MEPS 172:281-292 (1998)  -  doi:10.3354/meps172281

An extensive bloom of the N2-fixing cyanobacterium Trichodesmium erythraeum in the central Arabian Sea

Douglas G. Capone1,*, Ajit Subramaniam1, Joseph P. Montoya2,**, Maren Voss3, Christoph Humborg3, Anne M. Johansen4, Ronald L. Siefert4,***, Edward J. Carpenter5

1Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Solomons, Maryland 20688-0038, USA
2The Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138, USA
3Institut für Ostseeforschung, Seestr. 15, D-18119 Warnemünde, Germany
4Keck Laboratories, California Institute of Technology, Pasadena, California 91125, USA
5Marine Sciences Research Center, SUNY, Stony Brook, New York 11794, USA
**Present address: School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
***Present address: Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Solomons, Maryland 20688-0038, USA

ABSTRACT: We encountered an extensive surface bloom of the N2 fixing cyanobacterium Trichodesmium erythraeum in the central basin of the Arabian Sea during the spring inter-monsoon of 1995. The bloom, which occurred during a period of calm winds and relatively high atmospheric iron content, was metabolically active. Carbon fixation by the bloom represented about one-quarter of water column primary productivity while input by N2 fixation could account for a major fraction of the estimated 'new' N demand of primary production. Isotopic measurements of the N in surface suspended material confirmed a direct contribution of N2 fixation to the organic nitrogen pools of the upper water column. Retrospective analysis of NOAA-12 AVHRR imagery indicated that blooms covered up to 2 x 106 km2, or 20% of the Arabian Sea surface, during the period from 22 to 27 May 1995. In addition to their biogeochemical impact, surface blooms of this extent may have secondary effects on sea surface albedo and light penetration as well as heat and gas exchange across the air-sea interface. A preliminary extrapolation based on our observed, non-bloom rates of N2 fixation from our limited sampling in the spring intermonsoon, including a conservative estimate of the input by blooms, suggest N2 fixation may account for an input of about 1 Tg N yr-1. This is substantial, but relatively minor compared to current estimates of the removal of N through denitrification in the basin. However, N2 fixation may also occur in the central basin through the mild winter monsoon, be considerably greater during the fall intermonsoon than we observed during the spring intermonsoon, and may also occur at higher levels in the chronically oligotrophic southern basin. Ongoing satellite observations will help to determine more accurately the distribution and density of Trichodesmium in this and other tropical oceanic basins, as well as resolving the actual frequency and duration of bloom occurrence.

KEY WORDS: Trichodesmium · Nitrogen fixation · Cyanobacteria · Blooms · Arabian Sea · Remote sensing

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