Showing papers on "Productivity (ecology) published in 1988"

Sea ice microbial communities (SIMCO)

Abstract: Sea ice microbial communities (SIMCO) grow luxuriantly within several microhabitats of sea ice, indicating that the microorganisms comprising these communities are well adapted to the physicochemical gradients which characterize sea ice. We used SIMCO obtained from the bottom of congelation ice in McMurdo Sound, Antarctica, to test the hypothesis that low temperature limits microbial productivity in polar oceans and also to investigate the effect of salinity on rates of autotrophic and heterotrophic metablism. Substantial rates of carbon fixation, incorporation of thymidine, and uptake of glutamate occurred at the in situ temperatures of-1.9°C, with maximum rates at temperatures considerably warmer but below 15°C. Microalgae and bacteria of SIMCO are thus indicated to be psychrophiles. The relative rates of autotrophic and heterotrophic microbial growth (based on rates of fixation of 14CO2 by microalgae and incorporation of 3H-thymidine by bacteria, respectively) were similar and overlapped from 4° and 7°C. These data suggest that a recent hypothesis proposing the uncoupling of primary production and bacterial production in cold water, due to differential growth of phytoplankton and bacterioplankton at low temperatures, is refuted with respect to SIMCO. Maximum rates of carbon fixation by autotrophs of SIMCO occurred at salinities which characterized the ice from which the SIMCO were collected. In contrast, heterotrophs of SIMCO exhibited a more stenohaline response to variable salinity, with maximum incorporation of thymidine and uridine from 20‰ to 30‰. Adaptations by autotrophs and heterotrophs of SIMCO that permit substantial metabolism and growth at very low temperatures and variable salinities are significant when considering production and trophodynamics in polar oceans. Actively growing microorganisms in these unique communities contribute to overall production in polar oceans, provide carbon for food webs associated with sea ice, and upon release from melting ice may contribute to microbial blooms in marginal ice edge zones, which in turn support cryopelagic food webs.

Trophic Relations and Ontogenetic Niche Shifts in Aquatic Ecosystems

Abstract: One of the most pronounced and consistent patterns in freshwater lakes is that the biomasses of producers and consumers increase along a gradient of increased nutrient (phosphorous) loading. For example, phytoplankton biomass is positively correlated with total phosphorus concentration (Stockner and Shortreed 1985 and references therein), as is the biomass of Zooplankton (Hanson and Peters 1984; Pace 1984), and fish (Hanson and Leggett 1982). Given that the biomass of aquatic producers, herbivores, and carnivores each positively covaries with phosphorus loading, the abundances of adjacent trophic levels should also be positively correlated across productivity gradients. Indeed, such positive correlations between consumers and their resources are common; i.e., Zooplankton biomass is positively correlated with phytoplankton biomass (references in McQueen et al. 1986), plantivore density is positively related to Zooplankton biomass (Mills and Shiavone 1982), and fish density positively covaries with macrobenthos biomass (Hanson and Leggett 1982; Nakashima and Leggett 1975).

The annual cycle of sedimentation in Saanich inlet, British Columbia: implications for the interpretation of diatom fossil assemblages

Abstract: Data derived from monthly sediment traps at three depths near the head and mouth of Saanich Inlet yield a detailed record of the seasonal cycle of production and vertical flux to the sediments. The material is primarily composed of diatom frustules and silt-to-clay-sized lithic fragments, with dinoflagellates and naked algae present in summer samples. Diatoms dominate from April to September while clastics dominate from October to March. Particles occur primarily in loose flocs; pellets are common in early winter and in summer material, and consist of an indiscriminant mixture of the same particles seen in co-occurring flocs. The seasonal succession of taxa is similar at both sites and is transported to the sediment with little modification by dissolution. Carbon flux is a poor indicator or productivity, due to the strong effect of recycling in the surface waters during spring and uncertainty as to the source of the carbon accumulating in the traps. Although diatom flux is an adequate indicator of primary production, transforming each taxon to equivalent cell volume yields a more accurate picture of the seasonal cycle of production. Relative abundance (taxon percentage) data can be as useful as, and in some cases more accurate than, absolute flux (number per unit time), while number-per-gram may be highly misleading as an estimator of production. Lateral advection and benthic resuspension affect monthly data and annual valve-flux data, but appear to have no net effect upon annual percentage data.

