Oblique photographs of vegetation were taken with a 35-mm camera with real color film from airplanes between 1983 and 1989. Vegetation maps were traced on the positions after projective transformation to vertical photography. The distribution of the stand of Nymphoides peltata seemed to be restricted to a region of less than 1.5 m water depth. A rise of 1.0 m in the water level after a typhoon in 1986 caused a subsequent decrease in vegetation area of Nelumbo nucifera. The vegetation recovered in shallower regions after one year in 1987. The expansion rate of Nymphoides peltata and Nelumbo nucifera in one direction was in the range 3.8–10.0 m yr^-1 and 4.1–20.8 m yr^-1, respectively. Nelumbo nucifera, Nymphoides peltata and Trapa natans, having high leaf biomass to total biomass may therefore be more vulnerable to large flood and waves. Fluctuation in water level and spatial difference in water depth may also be important factors determining the vegetation area and survival of Nymphoides peltata, Nelumbo nucifera and Trapa natans.     

石灰岩地区连片出露石丛生境植物水分来源的季节性差异

以桂西北石灰岩地区连片出露石丛生境次生林群落为研究对象, 运用稳定性氢氧同位素技术结合IsoSource模型, 分析了5种典型植物(半落叶乔木粉苹婆(Sterculia euosma)、落叶乔木菜豆树(Radermachera sinica)、常绿灌木鹅掌柴(Schefflera octophylla)以及落叶灌木红背山麻杆(Alchornea trewioides)和紫弹树(Celtis biondii))水分来源的季节性差异。结果表明: 雨季, 除鹅掌柴同时利用部分前期雨水外, 其余4种植物均主要利用最近15天内的雨水(利用比例的平均值之和均超过80%)。旱季, 5种植物均主要利用最近一次雨水与前期雨水(一个月前)的混合(利用比例的平均值之和均超过80%), 其中乔木和常绿灌木对前期雨水的利用比例更高(利用比例的平均值均超过50%)。植被良好的石灰岩连片出露石丛生境中, 植物主要以不同时期的雨水为主要水源, 这可能与石灰岩发育的裂隙及其储水能力有关。储存在裂隙中的雨水通过植物蒸腾的方式返回大气, 这一良好的水文循环过程应得到充分的保护。     

Seedling morphological characteristics and seasonal growth of indigenous tree species transplanted into four plantations in South China

The successful transplantation of indigenous tree seedlings into established plantations requires an understanding of the conditions required by the seedlings. We evaluated seedling morphological characteristics and seasonal growth of three indigenous tree species (Castanopsis chinensis, Michelia chapensis, and Psychotria rubra) that were transplanted into four plantations (eucalyptus, mixed-native, mixed-legume, mixed-conifer) in South China; in each plantation, two treatments (understory vegetation and litter retained or removed) were applied before the seedlings were transplanted. Seedling leaf morphological characteristics and biomass allocation were determined at the end of the experiment, and seedling relative growth rate as indicated by change in height (RGRh) was determined during the experiment. Whether understory vegetation and litter were removed or retained, RGRh tended to be higher in the wet season than in the dry season. Leaf morphological characteristics and biomass allocation were significantly affected by species identity. The effect of the understory vegetation and litter treatments on seedling morphological characteristics such as specific leaf area, leaf area ratio, and root weight to total biomass ratio depended on species identity. Redundancy analysis showed that the three transplanted indigenous species differed in their responses to light conditions according to their tolerance to shade, and also differed in their responses to soil physical and chemical properties. Based on seedling seasonal growth patterns and morphological responses, we suggest that forest managers attempting to introduce seedlings of indigenous tree species should artificially supply water to increase seedling growth in the dry season. Also, the introduced tree species should be selected depending on the nature of the plantation; for example, C. chinensis and M. chapensis seedlings can be transplanted into mixed-legume plantations. Additionally, fertilizer with potassium and nitrogen should be used to improve seedling performance.     

