Limnology of Clear Lake
John A. Downing,
Jeff Kopaska, Rebecca Cordes, and Nicole Eckles
A. Introduction.
Clear Lake is typical of a large, shallow, corn-belt, kettle lake. The very shallow depth (maximum about 5.9 m or 19 ft) means that wind mixing and fish activities return nutrients from the sediments into the water column during the warm, summer season. This large input of nutrients from the watershed and the remobilization of sediment nutrients give Clear Lake a very high concentration of nutrients such as nitrogen and phosphorus. The mixed agricultural and urban watershed furnishes very high nutrient loads to the lake, some of which has been deposited into the sediment layers. These high nutrient inputs, coupled with the fish- and wind-induced mixing of sediments, are significant impediments to restoration efforts, since they have turned Clear Lake into a hypereutrophic ecosystem.
B. Physical
Characteristics.
Lake surface area: 1,468 hectares (3625 Ac)
Maximum depth: 5.9
meters (19.2 ft)
Mean depth: 2.9
meters (9.6 ft)
Volume: 42,054,656
m3 (34,080 Ac-ft)
Length of shoreline: 22.7 kilometers (14.1 mi)
Shoreline development index: 1.60
Watershed area: 4,888
hectares (12,079 Ac)
Watershed area/Lake area ratio: 2.3
Hydraulic residence time: 1.6 years
C. Historical
Data.
Clear Lake was one of fifteen Iowa lakes and reservoirs studied in the National Eutrophication Survey performed by U.S. EPA in 1974-1975. This survey indicated that Clear Lake was eutrophic, based on a combination of the following parameters: total phosphorus, dissolved ortho-phosphorus, inorganic nitrogen, Secchi disk measured water clarity, chlorophyll a and dissolved oxygen. Water samples were collected April 18, July 3, and September 23, 1974 from one or more depths at three locations in Clear Lake. From those samples, mean total phosphorus was 59 mg/L (N=13), mean inorganic nitrogen was 0.19 mg/L (N=13) and mean Secchi disk depth was 0.89 m (N=8). This survey ranked Clear Lake fourth out of fifteen lakes on a least to most eutrophic gradient according to measured in-lake parameters.
From data collected in Iowa’s 1979 lake classification survey, Clear Lake was classified as a eutrophic lake. The mean total phosphorus concentration was 110.5 mg/L (N=8), mean total Kjeldahl nitrogen was 1.3 mg/L (N=2), and mean Secchi disk depth was 0.7 m (N=5). Summer fishkills were estimated to be rare, but winter fishkills were estimated to occur in one year out of 100. An extensive winter fishkill occurred in 1978, and this was the first large-scale fishkill that had occurred in recorded history. A water quality index was calculated for the 106 Iowa lakes sampled in the 1979 lake classification study based upon Secchi disk depth, total phosphorus concentration, algal chlorophyll concentration, total suspended solids and winter fishkill frequency. The index ranked Clear Lake as the 28th poorest water quality of all lakes in the survey. Clear Lake’s major problems were water level maintenance, water removal by the city of Clear Lake and nonpoint source pollution from soil erosion and agricultural chemicals. This was suggested to be due to soil erosion from agricultural land in the watershed, considering that 76.9% of the watershed was under row crop production.
From data collected for Iowa’s 1994 lake classification survey (field collections occurred in 1990 and 1992), Clear Lake was classified as a eutrophic lake. The mean total phosphorus concentration was 155 mg/L (N=9), mean total nitrogen was 4.1 mg/L (N=9) and mean Secchi depth was 0.4 m (N=3). Winter and summer fishkills were estimated to be rare. A numerical system was developed to rank all Iowa lakes with mean depths of less than 4 m. It was based on a point system to rank Iowa lakes based on public benefit, need for restoration and restoration effectiveness. This system gave Clear Lake a ranking of 18th in terms of priority for restoration projects. A summary of lake nutrient changes over time is shown in Table 1.
