Showing papers on "Productivity (ecology) published in 2002"

Small Players, Large Role: Microbial Influence on Biogeochemical Processes in Pelagic Aquatic Ecosystems

Abstract: Although prokaryotes are small in size, they are a significant biomass component in aquatic planktonic ecosystems and play a major role in biogeochemical processes. A review of the recent literature shows that the relative importance of prokaryotes to material and energy fluxes is maximized in low-productivity (oligotrophic) ecosystems and decreases in high-productivity (eutrophic) ecosystems. We conclude that competition with eukaryotic autotrophs for dissolved nutrients and competition with phagotrophic heterotrophs and physical processes (sinking, photooxidation) for organic carbon (C) play important roles in determining the relative abundance and impact of prokaryotes in aquatic systems. Oligotrophic systems have low nutrient concentrations, with high proportions of dissolved nutrients in organic form, which favors prokaryotic heterotrophs over phytoplankton. Furthermore, a high proportion of the available organic C is dissolved rather than particulate, which favors prokaryotic heterotrophs over phagotrophic heterotrophs. In eutrophic systems, increased relative concentrations and loading of inorganic nutrients and increased relative concentrations of particulate organic C select for phytoplankton and phagotrophic heterotrophs over prokaryotic heterotrophs. Increased particle sinking fluxes and/or decreased excretion of organic carbon (EOC) may also decrease the relative importance of prokaryotic heterotrophs in eutrophic systems. In oligotrophic systems, interactions between autotrophs and heterotrophs are tightly coupled because the dominant heterotrophs are similar in size and growth rates, as well as having similar nutrient composition to the dominant autotrophs, small phytoplankton. In eutrophic systems, increased productivity passes through zooplankton that are larger and have slower growth rates than the autotrophs, leading to a greater potential for decoupled auto- and heterotrophic production and increased export production.

Consumer versus resource control of species diversity and ecosystem functioning

Abstract: A key question in ecology is which factors control species diversity in a community1, 2, 3. Two largely separate groups of ecologists have emphasized the importance of productivity or resource supply, and consumers or physical disturbance, respectively. These variables show unimodal relationships with diversity when manipulated in isolation4, 5, 6, 7, 8. Recent multivariate models9, 10, however, predict that these factors interact, such that the disturbance–diversity relationship depends on productivity, and vice versa. We tested these models in marine food webs, using field manipulations of nutrient resources and consumer pressure on rocky shores of contrasting productivity. Here we show that the effects of consumers and nutrients on diversity consistently depend on each other, and that the direction of their effects and peak diversity shift between sites of low and high productivity. Factorial meta-analysis of published experiments confirms these results across widely varying aquatic communities. Furthermore, our experiments demonstrate that these patterns extend to important ecosystem functions such as carbon storage and nitrogen retention. This suggests that human impacts on nutrient supply11 and food-web structure12, 13 have strong and interdependent effects on species diversity and ecosystem functioning, and must therefore be managed together.

Relevance of mangroves for the production and deposition of organic matter along tropical continental margins.

Abstract: Mangroves are highly complex ecosystems occupying a major part of tropical coastlines. High pri- mary productivity, efficient biological nutrient recyling and a permanent exchange with terrestrial and marine ecosystems are their common features. Despite the high production and export rates of leaf litter, mangrove detri- tus has been reported to be of minor importance in sus- taining marine food webs. The geographical distribution of mangrove-derived organic matter (OM) in marine sediments is found to be restricted to the vicinity of its source. Dissolved nutrient inputs from mangroves and rivers may fuel the production of marine OM. In this paper we assess the relevance of mangroves for the pro- duction and sedimentation of OM in the tropical coastal ocean based on data available from the literature and our own research results. We estimate the rates of carbon ac- cumulation in mangrove sediments and of carbon export to the coastal seas. From the rates of litter fall and export we calculate carbon accumulating in mangrove sedi- ments to be in the order of 23 ·1012 g C per year and mangrove carbon introduced into the coastal ocean to be in the order of 46·10 12 g C per year. They account for about 11% of the total input of terrestrial carbon into the ocean and 15% of the total carbon accumulating in modern marine sediments.

Estimating net primary productivity from grassland biomass dynamics measurements

Abstract: To address the need for a high quality data set based upon field observations suitable for parameterization, calibration, and validation of terrestrial biosphere models, we have developed a comprehensive global database on net primary productivity (NPP). We have compiled field measurements of biomass and associated environmental data for multiple study sites in major grassland types worldwide. Where sufficient data were available, we compared aboveground and total NPP estimated by six computational methods (algorithms) for 31 grassland sites. As has been found previously, NPP estimates were 2–5 times higher using methods which accounted for the dynamics of dead matter, compared with what is still the most commonly applied estimate of NPP (maximum peak live biomass). It is suggested that assumptions such as the use of peak biomass as an indicator of NPP in grasslands may apply only within certain subbiomes, e.g. temperate steppe grasslands. Additional data on belowground dynamics, or other reliable estimates of belowground productivity, are required if grasslands are to be fully appreciated for their role in the global carbon cycle.

