Bacterial responses to temperature and substrate concentration during the Newfoundland spring bloom

We postulate that microbial metabolism and production in cold waters are hmited by the ability of bacteria to transport and/or assimilate substrates at the low concentrations usually present. We measured rates of microbial activity in the water colunln and benthos in Conception Bay, Newfoundland and adjacent coastal waters, during 3 spring blooms. Chlorophyll abundance and distribution, photosynthesis, benthic respiratory rate and remineralization, bacterioplankton abundance, and production of bacterial blomass were measured in the water column, and the respuatory rate of benthic sediments was measured in situ by a free vehicle containing bell jars equipped w t h oxygen electrodes and syringe samplers. During 3 spring seasons, bacterioplankton numbers during the spring phytoplankton bloom exceeded 5 X lo5 ml-' only 15 % of the time. These numbers rank at the lower end of the range of bacterial numbers for the world's ocean. Bacterial productivity measured by 3 methods suggested average generation times of 30 to 86 d, although some samples in the chlorophyll maximum layer showed short generation times. Bacterial production and respiration, averaged over the entire water column, plus benthic aerobic respiration and denitrification, accounted for 3 % of primary production during the early bloom and 28 % of primary production during the late bloom. The difference between the early and late bloom is related to observed changes in primary production, not to increased microbial activity. Unless bacterial assimilation efficiency was very low, much less than half of the organic production of the spring diatom bloom was used by microbial processes dunng the early, highly productive phase of the bloom, while somewhat more than half of organic production was used in the short term during the later, less productive phase of the spring bloom. To test the hypothesis that bacteria require hlgher substrate concentrations at low temperature, water samples were amended with glucose and proteose-peptone and incubated for 2 wk at -1 to + 15 "C. Respiratory rate, measured at intervals of 1 to 2 d , increased with increasing temperature andlor substrate concentration. Analysis of vanance showed significant effects of temperature and substrate in all cases. In 3 of 4 experiments there were also significant effects due to the interaction of temperature and substrate