Iron-light interactions during the CROZet natural iron bloom and EXport experiment (CROZEX): II-Taxonomic responses and elemental stoichiometry

The CROZet natural iron bloom and EXport experiment (CROZEX) investigated the annual phytoplankton bloom that occurs in the vicinity of the Crozet plateau in the Polar Frontal Zone (PFZ) of the Southern Ocean. Shipboard manipulation experiments designed to investigate potential responses of phytoplankton community structure and elemental stoichiometry to iron (Fe) and/or light perturbation are compared to in situ data collected during CROZEX. The outcome of individual experiments was strongly influenced by initial phytoplankton community structure. For example Fe amendment of high (>8 μM) silicic acid waters resulted in a strong response by medium-sized diatoms, including Eucampia antarctica. In contrast Phaeocystis antarctica dominated the community response to increased Fe within an experiment initiated during the early declining stage of the bloom. Conversely, small diatoms responded more strongly to increased irradiance, while the population of very large diatoms remained relatively static. Consistent with experimental results, the intense natural blooms north of the Crozet plateau were observed to be dominated by either P. antarctica or medium-sized diatoms while small phytoplankton and large diatoms dominated lower-chlorophyll waters in the south. In situ data and results from experiments supported previous observations of lower nitrate to phosphate removal ratios for diatoms compared to P. antarctica. Experimental and in situ data also supported previous work showing that silicic acid to nitrate removal ratios can be reduced under conditions of enhanced Fe availability. Higher irradiance decreased this ratio still further. Interactions between Fe and light were thus likely to have contributed to the observed decoupling of the major element cycles between the intense bloom that occurs north of Crozet and more typical PFZ conditions to the South. Specifically, silicic acid became exhausted in both systems, with drawdown in the south potentially resulting from co-limitation of diatom growth by Fe and light. In contrast, increased Fe supply in the north enhanced nitrogen, phosphorous and carbon drawdown in ratios that were dependent on the community composition of the resultant bloom.