Mesocosm Approach Research Articles

Microplastics and riverine macroinvertebrate communities in a multiple-stressor context: A mesocosm approach

Growing use of synthetic materials has increased the number of stressors that can degrade freshwater ecosystems. Many of these stressors are relatively new and poorly understood, such as microplastics which are now ubiquitous in freshwater systems. The effects of microplastics on freshwater biota must be investigated further in order to better manage and mitigate their impacts. Our experiment provides the first empirical evaluation of stream invertebrate community dynamics in response to microplastics of different concentrations and sizes, in combination with fine sediment, a pervasive known stressor in running waters. In a 7-week streamside experiment using 64 flow-through circular mesocosms, we investigated the effects of exposure to three simulated microplastic influxes (polyethylene microspheres at four levels between 0 and 28,800 items/event) and the addition of fine sediment (to simulate a polluted stream environment). Invertebrate drift was monitored for 48 h immediately after each microplastic influx, and benthic invertebrate communities were sampled after 28 days of microplastic and sediment manipulations. Microplastic concentration, size and fine sediment all had significant factor main effects on several invertebrate drift response metrics, whereas few microplastic main effects were seen in the benthic community. However, interactive stressor effects were common in different combinations between sediment, microplastic size and concentration, suggesting multiple-stressor relationships between microplastics and fine sediment. Microplastic ingestion was witnessed in four of 12 taxa analysed: Hydrobiosidae, Deleatidium spp., Potamopyrgus antipodarum and Archichauliodes diversus. Our findings provide insights into how microplastics affect drift and benthic community dynamics of stream invertebrates in a field-realistic experimental setting and highlight areas requiring further study. These include investigations of invertebrate drift dynamics in response to other types of microplastics, the role invertebrate size may play in determining their vulnerability to microplastic pollution, and framing more microplastic research in a field-realistic multiple-stressor context.

Comparative metabolomics reveals how the severity of predation by the invasive insect Cydalima perspectalis modulates the metabolism re-orchestration of native Buxus sempervirens.

The recent biological invasion of box tree moth Cydalima perspectalis on Buxus trees has a major impact on European boxwood stands through severe defoliation. This can hinder further regrowth and threaten survival of populations. In a mesocosm approach and controlled larval density over a 2-month period, responses of B. sempervirens essential and specialized metabolites were characterized using metabolomics, combining 1H-NMR and LC-MS/MS approaches. This is the first metabolome depiction of major Buxus responses to boxwood moth invasion. Under severe predation, remaining green leaves accumulate free amino acids (with the noticeable exception of proline). The leaf trans-4-hydroxystachydrine and stachydrine reached 10-13% and 2-3% (DW), while root content was lower but also modulated by predation level. Larval predation promoted triterpenoid and (steroidal) alkaloid synthesis and diversification, while flavonoids did not seem to have a relevant role in Buxus resistance. Our results reveal the concomitant responses of central and specialized metabolism, in relation to severity of predation. They also confirm the potential of metabolic profiling using 1H-NMR and LC-MS to detect re-orchestration of metabolism of native boxwood after severe herbivorous predation by the invasive box-tree moth, and thus their relevance for plant-insect relationships and ecometabolomics.

Disinfection by-product formation potential in response to variability in dissolved organic matter and nutrient inputs: Insights from a mesocosm study

