Nitrogen uptake responses to a naturally Fe-fertilised phytoplankton bloom during the 2004/2005 CROZEX study

Abstract

Annual phytoplankton blooms are observed around most sub-Antarctic islands during austral spring and summer, but are absent in the surrounding high-nutrient low-chlorophyll (HNLC) ocean. The CROZEX study (2004/2005) tested the hypothesis that annual blooms occurring immediately north of the Crozet islands in the Polar Frontal Zone (PFZ) develop because of natural iron fertilisation, while to their south in HNLC waters, there is insufficient iron (Fe) to promote blooms. Size-fractionated nitrogen uptake (?N) experiments using 15N-NO3?, NH4+ and urea addressed three major goals. Firstly, measurements of NO3? uptake (?NO3?) aimed to establish whether ?NO3? responds to natural Fe fertilisation. Secondly, we compared regional ?NO3? rates, hypothesising that in Fe-fertilised regions, ?NO3? should exceed that in HNLC regions. Thirdly, by using satellite imagery, we extrapolated ?NO3? measurements made during a declining bloom to reconstruct seasonal ?NO3? by the spring bloom. Finally, we estimated the ‘new’ Fe demand required to support ?NO3?, comparing this with estimated Fe fluxes. Diatoms and colonial Phaeocystis dominated phytoplankton communities north of the islands, while to the south, Phaeocystis was absent. Total ?N was elevated north of the islands (400 ?mol m?2 d?1) relative to south of the islands (250 ?mol m?2 d?1). Nitrate uptake showed a clear response to Fe fertilisation, exhibiting a strong north (198 ?mol m?2 d?1) to south (74 ?mol m?2 d?1) gradient, while neither ?NH4+ nor ?urea showed such significant latitudinal gradients. The N–S integrated f-ratio gradient was 0.47–0.28 while specific N uptake (VNO3 d?1) rates were significantly higher in the Fe-fertilised region relative to those in the southern HNLC region. The potential for NH4+ inhibition of ?NO3? did not appear to be significant. High PON:chl-a ratios combined with relatively low 14C:15N uptake ratios suggested that most phytoplankton were relatively chlorotic and carbon stressed, with the exception of those growing actively within a cyclonic eddy where neither Fe nor light appeared to be limiting. Size-fractionated ?NO3? and f-ratios exhibited a complex response to NH4+ and Fe availability, with f-ratios in the >20-?m fraction being low (0.3) in the HNLC region, but significantly higher (0.7) in a localised diatom-dominated bloom in the northern Fe-fertilised region. In contrast, f-ratios in the <2-?m size class were similar everywhere (0.44), indicative of Fe-limitation for large-celled diatoms in the southern HNLC region. As a result of Fe-regulated ?NO3?, new production showed a N–S gradient of 24 to 15 mmol C m?2 d?1, very similar to carbon export determined from NO3? ‘draw-down’ and from 234Th measurements. The estimated DFe demand required to support seasonal ?NO3? in the northern region, based on conservative cellular Fe:N quotas, required surface (to 100 m) pre spring-bloom DFe concentrations of 0.75 nmol l?1. Our results support the hypothesis that phytoplankton blooms north of the islands are stimulated by natural Fe fertilisation, with a direct impact on ?NO3?, particularly for larger cells, resulting in higher new production rates relative to those from the Fe-limited HNLC region south of the Crozet islands.