Impacts of Soil Faunal Community Composition on Model Grassland Ecosystems

Abstract: Human impacts, including global change, may alter the composition of soil faunal communities, but consequences for ecosystem functioning are poorly understood. We constructed model grassland systems in the Ecotron controlled environment facility and manipulated soil community composition through assemblages of different animal body sizes. Plant community composition, microbial and root biomass, decomposition rate, and mycorrhizal colonization were all markedly affected. However, two key ecosystem processes, aboveground net primary productivity and net ecosystem productivity, were surprisingly resistant to these changes. We hypothesize that positive and negative faunal-mediated effects in soil communities cancel each other out, causing no net ecosystem effects.

Net primary productivity of a CO2-enriched deciduous forest and the implications for carbon storage

Abstract: A central question concerning the response of terrestrial ecosystems to a changing atmosphere is whether increased uptake of carbon in response to increasing at- mospheric carbon dioxide concentration results in greater plant biomass and carbon storage or, alternatively, faster cycling of C through the ecosystem. Net primary productivity (NPP) of a closed-canopy Liquidambar styraciflua (sweetgum) forest stand was assessed for three years in a free-air CO2-enrichment (FACE) experiment. NPP increased 21% in stands ex- posed to elevated CO2, and there was no loss of response over time. Wood increment increased significantly during the first year of exposure, but subsequently most of the extra C was allocated to production of leaves and fine roots. These pools turn over more rapidly than wood, thereby reducing the potential of the forest stand to sequester additional C in response to atmospheric CO2 enrichment. Hence, while this experiment provides the first evidence that CO2 enrichment can increase productivity in a closed-canopy deciduous forest, the implications of this result must be tempered because the increase in productivity resulted in faster cycling of C through the system rather than increased C storage in wood. The fate of the additional C entering the soil system and the environmental interactions that influence allocation need further investigation.

Desertification alters patterns of aboveground net primary production in Chihuahuan ecosystems

Abstract: The Chihuahuan desert of New Mexico, USA, has changed in historical times from semiarid grassland to desert shrublands dominated by Larrea tridentata and Prosopis glandulosa. Similar displacement of perennial grasslands by shrubs typifies desertification in many regions. Such structural vegetation change could alter average values of net primary productivity, as well as spatial and temporal patterns of production. We investigated patterns of aboveground plant biomass and net primary production in five ecosystem types of the Jornada Basin Long-Term Ecological Research (LTER) site. Comparisons of shrub-dominated desertified systems and remnant grass-dominated systems allowed us to test the prediction that shrublands are more heterogeneous spatially, but less variable over time, than grasslands. We measured aboveground plant biomass and aboveground net primary productivity (ANPP) by species, three times per year for 10 years, in 15 sites of five ecosystem types (three each in Larrea shrubland, Bouteloua eriopoda grassland, Prosopis dune systems, Flourensia cernua alluvial flats, and grass-dominated dry lakes or playas). Spatial heterogeneity of biomass at the scale of our measurements was significantly greater in shrub-dominated systems than in grass-dominated vegetation. ANPP was homogeneous across space in grass-dominated systems, and in most growing seasons was significantly more patchy in shrub vegetation. Substantial interannual variability in ANPP complicates comparison of mean values across ecosystem types, but grasslands tended to support higher ANPP values than did shrub-dominated systems. There were significant interactions between ecosystem type and season. Grasslands demonstrated higher interannual variation than did shrub systems. Desertification has apparently altered the seasonality of productivity in these systems; grasslands were dominated by summer growth, while sites dominated by Larrea or Prosopis tended to have higher spring ANPP. Production was frequently uncorrelated across sites of an ecosystem type, suggesting that factors other than season, regional climate, or dominant vegetation may be significant determinants of actual NPP.

Resource availability dominates and alters the relationship between species diversity and ecosystem productivity in experimental plant communities.

Abstract: Experimental evidence that plant species diversity has positive effects on biomass production appears to conflict with correlations of species diversity and standing biomass in natural communities. This may be due to the confounding effects of a third variable, resource availability, which has strong control over both diversity and productivity in natural systems and may conceal any positive effects of diversity on productivity. To test this hypothesis, I independently manipulated resource availability (soil fertility) and sown species diversity in a field experiment and measured their individual and interactive effects on productivity. Although fertility was a far stronger predictor of productivity than diversity, the effect of diversity on productivity significantly increased with fertility. Relative yield analyses indicated that plant mixtures of high fertility treatments significantly "overyielded," or were more productive than expected based on monoculture yields of component species. In contrast, plant mixtures of low fertility treatments had significantly lower-than-expected yields. The effect of diversity on productivity was also driven by sampling effects, where more species-rich mixtures were more likely to include particularly productive species. Unexpectedly, the strength of sampling effects was largely insensitive to fertility, although the particular species most responsible for sampling effects did change with fertility. These results suggest that positive effects of species diversity on ecosystem productivity in natural systems are likely to be masked by variation in environmental factors among habitats.

