Delineating the relative contribution of climate related variables to chlorophyll-a and phytoplankton biomass in lakes using the ERA5-Land climate reanalysis data.

Understanding the climatic drivers of eutrophication is critical for lake management under the prism of the global change. Yet the complex interplay between climatic variables and lake processes makes prediction of phytoplankton biomass a rather difficult task. Quantifying the relative influence of climate-related variables on the regulation of phytoplankton biomass requires modelling approaches that use extensive field measurements paired with accurate meteorological observations. In this study we used climate and lake related variables obtained from the ERA5-Land reanalysis dataset combined with a large dataset of in-situ measurements of chlorophyll-a and phytoplankton biomass from 50 water bodies to develop models of phytoplankton related responses as functions of the climate reanalysis data. We used chlorophyll-a and phytoplankton biomass as response metrics of phytoplankton growth and we employed two different modelling techniques, boosted regression trees (BRT) and generalized additive models for location scale and shape (GAMLSS). According to our results, the fitted models had a relatively high explanatory power and predictive performance. Boosted regression trees had a high pseudo R2 with the type of the lake, the total layer temperature, and the mix-layer depth being the three predictors with the higher relative influence. The best GAMLSS model retained mix-layer depth, mix-layer temperature, total layer temperature, total runoff and 10-m wind speed as significant predictors (p<0.001). Regarding the phytoplankton biomass both modelling approaches had less explanatory power than those for chlorophyll-a. Concerning the predictive performance of the models both the BRT and GAMLSS models for chlorophyll-a outperformed those for phytoplankton biomass. Overall, we consider these findings promising for future limnological studies as they bring forth new perspectives in modelling ecosystem responses to a wide range of climate and lake variables. As a concluding remark, climate reanalysis can be an extremely useful asset for lake research and management.

Stream invertebrate communities are primarily shaped by hydrological factors and ultimately fine-tuned by local habitat conditions.

The environmental factors that determine species richness and community structure in running waters have long been studied, but how these factors hierarchically and/or interactively influence benthic communities remains unclear. To address this research gap, we identified the principal abiotic factors that determine the taxonomic composition and functionality of stream macroinvertebrate communities and explored possible hierarchical and/or interactive patterns. We analyzed a large dataset from Greek rivers, and compared multiple macroinvertebrate metrics and traits between perennial and intermittent watercourses during wet and dry periods. We found that macroinvertebrates were primarily influenced by two ecological gradients: (i) aquatic vegetation-conductivity; and (ii) water temperature-canopy cover. Macroinvertebrates in perennial rivers were mainly influenced by the first gradient, whereas in intermittent rivers both gradients were important. Taxonomic richness and diversity were higher and temporally stable within years in perennial rivers, whereas in intermittent rivers, these metrics peaked during early summer, before the onset of streambed desiccation. The two environmental gradients determined the taxonomic richness and diversity in both spring and summer; however, a clear influence of hydrological factors (wetted width, water depth, flow velocity and discharge) was observed only in the intermittent samples. We conclude that the benthic invertebrate taxonomic richness and diversity in highly variable environments is primarily determined by hydrological variation and ultimately fine-tuned by local habitat factors. As climate change scenarios predict severe modification of hydrological and local habitat factors, this study concludes that in river management, hydrological restoration should be prioritized over other local habitat factors by maintaining natural hydrological variability, to ensure aquatic community richness and diversity.

Assessing the ecological effects of water stress and pollution in a temporary river - Implications for water management.

Temporary rivers are dynamic and complex ecosystems that are widespread in arid and semi-arid regions, such as the Mediterranean. Biotic communities adapted in their intermittent nature could withstand recurrent drought events. However, anthropogenic disturbances in the form of water stress and chemical pollution challenge biota with unpredictable outcomes, especially in view of climate change. In this study we assess the response of the biotic community of a temporary river to environmental stressors, focusing on water stress and pollution. Towards this aim, several metrics of four biotic groups (diatoms, macrophytes, macroinvertebrates and fish) were applied. All biotic groups responded to a pollution gradient mainly driven by land use, distinct functional groups of all biota responded to water stress (a response related to the rheophilic nature of the species and their resistance to shear stress), while the combined effects of water stress and pollution were apparent in fish. Biotic groups presented a differential temporal response to water stress, where diatom temporal assemblage patterns were explained by water stress variables of short-time response (15days), while the responses of the other biota were associated to longer time periods. There were two time periods of fish response, a short (15days) and a long-time response (60-75days). When considering management decisions, our results indicate that, given the known response of river biota to pollution, biomonitoring of temporary rivers should also involve metrics that can be utilized as early warnings of water stress.

Combined effects of water stress and pollution on macroinvertebrate and fish assemblages in a Mediterranean intermittent river.

Water stress is a key stressor in Mediterranean intermittent rivers exacerbating the negative effects of other stressors, such as pollutants, with multiple effects on different river biota. The current study aimed to determine the response of macroinvertebrate and fish assemblages to instream habitat and water chemistry, at the microhabitat scale and at different levels of water stress and pollution, in an intermittent Mediterranean river. Sampling was conducted at high and low summer discharge, at two consecutive years, and included four reaches that were targeted for their different levels of water stress and pollution. Overall, the macroinvertebrate fauna of Evrotas River indicated high resilience to intermittency, however, variation in community structure and composition occurred under acute water stress, due to habitat alteration and change in water physico-chemistry, i.e. water temperature increase. The combined effects of pollution and high water stress had, however, pronounced effects on species richness, abundance and community structure in the pollution impacted reach, where pollution sensitive taxa were almost extirpated. Fish response to drought, in reaches free of pollution, consisted of an increase in the abundance of the two small limnophilic species, coupled with their shift to faster flowing riffle habitats, and a reduction in the abundance of the larger, rheophilic species. In the pollution impacted reach, however, the combination of pollution and high water stress led to hypoxic conditions assumed to be the leading cause of the almost complete elimination of the fish assemblage. In contrast, the perennial Evrotas reaches with relatively stable physicochemical conditions, though affected hydrologically by drought, appear to function as refugia for fish during high water stress. When comparing the response of the two biotic groups to combined acute water stress and pollution, it is evident that macroinvertebrates were negatively impacted, but fish were virtually eliminated under the two combined stressors.