Although instantaneous measures of basic water
quality variables (temperature, salinity, dissolved oxygen, pH) were obtained
during the primary visit to each site, the continuous measures of these
parameters from the 25-hr instrument deployments provide the most comprehensive
information because they include numerous measures during both day and night
over two 
complete tidal cycles. Therefore, these data are used as the primary
data set in our analyses of these four water quality parameters. The other
measures of water quality (total and dissolved nutrients,
BOD5,
TSS, turbidity, TOC, total alkalinity, chlorophyll-a, and fecal coliform
bacteria) obtained at each site represent instantaneous measures collected
during the primary site visit.
The SCDHEC has developed State regulations 61-68 and 61-69 to protect the water quality of the state (SCDHEC, 2001b). The water quality standards include numeric and narrative criteria that are used for setting permit limits on discharges to waters of the state, with the intent of maintaining and improving surface waters “to a level to provide for the survival and propagation of a balanced indigenous aquatic community of flora and fauna and to provide for recreation in and on the water.” Occasional short-term departures from these conditions will not automatically result in adverse effects to the biological community. The standards also recognize that deviations from these criteria may occur due solely to natural conditions and that the aquatic community is adapted to such conditions. In such circumstances, the variations do not represent standards violations, and critical conditions of the natural situation, e.g., low flow, high temperature, minimum dissolved oxygen, etc., are used as the basis of permit limits.
All data collected by SCECAP from field observations and water samples are related to water quality standards for the state’s saltwater regions (SCDHEC, 2001b) where possible. Because SCECAP samples are limited to a summer index period and generally do not include multiple samples over time, the data are not appropriate for use in USEPA 303(d) or 305(b) reporting requirements. Additionally, there are no USEPA or state water quality standards for many of the parameters measured in this program. For those measures, values are compared to data compiled for a 5-year period (1993-1997) by the SCDHEC Bureau of Water in their routine statewide Ambient Surface Water Quality Monitoring Network (SCDHEC, 1998a). For this report, values exceeding the 75th percentile of all values measured (> method detection limit) in the state’s saltwater habitats indicate evidence of elevated concentrations and values exceeding the 90th percentile of all saltwater measures indicate high concentrations. The SCDHEC historical database on water quality was primarily obtained from larger open water bodies. Therefore, caution should be used in interpreting data obtained from tidal creek sites since high or low values observed for some parameters may represent “normal” conditions. For some water quality variables, such as dissolved nutrients and chlorophyll-a, criteria or guidelines published in other reports are used for comparison of conditions (e.g. Bricker et al., 1999; USEPA, in review) since no appropriate SCDHEC data were available.
Temperature data are collected primarily to relate with other water quality variables that are affected by this parameter. The average bottom water temperature based on the continuous 25-hr data collected at each site was 29.3oC for both the tidal creek and open water sites. This average was very comparable to the average temperatures observed in each habitat during the 1999-2000 survey (Van Dolah et al., 2002a). The range of mean bottom temperatures during 2001-2002 was 26.0 to 31.8oC among the tidal creek sites and 26.4 to 31.1oC among the open water sites (data online). The slightly greater variation in average bottom water temperature observed in the tidal creek habitats compared to the open water sites reflects the effects of solar heating on these shallow water sites. The instantaneous surface and bottom temperatures showed similar ranges and differences between habitats. The average difference between surface and bottom temperatures measured in either habitat type was < 0.2oC during both sampling years. Fauna inhabiting South Carolina estuaries are generally well adapted to the temperature ranges observed in this program.
Salinity influences the distribution and diversity of many invertebrate and fish species. Changes in salinity at a site can also provide a measure of stressful conditions if there is a large variation in concentrations over short time periods. The average bottom salinity of all tidal creek sites sampled during the 2001–2002 survey was 30.6 ppt and ranged from 9.5 to 37.4 ppt (Data Tables). The average bottom salinity among the open water sites was 29.5 ppt and ranged from 10.0 to 38.1 ppt. The salinities observed during this survey period were slightly greater than those observed in 1999 – 2000 (Van Dolah et al., 2002a, c), with 73% of the creek habitat and 63% of the open water habitat having an average bottom salinity of > 30 ppt (see figure below). This represents near full-strength seawater and reflects the effects of severe drought conditions that persisted throughout this sampling period. There was no significant difference between bottom salinities observed at the creek versus open water sites (p = 0.06).