Additional monitoring studies have occurred in recent years. In the summer of 1992, ISU was involved in a study on Clear Lake. Roger Bachmann, along with several Clear Lake residents, took samples on a weekly basis from May 1992 until August 1992. Average Secchi depths typically ranged from 0.305 m to 0.45 m. Chlorophyll concentrations were typically high at 84 mg/L relative to the Iowa average of 47 mg/L. The average summer phosphorus concentration was 100 mg/L, while the average total nitrogen and nitrate concentrations were 2.1 mg/L and 0.17 mg/L, respectively. In addition to the 1992 study, monitoring of Clear Lake was carried out by Dr. William Crumpton’s laboratory at ISU in both 1995 and 1996 (Appendix 4). Average chlorophyll concentration was 52 mg/L and average summer phosphorus concentration was 153 mg/L. As a result of these and other previous studies, long-term lake monitoring was recommended.
These data, taken together with the recent diagnostic work, show that Clear Lake has increased in nutrient concentration by nearly 3-fold since the early 1970’s, and has decreased in water clarity by nearly 3-fold during the same period (see Fig. 1). The most rapid drop in water clarity arose around 100-120 ppb (between 1980-1985). Currently, Clear Lake is increasing in total phosphorus concentration at a rate of 4 ppb/year. At the current rate of increase in total phosphorus, the concentration in Clear Lake would likely be 216 ppb by 2010, and 340 ppb by 2040. Projections such as this make many assumptions but are only meant to indicate the very rapid rate of change being seen in the water quality in this important lake.
D. Limnological data from diagnostic study.
a. Sampling Scheme. Clear Lake limnology was analyzed by sampling at three points, distributed from west to east across the lake (Fig. 2). This approach was used because Clear Lake has three somewhat distinct basins that function slightly differently. For example, the Little Lake can be considered a nearly separate waterbody. Analyses were, therefore, performed separately for each basin and the aggregate trends in water quality were examined by calculating volume-weighted averages for the entire lake. The baseline survey was conducted from July 1998 to September 2000. Previously established methods for diagnostic surveys were followed in regard to sampling frequency, and analytical techniques outlined in Standard Methods for the Analysis of Water and Wastewater (APHA 1994) were used. Average values of the measurements made in this study are presented in Tables 2-5. Additional tables of raw data are presented in Appendix 5.
b. Physics and Chemistry. Clear Lake is polymictic, meaning that it is nearly continuously mixed to the bottom by wind and wave action. The relative lack of stratification compared to other Iowa lakes is due to a somewhat shallow basin and broad expanse of water over which the wind can create turbulence due to wave and current energy. Relatively homogeneous water column temperatures are indicated by the nearly vertical lines on the isotherm graphs in Figure 3. Temperature decreases sharply and transparency increases in the autumn as the lake is cooled rapidly by wind mixing. Generally, temperatures are quite extreme in the lake, varying from >25°C in mid-summer to <4°C in winter. These extremes in heating and cooling result from the shallow depth and extreme degree of wind mixing of the water column, and are instrumental in the weathering and breakdown of sediments. In general, water clarity during the summer season averages between 0.3-0.4m (ca. 1 foot, Fig. 4).
Because of the intense mixing of the water column, surface oxygen concentrations are generally adequate throughout the year (Fig. 5). Because of intense sedimentary decomposition during warm weather and very weak stratification of water layers near the bottom, bottom waters are frequently somewhat low in dissolved oxygen during the summer and winter months. Concentrations of dissolved oxygen near the bottom rarely fall beneath the minima required to safely sustain sport fish.
