Showing papers on "Productivity (ecology) published in 1991"

The production of dissolved organic matter by phytoplankton and its importance to bacteria : patterns across marine and freshwater systems

Abstract: We analyzed published rates of extracellular release (ER) of organic carbon to determine the primary constraints on this process and its importance to bacteria. From 16 studies we extracted observations of ER, particulate primary production (PP), and phytoplankton biomass. In a regression model based on 225 observations, PP explained 69% of the variance in ER. From this model we estimate the average percent extracellular release (PER) to be 13% of total fixation. The slope of this relationship does not support the hypothesis that the PER declines with increasing productivity. Differences exist between marine and freshwater systems. In lakes, ER increases nonlinearly with productivity, resulting in very low PER in very eutrophic systems. In coastal marine and estuarine systems, ER increases linearly with productivity and the PER does not vary systematically. ER is not primarily related to phytoplankton biomass as predicted by passive diffusion models. Instead, ER appears to be constrained by the total availability of photosynthates. By comparing our model to an existing model of bacterial production and assuming a 50% growth efficiency, we estimate that ER amounts to less than half the C required for bacterial growth in most pelagic systems.

Paleoproductivity from benthic foraminifera abundance: Glacial to postglacial change in the west-equatorial Pacific

Abstract: Surface productivity is correlated with the rate of accumulation of benthic foraminifera on the deep-sea floor. As a rule of thumb, for each 1 mg of organic carbon arriving at the sea floor, one benthic foram shell >150 μm is deposited. The correlation can be used to reconstruct organic flux to the sea floor in the past, and hence the productivity of past oceans. Applying the appropriate equations to box core data from the Ontong Java Plateau in the western equatorial Pacific, we found that productivity during the last glacial maximum exceeded present productivity by a factor of between 1.5 and 2.0, with intermediate values for the mid-transition period. Accumulation of benthic foraminifera was depressed on top of the plateau during the glacial and transitional period, presumably because increased winnowing removed part of the food supply.

Facilitation and Interference of Quercus Douglasii on Understory Productivity in Central California

Abstract: Controversy over the effect of Quercus douglasii on the productivity of California grassland has been fueled by conflicting reports. In some studies, understory grassland productivity was < 25% of open grassland productivity, whereas in other studies understory productivity exceeded 200% of that of surrounding grassland. We examined light, temperature, soil nutrients, soil moisture, and fine tree root distributions under selected Q. douglasii trees (12 that appeared to have suppressive effects on understory productivity, and 12 that appeared to enhance understory productivity) in order to deter- mine how variations in these factors were associated with the differences in understory grassland productivities. We found that grassland productivity is likely to be facilitated by nutrient inputs via litterfall and throughfall under all trees, but that trees with low understory grassland productivities had substantially higher amounts of fine roots in the upper 50 cm of soil and much lower predawn xylem pressure potentials than trees with high understory productivities and presumably deeper root systems. Root exclosures reduced the negative effects of these trees on the dominant understory grass species, and further experiments indicated that the negative effects of the tree roots may partially result from allelopathic oak root exudates. Thus, shallow fine tree roots may inhibit understory productivity, and variations in Q. douglasii root morphology may explain the intertree variations in facili- tating/interfering effects on understory species.

Keystone species and mangrove forest dynamics: the influence of burrowing by crabs on soil nutrient status and forest productivity

Abstract: The density of the burrowing crab fauna in a mangrove forest was reduced, using pitfall traps, to test the hypothesis that decreased burrowing would lead to: (1) increased soil sulphide concentrations; (2) altered nutrient concentrations; and (3) decreased forest productivity and growth. Experiments were conducted in Rhizophora -dominated forests in north Queensland, Australia, over a 12-month period. Crabs were trapped and removed from the experimental plots during 1 week each month for a year. Soil chemical and forest growth parameters were measured at monthly intervals in the experimental and appropriate control plots. Over the course of the experiment, soil sulphide and ammonium concentrations increased to levels which were significantly higher in plots from which crabs were being removed in comparison to controls. No differences were observed for either phosphate or nitrate plus nitrite. Cumulative forest growth, as measured by stipule fall, was significantly less in removal plots than in controls. Additionally, trees in the removal plots had significantly less reproductive output than did trees in control plots. These results support the hypothesis that burrowing by crabs is an important process in Australian Rhizophora forests. It appears that burrowing affects soil aeration which in turn affects the productivity and reproductive output of Rhizophora . Knowledge of the ecology of grapsid crabs from other continents, however, is very limited. It remains to be seen if the roles played by grapsid crabs are as important elsewhere as in Australia. This is particularly true for the Caribbean region from which many of our paradigms concerning mangrove forest ecology were developed.

