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The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today.sup.1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements ([increment].sup.17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative [increment].sup.17O values (down to -0.880/00) observed in sulfates, except for those from the terminal Cryogenian period.sup.2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO.sub.2 and O.sub.2 and the photosynthetic O.sub.2 flux at this time.sup.3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O.sub.2 flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO.sub.2 (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O.sub.2 was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean.sup.4-6 and Phanerozoic primary production.sup.7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O.sub.2 over geologic time.sup.8.Triple oxygen isotope measurements of 1.4-billion-year-old sedimentary sulfates reveal a unique mid-Proterozoic atmosphere and demonstrate that gross primary productivity in the mid-Proterozoic was between 6% and 41% of pre-anthropogenic levels., Author(s): Peter W. Crockford [sup.1] [sup.2] [sup.3] , Justin A. Hayles [sup.4] [sup.5] , Huiming Bao [sup.5] [sup.6] , Noah J. Planavsky [sup.7] , Andrey Bekker [sup.8] , Philip W. [...]

Charles Elton introduced the 'pyramid of numbers' in the late 1920s, but this remarkable insight into body-size dependent patterns in natural communities lay fallow until the theory of the biomass size spectrum was introduced by aquatic ecologists in the mid-1960s. They noticed that the summed biomass concentration of individual aquatic organisms was roughly constant across equal logarithmic intervals of body size from bacteria to the largest predators. These observations formed the basis for a theory of aquatic ecosystems, based on the body size of individual organisms, that revealed new insights into constraints on the structure of biological communities. In this review, we discuss the history of the biomass spectrum and the development of underlying theories. We indicate how to construct biomass spectra from sample data, explain the mathematical relations among them, show empirical examples of their various forms, and give details on how to statistically fit the most robust linear and nonlinear models to biomass spectra. We finish by giving examples of biomass spectrum applications to production and fisheries ecology and offering recommendations to help standardize use of the biomass spectrum in aquatic ecology. Charles Elton a introduit le concept de la <> vers la fin des annees 1920, mais cette avancee remarquable dans la comprehension des motifs dependant de la taille du corps dans les communautes naturelles est demeuree lettre morte jusqu'a l'introduction de la theorie du spectre de taille de la biomasse par des ecologistes aquatiques, au milieu des annees 1960. Ces derniers avaient remarque que la somme des concentrations de biomasse d'organismes aquatiques individuels etait generalement constante pour des intervalles logarithmiques egaux de tailles du corps, des bacteries jusqu'aux plus grands predateurs. Ces observations constituaient le fondement d'une theorie des ecosystemes aquatiques reposant sur la taille du corps des organismes individuels, qui mettait en lumiere des facteurs agissant sur la structure des communautes biologiques. Dans la presente synthese, nous abordons l'histoire du spectre de biomasse et le developpement des theories sous-jacentes. Nous decrivons comment batir des spectres de biomasse a partir d'un echantillon de donnees, en expliquons les relations mathematiques et presentons des exemples empiriques de leurs diverses formes, ainsi que des details sur la maniere de caler les modeles lineaires et non lineaires les plus robustes sur des spectres de biomasse. Enfin, nous presentons des exemples d'applications des spectres de biomasse a la production et a l'ecologie des ressources halieutiques et formulons des recommandations pour aider a normaliser l'utilisation des spectres de biomasse en ecologie aquatique. [Traduit par la Redaction], Introduction The abundance of organisms in nature has been of interest to science since ecology emerged as a formal discipline of study. Elton (1927) introduced the 'pyramid of numbers' to [...]

ABSTRACT. Sediment cores from three lakes located in the northernmost region of Ungava, Quebec (Canada) were examined to define aquatic community and ecosystem variability during the Late Holocene period. A [...]

