The role of amino acids in the nitrogen cycle of Lake Mendota1

Abstract

Concentrations of most dissolved free amino acids (DFAA) in Lake Mendota, a eutrophic Wisconsin lake, were generally low (< 0.01 µM) but detectable and appeared to be controlled by bacterial uptake. Combined amino acids occurred in concentrations an order of magnitude higher than DFAA. Of 10 amino acids examined, serine and alanine were generally most prevalent in the DFAA fraction (glycine was not measured). Concentrations of dissolved amino acids (free and combined) showed little variation with depth or season, but the levels of particulate amino acids increased in the epilimnion during the warmer months when conditions were favorable for algal growth. In the hypolimnion concentrations of amino acids in particulate material were lower than in the dissolved fraction.Daily examination of shore samples suggested that the concentrations of the DFAA generally remained at trace levels, with substantially higher concentrations during a period of rapid algal decomposition following an Aphanizomenon bloom. Shore water samples from 3 m below the lake surface contained lower and more uniform DFAA concentrations from day to day than did surface samples.