Mountain lakes are often situated in protected natural areas, a feature that leads to their role as sentinels of global environmental change. Despite variations in latitude, mountain lakes share many features, including their location in catchments with steep topographic gradients, cold temperatures, high incident solar and ultraviolet radiation (UVR), and prolonged ice and snow cover. These characteristics, in turn, affect mountain lake ecosystem structure, diversity, and productivity. The lakes themselves are mostly small and shallow, and up until recently, have been characterized as oligotrophic. This paper provides a review and update of the growing body of research that shows that sediments in remote mountain lakes archive regional and global environmental changes, including those linked to climate change, altered biogeochemical cycles, and changes in dust composition and deposition, atmospheric fertilization, and biological manipulations. These archives provide an important record of global environmental change that pre-dates typical monitoring windows. Paleolimnological research at strategically selected lakes has increased our knowledge of interactions among multiple stressors and their synergistic effects on lake systems. Lakes from transects across steep climate (i.e., temperature and effective moisture) gradients in mountain regions show how environmental change alters lakes in close proximity, but at differing climate starting points. Such research in particular highlights the impacts of melting glaciers on mountain lakes. The addition of new proxies, including DNAbased techniques and novel stable isotopic analyses, provides a gateway to addressing novel research questions about global environmental change. Recent advances in remote sensing and continuous, high-frequency, limnological measurements will improve spatial and temporal resolution and help to add records to spatial gaps including tropical and southern latitudes.

Roughly 10% of the Earth's surface is permanently covered by glaciers and ice sheets and in mountain ecosystems, this proportion of ice cover is often even higher. From an ecological perspective, ice-dominated ecosystems place harsh controls on life including cold temperature, limited nutrient availability, and often prolonged darkness due to snow cover for much of the year. Despite these limitations, glaciers, and perennial snowfields support diverse, primarily microbial communities, though macroinvertebrates and vertebrates are also present. The availability and mass balance of key elements [(carbon (C), nitrogen (N), phosphorous (P)] are known to influence the population dynamics of organisms, and ultimately shape the structure and function of ecosystems worldwide. While considerable attention has been devoted to patterns of biodiversity in mountain cryosphere-influenced ecosystems, the ecological stoichiometry of these habitats has received much less attention. Understanding this emerging research arena is particularly pressing in light of the rapid recession of glaciers and perennial snowfields worldwide. In this review, we synthesize existing knowledge of ecological stoichiometry, nutrient availability, and food webs in the mountain cryosphere (specifically glaciers and perennial snowfields). We use this synthesis to develop more general understanding of nutrient origins, distributions, and trophic interactions in these imperiled ecosystems. We focus our efforts on three major habitats: glacier surfaces (supraglacial), the area beneath glaciers (subglacial), and adjacent downstream habitats (i.e., glacierfed streams and lakes). We compare nutrient availability in these habitats to comparable habitats on continental ice sheets (e.g., Greenland and Antarctica) and show that, in general, nutrient levels are substantially different between the two. We also discuss how ongoing climate warming will alter nutrient and trophic dynamics in mountain glacier-influenced ecosystems. We conclude by highlighting the pressing need for studies to understand spatial and temporal stoichiometric variation in the mountain cryosphere, ideally with direct comparisons to continental ice sheets, before these imperiled habitats vanish completely.

Generalist parasites can strongly influence interactions between native and invasive species. Host competence can be used to predict how an invasive species will affect community disease dynamics; the addition of a highly competent, invasive host is predicted to increase disease. However, densities of invasive and native species can also influence the impacts of invasive species on community disease dynamics. We examined whether information on host competence alone could be used to accurately predict the effects of an invasive host on disease in native hosts. We first characterized the relative competence of an invasive species and a native host species to a native parasite. Next, we manipulated species composition in mesocosms and found that host competence results did not accurately predict community dynamics. While the invasive host was more competent than the native, the presence of the native (lower competence) host increased disease in the invasive (higher competence) host. To identify potential mechanisms driving these patterns, we analyzed a two-host, one-parasite model parameterized for our system. Our results demonstrate that patterns of disease were primarily driven by relative population densities, mediated by asymmetry in intra- and interspecific competition. Thus, information on host competence alone may not accurately predict how an invasive species will influence disease in native species.

