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

Surface δ ¹⁵N _PON 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 δ ¹⁵N _PON was associated with an increase in the >20 μm size fraction, leading to a maximal difference of 6.23‰ between the >20 m and <20 m size fractions. Surface δ ¹³C _POC 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 δ ¹³C _POC relative to the first community. During growth of the second community, surface δ ¹³C _POC 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 δ ¹³C _POC associated with phytoplankton biomass accumulation, as well as the systematic offset in δ ¹³C _POC between the two phytoplankton communities.