7.0 Comparative Impacts of Hurricane Floyd on
Water Quality and Benthos
in the Lower Cape Fear Watershed
Michael A. Mallin, Martin H. Posey, Scott H.
Ensign,
Matthew R. McIver, Troy D. Alphin and G. Christopher Shank
7.1 Introduction
In recent years the lower Cape Fear watershed in eastern North Carolina has
been subject to a number of hurricanes. This region contains both rapidly urbanizing
coastal areas and large numbers of concentrated animal operations (CAOs) in the middle and
upper Coastal Plain. Anthropogenic development can magnify the effect of major storms on
water quality damage, as was the case for Hurricane Andrew in Biscayne Bay (Tilmant et al.
1994) and Hurricane Hugo in Charleston, S.C. (Van Dolah and Anderson 1991). While
Hurricane Bertha did little environmental damage in North Carolina in July 1996, Hurricane
Fran in September 1996 led to severe water quality impacts, largely as a result of
anthropogenic factors (Mallin et al. 1999a), as did Hurricane Bonnie in 1998 (Mallin et
al. 1999b).
On September 15, 1999, Category 2 Hurricane Floyd made landfall near
Wilmington, North Carolina (approximately 33055’ N latitude and 77050’
W longitude). The hurricane remained nearly stationary for several hours, deluging the
city with 19 inches of rain. It then moved slowly north-northwest, passing over the
Northeast Cape Fear River watershed and through the upper Coastal Plain. The hurricane
caused power outages and extensive flooding problems that were exacerbated by saturated
ground from the passage of Hurricane Dennis to the north of Wilmington two weeks previous.
A month later, rainfall from offshore Hurricane Irene also added to flooding problems.
Prior to the arrival of Hurricane Floyd, the UNCW Lower Cape Fear River
Program had been collecting monthly physical, chemical, and biological samples at 35
stations throughout the watershed (Fig. 1.1). Benthic invertebrates and fish were also
being sampled at a number of locations. We increased our sampling frequency at a number of
locations following the passage of Floyd to obtain more detailed information on its
aquatic impacts and subsequent ecosystem recovery. This report assesses the effects of
Hurricane Floyd and compares them with those of other recent hurricanes in the Cape Fear
system.
7.2 Methods
Regular monthly physical water quality sampling included temperature,
dissolved oxygen, pH, salinity/conductivity, and turbidity (measured by a YSI 6920
multiparameter water quality instrument). Light attenuation coefficients k were
calculated from vertical PAR measurements taken with a LiCor-1000 Datalogger coupled with
a spherical sensor. Nitrate+nitrite, ammonium, total Kjeldahl nitrogen (TKN), and total
phosphorus were measured using Standard Methods 4500-NO3-F, 4500-NH3-B, 4500Norg-B, and
4500P-E, respectively (APHA 1995). Orthophosphate was measured using EPA Method 365.5 on a
Technicon AutoAnalyzer. Five-day biochemical oxygen demand (BOD5) was assessed using
Standard Methods 5210-B, and fecal coliform bacteria were enumerated by membrane
filtration using Standard Methods 9222-D (APHA 1995). A complete set of samples was taken
from all stations October 4-7. Physical parameters and various other parameters were
sampled on September 17, 22, 23, 29, and 30. From September 22 until November 5 we
maintained a YSI Model 6920 multiparameter instrument in-situ approximately 1 m below the
surface at NCF117, changing instruments approximately every six days. In addition to water
quality samples, we obtained rainfall data from the State Climatologist Office in Raleigh,
N.C., and river flow data from the U.S. Geological Survey in Raleigh. Data on problems
with human sewage treatment plants and flooding or breaches at hog farms were obtained
from the North Carolina Division of Water Quality.
Benthic infauna were sampled at 4 sites in the Cape Fear River system
(Fig 1): NCF6, an oligohaline site in the Northeast Cape Fear River, NAV, an oligohaline
site in the mainstem Cape Fear River, M54, an upper mesohaline site below the city of
Wilmington, and M31, a lower mesohaline site that experiences salt water intrusions from
the adjacent Snow’s Cut. Benthic samples were taken quarterly from winter 1996
through fall 1999 using a Petite Ponar grab (Wildco, Saginaw, Michigan), 15 X 15 cm
opening, 15 cm depth. Hurricanes Fran, Bonnie and Floyd occurred before regular fall
sampling, so monthly grab samples were taken for 3 months after each to monitor recovery
of the benthic community in 1996, 1998 and 1999. Five grab samples were taken at each
station on each sampling date (grabs were kept only if the grab was full to standardize
the volume sampled). Samples were sieved through a 0.5 mm screen, preserved in 10%
formalin, and subsequently transferred to 70% ethanol for later identifications. Species
richness was measured as number of taxa (species or genus for most taxa, higher taxa for
oligochaetes, nemertea and platyhelminthes). Diversity was measured using the
Shannon-Weiner Index (Brower et al. 1998). Because both diversity and species richness are
influenced by sample size, and some grabs could not be collected on some dates because of
debris, these measures were made only for sample/date combinations where at least 3 good
grab samples were collected and were calculated based only on the first 3 grabs collected
at a site.
