Bacterioplankton dynamics in the McMurdo Dry Valley lakes, Antarctica: Production and biomass loss over four seasons

Research of the microbial ecology of McMurdo Dry Valley lakes has concentra ted primarily on phototrophs; relatively little is known about the heterotr ophic bacterioplankton. Bacteria represent a substantial proportion of wate r column biomass in these lakes, comprising 30 to 60% of total microplankto n biomass. Bacterial production and cell numbers were measured 3 to 5 times , within four Antarctic seasons (October to January), in Lakes Fryxell, Hoa re, and Bonney. The winter-spring transition (September to October) was inc luded during one year. Lake Fryxell was the most productive, but variable, lake, followed by Lakes Bonney and Hoare. Bacterial production ranged from 0 to 0.009 mu g C ml(-1) d(-1); bacterial populations ranged from 3.2 x 10( 4) to 4.4 x 10(7) cells ml(-1). Bacterial production was always greatest ju st below the ice cover at the beginning of the season. A second maximum dev eloped just above the chemocline of all the lakes, as the season progressed . Total bacterioplankton biomass in the lakes decreased as much as 88% betw een successive sampling dates in the summer, as evidenced by areal integrat ion of bacterial populations; the largest decreases in biomass typically oc curred in mid-December. A forward difference model of bacterial loss in the trophogenic zone and the entire water column of these lakes showed that lo ss rates in the summer reached 6.3 x 10(14) cells m(-2) d(-1) and 4.16 x 10 (12) cells m(-2) d(-1), respectively. These results imply that bacteria may be a source of carbon to higher trophic levels in these lakes, through gra zing.