Biogeochemistry of the sediments of the Pamlico and Neuse River estuaries, North Carolina

We completed the f i r s t general survey of the composition, distribution, and sources of surface sediments in the Pamlico ( P R E ) and Neuse River Estuaries ( N R E ) . The in ters t i t ia l (pore) waters of the sediments were also studied t o estimate the decomposition stoichiometry of or anic matter and the diffusive 9 flux of NHdf, FRP ( f i 1 terable reactive P ) , SO4 -, Si , DIC (dissolved inorganic carbon), and C1. These data were obtained monthly in the PRE and occasional ly in the NRE a t six stations on downriver transects from the freshwatersaltwater transition zone t o the mouth of each estuary a t Pam1 ico Sound. With few exceptions, the sediments of the two estuaries were quite simil a r . Mud deposits (>go% s i l t + clay) covered -90% of the area of b o t h , and the remainder was sandy sediments near shore or upriver of the transition zone. Sandy sediments were more widespread in the NRE than the PRE. Mud sediments t o 10 cm depth contained ( i n pmol g-1 dry mass) from 2560 t o 5135 total organic carbon (TOC), 282 t o 448 total Kjeldahl nitrogen (TKN), and 22 to 73 total phosphorus ( T P ) . There was a progressive decrease in TOC and TP concentrations downriver, b u t the TKN concentrations did not change. The PRE muds appeared to be relatively enriched in TP as compared w i t h those of the NRE. The contents of these fractions (per m2) in muds were 1.3 t o 5.4 times greater than those of sands, and are equivalent t o the amounts of carbon, nitrogen and phosphorus that phytoplankton take up from the water column i n 1 t o 4 years. TP a t the head of both estuaries was almost wholly composed of an acid extractable phosphorus ( E P ) fraction. EP decreased downriver suggesting that progressively more of the TP was associated with organic matter downriver. Exchangeable ammonium represented only 0.5% t o 2% of the TKN pool Stable carbon isotope data indicate that deposition of estuarine phytoplankton carbon governs the organic chemistry of the top meter of sediments in the lower two thirds of the estuaries, while in lower-salinity upriver areas, freshwater carbon i s evident. After deposition and burial, there i s l i t t l e alteration of 61% of organic matter, although higher isotope ratios of the DIC i n pore waters suggested that methanogens and other autotrophic bacteria are active. S p r i n g phytoplankton blooms associated w i t h nutrientrich runoff provide a significant input of organic carbon t o the sediments. Total annual diffusional f l u x of N H ~ + and RP from the sediments a t a l l stations in the PRE averaged 369 and 38 mnol m respectively, and represents 7.7 and 5.4% of average annual phytoplankton N and P uptake i n surface waters. Bioturbation and advection of pore waters, as well as large pulsed nutrient fluxes from anoxic bottom waters, could increase these estimates almost 5 fold. During high wintertime runoff, nutrient inputs from the watershed supplement phytoplankton N and P requirements by an amount similar to that from the sediments i n summer. Earlier reports and some of our data support Hargrave's (1973) model of increased importance of nutrient recycl i n g within the water column a t the expense of sedimentation and subsequent benthic metabolism as water depth increases. However, because only about half of the calculated N and P of phytoplankton sedimentation i s returned t o the water column, storage and recycling within the sediments must occur. Evidence for