Shifts in fish communities along the productivity gradient of temperate lakes - patterns and the importance of size-structured interactions

Abstract: SHIFTS IN FISH COMMUNITIES ALONG THE PRODUCTIVITY GRADIENT OF TEMPERATE LAKES - PATTERNS AND THE IMPORTANCE OF SIZE-STRUCTURED INTERACTIONS

Major role of the cyanobacterium trichodesmium in nutrient cycling in the north atlantic ocean.

Abstract: The diazotrophic cyanobacterium Trichodesmium is a large (about 05 by 3 millimeters) phytoplankter that is common in tropical open-ocean waters Measurements of abundance, plus a review of earlier observations, indicate that it, rather than the picophytoplankton, is the most important primary producer (about 165 milligrams of carbon per square meter per day) in the tropical North Atlantic Ocean Furthermore, nitrogen fixation by Trichodesmium introduces the largest fraction of new nitrogen to the euphotic zone, approximately 30 milligrams of nitrogen per square meter per day, a value exceeding the estimated flux of nitrate across the thermocline Inclusion of this organism, plus the abundant diazotrophic endosymbiont Richelia intracellularis that is present in some large diatoms, in biogeochemical studies of carbon and nitrogen may help explain the disparity between various methods of measuring productivity in the oligotrophic ocean Carbon and nitrogen fixation by these large phytoplankters also introduces a new paradigm in the biogeochemistry of these elements in the sea

Seasonal Food Limitation of Detritivores in a Montane Stream: An Experimental Test

Abstract: Many species of detritivorous invertebrates in small streams depend almost entirely on inputs of leaf litter for their nutritional requirements, however the concentration of this resource varies considerably seasonally. An experiment designed to test the hy- pothesis that productivity of coarse particle detritus feeders (shredders) is seasonally food limited was performed using replicate streamside channels that received one of three input rates of whole leaf detritus (rates equal to those naturally falling into montane streams, and two levels of increased inputs). Seven of the nine common shredder species attained significantly higher adult mass, higher densities, or both when food was supplemented. Larval densities of Malenka spp. and Brillia retifinis were significantly higher when additional food was added. Brillia retifinis densities were more than 10 x greater in high food additions than in the "natural" treatment; B. retifinis apparently fills a role as a fugitive in this system and is better able to track shifts in resource abundance by virtue of its short generation time. Zapada cinctipes and Z. haysi were more dense in high and intermediate food input treatments during the last half of the summer. Other taxa had nonsignificant trends to higher densities when extra food was provided. Most of the change in benthic densities can be attributed to decreased rates of emigration with increased food supply. Six of the eight taxa for which adult mass at emergence was measured were significantly more massive (4-46%) when food was added. Both sexes exhibited this increase in mass, but females gained proportionally more in most species. There were no detectable changes in the timing of adult emergence due to food manipulations. The biomass of most taxa increased in proportion to the overall increase in biomass of the common coarse-detritus consumers. The main exceptions to that pattern were a disproportionate increase in the percentage of the biomass represented by Brillia retifinis and a decrease in the representation of Zapada cinctipes. The responses of this community to food supplementation demonstrate food limitation of detritivores. The exponential increase in benthic biomass under food addition shows the scope for productivity if food were not limiting. These coexisting species benefitted from enhanced food supply on a spatial scale that is relevant for population level processes. The large seasonal variation in resource abundance under natural conditions creates bot- tlenecks during periods of low food supply, which constrain subsequent production even during periods when food is abundant.

