Physical
Limnology of Clear Lake
James Anthony,
Jordi Morell Farre, and John Downing
A. Introduction
Although analyses of external nutrient loads dominate lake restoration research, internal nutrient loads (nutrient loads arising from sources within the lake), have received comparatively little attention (Hamilton and Mitchell 1997). In shallow aquatic systems, however, internal nutrient loading may contribute significant proportions of the total nutrient load by diffusive flux of nutrients through the sediment-water interface (Søndergaard et al. 1999) as well as through the mobilization of nutrients by turbulent resuspension of sediments and nutrient-rich pore water (Kristensen et al. 1992; Reddy et al.1996; Nõges and Kisand 1999). Large-scale resuspension of sediments in shallow, lacustrine systems may be driven by wind-induced waves (Kristensen et al. 1992; Reddy et al.1996; Nõges and Kisand 1999) and recreational boat traffic (Yousef et al. 1980; Garrad and Hey 1987). The resuspension of benthic sediments may contribute to increased nutrient concentrations in the water column (Kristensen et al. 1992; Reddy et al.1996; Nõges and Kisand 1999), increased algal growth (Galicka 1992; Hawley and Lesht 1992; Søndergaard et al. 1992) and degradation of the light climate (Somlyódy 1982). Turbulent resuspension of sediments may also negatively impact macrophyte and fish communities (Jeppesen et al. 1990; McQueen 1990; Meijer et al. 1990) and sediment-mediated light limitation may facilitate domination of the phytoplankton community by potentially toxic cyanobacteria (Søndergaard et al. 1992).
In addition to negatively impacting aquatic communities, frequent resuspension of sediments may also maintain elevated trophic status long after external nutrient loads have been drastically reduced. This is due to the substantial concentrations of nutrients stored in lake sediments from periods of high external loading (Søndergaard et al. 1999). For example, total phosphorus concentrations in eutrophic Lake Søbygaard, Denmark remained unaltered 15 years after 80-90% reductions in external nutrient loads (Søndergaard et al. 1999). This persistence of elevated trophic status by diffusive flux and turbulent resuspension has been noted in other shallow, eutrophic lakes and is likely to be a common, although often overlooked, problem facing the management of these systems (Kristensen et al. 1992, Reddy et al. 1996; Nõges and Kisand 1999; Søndergaard et al. 1999).
Unfortunately, the mechanisms of diffusive flux and, especially, of turbulent resuspension of nutrients in shallow lakes are poorly understood (Phillips et al. 1994; Welch and Cooke 1995), making identification and quantification of internal nutrient loading quite difficult (Hamilton and Mitchell 1997; Søndergaard et al. 1999). Still, those studies that have been conducted on wind-induced resuspension indicate that rapid mobilization of phosphorus (P) and ammonium nitrogen (NH4-N), enhanced by photosynthetically-elevated pH levels (Bouldin et al. 1974), may be a common occurrence in shallow, eutrophic systems (Søndergaard et al. 1999). In fact, substantial wind-induced resuspension of sediments and nutrients may occur over 50% of the time in some shallow systems and may lead to variable nutrient and light limitation (Hamilton and Mitchell 1988).
Just as wind-induced waves may contribute to sediment resuspension, turbulence induced by recreational boat traffic may resuspend sediments and interstial water in some shallow lakes and rivers (Yousef et al. 1980; Garrad and Hey 1987). Boat-induced turbulence has been correlated to rapid increases in total dissolved solids, soluble reactive phosphorus, total phosphorus (Yousef et al. 1980), and turbidity (Yousef et al. 1980; Garrad and Hey 1987). It is therefore probable that, like wind, recreational boat traffic may lead to persistence of elevated trophic status, suppression of macrophyte and fish communities, and domination of the phytoplankton community by harmful cyanobacteria.
