Impact of nanoflagellate bacterivory on benthic bacterial production in the North Sea

Abstract

In situ grazing of benthic heterotrophic nanoflagellates on bacteria was studied in a wide range of sediment types in the North Sea during summer and winter. Grazing rates were measured using the fluorescently labelled bacteria (FLB) technique. Several factors may potentially influence flagellate grazing, viz. temperature, sediment grain size, bacterial abundance and production, flagellate abundance and biovolume. Flagellate grazing rates were related to these variables. Total grazing of the benthic flagellate population was estimated by multiplying flagellate abundance by maximum/minimum grazing rates. A comparison with the total amount of bacterial cells produced provided an estimate of the impact of flagellate bacterivory on benthic bacterial production.

Individual grazing rates ranged from 0 to 112 bacteria per flagellate per 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 39 bacteria per flagellate per hour. Minimum average values varied between 1 and 5 bacteria per flagellate per hour.

The percentages of benthic bacterial production accounted for by flagellate consumption differ greatly depending on the grazing rate used. Using maximum grazing rates, 2 to 23% of the bacterial production during summer was consumed, while 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, higher percentages of the bacterial production were consumed, which was probably due to the relatively lower bacterial production in winter compared to summer.