Shifts In Assemblage Composition Research Articles

The Structure of Rice Stemborer Assemblages: A Review of Species' Distributions, Host Ranges, and Interspecific Interactions.

This review describes global rice stemborer assemblages based on published species distributions, apparent host preferences, and reported shifts in assemblage composition in response to environmental factors. At least 56 moth (Lepidoptera: Crambidae, Pyralidae, Noctuidae) and fly (Diptera: Diopsidae, Chloropidae) species have been associated with rice; however, only 21 species are of potential, large-scale economic importance with a further 2 species of localized concern; most of the remaining species' associations with rice are based on dubious records without economic impacts on rice production. A list of stemborer-host associations indicates that rice stemborers are largely oligophagous on grasses (Poaceae), but a few species are polyphagous (also attacking Cyperaceae, Typhaceae, and some Eudicotyledon plants). Total stemborer abundance is determined by rice cropping patterns and management. Assemblage species richness is determined by geographical location, surrounding habitat (particularly as regards secondary and occasional species), and season. Evidence suggests that stemborer assemblage structure is largely determined through conditional interspecific competition. Regional assemblages typically include a single dominant lepidopteran species (primary species) that is largely restricted to rice and for which the climate is optimal; one or more secondary species that vary based on the age of rice attacked, rice anatomy, and the proximity to other habitats (including other crops); and occasional species that probably spill over from adjacent grasslands. The co-occurrence of lepidopteran with dipteran rice stemborers requires further research attention.

Big-cat hunting in the Bronze and Iron Ages of the Near East: a view from Tel Burna

In ancient Near Eastern iconography, panthers and lions were frequently used to express social status. The zooarchaeological remains of panthers and lions found in this region, however, are most commonly interpreted only as evidence for the management of dangerous animals. Starting with the faunal material from Iron Age Tel Burna, the authors collate and analyse zooarchaeological evidence for big cats across the Near East, from the Neolithic to the Iron Age (c. 9500–50 BC). The results show a shift in assemblage composition and find contexts starting in the Chalcolithic period, indicating the display of these animals by political leaders. The results also urge caution in the use of archaeological remains for reconstructing the natural ranges of big cats.

Plankton response to global warming is characterized by non-uniform shifts in assemblage composition since the last ice age

Biodiversity is expected to change in response to future global warming. However, it is difficult to predict how species will track the ongoing climate change. Here we use the fossil record of planktonic foraminifera to assess how biodiversity responded to climate change with a magnitude comparable to future anthropogenic warming. We compiled time series of planktonic foraminifera assemblages, covering the time from the last ice age across the deglaciation to the current warm period. Planktonic foraminifera assemblages shifted immediately when temperature began to rise at the end of the last ice age and continued to change until approximately 5,000 years ago, even though global temperature remained relatively stable during the last 11,000 years. The biotic response was largest in the mid latitudes and dominated by range expansion, which resulted in the emergence of new assemblages without analogues in the glacial ocean. Our results indicate that the plankton response to global warming was spatially heterogeneous and did not track temperature change uniformly over the past 24,000 years. Climate change led to the establishment of new assemblages and possibly new ecological interactions, which suggests that current anthropogenic warming may lead to new, different plankton community composition.

The Extremes of Disturbance Reduce Functional Redundancy: Functional Trait Assessment of the Shallow Antarctic Benthos

Climate-driven changes in disturbance are a major threat to ecosystem Functional diversity. The selective mechanisms underlying ecosystem response to disturbance are far from universal and remain the subject of scientific debate. Ice scouring of the shallow Antarctic benthos is one of the largest disturbance gradients in the natural environment and thus provides an opportunity to investigate how disturbance gradients influence functional structure of a biological assemblage. The Western Antarctic Peninsula, in particular, is a hotspot of climate-driven environmental change. Addressing how this system might respond to species loss is critical. Previous surveys across the shallowest 100 m of the seabed, detected unimodal changes in diversity and a shift in assemblage composition in response to disturbance gradients. This study investigated how functional traits and associated functional diversity change across the depth gradient. Our results revealed that selective mechanisms, such as disturbance filtering and inter-species competition, reduce functional redundancy at the extremes of the disturbance gradient. Our study highlights areas of potential vulnerability to future environmental change due to low functional redundancy. Threatening the important negative (mitigating) feedbacks on climate change, through blue carbon, currently provided by Antarctic continental shelf benthic assemblages.