Thalassia testudinum productivity and grazing by green turtles in a highly disturbed seagrass bed

Abstract: There has been an historical decline in the seagrass beds in Maho and Francis Bays, St. John, U.S. Virgin Islands: presently (1986) there are only five small seagrass beds in shallows water. These seagrass beds are highly disturbed by heavy boat usage and are intensively grazed by the green turtle Chelonia mydas L. Fifteen to 50 boats anchor each night in the bays: anchor scars cause a loss of up to 6.5 m2 d-1 or 1.8% yr-1 of the seagrass beds. Seagrasses regrew into such scars only minimally within a period of 7 mo. The size of the green turtle population was estimated at 50 subadults and their feeding behavior was determined by direct observation and radiotelemetry. The behavior of the green turtles differed from other observations published on the species. Here, the turtles grazed all available Thalassia testudinum, their preferred seagrass food, rather than creating discrete grazing scars, and spent all their waking hours (9 h per day) feeding. Areal productivity of T. testudinum leaves (33 to 97 mg dry wt m-2d-1) in the bays was at least an order of magnitude lower than published values or than the productivity of another, lessdisturbed seagrass bed on St. John, despite having comparable leaf-shoot density. Leaf shoots were stunted, fragile, achlorotic, and had only two leaves as opposed to the five leaves per shoot more typically seen. The green turtle population was near the estimated carrying capacity of T. testudinum, based on the standing crop and productivity of T. testudinum and the grazing rate of the turtles. The effect of disturbance of T. testudinum from boats and turtles was assessed by excluding these with emergent fences. Within 3 mo of protection, the areal and shoot-specific productivity of T. testudinum leaves as well as leaf size increased significantly compared to unprotected areas. Conservation efforts are recommended in Maho Bays and Francis because seagrass productivity is low, disturbance rates are higher than recovery rates, the turtles cannot increase further their feeding rate in order to compensate for such factors, and there are few alternate sources of T. testudinum on the north shore of St. John.

Comparative ecology of submersed grass beds in freshwater, estuarine, and marine environments'

Abstract: Worldwide, there arc 500-700 species of submersed angiosperms adapted to freshwater and estuarine environments compared with 50 species adapted to marine waters. In their evolution from freshwater ancestors, seagrasses have undergone extensive anatomical changes (e.g. reduction in floral and leaf structures, reduction of xylem tissue with a lacuna1 gas transport system), as well as physiological adaptations (bicarbonate utilization in photosynthesis). Seagrasses appear to have more annual production than do their freshwater counterparts because they develop greater standing crops and have the capacity to store photosynthetic products in extensive rhizome systems in the sediments. For example, maximum productivity of > 10 g C rnd2 d-* has been reported for tropical seagrass species (Cymodocea nodosa and Thalassia testudinum), but the maximum productivity of temperate freshwater species such as Myriophyllum or tropical freshwater species such as Hydrilla is usually 50% of their diets. Also, sea urchins (Diadema antilarum) consume plant material, creating bare halos around tropical patch reefs in the Caribbean Sea. It is difficult to generalize regarding brackish submersed aquatics in estuaries because their coverage is variable due to light limitation and algal overgrowth from eutrophication. Freshwater macrophytes seem rarely grazed by fish (except via exotic introductions of Tilapia or carp), but waterfowl use is often significant at the end of the growing season. Thus, trophic relations in freshwater macrophyte beds may be qualitatively different and much more pulsed than in seagrass systems, with more r-selection in lakes and more K-selection in marine environments.

Water column productivity attributable to displaced benthic diatoms in well-mixed shallow estuaries

Abstract: We attempted to determine the extent to which benthic diatoms contribute to water column primary productivity in shallow-water estuaries and to elucidate the primary mechanisms responsible for suspending the diatoms. A perliminary study conducted in Mugu Lagoon, California indicated that productivity of ocean water entering the lagoon during flood tides was often several orders of magnitude less than that of the same water mass about 3 h later. Benthic pennate diatoms displaced from the sediments into the water column accounted for the increase. A more detailed study was conducted in Barataria Estuary, Louisiana where, for one month, daily measurements were made of benthic and water column productivity and several other environmental variables. During the month, the relationship between water column and benthic primary productivity varied from strongly negative to weakly negative to positive. K-systems analysis indicated that factors comprised of wave height, meteorological tides, astronomical tides, and benthic productivity and standing crop accounted for the full range of variation in water column productivity. Benthic pennate diatoms, represented an average of 74% of the diatom taxa in water column samples. We conclude that the primary productivity of well-mixed shallow estuarine waters is often greatly aumented by displaced benthic algae.

