Showing papers on "Productivity (ecology) published in 1978"

Food-chain relationships in subtidal silty sand marine sediments and the role of meiofauna in stimulating bacterial productivity

Abstract: From bibliographic data the biomass correlations (organic dry weight) are constructed for the subsurface layer of a hypothetical 30 m deep silty sand station: 200 μg/ml macrofauna (including 120 μg/ml subsurface deposit feeders), 50 μg/ml meiofauna, 20 μg/ml Foraminifera, 1 μg/ml Ciliata and Flagellata, and 100 μg/ml bacteria. ATP-biomass is discussed.Meiofauna and Foraminifera contribute with 30 and 12% to the living biomass in the sediment, and it is assumed that their contribution to the food of deposit-feeding macrofauna is of a similar percentage. This is corroborated by productivity estimations.Bacteria are the main food of deposit feeding macrofauna, meiofauna, and microfauna. From different calculations it becomes evident that the productivity of bacteria in the sediment is far below figures achieved in experimental cultures: the conclusion is that sediment bacteria, in general, do not live under good environmental conditions.A rather large part of the bacterial population in the sediment seems to be in the stationary phase of life, and only a fraction of the total population exhibits high metabolic rates and rapid duplications. Only these active bacteria are of importance for the breakdown of relatively refractive organic matter in the sediment.In soft bottom marine sediments where the input of organic matter is higher than the remineralization rate, benthic animals stimulate by their activities and by nutrient cycling the decomposition of detritus via bacteria. Though meiofauna, in principle, feeds upon the same food resource as macrofauna, there is no real competition for food, because meiofaunal animals by their activities and by excreting metabolic end products induce a bacterial productivity which would not be there without them, and feed on it. There are a few examples where more specialized interactions exist between benthic animals and bacteria; these interactions have been termed "gardening". They could be highly important in the benthic ecosystem.

Production Ecology of British Moors and Montane Grasslands

Abstract: I. The Moor House Programme.- 1. Introduction and Site Description.- 2. Field Estimates of Primary Production.- 3. Physiological Aspects of Bog Production at Moor House.- 4. Ecology of Moorland Animals.- 5. Microbial Populations in Peat.- 6. Nutrient Availability and Other Factors Limiting Microbial Activity in the Blanket Peat.- 7. A Study of the Rates of Decomposition of Organic Matter.- 8. A Simulation of Production and Decay in Blanket Bog.- 9. A Model of Peat Bog Growth.- 10. The Blanket Bog as Part of a Pennine Moorland.- II. Supporting Studies-Dwarf Shrub Communities.- 11. The Productivity of a Calluna Heathland in Southern England.- 12. Production in Montane Dwarf Shrub Communities.- 13. Heather Productivity and Its Relevance to the Regulation of Red Grouse Populations.- III. Snowdonia Grasslands.- 14. Snowdonia Grassland: Introduction, Vegetation and Climate.- 15. Physiography, Geology and Soils of the Grassland Site at Llyn Llydaw.- 16. Primary Production, Mineral Nutrients and Litter Decomposition in the Grassland Ecosystem.- 17. The Role of Slugs in an Agrostis-Festuca Grassland.- 18. Sheep Population Studies in Relation to the Snowdonian Environment.- 19. The Grazing Intensity and Productivity of Sheep in the Grassland Ecosystem.- 20. The Distribution and Transfer of Energy and Nutrients in the Agrostis-Festuca Grassland Ecosystem.- References.

Breeding Patterns of Galápagos Penguins as an Indicator of Oceanographic Conditions

Abstract: Surface water changes associated with El Nino have been known to affect deleteriously top carnivores along coastal South America. Data on the breeding strategies of Galapagos penguins and other seabirds indicate that the biological effects of El Nino extend much farther west. The breeding biology of these seabirds is adapted to frequent changes in productivity which are associated with El Nino.

