Showing papers on "Productivity (ecology) published in 1996"

Productivity and sustainability influenced by biodiversity in grassland ecosystems

Abstract: THE functioning and sustainability of ecosystems may depend on their biological diversity1–8. Elton's9 hypothesis that more diverse ecosystems are more stable has received much attention1,3,6,7,10–14, but Darwin's proposal6,15 that more diverse plant communities are more productive, and the related conjectures4,5,16,17 that they have lower nutrient losses and more sustainable soils, are less well studied4–6,8,17,18. Here we use a well-replicated field experiment, in which species diversity was directly controlled, to show that ecosystem productivity in 147 grassland plots increased significantly with plant biodiversity. Moreover, the main limiting nutrient, soil mineral nitrogen, was utilized more completely when there was a greater diversity of species, leading to lower leaching loss of nitrogen from these ecosystems. Similarly, in nearby native grassland, plant productivity and soil nitrogen utilization increased with increasing plant species richness. This supports the diversity–productivity and diversity–sustainability hypotheses. Our results demonstrate that the loss of species threatens ecosystem functioning and sustainability.

Microphytobenthos: The ecological role of the “secret garden” of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production

Abstract: The microphytobenthos consists of unicellular eukaryotic algae and cyanobacteria that grow within the upper several millimeters of illuminated sediments, typically appearing only as a subtle brownish or greenish shading. The surficial layer of the sediment is a zone of intense microbial and geochemical activity and of considerable physical reworking. In many shallow ecosystems, the biomass of benthic microalgae often exceeds that of the phytoplankton in the overlying waters. Direct comparison of the abundance of benthic and suspended microalgae is complicated by the means used to measure biomass and by the vertical and horizontal distribution of the microphytobenthos in the sediment. Where biomass has been estimated as chlorophyll a, there may be negligible to large (40%) error due to interference by degradation products, except where chlorophyll is measured by high-performance liquid chromatography. The vertical distribution of microphytobenthos, aside from mat-forming species, is determined by the opposing effects of their vertical migration, which tends to concentrate them near the surface, and physical mixing by overlying currents, which tends to cause an even vertical distribution through the mixed layer of sediment. Uncertainties in vertical distribution are compounded by frequently patchy horizontal distribution. Under-sampling on small (<1 m) scales can lead to errors in the estimate that are comparable to the ranges of seasonal and geographic variation. These uncertainties are compounded by biases in the techniques used to estimate production by the microphytobenthos. In most environments studied, biomass (as chlorophyll a) and light availability appear to be the principal determinants of benthic primary production. The effect of variable light intensities on integral production can be described by a functional response curve. When normalized to the chlorophyll content of the surficial sediment, the residual variation in the data described by the functional response curve is due to changes in the chlorophyll-specific response to irradiance. Production by the benthos is often a significant fraction of production in the water column and microphytobenthos may contribute directly to water column production when they are resuspended. Thus on both the basis of biomass and biogeochemical reactivity, benthic microalgae play significant roles in system productivity and trophic dynamics, as well as such habitat characteristics as sediment stability. *** DIRECT SUPPORT *** A01BY074 00003

The Bering Sea Green Belt: shelf-edge processes and ecosystem production

Abstract: The concept of a highly productive habitat, or Green Belt, along the edge of the continental shelf in the Bering Sea is based upon compelling but fragmentary and often anecdotal observations of a variety of physical and biological features acquired from many sources over many years. Enhanced production at continental margins is not a novel concept, but in the case of the Bering Sea its importance has been overlooked during studies of the unusually broad continental shelf. The limited data reported from the vicinity of the shelf edge in the Bering Sea indicate that annual primary production can be as high as 175 to 275 g C m˜ year-, or approximately 60% greater than production in the adjacent outer shelf domain and 270% greater than in the oceanic domain. Estimates of annual secondary production at the eastern shelf edge also average approximately 60% higher than estimates for the outer domain and 260% higher than those for the oceanic domain. Physical processes at the shelf edge, such as intensive tidal mixing and transverse circulation and eddies in the Bering Slope Current, bring nutrients into the euphoric zone and contribute to enhanced primary and secondary production and elevated biomass of phytoplankton and zooplankton. Fishes and squids concentrate in this narrow corridor because of favourable feeding conditions and because of a thermal refuge from cold shelf-bottom temperatures that can be found at the shelf edge from fall to spring. The abundance of zooplankton, fishes and squids, in turn, attracts large numbers of marine birds and mammals. In aggregate, the observations suggest that sustained primary productivity, intense food web exchange and high transfer efficiency at the shelf edge are important to biomass yield at numerous trophic levels and to ecosystem production of the Bering Sea.