On productivity as a predictor of rodent and carnivore diversity

Abstract: Tilman's model predicts an asymmetric relationship, with ascending, peak, and descending parts, between plant species diversity and a resource gradient. I evaluated the applicability of Tilman's model to mammals by regressing rodent and carnivore species diversity on estimates of net aboveground primary productivity. Rodent diversity was highest at low productivity levels and declined as productivity increased. Although my data did not confirm the upslope and peak phases of Tilman's model, this may be because I did not sample from areas of extremely low productivity. Published studies from very dry desert habitats indicate that rodent diversity increases rapidly across a narrow gradient of very low productivity and peaks at moderately low levels. This suggests that rodents, which are largely herbivorous, exhibit an asymmetric productivity-diversity curve similar to that predicted by Tilman for plants. The rodent data are also consistent with the intermediate disturbance hypothesis, which predicts diversity as a peaked function of a disturbance gradient. Rainfall predictability, which may function as a disturbance, is inversely related to my plant productivity gradient. The regression curve for carnivores helps to distinguish between these two hypotheses. At a given locality carnivores and rodents should be exposed to about the same disturbance levels and according to the intermediate disturbance model should peak at about the same place. Since carnivores as a whole are about one trophic level above rodents, if plant productivity controls diversity the peak for carnivores should occur at about an order of magnitude of productivity higher than the peak for rodents. My carnivore diversity data exhibit upslope, peak, and downslope phases and are shifted to higher levels of productivity by about one order of magnitude. This provides evidence against the intermediate distur- bance hypotheses and corroborates predictions of Tilman's model as applied to mammals.

Population structure and energetics of the shallow-water antarctic sea star Odontaster validus in contrasting habitats

Abstract: Individuals and populations of Odontaster validus Koehler differed markedly among different habitats, as revealed in a study from October 1984 through January 1986 in McMurdo Sound, Antarctica. At McMurdo Station, individual sizes (wet weight) and population biomass (g wet wt m-2 and kJ m-2) decreased significantly with increasing depth. Individuals from shallow (10 to 20 m) habitats were in superior nutritional condition to those from deeper water (30 and 165 m), as shown by higher gonad and pyloric cecum indexes, and by higher lipid and energetic levels in the pyloric ceca. Moreover, gonadal output (reproductive output) was higher in shallow-water individuals. Higher levels of chlorophyll in the pyloric ceca and richer yellow to red coloration of the body wall in the shallow-water individuals indicate that they utilize the higher levels of primary production at shallow depths. At East Cape Armitage, where nearly permanent, thick, snow-covered ice most of the year resulted in very low levels of benthic primary production, the lowdensity sea stars were all very small and nutritionally similar to the deep-water individuals at McMurdo Station. At Cape Evans, where the generally snow-free sea-ice that broke up in mid-summer resulted in a luxurient benthic cover of diatoms and macroalgae, the sea stars were smaller than at McMurdo Station at comparable depths, but population densities were higher, resulting in 4 to 9 times greater biomass. Growth rates of sea stars fed in the laboratory were very low, especially compared to laboratory-reared temperate and tropical species; well-fed individuals need about 9 yr to reach 30 g wet weight, near the mean size of shallowwater individuals at McMurdo Station. No growth was detected in individuals caged at McMurdo Station for one year, suggesting even lower growth rates in the field. The stable size-frequency distributions at the different sites and depths throughout the year-long study suggest highly stable populations with low temporal variability in recruitment, migration and mortality. These data indicate that individuals and populations of O. validus quantitatively and qualitatively reflect the general level of productivity of a habitat. Differences noted in size, coloration, nutrition, and reproductive effort may be the result of long-term integration of local levels of primary production. These ubiquitous sea stars may serve as a biotic indicator of productivity in localized habitats around the continental shelf of Antarctica.

Productivity of the southern ocean: a closer look

Abstract: 1. 1. This paper reviews present knowledge of the standing crop and productivity of Antarctic phytoplankton and of the factors that govern primary production. 2. 2. Investigations have shown that (a) productivity of Antarctic waters varies by at least one to two orders of magnitude; and (b) generally, these waters display their richness mainly in the coastal regions. 3. 3. Recent studies of the autotrophic and heterotrophic pico- and nanoplankton are reviewed, and new data are presented. 4. 4. The relative abundances of these very small-sized classes of organisms are discussed, and their potentially important role in the emerging paradigm of the Antarctic food web is considered.