Changes in rainfall patterns driven by climate change affect the transport of dissolved organic matter (DOM) and nutrients through runoff to freshwater systems. This presents challenges for drinking water providers. DOM, which is a heterogeneous mix of organic molecules, serves as a critical precursor for disinfection by-products (DBPs) which are associated with adverse health effects. Predicting DBP formation is complex due to changes in DOM concentration and composition in source waters, intensified by altered rainfall frequency and intensity. We employed a novel mesocosm approach to investigate the response of DBP precursors to variability in DOM composition and inorganic nutrients, such as nitrogen and phosphorus, export to lakes. Three distinct pulse event scenarios, mimicking extreme, intermittent, and continuous runoff were studied. Simultaneous experiments were conducted at two boreal lakes with distinct DOM composition, as reflected in their color (brown and clear lakes), and bromide content, using standardized methods. Results showed primarily site-specific changes in DBP precursors, some heavily influenced by runoff variability. Intermittent and daily pulse events in the clear-water mesocosms exhibited higher haloacetonitriles (HANs) formation potential linked to freshly produced protein-like DOM enhanced by light availability. In contrast, trihalomethanes (THMs), associated with humic-like DOM, showed no significant differences between pulse events in the brown-water mesocosms. Elevated bromide concentration in the clear mesocosms critically influenced THMs speciation and concentrations. These findings contribute to understanding how changing precipitation patterns impact the dynamics of DBP formation, thereby offering insights for monitoring the mobilization and alterations of DBP precursors within catchment areas and lake ecosystems.

Do weathered microplastics impact the planktonic community? A mesocosm approach in the Baltic Sea

Microplastics (MPs) are ubiquitous pollutants of increasing concern in aquatic systems. However, little is still known about the impacts of weathered MPs on plankton at the community level after long-term exposure. In this study, we investigated the effects of weathered MPs on the structure and dynamics of a Baltic Sea planktonic community during ca. 5 weeks of exposure using a mesocosm approach (2 m3) mimicking natural conditions. MPs were obtained from micronized commercial materials of polyvinyl chloride, polypropylene, polystyrene, and polyamide (nylon) previously weathered by thermal ageing and sunlight exposure. The planktonic community was exposed to 2 μg L-1 and 2 mg L-1 of MPs corresponding to measured particle concentrations (10–120 μm) of 680 MPs L-1 and 680 MPs mL-1, respectively. The abundance and composition of all size classes and groups of plankton and chlorophyll concentrations were periodically analyzed throughout the experiment. The population dynamics of the studied groups showed some variations between treatments, with negative and positive effects of MPs exhibited depending on the group and exposure time. The abundance of heterotrophic bacteria, pico- and nanophytoplankton, cryptophytes, and ciliates was lower in the treatment with the higher MP concentration than in the control at the last weeks of the exposure. The chlorophyll concentration and the abundances of heterotrophic nanoflagellates, Astromoeba, dinoflagellate, diatom, and metazooplankton were not negatively affected by the exposure to MPs and, in some cases, some groups showed even higher abundances in the MP treatments. Despite these tendencies, statistical analyses indicate that in most cases there were no statistically significant differences between treatments over the exposure period, even at very high exposure concentrations. Our results show that weathered MPs of the studied conventional plastic materials have minimal or negligible impact on planktonic communities after long-term exposure to environmentally relevant concentrations.

Mesocosm experiments validate induction of Daphnia vertical migration by the fish‐derived kairomone 5α‐cyprinol sulfate

AbstractThe fish‐derived bile salt 5α‐cyprinol sulfate (CPS) has been identified as a kairomone inducing the predator avoidance behavior “diel vertical migration” (DVM) in Daphnia magna in response to fish. However, conclusions about the ecological significance of CPS have been derived from laboratory experiments only. Using a mesocosm approach, we investigate whether the role of CPS as a kairomone can be confirmed in the field. We demonstrate that CPS induces downward migration during daytime in a field‐derived Daphnia community mainly consisting of Daphnia longispina and Daphnia cucullata within the experiment. In the study lake, the actual population of D. longispina shows a similar pattern of DVM, while concomitant quantification of CPS by HPLC‐MS confirms that CPS in situ concentrations are sufficiently high for induction of daytime downward migration of D. longispina in this oligo‐mesotrophic lake. Together, these observations infer that CPS is a significant kairomone‐inducing DVM‐like behavior in Daphnia.

