NITRATE UPTAKE, NITRITE RELEASE AND UPTAKE, AND NEW PRODUCTION ESTIMATES

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.