Plant spinescence in arid southern Africa: does moisture mediate selection by mammals?

Summary The prediction that spinescence in plants increases with aridity, soil fertility and mammalian herbivory was examined at regional and local scales in southern Africa. Spinescence tended to increase with aridity. Within arid areas, vegetation of moist, nutrient-rich habitats was more spinescent than that of the surrounding dry plains. Spinescence in plants of drainage lines and pans in arid southern Africa occurs in a wide range of genera and appears to have been selected by the effect of large mammals which concentrate on these moist patches. It is concluded that spinescence may be selected by breakage as well as herbivory, and that in arid areas moisture may be important in mediating mammalian selection of spinescence.     

Water and carbon storage capacity in Isolatocereus dumortieri (Cactaceae) in an intertropical semiarid zone in Mexico

In the biosphere reserve Barranca de Metztitlán in Mexico, there is an extensive area with a semiarid scrub. The dominant species is the cactus Isolatocereus dumortieri. This is a key species in the ecosystem, because many species of birds, bats and insects are feeding from its nectar, pollen and fruits during the dry season thanks to their capacity to store water and carbon. However, there is no information about their potential to store water and carbon. The purpose of this study was to estimate the ability of I. dumortieri to store water and carbon. Water content per plant was estimated as 537.64 ± 71.59 L during the dry season and 692.24 ± 92.18 L during the wet season. In the same way the carbon stored per cactus was 16.75 kg ± 7.07 corresponding to 1.25 kg C m². The results shows the importance of I. dumortieri in maintaining the ecosystem services of the scrub vegetation.     

Growth response of Halimium halimifolium at four sites with different soil water availability regimes in two contrasted hydrological cycles

The relationship between water availability and plant growth response in Halimium halimifolium (L.) Willk throughout two years with contrasted precipitation (300 and 850 mm) was examined by measuring vegetative growth and midday leaf water potential in four sandy soil sites with different water availability in Doñana National Park, Spain. H. halimifolium, Cistaceae is mostly restricted to sandy substrates close to coastal Mediterranean areas. At Doñana National Park this species is the main component of stable sand vegetation. Vegetative growth started in March, ending in July. The maximum shoot elongation rate (110 mm/year) and number of branches (8.3 branches/year) occurred in plants growing in the most hygrophytic site (MN) in both the wet and dry years. Plants at this site exhibited higher midday water potentials throughout the year. In contrast, the minimum shoot elongation rate (40 mm/year) and midday water potential (–4.0 MPa) occurred in Monte Intermedio plants (MI) in the dry year, although the water table was shallower than in Monte Blanco (MB). In the wet year the minimum shoot and branch elongation were recorded in MB. The number of leaves per branch for a single main shoot was higher (55 leaves/branch) in the driest area (MB), but these leaves had the smallest area. ANOVA showed significant differences in growth response and midday water potential between the four sites. A stepwise multiple linear regression showed that midday water potential, water table depth and monthly average temperature were the variables most closely associated with shoot elongation rate. We conclude that under severe dryness, the expected patterns of greater growth in sites with better water supply may differ from compared with the expected growth pattern in normal conditions.     

Strong positive effects of termites on savanna bird abundance and diversity are amplified by large herbivore exclusion

