Results
Salinity
Salinity at the five sampling stations ranged from 33 - 37
psu. There were slightly higher
salinities at the inlet and Banks Channel stations than in Masons Creek and the
ICW (Table 2). The only evident
seasonality was slightly lower salinities November 2002 - February 2003,
probably a result of increased rainfall.
pH
During the study pH ranged from 7.6 to 8.2 among the
stations (Table 2), normal for euhaline and nearshore
marine salinities. There were no
seasonal or spatial patterns evident.
Dissolved oxygen
Dissolved oxygen ranged from 5.8 mg/L to 10.7 mg/L, with a
normal seasonal pattern of lowest concentrations in summer and highest in
winter. There were no occasions
when dissolved oxygen fell below the North Carolina State Standard of 5.0 mg/L.
Turbidity
Turbidity concentrations were generally low at all stations
(Table 2), and there was no evident seasonal pattern. The North Carolina brackish water turbidity standard of 25
NTU was exceeded on three occasions; on May 13, 2002 at MI-1 and MI-2, and on
August 15, 2002 at MI-3. May 13,
2002 was the only sampling period that yielded generally elevated turbidity
levels.
Total suspended solids (TSS)
Total suspended solids concentrations increased at all
sites following dredging (Table 2).
Although only three collections were made previous to dredging, these
collections occurred over a three-week period, and the standard deviation among
the three collections was generally quite low.
The post-dredging TSS concentrations were approximately twice as high at
all stations except MI-5 in the ICW, and the standard deviation of the twelve
collections was relatively low. Thus,
the increase was likely real and a result of inlet relocation (Fig. 1).
Total phosphorus (TP)
Total phosphorus data at first appearance shows apparently
larger post-dredging values compared with pre-dredging (Table
2). However, standard
deviations are very large, affected by unusually high TP levels on July 8, 2002. When comparing median values, the concentrations are much
closer and show no real difference. Median
TP values at MI-1 were 0.017 for pre-dredging versus 0.017 for post-dredging,
for MI-2 they were 0.029 for pre versus 0.024 for post, for MI-3 they were 0.023
for pre versus 0.018 for post, for MI-4 they were 0.010 for pre versus 0.014 for
post, and for MI-5 they were 0.019 for pre versus 0.025 mg/L for post.
Thus, there was no consistent pattern among stations for TP values.
Nitrate-N
Nitrate concentrations were characterized by high
variability among sample dates (Table 2) with no clear
pattern. Values were generally low
throughout, with the exception of a maximum of 0.5 mg/L at MI-3 on August 15,
2002, and a maximum of 0.2 mg/L on that same date at MI-1.
Chlorophyll a
Chlorophyll a concentrations, (a measure of algal
biomass), ranged from 0.6 to 5.2 mg/L,
with highest concentrations summer through fall 2002, and lowest during winter
months. There was little difference
among stations (Table 2), and these concentrations can be
considered typical of nearshore ocean waters in this region.
The North Carolina State standard of 40 mg/L
was not violated at any time.
Fecal coliform bacteria
Fecal coliform bacteria concentrations were generally low
at all stations (Table 2), ranging from 0 to 51 CFU/100 mL.
There was no apparent seasonality among coliform counts (Fig. 2).
The shellfishing water standard of 14 CFU/100 mL was exceeded on several
occasions: December 27, 2001 at MI-1 and MI-3, April 25, 2002 at MI-2, May 13,
2002 at MI-2, May 28, 2002 at MI-4, and September 26, 2002 at MI-1 and MI-3.
The human contact standard of 200 CFU/100 mL was not exceeded at any time
during the study.
Flow data
The flow data presented here should be considered limited
as it represents only snapshots in a strongly tidally influenced system.
The surface, middle, and bottom measurements were averaged to obtain the
data on Table 2. Additionally,
since there was an equipment malfunction only two data points are available for
pre-dredging results. Due to the
large variability, and the fact that during the latter part of the study some
stations were boat accessible only at high tide, few comparisons can be made.
However, the data for Mason Creek (Station MI-4) does show much greater
flow following inlet relocation than before, as one would expect.
The physical
attributes of the area around Mason Inlet and Mason Creek changed slowly over
time, but some patterns became evident. Shortly
after dredging, the inlet and creek stations were all deep enough for boat
passage at high tide. Station MI-1,
located in the inlet, was measured to be 3-4 meters deep at or near high tide. MI-2, located north of the inlet and closer to the marsh,
varied in depth but was usually approximately 1.5 meters deep.
MI-3, located in the sedimentation basin south of the inlet, was
generally shallower, about 1 to 1.5 meters deep.
The station located within the creek, MI-4, was about 3 meters deep after
dredging. MI-5 was located within
the Intracoastal Waterway and was not visibly affected by the dredging, though
some sedimentation may have occurred.
In the months
following dredging, it was observed that the inlet itself seemed narrower, with
waves breaking closer to the sound area. The
sand on the north end of Wrightsville Beach began to form a spit extending
outward in a northwest direction. Also,
it was noted that the area between the inlet and MI-2 became shallower with
shoaling occurring between the back of the barrier island and marsh area.
In February 2003, we were only able to get within 500 feet of MI-2, due
to shallow waters. The sedimentation basin filled in gradually, but eventually
made it impossible to sample the original site of MI-3. In February 2003, we were able to get within 1000 feet of the
site, sampling in less than 0.5 meters of water. Mason Creek also became shallower in the months following the
dredging. MI-4 had a depth of
1.5-2.0 meters from November 2002 to February 2003.
