Short-period hydrological regimes override physico-chemical variables in shaping stream diatom traits, biomass and biofilm community functions.

Despite increasing interest in hydrological effects on riverine ecosystems, few studies have documented the impact of hydrology on biofilm community functions, and those existing have typically focused on annual-based hydrological indices. In this study, we conducted monthly samplings during a year in five lowland streams with different flow regimes and investigated the impacts of hydrological conditions and physico-chemical variables on the trait composition of diatoms growing on artificial substrates, biomass (chlorophyll a and ash free dry weight), and biofilm community functions (biochemical processes, i.e., biofilm metabolism and nutrient uptake rates measured in the laboratory). Instead of the commonly used annual-based hydrological indices, we calculated indices for shorter periods (14 and ~28 days) of the hydrological regimes. Results of species-based variation partitioning showed that short-period hydrological indices (10.10 ± 7.18%) contributed more to explain species distribution than physico-chemical variables (5.90 ± 3.83%), indicating the dominant role of hydrology in structuring the diatom community. Specifically, we found different response patterns for different guilds and size classes to the hydrological and physico-chemical variables, and our results demonstrated that species tolerating high disturbance may be more appropriate as indicators of environmental disturbance than low-tolerant species. We also found dominant effects of short-period hydrological events on biomass and biofilm community functions. Despite an overall negative effect of high flow events and flow variations on biomass and biofilm community functions, positive effects on function-biomass ratios were also observed, indicating that the effects of flow regimes on biofilm are complex. In conclusion, our study highlights the importance of including short-period hydrological conditions in studies on environmental factors shaping benthic algae. Based on our results, we recommend use of short-period hydrological conditions when investigating the effects of flow regime on biofilm community composition and functions.

Probing the Response of the Amphibious Plant Butomus umbellatus to Nutrient Enrichment and Shading by Integrating Eco-Physiological With Metabolomic Analyses.

Amphibious plants, living in land-water ecotones, have to cope with challenging and continuously changing growth conditions in their habitats with respect to nutrient and light availability. They have thus evolved a variety of mechanisms to tolerate and adapt to these changes. Therefore, the study of these plants is a major area of ecophysiology and environmental ecological research. However, our understanding of their capacity for physiological adaptation and tolerance remains limited and requires systemic approaches for comprehensive analyses. To this end, in this study, we have conducted a mesocosm experiment to analyze the response of Butomus umbellatus, a common amphibious species in Denmark, to nutrient enrichment and shading. Our study follows a systematic integration of morphological (including plant height, leaf number, and biomass accumulation), ecophysiological (photosynthesis-irradiance responses, leaf pigment content, and C and N content in plant organs), and leaf metabolomic measurements using gas chromatography-mass spectrometry (39 mainly primary metabolites), based on bioinformatic methods. No studies of this type have been previously reported for this plant species. We observed that B. umbellatus responds to nutrient enrichment and light reduction through different mechanisms and were able to identify its nutrient enrichment acclimation threshold within the applied nutrient gradient. Up to that threshold, the morpho-physiological response to nutrient enrichment was profound, indicating fast-growing trends (higher growth rates and biomass accumulation), but only few parameters changed significantly from light to shade [specific leaf area (SLA); quantum yield (φ)]. Metabolomic analysis supported the morpho-physiological results regarding nutrient overloading, indicating also subtle changes due to shading not directly apparent in the other measurements. The combined profile analysis revealed leaf metabolite and morpho-physiological parameter associations. In this context, leaf lactate, currently of uncertain role in higher plants, emerged as a shading acclimation biomarker, along with SLA and φ. The study enhances both the ecophysiology methodological toolbox and our knowledge of the adaptive capacity of amphibious species. It demonstrates that the educated combination of physiological with metabolomic measurements using bioinformatic approaches is a promising approach for ecophysiology research, enabling the elucidation of discriminatory metabolic shifts to be used for early diagnosis and even prognosis of natural ecosystem responses to climate change.

Hydromorphology as a controlling factor of macrophytes assemblage structure and functional traits in the semi-arid European Mediterranean streams.

Macrophytes have a crucial impact on stream functioning. However, there is a significant gap of knowledge about how hydromorphological fluctuations affect their structural and functional responses in southern Mediterranean streams. In this study, we investigated the impact of hydromorphology on macrophyte stream assemblages in Cyprus and analysed their structural and functional responses. We collected macrophytes and hydromorphological data from 63 sites along a gradient from permanent to intermittent streams. We applied Principal Component Analysis (PCA) to identify and characterise stream sub-types. We performed an Indicator Species Analysis (ISA) and estimated taxonomical diversity indices to investigate whether differences among stream sub-types affect macrophytes assemblage structure. Functional responses to the flow regime were tested by allocating traits related to persistence, regeneration, dispersibility and ecological preferences for moisture, light, nutrients and salinity. The results indicated the existence of two permanent and two intermittent flow sub-types. A total of 25 indicator species were identified showing taxonomic variation in macrophyte assemblages among streams with different flow regimes. We demonstrated that flow intermittency promotes a higher number of indicator species with wider ecological preferences and traits allowing resilience to drought. Specifically, we found that macrophytes in the intermittent streams, survive during dry period through the establishment of dormant seed bank or through belowground organs. They also showed lower values of Specific Leaf Area and therefore a reduction of water loss through evapotranspiration. In the light of climate change, where droughts are anticipated to increase, more permanent streams will become intermittent especially in south Mediterranean countries, and new habitats will be released, including marginal zones. Our results showed that these alterations in stream hydromorphology will produce changes in macrophyte assemblages which might cause shifts in stream ecosystem functions and services. Therefore, our knowledge about the direction of these changes is crucial for future management and conservation plans.