Changes in the Size Structure of Algal Communities in Rivers of the Basin of Elton Lake under Salinity Gradient Conditions

Marine ecosystems have been heavily impacted by fishing pressure, which can cause major changes in the structure of communities. Fishing directly removes biomass and causes secondary effects such as changing predatory and competitive interactions and altering energy pathways, all of which affect the functional groups and size distributions of marine ecosystems. We conducted a meta-analysis of eighteen trawl surveys from around the world to identify if there have been consistent changes in size-structure and life history groups across ecosystems. Declining biomass trends for larger fish and invertebrates were present in nine systems, all in the North Atlantic, while seven ecosystems did not exhibit consistent declining trends in larger organisms. Two systems had alternative patterns. Smaller taxa, across all ecosystems, had biomass trends with time that were typically flat or slightly increasing. Changes in the ratio of pelagic taxa to demersal taxa were variable across the surveys. Pelagic species were not uniformly increasing, but did show periods of increase in certain regions. In the western Atlantic, the pelagic-to-demersal ratio increased across a number of surveys in the 1990s and declined in the mid 2000s. The trawl survey data suggest there have been considerable structural changes over time and region, but the patterns are not consistent across all ecosystems.

Indicators of change in the size structure of fish communities: A case study from the south coast of South Africa

Wildfires subject soil microbes to extreme temperatures and modify their physical and chemical habitat. This might immediately alter their community structure and ecosystem functions. We burned a fire-prone shrubland under controlled conditions to investigate (1) the fire-induced changes in the community structure of soil archaea, bacteria and fungi by analysing 16S or 18S rRNA gene amplicons separated through denaturing gradient gel electrophoresis; (2) the physical and chemical variables determining the immediate shifts in the microbial community structure; and (3) the microbial drivers of the change in ecosystem functions related to biogeochemical cycling. Prokaryotes and eukaryotes were structured by the local environment in pre-fire soils. Fire caused a significant shift in the microbial community structure, biomass C, respiration and soil hydrolases. One-day changes in bacterial and fungal community structure correlated to the rise in total organic C and NO(3)(-)-N caused by the combustion of plant residues. In the following week, bacterial communities shifted further forced by desiccation and increasing concentrations of macronutrients. Shifts in archaeal community structure were unrelated to any of the 18 environmental variables measured. Fire-induced changes in the community structure of bacteria, rather than archaea or fungi, were correlated to the enhanced microbial biomass, CO(2) production and hydrolysis of C and P organics. This is the first report on the combined effects of fire on the three biological domains in soils. We concluded that immediately after fire the biogeochemical cycling in Mediterranean shrublands becomes less conservative through the increased microbial biomass, activity and changes in the bacterial community structure.

Aquatic gastropods are important integral components of the macroinvertebrate community in freshwater ecosystems and play critical roles in freshwater ecosystems by contributing to biodiversity, nutrient cycling, and water quality. However, the variation of aquatic gastropods’ community structure under the combined effects of warming and nutrient enrichment remains largely unknown. To investigate this question, we performed an outdoor mesocosm experiment examining the interaction of warming (a 4.5 °C increase in mean temperature above ambient conditions) and nutrient enrichment (phosphorus addition) on the aquatic gastropods’ community and dominant population (Bellamya aeruginosa). We analyzed the changes in community dynamics (abundance and biomass), size structure, and stoichiometric traits (only B. aeruginosa). Results showed that phosphorus enrichment alone had a positive effect on the total abundance and biomass of gastropods, as well as the abundance and biomass of B. aeruginosa. Warming alone only produced a positive effect on total abundance. However, the combined effects of warming and phosphorus enrichment negatively affected the biomass and abundance of the whole gastropod community and the dominant gastropod population. The body mass of B. aeruginosa increased because of warming, whereas the body mass of the gastropod community negatively responded to warming. Phosphorus enrichment alone had no remarkable effects on body mass. The combined effects of warming and phosphorus enrichment negatively affected the whole community’s body mass but had no substantial effect on the body mass of B. aeruginosa. For body stoichiometric traits, warming or phosphorus enrichment alone produced positive effects on the nitrogen and phosphorus contents of B. aeruginosa. The combined effects caused adverse effects on the contents of the two elements. The effect of warming alone only decreased the ratio of nitrogen to phosphorus. Results suggested that the response levels in size structure between the gastropod community and the dominant population differed remarkably. Composition species shift was the main cause of the decrease in aquatic gastropods’ community size structure. The shift in species composition at the whole gastropod community level caused by warming and phosphorus enrichment may result in more complex and unpredicted consequences through cascade effects on the structure and function of freshwater ecosystems.

Nitrogen uptake regime and phytoplankton community structure in the Atlantic and Indian sectors of the Southern Ocean

Short-term variability of the phytoplankton community in coastal ecosystem in response to physical and chemical conditions' changes

Our previous investigation found elevated nitrogen deposition caused declines in abundance of arbuscular mycorrhizal fungi (AMF) associated with forest trees, but little is known about how nitrogen affects the AMF community composition and structure within forest ecosystems. We hypothesized that N deposition would lead to significant changes in the AMF community structure. We studied the diversity and community structure of AMF in northern hardwood forests after more than 12 years of simulated nitrogen deposition. We performed molecular analyses on maple (Acer spp.) roots targeting the 18S rDNA region using the fungal-specific primers AM1 and NS31. PCR products were cloned and identified using restriction fragment length polymorphism (RFLP) and sequencing. N addition significantly altered the AMF community structure, and Glomus group A dominated the AMF community. Some Glomus operational taxonomic units (OTUs) responded negatively to N inputs, whereas other Glomus OTUs and an Acaulospora OTU responded positively to N inputs. The observed effect on community structure implies that AMF species associated with maples differ in their response to elevated nitrogen. Given that functional diversity exists among AMF species and that N deposition has been shown to select less beneficial fungi in some ecosystems, this change in community structure could have implications for the functioning of this type of ecosystem.

