Excretion of nitrite during nitrate assimilation by phytoplankton is e
xamined in relation to estimates of new production. Evidence from the
literature indicates that nitrite excretion is exhibited by a wide var
iety of phytoplankton species in cultures and can represent up to 25,
45 and 50% of nitrate uptake in Thalassiosira pseudonana. Prorocentrum
minimum and Skeletonema costatum respectively, at irradiance levels w
hich are typical of the primary nitrite maximum in oligotrophic oceans
. In natural populations, nitrile excretion can represent up to 60 % o
f nitrate uptake in surface waters of upwelling areas. Conversely, nit
rite uptake can equal or exceed nitrate uptake in several species of m
arine phytoplankton in cultures as well as in natural populations. The
first phenomenon can lead to large Variations of the net/gross new pr
oduction estimates, as well as widely biased estimates of the f ratio.
For example, a nitrite excretion of 40 % of nitrate uptake will lead
to a 33 % underestimate of the f ratio in a range of values typical of
oligotrophic oceans. The effects of nitrite uptake taking place concu
rrently with nitrate uptake are more difficult to assess, but the over
all result is that f ratios in stratified oceanic areas are probably u
nderestimated by the N-15 tracer technique.