to dN/dt:(^) in which C = cells mol−1 TdR. A relatively clean example (Kirchman et al. 1982; Box Fig. 5.1.1) of
this relationship comes from a study of bacteria in salt-marsh water. Several other more empirical
formulations are also in use. A general grand mean value of marine estimates for C is 2 × 10^18 cells
mol−1 TdR (Ducklow & Carlson 1992), with a range of 1 to 4 × 10^18.
Box Fig. 5.1.1 Progressively increasing rates of tritiated-thymidine incorporation
by bacteria in a seawater sample (actually from Great Sippewissett Marsh,
Massachusetts) after dilution. Each point on the graph represents a relatively
short-term uptake determination. Points A and B are from replicate experiments.
Over the same interval, bacterial abundance increased from 1.6 × 10^5 to 30 × 10^5
ml−1.
(^) (After Kirchman et al. 1982.)
An interesting “field experiment” confirms a proportionality between tritiated
thymidine (TdR)-uptake (a measure of DNA synthesis) and bacterial abundance. In
May 1988, an expedition to the Gulf of Alaska arrived after the passage of a vigorous
storm, which deeply mixed the water column, reducing surface bacterial stocks to less
than half their usual level. Kirchman (1992) initiated a time-series of bacterial counts
and TdR-incorporation measures lasting 21 days (Fig. 5.6). The change in bacterial
abundance was not exponential, presumably because growth was partly balanced by
grazing. However, TdR-uptake was proportional to stock numbers (and thus, to
increase) for fully 15 days. This implies that at least the new bacteria added to the
post-storm residual were all actively dividing and that during this time the bacterial
production rate exceeded the grazing rate of bacterivores.
Fig. 5.6 Time-series of (a) near-surface bacterial counts, and (b) two bacterial