Vertical Distribution of Water Quality Parameters
3.1 Introduction
In aquatic ecosystems stratification of the water column, usually
detected as chemical or physical gradients, can have profound effects on biological
functions within the system. Stratification may result in chemical, physical, and
biological differences between the surface water and the bottom water. When sampling a
body of water it is desirable to determine if stratification exists, and if there are
subsequent vertical water quality parameter differences. These results can then be used to
determine if collection of both surface and bottom samples is warranted.
Due to high flow and turbulence, the Cape Fear River system (including
the lower Black River and lower Northeast Cape Fear River) appears to be a well-mixed
system throughout the lower riverine portion and estuary. To determine if the river is
well-mixed statistical analyses were performed to compare surface and bottom data for
several physical, chemical, and biological parameters.
3.2 Methods
Vertical comparisons between surface and bottom physical parameters
(temperature,salinity, dissolved oxygen, and turbidity) were made using data collected
monthly from June 1995-May 1996 in the Cape Fear River at NC 11, Acme, Indian Creek (IC),
Navassa (NAV), Marker 61 (M61), Marker 54 (M54), Marker 38 (M38), Marker 31 (M31), Black
River below Thoroughfare (BBT), the Northeast Cape Fear River at NC 117 (NCF117) and mile
6.4 (NCF6). For stations NAV, M61, and M54 we utilized additional data from August
1994-May 1995 from another study. Further comparisons were made for the estuarine stations
using only summer month data (June-September), when temperature and dissolved oxygen
stratification might be expected to be greatest. This was done to determine if the high
annual variability masked seasonal differences.
Comparisons for nutrients (TN, TP, ammonium) and chlorophyll a
were made using data collected monthly from June 1995-May 1996 at all of the above
stations. Again, at NAV, M61, and M54 we had additional data from August 1994-May 1995 for
TN, TP, and chlorophyll a and from January 1995-May 1995 for ammonium. Because of
extremely high flow at Markers 23 and 14 we often could not get bottom samples, and thus,
due to the paucity of comparative data we did not include these stations in the vertical
comparisons.
Physical parameters were measured on site (Chapter 1); thus, one value
was measured for each physical parameter during each month. For each parameter one t-test
was performed combining all monthly surface versus bottom values. Again, wwe also
separated out summer data for additional comparisons. The biological and chemical
parameters tested were sampled in triplicate and t-tests comparing surface and bottom
values were performed for each parameter for each individual month. Statistical tests were
performed using the Satistical Analysis System (SAS) (Schlotzhauer and Littell 1987).
3.3 Results
Physical Parameters
For all four of the physical parameters there were no overall significant differences (P<0.05) between the surface and bottom values at all stations (Tables 3.1,3.2). During the summer months there were likewise no significant vertical differences except for turbidity at M54 (df=3, p=0.042) where bottom turbidity was significantly greater than surface turbidity.
Biological Parameters
For the riverine stations overall, chlorophyll a surface and bottom concentrations in the river were significantly different 21% of the time, with the surface higher 18% of the time and the bottom higher 3% of the time (Table 3.3). The estuarine stations showed vertical differences 26% of the time, with the surface higher 13% of the time and the bottom higher 13% of the time (Tabel 3.4). The riverine station showing the most differences was IC (3 out of 6 times); of the estuarine stations M42 differed 3 out of 5 occasions, and M61 differed 7 out of 14 times. There was no consistant pattern as to the direction of these differences, however. At the rest of the stations differences occurred less than 20% of the time.
Chemical Parameters
For the riverine stations, total phosphorous surface and bottom
concentrations were significantly different on very few occasions, with no evident pattern
expressed (Table 3.3). In contrast, the estuarine stations showed vertical differences 40%
of the time, with the surface higher 6% of the time and the bottom higher 34% of the time.
