Surface active substances in the upper water column during a Southern Ocean Iron Fertilization Experiment (EIFEX)

Surface active substances (SAS) in the water column were measured by voltammetry using the electrochemical probe o-nitrophenol (ONP) during EIFEX, a mesoscale open ocean iron enrichment experiment in the Southern Ocean. SAS levels were low throughout the experiment (<0.005 - 0.03 mg L^-1 Triton X-100 equivalents). Initially SAS was extremely low in the photic zone, but as the phytoplankton bloom developed concentrations markedly increased throughout the upper 100 m (∼0.02 mg L^-1 Triton X-100 equivalents). Highest concentrations of SAS (>0.02 mg L^-1 Triton X-100 equivalents) were found at the end of the bloom particularly at density discontinuities where organic material may accumulate. Exudates from diatoms appeared to be the major source of SAS during EIFEX, either from direct extracellular release or in the action of being grazed upon by zooplankton.