Nitrogen dynamics in sandy freshwater sediments (Saginaw Bay, Lake Huron)

Sediment-water nitrogen fluxes and transformations were examined at two sit es in Saginaw Bay, Lake Huron, as a model for sandy freshwater sediments. S ubstantial ammonium release rates (74 to 350 mu mole NH4+/m(2)/h(1)) were o bserved in flow-through cores and in situ benthic chamber experiments. Sedi ment-water ammonium fluxes were similar at the inner and outer bay stations even though inner bay water are enriched with nutrients from the Saginaw R iver. The high net flux of remineralized ammonium into the overlying water from these sandy sediments resembles typical data for marine systems (11 to 470 mu mole NH4+/m(2)/h(1)) but were higher than those reported for deposi tional freshwater sediments (0 to 15 mu mole NH4+/m(2)/h(1); Seitzinger 198 8). 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 compar ed to 81 +/- 8% of the initial rates without clay addition. Zebra mussel ex cretion dominated 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 sediments did not increase ammonium or nitrate flux. Partial nitrifica tion of ammonium at the sediment-water interface was suggested by removal o f added (NH4+)-N-15 from lake water passing over dark sediment cores. Sedim ent-water fluxes of nitrogen obtained from flow-through sediment cores rese mbled those from in situ benthic chambers. However, extended static incubat ions in gas-tight denitrification chambers caused more of the regenerated n itrogen to be nitrified and denitrified than occurred with the other two me asurement systems.