Trait-based approaches are increasingly used to understand the responses of phytoplankton communities to changing environmental conditions. This paper analyses phytoplankton morphological traits (i.e. volume, surface to volume ratio) in a highly productive subtropical estuary (Rio de la Plata, South America). The goal was to investigate whether morphological variability, weighted by biovolume, supports the hypothesis that light is a key limiting factor for primary producers in this turbid and nutrient-rich estuary. A transect from nearly freshwater to marine stations was sampled with increasing mixing depth, decreasing turbidity and dissolved inorganic nitrogen concentration. Primary production (PP) and total biovolume were highest on the marine side of the turbidity front. In this area, morphologies of different high-order taxa converged towards smaller individual size and higher surface to volume ratio (S/V). Phytoplankton weighted average S/V increased with turbidity along the gradient, while individual size did so with salinity. High S/V and low morphological variability supported higher PP, appearing as responses to cope with light limitation in this environment. Multiple linear regressions indicated that morphological variability alone explained 72 % of PP, reinforcing the utility of trait-based approaches when targeting relations between phytoplankton communities and ecosystem functioning., Introduction Trait-based approaches are widely used in ecological studies to improve quantification and prediction of environmental change effects (Mc Gill et al. 2006; Violle et al. 2007). The scaling from [...]

Variation in terrestrial net primary production (NPP) with climate is thought to originate from a direct influence of temperature and precipitation on plant metabolism. However, variation in NPP may also result from an indirect influence of climate by means of plant age, stand biomass, growing season length and local adaptation. To identify the relative importance of direct and indirect climate effects, we extend metabolic scaling theory to link hypothesized climate influences with NPP, and assess hypothesized relationships using a global compilation of ecosystem woody plant biomass and production data. Notably, age and biomass explained most of the variation in production whereas temperature and precipitation explained almost none, suggesting that climate indirectly (not directly) influences production. Furthermore, our theory shows that variation in NPP is characterized by a common scaling relationship, suggesting that global change models can incorporate the mechanisms governing this relationship to improve predictions of future ecosystem function., Annual net primary production (NPP; [gm.sup.-2] [yr.sup.-4]) of woody plants is a major component of the terrestrial carbon cycle (1). Although it has long been known that NPP correlates with [...]

Herbivores mediate the abundances of primary producers both from the top-down, by consuming them, and from the bottom-up, by recycling nutrients. Whereas the top-down effects of herbivores on algae in marine ecosystems are well-documented, less is known about their roles as mediators of local-scale nutrient availability. We conducted a series of surveys and measurements of tide pools and the grazers in those pools between October of 2016 and June of 2017 at an intertidal site on the coast of Southern California, USA (33°35'16.3'N, 117°52'1.5'W). We surveyed grazer abundances in the field, measured biomass of representatives from four different grazer groups (littorine snails, limpets, chitons, and turban snails), measured ammonium excretion rates, and quantified ammonium accumulation rates in tide pools at our study site. We found that different grazer groups were characterized by different per-biomass ammonium excretion rates. Some grazer groups-turban snails and chitons-contributed more ammonium than predicted by their biomass, whereas other grazer groups-littorine snails and limpets-contributed less ammonium than predicted by biomass. Because of these differences between grazer groups, ammonium accumulation rates in tide pools at our study site were effectively predicted based on the ammonium excretion rates of the different grazer groups. However, ammonium accumulation rates were not related to total herbivore biomass. Our results highlight the importance of grazer identity-and particularly the role of species such as turban snails that contribute disproportionately to nutrient recycling-in understanding the contributions of grazers as mediators of bottom-up processes in marine systems., Author(s): Matthew E. S. Bracken [sup.1] , Jill M. Oates [sup.1] , Alexander J. Badten [sup.1] , Genevieve Bernatchez [sup.1] Author Affiliations: (Aff1) 0000 0001 0668 7243, grid.266093.8, Department of [...]

Regions of high primary production along the oligotrophic west coast of Australia between 34 and 22°S in May-June 2007 (midway through the annual phytoplankton bloom) were found around mesoscale features of the Leeuwin Current. At 31°S, an anticyclonic eddyforming meander of the Leeuwin Current had a mixed layer depth of >160 m, a depth-integrated chlorophyll a (Chl a)-normalised primary production of 24 mg C mg Chl [a.sup.-1] [day.sup.-1] compared to the surrounding values of 1,000 m of water with a shallow (, Introduction The Leeuwin Current is a narrow stream of tropical surface water (warm, low salinity and low nutrient) that flows southwards along the western margin of the Australian mainland (Cresswell [...]

