Seasonal Dynamics of the Deep-Chlorophyll Maximum in Castle Lake, California

Vertical profiles of chlorophyll concentration measured during 1980 in Castle Lake showed that a deep maximum developed immediately after ice thaw and persisted in the deep basin of the lake until autumn overturn. In the early portion of the ice-free season, low epilimnetic turbidity allows enough light to reach this deep-chlorophyll layer to produce a deep-primary productivity maximum. Photoautotrophic growth appears to maintain the deep-chlorophyll maximum early in the season whereas the accumulation of sinking organisms appears to do so later in the season. Although the deep-water phytoplankton have reduced rates of photosynthesis late in the season, they maintain their ability to photosynthesize immediately upon exposure to light. Consequently, the redistribution of deepwater chlorophyll at fall overturn can increase the chlorophyll concentration of the euphotic zone (0–15 m) by 58% which can potentially increase primary productivity in this zone by 81%.Key words: deep-chlorophyll maximum, primary productivity, aphotic viability