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

We encountered an extensive surface bloom of the N-2 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 N-2 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 N-2 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 10(6) km(2), 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 N-2 fixation from our limited sampling in the spring intermonsoon, including a conservative estimate of the input by blooms, suggest N-2 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, N-2 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.