A Surface Sediment Array to Monitor How Geochemical Gradients are Related to Hypoxic Conditions in Upper Narragansett Bay, RI

Warren Prell 1 , David Murray 1, Keira Heggie 1, and Emily Saarman 1

1 Brown University, Providence RI, 02912 E-mails: dmurray@brown.edu ; Warren_Prell@brown.edu ;Keira_heggie@brown.edu; emily_saa@hotmail.com

Surface sediments integrate the environmental-related inorganic and organic fluxes and thus provide reliable estimates of long-term conditions. However, the environmental inputs may be modified by depositional and oceanographic processes. The goal of this project is to assess the geochemical gradients in the mid-upper Narragansett Bay and the Providence River and how they are related to depositional processes and to areas of persistent hypoxia. To establish the database for measuring geochemical gradients, we used a standard Ponar grab to collect a high-resolution array of over 300 surface samples during the summer of 2003. Stations in the Providence River were collected at about 400 m intervals and stations in the mid-upper Bay were collected at about 800 m intervals. Samples were processed in the field, sub-sampled for geochemical analyses, and sieved at 2mm to retain the coarse fraction for benthic fauna and flora characterization. Preliminary analyses show distinct patterns of grain size and the expected correlation of higher organic carbon concentrations with fine-grained (dominantly silt and clay) samples. Sediments with >20% silt and clay average 3.95% organic carbon, with individual samples as high as 8.0%. Analyses of nitrogen concentrations, N isotopes, elemental composition, and biofacies will be integrated with the grain size and carbon concentration data.

Keywords: Sediments, Hypoxia, Narragansett Bay, Estuaries, Dissolved Oxygen, Spatial and Temporal Variations