Benthic infaunal assemblages adjacent to an ocean outfall in Australian marine waters: Impact assessment and identification of indicator taxa

An impact assessment of oceanic effluent releases from Belmont wastewater treatment works (WWTW) in Newcastle, Australia, was undertaken. Benthic infaunal assemblages in sandy sediments of ~25 m water depth were examined, at sites adjacent to the release point, and at increasing distances up to 2 km in both a NE and SW direction over five consecutive years (2016–2020). Localised impacts were evident for infaunal assemblages, with sites within 20 m of the outfall (“Impact” site types) exhibiting lower taxa richness and Shannon diversity, higher abundances of polychaetes and/or nematodes, higher polychaete ratios, and shifts in assemblage composition in comparison to sites at greater distances during some years. Taxa with increased localised abundances at the outfall were identified as indicators for monitoring impacts, including deposit-feeding polychaetes (Families Polygordiidae, Paraonidae and Dorvilleidae) and Phylum Nematoda. Future infaunal monitoring could include molecular tools and paired sediment analyses.

Joint Impacts of Drought and Habitat Fragmentation on Native Bee Assemblages in a California Biodiversity Hotspot.

Simple SummaryGlobal climate change is causing more frequent and severe droughts, which can have serious impacts on our environment. To examine how a severe drought in 2014 impacted wild bees in scrub habitats of San Diego, California, we compared bee samples collected before and after the drought. We also investigated whether habitat loss and fragmentation worsened the impacts of drought on wild bees by comparing samples collected from large natural reserves to those from small fragments of scrub habitat embedded in urban areas. Samples collected after the drought contained fewer bee species and fewer individual bees of most species, indicating that bee populations suffered losses during the drought. However, after-drought samples contained large numbers of Dialictus sweat bees, indicating that some bee species benefitted from environmental conditions present during the drought. The impact of drought on the composition of bee samples was three fold higher than the impact of habitat fragmentation, and habitat fragmentation did not appear to have exacerbated the impacts of drought. Our findings highlight the importance of studying how impacts of climate change compare with impacts of habitat loss and other threats to biodiversity conservation.Global climate change is causing more frequent and severe droughts, which could have serious repercussions for the maintenance of biodiversity. Here, we compare native bee assemblages collected via bowl traps before and after a severe drought event in 2014 in San Diego, California, and examine the relative magnitude of impacts from drought in fragmented habitat patches versus unfragmented natural reserves. Bee richness and diversity were higher in assemblages surveyed before the drought compared to those surveyed after the drought. However, bees belonging to the Lasioglossum subgenus Dialictus increased in abundance after the drought, driving increased representation by small-bodied, primitively eusocial, and generalist bees in post-drought assemblages. Conversely, among non-Dialictus bees, post-drought years were associated with decreased abundance and reduced representation by eusocial species. Drought effects were consistently greater in reserves, which supported more bee species, than in fragments, suggesting that fragmentation either had redundant impacts with drought, or ameliorated effects of drought by enhancing bees’ access to floral resources in irrigated urban environments. Shifts in assemblage composition associated with drought were three times greater compared to those associated with habitat fragmentation, highlighting the importance of understanding the impacts of large-scale climatic events relative to those associated with land use change.

Ecological filtering in scrub fragments restructures the taxonomic and functional composition of native bee assemblages.