7.3 Results
Rainfall and River Flow:
Hurricane Floyd dumped approximately 19 inches of rain on Wilmington,
the highest amount in the state (Bales et al. 2000). As it did elsewhere in the state,
Floyd caused severe flooding problems in the Cape Fear watershed. River flow in the Black
and northeast Cape Fear Rivers were at record one-day levels, and peak flows in the
mainstem Cape Fear were second only to those following Fran (Bales et al. 2000).
Light Attenuation:
Turbidity and dissolved organic materials (organic color) are
substances that can reflect or absorb (i.e. attenuate) light in the Cape Fear River
system. The heavy rains that affected the Piedmont and upper Coastal Plain led to the high
light attenuation values seen at NC11 (Table 7.1). These values were
comparable with those seen after Fran (which also strongly affected the Piedmont) and much
higher than those following Bonnie, which had little effect on the upper Cape Fear region.
Light attenuation was elevated compared with long-term averages in the lower river and
estuary as well, and generally ranged between those of Fran and those of Bonnie (Table
7.1). Turbidity is largely derived from the Piedmont and upper Coastal Plain, while highly
colored water is derived from the Black and Northeast Cape Fear Rivers. Other researchers
(L.B. Cahoon, UNCW; E. Koepfler, Coastal Carolina University) reported that a darkly
colored plume from the Cape Fear was clearly evident in the coastal ocean as far south as
mid-South Carolina for weeks after Hurricane Floyd. High light attenuation values will
suppress phytoplankton productivity, and the post hurricane low chlorophyll a
values found in the lower CFR estuary (Table
2.25) as well as offshore (L.B. Cahoon,
personal communication) demonstrated that this suppression did occur.
Biochemical Oxygen Demand:
Five-day BOD assessments were made on water collected at a number of
stations following Hurricane Floyd. BOD in the mainstem at NC11 was somewhat greater than
the long-term average, but values in the lower mainstem and the Black River were slightly
lower than average (Table 7.2). Only at
NCF117 on the Northeast Cape Fear River were BOD values elevated. However, these values
were low compared with BOD values recorded following Hurricanes Fran and Bonnie (Table
7.2). BOD levels after Bonnie were especially elevated compared with the average and
compared with those recorded after Floyd. It is likely that the extensive flooding led to
deposition of BOD-inducing materials (i.e. hog waste and human sewage) high onto river
floodplains, and dilution of those materials remaining in the river channels.
Dissolved Oxygen:
There were widespread decreases in dissolved oxygen following Hurricane
Floyd, although they were not as severe as those following Hurricanes Fran and Bonnie.
Hypoxia was encountered throughout the lower river and estuary, but near-anoxic conditions
only appeared in the Northeast Cape Fear River near Castle Hayne at station NCF117
(Fig. 7.1). This condition took three weeks to develop at NCF117, compared to anoxia appearing
within one week following Hurricanes Fran and Bonnie at that location (Mallin et al.
1999a; 1999b). Following Floyd, hypoxic conditions persisted for about a month in the
lower river and estuary, and for about six weeks in the Northeast Cape Fear River.
Nutrient Inputs:
Total nitrogen concentrations were normal in the tributary rivers and
low in the estuary following Floyd (Table
7.3). However, ammonium was elevated considerably in the Northeast Cape Fear River
compared with both the long-term average and values after Fran and Bonnie (Table 7.3).
Nitrate was much lower than normal at all stations sampled, as it was after Fran and
Bonnie as well (Table 7.3). Nitrate was likely low due to the hypoxic conditions that
followed the hurricane. Total P and orthophosphate were both higher than normal at most
stations in the blackwater tributaries, but concentrations were normal in the lower
mainstem river and the estuary (Table 7.3).
Benthos:
Benthic infauna, those animals living on or in the estuarine
sediments, form an important trophic link between production (microalgae, detritus) and
higher animals (fish, crabs and shrimp, and birds). As such, changes in this group may be
representative of broader ecosystem effects of disturbances such as hurricanes.
