New production and the f ratio around the Crozet Plateau in austral summer 2004–2005 diagnosed from seasonal changes in inorganic nutrient levels

Abstract

Recent mesoscale iron-fertilisation experiments suggest that iron may be an important micronutrient in HNLC regions but estimates of carbon export from such experiments are inconclusive. An alternative strategy to estimate export from such environments is to observe naturally productive ecosystems associated with topography. One such system is the Crozet islands and associated plateau (Crozet), at 46°S, 52°E. Each year a large bloom of phytoplankton occurs to the north of Crozet with a reduced bloom occurring to the south. We use nitrate data from the Crozet region collected during austral summer 2004–2005 to estimate new production (NP) via the Redfield ratio. Peak integrated values of up to 50 g C m−2 to the north of the plateau and up to 15 g C m−2 to the south are inferred. We estimate total integrated primary production (TP) using satellite techniques and calculate f for each station. Overall NP is linearly related to TP. However, f declines at very high levels of TP because nitrate usage ceases despite continuing PP and because nitrate levels increased from their postbloom low. This results either from a resupply of nitrate from beneath the thermocline due to mixing processes or to the mixed-layer ammonification and nitrification of accumulated organic nitrogen. We discount the first possibility because our estimates of the mixing flux of nitrate appear to be inadequate to cause the entire recovery in nitrate levels, and because any mixing flux of nitrate would likely be accompanied by a resupply of iron, which would induce NP to occur and erode the resupply of nitrate. Instead we consider the recycling of accumulated organic nitrogen to be a more likely explanation based on our observation of high organic nitrogen levels in the mixed layer north of Crozet during the cruise. The implications of this conclusion are that euphotic zone nitrification is a significant process, and that in this system new and export production are not equivalent. This recycling is sufficiently large that it reduces our estimate of NP north of the plateau to a level where it is equivalent to NP in the south. Whether a similar refertilisation of the mixed layer occurred in the south of the study region, which would be consistent with a meridional gradient in carbon export, is unknown due to the limited duration of the shipboard programme.