Legacy effects of earthworms on soil microbial abundance, diversity, and community dynamics

The earthworm species Lumbricus terrestris L. feeds on plant litter mixed with surrounding soil. Here, we analyzed with a mesocosm approach and soil incubations how that activity and subsequent ageing of casts (feces) affects the abundance and diversity of the soil microbiome. Earthworms were fed either with straw of sainfoin (SA, Onobrychis viciifolia; C/N ratio 22) or winter wheat (WW, Triticum aestivum, C/N ratio 101). The gut transit increased the abundances of bacteria and fungi, but reduced archaea. As indicated at the DNA and RNA level, main beneficiaries of the facilitated access to nutrients were members of Bacteroidota, especially Flavobacteriales with an estimated generation time of only 2 h. While Alphaproteobacteria were reduced, Gammaproteobacteria also increased in abundance and activity. SA was more nutritious for L. terrestris, and supported a higher bacterial abundance, probably because more N was available for growth and denitrification. During cast ageing, prokaryotic community compositions became increasingly similar to bulk soil communities. However, they remained distinguishable even after 168 d, suggesting that effects can last beyond a vegetation period. Dry-wet conditions preserved these differences better than continuous moisture. During ageing, more complex prokaryotic networks were detected with WW and dry-wet conditions. Thus, N and water limitations appeared to enhance cooperation rather than competition between the prokaryotes. Overall, this study demonstrates that earthworm soil interactions strongly affect the diversity and temporal dynamics of the soil microbiome. Legacy effects of earthworm activities should thus be kept in mind when investigating the environmental variation of soil microbiomes.

Mesocosm approaches to the examination of benthic–pelagic coupling, with emphasis on turbulence

AbstractBenthic and pelagic processes are closely coupled in shallow‐water estuarine and marine environments and in lakes yet benthic–pelagic coupling is difficult to simulate in mesocosms. Realistic water column (WC) and bottom turbulence often are not simultaneously mimicked in mesocosm experiments, leading to artifacts and omitting direct and indirect linkages and feedbacks between the sediment and the WC. The information on benthic–pelagic coupling mesocosm studies assembled illustrates the importance of realistic physics in mesocosms. Mesocosm approaches including both WC turbulence and bottom turbulence that are appropriately designed to resemble natural conditions can provide a deeper mechanistic understanding of feedback processes associated with biogeochemical, food web, and the habitat shifts necessary for models, and will help identify cause‐and‐effect relationships. Benthic–pelagic mesocosm experiments with realistic WC and bottom turbulence may fill the large gaps in our understanding of the responses of inorganic nutrient and organic matter fluxes between benthic habitats and the WC. Future applications of such benthic–pelagic coupling mesocosm studies are outlined.

Assessing the Impact of Hg-Contaminated Sediments Washing through Sentinel Species: A Mesocosm Approach

This study combines a traditional chemical characterization with a simultaneous biological evaluation through histological, immunohistochemical, and enzymatic observations to assess the efficiency and sustainability of soil washing on Hg-contaminated sediment in terms of the bioavailability of the contaminant before and after the treatment, as well as the potential drawbacks of the treatment that are not revealed by a simple chemical characterization of treated sediments on its own. Different extracting agents, that is, ethylenedinitrilotetraacetic acid (EDTA), ethylenediaminedisuccinic acid (EDDS), sodium thiosulfate, potassium iodide (KI), and iodine (I2), have been compared in this work to evaluate their efficiency in the removal of Hg from contaminated sediments. Speciation analysis was applied to assess the mobility of Hg from different fractions of aged sediments. Biological evaluation was carried out through the use of large mesocosms and Mytilus galloprovincialis as biological sentinels. Results from bench scale tests have shown Hg removal of up to 93% by means of the multi-step KI/I2 washing process of the sediment. Results from histological, immunohistochemical, and enzymatic analysis have shown significant differences in the degree of alteration of biological tissues and their functional integrity between organisms in contact with contaminated and restored sediments. The reduction in 5-HT3R immunopositivity in the mesocosm with treated sediments suggests a tendency for mussels to recover a healthy condition. This result was also confirmed by the measurement of the enzymatic activity of AChE in mussel gills, which was significantly reduced in organisms from the mesocosm with polluted sediments compared with those from the one with restored sediments. The proposed approach could help stakeholders all over the world select, at an early stage, the most efficient cleaning action from a more holistic perspective, including not only pollutant concentration and economic reduction but also a direct assessment of the ultimate impact of the selected process on the biological system.