The ocean is teeming with organisms so small you can't see them, populations of microorganisms called phytoplankton. Tiny they may be, but over recent decades these microscopic plant-like organisms have [...]

In many ecosystems, seasonal shifts in temperature and precipitation induce pulses of primary productivity that vary in phenology, abundance, and nutritional quality. Variation in these resource pulses could strongly influence community composition and ecosystem function, because these pervasive bottom-up forces play a primary role in determining the biomass, life cycles, and interactions of organisms across trophic levels. The focus of this research is to understand how consumers across trophic levels alter resource use and assimilation over seasonal and interannual timescales in response to climatically driven changes in pulses of primary productivity. We measured the carbon isotope ratios ([delta][sup.13]C) of plant, arthropod, and lizard tissues in the northern Chihuahuan Desert to quantify the relative importance of primary production from plants using [C.sub.3] and [C.sub.4] photosynthesis for consumers. Summer monsoonal rains on the Sevilleta Long Term Ecological Research (LTER) site in New Mexico support a pulse of [C.sub.4] plant production that has tissue [delta][sup.13]C values distinct from [C.sub.3] plants. During a year when precipitation patterns were relatively normal, [delta][sup.13]C measurements showed that consumers used and assimilated significantly more [C.sub.4]-derived carbon over the course of a summer, tracking the seasonal increase in abundance of [C.sub.4] plants. In the following spring, after a failure in winter precipitation and the associated failure of spring [C.sub.3] plant growth, consumers showed elevated assimilation of [C.sub.4]-derived carbon relative to a normal rainfall regime. These findings provide insight into how climate, pulsed resources, and temporal trophic dynamics may interact to shape semiarid grasslands such as the Chihuahuan Desert in the present and future. Key words: [C.sub.3] plants; [C.sub.4] plants; carbon; Chihuahuan Desert, New Mexico. USA; food web; lizard; plant photosynthetic type; precipitation; primary, production; pulse; resource; stable isotope.

Litter invertebrates are notoriously patchy at small scales. Here we show that the abundance of l0 litter taxa also varies 100-fold at landscape and regional scales across 26 forest stands in Peru and Panama. We contrast three hypotheses that link gradients of abundance to ecosystem biogeochemistry. Of 14 factors considered (12 chemical elements, plus fiber and litter depth), four best predicted the abundance of litter invertebrates. In the Secondary Productivity Hypothesis, phosphorus limits abundance via the conversion of detritus to microbial biomass. Two of four microbivore taxa, collembola and isopods, increased with the percentage of P (%P) of decomposing litter. However, percentage of S (correlated with %P) best predicted the abundance of collembola, oribatids, and diplopods ([r.sup.2] = 0.38, 0.33, 0.21, respectively). In the Structural Elements Hypotheses, N and Ca limit the abundance of silk-spinning and calcareous taxa, respectively. Mesostigmatids, pseudoscorpions, and spiders, all known to make silk, each increased with percentage of N of litter ([r.sup.2] = 0.22, 0.31, 0.26, respectively). Calcareous isopods, but not diplopods, increased with percentage of Ca of litter ([r.sup.2] = 0.59). In the Ecosystem Size Hypothesis, top predators are limited by available space. The abundance of the three remaining predators, chilopods, staphylinids, and ants, increased with litter depth ([r.sup.2]= 0.31, 0.74, 0.69, respectively), and food webs from forests with deeper litter supported a higher ratio of predators to microbivores. These results suggest that biogeochemical gradients can provide a mechanism, through stoichiometry and trophic theory, shaping the geography of community structure. Key words: calcium; decomposition; ecosystem size; invertebrates; litter; nitrogen; phosphorus; stoichiometry; trophic structure; tropical forests.

