Bear Lake is a large alkaline lake on a high plateau on the Utah-Idaho border. The Bear River was partly diverted into the lake in the early twentieth century so that Bear Lake could serve as a reservoir to supply water for hydropower and irrigation downstream, which continues today. The northern Rocky Mountain region is within the belt of the strongest of the westerly winds that transport moisture during the winter and spring over coastal mountain ranges and into the Great Basin and Rocky Mountains. As a result of this dominant winter precipitation pattern, most of the water entering the lake is from snowmelt, but with net evaporation. The dominant solutes in the lake water are Ca 2+ , Mg 2+ , and HCO 3 2‒ , derived from Paleozoic carbonate rocks in the Bear River Range west of the lake. The lake is saturated with calcite, aragonite, and dolomite at all depths, and produces vast amounts of carbonate minerals. The chemistry of the lake has changed considerably over the past 100 years as a result of the diversion of Bear River. The net effect of the diversion was to dilute the lake water, especially the Mg 2+ concentration. Bear Lake is oligotrophic and coprecipitation of phosphate with CaCO 3 helps to keep productivity low. However, algal growth is colimited by nitrogen availability. Phytoplankton densities are low, with a mean summer chlorophyll a concentration of 0.4 mg L ‒1 . Phytoplankton are dominated by diatoms, but they have not been studied extensively (but see Moser and Kimball, this volume). Zooplankton densities usually are low (<10 L ‒1 ) and highly seasonal, dominated by calanoid copepods and cladocera. Benthic invertebrate densities are extremely low; chironomid larvae are dominant at depths <30 m, and are partially replaced with ostracodes and oligochaetes in deeper water. The ostracode species in water depths >10 m are all endemic. Bear Lake has 13 species of fish, four of which are endemic.

Abstract Devonian reef systems are thought to represent the greatest phase of global reef development in the Phanerozoic. Despite this, ecological and environmental controls on the sedimentary nature of these vast systems have scarcely been investigated and remain enigmatic. The Late Devonian (Frasnian) Alexandra Reef System, exposed in the Northwest Territories of Canada, developed on a ramp that was situated on the western margin of Laurussia. The system consists of two reef complexes. The second reef complex developed basinwards of the first after sea level fell ~ 17 m. In contrast to stromatoporoid (± coral)-dominated reef facies in the first reef complex and the upper part of the second reef complex, reef facies in the lower part of the second reef complex are dominated by stromatoporoid-microbe associations. These include significant renalcid boundstone and stromatolite accumulations that are not found elsewhere in the reef system. It is concluded that the occurrence of the stromatoporoid-microbe reef facies indicates that a shift in the reef environment from oligotrophic to mesotrophic conditions took place. The mechanisms of nutrification were linked to the platform geometry, sea-level position, and oceanographic system, indicating that on carbonate ramps, systems tracts of falling sea level (forced regression) and sea-level lowstand may be particularly susceptible to nutrification. A nutrient-gradient model developed to explain different types of reef facies in the Alexandra Reef System indicates that trophic resources were an important control on the composition of Devonian reef-building communities, and that Devonian reefs and carbonate platforms were not highly susceptible to nutrient-invoked drowning.