AUTECOLOGY OF AN ULTRAPLANKTONIC SHADE ALGA IN LAKE TAHOE2

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Abstract
The ultraplanktonic green alga Monoraphidium contortum Korm. in Lake Tahoe (California‐Nevada) demonstrated several ecological and physiological attributes of a genetically adapted shade species. Monoraphidium contortum achieved maximum biomass during deep mixing in winter when light availability was at a minimum. During stratification it was found in maximum abundance in the deep euphotic region, 100–150 m. This species was also distributed through the deep aphotic zone where, despite prolonged darkness, it remained capable of immediate photosynthesis when re‐exposed to light levels in the euphotic zone. The spirally twisted cells were grazed by two calanoid copepods in Lake Tahoe as readily as much larger‐celled phytoplankton species of less complex morphology. Slow growth rates in combination with high susceptibility to copepod grazing may effectively exclude M. contortum from the upper 75 m, where it was rarely recorded. In culture it showed a marked incapacity to adjust to ‘sun’ conditions but was well adapted to low light regimes. Under a wide range of irradiances, photochemical capacity, photosynthetic capacity and growth rates were low, but cellular pigment content remained high. The ratio of Pmax to respiration was at the lower end of the range for shade plants. Genetically distinct sun and shade populations of phytoplankton may play a determining role in major shifts of community structure over depth and time in Lake Tahoe.