07 Tyrrell fbg026 (to/d)

The filamentous cyanobacteria belonging to the genus Trichodesmium represent the major group of N2-fixing planktonic organisms in the ocean [see the review by Capone et al. (Capone et al., 1997)]. Trichodesmium occurs at higher abundances in highly stratified, low-nutrient waters at tropical and subtropical latitudes, often forming smallto large-scale blooms which can be seen from space [e.g. (Subramaniam and Carpenter, 1994)], but are difficult to study due to their sparse distribution and ephemeral nature. It has been estimated that N2-fixation in the oligotrophic ocean may account globally for an annual input of ~80 Tg nitrogen (N) (Capone et al., 1997), although these estimates are subject to a high level of uncertainty, given our limited knowledge on the distribution of Trichodesmium and other N2-fixers in the open ocean. The magnitude and distribution of N2-fixation inputs of nitrogen to the ocean are important for a number of reasons. For example, current estimates of nitrogen diffusive fluxes from below the thermocline seem insufficient to sustain the measured rates of primary productivity in the oligotrophic regions of the eastern North Atlantic (Lewis et al., 1986) and the western South Atlantic (Planas et al., 1999). N2 fixation, where it occurs, has the potential for reconciling such imbalances. N2-fixation is also the critical feedback in the ‘nitrostat’, which controls the ocean’s fixed nitrogen content over longer time scales (Tyrrell, 1999). The Atlantic Meridional Transect (AMT) programme is a series of latitudinal transects between the UK (50°N) and the Falkand Islands (50°S), carried out since September 1995 on a bi-annual basis. The scientific work takes advantage of the opportunity presented by a supply ship travelling back and forth between the UK and Antarctica once a year. The length of these cruises, together with the fact that they are carried out during two seasons each year (although even greater frequency would of course be desirable), make them particularly appropriate to characterize the large-scale patterns of distribution of Trichodesmium in the open ocean. For instance, little