Activity of heterotrophic bacteria in the euphotic zone of the Celtic Sea

Abstract

As part of a study to determine the importance of picophytoplankton, we have used [methylL3~] thymidine incorporahon as an indicator of heterotrophic bacterial production in the Celtic Sea. Over a 4 yr period, a number of measurements were made on the same day that picophytoplankton production was measured by in situ incubations with 14C. 3H-thymidine incorporation rates are not closely coupled to primary production estimates of either total phytoplankton or picoplankton. Vertical profiles of the 2 processes are very different and there is no significant correlation between 3 ~thymidine uptake rate and primary production at any depth or when the rates are integrated to the base of the euphotic zone. Certain correlations were found: bacterial number u l t h chlorophyll a, and 3Hthymidine incorporation with bacterial number. However, the most significant correlations are between bacterial production and temperature; temperature explains 62 % of the variance in bacterial number and 35 % of the variance in 3 ~-t h y m ~d i n e incorporation of samples taken a t all depths throughout the euphotic zone. When the data are integrated throughout the euphotic zone, temperature explains 4 1 O/O of the variance in depth-integrated 3 ~-t h y n ~i d l n e incorporation rate. Using recently published factors for converting 3H-thymidine incorporation rates into estimates of bacterial production and data obtained in all seasons, we estimate that the production of heterotrophic bacteria is usually less than 10 %, but occasionally u p to 18 %, of the picoplankton production and is less than 6 % of the total phytoplankton production. The use of other conversion factors may result in higher estimates of bacterial production, but some published factors are shown to b e physiologically unreasonable.