Grazing effect on diversity of annual plant communities in a semi-arid rangeland: interactions with small-scale spatial and temporal variation in primary productivity

Abstract: Summary 1 The interactive effect of grazing and small-scale variation in primary productivity on the diversity of an annual plant community was studied in a semiarid Mediterranean rangeland in Israel over 4 years. The response of the community to protection from sheep grazing by fenced exclosures was compared in four neighbouring topographic sites (south- and north-facing slopes, hilltop and wadi (dry stream) shoulders), differing in vegetation, physical characteristics and soil resources. The herbaceous annual vegetation was highly diverse, including 128 species. Average small-scale species richness of annuals ranged between 5 and 16 species within a 20 × 20 cm quadrat, and was strongly affected by year and site. 2 Above-ground potential productivity at peak season (i.e. in fenced subplots) was typical of semiarid ecosystems (10–200 g m−2), except on wadi shoulders (up to 700 g m−2), where it reached the range of subhumid grassland ecosystems. Grazing increased richness in the high productivity site (i.e. wadi), but did not affect, or reduced, it in the low productivity sites (south- and north-facing slopes, hilltop). Under grazing, species richness was positively and linearly related to potential productivity along the whole range of productivity. Without grazing, this relationship was observed only at low productivity (< 200 g m−2). 3 The effect of grazing along the productivity gradient on different components of richness was analysed. At low productivity, number of abundant, common and rare species all tended to increase with productivity, both with and without grazing. Rare species increased three times compared with common and abundant species. At high productivity, only rare species continued to increase with productivity under grazing, while in the absence of grazing species number in the different abundance groups was not related to productivity. 4 In this semiarid Mediterranean rangeland, diversity of the annual plant community is determined by the interaction between grazing and small-scale spatial and temporal variation in primary productivity, operating mainly on the less abundant species in the community.

Direct estimation of aboveground forest productivity through hyperspectral remote sensing of canopy nitrogen

Abstract: The concentration of nitrogen in foliage has been related to rates of net photosynthesis across a wide range of plant species and functional groups and thus rep- resents a simple and biologically meaningful link between terrestrial cycles of carbon and nitrogen. Although foliar N is used by ecosystem models to predict rates of leaf-level photosynthesis, it has rarely been examined as a direct scalar to stand-level carbon gain. Establishment of such relationships would greatly simplify the nature of forest C and N linkages, enhancing our ability to derive estimates of forest productivity at landscape to regional scales. Here, we report on a highly predictive relationship between whole-canopy nitrogen concentration and aboveground forest productivity in diverse forested stands of varying age and species composition across the 360 000-ha White Mountain National Forest, New Hampshire, USA. We also demonstrate that hyperspectral remote sensing can be used to estimate foliar N concentration, and hence forest production across a large number of contiguous images. Together these data suggest that canopy-level N concentration is an important correlate of productivity in these forested systems, and that imaging spectrometry of canopy N can provide direct estimates of forest productivity across large landscapes.

Eco-Physiological Controls on the Productivity of Spartina Alterniflora Loisel

Abstract: The intertidal salt marshes of the Atlantic and Gulf coasts of the United States are dominated by the perennial grass, Spartina alterniflora Loisel. The ecology of salt marshes in which this species dominates has been extensively investigated because of the documented biogeochemical functions that these ecosystems perform and the resulting societal values they provide. Since many of the salt marsh-derived values originate, either directly or indirectly, from the presence of a vegetated marsh and its primary productivity, it has long been a major goal of salt marsh ecology to elucidate the determinants of the growth of Spartina. This paper reviews the interaction of the abiotic environment with key eco-physiological processes controlling the growth of this important plant species. The productivity of Spartina can vary on both spatial and temporal scales. Spatial differences in productivity on a local scale are primarily determined by abiotic factors, particularly the interaction of soil anoxia, soluble sulfide, and salinity, with plant nitrogen uptake and assimilation. Also, Spartina can induce a positive feedback on productivity by enhancing substrate aeration. The growth enhancing effects of marsh infauna, e.g., fiddler crabs, are mediated through these interacting abiotic variables. Productivity differences on regional scales are largely dependent on geographical differences in climate, tidal amplitude, and soil parent material. Temporal variation results from seasonal and annual variation in climatic and tidal controls that may influence marsh salinity and/or inundation. The concerted research of a large number of scientists has provided one of the most comprehensive and ecologically-relevant analyses of determinants of the primary productivity of any nonagricultural plant species.

Diversity, biomass and ecosystem processes in the marine benthos.

Abstract: Recent studies in terrestrial, plant-dominated systems have shown that reductions in diversity can affect essential ecosystem processes, especially productivity. However, the exact form of the relationship between diversity and ecosystem functions remains unknown, as does the relevance of these studies to other systems. We studied the relationships between macroinvertebrate species richness and ecosystem functions in a soft-bottom, intertidal system. We also considered, as a separate variable, the effects of macroinvertebrate biomass on ecosystem functions. A field experiment was conducted at Blackness, a mudflat in the Firth of Forth, Scotland, United Kingdom, using cages with different mesh sizes (195, 300, and 3000 μm) to establish low, medium, and high species richness treatments through differential colonization of defaunated sediments. Low, medium, and high biomass treatments were established by enclosing differing amounts of ambient sediment in defaunated plots. Other treatments controlled for the effects of defaunation and caging. The experiment ran for six weeks in the summer of 1999. All treatments contained species within the same five main functional groups of macroinvertebrate, but species' identity varied both within and between treatments (thus species richness was considered a random, rather than fixed, variable). A total of 27 macroinvertebrate species were sampled across all treatments; 37% of these occurred in the low, 52% in the medium, and 74% in the high diversity treatments. At the end of the experiment, the following physical variables were measured as indicators of ecosystem functions such as sediment stabilization and nutrient fluxes: sediment shear strength (a measure of sediment cohesiveness), water content, silt/clay content, organic content, redox potential (a measure of anoxia), nitrate, nitrite, phosphate and ammonium fluxes, and community respiration. Changes in biomass and species richness were found to have significant effects on oxygen consumption; these relationships were driven in particular by the presence of the largest species in our study, Nephtys hombergii. All other variables were not significantly affected by the treatments. These results support the null hypotheses of no relationship between ecosystem functions and diversity and biomass. However, our experiment was necessarily limited in both spatial and temporal scale; the implications of this when scaling up to larger scale generalizations are discussed. Our results suggest that diversity/biomass/ecosystem function relationships in the soft sediment benthos are likely to be very complex and may depend more on functional groups than species richness.

