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Sarah Fawcett - Research

Collecting seawater aboard the R/V Atlantic Explorer in the Sargasso Sea

1. Phytoplankton diversity and N use in upwelling systems

The ocean’s most productive regions are associated with eastern boundary currents where the upwelling of cold, nutrient-rich waters from depth stimulates phytoplankton blooms. The phytoplankton community in these regions tends to be dominated by diatoms, although the exact mechanism for diatom success has never been explicitly demonstrated. By simulating upwelling events in large mesocosms and monitoring nutrient uptake, biomass concentrations, and phytoplankton community composition over the course of the subsequent bloom, we can investigate the response of diatoms, and indeed the entire phytoplankton community, to upwelling conditions (fig. 1). We find that the biomass was initially dominated by the smallest phytoplankton, but the largest phytoplankton (which, based on light microscopy counts, comprised mainly diatoms) increased most rapidly to dominate biomass, nitrogen assimilation, and carbon fixation after a few days. While all phytoplankton size fractions achieved similar maximum specific nitrate uptake rates (V NO3 ), this occurred most rapidly and was maintained longest by the largest phytoplankton. This strategy appears to be the mechanism by which diatoms exploit upwelling conditions; their initial rapid acceleration and subsequent maintenance of V NO3 allows diatoms to dominate algal biomass in upwelling environments as it gives them access to a disproportionate fraction of the available nitrate pool. Pigment measurements and light microscopy independently documented changing phytoplankton abundance, with diatoms demonstrating a characteristic pattern of succession: Initially, the diatom community was dominated by a few ubiquitous oceanic and coastal species that are known to succeed under low nutrient conditions, and diversity was low. As nitrate became available, diversity increased due to an increase in the abundance of upwelling diatom species that were able to efficiently exploit the new environmental conditions. Once all available nitrate had been consumed, the diatom assemblage was completely dominated by upwelling species, and diversity decreased again.

Aboard the R/V Atlantic Explorer in the Sargasso Sea.

2. N sources to phytoplankton in the subtropical Sargasso Sea

In the vast subtropical ocean, intense surface stratification limits the nitrate supply from below such that regenerated N is assumed to fuel most phytoplankton growth. However, it is notoriously difficult to measure N uptake in such N-poor systems. One approach is to measure the naturally occurring ratio of nitrogen isotopes (quantified as δ15N ) in marine particles, with high particle δ15N implying phytoplankton growth on deep nitrate and low δ15N indicating dependence on regenerated N sources. Past measurements of bulk particle collections from the Sargasso Sea reveal a low δ15N , which has been interpreted as evidence of a system supported predominantly by regenerated N, in keeping with expectations for such a N-poor region of the ocean. However, bulk particle collections include only phytoplankton, but also heterotrophic organisms and detritus, and thus cannot address potential differences in N sources used by different phytoplankton.


To overcome this problem, we used flow cytometry to separate the important Sargasso Sea phytoplankton populations, not only from one another but also from any non-autotrophic material (fig. 2). Using a high-sensitivity method for nitrogen isotope analysis, we measure the individual δ15N of these sorted populations. For summertime particle collections we find that prokaryotic cyanobacteria (Prochlorococcus and Synechococcus) and eukaryotic phytoplankton have distinct isotope signatures, indicating that they rely on different N sources (fig. 3). The prokaryotes have a uniformly low δ15N, suggesting that they consume mostly regenerated N, and in keeping with expectations for the subtropical ecosystems. However, the more complex (but not much larger) eukaryotic phytoplankton have a high δ15N, implying that they are predominantly supported by deep nitrate; a simple calculation reveals that these eukaryotes rely on nitrate for more than 50% of their N. This finding is most surprising given that nitrate concentrations are undetectably low in Sargasso Sea surface waters during the summer, and also because the clearly different δ15N signatures of prokaryotes and eukaryotes implies that these taxa are somehow physiologically distinct, despite fairly minimal differences in their cell size.


The δ15N of the sorted eukaryotes is similar to both the subsurface nitrate supply and the organic matter sinking out of the surface ocean, suggesting that sinking material derives largely from eukaryotic, not prokaryotic, phytoplankton biomass. This implies that the Sargasso Sea’s biological pump is driven by eukaryotic phytoplankton, despite their being two orders of magnitude less numerically abundant than the prokaryotes. Our data have implications for paleoceanographic studies as they indicate a disconnect between the prokaryote-dominated biomass in the low-latitude surface ocean and the organic matter sinking to the seafloor, which appears to be predominantly of eukaryotic origin.

Further questions:

How do the sources of N to Sargasso Sea phytoplankton change seasonally?

How do stochastic events, such as mesoscale eddies, affect community composition and N sources to phytoplankton in the Sargasso Sea?

How does the elemental composition (specifically N:P) of different Sargasso Sea phytoplankton groups change with season (and thus nitrate supply)?

How does N use by phytoplankton in the Sargasso Sea compare to phytoplankton N use in more productive regions, such as the subpolar North Atlantic?

At the Bermuda Institute of Ocean Sciences.

Department: Geosciences

Bess Ward
(co-advised by Daniel Sigman, Princeton, and Michael Lomas, BIOS)

Position: Graduate Student

Area: Biological Oceanography

Office: M45 Guyot Hall

Phone: 609-258-6294


About Me


Topic 1:

   ■ Figure 1

(a) Drawdown of nitrate (NO3-) and accumulation of phytoplankton biomass over the course of an 8-day mesocosm experiment designed to simulate upwelling conditions. (b) Specific rates of nitrate uptake (VNO3) by the large and small phytoplankton size fractions, as well as the total phytoplankton community, during the mesocosm experiment. All phytoplankton size fractions achieved similar maximum VNO3, but this occurred most rapidly and was maintained longest by the largest phytoplankton, a fraction of the assemblage comprising predominantly diatoms. We conclude that this strategy is the mechanism by which diatoms exploit upwelling conditions: their initial rapid acceleration and subsequent maintenance of V NO3 results in diatom dominance of algal biomass in upwelling environments as it allows them access to a disproportionate fraction of the available nutrient pool.


Topic 2:

   ■ Figure 2

Cytograms representative of the phytoplankton populations sorted from collections at the Bermuda Atlantic Time-series Site (BATS) in July 2008 and 2009. (a) FSC1-H (s-polarized forward scatter; a qualitative proxy for cellular size and complexity) versus 650LP-H (red, chlorophyll) and (b) 650LP-H (red, chlorophyll) versus 580/30-H (orange, phycoerythrin). Phytoplankton populations of interest (Prochlorococcus, Synechococcus, and the pico- and nanoeukaryotes that were grouped together as ‘eukaryotes’) are indicated on the cytograms.

   ■ Figure 3

Natural abundance N isotopic composition (δ15N) of flow cytometrically sorted phytoplankton populations collected from the Sargasso Sea in July 2008. The mixed layer depth (MLD; dashed grey line) was ~20 m, characteristic of summertime stratification in this region, and the deep chlorophyll maximum (DCM; dashed green line) was ~85 m. Prochlorococcus (blue diamonds) and Synechococcus (red squares) δ15N was similar to bulk particle δ15N (black crosses), implying reliance of these populations on recycled N sources. In contrast, small eukaryotic phytoplankton (green circles) had a δ15N similar to the deep nitrate supply, suggesting that eukaryotes at BATS rely on the subsurface nitrate for more than 50% of their nitrogen.