As with temperature, the mean difference between the instantaneous surface and bottom salinities was relatively small (< 0.5 ppt for the tidal creeks and < 1.2 ppt for the open water sites) within each year (Data Tables). Salinity ranges observed at each site were also generally less than 15 ppt, except at four open water and five tidal creek sites. Two of those sites (RO01108 and RO01130) had greater than a 20 ppt range in salinity, which may represent stressful conditions (Holland et al., 2004). Until additional data are available, no criteria have been established by the SCECAP program to identify stressful conditions using salinity.
Comparison of the average bottom salinity concentrations observed in tidal creek and open water habitats during 2001-2002, and estimates of the percent of the state's coastal habitat that represented various salinity ranges based on the average of measurements obtained over 25-hours at each station.
Dissolved oxygen (DO) is one of the most critical water
quality parameters measured in this program. Low dissolved oxygen conditions
can limit the distribution or survival of most estuarine biota, especially if
these conditions persist for extended time periods (see
Diaz and Rosenberg,
1995;
USEPA, 2001 for reviews). Dissolved oxygen criteria established by the SCDHEC for “Shellfish Harvesting Waters” (SFH) and Class SA saltwaters are a
daily average not less than 5.0 mg/L and a low of 4.0 mg/L (SCDHEC, 2001b).
Class SB waters should have dissolved oxygen levels not less than 4.0 mg/L.
Since the SCECAP program was designed to sample only during a summer index
period when DO levels are expected to be at their lowest, DO measurements
collected in this program probably represent short-term worst-case conditions
that may not reflect conditions during other seasons or longer time-averaging
periods. However, SCECAP data provide useful measures of average DO
concentrations observed in South Carolina’s coastal habitats when DO levels may
be limiting, and it identifies areas within the state where this is occurring.
For the purposes of this study, mean or instantaneous DO concentrations > 4 mg/L
are considered to be good for summer time periods, values < 4 mg/L and > 3 mg/L
are considered to be fair (i.e., contravenes one portion of the state
standards), and average or instantaneous measures < 3 mg/L are considered to be
poor and potentially stressful to many invertebrate and fish species.
The average bottom DO concentration at the open water stations during the 2001–2002 survey was 5.0 mg/L, with approximately 89% of the state’s open water habitat having a mean DO > 4.0 mg/L based on the 25-hr instrument deployments (see figure below; Data Tables). Only one open water site (representing approximately 3% of the state’s open water habitat) had an average DO < 3.0 mg/L (RO01147). This site also had an instantaneous bottom DO < 3.0, with a surface water DO concentration of 4 mg/L.
The average bottom DO concentration observed at tidal creek sites was 4.5 mg/L, with 78% of this habitat having a mean DO value > 4 mg/L. The difference in mean DO values observed among the creek versus open water sites was highly significant (p < 0.001). Approximately 9% of the state’s tidal creek habitat had mean DO levels < 3.0 mg/L and 13% of this habitat had DO levels > 3 and < 4 mg/L. The mean values observed in creek and open water sites were similar to those observed during 1999-2000. In both survey periods, tidal creek sites generally had a much greater range in DO concentrations than the open water sites, as well as a higher percentage of sites with marginal or poor DO.
Although numeric state DO standards apply to all waters, the SCECAP data suggest that lower DO concentrations in tidal creeks may be normal during the summer months compared to larger water bodies. When making regulatory decisions in such situations, the practice of considering natural background conditions seems appropriate. Even so, creek sites with the mean DO levels < 3 mg/L may not fully support biological assemblages inhabiting those sites, especially during periods when DO levels are less than 2 mg/L (hypoxic conditions). Hypoxic conditions are known to be limiting to many estuarine and marine biota (Gibson et al., 2000).
