Ecosystem

Morphometry

BASIN DATA
Surface elevation:	1,657 m (5435 ft)
Lake surface area:	0.20 km2
Basin surface area:	0.81 km2
Ratio of drainage to lake surface area:	4.0
Maximum depth:	35 m
Average depth:	11.4 m
Lake volume:	2.29 x 106 m3
Shoreline development: (Ratio of shoreline distance to the circumference of circle with the same surface area)	1.36
Trophic status:	meso-oligotrophic

Basin

Castle Lake lies within a low lying basin that collects the runoff and groundwater and incorporates that water into the lake. As such, lakes are affected by features of the basin they are in. The part of the basin filled by water is called the lake basin. The shape of the lake basin affects its volume, area, depth, and the extent of different habitats, as well as the formation of currents and the thermocline. The type of bedrock, surrounding vegetation, and soils in the basin affect the amount and type of nutrients that reach the lake. The granite rocks around Castle Lake have little nutrients in terms of Nitrogen and Phospherous. Most of the Nitrogen that enters the lake is from the leaves of nearby alder trees and other vegetation. Phospherous also comes from terrestrial runoff, as phospherous is binded into the soil.

Climate

While there is interannual variation, Castle Lake enjoys four distinct seasons of the year. The influence of the climate on the processes occuring at Castle Lake is great. Please consult the references at the bottom of this page for further study.

Spring and Fall Mixing

In the fall and spring, "overturns" occur which equalize the temperature of the water in the lake. This mixing of the water temperatures brings up essential nutrients that accumulate at the bottom of the lake. This often initiates "blooms" of phytoplankton which stimulate productivity in the whole lake.

Summer Stratification

Castle Lake. During the summer, a phenomenon known as thermal stratification occurs in lakes. Sunlight heats the top of the lake, forming a layer of warm water over a cooler zone. These warm and cold zones are separated by a zone of mixed water where there is a sharp decline in temperature. Summertime swimmers often experience these temperature layers as their feet hit the sudden cool waters.

Winter Icecover

Castle Lake frozen in the early spring. With the onset of winter and continuous nights of 0° C temperatures, Castle Lake begins to freeze. Unlike other liquids which are heavier at 0° C, water is heaviest at 4° C. Therefore the bottom of the lake contains water at 4° C while colder water is layered above. This prevents Castle Lake from freezing all the way to the bottom and allows life to remain in the lake throughout the winter. When the ice is solid on Castle Lake, there are also opportunities for great ice skating or ice fishing.

Thermocline & Stratification

This is the zone in the lake where temperature changes most rapidly with depth. Since solar heating from above is typically the dominant heat source, lakes gain heat from their surface layers. If a temperate lake is deep enough, the deeper waters never warm up and stay at a consistant 4° C (water is most dense at 4° C) all year round. Castle Lake is such a lake.

During the winter the lake is covered with ice, which begins to melt in spring. Once the weather warms up and the sun begins to warm the surface layers, the entire lake becomes the same temperature at 4° C. When heating gets more intense, the warmer surface layers (less dense) float on top of the colder more dense layers. By late summer the surface waters reach 25° C, but at 10 M deep the water is only 10° C. The thermocline varies between 5 and 7 M deep during the summer. Having such a strong temperature (and density) gradient prevents water layers of different density to mix. In other words, the thermocline traps the lighter, warmer water on top of the heavier, colder water. Strong wind waves can shift the depth and gradient of the thermocline, but the main thermocline remains until fall. When sunlight decreases and the air gets colder, surface waters begin to cool and sink. This eventually destroys the thermocline and the lake mixes from bottom-up to reach a constant 4° C. Further cooling forms ice on the lake, which floats during winter. Lake mixing is critical for algal growth because nutrients vital to algae tend to sink to the deeper layers. Nutrients are mostly trapped in the deep layers during summer, while maximum solar energy, also vital for algae, is at the surface. This causes algae to be nutrient limited until the lake mixes and nutrients previously trapped in deep waters are delivered to the surface. The combination of adequate nutrients and sunlight make the algae bloom during this mixing events.

Climate References

• Goldman, C. R. and E. de Amezaga. 1984. Primary productivity and precipitation at Castle Lake and Lake Tahoe during twenty-four years, 1959-1982. Verh. Int. Ver. Theor. Angew. 22: 591-599.
• Goldman, C. R., A. Jassby and T. Powell. 1989. Interannual fluctuations in primary production: Meteorological forcing at two subalpine lakes. Limnol. Oceanogr. 34:310-323.
• Jassby, A. D., T. M. Powell and C. R. Goldman. 1990. Interannual fluctuations in primary production: Direct physical effects and the trophic cascade at Castle Lake, California. Limnol. Oceanogr. 35:1021-1038.