Functional and compositional responses of stream microphytobenthic communities to multiple stressors increase and release in a mesocosm experiment.

Field observations form the basis of the majority of studies on microphytobenthic algal communities in freshwater ecosystems. Controlled mesocosm experiments data are comparatively uncommon. The few experimental mesocosm studies that have been conducted provide valuable insights into how multiple stressors affect the community structures and photosynthesis-related traits of benthic microalgae. The recovery process after the stressors have subsided, however, has received less attention in mesocosm studies. To close this gap, here we present the results of a riparian mesocosm experiment designed to investigate the effects of reduced flow velocity, increased salinity and increased temperature on microphytobenthic communities. We used a full factorial design with a semi-randomised distribution of treatments consisting of two levels of each stressor (2 × 2 × 2 treatments), with eight replicates making a total of 64 circular mesocosms, allowing a nuanced examination of their individual and combined influences. We aimed to elucidate the responses of microalgae communities seeded from stream water to the applied environmental stressors. Our results showed significant effects of reduced flow velocity and increased temperature on microphytobenthic communities. Recovery after stressor treatment led to a convergence in community composition, with priority effects (hypothesized to reflect competition for substrate between resident and newly arriving immigrant taxa) slowing down community shifts and biomass increase. Our study contributes to the growing body of literature on the ecological dynamics of microphytobenthos and emphasises the importance of rigorous experiments to validate hypotheses. These results encourage further investigation into the nuanced interactions between microphytobenthos and their environment and shed light on the complexity of ecological responses in benthic systems.

Relating environmental pressures to littoral biological water quality indicators in Western Balkan lakes: Can we fill the largest gaps?

Along six transects in each of six lakes across the Western Balkans, we collected data for three groups of littoral biological water quality indicators: epilithic diatoms, macrophytes, and benthic invertebrates. We assessed the relationships between them and three environmental pressures: nutrient load (eutrophication), hydro-morphological alteration of the shoreline, and water level variation, separating the effect of individual lakes and continuous explanatory variables. Lake water total phosphorus concentration (TP) showed substantial variation but was not related to any of the tested biological indicators, nor to any of the tested pressures. We suggest that this may be due to feedback processes such as P removal in the lake littoral zone. Instead, we found that a gradient in surrounding land-use towards increasing urbanization, and a land-use-based estimate of P run-off, served as a better descriptor of eutrophication. Overall, eutrophication and water level fluctuation were most important for explaining variation in the assessed indicators, whereas shoreline hydro-morphological alteration was less important. Diatom indicators were most responsive to all three pressures, whereas macrophyte biomass and species number responded only to water level fluctuation. The Trophic Diatom Index for Lakes (TDIL) was negatively related to urbanization and wave exposure. This indicates that it is a suitable indicator for pressures related to urbanization, although a confounding effect of wave exposure is possible. Invertebrate abundance responded strongly to eutrophication, but the indicator based on taxonomic composition (Average Score Per Taxon) did not. Our results suggest that our metrics can be applied in Western Balkan lakes, despite the high number of endemic species present in some of these lakes. We argue that local water management should focus on abating the causes of eutrophication and water level fluctuation, whilst preserving sufficient lengths of undeveloped shoreline to ensure good water quality in the long run.

Biofilms in caves: easy method for the assessment of dominant phototrophic groups/taxa in situ.

