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 microorganisms is a potentially large source of organic N to aquatic ecosystems, particularly those that receive high loads of both terrestrial organic matter (with high C:N values) and DIN. We investigated this rarely studied process in such a system (the Hudson River Estuary, USA) using: (1) in situ incubations of terrestrially derived particulate organic matter; (2) laboratory microcosms with terrestrially derived dissolved 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 invertebrate and fish consumers to demonstrate how food web dynamics can be 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 1000 parts per thousand, respectively. Finally, an ecosystem budget suggests 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 ecosystem budgets suggest that MAD is large in the Hudson. Traditional food web analyses based on N-15 and C-13 which ignored MAD would result 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 important to consumers but a large part of organic N originates from heterotrophic rather than autotrophic assimilation.