Physiological and Growth Responses of Arctic Plants to a Field Experiment Simulating Climatic Change

Abstract: Field manipulations of light, temperature, nutrients, and length of growing season in directions simulating global environmental change altered biomass of the four most abundant vascular plant species in tussock tundra of northern Alaska. These species are Betula nana, Ledum palustre, Vaccinium vitis-idaea, and Eriophorum vaginatum. Biomass response reflected changes in both growth and mortality, with growth being stimulated by treatments that enhanced biomass,a and mortality being enhanced by all treatments (except in Vaccinium). Those species with highest leaf and stem turnover (the graminoid and deciduous shrub) initially showed large positive responses to nutrient addition. By contrast, slow-turnover evergeen species showed little initial change in production in response to our manipulations, and their long-term biomass responses were in the opposite direction to those of the responsive species. Short-term measurement of leaf expansion, photosynthesis, and phosphate uptake showed little correlation with net production of biomass change in response to manipulations because of compensatory mechanisms at levels of growth and allocation. Changes in nutrient distribution among species accounted for many of the long-term changes in biomass and productivity. Processes that are readily integrated at annual time steps (e.g., shoot growth, shoot mortality, allocation) were more useful than instantaneous physiological measurements in predicting decadalmore » vegetation changes because (1) compensating responses among physiological processes buffer plant responses at progressively longer time scales, (2) species interactions in the community buffer ecosystem processes such as productivity and nutrient cycling from changes in growth of individual species, and (3) different time lags between physiological, demographic, and ecosystem processes complicate modeling of long-term responses from short-term mechanism. 76 refs., 11 figs., 5 tabs.« less

Glacial to Interglacial Fluctuations in Productivity in the Equatorial Pacific as Indicated by Marine Barite

Abstract: An empirical correlation between marine barite (BaSO4) accumulation rate in core-top sediment samples from two equatorial Pacific transects (at 140°W and 110°W) and the estimated primary productivity of the overlying water column were used to evaluate glacial to interglacial changes in productivity. Fluctuations in barite accumulation rates down-core indicate that during glacial periods of the past 450,000 years, the productivity in the central and eastern equatorial Pacific was about two times that during intervening interglacial periods. This result is consistent with other evidence that productivity was high in the eastern and central equatorial Pacific during the last glacial.

Biodiversity and plant productivity in a model assemblage of plant species

Abstract: We examined productivity as a function of biotic diversity. We manipulated plant species richness as an experimental factor to determine if productivity (net above ground primary productivity or NPP) is affected by changes in plant diversity (species richness). We constructed 164 assemblages that varied in species richness and measured their biomass at the end of one growing season. The plants were drawn from a pool of 16 species of self-pollinating annual herbs common to English weedy fields. On average, species-poor assemblages were less productive. Results also showed, however, that species-poor assemblages had wider ranges of possible productivities than more diverse assemblages.

Productivity cycles of 200–300 years in the Antarctic Peninsula region: Understanding linkages among the sun, atmosphere, oceans, sea ice, and biota

Abstract: Compared to the rest of the world9s oceans, high-resolution late Holocene paleoclimatic data from the Southern Ocean are still rare. We present a multiproxy record from a sediment core retrieved from a deep basin on the western side of the Antarctic Peninsula that reveals a dramatic perspective on paleoclimatic changes over the past 3700 yr. Analyses completed include measurement of magnetic susceptibility and granulometry, bed thickness, particle size, percent organic carbon, bulk density, and microscopic evaluation of diatom and benthic foraminiferal assemblages and abundances. Downcore variability of these parameters demonstrates the significance of both short-term cycles, which recur approximately every 200 yr, and longer term events (≈2500 yr cycles) that are most likely related to global climatic fluctuations. In the upper 600 cm of the core, lower values of magnetic susceptibility (MS) are correlated with lower bulk density, the presence of thinly laminated units, specific diatom assemblages, and generally higher total organic carbon content. Below 600 cm, magnetic susceptibility is uniformly low, though variability in other parameters continues. The magnetic susceptibility signal is controlled primarily by dilution of ferromagnetic phases with biosiliceous material. This signal may be enhanced further by dissolution of magnetite in the magnetic susceptibility lows (high total organic carbon). The role of variable primary productivity and its relationship to paleoclimate is assessed through the diatom data. In particular, magnetic susceptibility lows are characterized by higher than normal abundances of Chaetoceros resting spores. Corethron criophilum and/or Rhizosolenia spp. also are found, as is a higher ratio of the most common species of Fragilariopsis versus species of Thalassiosira . These assemblages are indicative of periods of high primary productivity driven by the presence of a meltwater stabilized water column. The 200 yr cyclicity noted in other paleoclimatic records around the world suggests a global forcing mechanism, possibly solar variability. In addition to the cyclic changes in productivity, overall elevated productivity is noted below 600 cm, or prior to ca. 2500 yr B.P. This increased productivity may represent the tail end of a Holocene climatic optimum, which is widely recognized in other parts of the world, but as yet is poorly documented in Antarctica.