Evidence for lower productivity in the Antarctic Ocean during the last glaciation

Abstract: BOTH increased biological productivity and more efficient uptake of upwelled nutrients in high-latitude oceans have been proposed1–5 as mechanisms responsible for the glacial reduction in atmospheric concentrations of carbon dioxide deduced from ice-core measurements6–8. These glacial models invoke more efficient "biological pumping9 of carbon into the deep sea by increasing the uptake of 'excess' biolimiting nutrients in the Antarctic surface ocean9 or by reorganizing chemical circulation patterns within the ocean10,11. Here we challenge this conventional view with new evidence from tracers of palaeoproductivity preserved in Antarctic sediments. Records of the accumulation rates of diatom shells, the ratio of germanium to silicon in diatomaceous opal and the carbon isotope ratio in foraminiferal carbonate all suggest lower glacial productivity and less efficient uptake of nutrients. Although alternative interpretations are possible, our results support previous studies that indicate lower glacial productivity in the Southern Ocean12,13 and raise new questions about the role of ocean productivity in models of the causes (or remedies) for changes in atmospheric concentrations of carbon dioxide.

Pigment distribution and primary production in the western Mediterranean as derived and modeled from coastal zone color scanner observations

Abstract: A set of 114 coastal zone color scanner (CZCS) images of the western Mediterranean (mainly in the year 1981) have been processed and analyzed to describe the algal biomass evolution and estimate its potential carbon fixation. For that, the pigment concentration in the top layer, Csat, is used through empirical relationships to infer the depth-integrated pigment content of the productive column, 〈C〉tot. A spectral light-photosynthesis model driven by 〈C〉tot is operated with additional information, namely, about sea temperature and photosynthetically available radiation (computed from astronomical and atmospherical parameters then combined with a cloud climatology). This model also includes a standard set of physiological parameters which account for the light capture by algae and for the use of this radiant energy in photosynthesis. This model allows a climatology of ψ* the cross section for photosynthesis per unit of areal chlorophyll, to be produced and then convoluted with the biomass maps after they have been averaged and composited. On average and for the whole western Mediterranean, the pigment concentration in the upper layer is about 0.25 mg Chl m−3, leading to an areal mean concentration of 21 mg Chl m−2. The maximum (bloom) occurs in early May in all zones. Seasonal variations in algal biomass are much more marked in the northern part than in the southern part (apart from Alboran Sea); the south Tyrrhenian basin and the central part of the Algerian basin are steadily oligotrophic. The mean annual carbon fixation rate for the whole basin is about 94 g C m−2 yr−1, or 106 and 87, for the northern and southern basins when separately considered. The seasonality is expressed by a six-fold change in the production rate (between February and May) within the northern zone, whereas only a two-fold change occurs in the southern zone between the same months. These estimates, which compare well with previous episodic field data, considerably extend our knowledge of the temporal progression of productivity within the entire western Mediterranean and its various provinces.

Seasonal and hydrological control of phytoplankton nutrient limitation in the lower Neuse River Estuary, North Carolina

Abstract: Nutrient limitation of phytoplankton production was assessed monthly from 1987 through 1990 in the lower Neuse River Estuary, North Carolina, USA, a well-mixed, mesotrophic system Nutrient addition bioassays indicated that the lower estuary experienced a general state of nitrogen limitation, with especially pronounced limitation during summer months, a period of high phytoplankton productivity Bioassays conducted during spring months showed significantly greater stimulation of algal productivity with the addition of nitrogen and phosphorus than that found with nitrogen addition alone This CO-stimulation occurred during periods when surface-water dissolved inorganic nitrogen : dissolved inorganic phosphorus ratios were elevated above typical values of < 5 Seasonal patterns in ambient nutrient concentrations revealed nitrogen maxima associated with spring, fall, and winter runoff events, with summer minima Hydrologically driven nitrogen loading exerted a strong, yearround influence on primary production and nutrient limitation characteristics High-flow events acted to oversaturate the upper estuarine nutrient filtering capacity, resulting in increased delivery of nitrogen to the lower estuarine environment The phytoplankton community responded to increased flow and concomitant nutrient loadings by increasing production and b~omass levels, often very rapidly In this regard, hydrologic factors influencing nitrogen loading (terrigenous runoff, point source inputs, and wet and dry atmospheric deposition) are key determinants of the trophic state of this estuary

