The Effects of Fe on Plankton in HNLC Regions of the World’s Oceans

Ecosystem-scale productivity in many marine environments has been demonstrated over the last 15 years to be regulated by the biological availability of iron (Fe). Increasingly, more attention has been focused on coastal upwelling regions, like those off the coasts of California and Peru, since these areas are responsible for a disproportionately high contribution to new global production. A mosaic of nutrient gradients exists in upwelling regions, making them ideal sites for studies of nutrient affects on aquatic microbial communities. Paradoxically, both the California and Peruvian upwelling regions have also been shown to be Fe-limited. The present studies took place in the Peruvian upwelling region to demonstrate that alterations in Fe concentrations influence both the productivity and community structure of natural populations of plankton, and that these alterations have significant implications for biogeochemical cycling and the ‘biological carbon pump’.

The work within this dissertation has demonstrated that phytoplankton in the Peruvian upwelling region are limited by Fe. More importantly, this work has shown that phytoplankton community structure is influenced by Fe concentration. Because dissolved organic matter produced by phytoplankton is used by bacteria and correlations have been drawn between bacterial diversity and ability to consume diverse carbon substrates, the effects of iron concentration on bacterial diversity was also assessed. I have examined how the bacterial community responds to changes in iron concentration using both a natural iron gradient (a nearshore to offshore marine transect over the coastal shelf) and controlled shipboard experiments. I have determined that iron additions in this area increased bacterial diversity in on-deck bottle incubations. Samples collected in situ further demonstrated that iron and other factors cause changes in bacterial diversity, which highlights the complexity of interactions between bacteria and the physical characteristics of the upwelling region. These results are the first to show that bacterial diversity in marine systems is influenced by iron availability.