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Marine Ecology Progress Series

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MEPS 167:59-71 (1998)  -  doi:10.3354/meps167059

Microbial assimilation of DIN in a nitrogen rich estuary: implications for food quality and isotope studies

N. F. Caraco1,*, G. Lampman1, J. J. Cole1, K. E. Limburg2, M. L. Pace1, D. Fischer1

1Institute of Ecosystem Studies, Millbrook, New York 12545, USA 2Department of Systems Ecology, Stockholm University, S-10691 Stockholm, Sweden

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 15N. We also analyzed the 15N and 13C 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 δ15N values increased from -4 to 9o/oo. Similarly, in microcosms (where δ15N of DIN was 10000o/oo), δ15N of POM and DOM increased after 105 d to over 5000 and 1000o/oo, 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 15N and 13C 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.

N assimilation · Microbial · 15N · Estuary · Food web

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