Zooplankton effects on phytoplankton in lakes of contrasting trophic status

Abstract: Studies were carried out in three lakes of strongly contrasting trophic state to evaluate how the effects of zooplankton on phytoplankton vary as a function of lake productivity. Chlorophyll and total P concentrations differed by 2-3 orders of magnitude among ultra-oligotrophic Lake Tahoe, meso-oligotrophic Castle Lake, and strongly eutrophic Clear Lake. Three experimental designs involving short-term (4 d) manipulations of nutrients, ambient zooplankton, and the common crustacean grazer Duphnia were performed in each lake. Algal responses were assessed at both the “aggregate,” community (Chl, primary productivity, nutrient status) and species levels. Experiments in Lake Tahoe revealed a high degree of nutrient sensitivity but negligible grazing impacts by the very low densities of ambient zooplankton. Daphnia grazing had substantial impacts on Tahoe phytoplankton, however, indicating high susceptibility to grazing in this assemblage of relatively small-sized algal species. Castle Lake experiments revealed strong direct and indirect impacts of both ambient zooplankton and Duphnia on the nutrient-limited algal assemblage. The cyanobacteria-dominated algal community of Clear Lake was resistant to grazing impacts, responding relatively weakly only to the highest densities of Duphnin. Good correspondence between estimates of algal biomass made from chlorophyll measurements and microscopic examination were obtained, and, in Castle Lake, concordance between estimated community productivity tumover times made from species-specific growth determinations and community productivity measurements was observed. The contrasting responses to experimental zooplankton manipulations in the three lakes provide support for the view that the coupling between zooplankton and phytoplankton is strongest in lakes of intermediate productivity and imply that food-web alterations at the top of the food web are most likely to propagate to the level of the phytoplankton (and therefore lake water quality) in lakes of moderate trophic status.

Carbon budgets of temperate forest ecosystems.

Abstract: A summary of carbon-budget data from 30 forest stands is presented together with information on both above- and belowground ecosystem components. Only 10 of the stands had complete biomass data, 21 had complete productivity data and six had heterotrophic, or autotrophic respiration data, or both. The most comprehensive stand-level data have been collected for Pinus spp., Pseudotsuga menziesii, Abies amabilis and Liriodendron tulipifera. Only incomplete carbon budgets are available for most ecosystems studied, because measurements have been limited to one or several ecosystem processes (e.g., decay, productivity, storage). Based on the few complete C-budget studies that have been published, it is concluded that tree species differ in annual turnover of tissues.

Effects of Tidal Stage and Sun Angles on Intertidal Benthic Microalgal Productivity

Abstract: Motile benthic diatoms exhibit rhythmlc vertical migrations that are influenced by tidal and llght cycles. As a consequence of these periodlc migrations, corresponding periodicities in benthic microalgal production should occur. Using oxygen microelectrodes, hourly measurements of microalgal production were obtained from subaerially exposed cores collected from low-intertidal muddy sediments in North Inlet estuary, South Carolina, USA Microalgal productivity at low tide was twce that at high tlde (mean difference 52 %) and was significantly correlated with diurnal and tidal periodicities (r2 = 0.41; p < 0.0001). Production values ranged from 28.0 to 460.5 pm01 O2 mg chl a-' h' and maximum rates were achieved during mid-afternoon low tides. A curvilinear regression equation was constructed to simulate daily and monthly benthic microalgal production based on tidal and light cycles. Comparisons between predictions of the curvilinear equation and published data sets showed a reasonable agreement (r2 = 0.77), suggesting similar phenomena in other estuaries. Current benthic microalgal production models do not account for hourly variability in productivity, leading to potentially large errors when measurements are extrapolated over monthly and annual time scales. Although other physiological and abiotic factors also influence benthic microalgal productivity, much of the short-term variability in production rates may be simply attributed to migratory rhythms within estuarine sediments.