Selective pressure on ontogenic stages of blue mussels (Mytilus edulis, L.)

Our previous work confirmed that the higher survival, growth, and commercial performance of a mussel stock collected from a specific lagoon have been maintained over the last 25 years at a microgeographic scale despite intensive transfers of its spat to other local mussel sites. The objective of this study was to identify potential selection mechanisms supporting the quality of this stock. We used a mesocosm approach to examine the effect of maximum seawater temperature measured in the field on the different ontogenic stages of Mytilus edulis to simulate similar patterns in the laboratory. We demonstrated that thermal selection did not affect the development of peri-metamorphic stages (< 1 mm) since no effects were observed on growth, survival, or prodissoconch II size under natural thermal variations. However, our experimental results clearly indicated the existence of a thermal selective pressure (24 °C for 85 h) on 4.5–7 mm juvenile mussels, with growth performance enhanced in surviving individuals. Mortality was 37% higher compared to the control group, suggesting the elimination of more sensitive and less performant mussels. Interestingly, surviving individuals showed improved shell and tissue growth rates, by 22 and 51%, respectively. Our results support the hypothesis that the thermal selection of highly performant stock is size-dependent. The selective pressure demonstrated in this study along with our previous work highlighting the physiological and genetic differences between local stocks constitute the key mechanisms leading to the observed stable production performance over decades at a microgeographic scale.

Using convective mixing in mesocosms to study climate-driven shifts in phytoplankton community distributions

With climate change predicted to alter water column stability and mixing across the world’s oceans, a mesocosm experiment was designed to ascertain how a natural phytoplankton community would respond to these changes. As a departure from other mesocosm experiments, we used heating and cooling to produce four different climate-inspired mixing scenarios ranging from well-mixed water columns representative of typical open turbulence (ϵ = 3 x 10-8 m2/s3) through to a quiescent water column with stable stratification (ϵ = 5 x 10-10 m2/s3). This method of turbulence generation is an improvement on previous techniques (e.g., grid, shaker, and aeration) which tend to produce excessive dissipation rates inconsistent with oceanic turbulence observations. Profiles of classical physical parameters used to describe turbulence and mixing (turbulent dissipation rate, buoyancy frequency, turbulent eddy diffusivity, Ozmidov scale) were representative of the profiles found in natural waters under similar mixing conditions. Chlorophyll-a profiles and cell enumeration showed a clear biological response to the different turbulence scenarios. However, the responses of specific phytoplankton groups (diatoms and dinoflagellates) did not conform to the usual expectations: diatoms are generally expected to thrive under convective, turbulent regimes, while dinoflagellates are expected to thrive in converse conditions, i.e., in stable, stratified conditions. Our results suggest that responses to mixing regimes are taxon-specific, with no overwhelming physical effect of the turbulence regime. Rather, each taxon seemed to very quickly reach a given vertical distribution that it managed to hold, whether actively or passively, with a high degree of success. Future studies on the effects of climate change on phytoplankton vertical distribution should thus focus on the factors and mechanisms that combine to determine the specific distribution of species within taxa. Our convection-based mesocosm approach, because it uses a primary physical force that generates turbulence in open waters, should prove a valuable tool in this endeavor.