How plant diversity and legumes affect nitrogen dynamics in experimental grassland communities.

Abstract: Positive relationships between species richness and ecosystem processes such as productivity or nitrogen cycling can be the result of a number of mechanisms. We examined how species richness, biomass, and legume presence, diversity, and abundance explained nitrogen dynamics in experimental grassland plots in northern Sweden. Nitrogen concentrations and δ15N values were measured in plants grown in 28 mixtures (58 plots) including 1, 2, 4, 8 or 12 local grassland species over four years. Values for δ15N declined over time for all three functional groups (grasses, legumes, and non-leguminous forbs), suggesting greater reliance on N fixed by legumes over time by all species. Above ground percent nitrogen (%N) also declined over time but root %N and total N did not. Path analysis of above ground data suggested that two main factors affected %N and the size of the N pool. First, higher plant diversity (species richness) increased total N through increased biomass in the plot. Although in the first two years of the experiment this was the result of a greater probability of inclusion of at least one legume, in the last two years diversity had a significant effect on biomass beyond this effect. Second, percent legumes planted in the plots had a strong effect on above ground %N and δ15N, but a much smaller effect on above ground biomass. In contrast, greater plant diversity affected N in roots both by increasing biomass and by decreasing %N (after controlling for effects mediated by root biomass and legume biomass). Increased legume biomass resulted in higher %N and lower δ15N for both non-legume forbs and grasses in the first year, but only for grasses in the third year. We conclude that a sampling effect (greater probability of including a legume) contributed towards greater biomass and total N in high-diversity communities early on in the experiment, but that over time this effect weakened and other positive effects of diversity became more important.

Using simple environmental variables to estimate below-ground productivity in grasslands

Abstract: In many temperate and annual grasslands, above-ground net primary productivity (NPP) can be estimated by measuring peak above-ground biomass. Estimates of below-ground net primary productivity and, consequently, total net primary productivity, are more difficult. We addressed one of the three main objectives of the Global Primary Productivity Data Initiative for grassland systems to develop simple models or algorithms to estimate missing components of total system NPP. Any estimate of below-ground NPP (BNPP) requires an accounting of total root biomass, the percentage of living biomass and annual turnover of live roots. We derived a rela- tionship using above-ground peak biomass and mean annual temperature as predictors of below-ground biomass ( r 2 = 0.54; P = 0.01). The percentage of live material was 0.6, based on published values. We used three different functions to describe root turnover: constant, a direct function of above-ground biomass, or as a positive exponential relationship with mean annual temperature. We tested the various models against a large database of global grassland NPP and the constant turn- over and direct function models were approximately equally descriptive ( r 2 = 0.31 and 0.37), while the exponential function had a stronger correlation with the measured values ( r 2 = 0.40) and had a better fit than the other two models at the productive end of the BNPP gradient. When applied to extensive data we assembled from two grassland sites with reliable estimates of total NPP, the direct function was most effective, especially at lower productivity sites. We provide some caveats for its use in systems that lie at the extremes of the grassland gradient and stress that there are large uncertainties associated with measured and modelled estimates of BNPP.

Effects of seasonal and interannual climate variability on net ecosystem productivity of boreal deciduous and conifer forests

Abstract: The response of net ecosystem productivity (NEP) and evaporation in a boreal aspen (Populus tremuloides Michx.) forest and a black spruce (Picea mariana (Mill.) BSP) forest in Canada was compared u...