The instantaneous measures of bottom DO were, on average, slightly lower than the mean DO values obtained from the 25-hr deployment of water quality meters among both the open water and tidal creek sites (Data Tables). These differences were not statistically significant (p > 0.1) and a similar pattern was observed in 1999-2000 (Van Dolah et al., 2002a). There was also no significant difference between the surface and bottom measures when all sites were considered together within each habitat (mean differences were < 0.3 mg/L in either habitat, p > 0.08). However, as noted in the 1999-2000 survey, instantaneous DO measures resulted in a higher percentage of the state’s coastal water habitat coding as fair or poor (38% vs. 22% of the tidal creek habitat and 13% vs. 11% of the open water habitat). The instantaneous bottom DO measures at each site were only weakly correlated to the mean bottom DO obtained from the 25-hr instrument deployment (r2 = 0.25). While instantaneous measures of DO and other water quality parameters are the most reasonable approach for SCDHEC routine year-round assessment of coastal water quality, instantaneous measures do not appear to reflect the same DO conditions measured over both day and night during all tidal stages. Similarly, one summer-time measure may not accurately reflect long-term impairment of a site relative to low DO conditions.
Comparison of the average dissolved oxygen concentrations observed in tidal
creek and open water habitats during 2001-2002, and estimates of the percent of
the state's coastal habitat representing various DO ranges based on the average
of measurements obtained over 25-hrs at each station. Red indicates poor
DO conditions, yellow indicates fair DO conditions but below state standards,
light green represents good conditions that are considered acceptable for
supporting biota during summer months, and dark green represents good conditions
above the state DO standard.
Measures of pH provide another indicator of water
quality in estuarine habitats that has often been ignored by other sampling
programs at the state or national level. Measures of pH are based on a
logarithmic scale, so even small changes in the value can result in significant
stress to estuarine organisms (Bamber, 1987,
1990;
Ringwood and
Keppler, 2002). Unusually low or high pH values may indicate the presence
of pollutants (e.g., release of acids or caustic materials) or high
concentrations of carbon dioxide (Gibson et al., 2000).
Because salinity and alkalinity affect the pH of estuarine waters, SCDHEC has
established water quality standards that account for these effects. The pH in
Class SA and SB tidal saltwater areas should not vary more than one-half of a
pH unit above or below effluent-free waters in the same geologic area having a
similar salinity,
alkalinity
and temperature, and values should never be lower than 6.5 or higher than 8.5.
Shellfish Harvesting waters (SFH) should not deviate more than 0.3 units from
effluent-free waters. Based on these criteria, pH criteria were established for
SCECAP assessments using data collected from pristine environments sampled in
1999-2000 (e.g., Cape Romain, ACE and North Inlet National Estuarine Research
Reserves, SFH class saltwaters) to identify pH levels that were considered to
represent good, fair, and poor conditions for polyhaline waters (> 18 ppt;
Van Dolah et al., 2002a, c). For polyhaline,
effluent-free waters, the average pH in the 1999-2000 study was 7.6 (Van
Dolah et al., 2002a). Therefore, pH levels below 7.1 are below the 0.5 pH
unit variation allowed for effluent-free waters and are considered to be poor pH
conditions. Values below 7.4 pH units are considered to be only fair since they
represent the lower 10th percentile of all pH records observed for polyhaline
waters during the 1999-2000 survey. Values > 7.4 pH units are considered to be
good for polyhaline waters. Criteria are still not established for lower
salinity waters since the number of sites that had salinities < 18 ppt are still
too limited in number due to the extreme drought conditions experienced since
1999.
The overall average pH observed in 2001-2002 based on the 25-hr measures was 7.5 in tidal creek habitats and 7.7 in open water habitats (see figure below, Data Tables). The average instantaneous surface pH measures collected at all sites within each habitat type were within 0.1 pH unit of the average bottom pH based on the continuous measurements, and all average values were very similar to the averages observed in 1999-2000 (Van Dolah et al., 2002a, c). The difference in mean pH values was statistically significant between habitats (p < 0.001) with a higher percentage of the state’s creek habitat having pH values considered to be only fair or poor compared to the state’s open water habitat (see figure below). A similar trend was noted in 1999-2000 (Van Dolah et al., 2002a). None of the stations sampled in 2001-2002 had mean or maximum values that exceeded the maximum (8.5 pH units) or minimum (6.5 pH units) criteria established by SCDHEC, at any time during the 25-hr instrument deployment period at each site (Data Tables). Therefore, although we can’t apply the SCECAP criteria to the 10 sites with average salinities less than 18 ppt, those sites at least had pH values within the maximum range accepted by SCDHEC.
Comparison of the average bottom pH concentrations observed in tidal creek and open water habitats during 2001-2002, and estimates of the percent of the state's coastal habitat representing various bottom pH ranges based on the average of measurements obtained over 25-hrs at each station. Red indicates poor pH conditions below SCDHEC standards when compared to natural waters, yellow indicates fair pH conditions within the lower 10th percentile of historical pH values observed in pristine polyhaline waters, and green represents good pH relative to historical data for pristine polyhaline waters.
