Microbial growth and respiratory rates vary seasonally at all non-tropical latitudes. In addition, based on results from the PI's ongoing research, the minimal requirement for substrates by bacteria also varies seasonally. The interaction of these two factors results in a dramatic decrease in microbial food web activity at the time of the local minimum temperature. This is a transient condition and can be attenuated by a rise in temperature and/or increase in organic substrates, either directly or via zooplankton grazing. Results of ongoing research, over three spring bloom periods in Conception Bay, Newfoundland demonstrated that microbial loop activity is greatly diminished at the beginning to mid-bloom, increasing either after the bloom ceases (1986, 1990) or toward the end of a bloom that is extended by physical mixing so that zooplankton grazing intensifies (1988). Data from the U.S. southeastern continental shelf suggest that this concept applies to the subtropical ocean as well. In many continental shelf regions this transient period may represent the only autotrophic (primary production exceeds community respiration) period of the year. The length of this event becomes critical to the functioning of the system over the entire year. Even moderate global warming of surface waters could have a substantial impact on the organic nutrient status of the region. A combination of observations and experiments at sea, from high to subtropical latitudes, together with experimental studies, using bacterial and protozoan isolates in the laboratory, will help us define the combined effects of water temperature and substrate availabilities. These results should be of particular value in predicting the effect of global warming on marine food webs.
