Nitrate Diffusive Flux Cannot Support New Production During Quiescent Periods at Station ALOHA

John Dore

Abstract

The eddy-diffusive vertical flux of dissolved nitrate from deep waters into the euphotic zone has long been considered to be, in the absence of deep winter mixing, the primary source of "new" nitrogen supporting the exportable fraction of primary production. An analysis of nitracline and mixed layer depths at Station ALOHA during the "quiescent" period from HOT-23 through HOT-41 shows that deep mixing events are conspicuously absent, mixed layers never reaching below about 80 m and nitraclines only rarely rising up to 100 m. However, low-level nitrate measurements indicate the lack of a significant vertical flux of nitrate into the surface (< 100 m) layer, while this layer is responsible for > 90% of total primary production. A simple f-ratio model of this two-layer system reveals that f in the surface layer would have to be unreasonably low and f in the deep (100-200 m) layer unreasonably high to maintain the observed particulate flux from the euphotic zone if nitrate were fueling most of the new production. It is concluded that 4-6% of the surface layer production is "new" and must be supported by other sources of exogenous nitrogen. Speculations are given as to the other sources.