Errata: The pH values shown in the pie figure and text are in error. The correct values are as follows: RO, 91% ≥7.4, 9% >7.1&<7.4; RT, 82% ≥7.4, 14% >7.1&<7.4, 4% <7.1. Mean values apply to all stations statewide.
Nutrients:
Nutrient concentrations in
estuarine waters can become high due to runoff from upland urban and suburban
developments, agricultural fields adjacent to estuarine habitats, riverine input
of nutrient-rich waters from inland areas, and atmospheric deposition. High
nutrient levels can lead to eutrophication of estuarine waters resulting in
excessive algal blooms (including harmful algal blooms), decreased dissolved
oxygen, and other undesirable effects that adversely affect estuarine biota (Bricker
et al., 1999). Currently, there are no state standards in South Carolina
estuarine waters for the various forms of nitrogen (except ammonia) and
phosphorus. Therefore, the SCECAP data are compared to SCDHEC’s historical
database (SCDHEC, 1998a) to identify waters showing
evidence of elevated nutrients. Values below the 75th percentile of the
historical database are considered to be normal, values above the 75th
percentile and below the 90th percentile are considered to be moderately
enriched, and values above the 90th percentile are considered to be highly
enriched. Dissolved nutrient concentrations are also compared with guidelines
identified by NOAA (Bricker et al., 1999) and the
USEPA (in review).
Nitrogen:
Total nitrogen (TN), as measured by the SCDHEC laboratory, is
best represented by the sum of nitrate-nitrite and total Kjeldahl nitrogen (TKN).
Based on historical SCDHEC (1998a) data, TN values above 1.29 mg/L are
considered to be highly enriched since they represent the upper 90th percentile
of the historical records. Values > 0.95 mg/L and < 1.29 are considered to be
moderately enriched since they are above the upper 75th percentile of the
historical records and below the 90th percentile of those records. Values <
0.95 mg/L are considered to be normal. In 2001-2002, the average concentration
of TN was 0.53 mg/L among the tidal creek sites and 0.47 mg/L among the open
water sites (see figure below). In contrast to the 1999-2000 survey, this
difference was not statistically significant (p = 0.159) and the average values
observed in both habitats were lower than observed in 1999-2000 (Van Dolah
et
al., 2002a). Approximately 82% of the nitrogen was in the form of TKN (organic
fraction plus ammonia) when all stations were considered collectively. Average
nitrate-nitrite values in the creeks and open water sites were only 0.03 and
0.07 mg/L, respectively, which was similar to the values observed in 1999-2000.
Using the sum of the detectable values for nitrate-nitrite and TKN as an
indication of total nitrogen (TN) enrichment, only about 3% of the state’s creek
habitat and 4% of the state’s open water habitat had moderately elevated TN
concentrations using SCECAP criteria, and < 1% of either habitat had highly
enriched nutrient values (see figure below,
Data
Tables). These TN values observed in 2001-2002 are comparable to those
observed in open water habitats in 1999-2000 and lower than those observed
during that time period in tidal creek habitats. One of the two sites with high
TN values was located in a creek off the Old Chehaw River (RT01603) and the
other site was located in Winyah Bay (RO01113). Only the latter station also had
elevated concentrations of chlorophyll-a, another measure of possible estuarine
eutrophication (see Chlorophyll-a section).
Most of the dissolved nitrogen was in the form of dissolved organic nitrogen (DON) in both habitats (81% among all sites combined; Data Tables). Due to differences in analytical protocols used to estimate TN and TDN, combined with a high percentage of missing TN values in the 2001 data set, it is not possible to directly compare TN versus TDN values. However, based on the results obtained using the two procedures, it is likely that most of the TN measured at the SCECAP sites was in the form of TDN. Results obtained in 2000 also indicated that the majority of TN was in the form of TDN (Van Dolah et al., 2002a, c).
