IV) Results - Macronutrients

Research objectives, methods and initial site conditions

Biogeochemical controls on phytoplankton composition and productivity assessed

by C, N, and Si utilization in the eastern and western Equatorial Pacific1 4 5Adrian Marchetti , , Sophie Bonnet , Zackary Johnson and

1 5University of Washington, University of Southern California, University of Hawaii

2 3 1Diana E. Varela Veronica Lance , E. Virginia Armbrust2 3 4 University of Victoria, Duke University,

Eastern and western Equatorial Pacific nutrient amendment experiments (T = 48 hrs)

Figure 3. Size-fractionated and total A) chlorophyll a (Chl a) and B) particulate organic nitrogen (PON) responses 48 hrs after nutrient amendments. Dashed lines highlight the initial total values. Error bars are one standard deviation of the mean (n=3). ND, no data.

East (140° W) West (165° E)

East (140° W) West (165° E)

Figure 4. Size-fractionated and total A) dissolved inorganic carbon uptake (rDIC), B) chl a-specific rDIC and C) photosynthetic competency (F /F ) responses 48 hrs after nutrient amendments. Where provided, dashed lines v m

highlight the initial total values. Error bars are one standard deviation of the mean (n=3)

A) Biomass

B) Primary Productivity

East (140° W) West (165° E)

C) Nitrogen utilization

Figure 5. Size-fractionated and total A) nitrate uptake (rNO ), B) ammonium uptake (rNH ) and dinitrogen gas 3 4

uptake (rN ) responses 48 hrs after nutrient amendments. Where provided, dashed lines highlight the initial total 2

values. Where present, error bars are one standard deviation of the mean (n=3).

Eastern Equatorial Pacific nutrient amendment experiment (full time course)

B) Phytoplankton composition

A) Biomass, nutrient utilization and stoichiometry

Eastern Equatorial Pacific nutrient amendment experiment (full time course)

A) C)B)

Figure 2: Changes in total A) chlorophyll a (Chl a), particulate organic nitrogen (PON) and biogenic silica (BSi), B) dissolved inorganic carbon uptake (rDIC), nitrate uptake (rNO ), 3

ammonium uptake (rNH ) and silicic acid uptake (rSi[OH] ) and C) biomass and uptake 4 4

stoichiometries over a 96 hr time course after nutrient amendments. The greyed areas denote the time after NO was fully depleted from the nutrient amended treatments. The dashed lines 3

indicate Redfield/Brzezinski stochiometries. Where provided, error bars are one standard deviation of the mean (n=3).

Low iron concentrations in the pelagic equatorial Pacific limit primary productivity. The eastern Equatorial Pacific is a well-characterized high-nitrate, low-chlorophyll region (HNLC), however silicic acid concentrations are also low. The western Equatorial Pacific, known as the warm pool, is characterized as having both low iron and macronutrients in the surface waters which may co-regulate specific phytoplankton functional groups.

Our research objectives were:

A) To assess which nutrients or combinations thereof resulted in the largest enhancement in phytoplankton biomass and nutrient utilization and to compare these phytoplankton responses between the east and west.

B) To determine the changes in nutrient (C, N and Si) utilization rates and stoichiometry and the phytoplankton compositional shifts after the alleviation of nutrient limitation.

Conclusions

Acknowledgments

East (140° W)

West (165° E)

Figure 1: Ambient size-fractionated and total chlorophyll a (Chl a) and dissolved nitrate (NO ), silicic 3

acid (Si[0H] , phosphate (PO ) 4 4

and ammonium (NH ) 4

concentrations at the eastern and western Equatorial Pacific stations where the nutrient amendments were performed. The red dashed lines indicate the depths at which seawater was collected for the experiments. Where provided, error bars are one standard deviation of the mean (n=3)

A

B

C

A

B

A

B

C

Location Treatment Nutrients added

East (140 W, Eq) Ctl nothing added

Fe 4 nM FeCl3

FeSi 4 nM FeCl3

10 mM Na2SiO3

West (165 E, Eq) Ctl1 nothing added

Fe 4 nM FeCl3

N 2 mM NaNO3

1 mM NH4Cl

P 0.5 mM NaH2PO4

FeN 4 nM FeCl32 mM NaNO3

1 mM NH4Cl

FeP 4 nM FeCl3

0.5 mM NaH2PO4

FeNP 4 nM FeCl3

2 mM NaNO3

1 mM NH4Cl

0.5 mM NaH2PO4

Ctl2 added 50% filtered seawater (25m)

DW added 50% filtered seawater (200m)

Methods: Nutrient amendment experiments were performed during the EUCFe cruise in September 2006. Seawater was collected in 10L cubitainers using trace-metal clean techniques. The cubitainers were incubated on-deck at ambient surface seawater temperatures. All treatments were performed in triplicate and destructive sampling of cubitainers was implemented. Short-term (8 hr) uptake measurements of DIC, NO , NH , N , 3 4 2

and Si(OH) were performed using radio-isotope and stable-isotope techniques. Phytoplankton 4

composition was determined using flowcytometry and microscope cell counts.

Figure 6. Changes in abundance relative to initial (T ) cell concentrations over a 96 hr time course after nutrient 0

amendments. Dashed lines highlight the initial values. Error bars are one standard deviation of the mean (n=3).

of different functional groups of phytoplankton assessed by flowcytometry

Figure 7. Changes in diatom abundances 72 hrs after nutrient amendments (peak in biomass). Where provided, error bars are one standard deviation of the mean (n=3). L, large; s, small.

Pseudo-nitzschia spp. (L)

Pseudo-nitzschia spp. (s)

cf. Thalassiothrix sp.

small pennate spp.

other pennate diatoms

centric diatoms

We thank R. Marohl from UW for assistance with flowcytometry sample analyses. We are grateful to J. Murray from UW (chief-scientist of EUCFe 2006), the other scientists on this cruise and the crew of the R/V Kilo Moana for their efforts at sea. This study was partially funded by an NSF grant and a Gordon and Betty Moore Foundation Marine Microbiology Investigator Award held by EVA.

In the east,

1) FeSi additions resulted in the largest phytoplankton response with a 10-fold increase in chl a concentrations and 12-fold increase in rDIC relative to the control

2) large cells comprised of pennate diatoms (in particular, Pseudo-nitzschia spp.) were dominant, primarily utilizing NO3

3) there was 3-fold higher rSi(OH) in FeSi relative to Fe despite these treatments having 4

almost equivalent increases in biomass

4) enhanced BSi:PON and rSi(OH) :r(NO +NH ) in the FeSi relative to Fe could, in part, 4 3 4

be due to differences in diatom composition as more large diatoms were present in FeSi

5) after the alleviation of iron-limitation, rNO and rDIC were decoupled possibly due to 3

luxury uptake of NO3

In the west,

1) FeNP additions resulted in the largest phytoplankton response with a 3-fold increase in chl a concentrations and an 18-fold increase in rDIC relative to the control

2) small cells comprised of picophytoplankton (in particular, Prochlorococcus) were dominant, primarily utilizing NH4

3) Fe, N and FeN additions resulted in enhanced rDIC and rNH in small cells but only 4

minor increases in biomass

4) rN did not change within any of the nutrient amendments relative to the control; 2

however, there did appear to be a bottle effect in the west.