Clear Lake is well buffered, containing an average of about 150 mg/L of calcium carbonate alkalinity (range of averages 130-180). Alkalinity generally increases throughout the summer in the Little Lake, likely owing to evaporative loss of water and resultant concentration of dissolved salts in Ventura Marsh and this shallow basin (Fig. 6), but declines somewhat in the main lake due to extremely high rates of primary production during late summer. Lake pH becomes quite high in mid-summer as the phosphorus-driven primary production uses CO2 more rapidly than the alkalinity buffering system can supply it (Fig. 7). Maximum summer pH approaches 9.0. Some low pHs are observed in the bottom waters during mid-summer, due to intense decomposition of organic materials and bottom sediments, and declines rapidly throughout the autumn. Total dissolved solids peak in the spring as runoff dissolves terrestrial salts (Fig. 8), declining through dilution after spring rains, then declining further as alkalinity is depleted in late summer. Silicon (a dissolved mineral essential to the growth of beneficial algae) was highest near the beginning of the study and generally declined over the analysis period (Figs. 9-10). The silicon concentration declined fairly steadily over the study period, probably in response to intense inputs from heavy rains at the beginning of the study followed by steady sedimentation of algae.
In spite of the relatively small size of the watershed, Clear Lake has a very high concentration of suspended solids. The average for Iowa lakes (including natural and artificial lakes) is about 20-30 mg/L (0.02-0.03 g/L), with natural lakes usually containing less suspended solids than artificial impoundments (Fig. 11). Clear Lake averages much greater than this, with frequent lake-wide averages of >30 mg/L (0.03 g/L) (Fig.s 11-12). Highest suspended solids concentrations were found in mid-summer, indicative of the resuspension of lake sediments by wind and fish action. The lake sediments and watershed erosion are almost certainly the source of suspended solids during the summer months. This is indicated by the tendency for highest concentrations to be near the bottom (Fig.s 13-14) and the fact that one can observe the movement of sediment loads associated with high rainfall in June-July 1999 sequentially along the lakes axis from the Little Lake in June to the eastern-most sampling station in August (Figs. 12, 15).
c. Nutrient Concentrations. Nitrogen and phosphorus are the most important essential nutrients to the growth of algae and other organisms in lakes. Although Clear Lake is far from the most nutrient-rich lake in the state, it is extremely rich in nutrients when compared to some of Iowa’s other lakes (Fig. 16, Table 6) and lakes elsewhere in the world (Fig. 17). The rich nutrient environment is likely due to significant nutrient inputs from the agricultural watershed, as well as the regeneration of sedimentary nutrients throughout the summer season. Total nitrogen trends are characterized by large peaks of nutrient concentrations during periods of high run-off with concentrations highest in the Little Lake where most of the hydraulic load is received (Fig. 18). Late summer nitrogen concentrations generally decline, probably due to denitrification loss to the atmosphere under warm conditions at the oxic/anoxic interface near the sediment surface. Generally, however, the source of nitrogen is clearly indicated by the fact that total nitrogen concentrations decrease as water masses move from west to east. A paired t-test shows that total nitrogen concentrations in the Little Lake are an average of 0.7-0.8 mg/L higher than those in the central or eastern basins, respectively (p<0.0001). Total nitrogen levels decrease through autumn and winter as cool conditions slow nitrogen inputs from the watershed. Because the lake is large with respect to its watershed, algae growth is quite high and only a small fraction of the nitrogen occurs in the form of nitrate (Fig. 19), except in spring and early summer when watershed efflux is high. Generally, however, dissolved nitrogen is dominated by ammonium, because it is liberated from decomposing sediments (Fig. 20) and is mixed into the water by wind and fish. Under the current nutrient scenario, ammonia is likely to have adverse impacts on fish and other organisms in Clear Lake because unionized ammonia concentrations peak beyond the levels normally responsible for severe fish damage (Fig. 21). Unionized ammonia levels in Clear Lake are much higher than would be desirable for sustaining optimal sport fisheries. The prevalence of ammonia as a nitrogen form imply a constant supply of reduced nitrogen from decomposition or from other nutrient-rich, low oxygen sources.