Deep‐Sea Benthic Foraminiferal Assemblage Response to a Surface Ocean Productivity Gradient: A Test

Abstract: A transect of samples from the eastern equatorial Pacific, where surface ocean productivity is the only changing environmental variable, was used to test the response of benthic foraminiferal assemblages to an open-ocean productivity gradient. The transect runs along the East Pacific Rise from the highly productive waters near the equator to the oligotrophic South Pacific subtropical gyre. Surface sediment benthic assemblages show progressive change along the transect with strong compositional shifts corresponding to the rapid productivity gradient near the equator and to the position of the southern convergence, near 18°–19°S. The relationship of the assemblages to surface ocean productivity was tested by statistical regression of quantitative estimates of productivity on the assemblage principal component scores calculated for each sampling location. The regression yielded an r-squared of 0.87, demonstrating a clear relationship between surface ocean productivity and deep-sea benthic foraminiferal assemblage composition. The species most important to this relationship display a consistent response to oceanic productivity for long stretches of geologic time. This conservative behavior and the results of the regression analysis indicate that benthic foraminiferal assemblages can be used to estimate surface ocean productivity for the geologic past.

Differences in processing dynamics of fresh and dried leaf litter in a stream ecosystem

Abstract: SUMMARY. 1. Although the bulk of litter input to stream ecosystems is in the form of fresh leaves, current understanding of organic matter processing is largely founded on experimental studies made with pre-dried leaves. This paradox points to the critical need for evaluating to what extent those experiments with dried leaves reflect natural litter decomposition. 2. The mass loss rates, patterns of mass loss, and chemical changes during processing of fresh leaf litter were compared with air-dried leaf litter in a stream ecosystem. 3. Although overall mass loss rates were similar between treatments (k= 0.0213 day−1 and 0.0206 day−1), fresh leaves lost mass at a constant rate, whereas the decay of dried leaves proceeded in two distinct phases. Soluble organic carbon, phosphorus, and potassium were rapidly leached from dried litter, but were largely retained in fresh material for more than a week. Kinetics of concentrations of cellulose and changes in amounts of lignin remaining per leaf pack revealed further differences in decomposition dynamics between treatments, apparently related, either directly or indirectly, to differences in leaching behaviour. 4. Dynamics of nitrogen and protein contents were similar between treatments, indicating that microbial colonization was not greatly delayed on fresh leaves. 5. It is concluded that the retention of labile carbon and nutrients in fresh leaf litter facilitates their utilization by leaf-associated micro-organisms and invertebrates, resulting in an increased importance of biotic processes relative to physical processes such as leaching. 6. At the ecosystem level, retention of carbon and nutrients in streams would be increased, allowing greater overall productivity. Conversely, the availability of labile organic carbon would be reduced in compartments such as the epilithon, fine sediments, and the water column.

Calanus in North Norwegian fjords and in the Barents Sea

Abstract: The Physical environment of a North Norwegian fjord and of the Atlantic and Arctic domains of the Barents Sea are described. The seasonal variation of primary production and biomass of the most important copepod species are described in order to contrast regional differences in the timing of the plankton cycles. Analysis of the seasonal variation in the biomass of six different copepod species in Balsfjorden clearly demonstrate the importance of Calanus finmarchkus as a spring and early summer form, whereas Pseudoculanus acuspes, the most important smaller form, reaches the highest biomass later during the productive season. In the Atlantic part of the Barents Sea, C. finmarchkus is the dominant herbivorous form. The next most important species, Pseudocalanus sp. and M. longa, play a less important role here than in Balsfjorden. In the Arctic domain, the smaller copepod forms appear to have been replaced in trophodynamic terms by the youngest year-group (C-CIII) of C. glacialis, which prevails during the Arctic summer and autumn periods. The coupling between primary producers and Calanus on a seasonal basis is addressed through the grazing and the vertical organisation of the plant-herbivore community. The productivity of these two Calanus species is considered in relation to the seasonal and inter-annual variation in climate; although different mechanisms are utilised, cold periods tend to lower Calanus productivity both in the Arctic and the Atlantic domains of the Barents Sea. Interannual variations in Calanus biomass and productivity are discussed in the perspective of endemic and advective processes.