Because of their importance to internal nutrient loading, lake restoration time-scales, water quality, phytoplankton community structure, macrophyte suppression and fish community dynamics, we have examined the contributions of wind and recreational boat traffic to sediment resuspension in shallow, eutrophic Clear Lake, Iowa. These analyses not only elucidate potential means for sediment and nutrient resuspension in Clear Lake, but also provide insight into the degree of resuspension present. Our objectives are (1) to determine the influence, if any, of wind and recreational boat traffic on sediment resuspension, (2) to establish the frequency and degree of resuspension occurring in the lake and (3) to provide estimates of nutrient flux via sediment resuspension.
B. Methods
In addition to analyses of lake water samples for
ammonium and total phosphorus concentrations, we wished to establish a
relationship between total phosphorus and turbidity as measured by the YSI 6500
sonde. Three superficial sediment
samples were taken at the site of the submersed sonde (See Fig. 1) using a
0.0225 m2 Ekman grab. These
sediment samples were mixed and homogenized.
Sediments were then added in increasing amounts to samples of water from
Clear Lake in order to create a gradient of turbidity that might be
representative of increasing degrees of resuspension in the lake. These samples were subsequently analyzed for
total phosphorus as described above and for turbidity using a YSI 6500 sonde in
order to establish a predictive relationship between turbidity and TP in Clear
Lake. This relationship was used to
provide estimates of total phosphorus flux that may be attributed to resuspended
benthic sediments in the lake.
In order to ascertain the impact of wind direction
on the prevalence of sediment resuspension, we calculated the maximum effective
fetch (Lf), a measure of the water surface that may be acted upon by
wave action, following Håkanson and Jansson (1983). We measured the distance to the lakeshore from the sonde and the
3700 points throughout the lake along a radial of each recorded and
hypothetical wind direction (discussed above) as well as along 14 additional
radials deviating from the selected wind direction by ±6º, ±12º, ±18º, ±24º,
±30º, ±36º, and ±42º. The maximum
effective fetch was then calculated as:
Lf = (Sxi · cos gi)/(Scos gi) (1),
where
Lf is the maximum effective fetch in meters, gi is each ith deviation
angle, where gi =±6º,±12º,…±42º, and xi
is the distance in meters of the gith radial from the given
site to land.
Characteristics of wind-induced waves including wave height, period, wavelength, celerity and the maximum orbital velocity at the lake bottom were calculated according to Airy wave theory for irrotational waves traveling over a horizontal bottom at any water depth. The assumption of irrotationality simply requires that the individual particles comprising the water retain their orientation in space rather than spinning. Therefore, in the open water, there is no net transport of water particles but, rather, a transfer of energy (i.e. energy, not water, is actually displaced). It is this transfer of energy that becomes important in the translocation of sediments and erosion of shorelines (Komar 1972).
The wave heights, the distance from trough to crest,
induced by a given wind speed and direction were calculated according to Airy
wave theory as:
H
= w·[0.0026·(g·Lf / w2)0.47]/g
(2),
where
H is the wave height in meters, w is the wind speed in m·s-1, g is
the acceleration due to gravity (9.8 m·s-1), Lf is the
maximum effective fetch in meters, and 0.0026 and 0.47 are constants (Håkanson
and Jansson 1983). Wave heights were
calculated at the position of the sonde for all recorded wind speeds and at the
3700 points throughout the lake for the hypothetical wind speeds described
above.
Estimates of wave period, the time
interval between successive wave troughs or crests, induced by a given wind
speed and direction were calculated as:
T = w·[0.46·(g·Lf / w2)0.28]/g (3),
where
T is the wave period in seconds, w is the wind speed in m·s-1, g is
the acceleration due to gravity (9.8 m·s-1), Lf is the
maximum effective fetch in meters, and 0.46 and 0.28 are constants (Håkanson
and Jansson 1983). Wave period
calculations were performed for waves at the sonde and for the 3700 points
throughout the lake as discussed above for wave height.
The wavelengths for wave groups
passing the sonde and the 3700 lake-wide points were calculated as:
l = 1.56·T (4),
where
l is the wavelength in meters, 1.56 is a
constant, and T is the wave period in seconds (Håkanson and Jansson 1983). Estimates of wave celerity, or rate of
advancement of wave crests were then calculated as:
c = l / T
(5),
where
c is the celerity in m·s-1, l is the wavelength in meters
and T is the wave period in seconds (Håkanson and Jansson 1983).