Primary Production Processes in Arctic Bryophytes at Barrow, Alaska

Abstract: Mosses represent an important growth form in arctic areas where they contribute significantly to the production, biomass, and cover (Wielgolaski, 1972; Webber, 1974, 1978; Rastorfer, 1978). For example, bryophytes comprise 91% of the aboveground biomass in a sedge-moss meadow in Western Taimyr, and an average of 38% of the aboveground biomass for all IBP tundra sites reported by Wielgolaski (1972). In drier tundra areas, however, moss cover drops substantially (Wielgolaski and Webber, 1973). Despite their high levels of cover and biomass, aboveground productivity in arctic bryophytes is generally considerably lower than in vascular plants. Wielgolaski (1972) reports a range of aboveground production for bryophytes from 1 to 50 g dry wt m-2 yr -1 for numerous IBP tundra sites. This compares to values of 9 to 382 g dry wt m-2 yr-1 for vascular plants. In select antarctic areas, however, production of bryophytes may reach 1000 g dry wt m-2 yr-1 (Clarke et al., 1971).

Biomass and production in three contiguous forests in northern wisconsin

Abstract: Biomass and net primary production were measured in 3 forest communities in north- ern Wisconsin comprised mainly of Populus tremuloides, Betula papyrifera, Acer rubrum, and Acer saccharum. The estimated total aboveground biomass and production for living plants in the 3 stands, designated as (1) aspen, (2) aspen-maple-birch, and (3) maple-birch-aspen types, were 94.8, 95.6 and 119.0 t/ha (= 103 kg/ha) and 10.43, 8.45, and 8.69 t*ha-1tyr-1, respectively. Shrub and ground vegetation accounted for minor portions of the total standing crop. The tree:shrub:ground biomass ratios were 1:.019:.015 for aspen, 1:.009:.006 for aspen-maple-birch, and 1:.003:.006 for maple-birch-aspen. The contribution to stand production by shrubs and ground vegetation ranged from 17% of the total aboveground production in the aspen type to 9W in the other stands. Inputs to the production equation often neglected in stand analysis, i.e., plant losses by death, shedding, and consumption, amounted to a maximum of 4.5% of total aboveground production. A comparison of biomass data from forests in the lake states suggests that 100 to 200 t/ha (above- ground dry weight) is inclusive of most second-growth forests. The upper and lower bounds for mature forests were projected to be -600 and 60 t/ha. Estimates of aboveground productivity for a variety of forest ecosystems within the lake states had a narrow range from 7.1 to 10.4 t ha-1 year-'.

Productivity and production efficiency of an upper sonoran desert ephemeral community

Abstract: During years with good winter rainfall, ephemeral plants can contribute considerably to primary production in the upper Sonoran Desert. This study was designed to compare ephemeral community productivity and production efficiency within the different microhabitats created by trees, shrubs and open spaces in a Sonoran Desert ecosystem. Ephemerals were periodically harvested in the different habitats and dried for biomass and caloric determinations. The caloric data were compared to total solar input into the different microhabitats in order to determine production efficiencies of each stand. Productivity rates were also determined for the total community for the sample period, winters of 1972-74. Both productivity and production efficiency were highest under the shade of Cercidium. The highest production efficiency occurring in the shaded sites was 5.03% which approaches the theoretical maximum photosynthetic efficiency level. The production efficiency of the entire ephemeral community for the whole growing season was only 0.17% during an optimal year, demonstrating the influence of the environment in the interspaces on productivity and solar conversion in the desert. THE DESERT ecosystem is often considered to be bleak and unproductive when compared to more mesic or agricultural systems. This judgment is quite true if one measures productivity during long periods of drought; however, moderate changes in rainfall enhance desert productivity, while long periods of moisture input dramatically increase productivity of all desert plants. In the Sonoran Desert the "normal" periods of rainfall follow a bimodal pattern, occurring as convective thundershowers in the summer and long cyclonic storms in the winter. In the upper Sonoran Desert in central Arizona, the winter storms are the primary stimulant of primary productivity, much of it in the form of winter ephemeral plant growth. Although all desert plants respond to moisture input, the ephemerals, composed primarily of annual plants, are triggered by rainfall (Went, 1949) and are usually productive only for a very short period of time (Beatley, 1967). Production of desert annual plants has been shown to be closely related to amounts of precipitation (Norton, 1974).

