1998 Articles

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

Capone, Douglas G.; Subramaniam, Ajit; Montoya, Joseph P.; Voss, Maren; Humborg, Christoph; Johansen, Anne M.; Siefert, Ronald L.; Carpenter, Edward J.

We encountered an extensive surface bloom of the N, fixing cyanobactenum Trichodesrniurn erythraeum in the central basin of the Arabian Sea during the spring inter-monsooon of 1995. The bloom, which occurred dunng a period of calm winds and relatively high atmospheric iron content, was metabollcally active. Carbon fixation by the bloom represented about one-quarter of water column primary productivity while input by N₂ flxation 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 nltrogen 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.