The orbital diameter of surface
waves decline exponentially with depth.
Since we were interesting in benthic sediment resuspension, we needed to
calculate the orbital velocity of these waves near the lake bottom. The maximum orbital velocity of waves near
the lake bottom at the sonde and at the 3700 points throughout the lake were
therefore calculated as:
v = p·H /
T·e-2pd/l (6),
where
v is the maximum orbital velocity in m·s-1, p is pi (3.1416), H is the wave height in
meters, T is the wave period in seconds, d is the water depth in meters and l is the wavelength in meters (Smith and
Sinclair 1973).
During the course of our examination of sediment resuspension in Clear Lake, Iowa, measurements of turbidity varied from a maximum of 1202.1 NTU to a minimum of 9.4 NTU around a mean of 33.9 NTU. It is likely, however, that the 1202.1 NTU maximum is an aberrant observation due to interference in the measurement instrument’s path. Chlorophyll concentrations over the same period varied from a maximum of 49 mg·L-1to a minimum of 14 mg·L-1around a mean of 25 mg·L-1. Time-trends in both variables show considerable variability (Fig. 2). It is important to note, however, that measurements of turbidity are inextricably tied to those of chlorophyll a because the algal cells containing chlorophyll a are themselves a component of turbidity. Additionally, if resuspension is occurring, it is likely that a large proportion of the chlorophyll a indicated by the sonde is actually comprised of recently sedimented, rather than planktonic, algae. This close relationship is evident in the similarities in time-trends, or spectra, of both variables (Fig. 2).
Although variability in chlorophyll and turbidity measurements may superficially appear somewhat random (Fig. 2), closer examination of both time-trends reveals that, superimposed upon larger fluctuations, are notable diel patterns in both variables, with low values generally present during the evening and early morning and rising to daily maxima by early afternoon (Fig. 2). Although some of these diel patterns in turbidity may be strongly influenced by daily production of algae, the confounding of chlorophyll and turbidity measurements prevents a definitive diagnosis and it is possible that the rapid increases in chlorophyll concentrations are indicative of sedimented algal cells resuspended by wind or boat traffic. Variable and generally high values of turbidity early in the study may have been the result of the use of a turbidity probe that was subject to fouling. This probe was subsequently replaced with one that employs a cleaning mechanism to avoid such variability. Also notable is that the sonde ran out of battery power between September 9, 2000 and September 15, 2000 and all data for this period were lost.
Wind was variable with a mean of 6.8 m·s-1, a minimum recorded speed of 0.2 m·s-1, and a maximum record of 27.8 m·s-1. Wind direction was generally unstable at Clear Lake. Although the mean observed wind direction was 169.5°, sustained winds were observed from throughout a range from 0° to 360°. These values are representative of wind speed and direction records measured by our wind station atop the Clear Lake Municipal Water Treatment Facility (See Fig. 1). When daily mean wind speed data from the National Climactic Data Center were examined for the period of April – October in 1998, 1999, and 2000, the mean wind speed near Clear Lake was found to be 4.6 m·s-1with a maximum record of 11.3 m·s-1 and a minimum daily average wind speed of 0.6 m·s-1. The period between April and October was chosen to provide information representative of the seasonal no-ice state of the lake. It is only during this period that wind may influence the lake’s benthic sediments. Just as observed with chlorophyll and turbidity measurements, wind speed generally showed similar diel patterns superimposed upon larger, more long-term fluctuations (Fig. 3). Again, wind speeds tended to rise through the morning hours, before declining in late afternoon and evening (Fig. 3).
Unfortunately, the characteristics of the water quality and meterological variables measured do not lend themselves to straightforward statistical analyses. For example, the lack of independence between turbidity and chlorophyll is likely to mask a clearly defined and repeatable predictive relationship between wind and turbidity, even if one exists, since we cannot distinguish turbidity peaks caused by algal blooms from those induced by wind events. Variable and, likely, very slow settling velocities of small, unconsolidated particles as well as varying algal composition in the lake may also contribute to a varying background turbidity level that prevents the application of most statistical analyses. It is therefore not surprising that no significant relationships were observed among wind or estimated wave parameters and turbidity in the lake.
