| Summary: | A study was made of the heterotrophic bacteria of Lake Grasmere, an oligotrophic to mildly eutrophic freshwater lake, situated at an altitude of about 600 m in inland Canterbury, New Zealand. The maximum depth of the lake is 12 m and there are beds of Elodea canadensis around most of the lake to a depth of about 6-7 m. During the period of study, between 1969 and 1971, the lake was well oxygenated and did not stratify.
The following parameters were found to be positively correlated:
a) numbers of bacteria and total organic nitrogen in samples from over the inflow of a spring, which entered the lake beneath 2 m of water;
b) numbers of bacteria and total organic nitrogen in the open water;
c) numbers of bacteria over the spring and rainfall in the week before sampling.
Correlation coefficients were not significant between:
a) numbers of bacteria and nitrate nitrogen over the spring;
b) numbers of bacteria in the open water and rainfall in the week before sampling;
c) numbers of bacteria in the water over Elodea and rainfall in the week before sampling.
However, the size of the bacterial population in the open water in the autumn appeared to be influenced by the amount of rainfall in the previous winter.
There was some increase in the numbers of bacteria in the water over Elodea in the autumn and winter months. This response was attributed to the presence of Elodea, but was not recorded during the rest of the year. The total numbers of bacteria on Elodea, and numbers of those metabolizing, as detected by autoradiographic experiments, increased as the leaves aged.
In the open water, numbers of bacteria increased a little after blooms of zooplankton, but there was little or no response to changes in phytoplankton populations, or to the presence of Elodea canadensis. Bacteria in the mud rarely increased in numbers after the deposition of organic matter.
The numbers of viable bacteria in the open water were outnumbered by algae by a factor of between 4 and 34. Few, if any, bacteria were detected on algae or zooplankton by cultural methods. No evidence was obtained that the epiphytic bacteria, observed on some algae, were involved in their decomposition.
The kinds of bacteria in the water varied considerably at different times of the year and these seasonal changes were also reflected to some extent in the microflora of samples of mud and Elodea. In the water, pseudomonads, Alcaligenes/Achromobacter, flavobacteria and coryneforms were usually the main groups of bacteria found, but Cytophaga, Vibro extorguens, Enterobacteriaceae, Aeromonas/Vibrio and Micrococcaceae were abundant in some samples. Pseudomonads and non-motile flavobacteria were predominant in three samples of Elodea characterized. Samples of concentrated plankton had larger proportions of flavobacteria, coryneforms and Vibrio extorquens, but smaller percentages of pseudomonads and Micrococcaceae, than the samples of untreated water. The microflora of a sample from the surface layer of mud was similar to the population in the water at that time, but mud from up to 8 cm below the surface had larger proportions of Gram-positive bacteria.
In the autumn of one year, isolates of Enterobacteriaceae, which were unlike Escherichia coli, were common in several areas of the lake. This was attributed to a combination of factors, including the presence of waterfowl on the lake. An association of Vibrio extorquens with products of plankton, and of Cytophaga with increased concentrations of certain unspecified minerals was suggested.
Leaching of nutrients essential for bacterial metabolism from organisms which had recently died, and competition with the primary producers for nutrients, were suggested as possible reasons for the frequent lack of response by bacteria in the water and mud to the addition of organic matter.
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