Atmosphere-biosphere exchange of carbon dioxide in boreal forests

Abstract: An ecophysiological model of photosynthesis and respiration by forest ecosystems was used to examine CO2 fluxes in 23 mature boreal forests near Fairbanks, Alaska. Simulated soil respiration, photosynthesis, decomposition, and moss and tree productivity were consistent with observed data. Monthly ecosystem CO2 flux and net photosynthesis, averaged over the 23 sites, were correlated with atmospheric CO2 concentrations and δ13 ratios, respectively, at Barrow, Alaska, suggesting the boreal forests of Alaska play an active role in the seasonal dynamics of atmospheric CO2 at Barrow. Only one of the 23 stands was a source of CO2, and the 23 stands absorbed (mean ± SE) 1173±211 g CO2 m−2 yr−1. Observed productivity in these forests spans the range of productivity in the circumpolar boreal forest, suggesting the simulated CO2 fluxes are representative of the circumpolar boreal forest. If so, metabolic activity in the circumpolar boreal forest results in a significant annual uptake of CO2.

Organic carbon decomposition, primary and bacterial productivity, and sulphate reduction, in tropical seagrass beds of the Gulf of Carpentaria, Australia

Abstract: Seagrass (mainly Enhalus acoroides, Syringodium isoetifolium, Cymodocea serrulata, Thalassia hemprichii and Cymodocea rotundata) gross productivity, determined by a lacunal gas technique, ranged from 0.5-4.3 g C m. In comparison, gross community productivity ranged from 4-9 g C md. Macroalgae, epibenthic and epiphytic algae made the major contribution to gross primary production in the communities studied, contributing 40-90% of gross primary production. Most of the primary production was utilized by hydrolytic and fermentative bacteria and was ultimately oxidized by sulphate-reducing bacteria. Hydrolytic (aerobic) and fermentative bacterial productivity ranged from 3-13 g C md (integrated over a depth of 12 cm in the sediment). Highest rates of sulphate reduction coincided with the below-ground distribution of seagreass roots and rhizomes. Between 1.7-2.2 g C md was oxidized to CO by the sulphate-reducing bacteria, is equivalent to 30-80% of the net community productivity. -from Authors

Distribution, abundance and productivity of benthic invertebrates at the Berg River estuary, South Africa

Abstract: Twenty-five benthic invertebrate species were identified from samples taken monthly over 17 months at four sites on the Berg River estuary, South Africa. Gastropods and polychaetes dominated the macrofauna in terms of both numbers and biomass. Abundance of the dominant species fluctuated in response to seasonal growth of eelgrass Zostera capensis and filamentous alga Cladophora sp. Differences in distributions of invertebrates on the estuary were attributed to differences in physical properties of the substratum and in vegetation cover. Hydrobia sp., Ceratonereis erythraeensis and C. keiskama were the most important species in terms of biomass and accounted for an average of 75% of total biomass at all study sites. Biomass peaked during the austral winter, early spring and again in autumn. An increase in biomass in winter was due to somatic production, whereas spring and autumn increases were attributed to recruitment of juveniles following reproduction. Mean annual biomass for the whole estuary was 19·36 g m−2, and mean annual production 87·58 g m−2 year−1, yielding a net P B ratio of 4·52. Production and P B ratios of invertebrates in estuaries and coastal lagoons at temperate and subtropical latitudes were positively correlated with mean annual ambient temperature and negatively with distance from the equator. Production data are lacking from tropical estuaries.

