Mesocosm Experiment Research Articles

Microplastics at Environmentally Relevant Concentrations Had Minimal Impacts on Pelagic Zooplankton Communities in a Large In-Lake Mesocosm Experiment.

To assess the potential risks of contemporary levels of plastic pollution in freshwater ecosystems, a large-scale experiment was conducted over 10 weeks in a boreal lake at the IISD-Experimental Lakes Area (Ontario, Canada). Fragments of common polymers (polyethylene, polystyrene, and polyethylene terephthalate), each with distinct colors and buoyancies, were added as a single pulse to seven in-lake mesocosms in equal contributions in a range of environmentally relevant nominal concentrations (6-29,240 particles/L). Two additional mesocosms with no added microplastics were used as controls. Zooplankton ingested low levels of microplastics (mean of 0.06 particles/individual ± SD 0.07) and generally their total abundance and community composition were not negatively impacted. Temporary changes were however observed; total zooplankton abundance and abundance of calanoid copepods were temporarily stimulated by increasing nominal microplastic concentrations, and modest, short-term reductions in egg production of the cyclopoid copepod Tropocyclops extensus and abundance of copepod nauplii occurred. Collectively, these results suggest that microplastics could have complex impacts on zooplankton communities, stimulating some species while negatively impacting others.

Phytostabilisation of arsenic contaminated gold mine waste using the native species Juncus usitatus, Poa labillardieri and Themeda triandra

Arsenic (As) contaminated gold mine waste represents a significant issue for ecosystem and human health, as it is a known carcinogen and environmental toxin. Bendigo in Victoria, Australia, has reported As concentrations up to 22,000 mg/kg, exceeding the national soil guidance values (100 mg/kg) by 220-fold. To reduce exposure and potential human health risk, remediation or risk management strategies are required. Traditional remediation, such as excavation and landfill, often fail to improve soil conditions and are expensive. Biological remediation using plants (phytoremediation) represents a cost-effective strategy that results in the restoration of soil function and ecosystem services. There is a need to identify native plants for phytoremediation as current research focuses on fern species, which are not suitable for Bendigo. This study aimed to identify a native phytostabilising plant by conducting a mesocosm experiment involving Juncus usitatus, Poa labillardieri, and Themeda triandra. Plants grew in mine waste for 100 days; all survived and bioaccumulated As in the roots (phytostabilisation). Poa labillardieri grew significantly more biomass (roots 0.24 ± 0.01 g; shoots 0.22 ± 0.03 g (d/w)) and bioaccumulated the most As (~ 100 mg/kg plant biomass). The bacterial community was investigated to assess soil health and As association. The rhizosphere microbiota stabilised after root colonisation for Poa labillardieri and Themeda triandra, as indicated by richness and evenness changes. The two dominant phyla were Actinobacteriota and Proteobacteria, which displayed As tolerance. A keystone taxon, Latescibacterota, exhibited a positive relative association with As remediation. Poa labillardieri is a good candidate for future phytostabilisation trials.

Growth and Biomass Yield of Grey Sedge (Lepironia articulata Retz. Domin) under Different Shoot-Cutting Intervals in a Tropical Peatland

Grey sedge (Lepironia articulata Retz. Domin) is a plant endemic to tropical peatlands and is widely used as a handicraft and biodegradable product that brings income to local farmers. However, its habitat has been decreasing due to peatland degradation, which has forced local farmers to harvest L. articulata repeatedly in the same habitat. To examine the effects of repeated shoot cutting at different time intervals on L. articulata growth and biomass yield, a mesocosm experiment was conducted from June 2019 to March 2020 in a tropical peatland in Perigi village, Ogan Ilir District, South Sumatra, Indonesia, using a randomized block design with four treatments and three replicates. The treatments were as follows: P1 (cutting every 1 month), P2 (cutting every 2 months), P3 (cutting every 3 months), and P4 (cutting at 6-months). The results showed that P1 significantly reduced monthly shoot height, shoot diameter, shoot number, dry biomass, cumulative shoot number, and cumulative dry biomass. In contrast, considering L. articulata‘s regenerative growth, the growth and cumulative biomass yield of P3 (1453.5 ± 518.4 g m−2) were as good as those of P4. These results indicate that the harvesting interval should be longer than 3 months for the sustainable use of L. articulata in tropical peatlands without damaging its regenerative ability.