Measures of dissolved inorganic nitrogen (DIN) provide another estimate of possible estuarine eutrophication that is being used by the USEPA (in review). In the 2001-2002 survey, the average DIN concentrations at the tidal creek and open water sites were 0.11 and 0.13 mg/L, respectively. The USEPA (in review) considers DIN values between 0.1 and 0.5 mg/L to represent fair conditions and values above 0.5 mg/L to represent poor (or enriched) conditions. In our survey, only one site (RO01112) had a DIN value > 0.5 mg/L and there was no direct positive correlation with DIN and chlorophyll-a (see chlorophyll-a section). In fact, chlorophyll-a concentrations (one measure of possible eutrophication) were generally highest at stations with very low DIN concentrations. While this could be expected due to the utilization of DIN by phytoplankton, the DIN criteria used by the USEPA do not appear to be very indicative of possible eutrophic conditions in SC waters based on other measures we collect. Most of the DIN at station RO01112 was in the form of ammonia rather than nitrate/nitrite.
Comparison of the average total nitrogen (TN) concentrations observed in tidal creek and open water habitats during 2001-2002, and estimates of the percent of the state's coastal habitat representing various TN ranges that represent normal (green), moderately enriched (yellow), or highly enriched (red) values relative to SCDHEC historical data.
Based on SCDHEC historical survey data (SCDHEC, 1998a), average total phosphorus levels > 0.17 mg/L are considered to be highly enriched since they represent the upper 90th percentile of the historical observations. Values > 0.09 and < 0.17 mg/L are considered to be moderately enriched and represent concentrations above the 75th percentile and below the 90th percentile of historical records. Values < 0.09 mg/L are considered to be good. The average total phosphorus concentration (TP) measured by SCDHEC in 2001-2002 was 0.073 mg/L at the creek sites and 0.058 mg/L at the open water sites (see figure below). In contrast to the previous survey in 1999-2000, this difference was not statistically significant (p = 0.2) and values among the stations were generally lower. Only 5% of the state’s creek habitat and 1% of the state’s open water habitat had total phosphorus concentrations that exceeded the 90th percentile of the historical database collected by SCDHEC from 1993-1997 (SCDHEC, 1998a). Only four of the 20 sites with moderately enriched to highly enriched TP values also had elevated chlorophyll-a concentrations, which suggests that this measure may not be strongly related to phytoplankton enrichment in SC waters (see chlorophyll-a section).
Dissolved inorganic phosphorus (DIP) is used by the USEPA (in review) as another measure of possible estuarine eutrophication that may lead to undesirable phytoplankton blooms if DIP concentrations become excessive. The USEPA considers DIP levels less than 0.01 mg/L to be good for east coast estuaries. Levels between 0.01 – 0.05 mg/L are considered to be fair and concentrations greater than 0.05 mg/L are considered to be poor. The average DIP concentrations observed in tidal creek and open water habitats during this survey period were 0.033 and 0.029 mg/L, respectively. Approximately 12% of the state’s tidal creek habitat and 6% of the open water habitat had DIP concentrations greater than 0.05 mg/L. As noted for DIN, DIP values showed little correspondence to high chlorophyll-a concentrations, and the highest DIP concentrations that we have measured during SCECAP sampling since 2000 have generally had low chlorophyll-a concentrations (see chlorophyll-a section). While high DIP concentrations may be a useful indicator of possible estuarine eutrophication in other states or regions, the lack of any clear relationship between DIP and chlorophyll-a concentrations in South Carolina waters suggests that other nutrient measures collected by SCECAP should be given higher priority in our assessment of overall water quality.
Comparison of the average total phosphorus (TP) concentrations observed in tidal creek and open water habitats during 2001-2002, and estimates of the percent of the state's coastal habitat representing various TP ranges that represent normal (green), moderately enriched (yellow), or highly enriched (red) values relative to SCDHEC historical data.
Dissolved silica (DS) measurements are primarily collected for the National Coastal Assessment Program. Low silica levels can be a limiting factor in the production of certain forms of phytoplankton, primarily diatoms. Average silica concentrations in the 2001-2002 survey were 1.41 mg/L at tidal creek sites and 1.07 mg/L at open water sites. These DS concentrations represent relatively high values that should not be a limiting nutrient for phytoplankton species in South Carolina waters since the ratio of dissolved inorganic nitrogen to dissolved silica at all sites (mean ratio = 0.09) was well below the 1:1 ratio considered to be critical (Day et al., 1989).