Phytoplankton Productivity: Carbon Assimilation in Marine and Freshwater Ecosystems

Abstract: Contributors. Foreword. Acknowledgements. A Biography Of Einer Steemann Nielsen: the man and his Science. History Of The Study Of Plankton Productivity. Physiology And Biochemistry Of Photosynthesis And Algal Carbon Acquisition. Approaches To The Measurement Of Plankton Production. Diffusive Supply Of Inorganic Nutrients: Theory And Methodology. Variability Of Plankton And Plankton Processes On The Mesoscale. Assessment Of Primary Production On The Global Scale. Origins And Causes Of Interannual Variability Of Freshwater Phytoplankton. Interannual Variability Of Phytoplankton In Oceanic Systems. Ecosystem Function And Eutrophication. The Evolution Of The Productivity Of The Oceans And The Significance Of The Evolution Of Specific Groups. Regional--Scale Influences On The Long--Term Dynamics Of Lakes. Marine Productivity: The Footprint Of The Past And Steps Into The Future. Appendix: Steeman Nielsena s Publications. Index

Control of microbenthic communities by grazing and nutrient supply

Abstract: In periphyton communities, autotrophic algae and prokaryotes live in close spatial proximity to heterotrophic components such as bacteria and micro- and meiofauna. In factorial field experiments, we manipulated grazer access and nutrient supply to periphyton communities and measured the effects on algal, ciliate, meiofaunal, and bacterial biomass. We tested whether grazing macrozoobenthos affects all periphytic components (generalist consumption), whether nutrient effects propagate through the community, and whether interactions between the different periphyton groups allow for indirect feedback mechanisms. The experiments were conducted during three different seasons in a meso-eutrophic lake in Sweden (Lake Erken) and at an adjacent coastal marine site (Vaddo) of similar productivity, but with contrasting grazer fauna. We found strong direct effects of nutrients and grazing on algae at both sites. Algal biomass increased in fertilized treatments and was significantly reduced when grazers were present. The algae clearly dominated the system quantitatively and were positively correlated to the biomass of ciliates and meiofauna. The effects of grazing and nutrients were more complex for heterotrophs than for algae. Generally, the presence of grazers tended to increase the biomass of bacteria, ciliates, and meiofauna. Thus, macrograzers were not generalist consumers of the entire community, but mainly reduced algae. Furthermore, the results suggested strong indirect effects of grazing, presumably through changes in nutrient supply and algal size structure. Nutrient enrichment had weak and inconsistent effects on bacterial, ciliate, and meiofaunal biomass. There was thus no complete propagation of bottom-up effects through the community, and strong internal feedback mechanisms within the periphyton mediated the effects of macroconsumers and nutrient enrichment.

Energy budget and ecological role of mangrove epibenthos in the Caeté estuary, North Brazil

Abstract: Epibenthic community structure, somatic production and energy flow were studied in the Caete mangrove estuary in North Brazil on for 3 representative strata: high intertidal forest (F), small creeks in the forest (SC) and open mudbanks of large intertidal creeks (LC). Seven decapod crustaceans and 1 gastropod accounted for >95% of total epifaunal biomass, with highest values in the forest followed by large and small creeks (228.2, 103.6 and 69.7 kJ m -2 respectively). The leaf- consuming crab Ucides cordatus was clearly dominant in the forest, followed by the fiddler crabs Uca rapax and U. vocator. The large creek stratum was strongly dominated by the fiddler crab U. mara- coani, while in the small creek 4 species (Uca cumulanta, U. maracoani, Pachygrapsus gracilis and Eurytium limosum) contributed similar quantities to total biomass. Per area somatic production (P) and respiration (R) was highest in the large creek, followed by the forest and small creek stratum. Based on the contribution of each stratum, total biomass (332.8 kJ m -2 ), production (455.8 kJ m -2 yr -1 ) and assimilation (2959.6 kJ m -2 yr -1 ) were estimated for the whole area. While the herbivorous feed- ing guild with U. cordatus was the most prominent in terms of biomass (75% of the total), the detri- tivorous fiddler crabs and P. gracilis clearly dominated in terms of respiration and somatic production (60 and 90% respectively). Carnivores (Eurytium limosum and Thais coronata) contributed <2% to the epibenthic energy budget. A system picture emerges whereby energy flow is strongly dominated by herbivorous and detritivorous species. Both guilds probably promote mangrove primary produc- tion by (1) conserving nutrients in the system, (2) enhancing nutrient remineralization, and (3) oxy- genizing the soil through their burrowing and feeding activities. A resulting positive feedback loop between mangroves, crabs and bacteria could explain the very high mangrove productivity and the high efficiency with which mangrove primary production is assimilated by the crabs (almost 15%).

Fluctuations of macrobenthic populations: a link between climate-driven river run-off and sole fishery yields in the Gulf of Lions

Abstract: The Rhone river is the most important input to the Mediterranean Sea, responsible for 50% of the pri- mary productivity of the Gulf of Lions. A highly vari- able amount of 1-23×10 6 t year -1 of terrestrial material is exported to the sea by the Rhone and stocked on the continental shelf for the most part. Soft-bottom commu- nities off the Rhone delta were dominated by polychae- tes both in species richness and abundance, and exhibit- ed strong temporal fluctuations mainly related to flood- ing events. Floods caused pulses of organic matter fol- lowed, with different time lags, by peaks of polychaetes. Opportunistic, short-lived species, such as Mediomastus sp. and Aricidea claudiae, exhibited high short-term peaks in density and biomass a few months after flood- ing events. Conversely, long-lived species, such as Lao- nice cirrata and Sternaspis scutata, peaked in density and biomass with a time lag of 1-3 years, and their pop- ulation increase lasted for a few years. The common sole, Solea solea, is a voracious predator of polychaetes which represent >80% of its prey. A positive correlation was found between the mean annual discharge of the Rhone river and the annual commercial landings of S. solea with a time lag of 5 years in the two fishing har- bours (Sete and Martigues) located close to the Rhone delta. The long-term increase in food (i.e. polychaete density and biomass) after flooding events might favour the different stages of the sole life cycle, enhancing its population size for several years. Fluctuations of sole fishery yields in the Gulf of Lions could be influenced by climate, as the Rhone river flow is related to the North Atlantic Oscillation that drives precipitation over Western Europe.

