Abstract. Over the 20th century, surface water temperatures have increased in many lake ecosystems around the world, but long-term trends in the vertical thermal structure of lakes remain unclear, despite the strong control that thermal stratification exerts on the biological response of lakes to climate change. Here we used both neo-and paleoecological approaches to develop a fossil-based inference model for lake mixing depths and thereby refine understanding of lake thermal structure change. We focused on three common planktonic diatom taxa, the distributions of which previous research suggests might be affected by mixing depth. Comparative lake surveys and growth rate experiments revealed that these species respond to lake thermal structure when nitrogen is sufficient, with species optima ranging from shallower to deeper mixing depths. The diatom-based mixing depth model was applied to sedimentary diatom profiles extending back to 1750 AD in two lakes with moderate nitrate concentrations but differing climate settings. Thermal reconstructions were consistent with expected changes, with shallower mixing depths inferred for an alpine lake where treeline has advanced, and deeper mixing depths inferred for a boreal lake where wind strength has increased. The inference model developed here provides a new tool to expand and refine understanding of climate-induced changes in lake ecosystems.

The recent and rapid recession of alpine glaciers over the last 150 years has major implications for associated aquatic communities. Glacial meltwater shapes many of the physical features of high altitude lakes and streams, producing turbid environments with distinctive hydrology patterns relative to nival systems. Over the past decade, numerous studies have investigated the chemical and biological effects of glacial meltwater on freshwater ecosystems. Here, we review these studies across both lake and stream ecosystems. Focusing on alpine regions mainly in the Northern Hemisphere, we present examples of how glacial meltwater can affect habitat by altering physical and chemical features of aquatic ecosystems, and review the subsequent effects on the biological structure and function of lakes and streams. Collectively or separately, these factors can drive the overall distribution, diversity and behavior of primary producers, triggering cascading effects throughout the food web. We conclude by proposing areas for future research, particularly in regions where glaciers are soon projected to disappear.

A discussion of course-based undergraduate research experiences (CUREs) for non–science majors (nonmajors) that summarizes the state of knowledge of best practices for nonmajors CUREs, identifies future research priorities, and recommends tools to align research questions with student outcomes.

We compared phytoplankton diversity and productivity over various time scales in a set of lakes in the central Rocky Mountains of North America (fed by both glacial and snowpack meltwaters [GSF] and by snowpack alone [SF]) to better understand the influence of nitrogen-rich glacial meltwater on the structure and function of phytoplankton. Nitrate concentrations in GSF lakes were on average 44 times higher than in SF, even though only 0.01-0.59% of the catchment area of GSF lakes was covered by glaciers. In three lakes of each type, we determined the vertical distribution of algae, epilimnetic primary productivity rates, and nutrient-limitation patterns. Phytoplankton richness and community structure were compared between lake types in the nutrient enrichment experiments, as well as in the tops (i.e., modern) and bottoms (circa 1850) of sediment cores from four lakes of each type. Average primary productivity rates were five times higher in GSF lakes, but vertically integrated chlorophyll concentrations did not differ. However, algal biomass was higher in the epi-and metalimnion of GSF lakes, while it was equivalent in the hypolimnion of both lake types. Nutrient-limitation patterns differed between lake types, with GSF lakes limited by phosphorus and most SF lakes co-limited by nitrogen and phosphorus. Modern diatom species richness was lower in GSF compared with SF lakes, whereas differences between lake types were not apparent during the mid-19th century. Key differences in the structure and function of GSF compared with SF lakes imply that GSF lakes will take a different ecological trajectory if glaciers disappear.

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