We tested the hypothesis that nutrient sequestration and carbon flow through heterotrophic bacteria is relatively highest in oligotrophic systems and decreases with trophic state in 17 reservoirs in Texas and Ohio, U.S.A. The percentage of particulate carbon (C), nitrogen (N), and phosphorus (P) in bacteria (< 1 [micro]m) was highest in oligotrophic reservoirs and decreased with chlorophyll a, our measure of trophic state. Patterns of nutrient sequestration in bacteria were very similar to Minnesota (U.S.A.) natural lakes. Bacterial production (BPr) and primary production (PPr) ranged from 13 [micro]g C [L.sup.-1] d 1 to 172 [micro]g C [L.sup.-1] [d.sup.-1] and from 41 [micro]g C [L.sup.-1] [d.sup.-1] to 1695 [micro]g C [L.sup.-1] [d.sup.-1], respectively, and the BPr:PPr ratio decreased with trophic state. Inclusion of variables such as dissolved organic carbon (DOC), DOC:soluble reactive phosphorus, and DOC:dissolved inorganic nitrogen did not improve predictions of BPr : PPr over that based solely upon chlorophyll a. BPr : PPr was better predicted by DOC: particulate carbon (PC) and mixing depth ([Z.sub.mix]) than by chlorophyll a. The relationship between BPr : PPr and DOC : PC was primarily driven by the relationship between BPr : PPr and PC, which was a surrogate for trophic state. Unexpectedly, the strength of [Z.sub.mix] as a predictor BPr:PPr did not primarily reflect differences in the light environment. Our results support the hypothesis that the relative importance of heterotrophic bacteria is highest in oligotrophic systems and decreases with trophic state, but provide limited support for proposed mechanisms of this pattern. Patterns in microbial nutrient dynamics are similar in reservoirs and natural lakes.

A manipulative mesocosm experiment was done in a saline coastal lake in Australia to compare the top-down and bottom-up influences of jellyfish on primary production and planktonic assemblages. We hypothesized that non-zooxanthellate jellyfish (Catostylus mosaicus) would exert both 'top-down' (grazing) and 'bottom-up' (nutrient excretion) effects, whereas zooxanthellate jellyfish (Phyllorhiza punctata) would only exert top-down influences, as their dissolved excretory products are internally recycled to their zooxanthellae rather than released into the water column. Experimental treatments consisted of triplicate mesocosms that contained two C. mosaicus, two P. punctata, a combination of C. mosaicus and P. punctata, no jellyfish, and an open-water control (sampling outside mesocosms). Both species of jellyfish preyed heavily on mesozooplankton, initiating a top-down trophic cascade that resulted in increased production of the heterotrophic dinoflagellate Protoperidinium sp. However, no increase in primary production or phytoplankton biomass was observed in the treatments containing P. punctata, indicating that top-down processes did not extend to primary producers. Bottom-up excretion of nutrients, however, caused phytoplankton biomass to more than double in the C. mosaicus treatment compared to all other treatments. Increased primary production was due largely to a 10-fold increase in the diatom Chaetoceros sp. and was predominantly driven by C. mosaicus excreting phosphate (P[O.sup.3-.sub.4]), which was the limiting nutrient in the lake. Blooms of both zooxanthellate and non-zooxanthellate jellyfish will deplete mesozooplankton and alter the composition of microzooplankton assemblages via top-down processes. Excretion of nutrients by blooms of non-zooxanthellate jellyfish, however, can also greatly increase phytoplankton production and could favor algal blooms.