Primary productivity and its regulation in the equatorial Pacific during and following the 1991–1992 El Niño

Abstract: The cycling of carbon in the equatorial Pacific Ocean was investigated by the Equatorial Pacific (EqPac) Study in 1992. As part of that study in situ primary productivity was measured on survey and time-series cruises along 140°W from 12°N to 12°S with methods determined to be trace-metal clean. Primary productivity, chlorophyll and chlorophyll-specific productivity rates varied coherently in relation to two large-scale features: temporally, primary productivity was reduced during the El Nino dominated period (February–April 1992) and increased during the cool period (August–October 1992); and spatially enhanced primary productivity persisted close to the equator relative to the oligotrophic regions poleward of 10°N and 10°S. On the equator in October 1992 during the period of relatively cool water, primary productivity was about twice (125 mmol C m−2 day−1) the value during the peak warm period (60 mmol C m−2 day−1). The climatological mean equatorial productivity in the cold tongue has been recalculated to be about 75 mmol C m−2 day−1 (Chavez et al., 1996). The mean 1992 productivity on the equator (1°S–1°N) was about 25% higher than climatology (95 vs 75 mmol Cm−2 day−1) and about 3 times the value in oligotrophic waters poleward of 10°N and 10°S (95 vs 30 mmol C m−2 day−1). Higher chlorophyll-specific productivity during the cool period relative to the warm period (3.9 vs 2.4 mmol C mg chl−1 day−1) indicates that the increase in absolute productivity did not result solely from a biomass increase, but from a change in the nutrient-regulated specific productivity rate. The regulating nutrient was not a macronutrient, such as nitrate or silicic acid, because macronutrients (and light) were present in uptake-saturating concentrations during both the warm and cool periods of the 1992 EqPac study. Physiological constraint by a micronutrient, such as iron, is implicated as the factor regulating these productivity variations. The change in iron supply resulted from a change in equatorial circulation processes. During the warm period, El Nino-Southern Oscillation (ENSO)-driven changes in pycnocline topography depressed the Equatorial Undercurrent (EUC), thereby decreasing the amount of iron-rich EUC water entrained into equatorial upwelling and vice versa during the cool period. During the August–October cool period of generally increased productivity, two further episodic increases in specific productivity, biomass and diatom abundance occurred during intense and remotely forced upwelling events associated with a front or the passage of a frontal wave. In both mesoscale episodes, temperature and salinity show that the intensified upwelling reached more deeply into the already relatively shallow EUC. Productivity and biomass increases during both of these events were quantitatively similar to those in an in situ iron addition experiment (IronEx) carried out in equatorial Pacific waters in 1993. Variations in the supply of upwelled iron provided by the iron-rich EUC best account for the warm-cool period difference in phytoplankton productivity as well as the episodic increases in specific productivity, biomass and diatom abundance during intense mesoscale upwelling events seen in the dynamic equatorial region in the EqPac study.

Effects of summer grazing by caribou on composition and productivity of vegetation : community and landscape level

Abstract: 1 Changes in demography and studies on physical condition of the Riviere George caribou Rangifer tarandus herd have suggested that its size may be primarily regulated by the amount of forage available on the summer range. 2 We therefore document the impact of grazing and trampling on composition and productivity of two plant communities, the shrub tundra and stands of dwarf birch, within this range. Ungrazed sites were rare, but four previously located small areas were used as control sites. 3 For the shrub tundra, the lichen mat was absent in grazed sites and ground previously occupied by lichens was either bare, covered by fragments of dead lichens and mosses or recolonized by early succession lichen species. Ground cover of shrubs not eaten by caribou was lower in grazed sites than in ungrazed sites, and coverage of graminoids, forage shrubs and forbs did not differ significantly between grazed and ungrazed sites. 4 In stands of dwarf birch grazed by caribou, ground cover and leaf biomass of Betula glandulosa was significantly lower than in ungrazed sites. 5 Productivity of forage plant species over the summer range was estimated at 22.5 g m-2 year-l in an ungrazed condition compared to 10.3 g m-2 year' when grazed. 6 At the landscape level, caribou have fragmented the distribution of their food resource by reducing biomass of shrub tundra and stands of dwarf birch to a very low level. 7 The serious negative impact of migratory ungulates on plant productivity of their summer range may be explained by characteristics of the vegetation and the high carrying capacity of winter compared to summer ranges. Significant factors related to the vegetation are its low resilience and productivity and the absence of a response of vascular plants following removal of lichens.

The First five years in the reorganization of aboveground biomass and nutrient use following hurricane Hugo in the bisley Experimental Watersheds, luquillo Experimental Fortest, Puerto Rico

Abstract: Five years after Hurricane Hugo reduced the aboveground biomass by 50 percent in two forested watersheds in the Luquillo Experimental Forest of Puerto Rico, regeneration and growth of survivors had increased the aboveground biomass to 86 percent of the pre-hurricane value. Over the 5 yr, the net aboveground productivity averaged 21.6 Mg.ha-1 yr-' and was faster than most plantations and secondary forests in the area. Woodfall and associated nutrient fluxes never attained pre-storm values but by the fifth yr, mean daily total litterfall, and N, P, K, Ca, and Mg fluxes in litterfall were 83, 74, 62, 98, 75, and 81 percent of their pre-disturbance values, respectively. Aboveground nutrient pools of these nutrients ranged from 102 to 161 percent of their pre-disturbance values and were larger after 5 yr because of higher nutrient concentrations in the regeneration compared to the older wood that it replaced. The following sequence of ecosystem reorganization during this first 5 yr period is suggested. An initial period of foliage production and crown development occurred as hurricane survivors re-leafed and herbaceous vegetation and woody regeneration became established. During this period, 75 to 92 percent of the nutrient uptake was retained in the aboveground vegetation and there was a relatively low rate of aboveground carbon accumulation per mole of nutrient cycled. This initial period of canopy development was followed by a peak in aboveground productivity that occurred as early successional species entered the sapling and pole stages. This period was followed by the establishment of the litterfall nutrient cycle and an increase in the net productivity per mole of nutrient cycled. During this 5 yr period, the Bisley forest had some of the lowest withinstand nutrient-use-efficiencies and some of the highest levels of aboveground productivity ever observed in the LEE The study demonstrates that high levels of productivity and rapid rates of aboveground reorganization can be achieved with rapid within-system cycling and inefficient within-stand nutrient use.