Predicting the long-term consequences of habitat alteration for the preservation of biodiversity and ecosystem function requires an understanding of how ecological filters drive taxonomic and functional biodiversity loss. Here, we test a set of predictions concerning the role of ecological filters in restructuring native bee assemblages inhabiting fragmented coastal sage scrub ecosystems in southern California, USA. In 2011 and 2012, we collected native bees in scrub habitat belonging to two treatment categories: large natural reserves and small habitat fragments embedded in an urban landscape. We compared bee assemblages in reserve and fragment sites with respect to their taxonomic and functional alpha diversity, beta diversity, assemblage composition, and mean geographical range size estimated via distribution maps compiled for this study from digitized specimen records. We found multiple lines of evidence that ecological filtering drove bee diversity loss in fragments: a disproportionate loss of functional diversity relative to taxonomic diversity, shifts in assemblage composition driven largely by the preferential extirpation of reserve-associated indicator species, and disproportionate loss of range-restricted species. However, we found no evidence of taxonomic or functional homogenization across fragment bee assemblages, suggesting that filtering was not sufficiently strong to cause a subset of functional traits (and their associated species) to dominate assemblages in fragments. Our results suggest that ecological filtering altered bee assemblages in habitat fragments, even when such fragments contained well-preserved native plant assemblages, underscoring the importance of preserving large areas of natural habitat for the conservation of bees (especially range-restricted taxa) and their associated ecological functions.

Chironomid-based temperature reconstruction for the Eemian Interglacial (MIS 5e) at Sokli, northeast Finland

The Last Interglacial (Eemian, MIS 5e) can be considered a test-bed for climate dynamics under a warmer-than-present climate. In this study we present a chironomid record from the high latitude Sokli site (N Finland), where a long continuous sediment sequence from the last interglacial has been preserved from glacial erosion. The chironomid-analysis shows a diverse fauna, with dominance of warm-water indicators and shifts in assemblage composition that can be attributed to temperature, lake depth, productivity and habitat availability. Quantitative mean July paleotemperature estimates based on the chironomid data indicate overall mean July air temperatures up to 1 °C warmer than present. Two cooling events can be discerned, the Tunturi event, dated to about 127.5 ka BP, in the lower part of the sequence, and the Värriö event, dated to about 119 ka BP, associated with the beginning of a cooling trend in the upper part of the record. Warm conditions already at the onset of the interglacial contrast with a recent chironomid-based last interglacial temperature reconstruction from Denmark, which suggests a late onset of Eemian warming. The relatively small increase in inferred temperatures compared to present day temperatures at Sokli differs from other high latitude Eemian sites, and likely reflects the influence of the Atlantic Meridional Overturning Circulation in maintaining already elevated temperatures in Fennoscandia during interglacials.

Land-use heterogeneity by small-scale agriculture promotes amphibian diversity in montane agroforestry systems of northeast Colombia

Habitat fragmentation, the transformation of interconnected large habitats into several small patches, is generally considered to have a negative influence on biodiversity. Management policies should therefore favour strategies that minimize fragmentation. We studied amphibian diversity and compositional patterns in a complex montane agroforestry matrix of north-eastern Colombia to assess whether small-scale agroforestry is a viable way of reconciling crop production, farming, and biodiversity conservation. A total of five different land-use systems, including natural cloud forest, were compared. We used standardized visual and acoustic transect sampling routines in combination with field-based habitat assessments to establish amphibian assemblage data sets and corresponding habitat templets. Data sets were analysed using classical species richness and diversity statistics in combination with multivariate procedures to address compositional aspects. PER(mutational)MANOVA models were applied to identify environmental and land-use type-specific drivers of shifts in assemblage composition. Results show that agricultural management has a strong influence on amphibian species richness and diversity. With the exception of high intensity cattle pasture, diversity was generally highest in agriculturally used sites particularly in shaded plantations. Species composition differed markedly between different land-use types, indicating that there was not a single best land-use type that supported all species. This highlights the need for a landscape approach to biodiversity conservation in fragmented forests and consequently requires refocusing on conservation in management units as well as in the matrix. In human modified montane forest landscapes small-scale agriculture that generates landscape heterogeneity should therefore be promoted to effectively safeguard biodiversity.