Primary production ecology in southern Africa

Abstract: ‘Perhaps the most fundamental dimension of an ecosystem is its productivity — the rate of creation of organic material, by photosynthesis primarily, per unit area and time’ (Whittaker 1970). Of importance are the structural parameter, biomass, and the functional parameter, production (Odum 1962), both essential in the process of ecosystem analysis (Reichle 1975). Economic requirements for increased food and structural material production have long drawn attention to the importance of understanding and optimizing primary production within different ecosystems.

Organic matter production in rice field flood water

Abstract: To assess the contribution of organic matter produced in the flood water to the fertility of a rice soil, the primary productivity and the algal biomass therein were examined throughout one crop. Primary productivity was estimated from the diurnal curve of dissolved oxygen. Just after transplanting, an algal bloom developed due to fert,ilizer or ploughing or both. After submerged weeds occupied the whole paddy no distinct algal growth was found. At the ripening stage, the rice plant canopy suppressed the growth of aquatic plants. Benthic algal biomass did not change much throughout the crop period. The standing crop of algae ranged from 2 to 114 kg/ha by fresh weight, while the maximum standing crop of submerged weeds (Najas sp., Chara sp.) was 400 kg/ha by dry weight. The primary productivity of the flood water community was high (0.6-3.3 g 0 1 m-2day-l) and equivalent to productivity values in eutrophic lakes. The total gross primary production of the flood water community during the cropping p...

Growth, Mortality, Fecundity, Biomass and Productivity of Four Lake Populations of the Prosobranch Snail, Viviparus Georgianus

Abstract: Studies were carried out on natural populations of the freshwater prosobranch snail, Viviparus georgianus, located in 4 mesotrophic lakes in upstate New York Viviparus are among the dominant members of the benthic fauna at the second trophic level in these localities A growth survey involved regular sampling for measurements of growth, fecundity, mortality, and biomass production Viviparus georgianus were variable in ♀ life span, with 2 populations having a 36—mo maximum life and the remaining 2 populations having a 28—mo maximum life Maximum life span of ♂ ♂ was similar in all populations at 21—22 Analyses of growth and biomass production as organic carbon and total nitrogen were utilized to estimate standing crop biomass leading to productivity rates, and of changing C:N ratios during growth and reproduction Productivity ranged from 0911 to 8303 mg C/(m2°day) The ♀ portions of the populations were °5x as productive as the ♂ portions The lower life span, growth rates and productivity of ♂ ♂ are believed to be linked to increase ♂ mobility during the breeding period, with subsequent decrease in filter—feeding during spring primary productivity Reproduction in V georgianus was iteroparous and vivaparus, with a gestation time of °9 mo Fecundity varied seven—fold between the populations, with reproductive output paralleling the productivity rate Fecundity of ♀ ♀ approximates a 1:3:2 ratio for 1—, 2— and 3—year—old ♀ ♀ respectively Reproductive output with respect to ♀ age is discussed

Production and energy flow in the macrobenthos of two sandy beaches in South West India

Abstract: Estimates of the rates of secondary production by the individual macrofaunal invertebrates of two Indian beaches are presented and these are combined to give an estimate of the energy flow through the macrobenthos of the two beaches. The production by individual species and by the macrofauna as a whole is compared with that of other tropical, and of temperate beaches, and it is concluded that a similar biomass in the tropical beaches produces a rate of turnover of biomass some ten times that of the temperate beaches. Values of production to biomass ratios (P/B) and of elimination to biomass ratios (E/B) for the invertebrates of sandy substrata in various geographical areas are reviewed and the use of these ratios in comparisons of productivity, and their relationship to mortality and longevity are discussed.