Station M42 showed the most differences, 3 out of 5 occasions, M61 had differences 7 out
of 13 times, and M54 differed 5 out of 14 times (Table 3.4). Bottom TP was higher at these
estuarine stations on most of those occasions.
For the riverine stations, total nitrogen surface and bottom
concentrations were significantly different only 9% of the time, with the surface greater
3% and bottom greater 6% (Table 3.3). The estuarine stations likewise showed few vertical
differences, with no evident pattern (Table 3.4). The station showing the most
differences, 2 out of 5 occasions, was M42, and NCF6 had differences 2 out of 6 times. At
M42 the surface concentration was higher than the bottom on both occasions.
For the riverine stations, ammonium surface and bottom concentrations
were significantly different 21% of the time, with the surface greater 6% and bottom
greater 15% (Table 3.3). The estuarine stations showed vertical differences 41% of the
time, with the surface higher 14% of the time and the bottom higher 27% of the time. The
station showing the most differences, 4 out of 5 occasions, was M42, and M61 had
significant differences 4 out of 9 times, and NAV 3 out of 9 times. At M42 and NAV the
bottom concentration was higher than the surface during most of these differences.
3.4 Discussion
Both the riverine and the estuarine portions of the Cape Fear River
system that was included in our study appear to have very little stratification and little
or no surface- bottom differences in water temperature, salinity, and dissolved oxygen. As
discussed in Chapter 2, strong mixing prevents formation of anoxic bottom waters during
summer, when higher temperatures cause reduced oxygen carrying capacity of the water (Cole
1983). However, this same mixing also causes low dissolved oxygen conditions to prevail
throughout the entire water column. Turbidity showed no signifcant vertical differences in
the river. Estuarine turbidities, while not significantly different overall, displayed
greater bottom values on occasion. This was likely due to the processes of flocculation
and settling of sediments in upper estuaries (Wells and Kim 1991).
The strong mixing processes in the river led to a generally even
distribution of chlorophyll a throughout the water column. While the estuarine
stations showed a few significant vertical differences, there was no consistent pattern as
to direction. While phytoplankton abundance might be expected to be greater near surface,
with little benthic microalgal prouction due to strong light attenuation below (Nearhoof
1994), estuarine vertical mixing likely disperses cholophyll a throughout the water
column.
The strong vertical mixing in both the river and estuary led to similar
total nitrogen concentrations throughout the water column. Total phosphorous
concentrations were likewise well distributed vertically in the river. However, the
estuary had a much greater occurrence of significant vertical TP differences probably due
in part to periodic uneven vertical turbidity distribution, and a salt wedge that is often
evident. Phosphorus is readily bound to turbidity particles in rivers, particularly
organic materials, aluminum oxides, iron oxides, and apatite (Lebo 1991; NCRS 1995). When
fresh water and salt water meet, clay particles flocculate and sink to the bottom carrying
attached phosphorous with it (Lebo 1991; Wells and Kim 1991). Most differences in the
estuary occured at M42 and M61, upper estuarine stations where settling of particulates
may be strongest (Wells and Kim 1991). The periodic salt wedge was not consistent enough
to be seen in the salinity comparisons but it probably contributed to lower particulate
mixing at some stations during some months.
Ammonium concentrations were well-dispersed throughout the riverine
water column. Vertical differences were highest in the deepest estuarine stations where
the bottom water often displayed higher concentrations. This may be caused by reducing
conditions in or near the sediment and a more oxidizing environment near the surface. The
more shallow stations displayed no consistent vertical distribution pattern for ammonium
concenterations.
In summary, vertical water quality differences were insignificant in
the Cape Fear River. In the estuary, of the physical parameters only turbidity showed
periodic vertical differences (bottom greater than surface). This contributed to periodic
greater bottom-water TP concentrations at selected stations. Other chemical and biological
parameters showed no consistent vertical patterns. Thus, we discontinued taking bottom
samples in November 1995. The system appears to be well mixed and separate analysis of
surface water and bottom water does not yield additional water quality data.
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