Trends
in total phosphorus are dominated by inputs from the watershed and the
regeneration of phosphorus from decomposing sediments, summer wind mixing and
fish activities. Normally a lake will
show maximum phosphorus levels near ice-out, decreasing throughout the summer
until a second phosphorus peak is observed in autumn. In Clear Lake, however, phosphorus concentrations are lowest in
winter and early spring, reaching high levels during the warm-water season,
then declining when cold weather begins (Figs. 22, 23). Unlike many other lakes, there is no marked
trend in phosphorus concentration across the season, but lake-wide total
phosphorus hovers between 150-200 ppb.
These levels now place Clear Lake within the hyper-eutrophic category. It is likely that much of the supply of phosphorus
originates in the watershed, since much of the watershed is at the western end
of the lake and a paired t-test of inter-station differences in total
phosphorus shows that concentrations in the Little Lake are generally around an
average of 33 ppb higher than that in the central basin (p<0.0001). Nutrient concentrations generally decline as
the water masses move from west to east across the lake.
The
concomitant decrease in total nitrogen in mid-summer and increase in total
phosphorus from summer sediment mixing means that conditions favor the excess
growth of nuisance algae. Ratios of
total nitrogen to total phosphorus (N:P) are most frequently below the
acceptably high range (>30 as mass units) that would favor the growth of
most useful forms of phytoplankton. N:P
falls to very low levels by the end of summer (Figs. 24, 25). This is due to the excessively high rate of
input of phosphorus that keeps N:P quite low.
This favors the growth of nitrogen fixing forms such as the
cyanobacteria (formerly “blue-green algae”) that can be a nuisance and health
hazard when they grow in excess.
d. Phytoplankton Community Structure and Biomass. Phytoplankton in Clear Lake follow a seasonal pattern that is typical of temperate, shallow, hypereutrophic lakes (Fig. 26). Algal biomass is generally highest in mid-summer when it forms conspicuous “blooms” of algae coloring water an intense green color. Cyanobacteria (“blue-green algae”) and diatoms (Bacillariophyceae) make up the majority of the phytoplankton biomass for much of the active growing season (Figs. 27-31). Cyanobacteria dominance is especially acute in mid- to late-summer, when cyanobacteria make up >80% of the algal biomass. The dominance of Cyanobacteria gives Clear Lake a sometimes fluorescent green color. The taxa of Cyanobacteria involved in summer blooms are Anabaena, Spirulina and Oscillatoria (Figs. 29-31). These are taxa that are known to be able to produce some toxic materials, and these potentially toxic taxa compose between 27% of the phytoplankton in the Little Lake to 47% of the phytoplankton biomass at the central sampling station (overall average during the summer season: 35%). It is likely that the Cyanobacteria in Clear Lake exude some degree of toxin during growth, senescence or decay (Falconer 1999). Because such toxins can be harmful to invertebrates, fish, wildlife, livestock and humans, reduction of nutrient levels to eliminate Cyanobacterial dominance would be welcome. Cyanobacteria dominance in mid-summer arises due to very high nutrient concentrations and very low N:P ratios. Overall, about 38% of the phytoplankton in the lake is composed of Cyanobacteria on an annual basis (Table 7). Figure 32 shows that, when compared to world lakes in general, Clear Lake develops somewhat less Cyanobacteria than would normally occur at this high a level of total phosphorus concentration.
E. Trophic
condition of lake.
Based on both the historical data and the data collected during this study, Clear Lake is classified as hypereutrophic. This is reflected in the relatively high values for total phosphorus and total nitrogen, and low Secchi disk transparencies.
F. Algae.
The ratio of mean
total nitrogen to mean total phosphorus concentration on the lake is 13:1. This indicates that Clear Lake algae are
phosphorus limited throughout much of the ice-free season, but potentially
nitrogen limited at the height of summer stagnation.
G. Macrophytes.
Aquatic macrophytes were surveyed during the summers of 1999 and 2000. Transects perpendicular from the shoreline to the outer edge of macrophyte beds were established at 20 m intervals. Macrophytes were then quantified at 20 m intervals along each transect using a 1m2 quadrat. From this survey, we were able to determine species present and abundance of each species. These data were then compared to historical data collected from Clear Lake over the past 100 years.
