Another issue for ^14 C measurements of primary production is whether the
calculated rates represent gross or net primary production (Marra 2009). In short
incubations with little respiratory loss, the ^14 C technique estimates gross primary
production. In longer incubation, some or much of the ^14 C fixed and respired is
refixed, so that the measurement is greater than net production, but less than gross
production.
(^) An array of studies showed that the microherbivores (protozoa, crustacean nauplii,
rotifers) left in the incubation bottles of a ^14 C-uptake study (because they cannot be
filtered out without loss of phytoplankton) contribute a substantial fraction of grazing
in the field, and they continue to graze during an incubation. Their contribution varies,
but they can be eating one-third, even most, of the new photosynthate. Of course, at
first they are just grazing on the phytoplankton that have no ^14 C incorporated. As the
experiment runs, their impact gets greater and greater, making short incubations
preferable to long ones. This causes workers to use very high ^14 C activity in their
measurements. Landry and Hassett (1982) developed a dilution technique to account
for microzooplankton grazing during primary-production studies, and we will
consider it when we deal with microzooplankton grazing.
(^) Bender et al. (1999) compared gross O 2 production rates to (^14) C production in
several ocean regions. Gross O 2 production tends to be about two to four times higher
than net ^14 C production. The ^14 C estimates of primary production can be low if
respiratory losses of ^14 CO 2 and release of DO^14 C are not considered. Gross oxygen
production rates will also exceed ^14 C estimates when electrons are diverted from
photosystem II (PSII) and used for reduction of oxygen rather than being moved to
photosystem I (PSI) for synthesis of ATP (Fig. 3.1b). High levels of such oxygen
reduction are common in oligotrophic waters with low nutrient availability and high