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Diversity of primary producer is often surprisingly high, despite few limiting factors such as nutrients and light to facilitate species coexistence. In theory, the presence of herbivores could increase the diversity of primary producers, resolving this "paradox of the plankton." Little experimental evidence supports this natural enemies hypothesis, but previous tests suffer from several deficiencies. Previous experiments often did not allow for multigeneration effects; utilized low diversity assemblages of herbivores; and limited opportunities for new primary producer and herbivore species to colonize and undergo species sorting that favors some species over others. Using pond plankton, we designed a mesocosm experiment that overcame these problems by allowing more time for interactions over multiple generations, openness to allow new colonists, and manipulated higher diversity of primary producers and grazers than have previous studies. With this design, the presence of zooplankton grazers doubled phytoplankton richness. The additional phytoplankton species in grazed mesocosms were larger, and therefore likely more grazer resistant. Furthermore, phytoplankton richness in grazed mesocosms was similar to that observed in natural ponds whereas it was much lower in mesocosms without grazers. However, stoichiometric imbalance caused by variation in nitrogen : phosphorus ratios and light supply did not alter phytoplankton richness. Therefore, grazers enhanced primary producer richness more strongly than ratios of nutrient supply (even though both grazing and ratios of resource supply altered composition of primary producer assemblages). Taken together, these experimental and field data show that grazing from a diverse assemblage of herbivores greatly elevated richness of phytoplankton producers in pond ecosystems.

The vertical distribution and diel migration of Cryptomonas spp. were monitored continuously for 21 years in mesotrophic Cross Reservoir, northeast Kansas, USA. The movements of these motile algae were tracked on multiple dates during July–October of each year using in situ fluorometry and optical microscopy of Lugol’s iodine-preserved samples. Episodes of subepilimnetic diel vertical migration by Cryptomonas were detected and recorded on 221 different days between 1994 and 2014, with just 2 of these years (1998 and 2013) lacking any sampling events with deep peaks sufficiently large enough to track. Whenever a subepilimnetic layer of Cryptomonas was detectable, it was generally observed to ascend toward the bottom of the epilimnion beginning approximately at sunrise; to descend toward the lake bottom during the late afternoon and evening; and to remain as a deep-dwelling population until dawn of the following day. Moreover, there was high day-to-day consistency in the absolute water column depths at which the migrating algal cells would cease their ascending or descending movement. We believe this unique and remarkable dataset comprises the most detailed record of diel migratory behavior for any planktonic freshwater alga reported for a single freshwater lake.

The objective of this study was to investigate the microalgal dewatering efficiency of a belt filter system for feed concentrations below 10 g dry wt./L. A prototype belt filtration system designed for 50 g dry wt./L microalgal feed concentration was used for this investigation. The highest concentration of microalgal suspension available for testing on the prototype belt filtration system was 6 g dry wt./L obtained from biomass settling tanks at the Lawrence, Kansas domestic wastewater treatment plant. For preparation of feed suspension with concentrations below 10 g dry wt./L, microalgal cultivation was followed by flocculation. A mixed laboratory culture of freshwater species dominated by three eukaryotic green microalgae (Chlorella vulgaris, Scenedesmus sp., and Kirchneriella sp.) was cultivated in wastewater effluent. This was followed by flocculation which resulted in a microalgal feed suspension concentration of 4 g dry wt./L. Belt dewatering tests were conducted on microalgal suspensions with feed concentrations of 4 g dry wt./L and 6 g dry wt./L. The maximum microalgal recovery with the belt dewatering system was 46% from the 4 g dry wt./L, and 84% from the 6 g dry wt./L suspensions respectively. The results of this study indicate that microalgal suspension concentrations as low as 6 g dry wt./L can be recovered with a belt filter system improving the overall dewatering efficiency of the system.

Successful algal biomass cultivation at scale is a key limiting step for the production of algal biofuels and other high-value products, and crop protection against undesirable biomass losses will be a critically important component of commercialization efforts. If algal biomass production occurs in large, open, outdoor ponds, then it can be expected that invasions of these production facilities by multiple species of algae, invertebrate herbivores, and pathogens will only be a matter of time. This review identifies and discusses key aspects of community and ecosystem ecology that have direct relevance to the successful cultivation of algal biomass. We use experiences and examples from commercial agriculture to illustrate core ecological principles of crop cultivation that we believe can successfully be transferred to large-scale algal biomass production. We then discuss the degree to which herbivores and disease can significantly reduce potential yields, and the concepts of biological control. We also discuss the effects of crop species diversity and composition on algal biomass production, and explore the potential benefits of algal polycultures in large-scale algal biomass cultivation systems.

Microalgae represent one of the most promising groups of candidate organisms for replacing fossil fuels with contemporary primary production as a renewable source of energy. Algae can produce many times more biomass per unit area than terrestrial crop plants, easing the competing demands for land with food crops and native ecosystems. However, several aspects of algal biology present unique challenges to the industrial-scale aquaculture of photosynthetic microorganisms. These include high susceptibility to invading aquatic consumers and weeds, as well as prodigious requirements for nutrients that may compete with the fertiliser demands of other crops. Most research on algal biofuel technologies approaches these problems from a cellular or genetic perspective, attempting either to engineer or select algal strains with particular traits. However, inherent functional trade-offs may limit the capacity of genetic selection or synthetic biology to simultaneously optimise multiple functional traits for biofuel productivity and resilience. We argue that a community engineering approach that manages microalgal diversity, species composition and environmental conditions may lead to more robust and productive biofuel ecosystems. We review evidence for trade-offs, challenges and opportunities in algal biofuel cultivation with a goal of guiding research towards intensifying bioenergy production using established principles of community and ecosystem ecology.

