The acoustic Doppler current profiler (ADCP) data permitted separation of bi-directional (up- and downriver) influences (1, 2), making it possible to exclude wind-driven upstream movement in calculating net downstream volume of flow. The flow model is described by the following equations:

Kin <155 m³ s^-1: MB = 192.117 + 0.154846*Kin [1]

Kin ³ 155 m³ s^-1: MB = -27.364053 + 1.570529*Kin [2]

It was based on 3 years of ADCP data, restricted to transects with ³ 95% downstream flow (n = 28 dates; cross-estuary transects in January 1999 to August 2001; 53 cross-estuary transects generated 1,737 ADCP ensembles). The flow model was developed from 28 dates when cross-estuary transects, taken by using boat-mounted ADCP with bottom-tracking capability, met the criteria of 95% flow downriver. If more than 1 transect on a given date met the 95% criteria, the data were averaged (e.g., 12 transects on March 3rd, 2001, and 9 transects on April 4th, 2001). The number of ensembles in each transect depended on boat speed, water depth, ADCP configuration, and other parameters. A total of 1,737 ensembles were used to develop the flow model. A lag structure was not included in evaluating the hurricane impacts. Stations with ³ 90% available precipitation data (www.nc-climate.ncsu.edu) were used to create precipitation contours for the study periods. The comparative delivery volume profiles past the model gate for the 1996 and 1999 hurricane periods are shown in Fig. 8.

Data used to determine volumes for the outlined portion of the Neuse River Estuary (NRE) and Pamlico Sound were acquired from the Office of Special Projects of the National Oceanic & Atmospheric Administration - National Ocean Service (NOAA NOS, 2001) as part of the Coastal Assessment Framework. This was the most extensive and current bathymetric data set available for the Neuse and Pamlico Sound. Estuarine bathymetry was developed by the Special Projects Office of NOS as one component of a project to produce readily available bathymetry that can be used with the Coastal Assessment and Data Synthesis system. Although best known as a navigational tool, bathymetric data are a critical component of hydrodynamic models and can serve as the lower boundary of the water column in visualizing or calculating the volume, circulation and movement of water. To view the data index, refer to the NOAA Coastal Assessment and Data Synthesis System (http://cads.nos.noaa.gov).

Bathymetry for Pamlico Sound was derived from 65 surveys that included 648,402 soundings, with prioritization of recent data). The distance between soundings ranged from ~100 m to 600 m (mean distance of 104 m between soundings, http://cads.nos.noaa.gov). Ten surveys in the southwestern Sound dated before 1900. The remaining 55 surveys were completed from 1913 to 1980. The 90-m resolution, mass-point file for Pamlico Sound was downloaded from NOS and converted to a triangular irregular network (TIN) by using arcview 3.2a. Boundaries for Pamlico Sound and the NRE study area were determined by imputing boundary lines across open water at the intersections of U.S. Geological Survey hydrologic units and the estuarine shoreline. Basin volumes were estimated by calculating the volume of the TIN below a base height of 0.

1996 – One prestorm date (August 27) and three poststorm dates (September 17, October 18, November 19).

1999 – One prestorm date (September 8) and eight poststorm dates (September 6 and 29 October 11-13 and 21, November 11, and December 8).

Data from two other monitoring programs were referenced in this study. Available data from the modmon program in the NRE (http://www.marine.unc.edu/neuse/modmon) supported our analysis that there was a ~2.5-week period of low dissolved oxygen (DO) stress in the lower NSE after Hurricane Floyd. According to modmon data, bottom-water DO was £ 2 mg O₂ liter^-1 in a portion of the lower NRE, and bottom-water salinity was ³ 8-12 practical salinity units (psu). Data from the ferrymon program (www.ferrymon.org), describing conditions along a ferry route in Pamlico Sound, supported our analysis that mean chlorophyll a (chla) levels in western Pamlico Sound (WPS) generally have remained low in posthurricane years, as do other recently published studies (e.g., ref. 3).

