Vojtech Kolar

Direct predation is a major driver of the negative impact of the invasive round goby Neogobius melanostomus on native macrozoobenthic assemblages. However, the presence of N. melanostomus predators represents a possible means of reducing its negative impact via consumptive and non-consumptive effects (NCEs). However, the NCEs of native predators on the trophic interactions of N. melanostomus remain poorly understood. The functional response framework, which links prey density to the amount of prey consumed, was used to experimentally determine the foraging pressure of N. melanostomus on chironomid larvae and to test how key functional response characteristics, i.e., its shape, attack rate, and handling time, change under predation risk represented by chemical cues from heterospecific or conspecific prey preyed upon by European eel ( Anguilla anguilla ) and enriched with conspecific alarm cues. Predation risk negatively influenced the foraging of N. melanostomus relative to the risk-free control, but the effects differed between heterospecific and conspecific alarm cues. The attack rate and hence the foraging of N. melanostomus at low prey densities were significantly reduced by conspecific alarm cues relative to heterospecific alarm cues and the risk-free control. On the other hand, heterospecific alarm cues led to a longer handling time and consequently a lower maximum feeding rate relative to the risk-free control and conspecific alarm cues. The results show the potential of A. anguilla to reduce the negative impact of the invasive N. melanostomus on benthic prey through NCEs. Thus, ensuring that A. anguilla populations thrive in highly human-impacted, homogenized European freshwater ecosystems represents a promising approach for reducing the negative impact of N. melanostomus on native prey.

Aquatic macroinvertebrates are key components of freshwater ecosystems and are widely used as indicators of habitat quality, yet the ecological drivers of many protected species remain poorly understood. The diving beetle Graphoderus bilineatus (De Geer, 1774) is a strictly protected Natura 2000 species in Europe, with habitat requirements that vary geographically. While in most of its range the species is associated with standing waters, in Hungary it occurs predominantly in floodplain habitats, suggesting a role for hydrological dynamics on its local populations. Based on long-term field observations, we hypothesized that flood events, particularly spring floods, may influence the occurrence of the species. However, responses to flood events are inherently difficult to study due to their unpredictable distribution in space and time, which may not align with available data from monitoring and faunistic surveys conducted at irregular intervals. In this study, we evaluated whether such data are sufficient to detect relationships between flood dynamics and species occurrence. We analysed presence–absence records collected between 2000 and 2022 across four major river systems (Bodrog, Tisza, Danube and Drava rivers) in Hungary using Bayesian logistic regression models with hydrological predictors derived from water level data. Across all catchments, models including flood variables performed only marginally better than simpler models, and posterior estimates were associated with wide credible intervals. In the Bodrog catchment, higher flood levels were associated with lower predicted occurrence, while results for other rivers remained highly uncertain. Our results provide only weak and inconsistent support for the hypothesized role of floods and demonstrate that opportunistic presence–absence data are insufficient for robust ecological inference in this context. Given the conservation importance of G. bilineatus, we emphasize the need for standardized, hydrology-informed monitoring programmes to better understand its population dynamics and to support effective management.

Anthropogenic eutrophication poses a significant threat to freshwater environments globally. It also influences the population dynamics of invasive and native species, yet the cumulative effects of eutrophication and invasive species on native organisms are not well understood. We used invasive gibel carp (Carassius gibelio Bloch, 1782) and native crucian carp (Carassius carassius Linnaeus, 1758) as model taxa to investigate how eutrophication influences their interspecific competition. Carassius carassius is being outcompeted by invasive C. gibelio across most of its native range, and eutrophication may play a role in the competitive displacement of C. carassius. We explored how varying eutrophication levels influence growth and survival of both species in laboratory and mesocosm experiments, where we exposed them to different feeding rations and nutrient levels, respectively. We hypothesized that (1) C. gibelio benefits more from increased nutrient levels than C. carassius—which favors invasive C. gibelio over native C. carassius in interspecific competition—and that (2) the growth and survival of C. carassius deteriorate under interspecific competition with C. gibelio compared to intraspecific competition. Our experiments revealed that excess nutrient availability was more beneficial to the invasive C. gibelio. Overall, C. gibelio individuals grew more than C. carassius, and the difference between the two species increased with nutrient levels in both experiments, especially in the weight of smaller individuals. Moreover, interspecific competition led to lower survival in C. carassius but not in C. gibelio in the mesocosm experiment. This study shows that eutrophication may modulate interspecific competition, and cultural eutrophication of freshwater habitats may enhance the success of invasive species. Graphical abstract

Estimations of methane (CH₄) emissions are often based on point measurements using either flux chambers or a transfer coefficient method, which may lead to strong underestimation of the total CH₄ fluxes. In order to demonstrate more precise measurements of the CH₄ fluxes from an aquaculture pond, using a higher resolution sampling approach we examined the spatiotemporal variability of CH₄ concentrations in the water, related fluxes (diffusive and ebullitive) and relevant environmental conditions (temperature, oxygen, chlorophyll a) during three diurnal campaigns in a hyper-eutrophic fishpond. Our data show remarkable variance spanning several orders of magnitude while diffusive fluxes accounted for only a minor fraction of total CH₄ fluxes (4.1 %–18.5 %). Linear mixed-effects models identified water depth as the only significant predictor of CH₄ fluxes. Our findings necessitate complex sampling strategies involving temporal and spatial variability for reliable estimates of the role of fishponds in a global methane budget.

Multiple anthropogenic stressors influence the functioning of ponds and lakes, but their combined effects are often little understood. We ran two mesocosm experiments to evaluate the effects of warming (+4°C above ambient) and environmentally relevant concentrations of a mixture of commonly used pharmaceuticals, an emerging class of chemical contaminants, on tri-trophic food webs representative of pelagic communities in ponds and other small standing waters. We quantified the main and interactive effects of warming and pharmaceuticals on each trophic level and attributed them to the direct effects of both stressors and the indirect effects arising through biotic interactions. Warming and pharmaceuticals had stronger effects in the summer experiment, altering zooplankton community composition and causing delayed or accelerated emergence of top insect predators. In summer, both stressors and top predators reduced filter-feeding zooplankton biomass, while warming and pharmaceuticals had opposing effects on phytoplankton. In the winter experiment, the effects were much weaker and primarily limited to a positive effect of warming on phytoplankton biomass. Overall, we show that pharmaceuticals can exacerbate the effects of climate warming in freshwater ecosystems, especially during the warm season. Our results demonstrate the utility of community-level studies across different seasons for the risk assessment of multiple emerging stressors in freshwater ecosystems.

Estimations of methane (CH₄) emissions are often based on point measurements using either flux chambers or a transfer coefficient method which may lead to strong underestimation of the total CH₄ fluxes. In order to demonstrate more precise measurements of the CH₄ fluxes from an aquaculture pond, using higher resolution sampling approach we examined the spatiotemporal variability of CH₄ concentration in the water, related fluxes (diffusive and ebullitive) and relevant environmental conditions (temperature, oxygen, chlorophyll-a) during three diurnal campaigns in a hyper-eutrophic fishpond. Our data show remarkable variance spanning several orders of magnitude while diffusive fluxes accounted for only a minor fraction of total CH₄ fluxes (4.1–18.5 %). Linear mixed-effects models identified water depth as the only significant predictor of CH₄ fluxes. Our findings necessitate complex sampling strategies involving temporal and spatial variability for reliable estimates of the role of fishponds in a global methane budget.