We conducted continuous-flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH ) flux ranged from slight uptake (negative values) Nitrogen (N), a key nutrient limiting primary production in coastal ecosystems, can be introduced to coastal waters via tributaries, atmospheric deposition, or groundwater inputs as nitrates (NO or NO ), ammonium (NH ), and or-ganic N, or through N-fixation (Brock 2001;Paerl et al. 2002;Seitzinger et al. 2002). Available N is removed from coastal waters via physical transport, sediment burial, or conversion to gaseous forms, such as N 2 or N 2 O, via denitrification (Seitzinger et al. 1984;Brock 2001). An improved understanding of N dynamics in coastal systems is needed 1 Corresponding author (gardner@utmsi.utexas.edu). 2 Current address: Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea. AcknowledgmentsRick Kalke, Matt First, Melody Bernot, and Walter Dodds provided assistance for some of the field collections. Discussions and suggestions about N dynamics in Texas coastal regions provided by Ron Benner, Paul Montagna, and other colleagues are appreciated. We are grateful for the thorough and constructive observations and suggestions provided by two anonymous reviewers.

Variability of dissolved Fe 2+ and P H 2 S concentrations in porewaters were studied, using cathodic stripping voltammetry (CSV) microelectrodes, in sediments overlain by hypoxic waters in the summer from the southeastern region of Corpus Christi Bay, Texas (CCB) and the Mississippi River Bight (MRB), Louisiana. These measurements were complimented by sediment microcosm studies where oxygen concentrations in the overlying water were manipulated. Sulfate reduction rates, benthic oxygen demand, total reduced sulfide, porewater sulfate, and total organic carbon were also determined. Fe 2+ and P H 2 S were the major dissolved redox-reactive dissolved species in these sediments. During hypoxic conditions, an upward migration of porewater Fe 2+ and P H 2 S occurred, with Fe 2+ reaching much higher maximum concentrations than P H 2 S. Statistically significant (p<0:05) differences in both Fe 2+ and P H 2 S occurred between sediments at the CCB and MRB study sites. Although both sites were Fe-dominated, reactive and dissolved iron were higher while P H 2 S was lower at the MRB site. However, there were no statistically significant (p > 0:05) difference in regard to P H 2 S between microcosm and field monitoring studies. Results indicated that, for Fe 2+ and P H 2 S, relatively large and rapid changes occurred in both the concentrations and distributions of these important porewater constituents in response to relatively short-term changes in overlying water oxygen content. Model calculations indicated that conditions in the sediments can be responsible for the induction of hypoxic conditions in the formation of hypoxic overlying waters in <6 days at CCB and $20 days at MRB.

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