Flow alterations by dams shaped fish assemblage dynamics in the complex Mekong-3S river system

The Mekong, Sekong, Sesan, and Srepok (Mekong-3S) river system, a Ramsar wetlands of international importance and critical fish migration routes, is altered by dams that distort the seasonal flow dynamics, structuring dispersal and reproduction success of fishes. Here, we investigate the temporal responses of local fish beta diversity to hydrologic modification by the upstream functioning dams in five sites of the Mekong-3S system. The sampling design adopted (two sites on the Mekong River displaying relatively undisturbed flow and three sites in the 3S displaying a gradient in flow perturbation) allows us to focus on the effect of flow alteration on local fish assemblage compositions. By analysing 7-year daily fish monitoring data (06/2007–05/2014), we found that there have been overall declining trends in local species richness and abundance, with strong temporal variability in local beta diversity. Undisturbed sites are characterized by seasonal assemblage variability, while disturbed sites are characterized by aseasonal assemblage changes. Temporal shifts in assemblage composition suggest that dams alter seasonal flow patterns and favour generalist species. This study contributes to a better understanding of the temporal changes of tropical freshwater fish beta diversity in regulated and unregulated rivers. It is thus relevant for fisheries planning and conservation.

Explaining ecological shifts: the roles of temperature and primary production in the long‐term dynamics of benthic faunal composition

Predicting the ecological consequences of environmental change requires that we can identify the drivers of long‐term ecological variation. Biological assemblages can exhibit abrupt deviations from temporal trends, potentially resulting in irreversible shifts in species composition over short periods of time. Such dynamics are hypothesised to occur as gradual forcing eventually causes biological thresholds to be crossed, but could also be explained by biota simply tracking abrupt changes to their environment. Here, we modelled temporal variation in a North Sea benthic faunal assemblage over a 40‐year period (1972–2012) to test for changes to temporal trends of biota and determine whether they could be explained by underlying patterns in sea temperature and primary production. These extrinsic factors were postulated to influence community dynamics through their roles in determining and sustaining the metabolic demands of organisms, respectively. A subset of mainly large and long‐lived taxa (those loaded on the first principal component of taxa densities) exhibited two significant changes to their temporal trends, which culminated in a shift in assemblage composition. These changes were explained by an increase in pelagic primary production, and hence detrital food input to the seabed, but were unrelated to variation in sea temperature. A second subset of mainly small and short‐lived taxa (those loaded on the second principal component) did not experience any significant changes to their temporal trends, as enhanced pelagic primary production appeared to mitigate the impact of warming on these organisms. Our results suggest that abrupt ecological shifts can occur as biota track underlying variation in extrinsic factors, in this case primary production. Changes to the structure of ecosystems may therefore be predictable based on environmental change projections.

Unravelling the joint effects of flow regime, climatic variability and dispersal mode on beta diversity of riverine communities

Summary Differences in the composition of ecological communities among rivers with contrasting flow regimes have been detected in many regions worldwide. However, we know little of how climatic variation alters these spatial patterns for constituent species with varying dispersal modes and strengths. We predicted that beta diversity (spatial variation in composition) of macroinvertebrate and fish assemblages among streams with near‐perennial, mildly intermittent and highly intermittent flow regimes would be greater during prolonged drought than under wetter climate conditions. We hypothesised that changes in beta diversity between these climate phases would be greater for taxa with stronger dispersal ability. We tested these predictions with biomonitoring data from Australian subtropical rivers. Differences in average community composition among flow regimes were statistically significant for six groups of macroinvertebrates with differing dispersal (non‐swimmers, weak swimmers, strong swimmers, non‐fliers, weak fliers and strong fliers) and two groups of fish (potamodromous and diadromous), but not for locally dispersing fish. For each of these biotic groups, shifts in assemblage composition between supra‐seasonal drought and a subsequent non‐drought period were either statistically non‐significant or of similar magnitude among flow regimes. Therefore, beta diversity among flow regimes did not change significantly with climate phase. Beta diversity within flow‐regime classes was greater during the drought than afterwards or not significantly different between climate phases, depending on the flow regime and biotic group. Stronger powers of flight were associated with greater change in beta diversity between climate phases in near‐perennial streams, but stronger powers of swimming were not. Within‐regime beta diversity was driven more by replacement of taxa among samples than by nestedness, except for diadromous fish in near‐perennial streams. Our findings are consistent with the hypothesis that decreasing hydrological connectivity and greater environmental heterogeneity during prolonged drought increase beta diversity of stream communities. However, changes in beta diversity between climate phases were not as pronounced as we expected. The communities may have been somewhat drought resistant because the magnitude and duration of the drought were within the range of historical variability and because surface water persisted at study sites during drought‐imposed flow cessations. Our findings suggest that maintenance of refuge pools will be critical to lessening drought impacts on river biodiversity at landscape scales, particularly if drought duration and intensity increase in the future.

