Nitrogen Dynamics in Sandy Freshwater Sediments (Saginaw Bay, Lake Huron) Wayne S. Gardner 1,* , Longyuan Yang 2 , James B. Cotner 3,† , Thomas H. Johengen 4 , and Peter J. Lavrentyev 4,‡ 1 National Oceanic and Atmospheric Administration Great Lakes Environmental Research Laboratory 2205 Commonwealth Blvd. Ann Arbor, Michigan 48105 2 Nanjing Institute of Geography and Limnology Chinese Academy of Sciences 73 East Beijing Rd. Nanjing, Jiangsu 210008, P. R. China 3 Texas A&M University College Station, Texas 77843 4 Cooperative Institute for Limnology and Ecosystem Research (CILER) The University of Michigan Ann Arbor, Michigan 48109 ABSTRACT. Sediment-water nitrogen fluxes and transformations were examined at two sites in Sagi- naw Bay, Lake Huron, as a model for sandy freshwater sediments. Substantial ammonium release rates (74 to 350 μmole NH 4 +/ m 2 /h 1 ) were observed in flow-through cores and in situ benthic chamber experi- ments. Sediment-water ammonium fluxes were similar at the inner and outer bay stations even though inner bay waters are enriched with nutrients from the Saginaw River. The high net flux of remineralized ammonium into the overlying water from these sandy sediments resembles typical data for marine sys- tems (11 to 470 μmole NH 4 + /m 2 /h 1 ) but were higher than those reported for depositional freshwater sedi- ments (0 to 15 μmole NH 4 + /m 2 /h 1 ; Seitzinger 1988). Addition of montmorillonite clay (ca. 1 kg dry weight/m 2 ) to the top of the sandy cores reduced ammonium flux. Mean “steady-state” ammonium flux following clay addition was 46 ± 2 (SE) % of the initial rates as compared to 81 ± 8% of the initial rates without clay addition. Zebra mussel excretion dominanted ammonium regeneration in the inner bay where the bivalve was abundant, but addition of zebra mussel feces/psuedofeces (3.0 g dw/m 2 ) to sedi- ments did not increase ammonium or nitrate flux. Partial nitrification of ammonium at the sediment- water interface was suggested by removal of added 15 NH 4 + from lake water passing over dark sediment cores. Sediment-water fluxes of nitrogen obtained from flow-through sediment cores resembled those from in situ benthic chambers. However, extended static incubations in gas-tight denitrification chambers caused more of the regenerated nitrogen to be nitrified and denitrified than occurred with the other two measurement systems. INDEX WORDS: Ammonium flux, sediment-water nitrogen dynamics, nitrification, denitrification, Saginaw Bay, zebra mussels. J. Great Lakes Res. 27(1):84–97 Internat. Assoc. Great Lakes Res., 2001 * Present address: The University of Texas at Austin, Marine Science In- stitute, 750 Channelview Drive, Port Aransas, Texas 78373. Correspond- ing author: E-mail: gardner@utmsi.utexas.edu 84 † Present Address: Department Ecology, Evolution and Behavior, The University of Minnesota, St. Paul, Minnesota 55108. ‡ Present Address: Department of Biology, The University of Akron, Akron, Ohio 44325.