Introduction
Estuaries are the most productive ecosystems in the world. They are formed where
seawater on the rising tide mixes with freshwater in a semi-enclosed area like Charleston Harbor.
Nutrients wash downstream from the land, and move into the estuary from the open ocean.
Currents and tides bring in juvenile invertebrates and fish to grow and develop in the estuarine
waters. Coastal estuarine habitats, the natural environments in which organisms live, reproduce,
feed and find shelter include salt marshes, wetlands, tidal creeks and open water tidal rivers and
bays. The Charleston Harbor estuary habitat supports more than 80 species of plants, 250
species of birds, 570 species of invertebrates and finfish and 67 species of mammals - from tiny
worms to the magnificent dolphins that are often sighted in our tidal rivers.
Salt Marsh
Salt marshes fringe estuarine waters,
forming in the intertidal zone (between high
and low tide) along beaches and tidal rivers.
South Carolina has more salt marsh than any
other state on the east coast (about 400,000
acres) and Charleston County has the most
salt marsh of any county in South Carolina.
These productive marshes are
ecologically complex, with fluctuating levels
of salinity and temperature as the tides move
in and out. Most low salt marsh areas
(nearest to the water) are dominated by
smooth cordgrass (Spartina). A mix of plants
including Spartina and black needlerush
(Juncus) are found in the higher areas of marsh. While the leaves of Spartina are largely inedible,
the roots and stalks serve as protection for marsh animals. As the marsh grass decays in the fall,
it decomposes into a rich soup of nutrients providing food for both fish and invertebrates like
shrimp, blue crabs, stone crabs and clams. Fiddler crabs, marsh snails and marsh mussels are also
typical marsh inhabitants. In intertidal areas, the retreating tide exposes pluff mud. The resident
crabs, snails and worms burrow into the mud to escape heat and predators. Where tide pools
remain on the surface, animals like mummichugs (small fish) and grass shrimp thrive.
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The marshes also serve as high quality bird habitat, attracting, for example, blue herons
and American and snowy egrets, seaside sparrows, clapper rails and marsh rails as year-round
marsh residents. Waterfowl (ducks and geese) and wading birds find shelter in the marshes over
the winter. Other inhabitants of our coastal marshes include muskrats, rabbits, diamond back
terrapins and even alligators in less salty areas.
Tidal Creeks and Rivers
Tidal creeks begin in uplands, draining into larger creeks until they join a tidal river or
bay where their waters flow into the ocean. Tidal creeks represent about 17 percent of the state’s
estuarine waters by surface area. Numerous tidal creeks wind through the salt marshes of the
Charleston Harbor basin, forming a network of highly productive habitat and serving as critical
spawning areas and nursery habitat for fish, shellfish, birds and mammals. Seatrout, jack
crevalle, flounder, spadefish, spot, black drum, blue crab and brown and white shrimp are among
the many aquatic animals the mature in the shelter of these shallow, muddy-bottomed creeks
before moving into deeper waters.
Oyster Beds
The tidal rivers that flow through the
marsh areas support mussels and clams as
well as oysters. Oysters, common inhabitants
of intertidal areas along our rivers and creeks,
build reefs where small invertebrates like
juvenile shrimp and larval crabs find shelter
from predators. Many fish like flounder,
black sea bass and Atlantic spadefish use the
reefs as nursery habitat. Other fish species
come to the reef areas to feed.
Population Growth and Land Development
As humans move into our coastal watershed and change the way land is used, the ability
of estuarine habitats to function normally is challenged. Based on United States census data, the
eight coastal counties of South Carolina grew 49% between 1980 and 2000, and another 22%
through 2010. The Charleston Metropolitan Area (Charleston-North Charleston-Summerville) is
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one of the 10 fastest growing
metropolitan areas in the US.
The population grew from an
estimated 549,033 in 2000 to
664,607 in 2010, an increase
of 21 percent. Estimated
population growth from
2010 to 2011 is another 2.6
percent!
