Carpenter, E. J. SUNY Stony Brook, ecarpenter@ccmail.sunysb.edu
Montoya, J. P. Georgia Institute of Technology,
Burns, J. Chesapeake Biological Lab,
Mulholland, M. Chesapeake Biological Lab,
Subramaniam, A. Chesapeake Biological Lab,

 
EXTENSIVE BLOOM OF A N2 FIXING SYMBIOTIC ASSOCIATION IN THE TROPICAL ATLANTIC OCEAN
 
We encountered an extensive bloom of the colonial diatom Hemiaulus hauckii along a 2500 km cruise track off the NE coast of South America in autumn 1996. Each diatom cell contained the heterocystous, N2-fixing cyanobacterial symbiont Richelia intracellularis. Surface Richelia heterocyst (and filament) densities increased from less than 100 heterocysts L-1 to over 106 L-1 in the bloom. Total abundance ranged from 106 heterocyst m-2 outside the bloom to over 1010 m-2 within the bloom. Rates of primary production averaged 1.12 ? 0.1 g C m-2 d-1 (n = 11), higher than typical for open ocean waters. N2 fixation in the bloom added an average of 44 ? 18 mg N m-2 d-1 (n=14) to the water column. This flux together with N2 fixation by the cyanobacterium Trichodesmium could supply 13% of the instantaneous demand for N within the bloom and 39% of the N required to support typical rates of production in this region. Suspended particles and zooplankton collected within the bloom were depleted in 15N, reflecting the dominant contribution of N2 fixation to the planktonic N budget. Such blooms may represent an important and previously unrecognized source of new N to support primary production in nutrient poor waters, and furthermore, heterocystous cyanobacteria can make quantitatively important inputs of N in open ocean waters.
 
Day: Thursday, Feb. 4
Time:
Location: Sweeney Center
 
Code: CS62TH1038S