Activity patterns of Octopus tetricus (Mollusca: Cephalopoda) and their behavioural responses to fisheries trap and bait combinations

An understanding of octopus behaviour and their capture by fishing gears is required to inform efficient, sustainable, and ethical octopus fisheries. Here, the behaviour of Octopus tetricus was assessed in response to different bait and trap combinations in an outdoor mesocosm experiment. Eight wild octopuses were collected, maintained in individual tanks with flow-through seawater and aeration, and monitored with a 24-h video surveillance system. Six different traps and four different baits were presented to each octopus in various combinations during four sequential trials. Fine-mesh crab traps were the most successful in capturing octopus, accounting for 23 of the total 30 captures across all trials. Whereas solid trigger traps produced the greatest number of other interactions (e.g., octopus sitting on trap/in the entrance), averaging 43 interactions per trial, but were rarely triggered. Bait type did not influence octopus capture, trap interaction frequency, or octopus activity; however, only artificial bait types were trialled, and fresh natural baits may elicit a stronger response and should be investigated in future research. Generally, octopus were inactive, dedicating only 9.5% of their total time to active behaviours. Octopus activity varied with time of day, with peak activity during morning daylight (0800–1200) and the lowest activity during the dark hours of the very early morning (0000–0400). Additionally, capture numbers, trap interactions, and activity varied among individuals, with bolder personalities in some octopus. This natural variation among individual octopuses may lead to fishery-induced selection associated with the elevated capture frequency of bold or more active individuals.

Potential phosphorus mobilization from riparian vegetation following freezing

Phosphorus (P) rich runoff from agricultural landscapes is a major contributor to freshwater eutrophication. Vegetated riparian zones are often employed to retain P from field runoff before it enters streams. In cold regions, vegetation has the potential to release P to runoff following freezing; however, it is unclear if this differs with winter frost severity, riparian zone local topography, vegetation type, and/or exposure to flooding/inundation. To explore the vulnerability of riparian vegetation to winter P losses, this study quantified soil and vegetation P concentrations from 8 riparian zones in Canada at different topographic positions within each riparian site (upper/field edge, lower/water edge) in both the fall and spring seasons and related this to observed surface temperatures and water levels throughout the non-growing season. This was complemented by two laboratory mesocosm experiments in which (1) plant samples were subjected to moderate (−40C) and severe (−250C) simulated winter frost treatments to quantify changes in their water extractable P (WEP) content, and (2) mesocosms were inundated with water to determine if dissolved P concentrations in flood water differed when plants were left intact, clipped but left on the soil surface, or harvested. Greater soil and vegetation P concentrations were observed at upper locations (field edge), and this remained consistent following freezing; however, vegetation WEP concentrations increased with greater simulated frost severity. In the field, temperatures were moderated by snow cover and although differences with riparian zone position were apparent between fall and spring collected samples, changes in P pools did not appear to be related to frost severity or inundation in the field. The mesocosm experiment revealed that harvesting vegetation considerably reduced dissolved P concentrations in flood water. This study shows that vegetated riparian zones can act as a source of P to streams during the winter non-growing season, and highlights the potential for riparian vegetation management in reducing P losses from riparian zones in cold climates.

