MICROBIAL ASSIMILATION OF DIN IN A NITROGEN RICH ESTUARY - IMPLICATIONS FOR FOOD QUALITY AND ISOTOPE STUDIES

The assimilation of dissolved inorganic N (DIN) by heterotrophic micro organisms is a potentially large source of organic N to aquatic ecosys tems, particularly those that receive high loads of both terrestrial o rganic matter (with high C:N values) and DIN. We investigated this rar ely studied process in such a system (the Hudson River Estuary, USA) u sing: (1) in situ incubations of terrestrially derived particulate org anic matter; (2) laboratory microcosms with terrestrially derived diss olved organic matter; and (3) ecosystem budgets of bulk N and N-15. We also analyzed the N-15 and C-13 content of primary producers and inve rtebrate and fish consumers to demonstrate how food web dynamics can b e incorrectly interpreted if microbially assimilated DIN (MAD) is not considered. During 3 mo in situ incubations, %N content of terrestrial material increased by about 3-fold (C:N decreased from 70 to 25) and delta(15)N values increased from -4 to 9 parts per thousand. Similarly , in microcosms (where delta(15)N of DIN was 10 000 parts per thousand ), delta(15)N Of POM and DOM increased after 105 d to over 5000 and 10 00 parts per thousand, respectively. Finally, an ecosystem budget sugg ests that net MAD is up to 13 g N m(-2) yr(-1) which is 4-fold larger than net N assimilation by phytoplankton. Thus, both incubations and e cosystem budgets suggest that MAD is large in the Hudson. Traditional food web analyses based on N-15 and C-13 which ignored MAD would resul t in the conclusion that terrestrial organic matter was unimportant to consumers in the Hudson. When the large input of MAD is recognized, a likely interpretation becomes: terrestrial organic carbon is importan t to consumers but a large part of organic N originates from heterotro phic rather than autotrophic assimilation.