PHYTOPLANKTON ECOLOGY IN AN UNUSUALLY STABLE ENVIRONMENT (MONTEZUMA WELL, ARIZONA, USA)

Relatively minor annual amplitudes of change in certain major nutrients, and especially pH and water temperature were measured in the spring-fed system of Montezuma Well, Arizona during a 4 yr study. Phytoplankton diversity was low but for the most part, composition was spatially and temporally constant; total seasonal phytoplankton density was significantly correlated with regional incident light. Phytoplankton species composition changed briefly during and for a short period following teh summer monsoon. Ultraplankton (< 5 .mu.m) numerically comprised nearly 80% of the phytoplankton community throughout most of the year. The limited residence time of water in the Well may have provided a competitive advantage for cells with high surface area:volume ratios and correspondingly rapid division rates. Nannochloris bacillaris Naum. and Coccomyxa minor Skuja were perennial dominants. Diatom populations did not increase with annual increases in vernal solar radiation. Low pH, high dissolved CO2, and limited residence time for metabolic inhibitors are largely responsible for the reduced blue-green populations in the Well. The only flagellated photosynthetic group present in Montezuma Well was the Cryptophyta. Desmid populations were minimal, even though pH was consistently below circumneutral (6.5) and free CO2 concentrations high. The role of grazing by an amphipod, Hyalella montezuma, on annual phytoplankton abundance is examined.