3.0 Short-Term (Diel) Water Quality Variations in the
Lower Cape Fear River System

Michael A. Mallin and Scott H. Ensign

3.1 Introduction

    Water quality in lotic systems can be affected by both natural factors and pollution events. In order to fully understand the effects of the latter, an understanding of the former must be acquired. Natural factors include meteorological variations, diel (day vs. night) effects, and tidal effects in estuaries and lower rivers. The use of fixed in-situ instrumentation can be a powerful tool for gaining insight into natural short-term variability in water quality at a given location. The Lower Cape Fear River Program (LCFRP) utilized this technology to assess diel and tidal effects on water quality variability.
    From summer 1998 to summer 1999 the LCFRP conducted diel water quality monitoring at five stations, on four to six occasions per site. Prior to this research, little data existed concerning water quality dynamics over a short-term period (24 hr) in the LCFR system. The five stations in this study were chosen to represent the three main rivers in the LCFR system and two zones within the estuary. Station NC11 (Fig. 1.1) represents the mainstem of the Cape Fear River, above the major inputs from its Coastal Plain tributaries. Station B210 (Fig. 1.1) is located on the Black River, a fifth-order tributary of the mainstem Cape Fear River, and Station NCF117 is located on the Northeast Cape Fear River, the other fifth-order tributary of the mainstem Cape Fear River. Station NAV represents the oligohaline zone of the Cape Fear Estuary, and Station M42 is located in the mesohaline zone of the estuary (Fig. 1.1).

3.2 Methods

    At the five stations multi-parameter data logging sondes (YSI 6920) were placed at the surface and bottom of the water column and set to log data twice an hour. A lead mooring attached to a foam float was used as the deployment station, with the sondes attached approximately 0.5 m from the bottom and 0.25 m from the surface. Temperature, conductivity, salinity, dissolved oxygen, turbidity and pH were measured, with probes calibrated the morning of deployment. Data were uploaded from the sondes to a PC using YSI’s Ecowatch software.