Carbon:phosphorus homeostasis of aquatic bacterial assemblages is mediated by shifts in assemblage composition

Carbon:phosphorus homeostasis of aquatic bacterial assemblages is mediated by shifts in assemblage composition

Lionfish alter benthic invertebrate assemblages in patch habitats of a subtropical estuary

Invasive lionfish (primarily Pterois volitans) have spread throughout the Caribbean region, Gulf of Mexico, and the Southeast US coast, and resulting impacts on reef fish populations have been well documented. We examined whether lionfish can likewise affect benthic invertebrate communities, using an in situ caging experiment, in the Loxahatchee Estuary, Florida, USA. We found that lionfish caused significant declines in the three most abundant benthic invertebrate species, driving an overall shift in assemblage composition. For example, grass shrimp (Palaemonidae) abundance was reduced by nearly 90 % in the presence of lionfish. Species richness of benthic organisms was significantly higher when lionfish were present, suggesting potentially complex emergent effects of lionfish predation on benthic assemblages. Despite the fact that this experiment was conducted in just a single location using relatively small experimental units, we show altered benthic invertebrate communities could well be an additional outcome of the lionfish invasion.

Avian biodiversity in multiple-use landscapes of the Brazilian Amazon

Habitat loss and degradation is the most pervasive threat to tropical biodiversity worldwide. Amazonia sits at the frontline of efforts to both improve the productivity of tropical agriculture and prevent the loss of biodiversity. To date our understanding of the biodiversity impacts of agricultural expansion in Amazonia is restricted to findings from small scale studies that typically assess the importance of a limited number of land-use types. Here we investigate local and landscape-scale responses of Amazonian avian assemblages to land-cover changes across a gradient of land-use intensity ranging from undisturbed primary forest to mechanised agriculture in 36 drainage catchments distributed across two large regions of the eastern Brazilian Amazon. We found that species richness of forest-associated birds declined progressively along this gradient, accompanied by marked shifts in assemblage composition. We found significant changes in species composition, but not richness, between primary forests that had been subject to different levels of disturbance from logging and fire. Secondary forests retained levels of species richness intermediate between primary forests and production areas, but lacked many forest-dependent species. Production areas (arable crops, cattle pastures and plantation forests) all retained far fewer species than any forest habitat, and were largely dominated by taxa commonly associated with open areas. Diversity partitioning revealed that species composition varied the most among undisturbed forest transects, and steadily decreased with increasing forest degradation and land-use intensity. Our results emphasise the importance of protecting both remaining areas of primary forest in private lands, as well as protecting the same forests from further disturbance events.

Changes in Prokaryote and Eukaryote Assemblages Along a Gradient of Hydrocarbon Contamination in Groundwater

Groundwater biota are particularly sensitive to environmental perturbations such as groundwater contamination. The diversity of prokaryotic and eukaryotic biota has been examined along a gradient of chlorinated hydrocarbon (CHC) contamination in the Botany Sands, an urban coastal sand-bed aquifer (Sydney, Australia). Molecular techniques were used to analyze the richness and composition of prokaryote and eukaryote assemblages using 16S and 18S rDNA, respectively. Taxon richness did not change significantly along the gradient for either prokaryotes or eukaryotes; however, significant shifts in assemblage composition were evident for both groups. Assemblage changes were most strongly correlated with concentrations of the major CHC, cis-1,2-dichloroethene, but the concentrations of a number of the contaminants were also correlated, making it difficult to infer if effects were due to any particular contaminant. The presence of cis-1,2-dichloroethene and other secondary ethenes suggests in situ breakdown of the primary CHCs via natural attenuation. The current focus of management of the Botany aquifer is to stop the contaminant plume reaching the adjoining estuary. This approach is clearly justified given the changes evident in the microbial assemblages in the groundwater, which are a likely consequence of the contamination.

