Spatial and temporal variation in summer and winter populations of freshwater bacteria enumerated by scanning electron microscopy

Abstract

The aquatic bacteria of Sebesta Pond were classified and enumerated using scanning electron microscopy. Classification of the bacteria required the development of a key based on external morphology. These bacteria were placed into 58 morphological categories using the criteria of cell length, width, length-width ratio, surface texture, and appendages. The resolution of the scanning electron microscope made it possible to derive these morphological features. The spatial distribution (horizontal and vertical) of fifteen morphological categories and the total bacterial population were analyzed. The density of eight morphological categories varied significantly with depth in one or more of the vertical experiments. Two morphological categories were strict aerobes found in the epilimnion during summer stratification and throughout the water column during winter mixing. One morphological category was a strict anaerobe, found only in the anoxic hypolimnion. One morphological category was found in both aerobic and anaerobic environments, but had significantly higher densities in aerobic habitats. Another morphological category had higher densities associated with cold water found during winter mixing and the hypolimnion during summer stratification. Two of the morphological categories utilized niches occurring during periods of transition (early stratification). It was found that significant changes in the total bacterial population were influenced by the dominant morphological categories. Therefore, a change in the total bacterial population is not indicative of all the morphological categories. ..