The importance of dissolved organic nutrients in the biogeochemistry of oligotrophic gyres

Abstract

The aim of this thesis is to contribute to the observational database in order to address fundamental questions as to how dissolved organic nutrients influence N and P budgets, how they affect nutrient cycling and the sustainment of biological production within two major ocean oligotrophic gyres: the Southern Indian Ocean gyre and the subtropical North Atlantic gyre.
A transect across the Indian ocean at 32°S conducted in March/April 2002 was sampled for dissolved oxygen, inorganic and organic nutrients and phytoplankton pigments concentrations. A second cruise was undertaken in April/May 2004 across the oligotrophic North Atlantic Ocean at 24oN. A similar set of samples were collected on the second cruise and in addition the stable nitrogen isotopic signature of particulate organic matter was determined plus surface enzymatic activity and primary production incubation measurements.
The Indian ocean basin is characterized by low N:P ratios both in the inorganic and organic fractions with respect to phytoplankton nutrient requirements driven by an excess of denitrification over N2 fixation. The strongly non-Redfieldian TON:TOP ratio suggests a decoupling of TON and TOP remineralization processes and indicates a severe TON deficiency brought by the strong nitrate limitation that leads to a community demand for TON. TON and TOP Ekman meridional advection played a small but not trivial role in providing N and P into the gyre to support export production. It is estimated that the contribution of TON advection is of the same order as the new N supplied by N2 fixation into the gyre. In the Indian Ocean 40% of the net N transported by the overturning circulation across the boundary at 32°S, is attributable to organic nutrients. To close the N budget assuming the upper range estimates of denitrification to be correct, the inclusion of organic nutrient transport by the ITF is required. Hence, the Indian Ocean N budget conforms to a steady state where excess of denitrification over N2 fixation is compensated for by atmospheric, riverine and a hypothesized flux of DON in through the ITF.
In the permanently stratified North Atlantic subtropical gyre, nutrient supply pathways for the growth of phytoplankton are unclear and appear inadequate for the maintenance of the observed export production. Here the significance of two mechanisms, N2 fixation and the bioavailability of organic nutrients, has been investigated. The computation of a new geochemical proxy, TNex, for the determination of N to P anomalies suggests that N2 fixation is more important than previously thought. The spatial distribution of the isotopic composition of particulate organic matter shows a N2 fixation signal across most of the basin with the exception of the coastal margins and a central region. Thus no latitudinal gradient in diazotrophy, as suggested by other workers, is apparent. TON and TOP represented not only the major component of the upper ocean N and P pools but were also bioavailable to the community through the release of extracellular enzymes. The cycling of TON and TOP was decoupled, with higher turnover rates of the TOP pool as compared to the TON pool. Both TON and TOP could potentially sustain a significant fraction of primary production and TOP could potentially provide nearly all the P needed by N2 fixers. In conclusion, in the permanently stratified North Atlantic subtropical gyre, N2 fixation and the bioavailability of organic N and P appear to be major mechanisms for supplying nutrients and sustaining phytoplankton growth.

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