Long-term variability in North Sea zooplankton off the Northumberland coast: productivity of small copepods and analysis of trophic interactions

Abstract: All groups of meso- and macro-zooplankton in the North Sea off Northumberland, at a depth of 53 m, were studied during a 15-year period (1969–83); copepod productivity was estimated from biomass and growth rates. Phytoplankton were seasonally bi-modal with peaks in April and August–October; copepods were uni-modal peaking in June–July. The predatory zooplankters: larval fish, decapods, ctenophores, medusae (the summer-autumn predators) peaked between May and September, while chaetognaths and euphausiids (the winter predators) peaked in December–January. Copepods and the summer-autumn predators were seasonally and inter-annually positively correlated, and declined in abundance from 1974 to 1980. Euphausiids and chaetognaths on the contrary increased in abundance during these years, and were seasonally and inter-annually negatively correlated to the copepods. The mean annual abundance of copepods was positively related to the previous winter's minimum, and inversely related to the abundance of chaetognaths and euphausiids. Annual copepod productivity averaged 1260 kJ m-2 year-1, and showed no relationship to other groups of plankton.

The seasonal productivity cycle of phytoplankton and controlling factors in Lake Constance

Abstract: Annual phytoplankton productivity in Lake Constance is about 300 g C m−2, a value typical for mesoeutrophic lakes. Seasonal variations in phytoplankton biomass and productivity are exceptionally great because of a sequence of factors controlling the production process. During winter productivity is controlled by low energy inputs and high respiratory losses due to deep water column mixing. Biomass is low and water transparancy high. The spring phytoplankton growth is triggered by the thermal stabilization of the water column. The summer phytoplankton biomass maximum mainly depends on phosphorus availability. However, biomass yields comprise only 15–20% of values to be expected from the Redfield ratio because large proportions of POM are detritus and non-algal biota. Moreover, sedimentation during the second half of the year removes biomass from the euphotic zone. Water transparency and thus vertical distribution of algal photosynthesis is highly dependent on phytoplankton biomass. Self-shading causes considerably smaller seasonal variations in areal biomass and photosynthetic rates than in volume-based values. By light-shade adaptation effects of seasonal fluctuations in mean daily surface radiance fluxes on algal photosynthesis can to a significant extent be compensated for. At any given level of biomass daylength is the major determinant of daily production rates.

Phytoplankton production and seasonal biomass variation of seagrass, Ruppia maritima L., in a tropical Mexican lagoon with an ephemeral inlet

Abstract: Plankton metabolism andRuppia maritima biomass were measured seasonally during 1982–83 in El Verde Lagoon, a small coastal lagoon with an ephemeral inlet on the Pacific Coast of Mexico. Total net aquatic primary production was 521 g C m−2 y−1. The water column was slightly heterotrophic, with an annual P/R ratio of 0.89. Our analysis indicates that tropical and subtropical coastal lagoons with restricted or seasonal inlets have generally higher net aquatic primary productivity levels than lagoons with permanently open inlets. We hypothesize that this is due to retention of nutrients and plankton stocks during the dry season. The seasonal pattern of water column metabolism was related to rainfall and riverflow, with higher values generally occurring during the wet season. Net production and respiration were about three times lower during the 1982 dry season as compared to the 1983 dry season which received considerable rains due to abnormal climatic conditions. The biomass ofR. maritima ranged from zero to 620 g dry wt m−2. Growth occurred only during the dry season and there were two distinct biomass peaks representing two separate crops. The second crop was heavily epiphytized with nitrogen-fixing algae. There was an apparent succession in dominance of water column productivity over the year, withRuppia dominating during the dry season and phytoplankton more important during the wet season.

