Microplastics have become one of the most serious global threats to animal and human health. While their presence has been documented in all Earth water ecosystems, including remote mountain lakes, the observation that the abundance of microplastics is largely different across nearby lakes has rarely been examined. As part of a citizen science initiative, this study analyzed for the first time the abundance of microplastics in the surface of 35 glacial lakes of Sierra Nevada National Park in Southern Spain with the objective of determining the local factors that control their abundance. First, we described the shape, size, color and nature of microplastics. Second, we tested whether the number of microplastics differed between basins and analyzed environmental and morphometrical features of lakes affecting their abundance. We found that microplastics were common in most lakes, with a maximum abundance of 21.3 particles per liter that akin to some of the most microplastic polluted lakes worldwide. Fragments were the predominant shape (59.7%) followed by fibers (38.8%) and very scarce spheres (1.5%). Microplastics were observed for all size-fractions, but the abundance of particles <45 μm was higher, what advocates for the use of low pore-size filters to prevent underestimation of microplastics. While the mean abundance of microplastics did not differ among basins, their quantity was related to the presence of meadows surrounding the lakes. This result indicates that while atmospheric transport of microsplastics may equally reach all basins, differences in microplastics among nearby-lakes has an anthropic origin caused by mountaineers who find lakes with ample meadows much more attractive to visit relative to barren lakes. The staggering number in these remote lakes, headwaters of rivers that feed drinking reservoirs, is a major concern that warrants further investigation and the strict compliance with waste management laws to reduce the harmful impacts of microplastic contamination.

A continuing challenge for scientists is to understand how multiple interactive stressor factors affect biological interactions, and subsequently, ecosystems–in ways not easily predicted by single factor studies. In this review, we have compiled and analyzed available research on how multiple stressor pairs composed of temperature (T), light (L), ultraviolet radiation (UVR), nutrients (Nut), carbon dioxide (CO₂), dissolved organic carbon (DOC), and salinity (S) impact the stoichiometry of autotrophs which in turn shapes the nature of their ecological interactions within lower trophic levels in streams, lakes and oceans. Our analysis from 66 studies with 320 observations of 11 stressor pairs, demonstrated that non-additive responses predominate across aquatic ecosystems and their net interactive effect depends on the stressor pair at play. Across systems, there was a prevalence of antagonism in freshwater (60–67% vs. 47% in marine systems) compared to marine systems where synergism was more common (49% vs. 33–40% in freshwaters). While the lack of data impeded comparisons among all of the paired stressors, we found pronounced system differences for the L × Nut interactions. For this interaction, our data for C:P and N:P is consistent with the initial hypothesis that the interaction was primarily synergistic in the oceans, but not for C:N. Our study found a wide range of variability in the net effects of the interactions in freshwater systems, with some observations supporting antagonism, and others synergism. Our results suggest that the nature of the stressor pairs interactions on C:N:P ratios regulates the “continuum” commensalistic-competitive-predatory relationship between algae and bacteria and the food chain efficiency at the algae-herbivore interface. Overall, the scarce number of studies with even more fewer replications in each study that are available for freshwater systems have prevented a more detailed, insightful analysis. Our findings highlighting the preponderance of antagonistic and synergistic effects of stressor interactions in aquatic ecosystems—effects that play key roles in the functioning of feedback loops in the biosphere—also stress the need for further studies evaluating the interactive effects of multiple stressors in a rapidly changing world facing a confluence of tipping points.

We examined the response of the plankton community to strong changes in the volume of oligotrophic Lake La Caldera through the ice-free period of 1995-1997. As consequence of the changes in the precipitation regime, the lake volume diminished to 2 m depth during the extremely dry year of 1995. The increase in phosphorus availability for this year caused the diversification of the planktonic community, and the autotrophic:heterotrophic (A/H) ratio was below 1. In spite of the sporadic dust loads associated to atmospheric events, the quick recovery of the lake volumen in 1996 led to a severe phosphorus limitation. Phytoplankton community was dominated by Chrysophyceae (>90% of abundance) and large zooplankters were very scarce (<1 ind l-1). As a consequence, A/H ratio was over 1 and ciliates developed. Finally, towards the end of 1997, the zooplankton recovery led to a decrease of both A/H ratio and ciliates, and the lake returned to the conditions before drought. A decreasing tendency in both bacterial and phytoplankton densities, possibly as a consequence of top-down controls, was observed through the period of 1995-1997.