Salt Marsh Linkages to Productivity of Penaeid Shrimps and Blue Crabs in the Northern Gulf of Mexico

Abstract: Secondary production derived from coastal marshes of the northern Gulf of Mexico exceeds that of other regions in the United States and is exemplified by large fishery catches of penaeid shrimps (Farfantepenaeus aztecus, F. duorarum, and Litopenaeus setiferus – 66% of U.S.) and blue crabs (Callinectessapidus −25% of U.S.). We believe that this production arises from coastal wetlands, and is driven by wetland geomorphology and hydrology resulting from the delta building and wetland loss cycles of the Mississippi River. Quantitative surveys document that high densities of shrimps and blue crabs directly use northern Gulf marsh surfaces. Manipulative experiments demonstrate that such marshes provide these fishery species with increased resources for growth and with protective cover to reduce predator-related mortality. Thus, access to the marsh surface is an important component in controlling the link between secondary productivity and coastal wetlands. Marsh access is influenced by tidal flooding patterns, amount of marsh/water edge, and extent of connections between marsh systems and the Gulf. Low-elevation Gulf marshes are flooded nearly continually during some seasons and are extensively fragmented; such characteristics provide maximum access. By contrast, U.S. Atlantic coast marshes have less fragmentation and less flooding. These geomorphic and hydrologic differences coincide with differences in secondary production between the regions, e.g., marsh-derived fishery production is lower on the Atlantic coast. Despite the linkage between coastal wetlands and secondary production, the current rapid loss of wetlands in the Gulf does not appear to be causing a decline, but instead is associated with an increase in fishery productivity. This paradox may be explained by changes in access and habitat function during areal loss of wetlands. Wetland loss is accompanied by increased marsh inundation and fragmentation, expansion of saline zones, and shortened migratory routes. These processes extend the utilization of remaining marsh and support temporary increases in secondary production.

Community Structure and Functional Dynamics of Benthic Microalgae in Salt Marshes

Abstract: Benthic microalgae are a ubiquitous feature in sediments directly exposed to full sunlight or shaded by a vascular plant canopy in coastal salt marshes. Diatoms, cyanobacteria, and green algae are the dominant groups. Of these, diatoms are universally present and abundant, exhibit migratory rhythms driven mainly by light, and are by far the taxonomically most diverse group. Dense mats of cyanobacteria and secondarily green algae frequently develop where light levels are high. The more abundant species of all three algal groups are widely distributed within and among salt marshes of the United States and Europe. Standing crops of benthic microalgae beneath various vascular plant canopies exhibit mean annual values of 60 to 160 mg chl a m−2. Annual benthic microalgal production (BMP) has been shown to range from 28 g C m−2 y−1 beneath Juncus roemerianus to 314 g C m−2 y−1 beneath Jaumea carnosa. In general, BMP increases in a southerly direction in Atlantic coast marshes but is lowest in Gulf Coast marshes. In Atlantic and southern California marshes a significant portion of benthic microalgal production occurs when the overstory vascular plants are dormant. Experimental manipulations have shown that BMP and biomass beneath Spartina alterniflora are limited by nitrogen supplies and grazing activities. Manipulation of light appears to primarily affect the relative dominance of diatoms and cyanobacteria in the benthic microalgal assemblage. The ratio of annual BMP to net aerial production of the overstory vascular plant canopy is 10 to 60% in Atlantic and Gulf Coast marshes and 75 to 140% in a southern California marsh. The benthic microalgal portion of this two component productivity system has been shown by multiple stable isotope studies to be a major component of salt marsh food webs. Diatoms, in particular, are the preferred food item of a diverse array of invertebrate and fish species.

Microbial dynamics associated with leaves decomposing in the mainstem and floodplain pond of a large river

Abstract: Aquatic habitats of forested floodplain systems receive large inputs of allochthonous plant litter. We examined the decomposition of, and microbial productivity associated with, leaves of a common floodplain tree, Populus gr. nigra, in the mainstem and floodplain pond of a seventh order river in 2 consecutive years. Litter bags were submerged at both sites, retrieved periodically, and analyzed for litter mass loss, bacterial and fungal biomass, growth rate and production, and sporulation rates of aquatic hyphomycetes. Litter decomposition rates were similar in both sites and years (leaf breakdown coefficients k of 0.0070 to 0.0085 d–1), although microbial dynamics partly differed between sites. Species diversity of aquatic hyphomycetes was lower on leaves submerged in the pond (16 species) than in the river (21 species). Mycelial biomass was also significantly lower in the pond, with values <20 mgCg–1 of detrital C, whereas peaks of 50 and 80 mgC g–1 were reached in leaves in the mainstem. These differences contrast with the comparable fungal productivity at both sites (peak rates of 1.4 mg of mycelial C per g of detrital C per day in both years). This suggests that fungi were equally productive in both habitats but experienced greater losses in the pond. Bacterial numbers and biomass also showed the same basic pattern at both sites, although somewhat higher levels were reached in the pond (maximum of about 10^10 cells and 0.5 mg g–1 of detrital C). Bacterial- specific production rates fluctuated between 0.06 and 1.5 d–1 with lower values occurring in the floodplain pond. Although bacteria on leaves were clearly outweighed by fungi in terms of biomass, they accounted for a sizeable fraction of the total biomass (up to 11%), and up to 32% of the total microbial production. Our comparison of bacterial and fungal productivity thus points to a critical role of fungi in litter decomposition in aquatic habitats of river floodplain systems, while suggesting that bacteria must not be overlooked as important agents of litter decompositon in riverine environments.