Benthic diatom biomass, production and sediment chlorophyll in langebaan lagoon, South Africa

Abstract: Direct measurements were made of the biomass of benthic diatoms in Langebaan lagoon, a sheltered lagoon on the west coast of South Africa. Highest biomasses occurred in the most sheltered areas and reached 18·9 g m−2 in the top 30 cm of sediment. A large biomass was found even at depths of 10–20 cm and 20–30 cm, often exceeding that in the 0–10 cm depth range. This contrasts with chlorophyll estimates of biomass, in which chlorophyll-a declines rapidly below the photic zone. Very high sediment turnover rates by benthic macrofauna account for the presence of microalgae at depths of 30 cm below the sediment surface. Currents or waves (and hence particle size) are inversely correlated with diatom biomass. Chlorophyll-a concentrations in the surface layers of the sediment may reach 322 μg Chl-a g−1 sediment, but generally values range from 0 to 34 μg Chl-a g−1 sediment. Like diatom biomass, chlorophyll-a concentration increases with declining waves and currents. The highest chlorophyll-a concentrations occur in the top 1 mm of sediment, declining to 35% of surface concentrations 5 cm below the surface. Rates of carbon fixation by benthic microalgae in the top 5 mm of sediment were 17·38 mg C m−2 h−1 at an exposed sandy beach, and 69·54 mg C m−2 h−1 at a sheltered sand/mud beach. Benthic microfloral production rivals phytoplankton production and ranges from estimates of 63 g C m−2 year−1 (sand) to 253 g C m−2 year−1 (sand/mud). In the lagoon ecosystem, production by benthic microalgae amounts to 22% of the total primary production of carbon.

The role of habitat and productivity in structuring desert rodent communities

Abstract: In deserts a humped relationship seems to exist between mean annual rainfall, a measure of productivity, and species diversity of desert rodents. It has also been established that the diversity of desert rodents is affected by and correlated with the structure of the habitat. However, no attempt has been made so far to integrate the influence of both structure and productivity on species diversity. It was found that the structure of the habitat, measured as % perennial cover is significantly and negatively correlated with species diversity. A significant positive correlation exists between the productivity of the habitat, measured as % annual cover, and rodent diversity. Three hypotheses were tested: a) rodent diversity is determined only by productivity, b) structure enhances coexistence, and c) rodent diversity is determined by the area. The results of stepwise multiple regression rejected the first and third hypotheses. Both the structure and the productivity of the habitat are important factors in determining the diversity of rodents. It is still unknown to what extent structure and productivity interact to enhance new mechanisms of coexistence.

Interannual Variability in Estimated Primary Productivity of the Antarctic Marginal Ice Zone

Abstract: The significance of marginal ice zones as loci for primary productivity has been firmly established in recent years, yet little data exist on the variations within these areas over long time periods. Interannual variations in primary productivity of the marginal ice zone were calculated by combining actual field data on primary productivity, remote sensing data on ice position and concentrations, and a conceptual model of the dynamics of ice-edge phytoplankton blooms. The Ross Sea, the Weddell Sea, and the entire Southern Ocean were analyzed for the years from 1973–1986. Interannual variations in primary productivity within the marginal ice zone are shown to be substantial, with the maximum productivity being 50% greater than the minimum. In view of the importance of the marginal ice zone as a site of biological production and its temporal variations, an understanding of the causes of interannual variability of the ice-edge location and of biological processes within marginal ice zones is needed to assess the impact of these time/space variations on the food webs of the Southern Ocean.

Effects of Different Patterns of Supplemental Water and Nitrogen Fertilization on Productivity and Composition of Chihuahuan Desert Annual Plants

Abstract: The effects of supplemental water supplied as large events, 25 mm per month, or as frequent small events, 6 mm per week, and available nitrogen on density, aboveground biomass and species composition of an annual plant community were studied in the northern Chihuahuan Desert. The amendments of water and N had no effect on total annual plant density. However, supplemental water resulted in increased density of eight species and nitrogen fertilization produced increased densities in nine species. Total aboveground biomass was higher in the nitrogen fertilized plots; water amendments had no effect on dry matter production and there were no significant water-nitrogen interactions. Cluster analysis showed that species composition was similar on the fertilized plots and that the plots receiving 25 mm * month-' additional water in a single event differed most from the others. Species richness was highest in the 6 mm . week2, unfertilized plots, and lowest in the unwateredunfertilized plots. Biomass production of six of the 23 species recorded was significantly increased by nitrogen fertilization and biomass of two species was significantly reduced.

