J. Great Lakes Res. 33:219–231 Internat. Assoc. Great Lakes Res., 2007 Effects of Hydrological Flow Regime on Sediment-water Interface and Water Column Nitrogen Dynamics in a Great Lakes Coastal Wetland (Old Woman Creek, Lake Erie) Mark J. McCarthy 1,* , Wayne S. Gardner 1 , Peter J. Lavrentyev 2 , Kenneth M. Moats 2 , Frank J. Jochem 3 , and David M. Klarer 4 1 The University of Texas at Austin Marine Science Institute 750 Channel View Drive Port Aransas, Texas 78373 2 The University of Akron Department of Biology Akron, Ohio 44325 3 Florida International University Marine Biology Program 3000 NE 151 Street AC-1-379 North Miami, Florida 33181 4 Old Woman Creek National Estuarine Research Reserve 2514 Cleveland Road East Huron, Ohio 44839 ABSTRACT. Sediment-water interface nitrogen (N) transformations and water column ammonium cycling rates were measured along a stream to lake gradient at three sites within Old Woman Creek (OWC) and one near-shore Lake Erie site during two hydrological regimes: one with open flow to the lake after a rain event (July 2003), and another with a sand barrier blocking flow (July 2004). Net N 2 effluxes in OWC at all times and at the near-shore Lake Erie site in July 2003 suggest that sediments are a N sink via denitrification. Observed dissimilatory nitrate reduction to ammonium (DNRA) may counter- act some of this N removal, particularly when the creek mouth is closed. Upstream, a closed creek mouth led to higher sediment oxygen demand, net N 2 flux, potential DNRA, and potential denitrification rates. The lake site exhibited lower rates of these processes with the creek mouth closed except denitrification potential, which was unchanged. Denitrification in OWC appeared to drive N limitation in the lower wet- land when the sand barrier was blocking flow to the lake. Higher potential versus in situ denitrification estimates imply that water column NO 3 – limits and drives denitrification in OWC. Water column to sedi- ment regeneration ratios suggest that sediment recycling may drive primary production in the OWC inte- rior when the creek mouth is closed and new N inputs from runoff are absent, but more data are needed to confirm these apparent trends. Overall, hydrological regime in OWC appeared to have a greater impact on sediment N processes than on water column cycling. INDEX WORDS: Old Woman Creek, Lake Erie, sediments, denitrification, DNRA, regeneration. INTRODUCTION Coastal eutrophication has been implicated in several regional issues affecting human health and * Corresponding author. E-mail: markm@utmsi.utexas.edu the environment, including harmful and nuisance algal blooms, altered food webs, and hypoxia (i.e., Vitousek et al. 1997, Nicholls et al. 2001, Beeton 2002, Rockwell et al. 2005). Coastal wetlands serve as ‘metabolic gates’ removing excess nutrients from 219