Mortality partitioning between viral lysis and microzooplankton grazing in successive phytoplankton blooms using dilution and molecular methods

Phytoplankton play crucial roles in aquatic ecosystems, serving as the foundation of marine food webs and being responsible for ~50% of the world’s oxygen production. Predation by microzooplankton and viral lysis are the major sources of phytoplankton mortality, with the balance between these 2 processes affecting microbial food webs and biogeochemical cycles. However, determining the dominant mortality process in time and space remains an open question. This study investigated microzooplankton grazing and viral lysis rates during a mesocosm experiment in western Norway. High-resolution measurements were determined on phytoplankton groups using flow cytometry to observe changes in mortality rates and carbon flow during phytoplankton blooms. Digital droplet PCR was employed to detect Emiliania huxleyi and Micromonas spp. and associated viruses within environmental samples, to explore its use for determining mortality processes and understanding the impact on the phytoplankton community. The results showed that grazing and viral lysis dominated mortality at different times, with only one significant instance of both processes being observed. Microzooplankton grazing primarily affected picoplankton, while nanoeukaryotes and E. huxleyi were more susceptible to viral lysis. Molecular detection did not always match with abundances or rates determined by flow cytometry; however, it did provide insights into their dynamics throughout the mesocosm. These findings provide insights into the complex interactions between microzooplankton, viruses and phytoplankton communities. Understanding the balance between microzooplankton grazing and viral lysis can contribute to a more comprehensive understanding of carbon flow in aquatic ecosystems, which has significant implications for food webs and biogeochemical cycles.

Microbial necromass and glycoproteins for determining soil carbon formation under arbuscular mycorrhiza symbiosis

Arbuscular mycorrhizal fungi (AMF) form symbioses with most terrestrial plants and critically modulate soil organic carbon (C) dynamics. Whether AMF promote soil C storage and stability is, however, largely unknown. Since microbial necromass C (MNC) and glomalin-related soil protein (GRSP) are stable microbial-derived C in soils, we therefore evaluated how AMF symbiosis alters both soil C pools and their contributions to soil organic C (SOC) under nitrogen fertilization, based on a 16-weeks mesocosm experiment using a mutant tomato with highly reduced AMF symbiosis. Results showed that SOC content is 4.5 % higher following AMF symbiosis. Additionally, the content of MNC and total GRSP were 47.5 % and 22.3 % higher under AMF symbiosis than at AMF absence, respectively. The accumulations of GRSP and microbial necromass in soil were closely associated with mineral-associated organic C and the abundance of AMF. The increased soil living microbial biomass under AMF symbiosis was mainly derived from AMF biomass, and fungal necromass C significantly contributed to SOC accumulation, as evidenced by the higher fungal:bacterial necromass C ratio under AMF symbiosis. On the contrary, bacterial necromass was degraded to compensate for the increased microbial nutrient demand because of the aggravated nutrient limitation under AMF symbiosis, leading to a decrease in bacterial necromass. Redundancy analysis showing that bacterial necromass was negatively correlated with soil C:N ratio supported this argument. Moreover, the relative change rate of total GRSP was consistently greater in nitrogen-limited soil than that of microbial necromass. Our findings suggested GRSP accumulates faster and contributes more to SOC pools under AMF symbiosis than microbial necromass. The positive correlation between the contributions of GRSP and MNC to SOC further provided valuable information in terms of enhancing our understanding of mechanisms underlying the maintenance of SOC stocks through microbial-derived C.

Forbs from seasonal managed wetlands boost plankton production more than emergent graminoids by supplying novel labile detritus