The Inorganic Geochemical Record of the Northwest Arabian Sea: A History of Productivity Variation over the Last 400 k.y. from Sites 722 and 724

Abstract: High-resolution sampling from late Pleistocene (last 400 k.y.) sediments of Site 722 (upper 16 m) and Site 724 (upper 70 m), and subsequent inorganic geochemical analysis, has defined the history of productivity in the northwest Arabian Sea. Eolian dust input from the Arabian Peninsula and Somalia is characterized by the record of Ti/Al and Cr/Al. This dust record displays strong precessional periodicity (cycles at 25 k.y.) suggesting the Southwest Monsoon and associated winds play a key role in transporting terrigenous material from the land. High biological productivity results in the accumulation of biogenic CaCO3 and opal in the sediments, the latter having an unexpectedly minor contribution to the total mass flux. Due to dilution of the CaCO3 record by the terrigenous component, the record of biological productivity is best exemplified by Ba. Its record, together with that of other metals recording biological association and redox variability (Cu, Ni, Zn, V, U) clearly identifies the interglacial episodes as being more biologically productive. The striking agreement between Ba and the δ 1 8 θ record in planktonic foraminifers suggests that the supply of nutrients during these periods of high productivity is linked to ocean-wide changes in ocean fertility, and not just local upwelling conditions. High levels of phosphate accumulation in interglacial sediments is attributed to both diagenetic phosphorite formation and biogenic skeletal debris. This study provides a detailed record of productivity variation in the northwest Arabian Sea during the late Pleistocene.

Spatial distribution of freshwater mussels: the role of host fish and metabolic rate

Abstract: SUMMARY. 1. Three species of the superfamily Unionacea live in Bavarian trout streams: the freshwater pearl mussel Margaritifera margaritifera which usually occurs alone, and the unionids Unio crassus and Anodonta piscinalis which are frequently associated. 2. We found no evidence for competition between M. margaritifera and the unionids. Two types of habitat factors were analysed: (i) the occurrence of host fish, which does not explain the distributional relationships of mussels; and (ii) hydrochemical factors indicating a lower productivity in the rivers inhabited by M. margaritifera. 3. With respect to physiological differences, M. margaritifera is separated from the other two species by low growth constants and low metabolic rates. We conclude, therefore, that the metabolic rate is an important factor governing distribution of the species: M. margaritifera is able to grow in rivers with low food availability, whereas the higher metabolic rates of U. crassus and A. piscinalis presumably necessitate a richer food supply.

Changes in Abundance of Two Percids, Perca fluviatilis and Gymnocephalus cernuus, along a Productivity Gradient: Relations to Feeding Strategies and Competitive Abilities

Abstract: Changes in abundance of two percids, Perca fluviatilis and Gymnocephalus cernuus along a productivity gradient: Relations to feeding strategies and competitive ability

The effect of freshwater inflow on meiofaunal consumption of sediment bacteria and microphytobenthos in San Antonio Bay, Texas, U.S.A.

Abstract: If meiofauna are food-limited then they should respond with increased feeding rates when microbial production is stimulated. River inflow into estuaries is a source of organic matter that can be limiting to bacterial production, and nutrients that might limit primary production. Therefore, inflow should stimulate microbial primary and secondary production, and eventually meiofauna grazing rates should increase as a functional response to increased food availability and quality. To determine if meiofauna grazing rates were affected by inflow, two replicate stations were sampled in the upper, river-dominated end, of San Antonio Bay and contrasted with two replicate stations at the lower end of the estuary. The experiments were performed three times. Water column nutrients and sediment organic matter were higher in the upper end of the estuary than in the lower end. Benthic primary production was 2·5 times higher in the upper end than in the lower end. Benthic metabolism (measured by oxygen consumption) was also higher in the upper end, but bacterial production (measured by thymidine uptake) was not significantly different between the two ends. Grazing rates were 3·5 times higher on bacteria, and 2·5 times higher on microalgae in the upper end of the estuary than in the lower end, confirming our hypothesis that inflow would stimulate grazing rates. Grazing rates were dominated by juvenile molluscs (temporary meiofauna) which accounted for 39% of the microalgae and 68% of the bacteria ingested by the community. Juvenile molluscs were most prevalent in the upper, fresh-water zone. Harpacticoid copepods and nematodes had higher grazing rates in the lower end of the estuary. Grazing rates were higher on microalgae than on bacteria: 4% of the microalgae were removed per hour, compared to only 1% of the bacteria. Grazing rates on microalgae were 2·6 times higher than productivity, indicating meiofauna might be food-limited. Grazing on bacteria was low, and production (based on oxygen metabolism) exceeded grazing; thus bacterial food is not apparently limiting. Freshwater inflow can affect meiobenthic community structure, stimulate microbial production, and stimulate feeding rates by small invertebrates that can benefit by the increase in microbial production.

