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

Abstract

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. Copyright 2011 by the American Geophysical Union.