A.-J. Cavagna

Significant mixed layer nitrification in a natural iron‐fertilized bloom of the Southern Ocean

Nitrification, the microbially mediated oxidation of ammonium into nitrate, is generally expected to be low in the Southern Ocean mixed layer. This paradigm assumes that nitrate is mainly provided through vertical mixing and assimilated during the vegetative season, supporting the concept that nitrate uptake is equivalent to the new primary production (i.e., primary production which is potentially available for export). Here we show that nitrification is significant (~40-80% of the seasonal nitrate uptake) in the naturally iron-fertilized bloom over the southeast Kerguelen Plateau. Hence, a large fraction of the nitrate-based primary production is regenerated, instead of being exported. It appears that nitrate assimilation (light dependent) and nitrification (partly light inhibited) are spatially separated between the upper and lower parts, respectively, of the deep surface mixed layers. These deep mixed layers, extending well below the euphotic layer, allow nitrifiers to compete with phytoplankton for the assimilation of ammonium. The high contributions of nitrification to nitrate uptake are in agreement with both low export efficiency (i.e., the percentage of primary production that is exported) and low seasonal nitrate drawdown despite high nitrate assimilation.

Isotopic model of oceanic silicon cycling: The Kerguelen Plateau case study

A box model is presented describing the time evolution for the three stable Si isotopes (or total concentration and natural isotopic compositions), both in the dissolved and biogenic pools. Temporal variations are controlled by uptake, dissolution (both with isotopic fractionation), settling/export and mixing/advection (without isotopic fractionation). The basic building blocks of the model are combined to form a setup for the Kerguelen Plateau where distinct ‘‘plateau’’ and ‘‘out-plateau’’ areas exist and where measurements were made at the end of the growth season (early 2005, KEOPS cruise: Kerguelen Ocean and Plateau compared Study). In addition, we distinguished between surface (0–100 m) and subsurface (100–400 m) water. This resulted in a model composed of eight compartments, each containing three variables (the three Si isotopes) whose time evolution can be modelled. The model does not assume steady state, and can therefore be used to simulate transient events like blooms. We applied the model to simulate the 2004–2005 growth season. The model parameterisations were kept as simple as possible. Still, the KEOPS measurements were satisfactorily reproduced and estimates of instantaneous and seasonally integrated fluxes compared well with previous literature. & 2012 Elsevier Ltd. All rights reserved.