The U.S. National Marine Fisheries Service collected commercial statistics on the blue crab in North Carolina until 1978. North Carolina Department of Marine Fisheries (NC DMF) initiated and augmented the collection of hard blue crab landings data in 1978, and in 1994, NC DMF implemented a mandatory Trip Ticket record keeping program for each commercial harvest trip. The use of historical landings data in this study should be viewed cautiously and only as a general indicator of fishing trends, since they are influenced by different data collection methods, market demand, price, fishing effort, weather, availability of alternate species, regulations, and stock abundance. Fishery-independent indices of relative blue crab population abundance were used from two research surveys in North Carolina: (i) NC DMF Program 195 and (ii) an index of early juvenile abundance. NC DMF Program 195 was initiated in 1987 as a trawl survey to sample subadult and adult blue crabs in water >2 m in depth, primarily in Pamlico Sound (4). Program 195 is a stratified random survey design and samples 53 stations during June and September each year. The spatial coverage of sampling in Program 195 is comprehensive for Pamlico Sound, and ranges geographically from the mouth of Albemarle Sound to the Southwest portion of Pamlico Sound and the Neuse River.

The general direction of secondary, pelagic dispersal of early juvenile blue crabs in Pamlico Sound is westward, from initial settlement sites in seagrass beds along the Sound side of the Outer Banks to the western shore of Pamlico Sound (5, 6). Distribution and abundance patterns of early juvenile blue crabs were quantified with suction sampling techniques. Site-specific cohorts of early juvenile blue crabs were quantified over time to determine rates of juvenile loss or gain (5, 6). We examined the density of first and second benthic instars (J1-J2, respectively) in 1 month at a particular site, and compared this with fifth instar (J5) densities the following month at the same site. Thus, by sampling once per month during August to October 1996-1999, we were able to follow two cohorts per year during the fall recruitment season: Cohort 1, J1-J2 stage crabs in August and J5 stage crabs in September; cohort 2, J1-J2 stage crabs in September and J5 stage crabs in October (6). One would expect that the number of juveniles in a cohort would decrease over time due to mortality and emigration. In instances where a cohort increased over time (density of J5 crabs at month t + 1 > density of J1-J2 crabs at month t), we concluded that the cohort had been supplemented by pelagic immigrants. To distinguish an increase in cohort size over time due to postsettlement pelagic dispersal versus an increase due to natural variability, we categorized a cohort as increasing only when the 95% confidence interval (CI) of mean J1-J2 density did not overlap with the CI of J5 density (5). The frequency of secondary pelagic dispersal (6) was examined at four sites near each of the major inlets to Pamlico Sound (Oregon, Hatteras, Ocracoke, and Drum Inlets) and six sites within Pamlico Sound (Point Harbor, Manns Harbor, Engelhard, Swanquarter, Oriental, and Cedar Island). Thus, we examined eight cohorts over a 4-year study period (two per year, 1996-99) for juvenile patterns indicative of postsettlement pelagic dispersal.

Infaunal bivalves generally are tolerant of hypoxic events (7-12), and make use of anaerobic respiratory pathways and other mechanisms for survival in hypoxic conditions (8, 11, 13). The northern quahog (Mercenaria mercenaria) generally occurs at salinities ranging from 15 to 32 psu, but it can survive lower salinities and extended periods of hypoxia (³ 2 mg of O₂ per liter, days to weeks; refs. 8 and 14-16). The eastern oyster (Crassostrea virginica) can also survive extended hypoxia and low salinities (9). It normally occurs at salinities ranging from 5-40 psu, (9, 16, 17), but it can survive salinities as low as 2 psu for weeks (18, 19).

Benthic surveys from the lower NRE indicate that, in addition to occurrence of commercially important species M. mercenaria and C. virginica, clams of little commercial value such as the Baltic tellin (Macoma balthica) and the narrow tellin (Macoma mitchelli) are abundant (20, 21). M. balthica is considered to be tolerant of hypoxia, and even of anoxia (0.15 mg/liter) for days to weeks (22, 23). In the lower NRE, a previous study reported that 90% of M. balthica and M. mitchelli in a 100 km² area were killed when hypoxia lasted more than 3 weeks (20), but hypoxia apparently did not occur for that duration in the lower NRE after the 1999 hurricanes.

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