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

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Lucas, M.I., Seeyave, S., Sanders, R., Moore, C.M., Williamson, R., Stinchcombe, M.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2007
• Subjects: Antarctic; Austral; Antarc*; Crozet Islands
• Online Access: http://nora.nerc.ac.uk/id/eprint/149515/; https://doi.org/10.1016/j.dsr2.2007.06.017

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 (ρN) experiments using 15N-NO3−, NH4+ and urea addressed three major goals. Firstly, measurements of NO3− uptake (ρNO3−) aimed to establish whether ρNO3− responds to natural Fe fertilisation. Secondly, we compared regional ρNO3− rates, hypothesising that in Fe-fertilised regions, ρNO3− should exceed that in HNLC regions. Thirdly, by using satellite imagery, we extrapolated ρNO3− measurements made during a declining bloom to reconstruct seasonal ρNO3− by the spring bloom. Finally, we estimated the ‘new’ Fe demand required to support ρNO3−, 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 ρN was elevated north of the islands (400 μmol m−2 d−1) relative to south of the islands (250 μmol m−2 d−1). Nitrate uptake showed a clear response to Fe fertilisation, exhibiting a strong north (198 μmol m−2 d−1) to south (74 μmol m−2 d−1) gradient, while neither ρNH4+ nor ρurea 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 ρNO3− 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 ...