We investigate the effects of trophic lifestyle and two types of major evolutionary transitions in individuality—the endosymbiotic acquisition of organelles and development of multicellularity—on organellar and cellular metabolism and allometry. We develop a quantitative framework linking the size and metabolic scaling of eukaryotic cells to the abundance, size and metabolic scaling of mitochondria and chloroplasts and analyse a newly compiled, unprecedented database representing unicellular and multicellular cells covering diverse phyla and tissues. Irrespective of cellularity, numbers and total volumes of mitochondria scale linearly with cell volume, whereas chloroplasts scale sublinearly and sizes of both organelles remain largely invariant with cell size. Our framework allows us to estimate the metabolic scaling exponents of organelles and cells. Photoautotrophic cells and organelles exhibit photosynthetic scaling exponents always less than one, whereas chemoheterotrophic cells and organelles have steeper respiratory scaling exponents close to one. Multicellularity has no discernible effect on the metabolic scaling of organelles and cells. In contrast, trophic lifestyle has a profound and uniform effect, and our results suggest that endosymbiosis fundamentally altered the metabolic scaling of free-living bacterial ancestors of mitochondria and chloroplasts, from steep ancestral scaling to a shallower scaling in their endosymbiotic descendants.

Modern society is fueled by fossil energy produced millions of years ago by photosynthetic organisms. Cultivating contemporary photosynthetic producers to generate energy and capture carbon from the atmosphere is one potential approach to sustaining society without disrupting the climate. Algae, photosynthetic aquatic microorganisms, are the fastest growing primary producers in the world and can therefore produce more energy with less land, water, and nutrients than terrestrial plant crops. We review recent progress and challenges in developing bioenergy technology based on algae. A variety of high-value products in addition to biofuels can be harvested from algal biomass, and these may be key to developing algal biotechnology and realizing the commercial potential of these organisms. Aspects of algal biology that differentiate them from plants demand an integrative approach based on genetics, cell biology, ecology, and evolution. We call for a systems approach to research on algal biotechnology rooted in understanding their biology, from the level of genes to ecosystems, and integrating perspectives from physical, chemical, and social sciences to solve one of the most critical outstanding technological problems.

Genetically engineered (GE) microalgae are nearing commercial release for biofuels production without sufficient public information or ecological studies to investigate their possible risks. Blue-green algae (cyanobacteria) and eukaryotic green algae are likely to disperse widely from open ponds and, on a smaller scale with lower probability, from enclosed photobioreactors. With powerful molecular techniques, thousands of algal strains have been screened, hybridized, and redesigned to grow quickly and tolerate extreme conditions. Some biologists do not expect GE microalgae to survive in the wild. However, thorough ecological and evolutionary assessments are needed to test this assumption and, if the algae do survive, to confirm that their persistence is highly unlikely to cause environmental harm. Cyanobacteria are especially difficult to evaluate because of the chance of horizontal gene transfer with unrelated microbes. Before novel GE algae enter the environment, key biosafety and environmental risk iss...

The use of wastewater as a nutrient source for algal growth as a biodiesel feedstock could increase the viability of this process and reduce wastewater discharge of nitrogen- and phosphorus-bearing compounds. In this study, four algal bioreactors were fed continuously with municipal wastewater effluent over a 6-month period; these were monitored weekly to determine algal biomass and lipid production, nutrient removal, and carbon and nutrient availability. Algal biomass production ranged from 0.78 to 15.9 g dry weight m−2 d−1 over the experimental period. Algal reactors removed 19% of dissolved nitrogen and 43% of dissolved phosphorus from wastewater effluent. A stoichiometric analysis of particulates revealed that algal biomass had far lower C:P ratios (67:1) and N:P ratios (6:1), but higher C:N ratios (17:1), than Redfield values. These data suggest that the algae produced were rich in phosphorus, and that nitrogen was likely to be growth-limiting in these systems. Nonetheless, numerous studies suggest that algal biomass can be enhanced by providing CO2 in excess of normal atmospheric concentrations, and that algal biofuel production can contribute to CO2 sequestration efforts. To optimize nutrient removal in wastewater-fed algal reactors, and to determine the potential for CO2 sequestration, future reactor engineering research should continue to incorporate the expertise of ecological stoichiometry. © 2011 American Institute of Chemical Engineers Environ Prog, 2012

One of the most visible consequences of eutrophication in freshwater lakes is an increase in the biomass of cyanobacteria. While growth of cyanobacteria has been shown to be coupled to water column concentrations of total phosphorus (TP) in natural lakes, the ecological behavior of cyanobacteria in turbid impoundments is less well understood. Patterns of cyanobacterial biovolume in 7 turbid Kansas (USA) reservoirs exhibited overlap with published data from natural lakes in North America. Non-algal turbidity (NAT) influenced the relationship between cyanobacteria and TP in these systems. Relative to yields predicted from natural lakes, the biomass of cyanobacteria per unit TP was typically higher in the reservoirs, except in those systems that had exceptionally high levels of NAT (>∼2.0 2/m) where observed concentrations were mostly lower than predicted. A more consistent pattern was observed with respect to the effects of NAT on yields of chlorophyll-a per unit TP in the reservoirs: across almost...