Results for Clear Lake show that there exists a total of 12 different species of aquatic macrophytes in the lake. Species frequency of occurrence in quadrats show that Scirpus (Rush) is the most common 68%, followed by Typha (Cattail) 21%, Nuphar a. (Yellow Lily) 4.2%, P. nodosus (Floating Leaf Pond Weed) 3.4%, Nymphaea t. (White Lily) 2.2%, P. pectinatus (Sago Pond Weed) 0.4%. The rest were <1%.
To see changes in macrophyte distributions over the years, these results were compared to results from seven other surveys conducted on Clear Lake dating back to 1896. Results show that species abundance has steadily decreased from a maximum of 35 species surveyed in 1952, to 21 species in 1981, and is now 12 species. Frequency of occurrence of species in each quadrat was compared to data from 1981, which was the last time an aquatic survey was completed on Clear Lake. Results show a decrease in all of the species listed above except P. nodosus and Nuphar a., which show increases. The areal extent of macrophyte beds was measured by digitizing aerial photographs of Clear Lake from 1979 to 1999. Results show that there has been an overall decrease in macrophyte bed size by 49%.
We compared these data to historical Secchi disk readings and found a dramatic decrease in water clarity from 2.4 m in 1896 to 0.4 m in 2000 (Fig. 33). An increase in water clarity would lead to an increase in macrophyte species diversity and abundance in Clear Lake. If water clarity continues to degrade, macrophyte species diversity and abundance will continue to decrease. Macrophytes help stabilize sediments and create very useful fish and wildlife habitat.
H. Hydraulic budget for lake.
The method used in the analysis is based on the equation of continuity.
Fundamental Equation: DS = Inflow - Outflow
The equation in more detail is: DS = P + R - O - E + GW
Where:
S = change in storage (m3)
P = the precipitation falling directly onto the lake (m3)
R = the surface runoff from the watershed (m3)
E = the evaporation from the lake surface (m3)
O = the outflow over the spillway (m3)
GW = the groundwater seepage (m3)
P and R are positive quantities, E and O are negative quantities, GW and S may be either positive or negative. Precipitation records from the Mason City Municipal Airport (July 1998-September 2000, source: http://nndc.noaa.gov/?home.shtml). Evaporation records are from the National Weather Service, Climatological Data (May 1998-April 1999) for Ames. A pan coefficient of 0.74 was used (Kohler et al., 1959).
Runoff was determined using a rainfall volume and runoff coefficient method. Runoff coefficients were derived from measurement of rainfall and runoff in two subbasins (one agricultural, one urban) that were continuously monitored using flow meters. These runoff coefficients were directly assigned, or combined to determine, a runoff coefficient for the other subbasins in the watershed based upon percentages of agricultural or urban land use in the other subbasins. The entire study length was broken down into different time periods that had similar runoff coefficients. Water flux from each subbasin was determined by multiplying rainfall volume, drainage area and runoff coefficient for each time period. Periodic water fluxes (m3) were summed to determine total annual runoff.
Outflow was determined using discharges measured on the outflow stream. Outflow was measured in the field periodically (two times per month in April-September, once per month in October-March). Instantaneous outflow (m3/s) were multiplied by 86,400 to determine daily outflow (m3/day). These results were compared to lake stage data from the USGS gauging station at Clear Lake. From these results, a stage-discharge relationship was built for lake stage data for Clear Lake.
Groundwater seepage and other groundwater information are discussed in detail in Chapter 8 (page 180) of this report.
Table 8 summarizes the hydraulic budget for Clear Lake for a period of two years, ending July 31, 2000. The lake derives 49% of its water from rainfall onto the lakes surface and 43% of its water from runoff. Clear Lake loses 61% of its water through evaporation and 28% of its water through outflow. The lake flushes 0.69 times per year for a hydraulic retention time of 1.45 years. These data indicate that the lake overall has a positive water budget. The period that this budget covers is short due to the lack of inflow and outflow data other than that which was collected during the course of this study.