Factors affecting respiration and photosynthesis by the benthic community of a subtical siliceous sediment

Abstract: In a shallow, subtidal, siliceous sediment, benthic microalgal biomass (μg chlorophyll a cm-3) is influenced by light and physical sediment dynamics. The microalgal community is relatively dense, despite adverse conditions (7.0 μg chlorophyll a cm-3), and is able to respond rapidly to favorable conditions. Productivity of this community is significantly correlated (P≤0.05) with benthic light. In addition, productivity is influenced by temperature and bottom water NH4+ and PO4-3 concentrations, especially as the concentrations fall to levels approaching the Ks (halfsaturation constant) of the microalgal community. Metabolic activity in this environment is dependent upon a continuous supply of organic carbon. Temperature is significantly correlated with respiration rate, but other factors (e.g. biomass and organic matter supply) are important also. Community respiration responds to overlying phytoplankton productivity in the same manner as deep-water benthic environments. Bacterial enumeration using CFU (colony-forming units) does not measure accurately the number of in situ metabolically active bacteria.

Primary productivity of emergent macrophytes in a Wisconsin freshwater marsh ecosystem

Abstract: In 1972 primary production of various emergent macrophytes was examined in Theresa Marsh, a shallow, semimanaged impoundment in southeastern Wisconsin. Dominant macrophytes included Typha latifolia, Scirpus fluviatilis, Carex lacustris, Phalaris arundinacea and a shrub, Salix interior. Seasonal patterns of production as well as total production varied greatly among species. With estimates for litter loss and belowground production, annual net primary production ranged from 1181 g/m/sup 2//year for Carex lacustrris to nearly 3200 g/m/sup 2//year for Typha latifolia. Peak standing crop values were generally among the highest reported. Average productivities during the growing season, however, were relatively low, ranging from 6.31 to 10.52 g/m/sup 2//day for aboveground standing crops. Primary production was also estimated for transient species that occurred on mud flats following a marsh drawdown. The high primary production within the marsh appears to be based on high nutrient levels as indicated by the marsh water and soil chemistry.

Diatom productivity compared to other algae in natural marine phytoplankton assemblages1,2

Abstract: Total productivity rates and the component attributed to the diatom proportion in the crop were measured in natural water samples taken from the Scripts Pier (La Jolla, California) at ca, monthly intervals in 1975–76. Two other measurements were done in water samples taken from Saanich Intel, Vancouver Islands, British Columbia (Canada). The percent of photosynthesis by diatoms was compared with the proportion of diatom carbon in the crop. In all instances, percent diatom photosynthesis was higher than percent diatom carbon for dialoms were higher than those of other algae. It is hypothesized that, in the sea, diatoms can outcompete other algae. However, the assemblages consist of some tens of species coexisting, and no portion of the crop out competes the others to exclusion.

Primary Production, Mineral Nutrients and Litter Decomposition in the Grassland Ecosystem

Abstract: Measurements of seasonal changes in the standing crop of live vegetation are adjusted for transfers to herbivores, standing dead litter, and for litter decomposition, to estimate primary production of the grassland. The dynamics of nutrient concentrations are related to phenology of the grasses, and spatial variation in productivity to species composition, soil chemistry and intensity of sheep grazing.

Effect of slash and soil removal on the productivity of second rotation radiata pine on a pumice soil

Abstract: The effects of windrowing and skid site formation on site quality were examined in a 7-year-old, second rotation Pinus radiata stand growing on a yellow-brown pumice soil. Four site types were recognised — normal cutover, windrow, inter-windrow and skids. Standing volumes, after first thinning to 573 stems/ha, were 34.3, 40.7, 20.5 and 5.2m 8 /ha respectively. Relative to the normal cutover the overall windrowed area contained 7m 3 /ha less volume or the equivalent of a reduction in Site Index (mean top height in metres at age 20 yrs) of 2 m. Analysis of current season's foliage showed that N and B concentrations were lower on both skid and inter-windrow sites than on the windrow and normal cutover sites. Magnesium concentrations on the inter-windrow site were signifi­ cantly lower than those on all other sites. Levels of total N and exchangeable Mg were lower in the skid site and inter-windrow soil profiles than in the normal cutover profile. Using regressions of soil depth on total soil N it was calculated that c. 2.5 and 26 cm of soil had been scalped off the inter-windrow and skid sites respectively.

