Photosynthetic properties derived from Fast Repetition Rate (FRR) fluorometry (active fluorescence)

Southern Ocean JGOFS 	RR KIWI 09  		February 13 - March 19, 1998

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Methods:

Measurements were collected with a battery-powered FRR fluorometer with internal 
data logging (Chelsea Instruments, Ltd. FASTtracka); the instrument was 
attached at the base of the standard CTD/rosette frame during vertical profiles.  
All FRR fluorometer data was collected during downward casts; profiles were typically 
conducted at 10 m min-1 in the first 100 m of the water column and then 
40-60 m min-1 for the remainder of the cast.  Excitation conditions in the FRR 
fluorometer were set to provide 100 flashlets of light (~1 µs duration) 
spaced ~2.8 µs apart, followed by 20 additional flashlets 50 µs apart.  
To correct for variations in detector response during the 120-flashlet sequence, 
all fluorescence yields (signals divided by reference measurements of the excitation 
source) were scaled such that the yield for each flashlet was the same when 
analyzing a standard material with constant fluorescence yield, in this case 
chlorophyll extracted in methanol.  In addition, gain-dependent background 
signals (estimated based on measurements in waters deeper than ~100 m where 
fluorescence was below detection) were subtracted from all fluorescence 
measurements.  All reported data are from induction curve measurements in the 
instrument's "dark" chamber.   Note: Near surface measurements can be contaminated 
with surface light effects.  

Parameter values (Fo, Fm, Fv/Fm, sigma_PSII, tau_PSII) were determined using 
software provided by Chelsea Instruments (FRS v. 1.4).  All depth profiles of 
fluorescence properties were smoothed using a low-pass filter.  

The FASTtracka internal clock was synchronized to UTC several times 
during the cruise, and depth values for the profiles were determined by time 
synchronization with the CTD data.  

Reference: 

Sosik, H. M.  and R. J. Olson.  Phytoplankton and iron limitation of photosynthetic 
efficiency in the Southern Ocean during late summer.  Deep-Sea Res. Submitted.  

Heidi M. Sosik
Biology Department, MS 32              
Woods Hole Oceanographic Institution    
Woods Hole, MA  02543-1049
phone: 508-289-2311  fax: 508-457-2134
hsosik@whoi.edu