Abstract Opportunities to enhance ecosystem functions exist in human‐dominated working landscapes, but understanding of key drivers is often lacking in these novel environments. In Suisun Marsh, California, USA, impounded managed wetlands designed to promote waterfowl incidentally support higher plankton densities—a key resource for imperilled pelagic fishes—than adjacent tidal habitats. Managed wetland operations could produce plankton at critical periods for fish and the aquatic food‐web more broadly, but drivers of plankton production in managed wetlands are poorly understood. We proposed that decaying vegetation resulting from controlled flooding in managed wetlands is important for stimulating plankton blooms, but that the effect varies by plant species and functional type. We conducted a mesocosm experiment to test the effects of different inundated plants on plankton production, including three forb species, three emergent graminoid species and a control treatment without added plant material. Forbs promoted larger phytoplankton blooms and higher zooplankton production—by an order of magnitude—than both emergent graminoids and the control treatment. Emergent graminoids supported larger phytoplankton blooms, but not higher zooplankton production, than the control treatment. Phytoplankton blooms exhibited pulse dynamics in all treatments, with densities increasing initially and crashing by the end of the experiment. Phytoplankton blooms in forb and emergent treatments lasted similar durations, counter to the expectation that emergent graminoids would support diminished production over a relatively prolonged period. Among zooplankton taxa, Daphnia magna and rotifers were strongly associated with all three forbs species while the nonnative cyclopoid copepod Limnothoina tetraspina were associated with cattails (Typha domingensis) and the control. Forbs temporarily depleted dissolved oxygen concentrations during the first week of the experiment, suggesting a tradeoff by which decaying forbs boost plankton production at risk of initial hypoxia. Our results suggest that seasonal drying and flooding operations in managed wetlands are likely to enhance plankton production because dry periods promote forb growth and subsequent flooding introduces labile forb material to the aquatic food web. Results also suggest that seasonal managed‐wetlands act as novel floodplain‐analogues, by which flood pulses drive productivity. Further research is needed to quantify production benefits and tradeoffs to the aquatic ecosystem at the landscape scale and to identify optimal management regimes that will help conserve imperilled species.

Summer primary production of Arctic kelp communities is more affected by duration than magnitude of simulated marine heatwaves.

Fjord systems in the Norwegian Arctic are experiencing an increasing frequency and magnitude of marine heatwaves. These episodic heat stress events can have varying degrees of acute impacts on primary production and nutrient uptake of mixed kelp communities, as well as modifying the biogeochemical cycling in nearshore systems where vast areas of kelp create structural habitat. To assess the impact of future marine heatwaves on kelp communities, we conducted a 23 day mesocosm experiment exposing mixed kelp communities to warming and heatwave scenarios projected for the year 2100. Three treatments were considered: a constant warming (+1.8°C from the control), a medium magnitude and long duration heatwave event (+2.8°C from the control for 13 days), and two short-term, more intense, heatwaves(5 day long scenarios with temperature peaks at +3.9°C from the control). The results show that both marine heatwave treatments reduced net community production, whereas the constant warm temperature treatment displayed no difference from the control. The long marine heatwave scenario resulted in reduced accumulated net community production, indicating that prolonged exposure had a greater severity than two high magnitude, short-term heatwave events. We estimated an 11°C temperature threshold at which negative effects to primary production appeared present. We highlight that marine heatwaves can induce sublethal effects on kelp communities by depressing net community production. These results are placed in the context of potential physiological resilience of kelp communities and implications of reduced net community production to future Arctic fjord environmental conditions.

Phosphorus cycle in shallow lakes affected by crucian carp (Carassius auratus): Effects of fish density and size

Crucian carp (Carassius auratus) is an omni-benthivorous fish common in many shallow lakes in China. The presence of crucian carp can contribute to the nutrient cycles in lakes and thus affect water quality. In this work, a two-by-two factorial mesocosm experiment was performed with crucian carp of different sizes and densities, to investigate their effects on the cycle of phosphorus (P). Results showed that nutrients in particulate form increased in overlying water due to crucian carp disturbance, especially for treatments with higher fish densities and larger individuals. Smaller individuals at high density have a greater ability to promote P release from sediment, due to a stronger combined effects of physical disturbance and excretion. Accumulation of feces led to sediment anaerobiosis and the reductive dissolution of iron oxide-hydroxide, which were the main factors affecting the desorption of P. Our results quantify the endogenous P diffusion fluxes across the sediment-water interface attributed to different densities and sizes of crucian carp disturbance, and suggest controlling crucian carp at low density and small size to minimize their impact on sediment P flux.