Determining vulnerability of stream communities to climate change at the landscape scale

Summary1. As the climate changes, species are expected to shift to higher latitudes and altitudes where suitable habitat is available if dispersal is not constrained by geographic barriers. We analyse patterns of turnover in freshwater macroinvertebrate assemblages to identify which communities are most likely to be at risk from climate change, and the location of geographic barriers that could impede such adaptive range shifts.2. We analysed macroinvertebrate data from standard biological assessments at the family level, from surveys of all coastal basins of New South Wales, Australia, covering a latitudinal gradient of more than 1000 km. We used variance partitioning to separate the variation in composition explained by climate, among‐site distance, human disturbance and other stream factors.3. Montane stream assemblages showed high turnover in response to climatic variation. Turnover in coastal‐fringe streams was least affected by climate, but strongly correlated with distance and stream variables. Significant shifts in assemblage composition occurred between habitats within catchments and across catchment boundaries.4. Montane stream assemblages are most vulnerable to climate change because their distribution is most responsive to climatic factors, and elevated sites are isolated from one another, reducing the scope for altitudinal migration. Dispersal limitations in coastal‐fringe assemblages will also increase their vulnerability to habitat loss from sea‐level rise. For all stream classes, the separation of many neighbouring catchment assemblages, owing to either limited dispersal or the lack of suitable habitat, is likely to constrain adaptive range shifts. This would lead to an overall reduction in beta diversity among reaches and subsequently to a reduction in landscape‐level gamma diversity.

Observed impacts of climate change on terrestrial birds in Europe: an overview

Climate change is predicted to be the major threat to biodiversity by 2100, when the global average temperature may increase by 5--6 degrees, according to current predictions and trends in greenhouse gas emissions. However, many impacts of climate change on biodiversity have already been observed. These observed effects provide tangible evidence to motivate action to mitigate against further climate change. Focusing on Europe, this overview summarises recently published observed impacts of modern climate change on birds. Due to the sensitivity of birds to weather fluctuations, the high numbers of ornithologists throughout Europe and the tradition in the long-term study of bird populations, there is no doubt that climate change impacts on birds have already occurred. These impacts include changes in (i) phenology (e.g., breeding times), (ii) migration patterns (e.g., time of spring arrival from the wintering grounds), (iii) species distribution (e.g., poleward shift of range margins) and (iv) abundances (e.g., population declines of habitat-specialist birds). Although the overall evidence available is comprehensive, there is a challenge in disentangling the effects of climate change from those of other concurrent factors such as habitat loss and degradation, e.g. due to large-scale intensification of agriculture. Birds are coping with climate change by means of their phenotypic plasticity, but little evidence is available to prove that evolutionary adaptation is already taking place. Further climate change is likely to affect birds at many levels, from changes in body size to shifts in assemblage composition, from mismatches with their resources to the introduction of exotic species and new diseases: the interactions among these levels need to be considered in future research and to devise effective conservation strategies. The rapid development of this key topic in conservation biology and applied ecology makes it important to ensure long-term funding for research, review and dissemination.