Land development
along the coast is occurring
at six times the rate of
population growth. In
Charleston, for example, the
population grew by 40%
from 1973 to 1994 while size
of the urban area grew by 240%, creating urban sprawl and greatly increased impervious land
cover (e.g., roads, sidewalks, driveways and parking lots that prevent water from being absorbed
into the soil). The size of the urban area is expected to increase to 868 square miles by 2030,
covering 65 percent of the land area. Tidal creeks in particular are intimately linked to human
land-based activities through the stormwater that flows into them from developed land and
impervious surfaces. This stormwater runoff carries with it a cocktail of pollutants including
gasoline and oil, fertilizers, pet wastes, and sediments into our tidal creeks.
Because of the economic and ecological importance of estuarine habitats, particularly
tidal creeks, state and federal partnerships have been formed to assess their health and to
understand and learn to address the impacts of human development. The section that follows
describes one multi-year program that include sampling in our basin and discusses their results
and recommendations.
South Carolina Estuarine and Coastal Assessment Program (SCECAP)
SCECAP, established in 1999, is a joint program of South Carolina’s Department of
Health and Environmental Control and Department of Natural Resources (SCDNR) as well as
the NOAA Center for Coastal Environmental Health and Biomolecular Research (CCEHBR) in
Charleston. The program goals are to monitor the health of the coastal zone, to develop ways to
measure the condition of the coastal habitats, to report the results to the public and to use the data
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to enhance management and
regulatory decision-making. The
results of the SCECAP studies
are made available in a series of
reports, including the report
produced in 2011 which covered
the 2007-2008 sampling season
and summarized the overall
results from 1999-2008.
SCECAP sampling program
From 1999 to 2006, 5060 stations were sampled
annually along the South
Carolina coast for water quality,
bottom sediments (including
contaminants and toxicity),
benthic (bottom dwelling)
invertebrates, fish and large
crustaceans. Half of the samples
were collected in tidal creeks,
defined as water bodies with a
width of less than 100 meters
from salt marsh bank to salt
marsh bank.
The rest of the
samples were collected from tidal rivers, bays and sounds. In 2007-2008, the number of sites was
reduced to 30 along the entire state coastline. In all sampling years, the sites are sampled once
during the period June to August, a critical nursery habitat period.
Water Quality Index: Water quality parameters measured included dissolved oxygen
(DO), Total Nitrogen, Total Phosphorus, pH, fecal coliform bacteria and chlorophyll a (an
indicator of the amount of phytoplankton in the water too high a value may indicate the presence
of excess nutrients). All six of these measures were used to develop an overall Water Quality
Index (WQI) for each site.
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Sediment Quality Index: The
concentration of contaminants in bottom
sediments is measured, including metals, PAHs,
PCBs, PBDEs and pesticides (see p. x for
definitions and more information about these
pollutants). The concentration of sediment
pollutants is compared to published Effects
Range Median Quotients (ERM-Q), levels
known cause biological impacts on organisms.
Sediment toxicity was assessed using two
“bioassays,” which observe the survival of a
photo luminescent bacterium and a juvenile clam
over time when exposed to the sediment. Total organic carbon is also measured. These factors
are used to develop a Sediment Quality Index (SQI) for each site.
Benthic Index of Biological Integrity: Small benthic invertebrates are collected and
used to develop a Benthic Index of Biological Integrity (B-IBI). A B-IBI uses information about
the makeup of the sampled invertebrate community to assess the degree of human impact on the
health of the community. Small invertebrates are important biological indicators of habitat
quality because they are sensitive to environmental changes and usually sedentary. They are also
a key food source for fish, shrimp, and crabs.
The SCECAP program uses the
Water Quality Index, Sediment Quality Index
and B-IBI to develop an overall Habitat Quality
Index for each of the sites sampled.
Results
Water Quality Index: The Water
Quality Index for each site is reported as either
good, fair or poor. Overall, for the period 1999
to 2008, between 82 and 89% of the sites
sampled scored as good for water quality. The
variation from year to year was thought to be
related to the amount of rainfall for the individual sampling periods (i.e., years of lower rainfall
result in less stormwater runoff and improved water quality). Over the same period, one of the
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areas of the state with the highest incidence of fair to poor water quality was the upper Ashley
River.