Wastewater and warming effects on aquatic invertebrates: Experimental insights into multi-level biodiversity consequences

Wastewater effluents and global warming affect freshwater ecosystems and impact their crucial biodiversity. Our study aimed at characterizing individual and combined impacts of wastewater effluent and increased water temperature (as one aspect of climate change) on model freshwater communities. We tested the effect of experimental treatments on genetic diversity, survival, body weight, total lipid content, lipidome and metabolome of individual species as well as community composition and phylogenetic diversity. In a 21-day mesocosm experiment we assessed the responses of a simplified freshwater food web comprising of moss and seven species of benthic macroinvertebrate shredders and grazers (mayflies, stoneflies, caddisflies and amphipods) to four treatments in a full factorial design: control, increased water temperature, wastewater and a multiple stressor treatment combining increased temperature and wastewater. Physiological responses varied among taxa, with species-specific sensitivities observed in survival and lipid content. The lowest total lipid content was observed in caddisflies and a mayfly subjected to multiple stressor treatment. The effects of stressors were reflected in the altered metabolic pathways and lipid metabolism of the individual taxa, with differential treatment effects also observed between taxa. A notable decrease in phylogenetic diversity was observed across all experimental communities. Gammarus fossarum demonstrated a high susceptibility to environmental stressors at the genetic level. Hence, while commonly used indicators of ecosystem health (e.g. community composition) remained stable, molecular indicators (e.g. phylogenetic diversity, metabolome and lipidome) responded readily to experimental treatments. These findings underscore the vulnerability of macroinvertebrates to environmental stressors, even over relatively short exposure periods. They highlight the importance of molecular indicators in detecting immediate ecological impacts, offering valuable information for conservation strategies and understanding the ecological consequences in freshwater ecosystems.

Earthworms in an enhanced weathering mesocosm experiment: Effects on soil carbon sequestration, base cation exchange and soil CO2 efflux

Despite its attractiveness for long-term carbon dioxide removal (CDR), quantifying weathering and CDR rates for enhanced weathering is a significant challenge. Moreover, the role of soil organisms, such as earthworms, in enhancing silicate weathering (both physically and chemically) has been suggested, but there is limited quantitative data on how biota, especially earthworms, contribute to inorganic carbon sequestration. To address these gaps, we conducted a mesocosm experiment with earthworms and basalt.Results indicate increases in clay and cation exchange, causing a weathering rate of over 10−12 mol total alkalinity m2 s−1, in range with other basalt experiments. Basalt amendment increased dissolved inorganic carbon export by only 4 g CO2 m−2. During the 4.5-month experiment, we observed neither a change in organic nor in inorganic carbon content.In soils without earthworms, basalt amendment reduced soil CO₂ efflux by approximately 0.2 kg CO₂ m2, suggesting considerable CDR. This decrease was about two times larger than calculated inorganic CDR equivalents, suggesting changes in soil organic matter dynamics.Interestingly, earthworms reversed the basalt-induced reduction in soil CO₂ efflux. This reversal was partly due to reduced export of dissolved inorganic carbon but mainly driven by increased organic matter decomposition. Our study highlights the importance of including organic carbon dynamics when evaluating the CDR potential of enhanced weathering.

Effects of LDPE and PBAT plastics on soil organic carbon and carbon-enzymes: A mesocosm experiment under field conditions