Our measure of phytoplankton biomass in the water column is based on chlorophyll-a concentrations. Other phytoplankton pigments were also examined using HPLC analyses (see phytoplankton section). High chlorophyll-a concentrations provide an indication of possible estuarine eutrophication since phytoplankton respond rapidly to enriched nutrient concentrations and can form blooms that result in poor water quality (e.g., low DO, large DO variations) and the presence of harmful algal species. Bricker et al. (1999) and the USEPA (in review) consider chlorophyll-a concentrations above 20 µg/L to be high or poor, respectively. SCECAP sites with chlorophyll-a concentrations above 20 µg/L are also considered to be poor based on these studies. Chlorophyll-a values >12 µg/L represent the upper 75th percentile of all chlorophyll-a concentrations measured by the SCECAP program and are considered to be fair. Values < 12 µg/L are considered to be good.
The average chlorophyll-a concentration in creek habitats was 10.2 mg/L and 10.0 mg/L at the open water sites (see figure below). This difference was not statistically significant (p = 0.4) and represents relatively low concentrations based on our SCECAP database collected since 1999 (i.e., < 75th percentile). The CDF analysis indicated that only 7% of the state’s open water habitat and 1% of the state’s tidal creek habitat had chlorophyll-a concentrations > 20 mg/L, which is considered to be elevated by Bricker et al. (1999) and the USEPA (in review).
When chlorophyll-a concentrations were greater than 20 mg/L, the majority of those samples had TN concentrations > 0.5 mg/L. If additional data collected by this program support this pattern, the current thresholds representing enriched TN concentrations may be adjusted to better reflect the possibility of observing high phytoplankton concentrations. However, it is important to note that many samples with relatively high TN concentrations did not have high chlorophyll-a concentrations. The much weaker relationship between TP and chlorophyll-a suggests that this is not a limiting nutrient form in SC waters.
Comparison of TDN and TDP concentrations versus chlorophyll-a concentrations indicated that these variables were not correlated, and none of the samples with high chlorophyll-a concentrations had concentrations > 0.8 mg/L for TDN and 0.9 for TDP. These values are below the thresholds identified by NOAA as indicative of high nutrient concentrations that may result in algal blooms (Bricker et al., 1999).
Similarly, comparisons of DIN and DIP versus chlorophyll-a concentrations were also not correlated. The USEPA (in review) has developed criteria for these nutrients that correspond to good, fair, or poor levels of DIN and DIP. Using their criteria, only one of the sites sampled in 2000-2002 had poor (high) DIN concentrations and that site had a relatively low chlorophyll-a concentration. SCECAP sites with high chlorophyll-a concentrations always had DIN concentrations < 0.1 mg/L. In contrast, a high percentage of the SCECAP sites sampled in 2000-2002 had DIP concentrations considered to be poor by the USEPA. Only three of these sites also had chlorophyll-a concentrations the USEPA considers to be high. Rather, most of the SCECAP sites with high chlorophyll-a concentrations had DIP values < 0.03 mg/L. Thus, the USEPA criteria for DIN and DIP do not appear to be effective indicators of high phytoplankton concentrations indicating possible eutrophication.
Biochemical Oxygen Demand:
The five-day Biochemical Oxygen Demand (BOD5)
is a measure of the amount of oxygen consumed by the decomposition of
carbonaceous and nitrogenous matter, both natural and man-made wastes, in the
water column. Although
BOD5
is regulated by National Pollutant Discharge Elimination System (NPDES) permits
to protect instream dissolved oxygen concentrations, there are no freshwater or
saltwater standards for natural waters. Both the SCDHEC water quality
monitoring program and the SCECAP program include measurements of
BOD5
in order to obtain information on areas where unusually high values may occur,
but
BOD5
has been dropped from the integrated measure of water quality since there are no
clear guidelines or state criteria applicable for saltwater habitats. Based on
historical SCDHEC data (1998a),
BOD5
values > 2.6 mg/L are considered to be very high since they represent the upper
90th percentile of the historical observations. Values > 1.8 and < 2.6 are
considered to be moderately high since they are above the 75th percentile of
historical records but below the 90th percentile, and values < 1.8 mg/L are
considered to be normal.
Comparison of the average
five-day biochemical oxygen demand (BOD5) concentrations observed in tidal creek
and open water habitats during 2001-2002, and estimates of the percent of the
state's coastal habitat representing BOD5 ranges that represent normal(green),
moderately high (yellow) and very high (red) relative to SCDHEC historical data.