I. Lake bathymetry
A bathymetric survey of Clear Lake was conducted in July 2000. The survey was performed using digital, discriminating sonar and differentially corrected Global Positioning Systems (GPS). Lake elevation at the spillway crest was used as the baseline for lake depth, and over 77,000 distinct locations with their associated depths were used to create the new bathymetric map seen in Figure 34. These data were imported into SURFERÔ, a commercial geostatistical mapping package, to calculate the lake volume and generate bathymetric maps. The volume of the lake at spillway height in 2000 was found to be 42,054,656 m3 (34,108 ac-ft), and the surface area was 1,468 hectares (3,625 ac). The maximum depth of the lake was 5.9 m (19 ft), and the mean depth was 2.9 m (9.6 ft).
A bathymetric survey of Clear Lake was also completed in 1935. This map is shown in Figure 35. This earlier survey was digitized using ArcView and the data was exported for use in SURFERÔ. Using this data set, SURFERÔ determined the volume of the lake in 1935 to be 45,669,742 m3 (37,010 ac-ft) with a mean depth of 3.1 m (10.3 ft). An original lake volume was calculated using sediment depths from borings taken in 1935. The depths of soft sediments were added to the water depths recorded for the 1935 bathymetric survey to calculate original lake water depths. This map is shown in Figure 36. The original lake volume was determined to be 68,121,176 m3 (55,248 ac-ft) with a mean depth of 4.7 m (15.5 ft).
Sediment deposition in Clear Lake from 1935 to 2000 is shown in Figure 37. This figure was created by subtracting the 2000 depth map from the 1935 depth map. The change in depth between the two years is the result of sediment deposition. Sediment deposition from the original lake to 2000 was also calculated in the same manner and shown in Figure 38. Between 1935 and 2000, 3,537,706 m3 (2869 ac-ft) of lake volume was filled with sediment. Between the original lake and 2000, 25,990,520 m3 (21,079 ac-ft) of sediment was deposited with 2,731,853 m3 (2,216 ac-ft) of that sediment accumulating in the Little Lake (Fig. 39). This indicates an annual sediment deposition of 54,426 m3 (44 ac-ft) into Clear Lake between 1935 and 2000. The present volume represents an 8% reduction in lake volume from 1935 to 2000. From the original lake to 2000, Clear Lake has lost 38% of its volume. It is expected that sediment deposition rates will decrease with time, as trap efficiency of the lake declines. However, assuming constant rates of sediment delivery and deposition in the future, Clear Lake would completely fill with sediment in approximately 775 years.
Sediment deposition has resulted in the loss of one foot of mean depth since 1935. Considering that annual sediment deposition has been 54,426 m3 (44 ac-ft), and the surface area of the lake is 1,468 hectares (3,625 ac), the lake has been losing 3.7 mm/yr (0.94 in) of depth if this sediment were evenly distributed. This sediment has a density of 1440 kg/m3, which when multiplied by annual sediment deposition rate gives a result of just over 78 million kg/yr (>85,000 tons/yr) of sediment added to the lake. Thus, the Clear Lake watershed has lost 22,910 kg/ha/yr (10 tons/acre/yr) of sediment on average. This figure represents, of course, the aggregate of all erosional transport in the watershed (e.g., gully, sheet, rill, streambank, etc.), exclusive of shoreline erosion, less the sediment retransported down stream. It also does not account for erosional losses redeposited within the basin that were not yet transported into the lake. This figure thus represents a minimal estimate of erosional soil loss in the watershed. During the course of this study, tributary monitoring showed the sediment transport to the lake to be somewhat more than 1.1 million kg/yr (see Chapter 10), which equates to 327 kg/ha/yr (0.2 ton/acre/yr). Because these sediment loss flux rates are less than the overall average sediment accumulation rate, it seems that the rates we measured during 1998-2000 are lower than those over the lifetime of the lake. The decrease in watershed sediment loss rates may result from climatic variability and improved agricultural conservation practices implemented throughout the watershed.