Influence of temporal characteristics of physical phenomena on plankton dynamics, as shown by North-West European marine ecosystems

Abstract: This contribution shows that food chain structures, not only general productivity levels, are governed by temporal characteristics of nutrient dynamics at physical interfaces, or, in other words, by the temporal modulation of auxiliary energy transfers at ergoclines. The examples used, based upon both literature reviews and preliminary original results from a recent cruise, are taken from pelagic ecosystems in north-west European seas, where nutrient enrichment processes are periodic and tide-dependent. The period is that of the M2 tide at the Celtic Sea shelf-break, where time-integration in the ecosystem appears to result in enhanced general productivity in the form of a classical herbivore food chain. The period is fortnightly (neaps-springs alternation) at shelf-sea tidal fronts, where herbivores apparently cannot adapt themselves to short-lived phytoplankton outgrowths every second week; phytoplankton biomass is accordingly allowed to accumulate in hydrodynamic traps and recycled first by microheterotrophs, which are preyed upon, in turn, by larger-sized microphagous zooplankters.

The seasonal biomass and productivity of the submerged macrophytes in a polluted Wisconsin stream

Abstract: SUMMARY 1. We measured biomass and light/dark bottle productivity of macrophytes in a Wisconsin stream throughout one growing season. Except for a brief period in early spring when a Cladophora glomerata-filamentous algal community was dominant, Potamogeton pectinatus was the dominant macrophyte species in Badfish Creek. 2. Maximum community biomass was 710 g DW m−2, with a maximum above ground biomass of 620 g DW m−2 and a maximum below ground biomass of 120 g DW m−2. Annual productivity was estimated at 1435 g DW m−2 year−1, with a calculated P/B of 2.01. 3. In situ net production averaged 2.83g C g AFDW−1 h−1 Net positive carbon gain by the P. pectinatus community occurred when water temperatures were above 15°C, and daylength at least 12h. This is correlated to the onset of tuber germination in spring, and the point of maximal biomass decline in autumn.

Community oxygen metabolism in a shallow tidal estuary

Abstract: Comprehensive sampling of a shallow tidal estuary was performed hourly for 14 days, at three-month intervals throughout the annual temperature cycle. The project took place in the Newport River estuary located inside the Outer Banks of North Carolina, U.S.A. In all 26 parameters were monitored at a single station, including meteorology, hydrology, water chemistry and phytoplankton production physiology. The estuarine character in colder months was dominated by riverine input, while in warmer months the estuary was lagoonal, having limited input and exchange with the sea. A holistic evaluation of estuarine community O2 metabolism was developed using diffusion-corrected changes in O2 concentration. These data indicated positive integrated diel production (autotrophy) only in winter (February). The estuary was functionally heterotrophic during warmer months (May, August, November). Integrated short-term radiocarbon productivity estimates compared favorably with measurements of oxygen based diel gross production rates. Community O2 based respiration rates (heterotrophy) exhibited a strong positive relationship with water temperature. During summer, rates of heterotrophic metabolism and associated nutrient regeneration appeared to control autotrophic production.

Estimating phytoplankton productivity from light availability and biomass in the MERL mesocosms and Narragansett Bay

Abstract: Recent research has suggested that primary productivity in estuanne waters can be predicted as a function of phytoplankton biomass (chlorophyll a concentration) multiplied by light avalability in the photic zone (photic depth times incident irradiance). The apphcability of this function was confirmed using data from 1010 I4C incubations (during 1978 to 1984) from 4 diverse mesocosm experiments and Narragansett Bay. For each experiment 70 to 80 % of the variation in productivity was explained by the composite function. A regression equation developed from all experiments explained 82 % of the variation in primary productivity and was not statistically different from a previously reported equation based on comparable data from 4 estuarine regions (North and South San Francisco Bay, Puget Sound, and New York Bight). Phytoplankton production was correlated with biomass alone on a seasonal basis (summer, r = 0.87; non-summer, r = 0.77), with chlorophyll-specific productivity higher during summer (June to September) than non-summer (October to May). The decline in the slope of the relation between production and biomass during non-summer periods corresponded to the seasonal switch from summer dominance by nanoplankton (primarily flagellates) to non-summer dominance by net plankton (primarily diatoms) and coincided with decreased light availability in the photic zone. Annual productivity was also highly correlated with the mean annual product of chlorophyll a, photic depth and incident light ( r = 0.96).