1. An animal's elemental composition can be an important indicator of its physiological state and role in ecosystem nutrient cycling. We examined the interactive effects of bacterial (Pasteuria ramosa) infection and phosphorus (P)-poor food on the body stoichiometry of Daphnia magna. Daphnia were exposed to or held free of a bacterial parasite and fed algal food of different C:P ratios (100-500) over a 28-day period. 2. To assess the effects of exposure and infection on Daphnia stoichiometry, we measured their whole body content of carbon (C), nitrogen (N), and P on four different days (4, 8, 15, and 28) during the experiment. 3. We found strong effects of infection, food quality, and/or their interactions on the C, N, and P content of Daphnia, especially as the infectious disease progressed. At the end of the experiment, infected animals had significantly more C and less P in their bodies than uninfected conspecifics. Body N content of Daphnia consuming P-rich food was reduced by bacterial infection whereas Daphnia consuming P-poor algae showed increased body N content from infection. 4. Using a mass-balance model, we found that changes in N and P content of host bodies were largely accountable by disease-induced alterations to Daphnia reproduction (i.e. bacterial induced sterility) and the accumulation of Pasteuria spores in the body cavity. Our calculations also show that the observed increase in host C content could not be accounted for by loss of eggs or accumulation of bacterial spores. This instead must result from unidentified changes to underlying daphnid tissue C content. 5. These results demonstrate that intraspecific variation in zooplankton body stoichiometry can be caused by exposure to and infection by bacterial parasites. In addition, these effects were found to depend both upon the stage of the disease and upon the food quality consumed by the host.

Thanks to recent advances in molecular biology, one's knowledge of microbial co-occurrence patterns, microbial biogeography and microbial biodiversity is expanding rapidly. This MiniReview explores microbial diversity–productivity relationships in the light of what is known from the general ecology literature. Analyses of microbial diversity–productivity relationships from 70 natural, experimental, and engineered aquatic ecosystems reveal patterns that are strikingly similar to those that have long been documented for communities of macroorganisms. Microbial ecology and the general science of ecology are thus continuing to converge.

We examine co-occurrence patterns of microorganisms to evaluate community assembly ''rules.'' We use methods previously applied to macroorganisms, both to evaluate their applicability to microorganisms and to allow comparison of co-occurrence patterns observed in microorganisms to those found in macroorganisms. We use a null model analysis of 124 incidence matrices from microbial communities, including bacteria, archaea, fungi, and algae, and we compare these results to previously published findings from a meta-analysis of almost 100 macroorganism data sets. We show that assemblages of microorganisms demon- strate nonrandom patterns of co-occurrence that are broadly similar to those found in assemblages of macroorganisms. These results suggest that some taxon co-occurrence patterns may be general characteristics of communities of organisms from all domains of life. We also find that co-occurrence in microbial communities does not vary among taxonomic groups or habitat types. However, we find that the degree of co-occurrence does vary among studies that use different methods to survey microbial communities. Finally, we discuss the potential effects of the undersampling of microbial communities on our results, as well as processes that may contribute to nonrandom patterns of co-occurrence in both macrobial and microbial communities such as competition, habitat filtering, historical effects, and neutral processes.

The stoichiometric light:nutrient hypothesis (LNH) links the relative supplies of key resources with the nutrient content of tissues of producers. This resource-driven variation in producer stoichiometry, in turn, can mediate the efficiency of grazing. Typically, discussions of the LNH attribute this resource-stoichiometry link to bottom-up effects of light and phosphorus, which are mediated through producer physiology. Emphasis on bottom-up effects implies that grazers must consume food of quality solely determined by resource supply to ecosystems (i.e., they eat what they are served). Here, we expand upon this largely bottom-up interpretation with evidence from pond surveys, a mesocosm experiment, and a model. Data from shallow ponds showed the "LNH pattern" (positive correlation of an index of light : phosphorus supply with algal carbon : phosphorus content). However, algal carbon : phosphorus content also declined as zooplankton biomass increased in the ponds. The experiment and model confirmed that this latter correlation was partially caused by the various bottom-up and top-down roles of grazers: the LNH pattern emerged only in treatments with crustacean grazers, not those without them. Furthermore, model and experiment clarified that another bottom-up factor, natural covariation of nitrogen : phosphorus ratios with light : phosphorus supply (as seen in ponds), does not likely contribute to the LNH pattern. Finally, the experiment produced correlations between shifts in species composition of algae, partially driven by grazing effects of crustaceans, and algal stoichiometry. These shifts in species composition might shape stoichiometric response of producer assemblages to resource supply and grazing, but their consequences remain largely unexplored. Thus, this study accentuated the importance of grazing for the LNH; de-emphasized a potentially confounding, bottom-up factor (covarying nitrogen : phosphorus supply); and highlighted an avenue for future research for the LNH (grazer-mediated shifts in producer composition).

Resource theory and metabolic scaling theory suggest that the dynamics of a pathogen within a host should strongly depend upon the rate of host cell metabolism. Once an infection occurs, key ecological interactions occur on or within the host organism that determine whether the pathogen dies out, persists as a chronic infection, or grows to densities that lead to host death. We hypothesize that, in general, conditions favoring rapid host growth rates should amplify the replication and proliferation of both fungal and viral pathogens. If a host population experiences an increase in mortality, to persist it must have a higher growth rate, per host, often reflecting greater resource availability per capita. We hypothesize that this could indirectly foster the pathogen, which also benefits from increased within-host resource turnover. We first bring together in a short review a number of key prior studies which illustrate resource effects on viral and fungal pathogen dynamics. We then report new results from a semi-continuous cell culture experiment with SHIV, demonstrating that higher mortality rates indeed can promote viral proliferation. We develop a simple model that illustrates dynamical consequences of these resource effects, including interesting effects such as alternative stable states and oscillatory dynamics. Our paper contributes to a growing body of literature at the interface of ecology and infectious disease epidemiology, emphasizing that host abundances alone do not drive community dynamics: the physiological state and resource content of infected hosts also strongly influence host–pathogen interactions.

