Contrasting patterns of dissolved organic nitrogen release by two size fractions of estuarine plankton during a period of rapid NH/ consumption and N02-production

Rates of NH,' uptake and total and low molecular weight (LIMMJ, < 10000 Dalton) dlssolved organlc n ~t r o g e n (DON) release were measured on s~ze-fractlonated (<202 and < 1 2 urn) plankton Water was collected from the Choptank River, a subestuary of Chesapeake Bay, USA, In August 1990 and contained for 36 h in large carboys Penod~cally samples were removed from each carbo\ nutnent concentratlons were determined and the rates of NH,' uptake and total and LMW DON release were measured dunng 0 5 h lncubat~ons Approximately 7 ug-at NH,'were consumed in each carboy dunng the 36 h containment and up to 6 ug-at NO2 were produced whlch suggests that nitrification was occurring Rates of total DON release lvere s~gnificantly higher In the <202 pm plankton relatlve to the < 1 2 urn plankton llkely due to f e e d ~n g processes such as grazlng, associated with the larger size fractlon Evidence for feeding-lnduced DON release In the <202 urn plankton includes the small ratio of LMW DON to total DON rc,lease the Inclease in DON release rates at n ~g h t and the doubling of phaeoplgment concentratlons d u n n g the 36 h e x p e r ~m e n t In contrast in the < l 2 pm plankton, rates of LM DON release were not slgnif~cantly different from rates oi total DON release and rates of DON ielease decreased by over 95 in the dark In this case w e suggest that passive release from autotrophs was a more ~mportant release process