Seasonal changes in microbial heterotrophic activity in subarctic marine waters as related to phytoplankton primary productivity

Water samples were collected in and near Kasitsna Bay (Cook Inlet), Alaska over 18 mo (February 1979–August 1980). Seasonal changes in glucose and glutamate uptake rates were measured in these samples. During the second year of the study, the uptake rates of glycolate and acetate and primary productivity were also measured. Of the substrates tested, significant positive linear correlations were observed between glucose and glutamate uptake and primary productivity. A higher correlation was observed between glucose to glutamate uptake ratios and primary productivity. The seasonal patterns of glucose uptake, glucose-to-glutamate uptake ratios and primary productivity rates show that the ratios changes simultaneously with fluctuations in primary productivity rates. The glucose uptake patterns reveal a delay in the response of the glucose-utilizing microorganisms to the onset of the bloom. It is suggested that by measuring the uptake rates of the simple compounds by microheterotrophs, and comparing these rates with each other, one can determine the relative flux of these compounds through the system. If the major source of these compounds is material released into the seawater by phytoplankton, it is possible that this approach could characterize the flux of extracellular products. As more is known about how the composition of extracellular material relates to the physiological state of phytoplankton, such an approach may eventually be a useful indirect assessment of the physiological state of natural phytoplankton populations.Published as Technical Paper No. 6397, Oregon Agricultural Experiment Station