It is generally accepted that native communities provide resistance against invaders through biotic interactions. However, much remains uncertain about the types of ecological processes and community attributes that contribute to biotic resistance. We used experimental mesocosms to examine how zooplankton community structure, invertebrate predation, and nutrient supply jointly affected the establishment of the exotic Daphnia lumholtzi. We predicted that establishment would increase with declining biomass and diversity of native zooplankton communities and that an invertebrate predator (IP) would indirectly facilitate the establishment of D. lumholtzi due to its relatively long predator-deterring spines. Furthermore, we hypothesized that elevated nutrient supply would increase algal food availability and facilitate establishment. Only when the biomass and diversity of native zooplankton were significantly reduced, was D.␣lumholtzi able to successfully invade mesocosms. Although invertebrate predation and resource supply modified attributes of native zooplankton communities, they did not influence the establishment of D. lumholtzi. Overall, our␣results are consistent with observed population dynamics in invaded reservoirs where D.␣lumholtzi tends to be present only during the late summer, coinciding with historic mid-summer declines in native zooplankton populations. Lakes and reservoirs may be more susceptible to invasion not only by D. lumholtzi, but also by other planktonic species, in the late summer when native communities exhibit characteristics associated with lower levels of biotic resistance.

Ecological stoichiometry and food web theories focus on distinct mechanisms that shape communities. These mechanisms, however, likely interact in ways that neither theory alone addresses. To illustrate, we show how a model that tracks flow of energy and nutrients through two producers and two grazers reveals two indirect, interrelated roles for "neutrally inedible" producers. First, inedible producers can exert controls over the nutrient content of edible producers and indirectly influence whether grazers are nutrient or energy limited. Second, through these controls, inedible producers can shape community assembly by excluding grazers that are weak competitors for nutrients contained in edible producers. A mesocosm experiment revealed patterns consistent with both predictions: high abundances of inedible algae were accompanied by low phosphorus contents of edible algae and low abundances of the grazer Daphnia. Both lines of inference suggest that interactions between stoichiometry and plant heterogeneity may shape plankton communities.

We review the biogeography of microorganisms in light of the biogeography of macroorganisms. A large body of research supports the idea that free-living microbial taxa exhibit biogeographic patterns. Current evidence confirms that, as proposed by the Baas-Becking hypothesis, 'the environment selects' and is, in part, responsible for spatial variation in microbial diversity. However, recent studies also dispute the idea that 'everything is everywhere'. We also consider how the processes that generate and maintain biogeographic patterns in macroorganisms could operate in the microbial world.

Initial understanding of the links between nutrients and aquatic productivity originated in Europe in the early 1900s, and our knowledge base has expanded greatly during the past 40 yr. This explosion of eutrophication-related research has made it unequivocally clear that a comprehensive strategy to prevent excessive amounts of nitrogen and phosphorus from entering our waterways is needed to protect our lakes, rivers, and coasts from water quality deterioration. However, despite these very significant advances, cultural eutrophication remains one of the foremost problems for protecting our valuable surface water resources. The papers in this special issue provide a valuable cross section and synthesis of our current understanding of both freshwater and marine eutrophication science. They also serve to identify gaps in our knowledge and will help to guide future research. Knowledge of the links between nutrients and aquatic productivity began with the pioneering work of Weber (1907) on German peat bogs and with Johnstone’s (1908) studies of the North Sea. A crystallization of freshwater eutrophication concepts took place soon thereafter in Northern Europe, where the first trophic classification systems for surface waters were developed. These early classification systems were based on the intensity of aquatic organic matter production, as well as nutrient supply conditions and ecosystemlevel consequences of increased production (e.g., hypolimnetic oxygen depletion; Rodhe 1969). There was a lot of uncertainty in the subsequent 50 yr about the physical, chemical, and ecological details of the eutrophication process, and hot debates raged about the relative roles of different mineral nutrients as constraints on, or regulators of, primary productivity, especially the macronutrients nitrogen (N), phosphorus (P), and carbon (C). Work on the eutrophication process accelerated in the 1960s and 1970s. Particularly important was the landmark 1971 American Society of Limnology and Oceanography (ASLO) eutrophication symposium that culminated in the publication of the first special issue of Limnology and Oceanography (L&O) on nutrients and eutrophication, edited by G. E. Likens (Likens 1972a). This special issue was similarly stimulated by a symposium that the three of us 1

A current principle of ecological stoichiometry states that the nitrogen to phosphorus ratio (N:P) of primary producers should closely match that from environmental nutrient supplies. This hypothesis was tested using data from ponds in Michigan, USA, a freshwater mesocosm experiment, a synthesis of studies from diverse systems (cultures, lakes, streams, and marine and terrestrial environments), and simple dynamic models of producer growth and nutrient content. Unlike prior laboratory studies, the N:P stoichiometry of phytoplankton in Michigan ponds clustered around and below the Redfield ratio (7.2:1 by mass), despite wide variation in N:P supply ratios (2:1-63:1 by mass) and the presence of grazers. In a mesocosm experiment, the N:P stoichiometry of phytoplankton cells again deviated from a nearly 1:1 relationship with N:P supply. Phytoplankton seston exhibited lower N:P content than expected at high N:P supply ratios, and often higher N:P content than anticipated at low N:P supply ratios, regardless of herbivore presence. Similar devi- ations consistently occur in the N:P stoichiometry of algae and plants in the other diverse systems. The models predicted that both high loss rates (sinking, grazing) and physiological limits to nutrient storage capacity could attenuate producer stoichiometry. In the future, research should evaluate how limits to elemental plasticity of producers can influence the role of stoichiometry in structuring communities and ecosystem processes.

Nutrition is typically discussed in terms of maintaining a proper diet and avoiding nutrient deficiency diseases. However, nutrition can also be viewed from an ecological standpoint: mammalian hosts and their pathogens share a wide range of resource needs that are required to support normal metabolism and growth. The development of an infectious disease within a host involves the processes of invasion and resource consumption. Competition for growth-limiting resources can potentially occur between the invading pathogen and the cellular components of the host. Experimental studies confirm that nutrients can have profound effects on a host's response to an infection, and taking an ecological view of host nutrition may help to further inform biomedical practice.

