AME 38:269-282 (2005)  -  doi:10.3354/ame038269

Phytoplankton community structure changes following simulated upwelled iron inputs in the Peru upwelling region

Clinton E. Hare1, Giacomo R. DiTullio2, Charles G. Trick3, Steven W. Wilhelm4Kenneth W. Bruland5, Eden L. Rue5, David A. Hutchins1,*

1College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, Delaware 19958, USA
2Hollings Marine Laboratory, University of Charleston, 205 Fort Johnson, Charleston, South Carolina 29412, USA
3Department of Biology, University of Western Ontario, Biological and Geological Sciences Building, London, Ontario N6A 5B7, Canada
4Department of Microbiology, University of Tennessee, 1414 West Cumberland, Knoxville, Tennessee 37996, USA
5Institute of Marine Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
*Corresponding author. Email:

ABSTRACT: The effects of iron on phytoplankton community structure in ‘High Nutrient Low Chlorophyll’ regions of the ocean have been examined using both shipboard batch cultures (growouts) and open ocean mesoscale fertilization experiments. The addition of iron in these areas frequently results in a shift from communities dominated by small non-siliceous species towards ones dominated by larger diatoms. We used a new shipboard continuous culture experimental design in iron-limited Peru upwelling waters to examine shifts in phytoplankton structure and their biogeochemical consequences following simulated upwelled iron inputs. By allowing the added iron to pre-equilibrate with natural seawater ligands, we were able to supply iron in realistic chemical species at rates and concentrations similar to those found in upwelled waters off Peru. The community shifted strongly from cyanobacteria towards diatoms, and the extent of this shift was proportional to the increase in iron supply. Eukaryotic nanophytoplankton were the first to respond to the iron addition, followed by a community dominated by small pennate diatoms by Day 5. These community changes led to increased biogenic silica:particulate organic nitrogen (BSi:PON) and biogenic silica:particulate organic carbon (BSi:POC) production ratios, driven mainly by increases in diatom numbers with increasing iron. Our experiment demonstrated both similarities to and differences with parallel growout experiments and previous mesoscale fertilization experiments, and suggest that the shipboard continuous culture method can be applied to questions that cannot be easily addressed by either of these previous iron addition techniques.

KEY WORDS: Shipboard chemostat · Continuous culture · Phytoplankton · Community composition · Peru upwelling · Iron limitation

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