Reply by John Marra

Abstract

7Adding Eq. (1) and Eq. (2) we get G + DB = g (k-g) -1 B o (e (k-g)t -1) + B o (e (k-g)t -1) Eq. (3) Rearranging the right side of Eq. (3) we get k (k-g) -1 B o {e (k-g)t -1} = PP Eq. (4) Eq. (4) describes the primary production (i.e., net primary production according to the terminology of Williams, 1993). For more about the development of this equation, see Moigis (2000).According to Eq. (5), we then have the following relationship PP = G + DB Eq. (5) that shows an observed accumulation of the biomass of phytoplankton (DB) is only a fraction of the primary production. Eq. (5) shows that if G is not taken into consideration, we will obtain an underestimated, and thus wrong, estimate of PP. Hence, the statements given by Karl Banse (in KB02 and reference therein) that the grazing of zooplankton should be taken into consideration when predicting the biomass of phytoplankton should be seen as absolutely valid. JM03 had only correlated DB (DPOC) with his reported 14 C assimilation values. The conclusion that can be drawn from the above is that either JM03´s data should be seen as evidence that the 14 C method gives underestimated PP values, or that in his area of study there was no grazing by zooplankton. JM03 did not comment about grazing studies, but it can be assumed that in any natural waters there is always grazing (as pointed out by Karl Banse).Perhaps the discussions in the 1980's about the validity of the 14 C method, especially in open waters, should be reactivated. Otherwise, the situation that could occur is that productivity studies which have even been made with the clean radiocarbon method in open waters for the last two decades could be in error. , and had the opportunity also to estimate zooplankton grazing from microand mesozooplankton. Mesozooplankton removed between 1-5% of phytoplankton standing stock per day (Cowles et al., 1995). Mesozooplankton are of course unlikely to be included in the sample size typical of primary production incubations. For microzooplankton, the conclusion was that grazing was important, but only during mid-and late-stages of the bloom (Gifford et al., 1995). The activity of microzooplankton in incubations in bottles of 100-300 ml volume is not clear. As I indicated in JM03, grazing can't be assumed important at all places and times. My second point is related to the first. In Figure 1, I have plotted some other of the data from the North Atlantic Bloom Experiment, from Chipman et al. (1993). Along the x-axis is the daytime drawdown of ⌺CO 2 in the mixed layer, observed as the difference between dawn and dusk values from bottle cast samples, and expressed as an areal estimate of primary production. Daytime carbon assimilation based on 14 C uptake in incubation bottles suspended in situ, is plotted on the y-axis. Changes in ⌺CO 2 are probably the most direct estimate of net production we can make and include any heterotrophic Table 1. REFERENCESCitations showing that 14 C uptake closest to net production: processes. The sensitivity for the change in ⌺CO 2 is low; there ar...