Average BOD5 concentrations found at creek sites sampled in 2001-2002 were 0.6 mg/L and the average at open water sites was 0.4 mg/L (see figure above), which was much lower than the average values observed in the 1999-2000 survey (Van Dolah et al., 2002a, c). As in the 1999-2000 survey, this difference was not statistically significant (p = 0.5); only a slightly higher percentage of the state’s creek habitat had elevated BOD5 levels that exceeded the 75th and 90th percentiles of historical detectable observations when compared to open water habitat (figure above, Data Tables). High BOD5 concentrations may be indicative of poor water quality.
Fecal
Coliform Bacteria:
Coliform bacteria are present
in the digestive tracts and feces of all warm-blooded animals and public health
studies have established correlations between adverse human health effects and
the concentration of fecal coliform bacteria in recreational, drinking, and
shellfish harvesting waters. State fecal coliform standards to protect primary
contact recreation requires a geometric mean count that does not exceed 200
colonies/100 mL based on five consecutive samples in a 30 day period and no more
than 10% of the samples can exceed 400 colonies/100 mL. Fecal coliform criteria
established by the SCDHEC for “Shellfish Harvesting Waters” (SFH) to protect
for shellfish consumption requires that the geometric mean shall not exceed 14
colonies/100 mL and no more than 10% of the samples can exceed 43 colonies/100
mL (SCDHEC, 2001b). Since only a single fecal
coliform count was collected at each site, compliance with the standards cannot
be strictly determined, but the data can provide some indication of whether the
water body is likely to meet standards. Although not all of the waters sampled
are classified as “Shellfish Harvesting Waters,” for SCECAP, we consider any
sample with > 43 colonies/100 mL to represent fair conditions (i.e., potentially
not supporting shellfish harvesting) and any sample with > 400 colonies/100 mL
to represent poor conditions (i.e., potentially not supporting primary contact
recreation).
The average of fecal coliform measurements obtained during the 2001-2002 statewide assessments were 30.4 colonies/100 mL in the creeks and 13.3 colonies/100 mL at open water sites (see figure below). This difference was statistically significant (p = 0.01). The higher average for the tidal creek sites was largely due to the presence of > 300 colonies/100 mL at two sites (RT01628, RT022021). Using the SCECAP criteria and CDF analyses, approximately 73% of the state’s creek habitat was good, 24% was fair, and 3% was poor with respect to fecal coliform concentrations. Approximately 83% of the state’s open water habitat had good fecal coliform levels, 17% had moderately high fecal coliform concentrations, and no sites had coliform colony counts > 400 (Data Tables). The higher fecal coliform counts observed in creek habitats is most likely due to the proximity of these small drainage systems to upland runoff of both human and domestic wastes as well as wildlife sources, combined with the lower dilution capacity of creeks compared to larger water bodies. Greater protection of tidal creek habitats is warranted in areas where upland sources of waste can be identified and controlled.
Comparison of the average fecal coliform
concentrations observed in tidal creek and open water habitats during 2001-2002,
and estimates of the percent of the state's coastal habitat representing various
concentrations that are normal (green), moderately high (yellow) and indicative
of possible unsuitability for shellfish harvest, or very high (red) and
indicative of possible unsuitability for primary contact recreation.
Measures of water clarity
provide an indication of the amount of suspended particulate matter in the water
column. South Carolina’s estuarine waters are naturally turbid compared to many
other states. Exceptionally high turbidity levels may be harmful to marine
life. SCDHEC has recently developed a maximum saltwater state standard for
turbidity of 25 NTU. This corresponds to the 90th percentile of the SCDHEC
saltwater database, which was obtained primarily from the larger estuarine water
bodies. Therefore, values above 25 NTU are considered to be poor for this
program. The 75th percentile, representing partially elevated levels, is 15 NTU.
Values > 15 NTU and < 25 NTU are considered to be fair for SCECAP samples.
Average turbidities measured in the 2001-2002 survey by this program were 21 NTU in the tidal creeks and 15 NTU in the open water habitat (see figure below; Data Tables), which is almost identical to the averages observed in the 1999-2000 survey (Van Dolah et al., 2002a). The difference between habitats was statistically significant (p = 0.002). Based on the single measure of turbidity taken at each station, approximately 19% of the tidal creek habitat exceeded the State standard, whereas only 10% of the open water habitat exceeded the standard (see figure below; Data Tables). As noted by Van Dolah et al. (2002a, c), turbidity levels in tidal creeks may be naturally higher due to the shallow depths of these systems (i.e., surface samples are often within 1-2 m of the bottom) combined with re-suspension of the bottom sediments due to tidal currents.
