Chesapeake Bay (CB) and the northern Adriatic Sea (NAS) exhibit contrasting patterns in trophic status and thereby offer contrasting case studies in microbial ecology. In the surface mixed layer in both systems, the abundance and productivity of cyanobacteria and of heterotrophic bacteria exhibit summer maxima and winter minima. Available evidence implicates temperature as a major constraint during winter and spring, and supply of nutrients and/or dissolved organic carbon during summer. Bacterioplankton density is an order of magnitude higher in CB than in the NAS, while productivity varies over similar ranges in both systems, suggesting higher bacterioplankton turnover in the NAS. Where data exist, phages seem to play an important role in constraining cyanobacterial and heterotrophic bacterioplankton abundance. The ratio of viral‐like particles to bacteria was higher in the NAS, however, which may explain the higher turnover rate of heterotrophic bacteria. In the NAS, bacterial abundance has been in decline since the early 2000s, associated with declining nutrient loading, while changes in CB have not been documented. Projected climate‐driven changes in riverine nutrient loading and increases in the duration of the growing season are likely to increase contrasts observed in the microbial ecology of the two systems.
