Variation in particulate C and N isotope composition following iron fertilization in two successive phytoplankton communities in the Southern Ocean

Abstract

Surface δ15NPON increased 3.92 ± 0.48‰ over the course of 20 days following additions of iron (Fe) to an eddy in close proximity to the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. The change in δ15NPON was associated with an increase in the >20 μm size fraction, leading to a maximal difference of 6.23‰ between the >20 μm and <20 μm size fractions. Surface δ13CPOC increased 1.18 ± 0.31‰ over the same period. After a decrease in particulate organic matter in the surface layer, a second phytoplankton community developed that accumulated less biomass, had a slower growth rate and was characterized by an offset of 1.56‰ in δ13CPOC relative to the first community. During growth of the second community, surface δ13CPOC further increased 0.83 ± 0.13‰. Here we speculate on ways that carboxylation, nitrogen assimilation, substrate pool enrichment and community composition may have contributed to the gradual increase in δ13CPOC associated with phytoplankton biomass accumulation, as well as the systematic offset in δ13CPOC between the two phytoplankton communities.