Nearshore (littoral) habitats of clear lakes with high water quality are increasingly experiencing unexplained proliferations of filamentous algae that grow on submerged surfaces. These filamentous algal blooms (FABs) are sometimes associated with nutrient pollution in groundwater, but complex changes in climate, nutrient transport, lake hydrodynamics, and food web structure may also facilitate this emerging threat to clear lakes. A coordinated effort among members of the public, managers, and scientists is needed to document the occurrence of FABs, to standardize methods for measuring their severity, to adapt existing data collection networks to include nearshore habitats, and to mitigate and reverse this profound structural change in lake ecosystems. Current models of lake eutrophication do not explain this littoral greening. However, a cohesive response to it is essential for protecting some of the world's most valued lakes and the flora, fauna, and ecosystem services they sustain.

While deposition of reactive nitrogen (N) in the twentieth century has been strongly linked to changes in diatom assemblages in high-elevation lakes, pronounced and contemporaneous changes in other algal groups suggest additional drivers. We explored the origin and magnitude of changes in two mountain lakes from the end of the Little Ice Age at ca 1850, to ca 2010, using lake sediments. We found dramatic changes in algal community abundance and composition. While diatoms remain the most abundant photosynthetic organisms, concentrations of diatom pigments decreased while pigments representing chlorophytes increased 200–300% since ca 1950 and total algal biomass more than doubled. Some algal changes began ca 1900 but shifts in most sedimentary proxies accelerated ca 1950 commensurate with many human-caused changes to the Earth System. In addition to N deposition, aeolian dust deposition may have contributed phosphorus. Strong increases in summer air and surface water temperatures since 1983 have direct and indirect consequences for high-elevation ecosystems. Such warming could have directly enhanced nutrient use and primary production. Indirect consequences of warming include enhanced leaching of nutrients from geologic and cryosphere sources, particularly as glaciers ablate. While we infer causal mechanisms, changes in primary producer communities appear to be without historical precedent and are commensurate with the post-1950 acceleration of global change.

Nitrate concentrations in high-elevation lakes of the Colorado Front Range remain elevated despite declining trends in atmospherically deposited nitrate since 2000. The current source of this elevated nitrate in surface waters remains elusive, given shifts in additional nitrogen sources via glacial inputs and atmospheric ammonium deposition. We present the complete isotopic composition of nitrate (δ 15 N, δ 18 O, and Δ 17 O) from a suite of nitrate-bearing source waters collected during the summers of 2017−2018 from two alpine ecosystems to constrain the provenance of elevated nitrate in surface waters during the summer open-water season.The results indicate a consistent contribution of uncycled atmospheric nitrate throughout the summer (13−23%) to alpine lakes, despite seasonal changes in source water inputs. The balance of nitrate (as high as 87% in late summer) is likely from nitrate production within the catchment via nitrification of reduced nitrogen sources (e.g., thawed soil organic matter and ammonium deposition) and released with rock glacier meltwater. The role of microbially produced nitrate has become increasingly important over time based on historical surface water samples from the mid-90s to present, a trend coincident with increasing ammonium deposition to alpine systems.

Baron. 2020. The role of warm, dry summers and variation in snowpack on phytoplankton dynamics in mountain lakes.

We examined the importance of temporal variability in top–down and bottom–up effects on the accumulation of stream periphyton, which are complex associations of autotrophic and heterotrophic microorganisms. Periphyton contributes to primary production and nutrient cycling and serves as a food resource for herbivores (grazers). Periphyton growth is often limited by the availability of nitrogen and phosphorus, and biomass can be controlled by grazers. In this study we experimentally manipulated nutrients and grazers simultaneously to determine the relative contribution of bottom–up and top–down controls on periphyton over time. We used nutrient diffusing substrates to regulate nutrient concentrations and an underwater electric field to exclude grazing insects in three sequential 16–17 day experiments from August to October in montane Colorado, USA. We measured algal biomass, periphyton organic mass, and algal community composition in each experiment and determined densities of streambed insect species, including grazers. Phosphorus was the primary limiting nutrient for algal biomass, but it did not influence periphyton organic mass across all experiments. Effects of nutrient additions on algal biomass and community composition decreased between August and October. Grazed substrates supported reduced periphyton biomass only in the first experiment, corresponding to high benthic abundances of a dominant mayfly grazer (Rhithrogena spp.). Grazed substrates in the first experiment also showed altered algal community composition with reduced diatom relative abundances, presumably in response to selective grazing. We showed that top–down grazing effects were strongest in late summer when grazers were abundant. The effects of phosphorus additions on algal biomass likely decreased over time because temperature became more limiting to growth than nutrients, and because reduced current velocity decreased nutrient uptake rates. These results suggest that investigators should proceed with caution when extending findings based on short‐term experiments. Furthermore, these results support the need for additional seasonal‐scale field research in stream ecology.