IMPACT OF NANOFLAGELLATE BACTERIVORY ON BENTHIC BACTERIAL PRODUCTION IN THE NORTH-SEA

In situ grazing of benthic heterotrophic nanoflagellates on bacteria w as studied in a wide range of sediment types in the North Sea during s ummer and winter. Grazing rates were measured using the fluorescently labelled bacteria (FLB) technique. Several factors may potentially inf luence flagellate grazing, viz. temperature, sediment grain size, bact erial abundance and production, flagellate abundance and biovolume. Fl agellate grazing rates were related to these variables. Total grazing of the benthic flagellate population was estimated by multiplying flag ellate abundance by maximum/minimum grazing rates. A comparison with t he total amount of bacterial cells produced provided an estimate of th e impact of flagellate bacterivory on benthic bacterial production. In dividual grazing rates ranged from 0 to 112 bacteria per flagellate pe r hour. Grazing rates showed no significant differences between summer and winter and did not correlate significantly with any of the above- mentioned variables. Maximum average grazing rates ranged from 15 to 3 9 bacteria per flagellate per hour. Minimum average values varied betw een 1 and 5 bacteria per flagellate per hour. The percentages of benth ic bacterial production accounted for by flagellate consumption differ greatly depending on the grazing rate used. Using maximum grazing rat es, 2 to 23% of the bacterial production during summer was consumed, w hile in winter this ranged from 23 to 528%. Minimum estimates were 10 to 20 times lower: 0.2 to 3% in summer and 1 to 50% in winter. In wint er, higher percentages of the bacterial production were consumed, whic h was probably due to the relatively lower bacterial production in win ter compared to summer.