Recently, food web studies have started exploring how resources from one habitat or ecosystem influence trophic interactions in a recipient ecosystem. Benthic production in lakes and streams can be exported to terrestrial habitats via emerging aquatic insects and can therefore link aquatic and terrestrial ecosystems. In this study, we develop a general conceptual model that highlights zoobenthic production, insect emergence, and ecosystem geometry (driven principally by area-to-edge ratio) as important factors modulating the flux of aquatic production across the ecosystem boundary. Emerging insect flux, defined as total insect production emerging per meter of shoreline (g C x [m.sup.-1] x [yr.sup.-1]) is then distributed inland using decay functions and is used to estimate insect deposition rate to terrestrial habitats (g C x [m.sup.-2] x [yr.sup.-1]). Using empirical data from the literature, we simulate insect fluxes across the water--land ecosystem boundary to estimate the distribution of fluxes and insect deposition inland for lakes and streams. In general, zoobenthos in streams are more productive than in lakes (6.67 vs. 1.46 g C x [m.sup.-2] x [yr.sup.-1]) but have lower insect emergence to aquatic production ratios (0.19 vs. 0.30). However, as stream width is on average smaller than lake radius, this results in flux (F) estimates 2 1/2 times greater for lakes than for streams. Ultimately, insect deposition onto land (within 100 m of shore) adjacent to average-sized lakes (10-ha lakes, 0.021 g C x [m.sup.-2] x [yr.sup.-1]) greater than for average-sized streams (4 m width, 0.002 g C x [m.sup.-2] x [yr.sup.-1]) used in our comparisons. For the average lake (both in size and productivity), insect deposition rate approaches estimates of terrestrial secondary production in low-productivity ecosystems (e.g., deserts and tundra, [approximately equal to] 0.07 g C x [m.sup.-2] x [yr.sup.-1]). However, larger lakes (1300 ha) and streams (16 m) can have average insect deposition rates ([approximately equal to] 0.01-2.4 g C x [m.sup.-2] x [yr.sup.-1]) comparable to estimates of secondary production of more productive ecosystems such as grasslands. Because of the potentially large inputs of emerging aquatic insects into terrestrial habitats, ecosystem processes and terrestrial consumers can be influenced by insect inputs. The relative contribution of lakes and streams to this flux will vary among landscapes depending on the number and size of these ecosystems types on the landscape. Key words: allochthonous resources; emerging aquatic insects; food webs; insect dispersal; lake-land linkages; landscape ecology; spatial subsidies.

Attached algae play a minor role in conceptual and empirical models of lake ecosystem function but paradoxically form the energetic base of food webs that support a wide variety of fishes. To explore the apparent mismatch between perceived limits on contributions of periphyton to whole-lake primary production and its importance to consumers, we modeled the contribution of periphyton to whole-ecosystem primary production across lake size, shape, and nutrient gradients. The distribution of available benthic habitat for periphyton is influenced by the ratio of mean depth to maximum depth (DR = z/[z.sub.max]). We modeled total phytoplankton production from water-column nutrient availability, z, and light. Periphyton production was a function of light-saturated photosynthesis ([BP.sub.max]) and light availability at depth. The model demonstrated that depth ratio (DR) and light attenuation strongly determined the maximum possible contribution of benthic algae to lake production, and the benthic proportion of whole-lake primary production ([BP.sub.f]) declined with increasing nutrients. Shallow lakes (z [less than or equal to] 5 m) were insensitive to DR and were dominated by either benthic or pelagic primary productivity depending on trophic status. Moderately deep oligotrophic lakes had substantial contributions by benthic primary productivity at low depth ratios and when maximum benthic photosynthesis was moderate or high. Extremely large, deep lakes always had low fractional contributions of benthic primary production. An analysis of the world's largest lakes showed that the Shapes of natural lakes shift increasingly toward lower depth ratios with increasing depth, maximizing the potential importance of littoral primary production in large-lake food webs. The repeatedly demonstrated importance of periphyton to lake food webs may reflect the combination of low depth ratios and high light penetration characteristic of large, oligotrophic lakes that in turn lead to substantial contributions of periphyton to autochthonous production. Key words: average lake depth; chlorophyll; DOC; eutrophication; lake productivity; light attenuation; littoral-pelagic links; microphytobenthos; modeling periphyton production; morphometry; periphyton; primary production.