Species-area relationships have been observed for virtually all major groups of macroorganisms that have been studied to date but have not been explored for microscopic phytoplankton algae, which are the dominant producers in many freshwater and marine ecosystems. Our analyses of data from 142 different natural ponds, lakes, and oceans and 239 experimental ecosystems reveal a strong species-area relationship with an exponent that is invariant across ecosystems that span >15 orders of magnitude in spatial extent. A striking result is that the species-area relationship derived from small-scale experimental studies correctly scales up to natural aquatic ecosystems. These results significantly broaden our knowledge of the effects of island size on biodiversity and also confirm the relevance of experimentally derived data to the analysis and understanding of larger-scale ecological patterns. In addition, they confirm that patterns in microbial diversity are strongly consistent with those that have been repeatedly reported in the literature for macroorganisms.

Summary 1. Fine-scale physical and chemical gradients and deep photosynthetic microbial populations were assessed to provide an initial characterisation of a small, thermally stratified reservoir (Cross Reservoir, Kansas, U.S.A.) and its deep chlorophyll maxima (DCM). Factors were identified that may affect vertical positioning of subepilimnetic photosynthetic sulphur bacteria (PSB) in lakes. 2. Results indicate that Cross Reservoir is a mesotrophic, dimictic lake with large subepilimnetic chlorophyll maxima containing dense layers of PSB. Characteristics of the deep PSB community of Cross Reservoir strongly correlate with both light and nutrient gradients. 3. The deep bacterial community mostly contained single-celled and aggregating green sulphur bacteria, specifically free-living Chlorobium limicola and the conspicuous motile ectosymbiotic consortium known as ‘Chlorochromatium aggregatum’. The bacteria were within the anaerobic hypolimnion, beneath a metalimnetic plate of Cryptomonas spp. and within very low sulphide and light conditions [mean of 67 μgS L−1 and 0.05% photosynthetically active radiation (PAR)]. Pigment concentrations and fluorescence trends indicate that the bacteria made up a larger proportion of the DCM biomass than did phytoplankton in 1996. 4. Cross Reservoir shares characteristics with natural lakes world-wide that also include a deep PSB community containing dense layers of ‘C. aggregatum’. Correlation analyses indicate that PSB community positioning and density are related to light, sulphide supply, redox potentials and pH. A 2-factor principal components analysis (PCA) and other data trends supported these interpretations and indicated that PSB are sensitive to the thermal stability of the water column, are nitrogen limited and regulated more by sulphide or sulphide to light ratios than local levels of light. The sensitivity of these deep photosynthetic bacteria to environmental gradients, and their significance to some aquatic systems, demonstrate their potential as indicators of environmental disturbance.

Mechanisms that explain shifts in species composition over environmental gradients continue to intrigue ecologists. Ecological stoichiometry has recently provided a new potential mechanism linking resource (light and nutrient) supply gradients to grazer performance via elemental food-quality mechanisms. More specifically, it predicts that light and nutrient gradients should determine the relative dominance of P-rich taxa, such as Daphnia, in grazer assemblages. We tested this hypothesis in pond mesocosms (cattle tanks) by creating gradients of resource supply and predation risk, to which we added diverse assemblages of algal producer and zooplankton grazer species. We then characterized the end-point composition of grazer assemblages and quantity and elemental food quality of edible algae. We found that somatically P-rich Daphnia only dominated grazer assemblages in high-nutrient, no-predator treatments. In these ecosystems, P sequestered in producers exceeded a critical concentration. However, other grazers having even higher body P content did not respond similarly. These grazers were often abundant in low-nutrient environments with poorer food quality. At face value, this result is problematic for ecological stoichi- ometry because body composition did not correctly predict response of these other species. However, two potential explanations could maintain consistency with stoichiometric prin- ciples: species could differentially use a high-P resource (bacteria), or body composition might not always directly correlate with nutrient demands of grazers. Although our data cannot differentiate between these explanations, both suggest potential avenues for future empirical and theoretical study.

Summary 1 The relative importance of regional species pools and local ecological processes in governing landscape variation in plant species diversity and productivity was evaluated in a Kansas grassland. 2 We examined the impact of multispecies sowing treatments and experimental canopy disturbances on plant species diversity and ecosystem processes along a complex natural gradient of plant standing crop. 3 Data collected 4 years after sowing showed that plant invasion and diversity were seed limited in unproductive sites, but microsite limited in productive sites. Effects of sowing on plant diversity along the natural landscape gradient were paralleled by significant effects of sowing on measures of local plant production and community resilience to disturbance. 4 These results support the shifting limitations hypothesis (SLH) that landscape gradients in local plant diversity should reflect shifts in the major regulating factor, from species pools to local ecological processes, as one moves from sites of inherently low to inherently high productivity. 5 Our findings also indicate that diversity at the level of the available propagule pool acts to constrain ecosystem productivity and stability by mediating local community assembly, by determining the availability of key species, and by governing opportunities for functional compensation within the community. 6 In total, our results support an emerging view that community processes and ecosystem functions are dynamically linked and act reciprocally to constrain each other.