Zostera noltei response to transplantation into historically Hg-contaminated sediments (A mesocosm experiment): Growth, bioaccumulation and photosynthetic performance

The present study aimed to evaluate the effectiveness of seagrass recolonization as a nature-based solution for the recovery of a coastal area historically contaminated by mercury (Laranjo Bay, Ria de Aveiro, Portugal). A mesocosm approach was employed to assess the resistance of Zostera noltei to transplantation into contaminated sediments collected in-situ (0.5–20 mg kg−1 Hg). At each sampling time (15, 30, 60, 120 and 210 days), the resistance of transplanted Z. noltei was evaluated through growth parameters (biomass and coverage area), photosynthetic performance and elemental composition. Although some significant differences (p ≤ 0.05) were observed between treatments, essentially associated with the elemental composition of plant tissues, the most relevant variations were associated with seasonality. Overall, plants were found to not be affected by sediment contamination, under the tested concentrations, suggesting that recolonization with Z. noltei can be an effective restoration strategy for historically contaminated coastal areas.

Microbial mats as model to decipher climate change effect on microbial communities through a mesocosm study.

Marine environments are expected to be one of the most affected ecosystems by climate change, notably with increasing ocean temperature and ocean acidification. In marine environments, microbial communities provide important ecosystem services ensuring biogeochemical cycles. They are threatened by the modification of environmental parameters induced by climate change that, in turn, affect their activities. Microbial mats, ensuring important ecosystem services in coastal areas, are well-organized communities of diverse microorganisms representing accurate microbial models. It is hypothesized that their microbial diversity and metabolic versatility will reveal various adaptation strategies in response to climate change. Thus, understanding how climate change affects microbial mats will provide valuable information on microbial behaviour and functioning in changed environment. Experimental ecology, based on mesocosm approaches, provides the opportunity to control physical-chemical parameters, as close as possible to those observed in the environment. The exposure of microbial mats to physical-chemical conditions mimicking the climate change predictions will help to decipher the modification of the microbial community structure and function in response to it. Here, we present how to expose microbial mats, following a mesocosm approach, to study the impact of climate change on microbial community.

Exposure to fipronil, 2,4-D and vinasse influences macroinvertebrate assemblage structure: An experimental mesocosm approach

Sugarcane is one of the main monocultures in Brazil and widely uses herbicide 2,4-D and fipronil insecticide. In addition, vinasse can be mentioned as it is widely used in this plantation. These compounds occurring simultaneously in the aquatic environment can potentiate the deleterious effects on organisms. Thus, this study aimed to evaluate the composition, abundance and ecological indices of the benthic macroinvertebrate community, as well as its ability to reestablish itself considering environmental contamination by the pesticides Regent® 800WG (active ingredient - a.i. fipronil) (F) and DMA® 806BR (a.i. 2,4-D) (D) and vinasse (V), alone and in mixtures: pesticides - M and the three contaminants - MV. The study was conducted using open-air mesocosms. The macroinvertebrate community was monitored by colonization structures, the physical-chemical parameters, metals and pesticides were determined and the effects of contaminants were evaluated over the exposure time in 1, 7, 14, 28, 75 to 150 days. A multiple regression was performed between the water parameters and significant relationships were found between parameters associated with vinasse contamination (pH, total nitrogen, turbidity, and dissolved oxygen) and concentration of fipronil and the ecological variables studied. Over time, changes were observed in the composition of the community. The dominance and richness increased in treatments V and MV. The family Chironomidae and subclass Oligochaeta were more sensitive to the treatment V and MV, while individuals from the families Phoridae, Ephydridae and Sciomyzidae were occasionally found (depending on the experimental time) in these treatments. The insects were sensitive to treatments F and M, disappearing in these mesocosms after contamination, reappearing only after 75 days. The results reveal that sugarcane management practices associated with the use of pesticides and vinasse as fertilizer pose risks to the macroinvertebrate community with consequences for the trophic chains, given its importance in freshwater ecosystems and adjacent terrestrial environments.