The interactive effects of changing biodiversity of consumers and their prey are poorly understood but are likely to be important under realistic scenarios of biodiversity loss and gain. We performed two factorial manipulations of macroalgal group (greens, reds, and browns) and herbivore species (amphipods, sea urchin, and fish) composition and richness in outdoor mesocosms simulating a subtidal, hard-substratum estuarine community in North Carolina, USA. In the experiment where grazer richness treatments were substitutive, there were no significant effects of algal or herbivore richness on final algal biomass. However, in the experiment in which grazer treatments were additive (i.e., species-specific densities were held constant across richness treatments), we found strong independent and interactive effects of algal and herbivore richness. Herbivore polycultures reduced algal biomass to a greater degree than the sum of the three herbivore monocultures, indicating that the measured grazer richness effects were not due solely to increased herbivore density in the polycultures. Taking grazer density into account also revealed that increasing algal richness dampened grazer richness effects. Additionally, the effect of algal richness on algal biomass accumulation was far stronger when herbivores were absent, suggesting that grazers can utilize the increased productivity and mask the positive effects of plant biodiversity on primary production. Our results highlight the complex independent and interactive effects of biodiversity between adjacent trophic levels and emphasize the importance of performing biodiversity--ecosystem functioning experiments in a realistic multi-trophic context. Key words: amphipods; biodiversity; complementarity; diversity; ecosystem function; grazers; herbivory; macroalgae; production; richness.

Net primary production (NPP) is influenced by disturbance-driven fluctuations in foliar standing crop (FSC) and resource-driven fluctuations in rates of recruitment and growth, yet most studies of NPP have focused primarily on factors influencing growth. We quantified NPP, FSC, recruitment, and growth rate for the giant kelp, Macrocystis pyrifera, at three kelp forests in southern California, USA, over a 54-month period and determined the relative roles of FSC, recruitment, and growth rate in contributing to variation in annual NPP. Net primary production averaged between 0.42 and 2.38 kg dry mass*[m.sup.-2]* [yr.sup.-1] at the three sites. The initial FSC present at the beginning of the growth year and the recruitment of new plants during the year explained 63% and 21% of the interannual variation observed in NPP, respectively. The previous year's NPP and disturbance from waves collectively accounted for 80% of the interannual variation in initial FSC. No correlation was found between annual growth rate (i.e., the amount of new kelp mass produced per unit of existing kelp mass) and annual NPP (i.e., the amount of new kelp mass produced per unit area of ocean bottom), largely because annual growth rate was consistent compared to initial FSC and recruitment, which fluctuated greatly among years and sites. Although growth rate was a poor predictor of variation in annual NPP, it was principally responsible for the high mean values observed for NPP by Macrocvstis. These high mean values reflected rapid growth (average of ~2% per day) of a relatively small standing crop (maximum annual mean = 444 g dry mass/[m.sup.2]) that replaced itself approximately seven times per year. Disturbance-driven variability in FSC may be generally important in explaining variation in NPP, yet it is rarely examined because cycles of disturbance and recovery occur over timescales of decades or more in many systems. Considerable insight into how variation in FSC drives variation in NPP may be gained by studying systems such as giant kelp forests that are characterized by frequent disturbance and rapid rates of growth and recruitment. Key words: biomass; disturbance; giant kelp; growth; Macrocystis pyrifera; net primary production; nutrients; recruitment; resource supply; standing crop.

The relationship between ecosystem processes and species richness is an active area of research and speculation. Both theoretical and experimental studies have been conducted in numerous ecosystems. One finding of these studies is that the shape of the relationship between productivity and species richness varies considerably among ecosystems and at different spatial scales, though little is known about the relative importance of physical and biological mechanisms causing this variation. Moreover, despite widespread concern about changes in species' global distributions, it remains unclear if and how such large-scale changes may affect this relationship. We present a new conceptual model of how invasive species might modulate relationships between primary production and species richness. We tested this model using long-term data on relationships between aboveground net primary production and species richness in six North American terrestrial ecosystems. We show that primary production and abundance of non-native species are both significant predictors of species richness, though we fail to detect effects of invasion extent on the shapes of the relationship between species richness and primary production.

