As part of a MONitoring phase of MODMON , we collected surface sediment samples (top 5 cm) at 185 locations (shown above) extending from New Bern to the mouth of the Neuse River. These samples are in the process of being analyzed -- below are the results.

Porosity values >0.8 indicate that sediments are dominated by fine-grain (silt and clay) material; values <0.6 indicate a major sand component. Comparison of porosity and bathymetric maps show that fine-grained sediments tend to collect in the deepest portion of the channel where wave energy is attenuated with depth.

Concentrations are highest (>10%) in the mesohaline portion of the river (New Bern to Cherry Point) where major phytoplankton blooms are most common. Assuming that sedimentary organic matter has an oxidation state similar to carbohydrate (30 g organic matter/12 g organic carbon), surface sediments in the mesohaline region contain about 25% organic matter. These organic-rich sediments serve as a large reservoir of estuarine oxygen demand: the upper 2 cm of sediment contain >40 mol C m -2 whereas a 4 m deep water column fully saturated with oxygen contains only 1 mol O 2 m -2 . The cross-channel distribution of organic carbon reflects the influence of channel bathymetry on deposition of detrital material: organic-rich sediments are primarily associated with the high porosity, fine-grain sediments that collect in the deeper basins.

Like carbon, sedimentary nitrogen represents a vast nutrient storage reservoir. Remineralization of organic nitrogen in the upper 10 cm of sediment would release enough ammonium to raise water column concentrations to 3600 µM. The highest nitrogen concentrations in the deeper basins are associated with the high-porosity, fine-grain sediments.

d¹³ C values increase systematically from -28 to -21 ‰ between New Bern and Pamlico Sound, reflecting the transition from terrestrial to autochthonous organic matter, changes in isotopic composition of the dissolved inorganic carbon available for phytoplankton growth, and differential expression of the kinetic isotope effect. The absence of isotopically "light" organic carbon (d¹³ C<-26 ‰) downstream from Cherry Point suggests that organic matter produced by blooms is efficiently retained in the mesohaline region and not transported to the mouth of the estuary.

d¹⁵ N values depend on a variety of factors: the nitrogen species taken up by phytoplankton, the source and concentrations of the inorganic nitrogen species, and biogeochemical processes that affect nitrogen cycling (i.e., nitrification, denitrification, and nitrogen fixation). The high spatial variability seen in this plot are indicative of the many influences on the sediment d¹⁵ N signal. Since each of these factors vary with time and location, individual bloom events are likely to be marked by unique d¹⁵ N values that can be used to trace the fate of organic carbon and nitrogen produced during nuisance algal blooms. This tracing method may be effective if calibrated with in-situ measurements during future blooms.

The ratio largely reflects the relative contribution of terrestrial (C:N>20) and autochthonous (C:N~10) organic matter. The sharp gradient down-river from New Bern suggests that material from the watershed is a major source of sedimentary organic matter only near New Bern and at the mouths of the small tributaries.

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