Comparison of the average
turbidity concentrations observed in tidal creek and open water habitats during
2001-2002, and estimates of the percent of the state's coastal habitat
representing various turbidity ranges that represent good (green), fair (yellow), or
poor (red) values relative to SCDHEC historical data.
Alkalinity:
Alkalinity measurements were collected for the SCECAP program
to be consistent with SCDHEC's larger water quality monitoring program. There
are no state standards for alkalinity in saltwater and research is lacking on
how high or low alkalinity values affect estuarine biota. Until additional data
are gathered for this parameter in the SCECAP program, combined with better
information on how alkalinity should be interpreted, the data are only
summarized in
Appendix
2.4.
Integrated Water Quality Measure:
SCECAP has developed an integrated measure of water quality
using multiple parameters combined into a single index value. Six parameters
were used to develop the index of water quality for the 1999-2000 survey:
dissolved oxygen (DO), biochemical oxygen demand (BOD5),
fecal coliform bacteria, total nitrogen (TN), total phosphorus (TP), and pH. For
the 2001-2002 survey, BOD5
was dropped from the index because there are no documented criteria or
guidelines for BOD5
in estuarine waters and the effects of BOD5
in these systems are unknown. Chlorophyll-a was added to the index as a measure
of phytoplankton response to nutrient concentrations. An explanation of the scoring process is
provided by
Van Dolah et al. (2004).
Results of the 2001-2002 survey indicated that approximately 73% of the state’s creek habitat during this survey period was good, 22% had fair water quality, and 5% of the creek habitat had poor water quality (see figure below). In contrast, 88% of the state’s open water habitat had good water quality overall, 12% was considered to be only fair in quality, and none of the open water habitat sampled in this survey period had poor water quality. The specific location of creek sites with poor water quality, and the coding of each variable that comprises the integrated water quality score, is provided in Data Tables.
Proportion of the South Carolina's estuarine habitat that ranks as good (green), fair (yellow), or poor (red) using the integrated water quality score developed for the SCECAP program. This measure of overall water quality incorporates the six water quality parameters shown.
As noted in the 1999-2000 survey (Van Dolah et al., 2002a), the higher percentage of poor and fair water quality conditions in creeks indicates that these habitats are often more stressful environments, especially since many of these sites were in relatively pristine locations. The higher percentage of creek habitat with poor or fair conditions may also, in part, reflect the relatively greater effect of anthropogenic runoff into these smaller water bodies due to their proximity to upland sources and their lower dilution capacity. It may also be the result of using thresholds derived from SCDHEC’s historic database, which is composed predominantly of data from open water habitats. Now that four years of data are available SCECAP personnel will review the historical data available for both habitat types to identify whether the threshold criteria for some of the water quality parameters measured in creek habitats should be changed from those used in this report to reflect the greater natural variability in these habitats.
Due to the change in methods and thresholds in assessing overall water quality conditions in South Carolina’s estuaries, a re-evaluation of all survey data collected since 1999 was conducted on an annual basis to evaluate whether any trends were observed since the inception of SCECAP. While the probability-based sampling approach is not as suitable for trend analysis compared to fixed stations, it is possible to report changes in condition over time using this approach. In contrast to our two-year data summary periods, the annual assessment combines both the open water habitat and the tidal creek habitat, with appropriate weighting for each habitat type. The reader should note that by using this approach, the condition of tidal creeks contributes much less than the condition of open water habitat since tidal creeks comprise only about 17% of the states estuarine water surveyed by SCECAP (Van Dolah et al., 2002a).
Comparison of the state’s overall water quality condition on an annual basis indicated very little change over the four-year period. For all four years, more than 80% of the state estuarine waters rank as good in quality using the SCECAP criteria, and less than 5% of the estuarine waters are considered to be poor in quality. The lack of any major change in condition over time is probably due in part to the fact that all sampling has occurred during a major and unusual drought period. Return of climatic conditions to conditions with higher rainfall, resulting in more upland runoff, may change the water quality estimates considerably. The 2003-2004 survey should be indicative of estuarine water quality conditions during wetter years.