Bacterioplankton dynamics in a subtropical estuary: evidence for substrate limitation

Bacterioplankton abundance and metabolic characteristics were measured along a transect in Pensacola Bay, Florida, USA, to examine the factors that control microbial water column processes in this subtropical estuary. The microbial measures included (3)H-L-leucine incorporation, ectoenzyme activity (aminopeptidase, U-D-glucosidase, P-D-glucosidase) and bacterial abundance. Bacterioplankton abundance ranged from 1.8 to 15.3 x 10(9) l(-1) (average: 6.2 x 10(6)); highest abundances occurred during summer months, particularly in the upper estuary. Bacterial secondary production ranged from 20 to 273 mug C l(-1) d(-1) (average 115), aminopeptidase activities ranged from 34 to 356 nmol l(-1) d(-1) (average 165), alpha-D-glucosidase ranged from 0.4 to 61 nmol l(-1) d(-1) (average: 8.3), and beta-D-glucosidase ranged from 1.4 to 53.1 nmol l(-1) d(-1) (average: 10.5). Bacterioplankton exhibited strong seasonality, suggesting that temperature was an important driver of the observed variability. When normalized for bacterial biomass, metabolic rates exhibited a striking inter-annual pattern with lower rates during summer 2000 than 2001. This pattern was consistent with freshwater flows, which were much lower during 2000 than 2001, consequently lowering nutrient and dissolved organic carbon (DOC) supply to the estuary. These results underscore the importance of riverine flux of materials to support bacterial metabolism and suggest that bacterioplankton were substrate-limited during the low flow period. The empirical relationships between bacterioplankton and phytoplankton variables were similar to those found in literature synthesis studies, with the notable exception that bacterial abundances and production were higher than predicted from chlorophyll a concentration. One explanation for this departure is that these relationships are drawn largely from cool temperate environments and warm subtropical systems are underrepresented in the literature.