Diversity has been shown to be pivotal in ecosystem stability and resilience. It is therefore important to increase our knowledge about the development of diversity. The aim of this study was to investigate the temporal dynamics of the bacterial community structure in the rhizosphere of wheat plants growing in a soil in which the initial conditions for bacterial re-colonization were modified by mixing different amounts of sterilized with native soil at ratios of 19:1, 9:1, 4:1 and 1:1. Additional treatments comprised sterilized soil or native soil. Plant dry weight at day 20 decreased with increasing percentage of native soil in the mix. The bacterial community structure in the rhizosphere was assessed by polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) at days 3, 14 and 20 after planting. The bacterial community in the sterilized soil had a lower diversity and evenness than the native soil. Both diversity and evenness increased with time in the sterilized soil. Community structure in the different mixes changed over time and the changes were mix-specific. Principal component analyses of the DGGE banding patterns showed clear differences between the treatments particularly at day 3 and day 14 and revealed changes in community structure within a few days in a given treatment. The results of the present study show that bacterial communities rapidly re-colonize sterilized soil. During re-colonization, the community structure changes rapidly with a general trend towards higher diversity and evenness. The changes in community structure over time are also affected by the amount of sterile substrate to be re-colonized.

Molecular characterization of fungal community dynamics in the initial stages of composting

In order to study density-dependent effects of invertebrate planktivory, four different densities of Bythotrephes longimanus were inoculated into mesocosm enclosures with a mixed zooplankton community. Changes in size structure and abundance of zooplankton and phytoplankton communities were recorded over a period of 3 weeks. High densities of Bythotrephes were able to reduce total zooplankton abundance, which was mainly due to a decrease in the density of the relatively large species Holopedium gibberum. The density of the smaller species Bosmina longirostris was also reduced with increasing densities of Bythotrephes, whereas rotifer abundance remained largely unaffected. The mean size of Holopedium increased with increasing densities of Bythotrephes. Despite the decrease in total zooplankton biomass in high-Bythotrephes treatments, no effect of Bythotrephes density on primary producers was observed. Our experiment shows that predacious cladocerans may reduce macrozooplankton biomass, large as well as small species. Predation from invertebrate planktivores results in a zooplankton community consisting of larger individuals. Comparing our experimental densities with densities of Bythotrephes found in natural systems suggests that invertebrate planktivores may influence size structure and abundance of zooplankton communities even in lakes with planktivorous fish.

10 - Community Dynamics

Responses of stream macroinvertebrate communities to progressive forest harvesting: Influences of harvest intensity, stream size and riparian buffers

Here we considered two fundamental questions in community ecology regarding the relationship between seasonal changes in community structure and environmental gradients: (i) How does the magnitude of seasonal changes in community structure vary along an environmental gradient? (ii) How do the processes driving seasonal changes in community structure vary along an environmental gradient? To examine these questions, we investigated intertidal sessile assemblages inhabiting a notable vertical environmental gradient and fitted a transition probability matrix model to decadal time series data gathered at 25 plots along the Pacific coast of eastern Hokkaido, Japan. We found that the magnitude of seasonal changes in community structure was the largest at mid shore. The major processes driving seasonal changes in community structure changed vertically, reflecting the indirect influence of vertical changes in the physical environment on the vertical distributions of species. An unexpected finding was that the magnitude of seasonal changes in community structure did not reflect the strength of seasonal variation in the physical environment. One explanation may be that sessile organisms living on the high shore have a broad tolerance to environmental stress and are thus less sensitive to the large seasonal variation in physical stress.

Photoquadrats enable efficient and cost‐effective quantitative estimation of epibenthic communities. Despite their utility, however, there has been limited use of photoquadrats for such purposes in temperate north‐west Europe, where there is also a growing need for standardized approaches to marine monitoring. A rapid photoquadrat‐based methodology was trialled by scuba divers on a heterogeneous boulder‐slope habitat in a Special Area of Conservation (SAC) in Northern Irish waters. To investigate the monitoring potential of the method, the benthic community was randomly sampled in the summers of 2009 and 2010. The community was represented at species level (community composition) and by functional group (community structure), and the data‐extraction resolution was varied using different numbers of point intercepts per image (25, 50, and 100) to assess the balance of precision and efficiency with regard to detection of community change. The method was efficient in situ and yielded sufficient sample images for estimation of local benthic community diversity (number of species). The community varied significantly, with six distinct sub‐communities identified within the survey area. High spatial variability obscured detection of temporal changes in the overall community composition and structure. However, spatial variability was substantially reduced by testing only the dominant sub‐community, in which significant changes were detected between 2009 and 2010. The ability of the photoquadrat to detect individual taxa was related to data resolution: the more point‐intercepts sampled, the more taxa were discovered, but the data‐extraction effort was greater. After considering the ability of the photoquadrat method to quantify number of species and to detect change in community structure, as well as its precision and efficiency, the inspection of 50 point intercepts per sample image was found to be optimal. These findings demonstrate the benefits of photoquadrat‐based methods and highlight their potential as a standard approach to marine monitoring. Copyright © 2011 John Wiley & Sons, Ltd.

Seasonal changes in the zooplankton community and population structure in the northern Bering Sea from June to September, 2017