Predicting the dynamics of ecosystems requires an understanding of how trophic interactions respond to environmental change. In Antarctic marine ecosystems, food web dynamics are inextricably linked to sea ice conditions that affect the nature and magnitude of primary food sources available to higher trophic levels. Recent attention on the changing sea ice conditions in polar seas highlights the need to better understand how marine food webs respond to changes in such broad-scale environmental drivers. This study investigated the importance of sea ice and advected primary food sources to the structure of benthic food webs in coastal Antarctica. We compared the isotopic composition of several seafloor taxa (including primary producers and invertebrates with a variety of feeding modes) that are widely distributed in the Antarctic. We assessed shifts in the trophic role of numerically dominant benthic omnivores at five coastal Ross Sea locations. These locations vary in primary productivity and food availability, due to their different levels of sea ice cover, and proximity to polynyas and advected primary production. The [delta][sup.15]N signatures and isotope mixing model results for the bivalves Laternula elliptica and Adamussium colbecki and the urchin Sterechinus neumeyeri indicate a shift from consumption of a higher proportion of detritus at locations with more permanent sea ice in the south to more freshly produced algal material associated with proximity to ice-free water in the north and east. The detrital pathways utilized by many benthic species may act to dampen the impacts of large seasonal fluctuations in the availability of primary production. The limiting relationship between sea ice distribution and in situ primary productivity emphasizes the role of connectivity and spatial subsidies of organic matter in fueling the food web. Our results begin to provide a basis for predicting how benthic ecosystems will respond to changes in sea ice persistence and extent along environmental gradients in the high Antarctic. Key words: Antarctica; benthic food webs; coastal sea ice; omnivory; Ross Sea; spatial subsidies; stable isotopes.

Abstract: Net ecosystem production (NEP) trends among lakes have been ascribed to differences in nutrient and allochthonous carbon inputs, but little is known on how different habitats within lakes contribute [...]

A hypothesis for progressive nitrogen limitation (PNL) proposes that net primary production (NPP) will decline through time in ecosystems subjected to a stepfunction increase in atmospheric C[O.sub.2]. The primary mechanism driving this response is a rapid rate of N immobilization by plants and microbes under elevated C[O.sub.2] that depletes soils of N, causing slower rates of N mineralization.' Under this hypothesis, there is little long-term stimulation of NPP by elevated C[O.sub.2] in the absence of exogenous inputs of N. We tested this hypothesis using data on the pools and fluxes of C and N in tree biomass, microbes, and soils from 1997 through 2002 collected at the Duke Forest free-air C[O.sub.2] enrichment (FACE) experiment. Elevated C[O.sub.2] stimulated NPP by 18-24% during the first six years of this experiment. Consistent with the hypothesis for PNL, significantly more N was immobilized in tree biomass and in the O horizon under elevated C[O.sub.2]. In contrast to the PNL hypothesis, microbial-N immobilization did not increase under elevated C[O.sub.2], and although the rate of net N mineralization declined through time, the decline was not significantly more rapid under elevated C[O.sub.2].. Ecosystem C-to-N ratios widened more rapidly under elevated C[O.sub.2] than ambient C[O.sub.2] indicating a more rapid rate of C fixation per unit of N, a processes that could delay PNL in this ecosystem. Mass balance calculations demonstrated a large accrual of ecosystem N capital. Is PNL occurring in this ecosystem and will NPP decline to levels under ambient C[O.sub.2]? The answer depends on the relative strength of tree biomass and O-horizon N immobilization vs. widening C-to-N ratios and ecosystem-N accrual as processes that drive and delay PNL, respectively. Only direct observations through time will definitively answer this question. Key words: elevated C[O.sub.2]; net primary production; nitrogen cycling; temperate forest.

A temporal variability and resistance to perturbation of the biomass production of grassland communities from an experimental diversity gradient is studied. The result showed that with increasing species richness relative temporal variability of plant populations increased but that of communities decreased, supporting the insurance hypothesis and related theory.

