Project summary

1. Hypotheses

Prior results have documented eddy-driven transport of nutrients into the euphotic zone and the associated accumulation of chlorophyll. However, several key aspects of mesoscale upwelling events remain unresolved by the extant database, including: (1) phytoplankton physiological response, (2) changes in community structure, (3) impact on export out of the euphotic zone, (4) rates of mixing between the surface mixed layer and the base of the euphotic zone, and (5) implications for biogeochemistry and differential cycling of carbon and associated bioactive elements. This leads to the following hypotheses concerning the complex, non-linear biological regulation of elemental cycling in the ocean: 

H₁: Eddy-induced upwelling, in combination with diapycnal mixing in the upper ocean, introduces new nutrients into the euphotic zone. 

H₂: The increase in inorganic nutrients stimulates a physiological response within the phytoplankton community. 

H₃: Differing physiological responses of the various species bring about a shift in community structure. 

H₄: Changes in community structure lead to increases in export from, and changes in biogeochemical cycling within, the upper ocean. 

2. Scenarios 

There are several scenarios in which this chain of hypotheses could be linked or broken.  These include, but are not necessarily limited to, the following: 

S₁: Nutrient input to the euphotic zone simply increases the rate of production by the background species assemblage dominated by picoplankton; impacts on biogeochemical cycling are nil. 

S₂: Increased nitrate availability stimulates a bloom of diatoms; silica-rich organic material produced in the bloom sinks rapidly out of the euphotic zone once the nutrients are exhausted. 

S₃: Shoaling isopycnals transport DIP closer to the surface, facilitating nitrogen fixation by Trichodesmium or perhaps vertically migrating diatoms with symbiotic bacteria; nitrogen-rich organic material produced during the bloom is exported primarily in dissolved form. 

S₄: The eddy feature accommodates a change in community structure and biomass of consumers that produce rapidly sinking particles.

3. Objectives 

The following objectives are designed to test hypotheses H₁-H₄ and distinguish between the scenarios S₁-S₄ in which the chain of hypotheses are linked or broken. 

O₁:  Measure the enhancement of inorganic nutrient availability brought about by eddy-induced upwelling. 

O₂:  Measure the phytoplankton physiological response to increased nutrients. 

O₃:  Assess shifts in species composition associated with the eddy disturbance. 

O₄:  Quantify the impact of the eddy disturbance on upper ocean biogeochemical cycling: measure elemental inventories, primary production, and export. 

O₅:  Assess the interaction between eddy-driven isopycnal transport processes and diapycnal fluxes in and below the mixed layer (quantifies the reversibility of the eddy-induced transport; specifically, how is the oxygen anomaly “left behind” and eddy-driven new production event).

4. Project timeline 

2003

           July 1               Start Date
           July 23             First PI meeting, Woods Hole

2004

            February          Second PI meeting, ASLO/TOS Conference, Honolulu
            Summer            Field Work 

2005

            Winter              PI meeting: analysis of ’04 and planning for ‘05
             Summer            Field Work 

2006

            Winter              PI meeting: synthesis and manuscript preparation
            June 30            End date

5. Related Projects and Proposals 

Participation starting in 2004:

               Steinberg                      Zooplankton net tows

Davis                            VPR

Carlson                        prokaryotic community structure and DOM dynamics

Armbrust                      genetic diversity in eukaryotes

Oakey/Ledwell Finestructure 

Participating in 2005:

Shipe/Brzezinski           N and Si uptake

Fratantoni                     Gliders

Schofield                      Gliders 

Benitez-Nelson et al. Hawaiian Eddy Project “EFLUX”

http://www.soest.hawaii.edu/oceanography/eddy/