3.3 Results and Discussion

    Station NC11 was sampled in September and October 1998, and in March, May, and September of 1999 (Figs. 3.1-3.5). Station NC11 exhibited a tidal range of 0.7 m, but it was located well upstream of the oligohaline estuary and did not exhibit tidally driven variation in specific conductance or salinity. The water column here was well mixed, with few differences between surface and bottom measurements. The exception was pH, which was stratified on several occasions (September and October 1998 and September 1999). There were diel changes in surface temperature in which peaks occurred in the late afternoon and reached lowest temperatures in the early morning hours, probably due to solar irradiance. On three occasions (September and October 1998 and September 1999) NC11 demonstrated signs of increased phytoplankton productivity during daylight hours on the first day (Figs. 3.1, 3.2 and 3.5). Surface dissolved oxygen and pH showed concurrent daily maxima in the afternoon, with either no signal or a minor one the following morning. Increased water temperature appeared to be associated with these productivity peaks as well.
    Station B210 was sampled in July and December 1998, and in January, June, and August 1999 (Figs. 3.6-3.10). Station B210 had a tidal range of 0.5 m, but did not exhibit tidally driven changes in specific conductance or salinity. There were general diel changes in water temperature from maxima in the late afternoon of the first day, to minima in the morning of the second day. On two occasions (Figs. 3.7 and 3.10) there were general decreases in dissolved oxygen that followed water temperature. As with NC11, there were few differences in parameter concentrations between surface and bottom.
    NCF117 was sampled in July, September, and December 1998, and in February and June 1999 (Figs. 3.11-3.15). NCF117 is a freshwater station, but parameters are periodically influenced by tidal variation. The best example of this phenomenon is July 1998, when specific conductance, dissolved oxygen, and pH all showed clear tidal signals (Fig. 3.11). Clear diel signals were not apparent for water quality parameters, except for a slight temperature signal during some collections. This station generally shows the most severe impacts from hurricanes in the entire LCFR system (Mallin et al. 1999). This is clearly evident in the September 1998 sample, collected 12 days after Hurricane Bonnie. Dissolved oxygen was at near anoxic levels throughout the 24 hr period, showing the influence of the hurricane driven BOD load on the system (Fig. 3.12). Unusually low values for both specific conductance and pH showed the effects of heavy freshwater loads from rainfall and riparian swamps to this river.
    Station NAV was sampled in August, September, and November 1998, and in January, May, and August of 1999 (Figs. 3.16-3.21). Station NAV is an oligohaline station where the tidal range can reach nearly two meters. As such, the salinity varied considerably during our sampling, with a maximum range of 0 to 14 ppt in August 1998 (Fig. 3.16). Tidal movement was the dominating factor controlling water quality variability over the short term. The incoming tide led to concurrent increases in salinity, specific conductance, pH, and usually dissolved oxygen. However, an exception to this occurred in September 1998 following Hurricane Bonnie, when the incoming tide brought low dissolved oxygen upstream, likely from the Northeast Cape Fear River input downstream near Wilmington (Fig. 3.17). The only apparent diel signal was reflected by increased water temperature during the afternoons of the first day of deployment. Dissolved oxygen and pH were often higher in the bottom water than on the surface (especially so at high tide), probably a result of higher DO in the more saline waters carried upstream from the middle estuary. Turbidity showed much more variability near bottom than near the surface, with peaks often at high tide or on the rising tide.
    Station M42 in the mesohaline estuary was sampled in November 1998, and January, May, and August 1999 (Figs. 3.22-3.25). The tidal signal was even stronger at M42, ranging as much as 2.5 m in January 1999 with subsequent salinity variation from 3 to 17 ppt (Fig. 3.23). In general incoming tide led to increases in salinity, specific conductance, pH, and dissolved oxygen. As with Station NAV, bottom water turbidity pulses were often associated with high or rising tide, but pulses appeared with falling tide on occasion as well (Figs. 3.22, 3.23, 3.24, 3.25). There were slight surface diel signals for dissolved oxygen afternoon maxima in May and August 1999 (Figs. 3.24 and 3.25) potentially due to phytoplankton productivity and co-occurring with increased water temperature.

3.4 Summary

    Short term dissolved oxygen variability in the LCFR system was primarily controlled by tidal movement, at locations where tidal movement was strong. A diel signal for dissolved oxygen was not seen in three of the stations, and occurred only twice at NC11 and twice (faintly) at M42. Diel signals were common only for water temperature among all stations. Nearer the estuary, the tidal signal controlled dissolved oxygen, pH, salinity, specific conductance, and bottom water turbidity pulses. In the riverine stations the waters appeared well mixed, with stratification generally confined to the estuary where dissolved oxygen and pH were associated with higher salinity waters. These data confirm an earlier study by the LCFRP (Mallin et al. 1996) in which discrete depth water samples were collected at the surface and bottom for chemical parameters and statistically compared. It was also plain from this study that both diel and tidal variability can be overwhelmed by hurricane forced changes in water quality, particularly in the Northeast Cape Fear River.

Mallin, M.A., G.C. Shank, M.R. McIver and J.F. Merritt. 1996. Water Quality in the Lower Cape Fear Watershed, 1995-1996. Center for Marine Science Research, University of North Carolina at Wilmington, Wilmington, N.C.

Mallin, M.A., M.H. Posey, G.C. Shank, M.R. McIver, S.H. Ensign and T.D. Alphin. 1999. Hurricane effects on water quality and benthos in the Cape Fear Watershed: Natural and anthropogenic impacts. Ecological Applications 9:350-362.

3.6 Acknowledgments

    For funding we thank the Lower Cape Fear River Program, the North Carolina General Assembly, and the Water Resources Research Institute of the University of North Carolina (Project #70171). Field and laboratory assistance was provided by Jesse Cook, Virginia Johnson, Matt McIver, Doug Parsons, Chris Powell, Christian Preziosi, Brad Schroeder, Chris Shank, Ashley Skeen and Tracey Wheeler.

	Back to Table of Contents for 1999-2000 Annual Report
	Back to Lower Cape Fear River Program Homepage