Soil, biomass, and management of semi-natural vegetation. II. Factors controlling species diversity

Abstract: Using a wide range of conditions and plant community types, species diversity was investigated in relation to edaphic and non-edaphic site conditions, management, and biomass characteristics. Both standing biomass and aboveground production were investigated, and their effects compared. Three taxonomic assemblages were studied: (1) vascular plants only, (2) bryophytes also included, (3) terrestrial lichens included as well. Using a multivariate approach, both species richness and evenness could best be explained when only vascular plants were considered, emphasizing the role of taxonomic restrictions. The models best explaining species richness merely required abiotic conditions. This supports recent theories emphasizing the importance of an environmental regulation of the pool of (adapted) species from which the actual species are recruited. Explanatory soil properties were moisture and pH (both unimodal), and the soil available N:P ratio. Plots with large perimeter:area ratios had significantly more species than those with low ratios, indicating the importance of consistency in quadrat shape. Hump-shaped species richness relationships could be identified for both standing biomass and productivity, but they explained only a small part of the variation and were apparent only if soil and management effects were not accounted for. Unimodality (and notably the decreasing phase) was most pronounced when using maximum standing biomass, suggesting that the key factor is competition for light. At intermediate levels of standing biomass, the positive effects of habitat productivity and the negative effects of standing biomass itself are in balance, and high species numbers may be expected. When soil or management variables were allowed in the models, hump-shaped biomass relationships were no longer confirmed, suggesting that such relationships may arise from the covariation of biomass with other factors. Management explained a much larger part of the variation than the hump-shaped biomass relationship, suggesting that mowing and hay removal (both showing independent positive effects) regulate species richness in ways other than solely through the control of maximum standing biomass. Significantly higher evenness values were obtained at sites with low maximum biomass values, but only for the vascular plants. In addition, species evenness was positively related to the frequency of mowing. The numbers of both rare and endangered species were strongly curtailed by high standing biomass values, suggesting that these species are more susceptible to competitive exclusion than others. Through direct as well as indirect effects, management is confirmed to be beneficial not only for general species richness, but also for the occurrence of rare and endangered species.

Estimated biomass and productivity of natural vegetation on the tibetan plateau

Abstract: We developed a methodology for linking together data from forest and grassland inventories and ecological research sites, and provided a comprehensive report about live biomass and net primary productivity (NPP) on the Tibetan Plateau, the "Third Pole" of the earth where little information about plant biomass and production had been available outside China. Results were as follows. (1) The total live biomass of the natural vegetation in the Xizang (Tibet) Autonomous Region and Qinghai Province was estimated its 2,17 Gg dry mass, of which 72.9% was stored in forests where spruce-fir accounted for 65.1% (2) The total annual NPP of the natural vegetation in these two administrative regions, was estimated as 0.57 Gg dry mass. of which grasslands and forests accounted for 69.5% and 18.1%, respectively. (3) The alpine spruce-fir forests of the Tibetan Plateau had the highest maximum live biomass of the spruce-fir forests globally. with values up to 500-1600 Mg DM/ha (including both aboveground and belowground biomass), (4) The QZNPP model generally predicted NPP well for most of the biomes on the plateau, and simulated the various Chinese vegetation divisions. Model results showed a positive reinforcing effect of monsoon climate in China where the warmest season coincides with the wettest season. (5) The live biomass map for 117 counties of Xizang (Tibet) and Qinghai and the Potential NPP map for the whole plateau both showed the same decreasing trend from Southeast to northwest.

Biodiversity and ecosystem productivity: Implications for carbon storage

Abstract: Recent experiments have found that Net Primary Productivity (NPP) can often be a positive saturating function of plant species and functional diversity. These findings raised the possibility that more diverse ecosystems might store more carbon as a result of increased photosynthetic inputs. However, carbon inputs will not only remain in plant biomass, but will be translocated to the soil via root exudation, fine root turnover, and litter fall. Thus, we must consider not just plant productivity (NPP), but also net productivity of the whole ecosystem (NEP), which itself measures net carbon storage. We currently know little about how plant diversity could influence soil processes that return carbon back to the atmosphere, such as heterotrophic respiration and decomposition of organic matter. Nevertheless, it is clear that any effects on such processes could make NPP a poor predictor of whole-ecosystem productivity, and potentially the ability of the ecosystem to store carbon. We examine the range of mechanisms by which plant diversity could influence net ecosystem productivity, incorporating processes involved with carbon uptake (productivity), loss (autotrophic and heterotrophic respiration), and residence time within the system (decomposition rate). Understanding the relationship between plant diversity and ecosystem carbon dynamics must be made a research priority if we wish to provide information relevant to global carbon policy decisions. This goal is entirely feasible if we utilize some basic methods for measuring the major fluxes of carbon into and out of the ecosystem.