Biodiversity is declining worldwide from reductions in both species richness and evenness. Field experiments have shown that primary productivity is often reduced when richness of plant species is lowered. However, experiments testing richness effects have used evenness levels that are much higher than normally encountered in plant communities and have been based on the assumption that species extinctions are random. We experimentally varied, for the first time, both species richness (1-8 perennial species/[m.sup.2]) and species evenness (near maximal vs. realistically low) in grassland plots. Net primary productivity (NPP) and ecosystem C[O.sub.2] uptake declined when richness was reduced, and reductions were similar between evenness treatments. Richness effects were associated more with a selection effect than with complementarity (found only with high evenness). Importantly, extinctions in plots during the second year were not random, but were greater at low than at high evenness (i.e., with increased rarity) and in species with low aboveground growth rates. Thus, species evenness can affect grassland ecosystem processes indirectly by affecting species richness, and it will be imperative to understand how nonrandom extinctions affect NPP in future studies. Our results indicate that richness studies may not be biased by using mixtures with artificially high evenness levels, but the results also demonstrate that results from these studies are directly applicable only to communities in which plant extinctions are random. Key words: grasslands; prairies; primary productivity; rank-abundance relationships; species diversity; species evenness; species richness.

Algal [sup.14]C uptake and bacterial [sup.3]H leucine incorporation were measured over 20 months to assess the influence of macrophytes on the spatial distribution and magnitude of microbial production and the relative importance of algae versus macrophytes to whole-system energy flow in a southeastern U.S. wetland. Algal and bacterial production were determined for water column and plant-, sediment-, and wood-surface microhabitats in four zones defined by aquatic vascular plant composition: no macrophyte, floating leaved (Nymphaea odorata), heterophyllous (Proserpinaca palustris), or emergent macrophyte (Juncus effusus) zones. We combined production data with detailed habitat measurements to estimate production at meter-squared and whole-wetland scales and compared microbial C fixation to concurrently determined rates of macrophyte production. Production on plant surfaces was significantly lower than on wood and benthic sediments in all zones. At a meter-squared scale, 79% of algal production and 74% of bacterial production occurred on sediments, with epiphytes contributing

According to classical exploitation theory, an increase of primary productivity should result in increased herbivore grazing pressure, thus maintaining a low plant standing crop. However, field data obtained from a salt marsh revealed a maximal grazing pressure by hares, rabbits, and geese at intermediate levels of plant standing crop. Grazing pressure was relatively low in the more productive parts. We argue that this pattern is due to a low foraging efficiency of these herbivores in dense vegetation. In order to investigate this hypothesis, we examine a plant-herbivore model where grazing becomes less efficient in dense vegetation, and analyze the behavior of this model along a gradient of primary productivity. In systems of intermediate productivity, the model predicts that a plant-herbivore system may have two stable states. In one state, the herbivore maintains a low standing crop. The other state is dominated by a dense vegetation unsuitable for herbivore grazing. In systems of high productivity, the herbivore is unable to keep plant growth in check and a dense vegetation develops. Thus, in line with our field data but in contrast to classical exploitation theory, our model does not predict 'top-down' control in productive environments. Key words: foraging efficiency; functional response; geese; hare; herbivory; multiple stable states; plant-herbivore model; predator - prey theory; primary productivity; rabbit; salt marsh., INTRODUCTION Many models relating community structure to ecosystem functioning predict 'top-down' control along a gradient of primary productivity (e.g., Hairston et al. 1960, Fretwell 1977, Oksanen et al. 1981, DeAngelis [...]

HIGH-LATITUDE OCEANS ARE ECOLOGICALLY sensitive regions where impacts of climate change are believed to be largest and where early detection of change is possible. An obvious consequence of warming in [...]

An aggregative response of Dark-bellied Brent geese to increased productivity during growing season on agricultural fields is reported. Findings indicate that continuously grazed swards are more suitable for feeding.

Abstract. Progress in modeling the global carbon cycle is inhibited by the lack of a high-quality data set based upon field observations of net primary productivity (NPP) with which to [...]