Temporal and spatial patterns in the Ross Sea: Phytoplankton biomass, elemental composition, productivity and growth rates

Abstract: The temporal and spatial patterns of phytoplankton biomass, productivity, and particulate matter composition in the Ross Sea were assessed during cruises in January 1990 and February 1992. Biomass and primary productivity in the southern Ross Sea were greatest during mid-January, with surface chlorophyll concentrations, particulate organic carbon levels, and integrated primary productivity averaging 4.9 μg L−1, 0.54 mg L−1 and 2.63 g C m−2 d−1, respectively. Comparable mean concentrations and rates for February were 1.1 μg L−1, 0.29 mg L−1, and 0.78 g C m−2 d−1 (decreases of 76, 46, and 70%, respectively), indicative of the scale of temporal changes. A distinct south-north transition also was observed both in productivity and phytoplankton biomass, with the lowest values occurring in the northern Ross Sea. East-west gradients in phytoplankton biomass and composition occurred within the southern Ross Sea. The areal productivity of the Ross Sea ranged from 0.15 to 2.85 g C m−2 d−1 and is among the highest found in the entire Antarctic. Carbon:chlorophyll ratios were uniformly high but were highest (150) in 1990 in the diatom-dominated western Ross Sea. Surface growth rates were modest, averaging less than 0.2 day−1 during both seasons. We hypothesize that the marked seasonality in the region provides an environment in which net growth rates, although slow, are maximized through low loss rates and which allows biomass to accumulate in the surface layer. Furthermore, the temporal variations are quantitatively similar to the observed spatial variations. Therefore the dominant determinant of phytoplankton biomass and productivity at any one point on the Ross Sea continental shelf is the stage of the seasonal growth cycle.

Long-term monitoring and analyses of physical factors regulating variability in coastal Antarctic phytoplankton biomass, in situ productivity and taxonomic composition over subseasonal, seasonal and interannual time scales

Abstract: A 3 yr high-resolution temporal data base related to phytoplankton dynamics was collected during the austral spring/summer periods of 1991 to 1994 in shelf waters adjacent to Palmer Station, Antarctica. Here, the data base is used the variability in phytoplankton biomass, productivity and taxonomic composition over seasonal and interannual time scales; (2) to elucidate environmental mechanisms controlling these temporal patterns; and (3) to ascertain which phytoplankton markers are most suitable for detecting longer-term (i.e. decadal) trends in ton dynamics in coastal waters of the Southern Ocean. The Long-Term Ecological Research (LTER) coastal study sites showed high interannual variability in peak phytoplankton biomass to 494 mg a and integrated primary production (1.08 to 6.58 g C Seasonal and annual patterns in biomass and productivity were shown to be driven by shorter-time-scale physical forcing by local wind stress. Low daily wind speeds 10 m were associated with water-column stabilization. However, extended periods wk) of low wind stress were required for increased phytoplankton growth and biomass accumulation. Temperature data supports the view that water masses can be replaced on time scales of a less than a day to a few days in these coastal waters. Such disruptions are associated with abrupt changes in local primary production and may lead to sudden shifts in local phytoplankton community structure. Despite the high seasonal and interannual variability in biomass and associated productivity in this coastal environment, the replacement sequence of one dominant phytoplankton group by another was very similar on subseasonal time scales for all 3 years. We suggest that changes in phytoplankton successional patterns may be a more sensitive marker for detecting long-term trends in Southern Ocean ecosystems than either biomass or productivity indices, where short-term variability of the latter is as great or greater than interannual variations documented to date.

The relationship of floods, drying, flow and light to primary production and producer biomass in a prairie stream

Abstract: Factors related to autochthonous production were investigated at several sites along a prairie stream at Konza Prairie Research Natural Area. Primary production, algal biomass, litter input, and ability of floods to move native substrate were measured. Additional experiments were conducted to establish the influence of light and water velocity on primary production rates and recovery of biomass following dry periods. The study period encompassed two extreme (> 50 year calculated return time) floods, thus we were able to analyze the effects of scour on periphyton biomass and productivity. Biomass of sedimentary algae was reduced greatly by flooding and did not reach preflood amounts during the 2 months following the first flood. Rates of primary production associated with sediments recovered to levels above preflood rates within 2 weeks. Biomass of epilithic periphyton was not affected as severely as that of sedimentary algae. Little relationship was observed between water velocity and photosythetic rates. Production reached maximum rates at 25% of full sun light. Epilithic chlorophyll levels recovered within eight days following a dry period, and chl a was an order of magnitude greater on rocks than sediments 51 days after re-wetting. Estimated annual rates of primary production were 2.6 times greater in the prairie than in the forest reaches of the stream. The ratio of annual autochthonous:allochthonous carbon input was 4.81 for prairie and 0.32 for the forest. Periphyton production in prairie streams is resilient with regard to flooding and drought and represents a primary carbon source for the system.