Productivity-Hydrology-Nutrient Models of Forested Wetlands

Abstract: Modelling of forested wetlands depends on an understanding of the effects of hydrology and nutrient conditions on the primary productivity of the wetland ecosystem. Several studies, done over a decade and a half in the eastern half of the United States, suggest that flow-through forested wetlands are most productive and sluggish forested wetlands the least. Some investigators have attempted to translate these general findings into mathematical statements or other quantitative relationships. These quantifications include statistical relationships of primary productivity as a function of hydrology and for nutrient inflow, and parabolic curves depicting productivity as a function of a hydrologic variable (i.e. water depth, depth to water table, or flow-through conditions). A more detailed summary is given on a several-year project in western Kentucky, U.S., where determination of effects of wetland hydrodynamics and nutrient conditions on forested wetland productivity was a primary research goal. A preliminary forested wetland model, which simulates the influence of hydrology and nutrient conditions on wetland productivity, illustrates highest productivity with pulsing hydrology and least with sluggish, low nutrient conditions.

Evaluation of beef breeds for rangeland weaner production in Zimbabwe. 1. Productivity of purebred cows

Abstract: Purebred cows were evaluated for reproductive performance and live weight, and for pre-weaning growth and viability of crossbred progeny sired by unrelated breeds. The cows comprised three indigenous breeds, Mashona, Nkone and Tuli and four exotic breeds Africander, Brahman, Sussex and Charolais. Over 3 years, the mean calving rates % (s.e.) were 55·5 (3·0), 76·0 (3·8), 63·1 (4·5), 69·6 (3·5), 70·0 (3·8), 60·0 (4·3) and 67·0 (5·0) for Africander, Mashona, Nkone, Tuli, Brahman, Sussex and Charolais cows, respectively. Bos taurus and Brahman cows were heavier at parturition and at calf weaning than indigenous cows. Mashona cows were lightest at both stages. Calves of Bos taurus cows were heaviest at birth and those of Mashona lightest. Calves of Brahman cows had the most rapid growth rate and at weaning (240 days) weighed in kg (s.e.) 207 (3·1), compared with 184 (2·1), 172 (2·2), 187 (2·6), 184 (2·1), 178 (2·7) and 186 (2·9) for progeny of Africander, Mashona, Nkone, Tuli, Sussex and Charolais, respectively. Pre-weaning survival rates of the breeds were not significantly different but viability was higher in progeny of indigenous than exotic breeds. The productivity estimates combining calving rate, cow weight, calf weaning weight and pre-weaning viability demonstrated the superiority of indigenous breeds and the Brahman over Africander and Bos taurus breeds.

Growth characteristics of Small and large free-living and attached bacteria in Lake Constance

Abstract: The growth characteristics of small (0.2–1.0μm) and large (1.0–3.0 (μm) free-living and attached bacteria were studied in Lake Constance by comparing the spatial and seasonal dynamics of their biomass turnover time (ratio of biomass/production). The biomass of small free-living bacteria usually turned over significantly faster than that of large free-living bacteria throughout the water column. The turnover of attached bacterial biomass was characterized by large fluctuations. Occasionally, in aphotic water layers, it was as long as that of large free-living bacteria, but when large amounts of decaying organic particles were present, it was shorter than that of small free-living cells. Biomass turnover times of free-living bacteria were in the same range as their generation times, which were estimated from the increase in bacterial abundance in 3μm prefiltered samples. The biomass turnover time of actively metabolizing bacteria was comparable to the generation time of actively metabolizing cells. These results indicate that the biomass turnover time is a useful indicator of the growth of different bacterial fractions, as it reflects their different amounts of participation in microbial processes of aquatic ecosystems.

Vegetation, biomass and productivity of seral grasslands of Cherrapunji in north-east India

Abstract: This study deals with the composition of vegetation, biomass and productivity of four different grassland types at Cherrapunji, India. Varied levels of degradation of the climax mexed evergreen forest are represented by these grasslands. Where the soil profile is deep, the grasslands are maintained through frequent fire. At each site, belowground production is more than double aboveground production. Annual turnover rates for the belowground biomass in these grasslands are more dynamical than their temperature counterparts. These grasslands are maintained under high stress conditions of thin soil cover, highly leached nutrient-deficient soils, frequent fires, and low soil moisture retention in spite of a high annual rainfall of 10372 mm.

Potential Contribution of Natural Hydrocarbon Seepage to Benthic Productivity and the Fisheries of Atlantic Canada

Abstract: Bacterial processes involving methane and other low molecular weight hydrocarbons may be a substantial contribution to the overall benthic productivity in areas where natural seepage from the seabed occurs. This process may be sufficiently extensive on the continental shelf off the east coast of Canada to be of importance to the fishery.