Large-scale latitudinal distribution of Trichodesmium spp. in the Atlantic Ocean

The Atlantic Meridional Transect (AMT) programme is a series of bi-annual cruises between the Falkland Islands (50°S) and the UK (50°N). Measurements of the abundance of theN₂-fixing, colonial cyanobacterium Trichodesmium along this transect in the years 1995-1999 reveal that it is especially abundant between 0 and 15°N, but by contrast almost completely absent between 5 and 30°S. The cruise path between 0 and 15°N lies close to 20°W, on the African (eastern) side of the Atlantic. The maximum colony abundances we observed (∼100 000 colonies m^-2) are greater than those reported in many other studies. The results of different methods for assessing Trichodesmium abundance are compared. Possible correlations between Trichodesmium abundance and several physical and chemical variables were examined to try and elucidate the factors controUing N₂ fixer occurrence. High Trichodesmium abundance was found to be correlated with shallow mixed layer depth and high estimated iron deposition to the surface ocean, but not with temperature, nitrate or the concentration of total dissolvable iron in sea water.