Although the effects of plastic residues on soil organic carbon (SOC) have been studied, variations in SOC and soil carbon-enzyme activities at different plant growth stages have been largely overlooked. There remains a knowledge gap on how various varieties of plastics affect SOC and carbon-enzyme activity dynamics during the different growing stages of plants. In this study, we conducted a mesocosm experiment under field conditions using low-density polyethylene and poly (butylene adipate-co-terephthalate) debris (LDPE-D and PBAT-D, 500–2000 μm (pieces), 0%, 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%), and low-density polyethylene microplastics (LDPE-M, 500–1000 μm (powder), 0%, 0.05%, 0.1%, 0.5%) to investigate SOC and C-enzyme activities (β-xylosidase, cellobiohydrolase, β-glucosidase) at the sowing, seedling, flowering and harvesting stages of soybean (Glycine Max). The results showed that SOC in the LDPE-D treatments significantly increased from the flowering to harvesting stage, by 12.69%–13.26% (p < 0.05), but significantly decreased in the 0.05% and 0.1% LDPE-M treatments from the sowing to seedling stage (p < 0.05). However, PBAT-D had no significant effect on SOC during the whole growing period. For C-enzyme activities, only LDPE-D treatments inhibited GH (17.22–38.56%), BG (46.7–66.53%) and CBH (13.19–23.16%), compared to treatment without plastic addition, from the flowering stage to harvesting stage. Meanwhile, C-enzyme activities and SOC responded nonmonotonically to plastic abundance and the impacts significantly varied among the growing stages, especially in treatments with PBAT-D (p < 0.05). These risks to soil organic carbon cycling are likely mediated by the effects of plastic contamination and degradation soil microbe. These effects are sensitive to plastic characteristics such as type, size, and shape, which, in turn, affect the biogeochemical and mechanical interactions involving plastic particles. Therefore, further research on the interactions between plastic degradation processes and the soil microbial community may provide better mechanistic understanding the effect of plastic contamination on soil organic carbon cycling.

Nutritional Quality of Basal Resource in Stream Food Webs Increased with Light Reduction—Implications for Riparian Revegetation

Biofilms are considered a basal resource with high nutritional quality in stream food webs, as periphytic algae are abundant of polyunsaturated fatty acids (PUFAs). PUFAs are essential for growth and reproduction of consumers who cannot or have very limited capacity to biosynthesize. Yet, how the nutritional quality based on PUFA of basal food sources changes with light intensity remains unclear. We conducted a manipulative experiment in mesocosms to explore the response and mechanisms of nutritional quality to shading, simulating riparian restoration. We found a significant increase in PUFA% (including arachidonic acid, ARA) under shading conditions. The increased PUFA is caused by the algal community succession from Cyanobacteria and Chlorophyta to Bacillariophyta which is abundant of PUFA (especially eicosapentaenoic acid, EPA; docosahexaenoic acid, DHA). On the other hand, shading increased PUFA via upregulating enzymes such as Δ12 desaturase (FAD2, EC:1.14.19.6) and 3-ketoacyl-CoA synthase (KCS, EC:2.3.1.199) in the biosynthesis of unsaturated fatty acid elongation pathways. Our findings imply that riparian reforestation by decreasing light intensity increases the nutritional quality of basal resources in streams, which may enhance transfer of good quality carbon to consumers in higher trophic levels through bottom-up effects.

Response of submerged macrophytes of different growth forms to multiple sediment remediation measures for hardened sediment.

Climate change and intensified human activities have disrupted the natural hydrological regime and rhythm of river-connected lakes, extending the dry season, increasing water loss, and exposing previously submerged lake floors. This exposure has led to significant sediment hardening, which directly impacts submerged macrophytes. However, strategies to mitigate the negative effects of hardened sediments and promote the growth and development of submerged macrophytes remain largely unexplored. In this study, we selected typical hardened sediment from Dongting Lake to investigate the response of different growth forms of submerged macrophytes to multiple sediment remediation measures (loosening and litter addition) using a mesocosm experiment. The results indicated that loosening alone uniformly benefited all submerged macrophytes by increasing total biomass, relative growth rate (RGR), and the root/shoot ratio. Additionally, loosening altered the root traits of submerged macrophytes, promoting maximum root length (MRL) while reducing average root diameter (ARD). Moreover, different submerged macrophytes exhibited species-specific responses to the combination of loosening and litter addition. Notably, the combination of loosening and adding Miscanthus lutarioriparius litter had an antagonistic effect on the growth of Potamogeton wrightii and Myriophyllum spicatum. The response of functional traits of submerged macrophytes with similar growth forms to the same treatment was consistent. Our findings suggest that future sediment remediation efforts should consider matching specific treatments with the growth forms of submerged macrophytes to achieve optimal outcomes.