Differences in productivities between the Great Australian Bight and the Gulf of Carpentaria, Australia, in summer

Abstract: Phytoplankton standing crop (chlorophyll a) and primary productivity were recorded, and zooplankton biomass was estimated in the two large bays of Australia, the Great Australian Bight on the south coast (December, 1965) and the Gulf of Carpentaria on the north coast (December, 1968). In the Gulf of Carpentaria, the phytoplankton standing crop (average, 27.3 mg chlorophyll a m-2) and primary productivity (average, 133.1 mg C m-2 h-1), as well as zooplankton biomass (average, 305.3 mg wet weight m-3) are much higher than in the Great Australian Bight (12.1 mg chlorophyll a m-2, 18.2 mg C m-2 h-1, 7.1 mg wet weight m-3, respectively). The unexpectedly low productivity values in the Great Australian Bight are attributable to environmental conditions of this bay, which obtains neither replenishment of nutrients from the land nor receives upwelling of deep water.

Land floras: the major late phanerozoic atmospheric carbon dioxide/oxygen control.

Abstract: Since at least the late Mesozoic, the abundance of terrestrial vegetation has been the major factor in atmospheric carbon dioxideloxygen fluctuations. Of modern ecosystem types occupying more than 1 percent of the earth's surface, productivity/area ratios of terrestrial ecosystems (excepting tundra and alpine meadow, desert scrub, and rock, ice, and sand) exceed those of marine ecosystems and probably have done so for much of late Phanerozoic time. Reduction of terrestrial ecosystems during marine transgression would decrease the world primary productivity, thus increasing the atmospheric carbon dioxide concentration and decreasing the oxygen concentration. Regression would produce opposite effects.

The Grazing Intensity and Productivity of Sheep in the Grassland Ecosystem

Abstract: Within the Agrostis-Festuca grassland of the main site, the variation in numbers of sheep, their food consumption, egestion and production are described as part of the analysis of the relationship between primary production and herbivores.

Primary Productivity of a Hypersaline Antarctic Lake

Abstract: Although viable unicellular green algae are found in Deep Lake (a perenially ice-free hypersaline lake in Antarctica, 68o 34'S., 78o 11'E.), attempts made throughout a year to detect primary production using 14C failed. It is concluded that the rate of production in the limnetic zone does not exceed 0.15 mg C m-3 h-1. Only one positive measurement of carbon fixation was obtained: a sample collected near the bottom after the sediments had been disturbed gave a value of 0.18 mg C m-3 h-1. It was calculated that the upper limit of phytoplankton production was 3.3 g C m-1 year-1 and that total primary production could not exceed 10 g C m-2 year-1. Thus Deep Lake is one of the least productive lakes yet recorded. The combined effects of nutrient deficiency, hypersalinity, low temperatures and annual extremes in the availability of light in restricting the species diversity, population and productivity are stressed. One experiment in the nearby freshwater Watts Lake indicated a primary productivity in the limnetic zone ranging from 0.20 to 0.94 mg C m-3 h-1.

Seasonal dynamics of epiphytic algal growth in a marsh pond: productivity, standing crop, and community composition

Abstract: The seasonal growth of epiphytic algae colonizing artificial cellulose acetate substrates positioned in a stand of Scirpus acutus Muhl. and in a zone of Potamogeton pectinatus L. in a marsh pond was quantified in terms of 14C photosynthetic uptake, cell volume, cell surface area, dry weight, and chlorophyll a, protein, carbohydrate, and lipid content. Standing crop and productivity increased at both sites in September and October, after generally low summer growth with the exception of the occurrence of heterocystous blue-green algae at the Potamogeton site in July. Factor analysis of interrelationships among the various parameters suggested that cell surface area was more directly related to productivity and various standing-crop parameters than was cell volume.

