WATER QUALITY IN THE LOWER CAPE FEAR RIVER SYSTEM, 1997-1998

by

Michael A. Mallin, Martin H. Posey, Mary L. Moser, G. Christopher Shank,
Matthew R. McIver, Troy D. Alphin, Scott H. Ensign and James F. Merritt

Executive Summary

    Multiparameter water sampling for the Lower Cape Fear River Program (LCFRP) has been ongoing since June 1995. The LCFRP currently encompasses 35 water sampling stations throughout the Cape Fear, Black, and Northeast Cape Fear watersheds. The LCFRP parameters include physical, chemical, and biological water quality measurements, analyses of the benthic macroinvertebrate communities, and assessment of the fish communities. Principal conclusions of the UNCW researchers conducting these analyses are presented below, with emphasis on the period June 1997-May 1998.
    There were no major anthropogenic or meteorolgical disturbances in the basin during this period. Consequently, the data probably reflect baseline water quality conditions. Despite this, low dissolved oxygen (DO) continued to remain a problem in the system. Hypoxic conditions (DO < 5 ppm) occurred at a number of stream stations throughout the watershed from June through October. Along the river mainstem there was a distinct summer oxygen sag which reached its minimum at Horseshoe Bend, between Navassa and Channel Marker 61(Port of Wilmington). Summer DO levels at Station NC11(representing water entering the LCFR basin) were approximately 5.5 ppm, but fell to 3.5 ppm in the sag area. One cause of the sag is BOD loading from industrial point sources, and a second cause is low DO blackwater entering the system from the Black River. Fecal coliform bacteria levels remain a periodic problem as levels in the tributary stream stations often exceeded the state recreational contact water standard following rain events. Fecal coliform sampling also indicated that effluent from selected municipal and industrial point sources exceeded standards periodically.
    Generally high levels of turbidity (>25 NTU) were often present at NC11 and nearby stations, indicating a considerable turbidity load coming downstream from the Piedmont area, a result of non-point source runoff in the upper and middle watershed. There was a second peak turbidity area in mid-estuary near Channel Marker 54 (the estuarine turbidity maximum) where electrochemical processes cause flocculation and subsequent settling of clay particles. Turbidity in the estuary was statistically correlated with river flow, indicating the importance of Piedmont loading of turbidity into the system. Turbidity was also significantly correlated with total nitrogen and phosphorus, demonstrating that these particles can serve as downstream carriers of nutrients.
    High levels of nitrate enter the system at NC11, probably from agricultural non-point source runoff. Additionally, municipal and industrial point sources periodically loaded high nitrate concentrations into tributary streams. These point sources also produced high orthophosphate loads. Phosphate loading to the mainstem is somewhat lower than that of the Neuse and Pamlico Rivers, resulting in generally high inorganic nitrogen to phosphorus ratios. Much of the nutrient load in the two blackwater rivers was organic, as opposed to the predominantly inorganic mainstem Cape Fear load, indicating the ability of the extensive streamside wetlands to improve water quality. Low turbidity in the blackwater rivers compared with high turbidity in the mainstem also demonstrated this wetlands function.
    Chlorophyll a concentrations were generally low to moderate in the LCFR system. However, during low flow conditions dense algal blooms did occur at several tributary stations. Estuarine chlorophyll levels peaked just downstream of the turbidity maximum area, where light was less strongly attenuated. Peak annual chlorophyll levels occurred from July-September, and there was a significant correlation between chlorophyll and water temperature. For the three-year period 1995-1997 there was a highly significant inverse correlation between chlorophyll a and both river flow and Piedmont rainfall. Higher rainfall in the Piedmont and Upper Coastal Plain leads to increased river flow and elevated turbidity and supressed phytoplankton growth in the estuary. During low flow periods there is less turbidity, greater light penetration, and greater phytoplankton production.
    There are two patterns concerning factors controlling estuarine phytoplankton growth; spatial and temporal. Spatially, phytoplankton growth is limited in the turbid upper estuary by low light penetration. Mid-estuary appears to be a transition zone between light and nutrient limitation. In the clearer waters of the lower estuary, nutrient availability limits phytoplankton growth. Temporally, there is seasonal switching of limiting factors. In fall and winter there is elevated flow and turbidity, and light availability limits phytoplankton growth. In spring the lower estuary becomes sensitive to phosphorus inputs, and in summer phytoplankton growth is sensitive to nitrogen inputs.
    The Cape Fear is the largest river in North Carolina with an open connection to the sea, allowing for generally high flushing and flow velocities. It is evident that water quality in the estuary is strongly linked to events in the middle and upper watershed, including the distant Piedmont. Correlation analyses have demonstrated that rainfall at Greensboro in the upper watershed (after a lag time) is strongly related to flow in the lower river. Furthermore, this analysis has shown that rainfall and river flow are both significantly correlated with turbidity and nutrient concentrations in the estuary. These results show that non-point source runoff is an important source of turbidity, nutrients (and probably other pollutants) to the estuary.      Results of the benthos program indicate that the benthic fauna in the Cape Fear River appears to be typical in species composition, spatial patterns and temporal variability to benthic communities in other river-dominated estuaries in the southeastern and mid Atlantic areas of the United States. As is typical of such estuaries, the fauna is dominated by a variety of opportunistic and widespread taxa that are capable of recovery from a variety of disturbances. From a benthic community perspective, these patterns suggest that the Cape Fear River is subject to periodic disturbances (salinity, storm inputs, and possibly sediment, DO and organic stresses), but that the general condition of the Cape Fear River is similar to that of other estuaries with similar levels of development. Functional approaches to benthic ecology indicated general stability in dominant feeding and living mode characteristics. Strong responses in the benthic community to Hurricane Fran indicate that the estuarine system can be negatively affected by upstream inputs, possibly with strong ecosystem consequences, and long-term trends in estuarine health need to be closely monitored. Strong temporal variability in faunal composition and abundances emphasizes the need to conduct monitoring over several seasons and years in order to understand and identify important ecosystem dynamics.
    Fisheries sampling continued to document the long-term effects of anoxic conditions that occurred in September 1996, following the passage of Hurricane Fran. Seasonal trends in fish diversity and abundance indicated that Northeast Cape Fear River fish populations continued to recover through the summer of 1997, while fish populations in the Cape Fear River (which experienced less extreme hypoxia) did not change as dramatically. Overall disease incidence was 1.5%, and was approximately twice as high in periods of warm water temperature (April-October). As in early 1996, the data from this past year indicated that disease incidence is highest among large resident fishes collected using gillnets and the boat electroshocker. As in previous gillnet surveys, non-native species dominated gillnet collections in the upper Cape Fear River stations. However, lower relative abundance of non-native fishes in the Northeast Cape Fear River this year may reflect recovery of native populations after the passage of Hurricane Fran. Commercial drift nets were both more effective and selective than commercial set nets. Commercial fishermen reported higher incidence of fish disease in gillnets set at the same stations as research nets in the period from February - April 1998. However, disease incidence in these gillnets was low (0.9%) relative to overall disease incidence in fish collected using research gear.

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