Despite the inadequacy of conventional statistical measures to treat these complex data, examination of spectral trends makes it increasingly plausible that wind may influence sediment resuspension in Clear Lake (Fig. 3). The locations of peaks and troughs in wind speed are, in many cases, remarkably similar to those of turbidity, indicating that much of the diel and long-term variability in turbidity in Clear Lake may be related to wind events (Fig. 3). Calculations of wave orbital velocities near the bottom of the lake for the 3700 lake-wide points corroborate the implications of spectral analyses by indicating that large proportions of the lake bottom may become prone to wave velocities strong enough resuspend sediments ranging in size from silts to pebbles (Table 1; Figs. 4 – 12).
Maps of wave velocities near the bottom of Clear Lake indicate that large portions of the lake bottom may become prone to wave velocities capable of substantial resuspension. At wind speeds near the mean daily wind speed indicated by NCDC data (~5 m·s-1), little wind-driven resuspension is likely to occur in Clear Lake when winds are along the prevailing wind axis (Fig. 4) or along the other wind axes examined. Bottom velocities capable of resuspending sediments become prevalent along the lake margins and in the shallow, western basin of the lake when wind speeds along the prevailing wind axis reach 10 m·s-1 (Fig. 5). Dramatic increases in bottom velocities and the area of the bottom involved in sediment resuspension (Table 1) are notable, however, when wind speed along the prevailing wind axis climbs to 15 m·s-1, 20 m·s-1 and 25 m·s-1 (Figs. 6-8). At these wind speeds, a majority of the lake bottom becomes mobile, and wind-induced sediment resuspension may become a lake-wide phenomenon. The same dramatic increase in the potential for sediment entrainment is notable for 10 m·s-1 and 15 m·s-1 winds along North-South, East-West, and Northeast-Southwest axes (Figs. 9-11). The potential for sediment resuspension is particularly notable in areas where fetch becomes large or where depth becomes shallow. This is especially true for the shallow margins of the lake as well as for the small western basin of the lake, which is exposed to strong potential for resuspension even at relatively low wind velocities.
The rate of occurrence of substantial resuspension events such as those described above is difficult to quantify. Our wind data, however, indicate that winds during our study period ranged from 5 - 10 m·s-1, 10 - 15 m·s-1, 15 - 20 m·s-1, and 20 – 25 m·s-1, 43.4%, 14.9%, 5.8% and 1.1% of the study period, respectively. This implies some degree of resuspension occurring over 60% of the time at Clear Lake. These estimates may be slightly biased, however, as our data may include substantial wind gusts which, if present only in short duration, may not lead to large-scale wind resuspension. Data from the NCDC suggest mean daily wind speeds from 5 – 10 m·s-1may occur 45.1 % of the time while winds exceeding 10 m·s-1 occur only 2.3 % of the time. This may, however, lead to a large underestimate of the frequency and magnitude of resuspension occurring at the lake, however, as these daily mean wind data, may mask the presence of some high wind periods.
During September 27, 2000, wind
speeds at Clear Lake ranged from a minimum of 14 m·s-1
at 7:30 AM to maximum of 24 m·s-1at 3:00 PM (Fig.
12a). Winds increased throughout the
day before beginning to diminish after the 3:00 PM
maximum (Fig. 12a). Sonde
measured turbidity ranged from a minimum of 30 NTU at
7:30 AM to a maximum of 48 NTU at 3:00 PM,
before declining thereafter (Fig. 12a).
Similar patterns were observed in ammonium-nitrogen concentrations,
which increased from a minimum of 739 mg·L-1at 7:30 AM to a maximum of 1052 mg·L-1at
2:00 PM (Fig. 12b).
Concentrations of unionized ammonia (NH3) increased from 61
to 115 mg·L-1. Although these concentrations are just below
those necessary for acute fish damage (120 ug/L) they are low only due to the
low temperatures in the lake in September (~13°C). Had the water temperature been closer to
that observed in the summer months (~25°C), unionized ammonia
would have increased from 126 to 221 mg·L-1, reaching concentrations far beyond
those necessary for acute fish damage.