Additionally, benthos are relatively sedentary after settlement, often constrained to an
area of only a few meters throughout their life, and can be relatively long-lived, with
lifespans of several months to a year for some species. This site-specificity and
longevity makes this group particularly susceptible to environmental fluctuations in the
estuary. Factors that may be particularly important in affecting survival and growth of
benthos include dissolved oxygen levels (Boesch et al. 1976, Dauer 1984, Aschan and
Skullerud 1990, Tsutsumi 1990, Weston 1990, Mallin et al. 1999a), inputs of nutrients
(Beukema 1991, Posey et al. 1995, 1999), changes in productivity on the scale of weeks or
more (Posey et al. 1999), increased sedimentation, changes in sediment characteristics
(Wilson 1991), and increased turbidity (Rhoads and Young 1970). As indicated above, there
were significant changes in many of these physical parameters associated with the multiple
hurricane events in the Cape Fear system, so one might predict concordant changes in the
benthic community as well.
Like most estuarine systems, diversity and species richness show some
fluctuations over time in the Cape Fear Estuary, reflecting seasonal variations in
physical conditions and predators and interannual variations in recruitment patterns
(Boesch et al. 1976, Dauer 1984; Fig
7.2). There was a trend towards lower species
richness and diversity after the first hurricane of the series (Hurricane Fran), but there
was no long term, estuarine-wide trend for either species richness or diversity over time
with successive hurricanes. However, when sites are examined separately, there was a
steady decline in species richness at the lowest estuarine site, M31, over the four-year
period (Fig 7.3). Unlike the other three sites, this site is dominated primarily by
mesohaline polychaetes such as Mediomastus, Streblospio, Nereis and Laeonereis,
with many rarer polychaetes present on some sampling dates (Table 7.4). Many of these species cannot
maintain populations with prolonged freshwater exposure and we believe the decline in
species richness over time at M31 reflects loss of some of the more marine-oriented taxa
with repeated freshwater events, combined with possible stress from hypoxia.
Total faunal abundance varied strongly over time and responded
differently among various hurricane events. However, decreases in total benthic abundances
occurred following each hurricane (Fig.
7.2). The identity of dominant taxa showed some
responses to hurricanes, with insects and amphipods showing some increase after hurricanes
in the upper sites relative to the non-hurricane year and to summer patterns. Both taxa
groups are able to exhibit quick population responses to disturbance events because of
their direct development (amphipods) or dispersal by winged adults (insects) (Mallin et
al. 1999a). The most obvious effect at lower sites was the loss of selected taxa after
hurricanes, especially the polychaete Streblospio and juvenile clams. Only one
site, the upper estuarine site in the mainstem Cape Fear River (NAV), showed consistency
over time in dominance patterns, with oligochaetes being the most common taxa during all
seasons.
Fecal Coliform Bacteria – The Potential Human Health Issue:
Following Hurricane Floyd there were low fecal coliform counts
throughout the watershed (Table 7.5).
This was in contrast to the very high fecal coliform counts seen following Bonnie.
However, we were unable to sample fecal coliforms for nearly three weeks after Floyd,
which may have been sufficient time for significant mortality or settling of these
bacteria. We note that samples collected two weeks after Fran did not contain unusually
high counts, while collections one week after Bonnie showed very high counts (Table 7.4).
Also, the unusually high flooding may have diluted even polluted waters. We also note that
NCDWQ personnel found high fecal coliform counts (2,500 CFU/100 mL) in the upper Northeast
Cape Fear River within one week of Floyd’s landfall (B. Wrenn, NCDWQ, personal
communication).
Regardless of the low counts in the river, streams and lakes within the
City of Wilmington showed unusually high fecal coliform counts within a week after Floyd.
As part of the Wilmington Watersheds Program UNCW has been collecting water quality
samples on a monthly basis throughout Wilmington’s watersheds since 1997. Very high
counts were found in Greenfield Lake, Burnt Mill Creek, Barnard’s Creek and Smith
Creek after Floyd, and lesser but still elevated counts were seen in those locations after
rains from Hurricane Irene impacted the city (Table
7.6). Sewer overflows and extensive urban runoff were the most likely causes of these
high counts.