Food Web Structure and Littoral Zone Coupling to Pelagic Trophic Cascades

Abstract: Lake productivity ultimately depends on phosphorus (P) supply rates (Schindler, 1977). Community interactions, especially size-selective predation by fishes and size-dependent rates of grazing and nutrient recycling by zooplankton, determine the efficiency and rate with which P inputs are translated to ecosystem productivity (Carpenter and Kitchell, 1993). Limnologists are now trying to determine how trophic cascades interact with the P cycle to influence lake productivity (McQueen et al., 1986; Elser and Goldman, 1991; Carpenter et al., 1991). One informative approach analyzes food web interactions in P currency, thereby integrating the direct and indirect effects of fishes on lake P cycles.

A demographic model for a population of the endangered lesser kestrel in southern Spain

Abstract: 1 The lesser kestrel (Falco naumanni) has experienced a dramatic decline in the last 20 years in the Western Palearctic To help in making decisions for the recovery of the species, a matrix projection model has been developed using demographic data from an intensively monitored population in southern Spain in 1988-93 Survival rates were estimated using the Jolly-Seber modelling approach and Program SURGE 2 The growth rate of the population was λ = 0959 ± 004 The upper 95% confidence limit of λ is 10398, and thus our estimate of λ is not significantly different from that of a stable population (ie λ = 1) Using the mean value for λ, the probability of extinction for this population, now consisting of 1000 breeding pairs, is 35% in 100 years and 98% in 200 years 3 A sensitivity analysis indicates that population growth was most sensitive for changes in adult survival, followed by juvenile survival, productivity of fledglings, proportion of adults that attempt breeding and age at first breeding 4 Adult and juvenile survival do not seem easily amenable to management, but the small improvement (85%) that could be reached enforcing protective laws substantially reduce the probability of extinction of the population 5 Productivity is less than half its potential maximum due to massive nestling mortality Increasing food availability around the breeding colonies through habitat management (eg leaving uncultivated strips around fields and favouring cereal crops), or introducing the species in areas containing suitable habitat may substantially increase productivity Combining these measures with the achievable improvement (85% increase) in adult and juvenile survival maximizes the long-term survival of lesser kestrel populations

Balance Between Autotrophic and Heterotrophic Components and Processes in Microbenthic Communities of Sandy Sediments: A Field Study

Abstract: The microscopic community of a microtidal sandy sediment on the Swedish west coast was studiedin situat two depths (0·5 and 4 m) on four occasions (January, April, August and October). Biomass of microalgae, bacteria, ciliates and meiofauna, as well as primary and bacterial productivity, were quantified. Meiofaunal grazing on algae and bacteria was measured simultaneously by radiolabelling intact sediment cores. Autotrophic biomass dominated the microbial community at both depths and on all sampling occasions, accounting for 47–87% of the microbial biomass. Meiofauna contributed 10–47%, while bacteria and ciliates together made up less than 6%. The microflora was dominated by attached (epipsammic) diatoms, but occasional ‘ blooms ’ of motile species occurred. Vital cells of planktonic diatoms contributed to benthic algal biomass in spring. Primary productivity exceeded bacterial productivity in April and August at both depths, while the balance was reversed in October and January. Meiofauna grazed between 2 and 12% of the algal biomass per day, and between 0·3 and 37% of the bacterial biomass. Almost an order of magnitude more algal (17–138 mg C m−2) than bacterial (0·1–33 mg C m−2) carbon was grazed daily. At the shallow site, primary productivity always exceeded grazing rates on algae, whereas at the deeper site, grazing exceeded primary productivity in October and January. Bacterial productivity exceeded grazing at both depths on all four occasions. Thus, meiofaunal grazing seasonally controlled microalgal, but not bacterial, biomass. These results suggest that, during summer, only a minor fraction (<10%) of the daily microbenthic primary production appears to enter the ‘ small food web ’ through meiofauna. During spring and autumn, however, a much larger fraction (≈30–60%) of primary production may pass through meiofauna. During winter, meiofaunal grazing is a less important link in the shallow zone, but at sublittoral depths, algal productivity may be limiting, and meiofauna depend on other food sources, such as bacteria and detritus.