Photosynthesis and respiration of Spartina and Juncus salt marshes in North Carolina: Some models

Abstract: In situ ecosystem gas exchange measurements were taken monthly for a 15-month period during low tide forJuncus roemerianus Scheele and short, medium, and tall height forms ofSpartina alterniflora Loisel. in three salt marshes near Southport, North Carolina. Multiple regression analysis was used to obtain empirical relationships betwen gas exchange values and the physical and biotic variables measured. Preliminary models for net ecosystem photosynthesis, ecosystem respiration, and respiration of aboveground standing crop were developed. Validation of models was carried out in the following manner: (1) net primary productivity of aboveground standing crop was calculated from model data and compared with harvest estimates of net primary productivity for the same year; and (2) carbon exchange and energy efficiency values were compared with literature values. In general, theSpartina models, because of their larger data base, were more useful than theJuncus models. Annual primary productivity of aboveground standing crop calculated from gas exchange values was 1.8 to 3.6 times greater forSpartian than values calculated from harvest data by Smalley's method.Juncus values were approximately equal. Since values calculated from carbon data and harvest data are not entirely equivalent, required adjustments to the carbon values are discussed. Both species have two growth periods—one in ths pring and one in the fall—with the spring growth in excess of that in the fall. Net ecosystem productivity was highest in the spring and lowest in the summer. Ecosystem respiration amounted to 71% of gross annual photosynthesis. The aboveground standing crop contributed approximately 35% to this ecosystem respiration and the soil, the remaining 36%. Efficiency for gross photosynthesis per unit of solar radiation ranged from 0.22 to 1.11%. Efficiencies for net primary production of aboveground standing crop ranged from 0 to 0.82% and those of net ecosystem production from 0 to 0.56%.

Regeneration of Nitrogen by Zooplankton and Fish in the Northwest Africa and Peru Upwelling Ecosystems

Abstract: The availability of nutrients and light constitutes the dominant controlling factors of the levels of primary production in the ocean. In the lower latitudes where most coastal upwelling areas are located, the amount of light is seldom below the critical level to inhibit productivity, so nutrients are often the limiting factor in phytoplankton growth (Koblentz-Mishke et al., 1970). Nutrients utilized in primary productivity are derived from two sources in upwelling areas. Nutrients are introduced to the euphotic zone from the depths by the physical processes that create upwelling, and nutrients are recycled by biologic organisms that inhabit the area. Nitrate introduced into the euphotic zone by upwelling supports “new productivity”, while ammonium and other excretory products regenerated by zooplankton and nekton support “regenerated productivity” (Dugdale and Goering, 1967).

Phytoplankton Primary Productivity and Population Efficiency Studies in a Prairie‐Parkland Lake Near Edmonton, Alberta, Canada

Abstract: The standing crop and primary productivity of a small eutrophic, prairie-parkland lake were measured. In general, both standing crops and primary productivity were large, 29.4 and 73.09 mg chlorophyll a m−3 and m−2 and 78.71 and 196.77 mg C hr −1m−3 and m−2 respectively. Productivity decreased with increasing depth, therefore, decreasing light intensity. Relations between productivity and chlorophyll a content, productivity and light intensity, phytoplankton productivity efficiency and light intensity, productivity and water temperature were investigated, as was the photosynthetic index. Experiments designed to determine the photosynthetic capacity of the phytoplankton distinguished between actively growing and senescent populations. The latter were present during the winter ice cover.

Seasonal patterns of co2 and water vapor exchange of juncus roemerianus scheele in a georgia salt marsh