Sediment Quality Index: For the overall sampling period from 1999-2008, Charleston
Harbor was among the areas with the greatest incidence of poor sediment quality. Generally, the
overall sediment contaminant
concentrations seen in the study
were not high in relation to levels
that are known to cause
biological impacts.
Benthic Index of
Biological Integrity: For the
2007-2008 sampling period, the
percent of habitat sampled that
scored in the good biological
integrity category was the highest
(95%) seen during the sampling
program. The lowest (65%) was
seen in the 2003-2004 sampling
period. The improvement in
2007-2008 was again thought to be due to lower rainfall during the period. During periods of
low rainfall, salinity is increased, resulting in an increase in the number of species that can
inhabit the estuarine areas and raising the B-IBI score. During the 2007-2008 sampling period,
one of the two stations statewide scoring as poor was Clouter Creek, about one-half mile from its
confluence with the Cooper River. Several Charleston Harbor sites have also scored as poor
during the 1999-2006 sampling periods.
Commercially and Recreationally Important Species: Fish species collected over the
course of the 1999-2008 collection period throughout the state included spot, Atlantic croaker,
weakfish, silver perch and Atlantic spadefish. Crustaceans collected included blue crab, and
white and brown shrimp. All of these species were more abundant in tidal creeks than in open
water habitats. There was some evidence of declining numbers and density of fish over time,
especially spot, weakfish and croaker.
Overall Habitat Quality Index for the South Carolina Coast: A Habitat Quality Index
developed for the SCECAP program integrates the Water Quality Index, the Sediment Quality
Index and the Benthic Biological Integrity Index scores for each sampling site. Overall, for the
state, more tidal creek habitat was considered to be in fair or poor condition (23% fair, 7% poor)
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compared to open water habitat (3% fair, 3 % poor) for the 2007-2008 period. For the
1999-2008 period, the rivers draining into Charleston Harbor showed a persistent pattern of
degraded habitat quality (see the map below). This habitat degradation is likely the result of a
combination of historical industrial activity and high-density urban development.
Clearly, the SCECAP study supports our understanding that human development and land
use patterns have an impact on estuarine habitat quality, particularly on sensitive and valuable
tidal creek habitat.
Tidal Creeks: Sentinel Habitat
In our watershed, researchers identified the upper reaches (headwaters) of tidal creeks as
“sentinel habitats,” acting as first responders to the physical, chemical, and biological changes
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resulting from human activities. Two major projects have assessed the impacts of coastal
development on tidal creeks in South Carolina. The resulting data have been used to develop and
verify a model that demonstrates the physical and biological responses to human-induced
changes in tidal creeks.
Charleston Area Tidal Creeks
(1994-2002)
During the period from
1994 to 2002, 23 headwater tidal
creeks were sampled along the
South Carolina coast. Eighteen of
these were located in the
Charleston Harbor Watershed.
The project was conducted by the
South Carolina Department of
Natural Resources in
collaboration with NOAA’s
Center for Coastal
Environmental Health and
Bimolecular Research. The
research team was led by Dr. Fred Holland, now retired, formerly the Director of Hollings
Marine Laboratory at Fort Johnson on James Island.
The research team selected local tidal creeks to represent four types (forested, suburban,
urban, and industrial) based on the land use characteristics of the area draining to each individual
creek (its watershed). Habitats north of Charleston (Crab Hall/North Inlet and Murrell’s Inlet)
and south of Charleston (Marine Corps Air Station in Beaufort and Sawmill) were also sampled
for comparison. All other creeks sampled were in the Charleston Harbor Watershed with the
exception of Long Creek, which is slightly to the west (near Bohicket Creek on Wadmalaw
Island). See the map below. Two of the creek’s experienced additional development during the
period from 1992 to 1999, changing their watershed land use type.
In addition to land use and population density in each creek’s watershed, the study also
looked at water quality, sediment contaminants (heavy metals, PAHs, PCBs and pesticides), and
fecal coliform bacteria. The bottom-dwelling (benthic) community in each creek was evaluated
for the relative abundance of stress-sensitive invertebrate benthic species (animals that tend to
decrease in polluted water bodies) and stress tolerant benthic species. These benthic species are
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used to indicate the extent of pollution impacts in a water body (biological indicator species).
