Short-term variability of heterotrophic bacterioplankton during upwelling off the NW Iberian margin

Abstract

Short-term variability of heterotrophic bacterioplankton was studied in a recently upwelled water mass at the NW Iberian margin (August 1998). Bacterioplankton abundance (BA), biomass (BB), production (BP), and specific production (SBP) were monitored during two Lagrangian drift experiments, one along the shelf-edge, the other off-shelf along an upwelling filament. Other measurements included chlorophyll a (Chl a), primary production (PP), suspended particulate organic carbon (POC) and nitrogen (PON), and dissolved organic carbon (DOC) and nitrogen (DON). Although primary production was significantly higher during the shelf-edge drift experiment, bacterial biomass in the euphotic zone (2.68 to 22.20μgC.l −1) was not significantly different from that in the offshore filament. In contrast, bacterial production (0.13-3.52μgC.l −1.d −1), estimated using an empirically determined 14C-leucine to carbon conversion factor, and bacterial growth rates (doubling time, DT: 3.9–29.7d), were significantly higher during the shelf-edge drift (BP: 1.50±0.11 versus 0.50±0.02μgC.l −1.d −1; DT: 6.9±0.3 versus 16.2±0.9 d; p<0.01). Depth-integrated BB over the euphotic zone comprised 15±1% of phytoplankton biomass during shelf-edge drift and 39±4% under the more oligotrophic conditions in the filament. However, daily BP to net primary production ratios were not significantly different in the two regions (6±1% versus 7±1%). BA, BB, BP and SBP were enhanced in the later part of the shelf-edge drift following a pronounced increase in both PP and gross DOC production, suggesting that phytoplankton was a source of substrates for bacteria in recently upwelled waters. This contrasted with the filament drift in which short-term variability of bacterioplankton was much less pronounced and there was no correlation between BP and PP. In both regions, SBP and DOC in the euphotic zone were significantly correlated (p<0.005) indicating some regulatory effect of DOC over bacterial activity. Bacterial carbon demand in the euphotic zone ranged from 22.5 to 44.0μmolC.m −3.h −1, and 10.6 to 11.3μmolC.m −3.h −1 and represented 62% and 43% of overall gross DOC production during shelf-edge and filament drift experiments. During the filament drift, the relatively high bacterial doubling times coupled with high percentage of non-metabolised DOC (ca. 60%) suggested control of bacteria by either bioavailability of DOM or inorganic nutrients. Between 40 and 60% of gross DOC production was not metabolised by heterotrophic bacteria, thus allowing DOC net accumulation and off-shore advection.