Vast areas of the African savanna landscapes are characterized by tree‐covered Macrotermes termite mounds embedded within a relatively open savanna matrix. In concert with termites, large herbivores are important determinants of savanna woody vegetation cover. The relative cover of woody species has considerable effects on savanna function. Despite the potentially important ecological relationships between termite mounds, woody plants, large herbivores, and birds, these associations have previously received surprisingly little attention. We experimentally studied the effects of termites and large herbivores on the avian community in Lake Mburo National Park, Uganda, where woody vegetation is essentially limited to termite mounds. Our experiment comprised of four treatments in nine replicates; unfenced termite mounds, fenced mounds (excluding large mammals), unfenced adjacent savanna, and fenced savanna. We recorded species identity, abundance, and behavior of all birds observed on these plots over a two‐month period, from late dry until wet season. Birds used termite mounds almost exclusively, with only 3.5% of observations occurring in the treeless intermound savanna matrix. Mean abundance and species richness of birds doubled on fenced (large herbivores excluded) compared to unfenced mounds. Feeding behavior increased when large mammals were excluded from mounds, both in absolute number of observed individuals, and relative to other behaviors. This study documents the fundamental positive impact of Macrotermes termites on bird abundance and diversity in an African savanna. Birds play crucial functional roles in savanna ecosystems, for example, by dispersing fruits or regulating herbivorous insect populations. Thus, the role of birds in savanna dynamics depends on the distribution and abundance of termite mounds.     

BATHYMETRIC STUDIES ON THE PONGOLO RIVER FLOODPLAIN

SUMMARY

Thirteen of the major pans on the Pongolo river floodplain have been studied. Most are shallow (>2.5 m) and regularly lose up to 70% of their water each year, although few dry out. Floods inundate considerable areas (1000 ha) around pans situated in the flatter parts of the floodplain. Whilst inundated, these areas provide an allochthonous energy and nutrient source for the pan and, as the water levels recede, the exposed Cynodon dactylon (L.) Pers lawns provide grazing for cattle. The suitability of the various pans for fish production is discussed.     

Assessment of the impacts of hydrological fluctuations and salt pans abandonment on Greater flamingos in the Camargue, South of France

Flamingos forage in both commercial salt pans and natural marshes and lagoons along the French Mediterranean coast. In order to assess the impact of changes in management of commercial salt pans and hydrological fluctuations on this flagship species, we evaluated the foraging areas of breeding flamingos using the resightings of 283 breeding flamingos marked with dye at the colony in 1987 and 1989, two years with contrasting hydrological conditions. Teams of observers searched all suitable habitats within 80 km of the colony during the four days following marking and recorded presence of off-duty flamingos. About one-third of the birds were found within 10 km of the colony, but some were seen up to 70 km away. About 24–54% of the birds were found in permanent brackish lagoons and 18–60% in the salt pans, the two most important habitats. In 1989, a dry year with lower water levels in the natural wetlands, the proportion of breeding flamingos using salt pans was twice as high [53%, range (47–60%)] as in 1987 [26%, range (18–29%)], this habitat thus acting as a refuge. Most of the feeding areas shown to be important for flamingos breeding in the Camargue are thus susceptible to variations according to rainfall and to transformations or drying out if the salt pans are abandoned. Our results provide essential benchmarks to reconsider the conservation of this flagship species when management of commercial salt pans changes.     

Water relations and gas exchange in Coespeletia moritziana (Sch. Bip) Cuatrec., a giant rosette species of the high tropical Andes

Giant rosettes are ones of the most striking features of the vegetation in the high tropical Andes, with Coespeletia moritziana reaching the highest altitudes up to 4,600 m a.s.l. Different from other giant rosettes, this species grows on rock outcrops with poorly developed soils and where water availability may be limited. Two questions are addressed in this study: How does this species respond in terms of water relations to maintain favorable gas-exchange conditions? Considering that adult plants rely on a water-reserving central pith, how do early stages respond to this environment??s extreme conditions? Water relations and gas-exchange studies were carried out on juveniles, intermediate and adult C. moritziana plants during wet and dry seasons in Páramo de Piedras Blancas at 4,200 m a.s.l. Adult plants maintained higher leaf water potentials (??_L) during the wet season, however, no differences between stages were found for the dry season. Minimum dry season ??_L were never near the turgor loss point in any of the stages. Juveniles show a more strict stomatal control during the dry season to maintain a favorable water status. Net photosynthesis significantly decreased in intermediate and juvenile stages from wet to dry seasons. Our results suggest that C. moritziana resists more extreme conditions compared to other Andean giant rosettes