Assessing ecological responses of exposure to the pyrethroid insecticide lambda-cyhalothrin in sub-tropical freshwater ecosystems

Pyrethroid insecticides are widely detected in aquatic ecosystems due to their extensive use in agriculture and horticulture, which could pose a potential risk to aquatic non-target organisms. While previous ecotoxicological studies have been conducted mainly with standard tests and local species under temperate conditions, scarce information is available on the effects of pyrethroid insecticides on communities and ecosystems under (sub-)tropical conditions. A single application of lambda-cyhalothrin at concentrations of 0, 9, 30, and 100 ng/L was evaluated in outdoor mesocosms under sub-tropical conditions. Lambda-cyhalothrin was found to dissipate rapidly in the water column, with only 11 % and 7 % of the remaining dose measured at 1 and 3 days after application, respectively. Lambda-cyhalothrin concentrations disappeared considerably faster from the water compartment compared to temperate conditions. Consistent decreases in abundance were observed for Lecane lunaris at the medium and higher treatments (NOEC = 9 ng/L) and at the highest treatment (NOEC = 30 ng/L) for Keratella tropica. On the contrary, two taxa belonging to Cladocera (i.e., Ceriodaphnia sp. and Diaphanosoma sp.) showed the most prominent increase in abundance related to the lambda-cyhalothrin treatments. At the community level, a consistent no observed effect concentrations (NOECs) of 9 ng/L could be calculated for the zooplankton community. A marginal significant overall treatment related effect was observed for the macroinvertebrate community. The results of species sensitivity distribution (SSD) analysis based on results of acute toxicity experiments conducted alongside the mesocosm experiment and obtained from the literature indicated that macroinvertebrates from temperate regions may be generally more sensitive than their counterparts in (sub-)tropical regions. Overall, these findings suggest that environmentally relevant concentrations of the pyrethroid insecticide lambda-cyhalothrin may lead to different ecological outcomes in freshwater ecosystems in the (sub-)tropics relative to temperate regions.

Grasshoppers do not flee from their alarmed predators but from non-alarmed ones.

Alarm calls produced by basal prey have a well-known informative value. In multi-predator communities, mesopredators, when faced with top predators, may emit alarm calls that could inform basal prey about their lowered predation risk. To test this unexplored possibility, we conducted one field and one mesocosm experiment in which we simulated alarm and non-alarm calls from little owls (Athene noctua) as mesopredators and measured their effects on grasshoppers as prey of little owls but not of top predators. In the field experiment, we found that grasshopper species were significantly more abundant in patches where we simulated either the presence of scared little owls (alarm treatment) or no owls (control treatment) compared to patches where the presence of non-scared little owls (non-alarm treatment) was simulated. In the mesocosm experiment, locusts (Locusta migratoria) moved significantly more to exposed areas when we simulated the presence of scared little owls (alarm treatment) or of a granivorous bird (control treatment), while they moved to sheltered areas when we simulated the presence of non-scared owls (non-alarm treatment). These results show that prey could cue on predators' calls to assess their predation risk and make decisions, revealing unprecedented potential ecological consequences of alarm calls in invertebrate communities.

EDNA State and Medium Affect DNA Degradation Patterns in Seminatural Systems of Southern African Waterholes

ABSTRACTEnvironmental DNA (eDNA) has evolved into a valuable asset of the ecologists' toolkit, enabling time‐ and cost‐efficient biodiversity assessments in a wide variety of ecosystems. Since eDNA can be isolated from a broad range of environmental substrates, its persistence times in those media are of decisive importance for drawing inferences about species presence. For the first time, we characterize eDNA persistence in water and sediment samples of seminatural waterholes in a savanna system in South Africa to gain a better understanding of eDNA decay in these waterbodies. Using mesocosm experiments, we tracked eDNA decay in two different DNA states (extracellular and membrane bound), during two different seasons (wet and dry), and from two different substrates (surface water and sediment). Extracellular DNA degraded rapidly in a first‐order exponential decay fashion and membrane‐bound DNA exhibited a slower decline with more intricate patterns, involving initial reduction followed by a subsequent increase in measured DNA concentrations. The latter we attribute to cell disassociation and cell lysis at 24–48 h after introduction into the environment. Higher stochasticity of membrane‐bound DNA capture in the dry season highlights the need for higher sampling efforts in natural systems in which eDNA is presumably more patchily distributed. Additionally, we observed longer eDNA persistence in sediments than in water samples, presumably due to better protection from nucleases. We could not reveal any effects of environmental parameters on eDNA decay, emphasizing that further research is needed to better understand eDNA dynamics in those waterbodies in order to exploit their full potential for eDNA‐based bioassessments in those systems.

