Ammonium regeneration by microplankton in an upwelling environment

Size-frachonated measurements of ammonium flux In the southern Benguela upwelllng system provided evldence that regenerahon by microplankton (<200um) was concentrated In the upper water column and generally supported the majonty of phytoplankton demands Based on the present study and literature data lt was demonstrated that ammonium reminerahzabon was dependent on the concentrahon of particulate nitrogen present when viewed on a broad scale The < 15 pm size class, including bactena and nanoflagellates proved to be the predominant mineralizers accounting for 95 % of total microplankton regeneration on average Although ammomf~cahon by heterotrophic p~coplankton (presumably mainly bactenal) was Important in some instances bacte~nvorous nanoflagellates (1 to 15 urn) appeared to supply the bulk of mlcroplankton excretion, i e a mean of 87 % in surface waters and 55 % near the base of the euphot~c zone Bactena were relatively more active at depth than at the surface although in the majonty of cases absolute rates were fastest In surface watersThe posslbdlty e x s t s that bactenal reminerallzation was underesbmated in the < 1 um fracbon due to the absence of nanoflagellates dunng the ~ncubahon Nevertheless, nanoflagellate feeding compnses a n essenhal component of the reminerahzat~on process Ammonlum uptake by the dlfferent size classes was dlrectly proportional to chlorophyll a concentrations Results of thls study are cons~stent wlth the proposed cruclal role for small protozoans as intermediaries between bacterla and hlgher trophic levels and as m~nerallzers of nutrients for pnmary producers O Inter-Research/Printed in F. R. Germany