Effect of inorganic sediment on whole-stream productivity

Abstract: Sediment from agricultural, logging, and mining activities impairs more miles of rivers and streams in the United States than any other type of pollutant, including bacteria, nutrients, oxygen-depleting substances, and metals. However, specific impacts of sediment to streams have not been well studied or understood. To study the effect of inorganic sediment on plant and animal communities in stream ecosystems, we added clay to outdoor experimental streams 520 m long and 3.5 m wide at the Monticello Ecological Research Station (MERS). The streams take water from the Mississippi River and are designed to represent higher order streams in the upper midwestern United States. The sediment loading rates were 300, 200, 100, and 50 mg l−1. Our first dosing period (mid-August to November 1994) began at the start of a fall bloom in autotroph productivity, and the second (May to August 1995) began before the summer communities were established. During both treatment seasons, the addition of clay significantly increased turbidity and sedimentation, and decreased light penetration in treated streams corresponding roughly to 30–35, 25–30, 15–20, and 5–10 NTU, respectively. In general, the macrophyte and periphyton communities responded quickly after only a few weeks exposure to the sediment additions. Whole-stream respiration was significantly lower in treated streams, decreasing as the amount of sediment added increased. Periphyton biomass (chl a) on tiles and percent cover of macrophytes was significantly lower in treatment streams than in controls. In contrast to expectations and previous findings in two sets of field streams, total whole-stream productivity in the MERS streams was not significantly lower in streams receiving sediment loads than in control streams because the overall photosynthetic efficiency by the plant community compensated for the loss in irradiance.

Microbial respiration within a floodplain aquifer of a large gravel‐bed river

Abstract: 1. Aerobic respiration, productivity and the carbon turnover rate of microbial biofilms were determined at hyporheic and phreatic sites in the Kalispell Valley alluvial aquifer along a transect extending 3.9 km laterally from the main channel of the Flathead River, a sixth order river in Montana (U.S.A.). The effect of experimentally increasing bioavailable organic carbon (acetate) on the respiration rate of biofilms in this carbon-poor [dissolved organic carbon (DOC) < 2 mg L -1 ] aquifer was also measured. 2. Chambers containing natural substratum were placed in-situ and allowed to colonise for 20 weeks. After 4, 12 and 20 weeks, they were taken to the laboratory where oxygen flux was measured in a computer-controlled, flow-through respirometry system. 3. Respiration ranged from 0.01 to 0.33 mg O 2 dm -3 h -1 across sites, with means ranging from 0.10 to 0.17 mg O 2 dm -3 h -1 . Productivity estimates ranged from 0.18 to 0.32 mg C dm -3 day -1 (mean 0.25, SE 0.03). The total organic carbon (TOC) of the microbial biofilms ranged from 18.2 to 29.7 mg C dm -3 . Turnover rate ranged from 3.2 to 5.6 year -1 with a mean of 4.2 year -1 . 4. At the hyporheic site very close to the river, respiration did not significantly increase when samples were supplemented with labile carbon. Respiration increased with increasing DOC addition at hyporheic sites more distant from the river, suggesting a carbon-limitation gradient within the hyporheic zone. Microbes at the phreatic site did not respond to increasing DOC addition, suggesting that the phreatic biofilm is adapted to low carbon availability. 5. Comparing the volume of the alluvial aquifer (about 0.7 km 3 ) to that of the river benthic sediments (to 0.25 m depth, which amounts to about 1.6 x 10 -4 km 3 ) within the Flathead Valley, leads to the conclusion that interstitial microbial productivity is orders of magnitude greater than benthic productivity. Alluvial aquifers are often voluminous and microbial production is an enormous component of ecosystem production in rivers such as the Flathead.

Dry Heath Arctic Tundra Responses to Long-term Nutrient and Light Manipulation

Abstract: Long-term fertilization studies in several arctic ecosystems have demonstrated dramatic responses of plant community structure with concomitant changes in ecosystem properties. Although these results are well documented in moist tussock and wet sedge tundra, dry heath tundra has been less studied. In an Alaskan dry heath arctic tundra, we conducted a biomass harvest of plants that received additional nitrogen (N, 10 g m-2 yr-1) and/or phosphorus (P, 5 g m-2 yr- 1) or reduced light (50% of ambient) for 8 yr. We expected responses to be similar to those of other arctic tundra communities with increased biomass resulting from added nutrients and species responding individualistically to generate the community-level response. However, total vascular biomass did not change in the dry heath tundra in response to any treatment, although individual species and functional group biomass differed from controls. Aboveground productivity, estimated using new apical growth, significantly increased in the N and N+P plots caused by significantly greater abundance of a tussock-forming grass, Hierochloe alpina. The lowest species richness was recorded in the N alone plots, where a deciduous shrub, Betula nana, had its greatest biomass, and richness also declined in N+P plots. Plots that received P alone had similar biomass and species richness to