Nitrate uptake, nitrite release and uptake, and new production estimates

Abstract

Excretion of nitrite during nitrate assimilation by phytoplankton is examined in relation to estimates of new production. Evidence from the literature indicates that nitrite excretion is exhibited by a wide variety of phytoplankton species in cultures and can represent up to 25, 45 and 50% of nitrate uptake in Thalassiosira pseudonana, Prorocentrum minimum and Skeletonema costa turn respectively, at irradiance levels which are typical of the primary nitrite maximum in oligotrophic oceans. In natural populations, nitrite excretion can represent up to 60% of nitrate uptake in surface waters of upwelling areas. Conversely, nitrite uptake can equal or exceed nitrate uptake in several species of marine phytoplankton in cultures as well as in natural populations. The first phenomenon can lead to large variations of the net/gross new production 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 concurrently with nitrate uptake are more difficult to assess, but the overall result is that f ratios in stratified oceanic areas are probably underestimated by the 15N tracer technique.