Biological invasions are becoming more common, yet the majority of introduced exotic species fail to establish viable populations in new environments. Current ecological research suggests that invasion success may be determined by properties of the native ecosystem, such as the supply rate of limiting nutrients (i.e. trophic state). We examined how trophic state influences invasion success by introducing an exotic zooplankter, Daphnia lumholtzi into native plankton communities in a series of experimental mesocosms exposed to a strong nutrient gradient. We predicted that the attributes of nutrient-enriched communities would increase the likelihood of a successful invasion attempt by D. lumholtzi. Contrary to our original predictions, we found that D. lumholtzi was often absent from mesocosms that developed under high nutrient supply rates. Instead, the presence of D. lumholtzi was associated with systems that had low nutrients, low zooplankton biomass, and high zooplankton species diversity. Using generalized estimating equations (GEE) and multivariate species data, we found that the presence-absence of D. lumholtzi could be explained by variations in zooplankton community structure, which was itself strongly influenced by nutrient supply rate. We argue that the apparent invasion success of D. lumholtzi was inhibited by the dominance of another cladoceran species, Chydorus sphaericus. These results suggest that the interaction between trophic state and species identity influenced the invasion success of introduced D. lumholtzi.

Summary 1. The exotic cladoceran Daphnia lumholtzi has recently invaded freshwater systems throughout the United States. Daphnia lumholtzi possesses extravagant head spines that are longer than those found on any other North American Daphnia. These spines are effective at reducing predation from many of the predators that are native to newly invaded habitats; however, they are plastic both in nature and in laboratory cultures. The purpose of this experiment was to better understand what environmental cues induce and maintain these effective predator-deterrent spines. We conducted life-table experiments on individual D. lumholtzi grown in water conditioned with an invertebrate insect predator, Chaoborus punctipennis, and water conditioned with a vertebrate fish predator, Lepomis macrochirus. 2. Daphnia lumholtzi exhibited morphological plasticity in response to kairomones released by both predators. However, direct exposure to predator kairomones during postembryonic development did not induce long spines in D. lumholtzi. In contrast, neonates produced from individuals exposed to Lepomis kairomones had significantly longer head and tail spines than neonates produced from control and Chaoborus individuals. These results suggest that there may be a maternal, or pre-embryonic, effect of kairomone exposure on spine development in D. lumholtzi. 3. Independent of these morphological shifts, D. lumholtzi also exhibited plasticity in life history characteristics in response to predator kairomones. For example, D. lumholtzi exhibited delayed reproduction in response to Chaoborus kairomones, and significantly more individuals produced resting eggs, or ephippia, in the presence of Lepomis kairomones.

Primary productivity is a key determinant of biodiversity patterns in plants and animals but has not previously been shown to affect bacterial diversity. We examined the relationship between productivity and bacterial richness in aquatic mesocosms designed to mimic small ponds. We observed that productivity could influence the composition and richness of bacterial communities. We showed that, even within the same system, different bacterial taxonomic groups could exhibit different responses to changes in productivity. The richness of members of the Cytophaga-Flavobacteria-Bacteroides group exhibited a significant hump-shaped relationship with productivity, as is often observed for plant and animal richness in aquatic systems. In contrast, we observed a significant U-shaped relationship between richness and productivity for a-proteobacteria and no discernable relationship for b-proteobacteria. We show, for the first time, that bacterial diversity varies along a gradient of primary productivity and thus make an important step towards understanding processes responsible for the maintenance of bacterial biodiversity.

Treated wastewaters in the United States contain detectable quantities of surfactants, antibiotics, and other types of antimicrobial chemicals contained in pharmaceutical and personal-care products (PPCPs) that are released into stream ecosystems. The degradation characteristics of many of these chemicals are not yet known, nor are the chemical properties of their byproducts. They also are not currently mandated for removal under the U.S. Clean Water Act. Three representative PPCPs were individually tested in this study using a series of laboratory dilution bioassays: Ciprofloxacin (an antibiotic), Triclosan (an antimicrobial agent), and Tergitol NP 10 (a surfactant), to determine their effects on natural algal communities sampled both upstream and downstream of the Olathe, KS wastewater treatment plant (WWTP). There were no significant treatment effects on algal community growth rates during the exponential phase of growth, but significant differences were observed in the final biomass yields (p0.001). All three compounds caused marked shifts in the community structure of suspended and attached algae at both the upstream and downstream sites (p0.05). Increasing the concentrations of all three compounds over a 3 orders of magnitude range also caused a consistent decline in final algal genus richness in the bioassays. Our results suggest that these three PPCPs may potentially influence both the structure and the function of algal communities in stream ecosystems receiving WWTP effluents. These changes could result in shifts in both the nutrient processing capacity and the natural food web structure of these streams.

The supplies of nutrients, and their elemental stoichiometry, can have significant impacts upon the structure and function of microbial communities. This review focuses on the effects of nutrient supplies on the biodegradation of organic matter, and on the dynamics of host-pathogen interactions. Analyses of data from the literature suggest significant effects of nitrogen:phosphorus supply ratios on the microbial decomposition of organic matter, and it is argued that these stoichiometric effects may have important implications for the fate and fluxes of carbon in natural ecosystems. In addition, it is shown that the supplies of nitrogen and phosphorus to the host can strongly influence the outcome of infections in both terrestrial and aquatic plants, suggesting that resource availability and resource supply ratios potentially may have significant effects on the health of many plant communities.

Ecological Stoichiometry theory predicts that the production, elemental structure and cellular content of biomolecules should depend on the relative availability of resources and the elemental composition of their producer organism. We review the extent to which carbon- and nitrogen-rich phytoplankton toxins are regulated by nutrient limitation and cellular stoichiometry. Consistent with theory, we show that nitrogen limitation causes a reduction in the cellular quota of nitrogen-rich toxins, while phosphorus limitation causes an increase in the most nitrogen-rich paralytic shellfish poisoning toxin. In addition, we show that the cellular content of nitrogen-rich toxins increases with increasing cellular N : P ratios. Also consistent with theory, limitation by either nitrogen or phosphorus promotes the C-rich toxin cell quota or toxicity of phytoplankton cells. These observed relationships may assist in predicting and managing toxin-producing phytoplankton blooms. Such a stoichiometric regulation of toxins is likely not restricted to phytoplankton, and may well apply to carbon- and nitrogen-rich secondary metabolites produced by bacteria, fungi and plants.