Phytoplankton dynamics in the stratified water column of Lake Bonney, Antarctica I. Biomass and productivity during the winter-spring transition

Abstract: Phytoplankton populations in perennially ice-covered Lake Bonney, Antarctica grow in a unique non-turbulent environment. The absence of turbulence generated by winds or major streams, combined with strong vertical gradients in temperature and nutrients, create vertically stratified environmental conditions that support three discrete phytoplankton populations in the east lobe of this lake. Phytoplankton biomass and photosynthesis were measured in the east lobe of Lake Bonney during the winter-spring transicion (September) to mid-summer (January). During this period, irradiance beneath the ice increased from 0.03 to 1.9 mol quanta m−2 d−1. Chlorophylla concentrations ranged from 0.03 to 3.8 μl−1 within the trophogenic zone (just beneath the permanent ice cover to 20 m) and photosynthesis ranged from below detection to 3.2 μg Cl−1 d−1. Our results indicate: (1) phytoplankton photosynthesis began in late winter (before 9 September, our earliest sampling date); (2) maxima for phytoplankton biomass and production developed sequentially in time from the top to the bottom of the trophogenic zone, following the seasoral increase in irradiance; and (3) the highest photosynthetic efficiencies occurred in early spring, then decreased over the remainder of the phytoplankton growth season. The spring decrease in photosynthetic rates for shallower phytoplankton appeared to be related to nutrient availability, while photosynthesis in the deeper populations was solely lightdependent.

Contribution by Macroalgal Mats to Primary Production of a Shallow Embayment Under High and Low Nitrogen-loading Rates

Abstract: The limits on primary production in areas undergoing eutrophication may be set by indirect effects of nitrogen loading, i.e. decreasing irradiances, associated with proliferating opportunistic algae. Using in situ photon flux density (PFD) availability estimates within unattached algal mats and photosynthetic parameters determined from photosynthesis vs. irradiance (P vs. I) curves generated for the dominant components of mat assemblages, Cladophora vagabunda and Gracilaria tikvahiae , seasonal net mat production rate for estuaries (Waquoit Bay, Massachusetts, U.S.A.) receiving high (Childs River) and low (Sage Lot Pond) N-loading rates were determined. Although abundance of C. vagabunda was 2× greater than G. tikvahiae , the former species contributed only about 50% of total mat productivity, due largely to rapid light attenuation within the dense algal mat. While mat production was low and similar at both sides during winter (≈0·35 g C m −2 day −1 ), for other seasons, the net mat daytime productivity at Childs River, the N-loaded site, was 2·5× higher than rates determined for Sage Lot Pond. Although annual net daytime production at Childs River (1094 g C m −2 year −1 ) was comparable to estimates for other algal mat assemblages in eutrophic systems, primary production of the Waquoit Bay system was found to become self-limiting as available PFD controls maximum productivity. In photosynthetically inactive portions of the algal mat, carbon release was estimated from tissue-loss measurements at 0·14 g C m −2 day −1 for C. vagabunda and 0·05 g C m −2 day −1 for G. tikvahiae . Annual in situ C release of 73 g C m −2 is ≈20% of annual net mat production (fixed carbon not respired by algae) in this embayment. Although both estuaries showed net autotrophy year round (Pg:R>1), the high metabolic cost of a large, inactive mat resulted in lower Pg:R ratios at Childs Rivers than at Sage Lot Pond, particularly during the summer period of peak production. Thus, it is predicted that the Waquoit Bay system will experience an overall decline in Pg:R ratios and consequent increase in anoxic events as eutrophication continues.

Changes in Live Plant Biomass, Primary Production, and Species Composition along a Riverside Toposequence in Arctic Alaska, U.S.A.

Abstract: In the arctic landscape, vegetation composition and structure are strongly affected by topographic position and associated variation in microclimate. Along a single riverside toposequence in northern Alaska, six distinct plant communities were studied including a riparian shrub community, a wet sedge tundra, a footslope Equisetum community, a hillslope shrub/lupine community, a hilltop birch-heath community, and a moist tussock tundra. Total live plant biomass varied threefold along the toposequence (450-1400 g m-2) while live vascular plant biomass (including belowground stems and rhizomes but not roots) varied sevenfold (160970 g m-2). Aboveground vascular plant production varied less than fourfold (80265 g m-2). Although all six communities showed some signs of nutrient limitation, measures of soil nutrient availability were highly variable among communities. Contrary to expectations, the most productive community along the toposequence was the hillslope shrub/lupine community, where a late-lying snowbank delayed the start of the growing season by 2 wk. The second most productive community was the hilltop birch-heath, which was exposed to winter winds and where snow melted earliest; most of the production in this community occurred in relatively protected depressions where there were dense accumulations of plant mass. A conclusion is that soil fertility, soil thaw, and protection from wind are more important than length of the snow-free season in regulating productivity along the toposequence. Also contrary to expectations, overall production:live biomass ratios of the six communities varied little despite large differences in growth form composition among communities and in biomass turnover among growth forms. High-biomass, highly productive communities had the lowest production:live biomass ratios, and thus the lowest biomass turnover. Because production and live biomass were linearly correlated over the entire range sampled, it may be possible to use live biomass and/or leaf area as a reasonably accurate predictor of productivity at the landscape or regional level in the Arctic.

