Eloísa Ramos-Rodríguez

Human-induced climate change is accelerating and is profoundly impacting remote aquatic ecosystems. In the Sierra Nevada, these climatic shifts—exacerbated by increased Saharan dust inputs—are likely driving major transformations in lake environments. This study presents paleolimnological research reconstructing ecological and environmental trends in Sierra Nevada lakes and their catchments over the past two centuries. We analyzed multiple independent paleoindicators from high-resolution, chronologically dated sediment cores, including spectrally inferred chlorophyll-a, sedimentary ancient prokaryotic DNA, and subfossil remains of diatoms, cladocerans, and chironomids. A compilation of the paleolimnological research conducted in Sierra Nevada lakes was made, examining the effects of environmental variables on indicators of the lake structure and function, which were grouped accordingly. Results reveal that ecological changes began subtly over a century ago, intensifying in the 1960s–1970s, coinciding with regional warming, reduced precipitation, and intensified Saharan dust deposition. Generalized linear models (GLM) and Redundancy Detrended Analyses (RDA) were applied to analyze the effects of climate variables on community assemblages. Results indicate major shifts in aquatic community composition and recent algal biomass increases, with temperature emerging as the dominant driver, and Saharan dust and precipitation as secondary factors. Species shifts indicate rising water temperature and alkalinity and greater eutrophication. Microbial communities show significant restructuring, with increased archaeal and bacterial diversity and abundance, suggesting heightened microbial activity and functional changes in biogeochemical processes. The synchrony among proxies and climate variables across sites suggests a region-wide phenomenon. These changes are linked to prolonged ice-free seasons, elevated lake water temperatures, and reduced hydrological input, factors that influenced lake volume and water residence time, and reflect the increasing severity of summer droughts in the Sierra Nevada highlands over the past five to six decades. Continued global warming, reduced precipitation, and increased Saharan dust transport are expected to intensify these ecological shifts in the future.

The food web in the fishless lakes of Sierra Nevada is largely shaped by several environmental factors, primarily depth and morphometry. Over the past 50 years, numerous studies have explored the trophic compartments of these ecosystems, with a primary focus on the taxonomy and ecology of planktonic species and coleopterans. This paper synthesizes research in these areas. In terms of taxonomy, several species have been recorded for the first time worldwide, such as the chrysophyte Chromulina nevadensis. Other plankton species have been newly documented in Spain, including the diatom Achnanthidium macrocephalum and the rotifer Trichocerca relicta, while Daphnia cf. pulex have been reported for the first time in European high mountain lakes. Ecological research has also focused on biodiversity, life history strategies, stoichiometry and trophic interactions. Key findings include: (i) low species richness and the Shannon-Wiener diversity in planktonic communities; (ii) temperature and mineralization as likely primary drivers of zooplankton composition; (iii) evidence supporting the concept of rheostasis in zooplankton; (iv) pioneering insights into links between organismal stoichiometry and life-history strategies; (v) broad trophic niches in dytiscid beetles, which primarily feed on zooplankton; (vi) discovery of a mutualistic relationship between an epibiont green algae and Daphnia; and (vii) the significant role of multiple interacting environmental factors on the algae-bacteria relationship.

Calcium (Ca) is an important driver of community structure in freshwaters. We examined the combined effects of increased temperatures and variations in food quantity on the tolerance to low Ca of Daphnia pulex. The aim was to predict the impact of climate warming on this keystone zooplanktonic species in cold-climate lakes. We conducted a factorial life-history experiment in a clone of North American Daphnia cf. pulex to analyse the interaction effects of a temperature increase (17.5 °C–21 °C) within their physiological preferred range and expected by climate warming over the next few decades and a narrow Ca gradient (0.25–1.74 mg Ca L⁻¹) under stressful vs. abundant food conditions. We found a striking positive synergistic effect of Ca and temperature on D. pulex reproduction at high food conditions. Although the increase in temperature to 21 °C greatly reduced survival, high energy allocation to reproduction at high food levels allowed the population to succeed in poor Ca (−1). Results suggest that climate warming and higher food availability will make the populations of many cold and Ca-limited lakes more tolerant to low Ca levels with higher growth population rates, thereby altering zooplanktonic community structures and inducing potential cascading effects on the food web.

