Sediment-water nitrogen fluxes and transformations were examined at two sites in Saginaw Bay, Lake Huron, as a model for sandy freshwater sediments. Substantial ammonium release rates (74 to 350 μmole NH4+/m2/h1) were observed in flow-through cores and in situ benthic chamber experiments. 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 systems (11 to 470 μmole NH4+/m2/h1) but were higher than those reported for depositional freshwater sediments (0 to 15 μmole NH4+/m2/h1; Seitzinger 1988). Addition of montmorillonite clay (ca. 1 kg dry weight/m2) 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 dominated ammonium regeneration in the inner bay where the bivalve was abundant, but addition of zebra mussel feces/pseudofeces (3.0 g dw/m2) to sediments did not increase ammonium or nitrate flux. Partial nitrification of ammonium at the sediment-water interface was suggested by removal of added 15NH4+ 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.
Bibliographical noteFunding Information:
and in doing field experiments, H. Bootsma for ana- lyzing nitrate in the core experiments, S. Seitzinger for providing the montmorillonite , clayJ. R. John- son, J. . Cavaletto, Fand M. McCarthy for technical assistance and S. An for comments on the manu- script. This research was supported in part by NOAA Grant # NA16RGO457 to J. Co,t nerexasT A&M Univ.e rsiThe tyChinese Ecosystem Research Network, Chinese Academy of Sciences, provided financial support for L. ang Yto participate in this cooperative project. The Nancy Lee and Perry R. Bass Regents Chair in Marine Science, at The Uni- versity of exas TMarine Science Insitute, supported manuscript preparation. This paper is GLERL Con- tribution #1184 and UTMSI Contribution #1174.
- Ammonium flux
- Saginaw bay
- Sediment-water nitrogen dynamics
- Zebra mussels