Soil management effects of extensive pastures, intensive pastures and sugarcane crops on the availability of metals and nutrients in freshwater: A realistic mesocosm approach

Changes in land use have led to important modifications in aquatic ecosystems around the world. In Brazil, extensive pastures are being converted to sugarcane crops to avoid deforestation, while the remaining pastures are being intensified to compensate for the loss of area. In this study, the hypothesis that conventional soil management practices conducted on intensive pasture and sugarcane crops increase the availability of contaminants in freshwater was tested. Thirteen mesocosms embedded in extensive pasture (EP), intensive pasture (IP), and sugarcane crops (C) systems were monitored for 392 days. The monitoring included the measurement of in situ parameters (pH, dissolved oxygen, electrical conductivity, turbidity, and chlorophyll), nutrients (total nitrogen, nitrite, nitrate, ammonium ion, total phosphorus, total dissolved phosphorus, inorganic phosphorus, and silicate), and metals (aluminum, cadmium, copper, manganese, nickel, lead, potassium, and zinc). Soil management practices were performed in IP and C treatments at full-scale (tillage, application of fertilizers, vinasse and pesticides, and cattle pasture). The patterns observed for each treatment were obtained developing Principal Component Analysis over time. The mesocosms in the EP treatment did not experience large variations in water quality over time since no soil management was performed in this treatment. However, we found that fertilizer and vinasse application was the major precursor to the deterioration of water quality over time. After the first fertilizer application, we found eight and 16 times higher concentrations of total phosphorus, and seven and 45 times higher concentrations of total nitrogen in the IP and C treatments, respectively, in comparison to their concentration in the EP treatment. The application of vinasse in sugarcane crop increased water conductivity, potassium, and manganese concentrations in mesocosms. Dissolved oxygen and pH values decreased over time in all treatments. Overall, soil management practices resulted in greater deterioration of water quality in the sugarcane treatment than in the intensive pasture.

Experimental effects of warming and epiphyte grazing on the ecophysiology of two seagrass morphotypes

Seagrasses form important ecosystems providing essential provisioning and regulating services. They exhibit acclimative properties to contend with environmental change while intrinsic responses allow seagrasses to persist in sub- and supra-optimal temperatures, and are critical to understanding their ability to cope with a changing climate. An indoor mesocosm approach was used to investigate the effects of warming, i.e. mean summer (24 °C) and extreme (30 °C) temperatures, and epiphyte grazing on large-leaved and small-leaved morphotypes of a dwarf eelgrass (Zostera capensis) from a temperate lagoon habitat. Both morphotypes displayed similar responses under extreme temperatures with greater losses observed in shoot densities and total biomass compared to summer mean temperatures. However, higher photosynthetic rates and accumulations of carbon, nitrogen and phenolics were observed in small- compared to large-leaved plants. This suggests that small-leaved populations possess a larger propensity to store resources - a potential for resilience. Large-leaved populations appeared to have channelled resources towards growth suggesting a greater susceptibility to stress from e.g. fouling. Epiphyte biomass increased under extreme temperatures and, even though grazing had a positive effect on seagrass biomass under summer mean conditions, grazers did not regulate algal growth under extreme temperatures which may have been too rapid for grazer control. These findings indicate that seagrasses will be negatively affected by extreme warming with ensuing reductions in biomass, increasing their susceptibility to stressors. However, acclimation strategies including carbon and nitrogen stores provide high resilience potential and attest to a system's ability to cope with stress.

Experimental assessment of salinization effects on freshwater zooplankton communities and their trophic interactions under eutrophic conditions

Freshwater ecosystems are becoming saltier due to human activities. The effects of increased salinity can lead to cascading trophic interactions, affecting ecosystem functioning and energy transfer, through changes in community and size structure. These effects can be modulated by other environmental factors, such as nutrients. For example, communities developed under eutrophic conditions could be less sensitive to salinization due to cross-tolerance mechanisms. In this study, we used a mesocosm approach to assess the effects of a salinization gradient on the zooplankton community composition and size structure under eutrophic conditions and the cascading effects on algal communities. Our results showed that zooplankton biomass, size diversity and mean body size decreased with increased chloride concentration induced by salt addition. This change in the zooplankton community did not have cascading effects on phytoplankton. The phytoplankton biomass decreased after the chloride concentration threshold of 500 mg L−1 was reached, most likely due to direct toxic effects on the osmotic regulation and nutrient uptake processes of certain algae rather than as a response to community turnover or top-down control. Our study can help to put in place mitigation strategies for salinization and eutrophication, which often co-occur in freshwater ecosystems.