7.4 Discussion
While the flooding from Hurricane Floyd was
devastating to human property and cost up to fifty lives statewide, the effects on water
quality were generally less severe compared to those documented after Hurricanes Fran and
Bonnie (Burkholder et al. 1999; Mallin et al. 1999a; 1999b). Both Hurricanes Fran and
Bonnie caused massive fish kills in the Northeast Cape Fear River from severe hypoxia and
anoxia caused by the heavy BOD loads. That was not the case with Floyd. The hypoxia after
Floyd was both delayed and milder than that following the other two hurricanes, and the
effect on fish appeared to be much less severe. The largest fish kill we found occurred
three weeks after Floyd, and consisted of only 18 carcasses at NCF117, the scene of
large-scale kills on other occasions. We suspect that the flooding caused the smaller fish
to move up over the floodplain, where abundant food sources are found (Junk et al. 1989)
while dilution of BOD-causing materials in the river channels ameliorated the effect of
hypoxia on the larger fish in the channels.
To summarize responses by the benthic community to multiple hurricanes
in the Cape Fear River system, long-term declines in species richness occurred at the
lowest site (M31) and there were short-term, though variable, effects on species
composition and abundance at most sites. However, the benthic community appeared
remarkably resilient to repeated hurricane events with few major long-term changes
apparent. This is probably related to the opportunistic lifestyle of the dominant species
found in this estuary (Dauer 1984, Posey et al. 1996, Mallin et al. 1999a). Maranzellaria,
Mediomastus, the dominant insect larvae, and oligochaetes are well-known to be
opportunistic taxa that can quickly colonize an area after a disturbance and exhibit rapid
recovery in population numbers (Boesch et al. 1976, Aschan and Skullerud 1990, Whitehurst
and Lindsay 1990, Posey et al. 1999). The dominance of these taxa in the Cape Fear estuary
is typical of many river-dominated estuarine systems, but may also reflect the long
history of disturbance from channelization, dredging, and upstream development and
agricultural runoff that has occurred. The benthic community in the Cape Fear estuary may
reflect those taxa that have already undergone selection for periodic, strong disturbance
events.
Floodwaters in the lower rivers and estuary showed low fecal coliform
counts when examined three weeks after Floyd. In contrast, many LCFRP stations exhibited
high fecal coliform counts one week following Hurricane Bonnie, demonstrating that there
is an important human health concern in post-hurricane water bodies. Additionally,
watersheds within the Wilmington City limits showed very high fecal coliform counts a week
after Floyd’s passage. Post-hurricane sources of fecal coliforms in the rivers
include swine waste from flooded or breached lagoons, runoff from sprayfields, and
rerouted human sewage that was incompletely or not treated. Sources in the city include
sewage overflows and urban runoff (likely from dogs and other animals). The increase in
potential exposure to pathogenic microbes appeared to lead to significant increases in
gastrointestinal and respiratory illnesses in eastern North Carolina during a week-long
period a month after Floyd that was statistically compared with the same week in 1998 (CDC
2000). We recommend that civic health authorities post warnings at all public boat ramps
and in the newspapers that water may not be safe for human contact for a period of at
least three weeks following hurricane events.
Hurricanes Fran and Bonnie, and to a lesser extent Floyd, caused water
quality degradation and increased risk of human exposure to potential microbial pathogens.
The lower Cape Fear watershed contains many point-source sewage outfalls and the largest
concentration of industrial-style swine farms in North Carolina (Mallin 2000). Hurricane
impacts to these pollutant sources led to large inputs of nutrients, oxygen-consuming
organic wastes (BOD) and fecal bacteria into the river system. Following the hurricanes
the most severe damage, as measured by water quality indicators and losses in the benthic
community, was in the Northeast Cape Fear River. This tributary hosts both point sources
and numerous CAOs; additionally, lower flows than the Cape Fear mainstem and strong tidal
action helps to retain pollutants for extended periods. While the river hydrology cannot
be changed, better management of animal wastes from CAOs on the floodplain and reliable
backup generating systems to treat human waste during storm events will help reduce
post-storm pollutant impacts to the biotic communities.
7.5 Acknowledgments
Funding was provided by the Lower Cape Fear River Program and the Water Resources Research Institute (Project # 70171). For field and laboratory help we thank Booty Baldridge of Cape Fear River Watch, Jesse Cook, Virginia Johnson, Doug Parsons, Christian Preziosi, Ashley Skeen, and Tracey Wheeler. Helpful information was provided by Rick Shiver, Jimmie Overton, Kent Wiggins of the North Carolina Division of Water Quality, Ryan Boyles and Wendy Sellers of the State Climate Office at North Carolina State University, and Alex Cardinell of the U.S. Geological Survey.
7.6 References Cited
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