Different model assumptions about plant hydraulics and photosynthetic temperature acclimation yield diverging implications for tropical forest gross primary production under warming

AbstractTropical forest photosynthesis can decline at high temperatures due to (1) biochemical responses to increasing temperature and (2) stomatal responses to increasing vapor pressure deficit (VPD), which is associated with increasing temperature. It is challenging to disentangle the influence of these two mechanisms on photosynthesis in observations, because temperature and VPD are tightly correlated in tropical forests. Nonetheless, quantifying the relative strength of these two mechanisms is essential for understanding how tropical gross primary production (GPP) will respond to climate change, because increasing atmospheric CO2 concentration may partially offset VPD‐driven stomatal responses, but is not expected to mitigate the effects of temperature‐driven biochemical responses. We used two terrestrial biosphere models to quantify how physiological process assumptions (photosynthetic temperature acclimation and plant hydraulic stress) and functional traits (e.g., maximum xylem conductivity) influence the relative strength of modeled temperature versus VPD effects on light‐saturated GPP at an Amazonian forest site, a seasonally dry tropical forest site, and an experimental tropical forest mesocosm. By simulating idealized climate change scenarios, we quantified the divergence in GPP predictions under model configurations with stronger VPD effects compared with stronger direct temperature effects. Assumptions consistent with stronger direct temperature effects resulted in larger GPP declines under warming, while assumptions consistent with stronger VPD effects resulted in more resilient GPP under warming. Our findings underscore the importance of quantifying the role of direct temperature and indirect VPD effects for projecting the resilience of tropical forests in the future, and demonstrate that the relative strength of temperature versus VPD effects in models is highly sensitive to plant functional parameters and structural assumptions about photosynthetic temperature acclimation and plant hydraulics.

Environmental DNA Particle Size Distribution and Quantity Differ Across Taxa and Organelles

ABSTRACTThe use of environmental DNA to detect species is now widespread in freshwater ecology. However, the detectability of species depends on many factors, such as the quantity of eDNA particles available in the environment and their state (e.g., free DNA fragments, organellar, or aggregated DNA particles). To date, the most advanced knowledge of the production and state of DNA particles concerns teleosts. Most often, these studies target mitochondrial genes, since they are present in multiple copies in a cell. However, it is likely that the characteristics of eDNA molecules vary greatly among taxa and genetic compartments, with direct consequences for species detection. Using an indoor mesocosm experiment, we compared the rate of mitochondrial and nuclear eDNA production and particle size distribution (PSD) of four distinct and common aquatic taxa (zebrafish, tadpole, isopod and mollusk). The tank water was filtered through a series of filters with decreasing porosity and mitochondrial and nuclear eDNA at each size fraction were quantified by qPCR. We found that the production and the size of eDNA particles varied greatly among taxa and genetic compartments. For most taxa, the number of nuclear eDNA particles released in water was higher than that of mitochondrial origin. The PSD of mt‐eDNA showed a pattern common to all taxa: the relative number of particles increased from the smallest size fractions (0.2 μm and less) to the largest (over 1.2 μm), while the distribution of nu‐eDNA was very different from one taxon to another. We also observed a high temporal variability in the quantity of eDNA particles and in PSD, although the latter was more complex to model. These results call for caution in how to sample and analyze eDNA in aquatic environments, particularly for organisms that emit small particles in small quantities such as isopods.