Biological Oceanography

(^) The ^14 C method has been used all over the world since 1952. There is a large mass
of data, and much of what we know about the vertical and geographical distribution of
primary productivity comes from it. However, it has always been recognized that the
method has some uncertainties associated with it, and during the 1980s even the
general levels measured were questioned. The recognized problems fall into two
classes:
(^1) Errors of the method itself, that is, procedural problems; and
2 Comparison of the results to field measures of variables that imply values
different from those of the ^14 C technique.
(^) The simple outcome of the substantial reassessment, which started in about 1978
(Peterson 1980), is that some of the classical (pre-1980) ^14 C-uptake values were too
low. Fitzwater et al. (1982) introduced an elaborate cleaning of the bicarbonate
inocula with ion-exchange medium (Chelex®), incubation in polycarbonate bottles,
and maintenance of high cleanliness with respect to trace metals, particularly copper
and zinc, in all aspects of the work. Rates compared to the standard inocula in glass
bottles were two to eight times higher. The implication is that trace-metal
contamination can poison phytoplankton, particularly in oceanic habitats. Trace-metal
concentrations well above habitat levels were contaminants of early ^14 C-bicarbonate
used as tracer in productivity studies. The results from newer clean techniques are
consonant with demonstrations of substantial sensitivity of phytoplankton to trace-
metal poisoning. Methods closely akin to that of Fitzwater et al. (1982) have become
standard. On the whole, new numbers are higher than old. For example, a subarctic
Pacific study by Welschmeyer (1993) (Fig. 3.4) shows that clean methods in use after
1980 give higher results than those conventional in the 1960s and 1970s. The change
for this comparison is almost a factor of two. In even more oligotrophic habitats, such
as the Pacific central gyres, the increase is greater.
Fig. 3.4 Depth-integrated primary productivity in the subarctic Pacific compared to a
generalized curve of PAR (“photosynthetically active radiation”). Filled circles are
modern ^14 C-uptake data from a trace-metal clean technique (1984–1988); open circles
are data collected before 1980 with the then-standard ^14 C-uptake method.
(^) (After Welschmeyer 1993.)