Concentrations of total phosphorus in the lake increased from a minimum
of 82 mg·L-1 at 7:30 AM to a maximum of 186 mg·L-1at
2:00 PM before declining (Fig. 12c).
The similarity in trends of the 71% increase in wind speed and the 60%
increase in turbidity, 42% increase in ammonium and 126% increase in total
phosphorus support the role of wind in sediment resuspension and substantial
increases in water column nutrient concentrations. In fact, on September 27, 2000, statistical analyses of the data
reveal positive correlations between wind speed and
turbidity (r2 = 0.65), ammonium (r2 = 0.27), and total
phosphorus (r2 = 0.51).
Concentrations of total phosphorus increased rapidly when benthic sediments were added to water taken from Clear Lake. The turbidity gradient created ranged from 32 to 160 NTU, while TP concentrations ranged from 143 to 876 mg·L-1. The relationship between the two variables, with TP as the independent variable (y = 3.975x + 55.4) was used to estimate TP concentrations in the water from turbidity measurements provided by the sonde. These predictions are, however, based upon turbidity comprised primarily of sediment and are unlikely to account for variability of background phosphorus from other sources. The slope of the laboratory-derived relationship between turbidity and TP (4.0), however, is very similar to that derived in situ (3.9) on September 27, 2000. It is therefore likely that the magnitude of change in phosphorus (DTP) predicted by the laboratory-derived relationship is an accurate depiction of the change in TP in the lake, while predictions of the actual TP concentrations may be slight overestimates. Total phosphorus concentrations predicted from turbidity in Clear Lake through our study period are quite variable with a daily phosphorus flux often surpassing 100 mg·L-1 (Fig. 12d). Some very rapid fluctuations and very high concentrations of TP are likely related to turbidity spikes that may have been erroneously high due to bubbles or other obstructions in the instrument’s path.
Although substantial evidence seems to support the role of wind in the resuspension of sediments and nutrients in Clear Lake, not all of the flux of turbidity and TP may be attributed to wind-induced wave energy. Through the analysis of the passage near the buoy (generally within 100 meters) of 2287 boats, it appears as though recreational boat traffic may play an important role in the entrainment of benthic sediments. Most of the 2287 boats observed were traveling at a sufficient velocity to produce wakes. Violations of the no wake zone (91.5 meters from shore) were frequent (Fig. 13a and b) and many boats traveled at high speeds along the margin of the no wake zone (Fig. 13c). Although the same problems that plague statistical analyses of wind-induced resuspension preclude a direct statistical analyses of boat traffic (no significant correlations exist), spectral trends in mean hourly turbidity do appear to be closely related to those of the mean number of boats passing the sonde per 10 minute interval (Fig. 14a). While spectral trends in wind over the same period seem to follow those of turbidity with some correspondence, it is important to note that wind speeds during the period are generally low and, over time, wind generally decreased while turbidity and boat traffic show overall increases through time. The role of boat-induced turbulence in the resuspension of sediments in Clear Lake is also anecdotally supported by observations of sediment plumes following the passage of boats (Fig. 14b).
D. Discussion
Although the nature of the time-series data we collected prevents direct statistical analyses among wind, recreational boat traffic, and turbidity, spectral analyses of these data support the potential for wind and boat traffic to influence sediment resuspension in Clear Lake. Downing and Ramstack (2001) have also shown turbidity maxima near the lake’s bottom indicating that resuspension of sediment may be prevalent. Maps of wave velocity near the lake’s bottom also indicate that, during strong wind events, sufficient energy may exist to lift even relatively large sediment particles from a large proportion of the lake bottom. Turbulent resuspension of benthic sediments and nutrients may therefore be frequent occurrences that, without remediation, may lead to increased restoration time-scales as resuspended nutrients maintain the lake’s elevated trophic status.