The ability of aquatic organisms to sense the surrounding environment chemically and interpret such signals correctly is crucial for their ecological niche and survival. Although it is an oversimplification of the ecological interactions, we could consider that a significant part of the decisions taken by organisms are, to some extent, chemically driven. Accordingly, chemical contamination might interfere in the way organisms behave and interact with the environment. Just as any environmental factor, contamination can make a habitat less attractive or even unsuitable to accommodate life, conditioning to some degree the decision of organisms to stay in, or move from, an ecosystem. If we consider that contamination is not always spatially homogeneous and that many organisms can avoid it, the ability of contaminants to repel organisms should also be of concern. Thus, in this critical review, we have discussed the dual role of contamination: toxicity (disruption of the physiological and behavioral homeostasis) vs. repellency (contamination-driven changes in spatial distribution/habitat selection). The discussion is centered on methodologies (forced exposure against non-forced multi-compartmented exposure systems) and conceptual improvements (individual stress due to the toxic effects caused by a continuous exposure against contamination-driven spatial distribution). Finally, we propose an approach in which Stress and Landscape Ecology could be integrated with each other to improve our understanding of the threat contaminants represent to aquatic ecosystems.

Organisms with wide environmentally-induced morphological plasticity and cosmopolitan distribution, e.g., the common freshwater rotifer Keratella cochlearis, are ideal models to study the evolution of plastic polymorphisms and the capacity of zooplankton to adapt to local selection conditions. We investigated population-level differences (population-by-environment interaction) in sensitivity to food availability and temperature-induced phenotypic plasticity between two clones of K. cochlearis (Gosse) isolated from neighboring populations in Ruidera Natural Park (Spain) with different trophic statuses: Tinaja (TIN) lake (mesotrophic) and Cueva Morenilla (MOR) lake (eutrophic). Using common-garden experiments, each clone proved to have a different sensitivity to food availability, with substantial phenotypic differences between them. When rotifers grew at moderate temperature (15.6°C), low food levels were more efficiently used by the TIN versus MOR clone, whereas high food levels were more efficiently used by the MOR versus TIN clone. The posterior spine was much longer and the lorica wider in the TIN versus MOR clone, with no difference in lorica length. Phylogenetic analysis based on COI sequences showed that both populations have the same haplotype. This is the first study to show possible local adaptation by a rotifer species to habitats that consistently differ in food availability. We also detected an intriguing deviation from the expected negative relationship between posterior spine length and temperature. Our experimental results indicate that intermediate temperatures may activate the gene responsible for spine elongation in K. cochlearis. This suggests that rotifers in nature could use water temperature as proxy signal of a change in predation risk before defense is needed.

Under ongoing climate change and increasing anthropogenic activity, which continuously challenge ecosystem resilience, an in-depth understanding of ecological processes is urgently needed. Lakes, as providers of numerous ecosystem services, face multiple stressors that threaten their functioning. Harmful cyanobacterial blooms are a persistent problem resulting from nutrient pollution and climate-change induced stressors, like poor transparency, increased water temperature and enhanced stratification. Consistency in data collection and analysis methods is necessary to achieve fully comparable datasets and for statistical validity, avoiding issues linked to disparate data sources. The European Multi Lake Survey (EMLS) in summer 2015 was an initiative among scientists from 27 countries to collect and analyse lake physical, chemical and biological variables in a fully standardized manner. This database includesin-situlake variables along with nutrient, pigment and cyanotoxin data of 369 lakes in Europe, which were centrally analysed in dedicated laboratories. Publishing the EMLS methods and dataset might inspire similar initiatives to study across large geographic areas that will contribute to better understanding lake responses in a changing environment.