全球CO2水平升高对浮游植物生理和生态影响的研究进展

工业革命以来由于化石燃料的大量燃烧,大气CO₂水平不断增加,预计在21世纪末将增至现有水平的两倍,达到750 μL/L。作为全球初级生产力的重要贡献者,浮游植物应对CO₂水平升高的生理生态响应必然会对水生生态系统和碳、氮等元素的生物地球化学循环产生重要影响。全球CO₂水平的升高将显著改变水体的碳化学环境,淡水生态系统(湖泊和河流)由于容量小变化比海洋更为显著。水体碳化学环境的改变首先会影响浮游植物个体,在高CO₂水平下,浮游植物的细胞会有变小的趋势,并且细胞的光合作用强度也会有不同程度的增加,其中细胞较小或者不具有碳浓缩机制(CCM)的浮游植物增加较多,此外浮游植物细胞的化学元素计量值也将显著改变。随后浮游植物个体水平上的变化会进一步影响水生生态系统,例如水体初级生产力水平的提高,浮游植物、浮游动物群落结构组成以及水体微食物网结构的变化等。此外浮游植物对CO₂水平升高的生理生态响应程度还与水体的营养水平有关。总结了大气CO₂水平升高对浮游植物生理生态影响的研究方法,展望了未来可能的研究方向。;With the increase of global carbon emissions since the industrial revolution, atmospheric CO₂ level is expected to be twice higher than the present level at the end of this century, reaching to 750 μL/L. As atmospheric CO₂ is the most important source of soluble CO₂ in water due to the deliberate CO₂ sequestration, global CO₂ elevation would significantly alter carbonate chemical environments of aquatic ecosystems. When atmospheric CO₂ concentration doubles at the end of century, it is predicted that pH value of surface oceans would drop approximately 0.3 units, the concentration of bicarbonate ion would increase by 6%, and carbonate ion concentration would decrease by 50%. Considering the smaller buffering capacity of freshwater ecosystems than that of oceans, carbonate chemical environments of freshwater lakes and rivers may change more notably after the increase of CO₂ level. The shift of carbonate chemistry would in turn have dramatic effects on aquatic ecosystems. Phytoplankton has been used widely as an indicator of changes of aquatic ecosystems because this relatively short-lived organism responds rapidly to subtle changes. Meanwhile, it contributes approximately 50% of the total global primary productivity, which plays a vital role in global carbon cycling. Therefore, the physiological and ecological responses of phytoplankton to global CO₂ elevation would have great significance on water ecosystems and biogeochemical cycle. Elevated carbon resource could enhance the photosynthetic activity of phytoplankton, particularly small phytoplankton or non-CCM (non-carbon concentration mechanism) phytoplankton. When CO₂ level increases, the cell size of phytoplankton would potentially decrease, and the elemental ratio of carbon to nitrogen in phytoplankton cell would increase up to more than 10%. Then, those changes in the individual phytoplankton cell would cause certain variations in the ecological level. Primary productivity would rise significantly as a result of enhanced photosynthetic activity of phytoplankton. The shift in assemblage composition of phytoplankton, and the variation of elemental ratio could affect the edibility of phytoplankton, which would alter the abundances and the community structure of zooplankton. Furthermore, the accumulating concentration of organic carbon in water would promote the growth and reproduction of heterotrophic bacterioplankton, which would strengthen the competition between bacterioplankton and phytoplankton for nutrients, such as nitrogen and phosphorus. More importantly, elevated CO₂ effects differ between oligotrophic and eutrophic water body. It is expected that effects on algal productivity in eutrophic aquatic ecosystems could be potentially larger, since in eutrophic water the input of nitrogen and phosphorus to the global biochemical cycle exceeds that of the carbon input by several orders of magnitude. Finally, we summarized the development of the methodology for studying the effect of enhanced CO₂ level on water ecosystems. Long-term in situ simulating experiment would be the best approach for studying the effect of enhanced CO₂ on natural phytoplankton community. In future research, picophytoplankton will attract more attention, since they are sensitive to the change of CO₂ level compared with large ones because of their high surface-area-to-volume ratio. In addition, more efforts should be exerted for freshwater ecosystems with respect to elevated CO₂ effects, as they are far less investigated than marine ecosystems even though they are closely linked to the survival of human being.

Long‐term fish assemblages of inner bends in a large river

AbstractWe sampled fishes at 17 inner bend sites on the Wabash River in 2008 to compare with collections from 1977 to 1997. We used the same seine collection methods as previous years and collected a total of 37 species. Mean site Shannon‐Wiener diversity, species richness, evenness and abundance for all years were similar. We used multivariate analyses to test for patterns in fish assemblage structure among all sites and all years. The multivariate analyses resulted in distinct assemblages for each collection‐year, suggesting shifts in assemblage composition among years. We used separate multivariate analyses to examine fish assemblage variation within individual years. Variation that corresponded to an upstream–downstream pattern was present in 1977 and 2008, but not in 1997. A hydrologic analysis based on daily discharge revealed that eight large flood events occurred from 1928 to 2007, with four of these events during the recent 20 years. We quantified substrate variation at the 17 sites in 2008 and identified a longitudinal gradient in dominant substrate categories with gravel upstream and sand downstream that was correlated with the first axis of the 2008 fish assemblage ordination. We suggest that observed changes in fish assemblages in decadal periods were from hydrologic impacts of large floods on local habitats. Copyright © 2010 John Wiley & Sons, Ltd.