Satellite bio-optical and altimeter comparisons of phytoplankton blooms induced by natural and artificial iron addition in the Gulf of Alaska

An iron fertilization experiment conducted during the summer of 2012 dumped over 100 tons of an iron-containing substance into surface waters of a Haida eddy in the eastern North Pacific to stimulate a large phytoplankton bloom. Announced as a privately funded ocean fertilization effort to increase salmon returns, it attracted considerable press coverage, caused much public controversy and has been widely denounced by the science community. Here, we use available satellite bio-optical measurements from the MODIS/Aqua instrument and AVISO altimeter dynamic height data to examine the timing, magnitude and extent of this artificial iron fertilization experiment, comparing it with natural fertilization events such as volcanic ash deposition and mesoscale eddy transport in the Gulf of Alaska. With respect to other Haida eddies over the past 10 years, this event produced the highest chlorophyll concentrations observed late (> 400 days) in eddy life history, produced the strongest bloom ever observed in an eddy in late-summer and induced this late-summer bloom earlier in the season than any other eddy. With respect to the local area of the fertilization, this experiment induced the most intensive phytoplankton bloom of the past 10 years, ~ 2 × stronger than that caused by Kasatochi volcano in 2008, ~ 5 × that typically observed in the region, including any induced by passing eddies of previous years. Due to its limited spatial and temporal scales, however, estimated total annual carbon drawdown over the Gulf of Alaska by this experiment is one order of magnitude smaller than the Kasatochi volcano and mesoscale Haida eddies. The target eddy followed a path typical of previous Haida eddies, but with relatively weak dynamic height and rotational circulation. Satellite-based calculations also suggest that only a small fraction of the dumped iron might have been taken up by phytoplankton. The extent to which this localized experiment may impact higher trophic levels such as salmon remains uncertain.