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Transport and mixing of momentum and salt in an estuary varies in time and space due to river discharge, changes in tidal amplitude and phase, wind stress, and lateral mixing processes, such as secondary circulation. This dissertation focuses on observing, describing, parameterizing, and quantifying secondary circulation in a sinuous coastal plain estuary using acoustic Doppler current meters. This endeavor is made to improve our general understanding of secondary circulation, to identify its primary driving forces, and to better parameterize key physical processes necessary for further study in current numerical models. Secondary circulation is used to describe several mechanisms whose result is to vertically overturn the water column along the transverse axis of a channel. Secondary circulation is commonly generated by one or more of the following mechanisms: channel curvature, unusual bottom topography or channel geometry, planetary rotation, and/or the differential advection of density. Data for this dissertation was collected in the naturally sinuous Satilla River in southeast Georgia. A shallow coastal plain estuary, the Satilla is a partially-mixed estuary characterized by 2 meter range semidiurnal tides. It has a strong neap-to-spring axial current inequality and strong neap-to-spring vertical salinity differences. The balance of mechanisms responsible for the strength and location of secondary flow in the Satilla River varies with spatial location, phase of the tide, and lateral cross-channel position. A simple steady-state momentum balance between the Coriolis acceleration, centrifugal acceleration, the lateral baroclinic gradient, and bottom stress is sufficient to explain secondary circulation in the Satilla River under a wide variety of conditions. The primary momentum balance for this river is a three-way balance between the centrifugal acceleration, bottom stress, and the lateral baroclinic gradient. The dominant mechanism that drives the local secondary circulation depends on the phase of the tide and the lateral placement of the acoustic current profiler.

In the past couple of decades, the discovery that iron is a limiting nutrient in large regions of the ocean has spurred much research into characterizing the biogeochemical controls on iron cycling. While Fe(II) is soluble at circumneutral pH, it readily oxidizes to Fe(III) in the presence of oxygen. Fe(III) is highly insoluble at circumneutral pH, presenting organisms with a bioavailability paradox stemming from the physiological challenge of using a solid phase mineral for assimilatory or dissimilatory purposes. Interestingly, dissolved organic-Fe(III) complexes can be stable in seawater in the presence of oxygen, and an active flux of these complexes has recently been measured in estuarine sediments. Their sources and biogeochemical role, however, remain poorly understood. In this work, a suite of field and laboratory techniques were developed to quantify diagenetic processes involved in the remineralization of carbon in marine sediments in situ, investigate the role of these organic-Fe(III) complexes in sediment biogeochemistry, and characterize the composition of the ligands possibly involved in the solubilization of Fe(III) in marine sediments. The first-of-its-kind in situ electrochemical analyzer and HPLC was used to better constrain diagenetic processes that may lead to the formation of dissolved organic-Fe(III) complexes in the Altamaha estuary and Carolina slope. An intensive study of the Satilla River estuary reveals that dissimilatory iron-reduction contributes to the formation of sedimentary organic-Fe(III) complexes, which are demonstrated to serve as an electron acceptor in subsequent incubations with a model iron-reducing microorganism. Similar observations in deep-sea slope and abyssal plain sediments fed by the Mississippi and Congo Rivers suggest that dissimilatory iron reduction may represent an important component of carbon remineralization in river-dominated ocean margin sediments that may be currently underestimated globally. To confirm that these organic-Fe(III) complexes are produced during microbial iron reduction, novel separation schemes were developed to extract and identify Fe(III)-binding ligands from sediment pore waters. Preliminary results reveal the presence of a few select low-molecular weight compounds in all pore waters extracted, suggesting they might be endogenous ligands secreted by iron-reducing bacteria to non-reductively dissolve Fe(III) minerals prior to reduction.