Nutrient limitation of phytoplankton growth in the central equatorial Pacific Ocean

Abstract

Phytoplankton play a key role in removing carbon dioxide from Earth’s atmosphere by ultimately exporting organic carbon to the deep ocean through the process known as the biological pump. Larger phytoplankton sink more rapidly than smaller, resulting in more carbon export as they are less likely to be fully remineralized. However, phytoplankton growth is highly dependent on the supply of key nutrients. In this study, I examined the effects of limiting nitrate, silicate, and iron on phytoplankton growth. To achieve this, I incubated samples from 4° S and 0° along the 167° W meridian for 96 hours in different nutrient treatments. I filtered subsamples of each incubation through a size-fractionated manifold every 24 hours and measured the chlorophyll α concentrations in three different size fractions to determine growth rates. Growth rates were not statistically different between treatments across all size fractions at both 4° S and the equator, but growth rates did statistically differ between 4° S and the equator in most treatments. Nutrient analysis showed an N:P usage of 15.0 ± 0.93:1 at 4° S and 9.09 ± 1.34:1 at the equator, and an Si:P usage at 4° S of 6.2 ± 2.19:1 over the control and nitrate-depleted treatments and 1.74 ± 0.10:1 over the two silicate-depleted treatments; at the equator, these ratios were 8.89 ± 0.80:1 over the control, nitrate-depleted, and iron-depleted treatments and 3.81 ± 0.78:1 over the two silicate-depleted treatments. The results of this study suggest that controls on phytoplankton growth are more varied and complex than just nutrient availability, and more work is necessary to determine what actually limits growth in this area.