Species-Area Curves and Diversity-Productivity Relationships in Beaver Meadows of Voyageurs National Park, Minnesota, USA

Abstract: Species richness (S) increases with area (A) as shown in the well-known power function proposed by Arrhenius: S=cA 2 . We show that the exponent of the species-area curves, z, was negatively correlated with aboveground annual production (biomass) but the coefficient of the curves, c, was positively correlated with production in two communities dominated by grasses and sedges in beaver meadows in northern Minnesota. The parameters of the species-area power function were not correlated with water table depth nor with soil nutrient availabilities. We then show that the often-reported unimodal relationship between richness and productivity can be derived from the negative correlation of z and the positive correlation of c with productivity. We also show that the productivity corresponding to maximum richness declines with increasing area sampled. All other correlations of c and z with productivity yield either monotonically increasing or decreasing richness with increasing productivity. The relationships between richness and productivity depend on the area censused and the correlations of the coefficient and exponent of the species-area curve with productivity. Interpretations of diversity-productivity relationships are sensitive to plot size and require specification of the species-area relationship for the community in question.

Phytoplankton biomass and production in shelf waters off NW Spain: spatial and seasonal variability in relation to upwelling

Abstract: Chlorophyll-a and primary production on the euphotic zone of the N-NW Spanish shelf were studied at 125 stations between 1984 and 1992. Three geographic areas (Cantabrian Sea, Rias Altas and Was Baixas), three bathymetric ranges (20 to 60 m, 60 to 150 m and stations deeper than 200 m), and four oceanographic stages (spring and autumn blooms, summer upwelling, summer stratification and winter mixing) were considered. One of the major sources of variability of chlorophyll and production data was season. Bloom and summer upwelling stages have equivalent mean and maximum values. Average chlorophyll-a concentrations approximately doubled in every step of the increasing productivity sequence: winter mixing — summer stratification — high productivity (upwelling and bloom) stages. Average primary production rates increased only 60% in the described sequence. Mean (± sd) values of chlorophyll-a and primary production rates during the high productivity stages were 59.7 ± 39.5 mg Chl-a m−2 and 86.9 ± 44.0 mg C m−2 h−1, respectively. Significant differences in both chlorophyll and primary production resulted between geographic areas in most stages. Only 27 stations showed the effects of the summer upwelling that affected coastal areas in the Cantabrian Sea and Rias Baixas shelf, but also shelf-break stations in the Rias Altas area. The Rias Baixas area had lower chlorophyll than both the Rias Altas and the Cantabrian Sea areas during spring and autumn blooms, but higher during summer upwelling events. On the contrary, primary production rates were higher in the Rias Baixas area during blooms in spring and autumn. Mid-shelf areas showed the highest chlorophyll concentrations during high productivity stages, probably due to the existence of frontal zones in all geographic areas considered. The estimated phytoplankton growth rates were comparable to those of other coastal upwelling systems, with average values lower than the maximum potential growth rates. Doubling rates for upwelling and stratification stages in the northern and Rias Altas shelf areas were equivalent, despite larger biomass accumulations during upwelling events. Low turnover rates of the existing biomass in the Rias Baixas shelf in upwelling stages suggests that the accumulation of phytoplankton was due mainly to the export from the highly productive rias, while the contribution of in situ production to these accumulations was relatively lower.

Phytoplankton composition in relation to primary production in Chesapeake Bay

Abstract: The dominant phytoplankton taxa during seasonal periods of peak primary productivity were identified during a 4 yr study (July 1989 to June 1993) in Chesapeake Bay. Maximum phytoplankton abundance occurred from late winter to early spring, and was dominated by a few species of centric diatoms. This development was followed by more diversified assemblages of diatoms and phytoflagellates that produced additional concentration peaks in summer and fall; all these maxima were accompanied by concurrent productivity peaks. High summer productivity resulted when the phytoplankton concentrations of diatoms and phytoflagellates were augmented by an increased abundance of autotrophic picoplankton. There was variability in both the seasonal and annual growth maxima of these algal populations and in total productivity. Higher cell concentrations and productivity were associated with higher nutrient levels on the western side of the bay, at sites adjacent to major tributaries. Periods of highest productivity were in spring and summer, ranging from 176 to 346 g Cm-2yr-1 over the 4 yr period, with a mean annual productivity of 255 g Cm-2yr-1. The bay stations rates ranged from 82 to 538 g Cm-2yr-1.

Late Neogene Oceanographic Change along Florida's West Coast: Evidence and Mechanisms

Abstract: Evidence from vertebrate and invertebrate fossil assemblages and isotopic analyses supports the hypothesis that during the Pliocene biological productivity in the eastern Gulf of Mexico was considerably higher than during the Pleistocene and Recent. Late Pliocene faunal changes in the eastern Gulf, Western Atlantic, and possibly elsewhere may have resulted, at least in part, from this shift in productivity conditions. Even if marine temperatures declined, paleontological and isotopic data appear to require a change in productivity in the Late Pliocene. This putative productivity decline may have been caused by some combination of causes at three geographic scales: (1) globally-marine productivity may have fallen due to changes in continental weathering; (2) regionally-North Atlantic productivity may have fallen as a result of initiation of North Atlantic Deep Water formation (possibly a consequence of formation of the Central American Isthmus, CAI) and resulting net transfer of nutrients to the Pacific; (...