Abstract: CO2 and water vapor exchange studies of intact plants of black needle rush (Juncus roemerianus Scheele) were conducted in an undisturbed marsh community on Sapelo Island, Georgia. The seasonal patterns of the light and temperature responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration were determined five times over the year. Internal resistances to CO2 uptake were also evaluated. Net photosynthesis was highest in early spring, but declined only slightly through the year. A distinct and moderate temperature optimum of net photosynthesis was observed with decreasing rates above 30 C. Leaf conductances to water vapor were similar at all seasons and were high at cooler temperatures and decreased with increasing temperature. Transpiration was relatively high and constant during all seasons. The water-use efficiency of photosynthesis was high below 25 C, but decreased sharply above that temperature. Dark respiration was relatively low. Seasonal changes reflected changes in leaf density. Decreasing stomatal conductances and increasing respiration rates reduced net photosynthesis at higher temperatures. The stomatal resistance increased and internal resistances to CO2 uptake decreased over the year, but the total resistance remained constant. The internal resistance to CO2 uptake was consistently higher than the stomatal resistance. These seasonal response patterns show that J. roemerianus is well adapted to the seasonal changes in ambient temperature and irradiance and other microenvironmental factors in the high marsh. These physiological characteristics permit this C3 species to maintain a high productivity in a seasonally hot and stressful environment. THE SALT MARSHES of the Atlantic and Gulf coasts of the U.S. are among the most highly productive natural ecosystems (Odum, 1961; Whittaker, 1975). These marshes are characterized by an assemblage of few species of vascular plants in distinct zonation patterns (e.g., Cooper, 1974). Juncus roemerianus Scheele is an important vascular plant species in salt marshes from New Jersey to north Florida on the Atlantic coast and from south Florida to Texas on the Gulf coast (Cooper, 1974; Eleuterius, 1976a). In the Georgia marshes, J. roemerianus generally dominates the upper or high marsh regions which are not flooded with each tidal cycle. There are several estimates of the productivity of J. roemerianus (e.g., Keefe, 1972; Williams and Murdoch, 1972; Gabriel and de la Cruz, 1974), and the high productivity of this species in the Georgia marshes (2,156 g DW m-2 yr-t) may contribute significantly to the overall productivity of the 1 Received for publication 5 July 1977; revision accepted 6 March 1978. Research was supported in part by the National Sea Grant Program, U.S. Dept. of Commerce, Grant No. 04-5-158-4, and by the National Science Foundation, Grant No. DEB7706632. The assistance of A. E. Antlfinger, Dr. J. L. Gallagher, and the staff of the University of Georgia Marine Institute is gratefully acknowledged. Contribution No. 357 from the University of Georgia Marine Institute, Sapelo Island, Georgia 31327. marsh ecosystem of Georgia (Gallagher et al., 1978). However, there is little information concerning the physiological ecology of this species. Gallagher (1975) applied a single pulse of ammonium nitrate to J. roemerianus in a Georgia salt marsh and found no significant changes in live or dead aerial biomass or elemental composition either 10 weeks or 1 year after treatment. He suggested thatJ. roemerianus was growing near the asymptote of its nitrogen response curve. Rozema (1976) studied the responses of four European Juncus species to a range of salt concentrations in solution culture and found differences in the response to salinization as well as different physiological mechanisms by which the adaptations were achieved. This study was designed to begin investigations on mechanistic explanations for the variations in productivity and distribution of J. roemerianus along environmental gradients in the southeastern salt marshes. The seasonal temperature and light responses of photosynthesis, respiration, transpiration, leaf conductance, CO2 diffusion resistance and water-use efficiency measured on in situ J. roemerianus plants were investigated in this study. The daily patterns of photosynthesis and transpiration and estimates of productivity extrapolated from these gas ex-

The limnology of Cabora Bassa, Moçambique, during its first year

Abstract: SUMMARY. Cabora Bassa, a new impoundment in the Middle Zambezi River, was closed in December 1974, filling rapidly to 12m below full supply by May 1975. The lake, with five basins, lies along a west–east axis almost parallel to prevailing south-easterly winds. Limnological records, from several stations along the lake axis, indicated homoiothermal conditions until September, the hot dry season. From then until records ceased in December, thermoclines and oxyclines were present in the deeper eastern basins. Hydrogen sulphide was recorded in hypolimnial waters towards the end of the year. Open water of the shallow, wind- and wave-swept eastern basins remained oxygenated throughout the year. Water transparency increased fivefold from April to December, increasing too from east to west. pH was alkaline becoming more acid in cool winter months and in bottom waters of stratified basins. Conductivity ranged from 95 to 119 °S cm−1 with higher values in shallow lake margins. Chemical content of surface waters showed little seasonal or spatial variation and was, in general, comparable with pre-impoundment levels. Duration of the initial ‘productive phase’ will probably be shorter than Kariba due to more rapid lake water exchange but indications are that productivity will be higher in the maturation phase.