Urea distribution and uptake in the Atlantic Ocean between 50° N and 50°S.

We investigate the distribution of urea and its uptake by phytoplankton during 3 meridional transects of the Atlantic Ocean between 50°N and 50° S. Significant relationships were identified between urea uptake and Prochlorococcus abundance (p < 0.01) in the northern subtropical Atlantic, where Prochlorococcus appears likely to dominate urea uptake, and between urea concentration and the <200 µm microplankton biomass fraction (p < 0.005) in the South Atlantic, which may be associated with the production of urea. These results suggest that the distribution of urea in the subtropical ocean may be controlled by regional imbalances between urea consumption and urea production. In parallel with these simple relationships significant spring-autumn seasonal changes in the distribution of urea were identified in southern subtropical and tropical latitudes. Urea was twice as abundant during local spring than during local autumn in the subtropical South Atlantic but 2.5 times more abundant in equatorial waters during the boreal autumn period. Euphotic zone integrated urea uptake rates also varied seasonally, being considerably higher in the North Atlantic temperate and subtropical latitudes during the boreal spring whilst in the subtropical South Atlantic urea uptake peaked in local autumn.