No description available.

No description available.

No description available.

No description available.

An extensive wave monitoring program was conducted in the lower Chesapeake Bay from fall 1989 through spring 1995. From October 23, 1992 - April 20, 1993 the Virginia Institute of Marine Science deployed an aluminum tetrapod containing wave and current sensing and recording equipment in the Thimble Shoal region of the lower Chesapeake Bay. Measurements were made in half hour bursts every three hours to document the area’s dynamic wave characteristics. It was shown previously by Boon et al. (1990 - 1996) that a bimodal frequency distribution of waves in the Thimble Shoal region existed, which was not seen in other areas of the lower Chesapeake Bay. This study examines the temporal changes in the wave energy spectra at Thimble Shoal, fall 1992 - spring 1993, providing a more complete description of the wave climate of the lower Chesapeake Bay. I used Q-mode Factor Analysis to examine the temporal changes in the wave energy spectra. This method of analysis reduced the dimensionality of the large data set by decomposing the data into its basic modal components. Using this technique, four primary modes of spectrum shape at this site were described. These modes include calm, bimodal, local and a mode that is modulated by non-local events. Identification of these components (modes) and the systematic variation between them provides important insights about the wave climate of the lower Chesapeake Bay. The onset of a local wind event starts a systematic variation between the modes. The dominance of the calm mode is quickly reduced by the local storm waves. The bimodal mode is seen as a transition from dominance in the previous two. The fourth mode is dominated by very low frequency waves and appears to be modulated by non-local events. This variation is seen for every local wind event, regardless of type or size.

No description available.

No description available.

Nitrogen (N) has been shown to limit primary production in many estuarine systems, including the New River Estuary (NRE), NC, a moderately eutrophied system with large areas of photic sediment. The NRE receives major inputs of allochthonous nutrients from agriculture and confined animal feeding operations as well as other sources. Autochthonous sources of N in the NRE include both remineralization and Nfixation. Whereas allochthonous sources are usually most important in winter/spring and during periods of high fresh water discharge, autochthonous sources are likely to become more important in summer. N-fixation, which can be performed by both autotrophic cyanobacteria and heterotrophic bacteria, was shown to vary in response to light levels, organic matter and temperature. To assess the importance of N-fixation at a system-wide scale we sampled seasonally along the estuarine gradient at multiple water depths (with a range of light availabilities) and multiple sediment depths. Benthic N-fixation activity was determined using the acetylene reduction method. Molecular characterization of the microbial communities along with the molybdate inhibition technique were used to verify the relative importance of autotrophic to heterotrophic sulfate reducing N-fixers. The majority of benthic NFix in the NRE was performed by sulfate-reducing bacteria. Although highest rates of benthic N-fixation were in the top 0 – 1 cm, a substantial portion occurred down to 10 cm in shallow and deep water samples. N-fixation rates were highest in the mid and lower estuarine sites during summer, with estimates as high as 2407 μmol N m-2 d-1. Benthic N-fixation rates varied seasonally and were a significant source of autochthonous N to the NRE, contributing up to 19% of total new N inputs during spring.

The physical exchanges between shelf and slope water masses are important drivers of biological productivity in the shelfbreak region of the Mid-Atlantic Bight (MAB). Based on two ocean glider surveys that were conducted in Autumn 2013, and concurrent wind and satellite based sea surface height observations, this study investigates the dynamic mechanisms of wind, surface height variation, water column hydrographic structure, and canyon topography in driving shelf-slope water mass exchanges across the shelfbreak near Norfolk Canyon and Washington Canyon in the MAB. Over the outer shelf, sea surface height variation and wind are important drivers of cross-shelfbreak transport through geostrophic and Ekman mechanisms. Opposing flow in the different layers of the water column leads to shelf-slope water mass exchange. Over submarine canyons, strong upwelling favorable wind in combination with flat sea surface can cause reversed flow along the MAB shelfbreak and thereby induce canyon upwelling of slope water. In addition, the interfaces of shelf and slope water masses are expected to be conducive to double diffusion, which in turn can drive thermohaline intrusions and further enhance shelf-slope exchanges. These shelf-slope exchange processes can contribute to a net salt flux onto the shelf and support enhanced sub-surface primary production in the shelfbreak region

The Ayeyarwady and Thanlwin Rivers, which primarily drain Myanmar, are together one of the largest point sources of freshwater and sediment to the global ocean. Much of the estimated 600 Mt of river sediment annually carried by the combined Ayeyarwady and Thanlwin River system is delivered to a wide continental shelf in the northern Andaman Sea. Called here the Ayeyarwady-Martaban continental shelf, this area is influenced by strong tides, monsoon conditions, and periodic cyclones; however, the processes that dominate dispersal of fluvial material in the coastal ocean of this system remain poorly understood. The shelf exhibits a dramatic asymmetry of the surface morphology and sediment texture in the east–to–west direction, and recent field observations indicate that sediment accumulation rates increase and then decrease offshore of the western portion of the Gulf of Martaban. A three-dimensional coupled hydrodynamic and sediment transport model was used to explore the oceanographic processes responsible for sediment dispersal off the Ayeyarwady-Thanlwin Rivers. Model runs were developed using ROMS (Regional Ocean Modelling System) and SWAN (Simulating WAves Nearshore) to represent oceanographic conditions in the region and suspended sediment transport. The model setup was applied to two cases: one month representative of the winter northeast monsoon, and one month representative of the summer southwest monsoon. Model estimates of sediment dispersal and transport during summer and winter monsoon conditions were analyzed, and compared to spatial patterns found in field measurements, as well as satellite imaging. Within the Gulf of Martaban, both the surface and bottom currents were strongly tidally driven; therefore, seasonal signals were less prevalent. Over the Ayeyarwady delta region, the seasonal signal was large in both wave energy and surface currents, which had a distinct bidirectional pattern: flowing eastward during the SW monsoon and westward during the NE monsoon. Bottom currents over the Mouths of the Ayeyarwady had less seasonality. During the southwest monsoon, wave energy was higher over the Ayeyarwady Delta, leading to increased resuspension of sediment that was subsequently carried into the Gulf near the coast. Sediment resuspension was strongly tidal-dominated within the Gulf of Martaban, with asymmetric tidal trapping making the Gulf region conducive to high turbidity. During the NE monsoon, the Gulf acted as a "mixing-bowl," where there were high sediment fluxes during flood and ebb tides, but very little net export. Meanwhile, the SW monsoon had high tidal fluxes as well as a large net export of sediment out of the Gulf.