Drastically increased phosphorus concentrations in Clear Lake have resulted in decreases in many aspects of the quality of the Clear Lake ecosystem. Judging from trends in water clarity, Clear Lake was likely oligotrophic-mesotrophic at the turn of the century, mesotrophic until the mid 1970s, then moving from eutrophic in the mid-1970s to near hyper-eutrophic in the late 1990s. The implications of this change for many aspects of the Clear Lake ecosystem are listed in Table 9. Phosphorus concentrations of the magnitude seen in Clear Lake during this study are very poor for continued quality of recreational use. If trends continue in this vein, users of Clear Lake should expect further degradation of water clarity, reduced oxygen levels, frequent blooms of toxic algae, increased survival and persistence of fecal and potentially pathogenic bacteria, accelerated filling and siltation, mobile toxins, increased impacts of ammonia on the quality of fish and other aquatic organisms, continued declines in biodiversity and year-to-year stability, degraded fish and wildlife habitat, decreased fish production and a fish community highly dominated by rough fish.
The increase in total phosphorus concentration in the lake has yielded a profound increase in algal abundance. The dense algae that have bloomed in Clear Lake have decreased water clarity to the point that rooted aquatic vegetation has declined substantially. Turbid waters with toxic algae favor the growth of resistant fishes like carp and bullhead that perturb sediments and uproot vegetation. Sediment resuspended by fish and increased wind mixing in the absence of rooted vegetation further decreases water clarity further reducing the ability of aquatic plants to cleanse waters and stabilize sediments. Resuspended sediments lead to increased phosphorus concentrations that have favored even more algae growth. Projected increases in phosphorus concentrations indicate that, in the absence of remedial measures, Clear Lake will continue to decline in quality and utility as a recreational resource.
In order to improve Clear Lake, three fundamental changes would need to take place:
Such changes would give rise to gradual improvements in the lake, the course of which is likely to span 5-30 years before substantial improvements would be achieved.
Knowledge of the hydraulic and nutrient budgets as well as various limnological details allow computation of future water quality under various scenarios of improved watershed characteristics. First, it is important to understand that the phosphorus concentration in the lake is principally a function of the rate of phosphorus input and the rate of flushing of the lake, and secondarily a function of return of sedimentary phosphorus to the water column. One can thus calculate the expected change in water quality (i.e., phosphorus concentration) by calculating the impact of a reduction in phosphorus input. There are many models that enable these calculations but normally it is wise to fit several of them to find which yields the best prediction of the current total phosphorus concentration from calculated inputs and hydrology. We examined the fit of more than a dozen such models and found that the Canfield/Bachmann (Canfield & Bachmann 1981) yields a prediction of current phosphorus concentration at spring circulation that is the best, and is within 2% of the actual phosphorus concentration. This model is thus likely to predict the phosphorus concentration under future remedial states.
Next, it is possible to alter the parameters of the equations to examine the predicted change in total phosphorus concentration that would result from various levels of decrease in phosphorus inputs (Fig. 40). Apparently, it would take around a 60% reduction in total phosphorus inputs to bring the lake back to the total phosphorus concentrations that were seen in the late 1970s and early 1980s. Using the normal relationship between water clarity and total phosphorus seen in other Iowa lakes (Bachmann et al. 1994), we can predict changes in water clarity that would be expected following these scenarios (Fig. 41). This analysis suggests that a 60% reduction in total phosphorus loading to Clear Lake should bring water clarity to the 0.8-1.2 m level, once lake conditions equilibrate. This water clarity level is somewhat conservation because increased water clarity and carp management taken together would greatly reduce suspended solids in the water column, affording even greater increases in water clarity. It is likely, therefore, that such a management scenario could bring water clarity in Clear Lake back to pre-1970 levels, allowing marked increases in the entire lake as an ecosystem and recreational resource.
References
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TABLE 1. Summary of historical nutrient
data collected on Clear Lake.