Seine samples of fish and crustaceans were also collected from each site.
Charleston Results
Holland’s research team
found population density was
significantly associated with the
percentage of land covered by
impervious surface in suburban and
urban creek watersheds. As might be
expected, industrial creek watersheds
tended to have lower population
density associated with high
impervious surface area.
Generally, the levels of trace
metals in the sediments (copper,
chromium, lead, zinc, cadmium and
mercury) were 2 to 10 times higher in
the urban and industrial tidal creeks.
These creeks also had higher
concentrations of PAHs, PCBs, and
the pesticide DDT when compared to
tidal creeks in suburban and forested
areas. Biologically, there were fewer
stress-sensitive species and more
Map showing the location of sites sampled during the stress-tolerant species in the creeks
1994-2002 tidal creek project. Forested/reference creeks are
from watersheds with high impervious
indicated by a circle and developed/impacted creeks by a
Economically
diamond. The insets show relatively pristine sampling sites to surface cover.
the North and South.
important species of shrimp (brown,
white, and pink) and the bottomfeeding fish spot declined with increasing impervious cover. In some samples, grass shrimp, a
key prey species for economically important fish like red drum, spotted sea trout, and southern
flounder, also declined with increasing impervious cover. Additionally, measured concentrations
of fecal coliform bacteria increased as percent impervious cover increased.
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North Carolina, South Carolina, and Georgia Tidal Creeks (2005-2006)
A similar tidal creek research project was undertaken in 2005-2006. The goal of the
2005-2006 project was to assess the relationship between development and land use changes and
the ecosystem status of 19 tidal creek systems from southern North Carolina to southern Georgia.
Of these 19 tidal creeks systems, 7 were located within the Charleston Harbor Watershed. The
study was conducted by NOAA’s Hollings Marine Laboratory and the NOAA CCEHBR,
SCDNR and the South Carolina Sea Grant Consortium, with funding from NOAA’s Ocean
Human Health Initiative. Dr. Denise Sanger, currently with SCDNR, led the research team.
Within each tidal creek system, creeks were divided into two sections, the intertidal
(shallower, narrow) headwater section and the subtidal (deeper, wider) sections for comparison.
The land-use type for each creek was characterized as suburban, urban, industrial, or
undeveloped. The sampled parameters included dissolved oxygen, salinity, nutrients, chlorophyll
a, sediment contaminants (metals, PAHs, PCBs, pesticides, PBDEs (flame retardant chemicals of
emerging concern), pathogens, biological community, and shellfish tissue contaminants. Samples
were collected during from June through August in 2005 and 2006.
North Carolina, South Carolina, and
Georgia Results
The overall results of the
2005-2006 Southeast Tidal Creek
Project supported the conclusions of the
earlier South Carolina-focused
sampling. Tidal creeks are sensitive to
coastal development and can serve as
early warning sentinels of water quality
and habitat degradation in tidal rivers
and bays. As the percent impervious
cover in a creek watershed increased,
so did indicators of deteriorating
environmental quality. An important additional finding was that smaller intertidal (headwater)
creeks were more sensitive to land use and impervious surface in their surrounding watersheds
than the subtidal creeks located further downstream towards the creek mouths. This is likely due
to the greater amount of runoff upland and the tidal flushing and dilution in larger portion of the
creek downstream.
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Focusing on the results in the Charleston Harbor Watershed, the intertidal portions of the
most urbanized creeks (particularly New Market and Shem Creeks) had the highest levels of
PAHs, PCBs, and pesticides seen across the multi-state study. New Market sediments had the
highest levels of PCBs (107 ng/g dry weight). Orangegrove Creek on the Ashley River had
elevated levels of total DDT, Chromium, and Copper, likely reflecting the legacy of industrial
contamination. Of all the sites sampled in the southeast, PDBEs were found only in the
Charleston area sites (James Island, Orangegrove, Bulls, Shem and New Market – all urban and
suburban watersheds). These intertidal creeks are important sentinel areas for these emerging
contaminants and potentially for other emerging contaminants as well.