Shelter availability reduces the effects of the invasive Red Swamp Crayfish (Procambarus clarkii) on eelgrass-dominated clear-water lakes: a mesocosm approach

Shelter availability reduces the effects of the invasive Red Swamp Crayfish (Procambarus clarkii) on eelgrass-dominated clear-water lakes: a mesocosm approach

Top-down and bottom-up effects of fish on a macrophyte-mediated trophic network: a mesocosm approach

Top-down and bottom-up effects of fish on a macrophyte-mediated trophic network: a mesocosm approach

Seasonally driven internal P and N nutrient (re)cycling strategies of beech saplings are element specific

Excessive N deposition leads to the conversion of previously N limited to N saturated forest ecosystems. The input of N can result in severe N/P imbalance and N-induced P deficiency. The present study investigates whether N addition induces P deficiency in beech saplings or if this can be counteracted by enhanced internal P (re)cycling. Furthermore, it was tested whether addition of P can mitigate N induced P deficiency. In addition, higher need of P due to enhanced growth may be realized either by enhanced P uptake or by improved internal P (re)cycling. This assumption was tested with beech saplings growing at higher light intensity. Therefore, a mesocosm approach with beech saplings and soil (O and A horizon) originating from a P-poor forest was conducted. Mesocosms were cultivated for two growing seasons in a garden. Addition of N, P, or combined addition of N and P took place during the first year and another group of saplings was exposed to higher light intensity to enhance CO 2 assimilation and growth. Soil horizons and beech saplings were harvested during the second year of growth in spring, summer, autumn, and winter. The results show that internal (re)cycling of N and P is different and mostly determined by the season. The seasonal (re)cycling of P was driven by the metabolic P demand of tissues/organs characterized by shifting P between the perennial tissues bark and wood of branches and coarse roots and deciduous leaves, fine roots and long-distance transport paths without using major storage resources. In contrast, seasonal (re)cycling of N is characterized by N storage in perennial tissues during dormancy and by N mobilization from the entire trunk, i.e. branch, stem and coarse roots, in spring. Furthermore, these seasonal dynamics were found to be independent of the treatment. • P (re)cycling comprises the shift of P to the places of demand without P storage. • N storage and re-mobilization characterizes saplings seasonal (re)cycling of N. • High light induces N deficiency but sufficient P supply sustained by P (re)cycling. • N/P ratios used as indicator of nutrient imbalance show large seasonality. • Missing root establishment in the Of+Oh prevents P improvement of saplings.

Resilience of a microphytobenthos community from the Severn Estuary, UK, to chlorination: A mesocosm approach

The Severn Estuary is a large macrotidal estuary which includes an extensive mudflat with microphytobenthos (MPB) playing a key role in the ecosystem. This study evaluated the impact of chlorination at two different dosing levels (0.05 and 0.5 mg/l as total residual oxidants, TRO, representative of potential concentrations in the mixing zone and within the cooling water systems of a power station) on a MPB community representative of the Severn Estuary. Biomass and diversity were not negatively impacted while physiology was partially affected at the beginning of the experiment, and it recovered towards the end of the experiment. Further investigations for diversity are needed to consolidate our findings. In conclusion our results show that MPB is resilient to chlorination up to a concentration of 0.5 mg/l which is much higher (>10 times) than what might be expected near the chlorinated discharges for most coastal power stations.