Crater Lake was formed by the cataclysmic collapse of Mount Mazama approximately 7,700 years ago, creating the deepest lake in the United States at 1,949 feet. It is a closed hydrologic system; water levels are maintained solely by precipitation and seepage. This isolation and lack of sediment input contribute to the water’s extraordinary clarity and purity. The water is perpetually cold, maintaining a temperature near \(39^{\circ} \text{F}\) below the upper mixed layer, classifying the lake as ultraoligotrophic, or extremely poor in nutrients.
The Introduced Fish Population
Crater Lake was naturally barren of fish until the late 19th century due to its isolated, nutrient-poor nature. Between 1888 and 1941, humans introduced six species to enhance recreational value. Only two species survive today: the Rainbow Trout (Oncorhynchus mykiss) and the Kokanee Salmon (Oncorhynchus nerka). Both species now sustain their populations through natural reproduction.
The Kokanee Salmon, a landlocked, dwarf form of Sockeye Salmon, is the most abundant fish species. These fish primarily inhabit the open waters, or pelagic zone, where they feed heavily on zooplankton and small insects. Rainbow Trout are less numerous but typically larger, averaging between 10 and 14 inches in length.
The two species maintain stable populations by partitioning their food resources and habitat. Rainbow Trout mostly remain near the shore, feeding on large-bodied invertebrates, aquatic insects, and terrestrial insects. This division of resources helps mitigate direct competition. These non-native fish function as the top predator, influencing the structure of the invertebrate communities they consume.
Invertebrate and Planktonic Life
The base of the lake’s food web includes zooplankton and benthic invertebrates. The zooplankton community consists primarily of two species of cladocerans (small crustaceans) and at least nine species of rotifers (microscopic, multicellular animals). These small organisms are the main food source for the Kokanee Salmon.
The distribution of zooplankton in the water column changes with the seasons. During the summer, when the lake is thermally stratified, zooplankton density is extremely low in the warm upper 65 feet. To avoid fish predation and warmer temperatures, the highest densities are found deeper, typically between 80 and 120 meters.
In the nearshore benthic zone, aquatic insects provide a steady food source for Rainbow Trout. These include the larvae and pupae of midges, along with other large-bodied aquatic invertebrates. Small aquatic snails and amphipods are also common in the shallower waters to depths of about 100 meters, contributing significantly to the fish diet.
The lake also hosts unique endemic species, such as a newly identified aquatic mite in the genus Algophagopsis. These small, bottom-dwelling organisms, along with oligochaete worms and chironomid midges, form a crucial layer of the food web in the deeper, cold waters.
Unique Flora and Deepwater Ecosystems
The lake’s deep blue color is a direct result of its pure water. This extreme transparency allows light to penetrate to exceptional depths, supporting primary producers far below the surface. The lake’s primary plant-like life is phytoplankton, with over 150 different species detected.
The phytoplankton are responsible for the lake’s primary production. Because of the water’s clarity, the maximum rate of photosynthesis occurs far below the surface, usually between 40 and 80 meters. Furthermore, algae species adapted to very low light levels form a deep-water chlorophyll maximum at depths between 120 and 140 meters.
The most remarkable flora are the dense, deep-water moss beds. These communities, composed largely of the moss Drepanocladus aduncus, form extensive beds up to several feet thick. Living specimens have been found attached to the sediment and rock walls at depths reaching 140 meters.
The moss survives at these extreme depths due to the lake’s clarity and its ability to thrive in low-light conditions. Dense beds are typically observed between 30 and 80 meters, but deeper populations survive where light penetration is minimal. These moss beds also support a diverse collection of epiphytic algae, representing a unique ecosystem on the lake’s floor.