The study is a first attempt to generate a series of Total Phosphorus–Chlorophyll a (TP–Chl a) predictiveOC models relating the quantitative responses of four algal size fractions to phosphorus gradients. The study was carried out in 27 glacial lakes from two regions in southern Quebec, the Laurentians and the Eastern Townships, and covered a relatively modest range of trophic conditions (TP, 3–34 µg P L-1; Chl a, 0.3–7.6 µg L−1). Algal biomass was estimated using measurements of Chl a, and the total Chl a was divided into four operational size fractions: picophytoplankton 20 mm (inedible fraction). We tested the hypothesis that the slopes of the TP–Chl a regression models developed for algal size fractions would increase consistently from the smallest to the largest algal size fraction, as suggested by the first half of the sigmoidal TP–Chl a models. Although there was no consistent trend in the magnitudes of the slopes of TP–Chl a relationships for picophytoplankton (slope = 1.14), nanophytoplankton (0.93), and microphytoplankton (1.22), Chl a concentrations in the largest size fraction increased more rapidly with phosphorus enrichment than in either of the smaller fractions. When included as an additional variable, lake water alkalinity improved the prediction of Chl a and presented differential effects on size fractions. The effect of TP enrichment on microphytoplankton is more pronounced in well-buffered lakes, whereas TP enrichment has a stronger effect on nanophytoplankton in low-alkalinity lakes. The effects of alkalinity may be the result of either a pH influence on phytoplankton carbon uptake or a stronger top-down grazing effect on small algae in well-buffered lakes.

The ability to compete for the limited nutrients available to the microorganisms of dental plaque is a strong ecological determinant of the structure of the subgingival ecosystem. This paper introduces a new concept from the field of ecology, resource-ratio theory, and applies it to the dynamics of microbial dental plaque with emphasis on the putative periodontal pathogens. Resource-ratio theory is a mechanistic theory of resource competition that utilizes pairs of growth-limiting nutrients in a stoichiometric fashion to predict zones of competitive dominance, exclusion, and coexistence for organisms competing for these resources. Once these resource pairs are identified for plaque organisms, resource-ratio theory may provide predictions of changes in the microbial community structure of plaque based on directional changes in their resource supply ratios.

The host organism is a complex mosaic of cell populations that requires adequate supplies of nutrients for maintenance, growth and proliferation. Because many nutrient requirements may be shared by host cells, pathogens and indigenous microflora, all these cells may potentially compete for growth-limiting resources. Ecological theory can explain some of the dynamics commonly seen in host-pathogen interactions; and mechanistic resource-consumer theory provides an instructive framework for viewing the disease process.

Pathogens and their host organisms share a wide range of resource needs that are required to support normal metabolism and growth. Because the development of infectious disease on or within the host involves the processes of invasion and resource consumption, competition for growth-limiting resources potentially may occur between pathogens and cellular or sub-cellular components of the host ecosystem. Examples from the plant, animal, and microbiological literature provide unambiguous evidence that external resource supplies to the host organism can have profound effects on the outcome of infection by a broad diversity of bacterial, fungal, metazoan, protozoan, and viral pathogens.

D espite many decades of research in experimental and clinical medicine, it remains difficult to predict the outcome of infectious disease. A number of factors influence the growth and proliferation of a disease-causing organism inside its host, including the initial inoculum size; virulence and resistance to antibiotics of the invading pathogen; and age, sex, genetic composition, immunocompetence, and nutritional status of the host animal. Of these many factors, the external supply of nutrients to the host is potentially one of the more easily controlled aspects of the disease process. In humans, dietary nutrients have a profound effect on the host response to infection, and preliminary studies with defined nutritional formulas have shown significant benefits for critically ill patients (Peck et al. 1991). It should thus in principle be possible to tailor the patient's nutritional support to specific diseases; however, advances in the proper nutritional management of the host require a better understanding of the disease process (Abumrad and Frexes-Steed 1990). This article explores the relationships between host nutrition and the growth and proliferation of disease-causing

Summary 1. Spatial variation in species composition within and among communities may be caused by deterministic, niche-based species sorting in response to underlying environmental heterogeneity as well as by stochastic factors such as dispersal limitation and variable species pools. An important goal in ecology is to reconcile deterministic and stochastic perspectives of community assembly and to assess the contribution of each class of processes to community dynamics and structure. 2. We present an 8-year field experiment of grassland secondary succession that documents plant community differentiation in response to an experimental gradient of nitrogen (N) fertilization, factorially crossed with a manipulation of the available species pool achieved using a multi-species seed sowing treatment. We evaluate the hypothesis, adapted from meta-community theory, that seed availability limits the contribution of niche-based, species sorting to patterns of community variation along an environmental gradient. 3. The magnitude of species sorting and community differentiation observed in response to N fertilization (measured as N effect size on multivariate community composition) became progressively more distinct and more statistically significant over time in experimental plots as succession proceeded. However, this response was significantly more pronounced among plots that had been exposed to experimentally enriched propagule pools. 4. Synthesis. Our findings support the hypothesis that dispersal limitations and species pools can mediate the contribution of niche-based, species–environment sorting to plant community development and limit the extent to which underlying resource gradients become deterministically expressed in patterns of vegetation composition. We suggest that continued habitat destruction and fragmentation in the former prairie landscape where this study was conducted would further reduce native species pools and habitat connectivity, diminishing opportunities for species–environment sorting and compromising the capacity of these grassland systems to respond to environmental change.

