Spatial and temporal distribution of Trichodesmium blooms in the world's oceans

Nitrogen fixing organisms, such as the cyanobacterium Trichodesmium, directly affect the oceanic nutrient inventory through the addition of new nitrogen to the ocean ecosystem and therefore have an important role in the strength and functioning of the biological carbon pump. Nonetheless, little is known about the distribution of Trichodesmium beyond limited shipboard observations. Even less is known about the occurrence and characteristics of very intense, transient blooms of this organism that have been observed historically throughout the world oceans. A new method for discriminating the occurrence of blooms from satellite ocean color data is used here to make the first global maps of Trichodesmium bloom occurrence and to examine their spatial and temporal distribution. As expected, Trichodesmium blooms are rare, occurring <5-10% of the time over most of the tropical and subtropical oceans for the time period examined (1998-2003). Areas of greatest persistence are found in the eastern tropical Pacific and the Arabian Sea, and reach recurrence levels of >30%. Many of the retrieved patterns are consistent with previously reported blooms, though differences exist. A strong seasonal cycle is observed in the Indian Ocean, probably related to monsoonal forcing, with weaker seasonal changes elsewhere. Estimated global nitrogen fixation rates by Trichodesmium blooms is similar to 42 Tg N yr(-1) which is biogeochemically significant on regional and global scales. Further, an estimate of the rate of Trichodesmium nitrogen fixation under nonbloom conditions is an additional similar to 20 Tg N yr(-1) suggesting that Trichodesmium is likely the dominant organism in the global ocean new nitrogen budget.