Table 2. Parameter
concentrations before and after Mason Inlet relocation, presented as mean
+ standard deviation (fecal coliform bacteria presented as geometric
mean). Pre-n=3, Post-n=12
_______________________________________________________________________________________
Station
MI-1
MI-2
MI-3
MI-4
MI-5
_______________________________________________________________________________________
Salinity (psu)
Pre
36.6+0.3
36.6+0.3
36.6+0.3
36.3+0.7
35.7+1.2
Post
36.0+0.7
36.6+0.7
36.0+0.7
35.4+1.2
35.4+1.2
pH
Pre
8.0+0.2
8.0+0.1
8.0+0.1
7.9+0.2
8.0+0.1
Post
7.9+0.2
8.1+0.1
8.1+0.1
8.0+0.1
8.0+0.1
DO (mg/L)
Pre
8.5+0.8
8.2+0.2
8.2+0.2
8.3+0.1
8.5+0.5
Post
7.8+1.1
7.7+1.2
7.7+1.1
7.3+1.4
7.3+1.5
Turbidity (NTU)
Pre
3+2
3+2
3+2
5+7
2+2
Post
7+7
5+8
10+11
5+5
5+3
TSS (mg/L)
Pre
23.1+0.3
26.3+3.4
23.1+2.2
25.0+2.1
29.4+13.4
Post
47.3+9.9
51.7+8.2
50.4+7.7
50.8+6.4
49.9+8.0
TP (mg/L)
Pre
16+4
41+27
29+12
17+16
16+7
Post
61+137
32+32
65+108
34+45
39+42
Nitrate-N (mg/L)
Pre
45+13
28+8
44+9
64+33
70+40
Post
38+55
15+11
63+146
20+13
43+55
Chlorophyll a (mg/L)
Pre
1.3+0.3
1.3+0.3
1.3+0.7
1.1+0.7
0.9+0.3
Post
2.0+1.2
1.7+0.9
1.8+1.0
2.3+1.6
2.5+1.6
Fecal coliforms (CFU/100 ml)
Pre
4
4
5
2
2
Post
2
2
2
3
2
Flow (m/s)
Pre
0.74+0.40
0.39+0.08
0.32+0.04
0.08+0.07
0.39+0.00
Post
0.51+0.44
0.28+0.17
0.18+0.23
0.47+0.23
0.20+0.10
______________________________________________________________________________________
Howe Creek data were examined to see if the relocation of Mason Inlet had any effect on tidal creek water quality. We used fecal coliform counts as the measure because fecal coliform counts are typically inversely related to salinity (Mallin et al. 2000a), and we used measured salinity and rainfall data to help explain the results. Dredging the mouth of Futch Creek had led to both increased salinity and decreased fecal coliform counts in that nearby tidal creek (Mallin et al. 2000b). Fecal coliform bacterial counts had been collected from August 2001, before dredging, through the dredging and are scheduled to continue until at least July 2003, over a year following dredging. In this report we examine the six months of pre-dredging data collected from August 2001 though January 2002 and compare it with six months of post-dredging data collected from August 2002 through February 2003 (October 2002 was not sampled). Salinity data were collected on station during the fecal coliform collections.
The data show that
fecal coliform counts for the lower four stations in the creek showed little
change in either direction following dredging (Table 3).
The lower two stations continued to show low coliform counts and the
middle two stations moderate coliform pollution.
However, the uppermost station (essentially as far upstream as one can
safely take a boat on high tide) showed a distinct decrease in terms of the
geometric mean of the six months from 325 to 185 CFU/100 mL.
Table 3. Fecal coliform
bacterial concentrations in Howe Creek before and after Mason Inlet relocation,
presented as geometric mean. Pre-August 2001-January 2002, n=6, Post-August
2002-February 2003, n=6. Sampling
date salinity data are presented as mean + standard deviation.
______________________________________________________________________________________
Station
HW-M
HW-FP
HW-GC
HW-GP
HW-DT
______________________________________________________________________________________
Fecal coliforms (CFU/100 ml)
Pre
2
2
11
87
325
Post
4
4
14
74
185
Salinity
Pre
35.7+0.9
35.5+1.0
34.8+1.8
27.5+9.5
15.4+13.5
Post
31.2+3.8
33.0+2.3
29.4+5.2
20.0+10.3
2.5+1.9
______________________________________________________________________________________
Salinity data collected at the time fecal coliforms were sampled in Howe Creek showed that lower salinities prevailed following the dredging of the new Mason Inlet channel (Table 3). Thus, the reduction in fecal coliform bacterial counts in upper Howe Creek is not likely associated with salinity increases associated with the moving of Masons Inlet. It is more likely that fecal coliform counts in upper Howe Creek are associated with local runoff factors, especially rainfall variability. To test this we performed correlation analyses among untransformed fecal coliform counts, salinity, and total rainfall for the 72-hour period preceding fecal coliform collections at HW-DT. There was no significant correlation between fecal coliforms and salinity. However, there was a significant correlation (r = 0.644, p = 0.024) between fecal coliform abundance and rainfall. Thus, rainfall-driven runoff variability was the factor most likely responsible for the decrease in fecal coliforms in upper Howe Creek following inlet relocation. Extensive water quality data for Howe Creek can be found at our Tidal Creeks website: http://www.uncwil.edu/cmsr/aquaticecology/TidalCreeks/Index.htm