Managing the Impacts of Development and Land Use
The Tidal Creek Projects support our understanding that increasing human population
density results in changes in land use patterns which lead to altered water movement and
ultimately chemical and microbial contamination and changes to living resources. The results of
these studies have been synthesized and used to develop management guidance for land-use
decision makers and citizens. The following sections summarize material from the 2008
document Tidal Creek Habitats: Sentinels of Coastal Health.
Impervious Cover
When percent impervious surface is 10 to 20% or below, tidal creeks are able to function
normally, physically, chemically, and biologically. When impervious cover exceeds 10 to 20 %,
the natural infiltration process that filters pollutants is decreased and pollutants are rapidly
transported into water bodies. For example, in a forested ecosystem 5% of rainfall may runoff
into tidal creeks. The remaining 95% evaporates or infiltrates into the soil. In a developed
watershed 15-75% of the rainfall runs directly off the land. Thus, for a given rainfall event,
runoff is 3 to 25 times greater in a developed versus a forested tidal creek watershed.
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Linking Humans, Environmental Change, and Living Resources
As a result of the Tidal Creek Projects, a model was developed to explain how human
population growth is linked to the physical and chemical changes in the environment. Our
crowded coasts and the impervious land cover and stormwater runoff associated with human
development alter the water flow and quality, change salinity, alter sediment characteristics and
increase contaminant levels. As a result, the living community in tidal creeks is impacted,
decreasing shrimp abundance and reducing food for important fish species.
Note that, in addition to ecological costs, the societal costs of impervious land cover
greater than 20 percent include increased flooding due to increases in volume and speed of runoff
into tidal creeks, beach and shellfish bed closures, public health risks due to bacterial
contamination and ultimately loss of revenue from fisheries and recreational activities.
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Take Action! Reduce Impervious Surface
Human population density is a major stressor for tidal creeks and for the ecological
function of estuarine environments. Urbanization of coastal watersheds increases the risk of
biological degradation and reduces our ability to use estuarine resources safely.
Given the projected increases in population expected in our watershed over the coming
decades, it is essential to take action to reduce urban sprawl and minimize the impacts of
stormwater runoff from existing impervious surfaces. The research conducted for the Tidal Creek
Projects support a set of solutions to manage and reduce impervious surface at the regional,
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neighborhood, and household level. Regional scale solutions include purchasing development
rights and requiring new development to be mixed-use and preserve open space. At the
neighborhood scale solutions range from developing standards to prevent pollutants from
entering tidal creeks to monitoring water quality conditions in tidal creeks and reporting the
results to the public. Charleston Waterkeeper’s Recreational Water Quality Monitoring Program
is one such solution. At the household scale solutions include:
•
•
•
•
•
•
•
•
Minimize the amount of new impervious cover that is created on one’s property
Use pervious alternatives such as porous concrete and pavers
Direct surface water runoff into swales and vegetated buffers to trap pollutants and
increase infiltration allowing slower movement of stormwater into creeks
Maintain naturally vegetated open spaces and buffers to decrease the amount and rate of
runoff
Design and implement rain gardens and/or constructed wetlands to provide added
treatment of stormwater.
Follow directions when applying fertilizers and pesticides as well as properly strong and
disposing of hazardous household products
Dispose of trash properly and recycle when possible
Pick up and properly dispose of pet wastes
Conclusions
Both the SCECAP program and the Tidal Creek Projects make the connection between
land development and the societal and environmental costs very clear. Our salt marshes and tidal
creeks serve as home for some part of the life cycle of three-quarters of all of our commercially
harvested seafood. Both as individuals and as a society, we must act as good stewards of our
estuarine habitats by: (1) encouraging the SCDHEC and SCDNR to continue the ongoing holistic
estuarine monitoring program (SCECAP) to assess the condition of our coastal habitats, and (2)
implementing the recommendations of the Tidal Creek Projects to manage and reduce polluted
runoff from impervious surfaces.
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