The role of the microbial loop in Antarctic pelagic ecosystems

Abstract: The Paradigm pelagic food web organization in Antarctic waters is undergoing fundamental revision evidence that large fractions of material and energy flow through the microbial food web. because of the unique Antarctic ecosystem conditions, the microbial food web performs some roles that are fundamentally different from those in oligotrophic temperate and tropical waters: 1) during winter, bacterial production, at the expense of slow-turnover DOM (dissolved organic matter) from the previous summer, could be a significant factor in the survival of over wintering animal populations; 2) microbial regeneration of ammonium in nitrate-replete Antarctic waters may spare the reductants necessary for nitrate assimilation and thus enhance primary productivity of deep-mixed light-limited phytoplankton; and 3) the small diatoms and phytoflagellates which dominate the Antarctic pelagic primary production are apparently directly digestible by the metazoan herbivores, whereas cyanobacteria which dominate the primary productivity in lower latitude oligotrophic waters are not digestible by the metazoan herbivores. These roles performed by the microbial loop may, in part, explain why Antarctic waters, in contrast to the lower latitude oligotrophic waters, have high levels of tertiary productivity despite low primary productivity.

Coupling between ice microalgal productivity and the pelagic, metazoan food web in southeastern Hudson Bay: a synthesis of results*

Abstract: A multidisciplinary study of the ice algal production cycle in southeastern Hudson Bay in spring, 1986, provided an exceptional opportunity to observe trophic interaction between ice algae and metazoan zooplankton. During the ice algal bloom, females of the biomass-dominant copepod species, Calanus glacialis and Pseudocalanus spp. (predominantly P. minutus), grazed at night near the ice-waste interface. Feeding activity of these copepods significantly increased after the start of ice melt in mid-May when ice algae were released in large quantities into the water column. Copepod egg production was negligible during the ice algal bloom, then increased by approximately two orders of magnitude by mid-June. While changes in Calanus productivity can be attributed to increases in female-specific egg production rates alone, Pseudocalanus productivity was also substantially augmented by the appearance in late May of females of a second species, P. newmani. The data are consistent with the hypothesis that the major sources of nutrition for copepod production during this period are scdimenting ice algae (during and immediately after the bloom at the ice-water interface) and diatoms seeded from the interfacial layer and actively growing in the water column (in late May and Junnc). Larvae of Arctic cod and sand lance, the yolk-sac stages of which were found in samples immediately after the onset of ice melt, were in a good position to feed on the resulting high concentrations of copepod nauplii in mid-June. These results therefore suggest a strong coupling between the ice algal production cycle and production cycles in the pelagic food web in this region.

Seasonal variation of biomass and production dynamics for above- and belowground components of a Spartina alterniflora marsh in the euhaline sector of Paranaguá Bay (SE Brazil)

Abstract: The seasonal variation of biomass and the annual net primary production for above- and belowground components of Spartina alterniflora Loisel intermediate form were evaluated by harvest methods over a 14-month period in Paranagua Bay (SE Brazil). Belowground components were characterized by a larger biomass, ranging from 172±48 g m −2 at the end of summer to 569±211 g m −2 in late spring. The biomass of live aboveground tissue was usually higher than dead tissue, ranging from 51±21 g m −2 in winter to 116±36 g m −2 in summer. The overall mean root to shoot ratio was 4·8, ranging from 1·7 in the summer to 8·5 in late spring, reflecting storage and translocation events. Net belowground primary production was 358 g m −2 year −1 , largely exceeding aboveground production, which ranged from 101 to 179 g m −2 year −1 , depending on how data were handled. Turnover of above- and belowground material, expressed by a production to maximum biomass ratio, was fast, ranging from 0·66 to 1·52 years, respectively. In comparison with temperate Spartina marshes, the low productivity and low biomass of marshes of the high-energy sector of Paranagua Bay are probably a result of different growth strategies, high salinities and fast sediment accretion, which usually lead to their ultimate replacement by mangroves.