Lessons from 30 years of multiple-stressor experiments on microbial plankton in a Spanish high-mountain lake

Abstract

The impact of multiple environmental stressors on freshwater ecosystems has been linked to the study of global change and its effects on biotic communities since the last quarter of the 20th century. In the high mountain lakes of Sierra Nevada National Park (Spain), these studies emerged in the early 1990s, initially aiming to determine whether the most influent environmental stressors in the region (ultra-oligotrophy and high UV radiation) could explain the unique properties of the microbial communities in these systems (a simple, phytoplankton-dominated structure). Later, the focus shifted toward understanding how environmental stressors associated with global change could shape their trophic structure and functioning. In this review, we analyze the 35 experimental studies carried out in the Lakes of Sierra Nevada, with the aim of synthesizing the existing information on the combined effects of different stressors in these ecosystems. The findings highlight a predominant interest in studying the effects of nutrients and UV radiation on phytoplankton and bacterial communities. Interestingly, we report an increase in positive effects on variables such as chlorophyll-a, primary production, and bacterial production as the number of interacting stressors increases. The combination of stressors leads to a higher proportion of non-additive effects (-90%) over additive ones (-10%), with antagonistic effects predominantly affecting autotrophic variables and synergistic effects affecting heterotrophic ones. Taken together, the evidence indicates that while the microbial network structure in Sierra Nevada lakes shows a certain resistance to multiple stressors, their chronic effects may reveal an incipient shift toward a greater prominence of the bacterial compartment, ultimately transforming the microbial food web dynamics in these ecosystems.