An analysis of data compiled from the literature confirms a strong inverse relationship between annual rates of nitrogen fixation and the soil nitrogen content in agricultural and pastoral ecosystems. However, this inverse relationship is strongly modified by the rate of application of phosphorus fertilizer, which strongly influences the activities of both symbiotic and non-symbiotic nitrogen fixing organisms. In the case of symbiotic legumes, the response of N-fixation to N and P is in part a result of changes in legume dominance within the plant community. These results, as well as supporting data presented from a review of experiments on nitrogen fixation in a variety of other terrestrial and aquatic ecosystems, provide important support for the hypothesis that phosphorus availability is a key regulator of nitrogen biogeochemistry.

An analysis of data from four turbid North Carolina reservoirs showed that both the summer mean biomass and relative biomass of planktonic blue-green algae (Cyanophyta/Cyanobacteria) are frequently lower than would be predicted by models developed from natural north temperate lakes. These results are consistent with the hypothesis that the growth of blue-green algae in these reservoirs is reduced in the presence of high concentrations of non-algal turbidity.

Pelagic crustacean zooplankton were collected from 336 Norwegian lakes covering a wide range of latitude, altitude, lake area, mean depth, production (as chlorophyll a), and fish community structure. Mean zooplankton species richness during the ice-free season was generally low at high latitudes and altitudes. Further, lower species richness was recorded in western lakes, possibly reflecting constraints on migration and dispersal. However, despite obvious spatial limitations, geographic boundaries were only weak predictors of mean zooplankton richness. Similarly, lake surface area did not contribute positively to mean richness such as seen in other ecosystem surveys. Rather, intrinsic factors such as primary production and fish community (planktivore) structure were identified by regression analysis as the major predictors of zooplankton diversity, while a positive correlation was observed between species richness and total zooplankton biomass. However, in spite of a large number of variables included in this study, the predictive power of multiple regression models was modest (

A long-term (28-year) data set was used to investigate historical changes in concentrations of phosphorus (P), nitrogen (N), N:P ratios, and Secchi disk transparency in a shallow subtropical lake (Lake Okeechobee, Florida, USA). The aim was to evaluate changes in the risk of N2-fixing cyanobacterial blooms, which have infrequently occurred in the lake's pelagic zone. Predictions regarding bloom risk were based on previously published N:P ratio models. Temporal trends in the biomass of cyanobacteria were evaluated using phytoplankton data collected in 1974, 1989-1992, and 1997-2000. Concentrations of pelagic total P increased from near 50 microg l-1 in the mid-1970s to over 100 microg l-1 in the late 1990s. Coincidentally, the total N:P (mass) ratio decreased from 30:1 to below 15:1, and soluble N:P ratio decreased from 15:1 to near 6:1, in the lake water. Published empirical models predict that current conditions favor cyanobacteria. The observations confirm this prediction: cyanobacteria presently account for 50-80% of total phytoplankton biovolume. The historical decrease in TN:TP ratio in the lake can be attributed to a decreased TN:TP ratio in the inflow water and to a decline in the lake's assimilation of P, relative to N. Coincident with these declines in total and soluble N:P ratios, Secchi disk transparency declined from 0.6 m to near 0.3 m, possibly due to increased mineral turbidity in the lake water. Empirical models predict that under the turbid, low irradiance conditions that prevail in this lake, non-heterocystous cyanobacteria should dominate the phytoplankton. Our observations confirmed this prediction: non-N2-fixing taxa (primarily Oscillatoria and Lyngbya spp.) typically dominated the cyanobacteria community during the last decade. The only exception was a year with very low water levels, when heterocystous N2-fixing Anabaena became dominant. In the near-shore regions of this shallow lake, low N:P ratios potentially favor blooms of N2-fixing cyanobacteria, but their occurrence in the pelagic zone is restricted by low irradiance and lack of stable stratification.

We examined effects of fertilization and haying on grassland biodiversity, primary production and other ecosystem properties in a hay management experiment conducted in northeastern Kansas. We examined relationships between biodiversity and ecosystem productivity across our experimental landscape and evaluated relationships of biodiversity and ecosystem properties to a remotely-sensed Normalized Difference Vegetation Index (NDVI). Fertilization increased production, measured as standing crop biomass and seasonally integrated NDVI and altered a range of ecosystem traits above- and belowground. Shifts at the ecosystem level in response to fertilization were accompanied by declines in plant and arthropod diversity, and negative relationships of both to gradients of standing crop and NDVI, the magnitude of which was altered by haying disturbance in the case of plants. Bacterial diversity showed no response to fertilization, but was increased by haying. In contrast to plants and arthropods, bacterial diversity...

Summary 1. Over the past decade, ecologists have tried to determine how changes in species composition and diversity affect ecosystem structure and function. Until recently, the majority of these studies have been conducted in terrestrial ecosystems and have not taken into account environmental variability. The purpose of this research was to determine how species identity and diversity in the freshwater zooplankton affected biomass of algae and zooplankton at two levels of nutrient enrichment. 2. Several species of cladocerans were grown alone and together in microcosms at both ambient and raised phosphorus concentrations to determine if the effects of consumer identity and diversity were nutrient dependent. 3. Total zooplankton biomass was greater, while algal biomass was lower, in mixed culture than in monoculture. The effects of zooplankton diversity on algal biomass, however, were only observed at raised phosphorus concentrations, suggesting that diversity effects were nutrient dependent. Specifically, diversity effects appeared to be related with biological mechanisms such as complementarity in resource use and/or facilitation. 4. More diverse communities of zooplankton appear to be better able to control algae than single species of zooplankton at high nutrient concentrations; therefore, zooplankton diversity may provide a buffer against eutrophication in freshwater ecosystems.