Rapid Holocene hydrologic change along boreal treeline revealed by δ 13 C and δ 18 O in organic lake sediments, Northwest Territories, Canada

Abstract: Analysis of δ18Ocellulose, δ13Corganic matter, and δ13Ccellulose at about 100 year intervals from organic matter deposited in Toronto Lake, Northwest Territories, Canada, revealed an 8000-year history of rapid, post-glacial hydrologic change at the treeline zone. Several mid-Holocene phases of enriched δ13Corg and δ13Ccell, caused by elevated lake productivity, declining [CO2(aq)], and closed basin conditions, were abruptly terminated by intervals of open hydrology recorded by sharply depleted δ18Ocell. Two of these events, at 5000 and 4500 BP, are correlated with increased total organic content and Picea mariana pollen concentration, which indicate that high levels of productivity were also accompanied by northern treeline advances. A third treeline advance at about 2500 BP is also marked by an apparent outflow event from Toronto Lake, but this was not associated with δ13Corg/cell enrichment in the sediment record because rapid and substantial lake water renewal probably prevented productivity-driven enrichment of the dissolved inorganic carbon and replenished the CO2(aq) supply to thriving phytoplankton. However, high sediment organic content during this period suggests increased productivity. Increases in the inflow:evaporation ratio at about 6500 and 3500 BP were also sufficient to cause Toronto Lake to overflow but the prevailing climate during these periods apparently did not favour appreciable northward treeline migration or changes in lake productivity.

Top-down impact through a bottom-up mechanism: the effect of limpet grazing on growth, productivity and carbon allocation of Zostera marina L. (eelgrass)

Abstract: The unusual appearance of a commensal eelgrass limpet [Tectura depicta (Berry)] from southern California at high density (up to 10 shoot−1) has coincided with the catastrophic decline of a subtidal Zostera marina L. meadow in Monterey Bay, California. Some commensal limpets graze the chloroplast-rich epidermis of eelgrass leaves, but were not known to affect seagrass growth or productivity. We evaluated the effect on eelgrass productivity of grazing by limpets maintained at natural densities (8±2 shoot−1) in a natural light mesocosm for 45 days. Growth rates, carbon reserves, root proliferation and net photosynthesis of grazed plants were 50–80% below those of ungrazed plants, but biomass-specific respiration was unaffected. The daily period of irradiance-saturated photosynthesis (H sat) needed to maintain positive carbon balance in grazed plants approached 13.5 h, compared with 5–6 h for ungrazed plants. The amount of carbon allocated to roots of ungrazed plants was 800% higher than for grazed plants. By grazing the chlorophyll-rich epidermis, T. depicta induced carbon limitation in eelgrass growing in an other-wise light-replete environment. Continued northward movement of T. depicta, may have significant impacts on eelgrass production and population dynamics in the northeast Pacific, even thought this limpet consumes very little plant biomass. This interaction is a dramatic example of top-down control (grazing/predation) of eelgrass productivity and survival operating via a bottom-up mechanism (photosynthesis limitation).

Effects of conservation reserve program field age on avian relative abundance, diversity, and productivity

Abstract: ABSTRACr.-Introduced grass dominated Conservation Reserve Program (CRP) fields were monitored in summer 1992 in Gratiot County, Michigan, to determine the relationship between field age and avian relative abundance, diversity, and productivity. Younger CRP fields (1-2 years old), best described as a combination of forbs and bare ground, had the greatest diversity and relative abundance of avian species. Older CRP fields (3-5/6 years old) were a combination of grasses and deep litter cover and had the greatest avian productivity. We recommend that after 3-5 growing seasons CRP fields be manipulated to provide a variety of successional stages to maintain simultaneously high avian relative abundance, diversity, and productivity. Received 6 Nov. 1995, accepted I May 1996.

Relationships between graminoid growth form and levels of grazing by caribou (Rangifer tarandus) in Alaska

Abstract: Herbivores and their forage interact in many ways, in some instances to the benefit or detriment of herbivore and vegetation. Studies of wildebeest (Connochaetes taurinus) in Africa and snow geese (Chen caerulescens) in the Arctic have suggested that these grazers enhance graminoid production in certain sites by repeatedly using them. Other studies have concluded that herbivores are sensitive to local variation in forage quality and quantity, and preferentially use those sites that are intrinsically more productive. In this study, caribou (Rangifer tarandus) were observed foraging at different densities on two adjacent Alaskan ranges, within which particular feeding sites contained predictably high, medium, or low densities of caribou. Vegetation from one high- and one low-use site on each of the high- and low-density ranges was sampled and monitored for productivity, measured as re-growth following clipping, with the objectives of determining which forage characteristics influence usage by grazers and whether the productivity and nature of graminoid growth after clipping were related to grazing history. Forage biomass density (g/m3), shoot density (number/m2), stand densities of nutrients and minerals (g/m3), and forage concentrations of nutrients and minerals (g/100 g tissue) correlated positively with use of sites by caribou. Productivity was independent of previous use by grazers, but consistent within ranges. These results indicate that caribou are sensitive to local variation in forage quality and quantity, preferentially use those sites with higher returns of nutrients and minerals, and have the potential to enhance graminoid growth on sites that are inherently more productive.