Domination of certain aerophytic phototrophic group or specific taxon in biofilms is connected with biofilm features recognised in situ. Well-developed, gelatinous, olive to dark-green biofilms are composed mostly of coccoid cyanobacterial forms. The same features, characterised biofilms dominated by one coccoid taxon, except the latter were vividly coloured. Gloeobacter caused the appearance of purple, Gloeocapsa representatives yellow and Chroococcidiopsis black biofilm. The brown to the dark colour of heterocytous biofilms was mainly caused by Nostoc. Simple trichal Cyanobacteria were occasionally present in biofilm, except in one blue-coloured sample. According to the principal component analysis (PCA), well-developed and gelatinous biofilms were correlated with Cyanobacteria, while scanning electron microscopy (SEM) revealed richness of extracellular polymeric substances (EPS) in such biofilms. Biofilm with calcified cyanobacterium (Geitleria cf. calcarea) was also found. Chlorophyta-abundant biofilms (many rich in Desmococcus), thinner than cyanobacterial, were predominantly green and occasionally yellow and blue. Many were dry when observed in situ (confirmed with PCA), with few being moistened (i.e. Klebsormidium-dominant). Diatom biofilms were usually developed on sediment, mosses or near seeping water (demonstrated by PCA) and were also thinner than cyanobacterial ones. Compared to cyanobacterial biofilms, SEM showed less developed EPS in those rich in diatoms and green algae, where microorganisms are more exposed to the environment. The study demonstrates an easy method for biofilm assessment based on visual characterisation and provides encouragement for more frequent biofilm investigation in caves that can be important from an ecological, biological, biotechnological point of view and which assessment can have an irreplaceable role in potential monitoring and protection.

Littoral eutrophication indicators are more closely related to nearshore land use than to water nutrient concentrations: A critical evaluation of stressor-response relationships.

Biological assessment metrics and water chemistry measurements are used to quantify the link between stressors and their effects on lake ecosystems, for the Water Framework Directive. However, correlations between metrics and water chemistry are often poor. This is seen as major weaknesses of Water Framework Directive-related monitoring and assessment. We analyzed macrophytes, benthic algae, benthic macroinvertebrates, water chemistry and sediment total phosphorus content in the littoral of six lakes in the Western Balkans and used CORINE land use data to estimate nutrient enrichment via runoff from the adjacent land. Lakes with a higher estimated phosphorus runoff from the adjacent land did not have higher littoral water nutrient concentrations, but littoral diatom assemblages indicated more eutrophic conditions. These lakes also had higher abundances of littoral benthic primary producers, which in turn were associated with low concentrations of dissolved nutrients, but only in autumn, not in spring. This is consistent with primary producers taking up nutrients during the summer growth season. In lakes with high abundances of benthic primary producers, it is likely that the littoral vegetation plays a large role in the transfer of nutrients from the water to the benthos. This process impairs correlations between biological metrics and water nutrient concentrations. Our results suggest that CORINE land cover may be more useful to characterize littoral nutrient enrichment than lake water chemistry. Increased benthic primary producer biomasses and "eutrophic" diatom indices may indicate littoral nutrient enrichment even if water nutrient concentrations are low.

Alkaline soda Lake Velika Rusanda (Serbia): the first insight into diatom diversity of this extreme saline lake.

Alkaline soda lakes are unique habitats found in specific geographic regions, usually with dry climate. The Carpathian Basin is one of those regions very important for habitat and biodiversity conservation in Europe, with natural soda lakes found in Austria, Hungary and Serbia. In comparison to other two countries from Central Europe, algal biodiversity studies of saline soda lakes in Serbia are scarce. Lake Velika Rusanda has the highest measured salinity of all saline lakes in the Carpathian Basin and there were no reports of its diatom species richness and diversity till now. We conducted 2-year investigation programme to study biodiversity and seasonal dynamics of diatoms in this lake. A total of 27 diatom taxa were found, almost all of them attached to reed and much less in benthos and plankton. Five new diatom species for Serbia were recorded, Craticula halopannonica, Navicymbula pusilla, Hantzschia weyprechtii, Nitzschia thermaloides and Navicula staffordiae. The last mentioned is new for Europe as well. Lake Velika Rusanda is inhabited mostly by alkaliphilous and halophilic diatoms. Since diatoms are used as bioindicators in soda lakes, our results will improve their further application in ecological status assessment of these fragile habitats in the Carpathian Basin.