The degree of nutrient flux evident during wind resuspension events is of substantial magnitude. Concentrations of total phosphorus may more than double, increasing by over 100 mg·L-1, in less than 12 hours, leading to rapid reductions in N:P ratios. Fortunately, as observed in other shallow, eutrophic lakes (Kristensen et al. 1992), resuspended total phosphorus seems to drop out of the water column with turbidity during times of low wind, implying that suppression of wind-induced waves could suppress phosphorus flux from the sediments. The resuspension of unionized ammonia, however, represents perhaps a more insidious threat to the aquatic community as concentrations may frequently exceed levels dangerous to fish and wildlife. During the summer months, when water temperature and pH are high, turbulent resuspension of high concentrations of unionized ammonia could lead to ammonia-, rather than oxygen-mediated fish kills.
Areas of the lake characterized by shallow depths and long fetch may be particularly sensitive to resuspension from wind-induced waves, especially as wind speeds surpass 10 m·s-1. This is perhaps most evident in the lake’s small, shallow, western basin. Resuspension here is likely to be a common occurrence even at relatively low wind velocities due to the basin’s shallow depth and fine, unconsolidated sediments. In fact, the frequent brown hue of the water in this smaller basin suggests sediments are often in suspension. Nutrient flux from resuspended sediments and phosphorus- and ammonia-rich pore water is likely to be high here and it is probable that prevailing currents transport large sediment and nutrient loads from the smaller western basin into the lake’s larger basins to the east. This is, in fact, observable in the sediment plumes that may often be observed passing through the narrow strait between basins.
Shallow areas of Clear Lake may also be quite susceptible to sediment resuspension induced by recreational boat traffic. It is important to note, however, that increased resuspension due to boat traffic was evident even in relatively deep water (over three meters) at the site of the sonde. Because maps of bottom velocity also indicate that wind-driven sediment resuspension may be most prevalent in the shallow areas around the lake’s margin and in the small western basin, it is plausible that the frequent violation of the no wake zones in these areas may enhance and contribute to resuspension or may prevent resuspended particles from resettling. Through this mechanism, wind and boat traffic may act together to resuspend benthic sediments and maintain their suspension in shallow water.
Internal nutrient loading through turbulent resuspension of sediments is a substantial problem in Clear Lake and, without change, will likely lead to further degradation of the lake’s water quality, recreational value, and its fish and wildlife communities. Potentially hazardous blooms of cyanobacteria may become more common as resuspended sediments degrade the light climate, suppress macrophyte growth, and lower N:P ratios. As sediments continue to fill the lake, reducing water depth, more of the lake’s bottom will become susceptible to resuspension, thereby increasing water column nutrients including total phophorus, ammonium, and toxic ammonia. Lake restoration time-scales may also be prolonged if sediment resuspension continues to occur unabated in the lake.
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Table 1. The area (m2) of the bottom subject to wave velocities
capable of resuspension and the total % of the bottom capable of resuspension
and, for the prevailing wind axis (*), the % bottom capable of resuspending
particles of increasing diameters. Estimates
of the area and % of the bottom prone to mobility during wind events are based
upon lake surface area and are therefore slight underestimates.
Wind Axis
|
Wind Speed |
Total
Mobile Area (m2) |
Total Mobile Area (% of surface area) |
Silt (<0.063mm) |
Fine
Sand
(0.063 – 0.5mm) |
Coarse Sand (0.5 – 1.0mm) |
Very
Coarse Sand (1.0 – 2.0mm) |
Granules (2.0 – 4.0mm) |
Pebbles (4.0 – 64mm) |
NW-SE*
(330°-170°) |
5 m·s-1 |
28825 |
0.2 |
0.2 |
- |
- |
- |
- |
- |
NW-SE* (330°-170°) |
10 m·s-1 |
1804562 |
17.2 |
12.5 |
4.6 |
- |
- |
- |
- |
NW-SE* (330°-170°) |
15 m·s-1 |
8370631 |
58 |
33.8 |
19.7 |
4.2 |
0.3 |
- |
- |
NW-SE* (330°-170°) |
20 m·s-1 |
13420174 |
93.0 |
33.7 |
42.1 |
12.5 |
4.1 |
0.8 |
- |
NW-SE* (330°-170°) |
25 m·s-1 |
14284528 |
99.0 |
9.7 |
45.7 |
28.9 |
9.9 |
3.9 |
1.0 |
NE-SW (45°-225°) |
10 m·s-1 |
3964036 |
27.5 |
|
|
|
|
|
|
NE-SW (45°-225°) |
15 m·s-1 |
12183123 |
84.5 |
|
|
|
|
|
|
N-S (0°-180°) |
10 m·s-1 |
3736538 |
25.9 |
|
|
|
|
|
|
N-S (0°-180°) |
15 m·s-1 |
12324986 |
85.4 |
|
|
|
|
|
|
E-W (270-90°) |
10 m·s-1 |
2568821 |
17.9 |
|
|
|
|
|
|
E-W (270°-90°) |
15 m·s-1 |
8638721 |
59.9 |
|
|
|
|
|
|
Figure 1. Locations of the sonde (·), Clear Lake Municipal
Water Treatment Facility (·), and 3700 points used for
calculation of physical wave characteristics (·).
Figure 2.
Turbidity (NTU) (·), chlorophyll a concentrations
(mg·L-1) (·), and wind speed (m·s-1)
(·) observed at Clear Lake
between July 25, 2000 and October 19, 2000.
The data logging sonde ran out of battery power between September 9,
2000 and September 15, 2000 leading to data loss for that period. Although data are variable, closer
examination (cut away) reveals diel fluctuations in all three variables are
superimposed on larger-scale trends.
Figure
3. Peaks and troughs in
spectral trends in Turbidity (NTU) (¾) and wind speed
(m·s-1)
(¾), both averaged over one hour intervals, are similar, implying a role
of wind–induced turbulence in sediment resuspension at Clear Lake.
Figure 4. Wave
velocities (cm·s-1) at
the lake bottom during a 5 m·s-1wind
event along the prevailing Northwest-Southeast (330°-170°) wind axis.
Figure 5. Wave velocities (cm·s-1)
at the lake bottom during a 10 m·s-1wind
event along the prevailing Northwest-Southeast (330°-170°) wind axis.
Figure 6. Wave
velocities (cm·s-1) at
the lake bottom during a 15 m·s-1wind
event along the prevailing Northwest-Southeast (330°-170°) wind axis.
Figure 7. Wave
velocities (cm·s-1) at
the lake bottom during a 20 m·s-1wind
event along the prevailing Northwest-Southeast (330°-170°) wind axis.
Figure 8. Wave
velocities (cm·s-1) at
the lake bottom during a 25 m·s-1wind
event along the prevailing Northwest-Southeast (330°-170°) wind axis.
Figure 9. Wave
velocities (cm·s-1) at
the lake bottom during 10 m·s-1and
15 m·s-1 wind events
along a North-South (0°-180°) wind axis.
Figure 10. Wave
velocities (cm·s-1) at
the lake bottom during 10 m·s-1and
15 m·s-1wind events along
an East-West (270°-90°) wind axis.
Figure 11. Wave
velocities (cm·s-1) at the
lake bottom during 10 m·s-1and
15 m·s-1wind events along
a Northeast-Southwest (225°-45°) wind axis.
Figure 12. Flux
of (a) turbidity (NTU) (¾), (b) ammonium (mg·L-1) (¾), and (c) total phosphorus
(mg·L-1) (¾) during a significant wind
event (10 m·s-1) (¾) on September 27,
2000. Total Phosphorus (d) flux (¾) between August 4, 2000 and
October 19, 2000 was estimated from a laboratory-derived relationship between
water and sediments from Clear Lake. These
predictions of total phosphorus are likely to be overestimates since they do
not account for variable background phosphorus from external sources. The change in predicted TP concentrations
over time is, however, likely to be accurate.
Figure
13. Violations of the no wake
zone on Clear Lake, Iowa.
Figure
14.
a. Trends in the mean number of boats passing the sonde per 10
minute interval (¾), turbidity (NTU) (¾), and wind speed (10m·s-1) (¾) between August 25, 2000
and September 3, 2000.
b. A plume of resuspended sediment behind a
motorboat in the western basin