Parameter |
1974 Study |
1979 Survey |
1990 Survey |
Total
Phosphorus |
59
mg/L |
110.5
mg/L |
155
mg/L |
Nitrogen |
0.19
mg/L (inorganic N) |
1.3
mg/L (TKN) |
4.1
mg/L (TN) |
Secchi
depth |
0.89
m |
0.7
m |
0.4
m |
TABLE 2. Summary table of measurements
made on all Clear Lake sampling stations during the diagnostic study between
July 1998 and September 2000. All
dates, depths and stations combined.
Parameter |
Units |
Mean |
Standard Error |
n |
Total
Phosphorus |
mg/L as P |
188 |
4 |
659 |
Total
Nitrogen |
mg/L
as N |
2.39 |
0.06 |
659 |
Nitrate-Nitrogen |
mg/L
as N |
0.29 |
0.01 |
475 |
Ammonia-Nitrogen |
mg/L
as N |
0.20 |
0.02 |
475 |
Chlorophyll
a |
µg/L |
42 |
6 |
111 |
Secchi
depth |
m |
0.41 |
0.01 |
111 |
Alkalinity |
mg/L
as CaCO3 |
143 |
2 |
390 |
Dissolved
Oxygen |
mg/L |
9.2 |
0.4 |
611 |
Specific
Conductance |
mmhos/cm |
331 |
5 |
636 |
Total
Suspended Solids |
mg/L |
60 |
13 |
579 |
pH |
neg.
log H+ conc. |
8.40 |
0.02 |
636 |
TABLE 3. Summary table of measurements
made on the Little Lake Site in Clear Lake during the diagnostic study between
July 1998 and September 2000. All dates
and depths combined.
Parameter |
Units |
Mean |
Standard Error |
n |
Total
Phosphorus |
mg/L as P |
210 |
10 |
156 |
Total
Nitrogen |
mg/L
as N |
2.9 |
0.2 |
156 |
Nitrate-Nitrogen |
mg/L
as N |
0.41 |
0.05 |
110 |
Ammonia-Nitrogen |
mg/L
as N |
0.23 |
0.05 |
110 |
Chlorophyll
a |
µg/L |
49 |
11 |
39 |
Secchi
depth |
m |
0.34 |
0.02 |
37 |
Alkalinity |
mg/L
as CaCO3 |
143 |
4 |
94 |
Dissolved
Oxygen |
mg/L |
12 |
2 |
145 |
Specific
Conductance |
mmhos/cm |
308 |
7 |
149 |
Total
Suspended Solids |
mg/L |
62 |
10 |
137 |
pH |
neg.
log H+ conc. |
8.54 |
0.03 |
149 |
TABLE 4. Summary table of measurements
made on the Central Lake Site in Clear Lake during the diagnostic study between
July 1998 and September 2000. All dates
and depths combined.
Parameter |
Units |
Mean |
Standard Error |
n |
Total
Phosphorus |
mg/L as P |
184 |
7 |
252 |
Total
Nitrogen |
mg/L
as N |
2.25 |
0.07 |
252 |
Nitrate-Nitrogen |
mg/L
as N |
0.26 |
0.01 |
183 |
Ammonia-Nitrogen |
mg/L
as N |
0.21 |
0.03 |
183 |
Chlorophyll
a |
µg/L |
33 |
9 |
36 |
Secchi
depth |
m |
0.45 |
0.03 |
37 |
Alkalinity |
mg/L
as CaCO3 |
144 |
3 |
147 |
Dissolved
Oxygen |
mg/L |
8.37 |
0.18 |
234 |
Specific
Conductance |
mmhos/cm |
341 |
11 |
243 |
Total
Suspended Solids |
mg/L |
66 |
27 |
219 |
pH |
neg.
log H+ conc. |
8.38 |
0.02 |
243 |
TABLE 5. Summary table of measurements
made on the East Lake Site in Clear Lake during the diagnostic study between
July 1998 and September 2000. All dates
and depths combined.
Parameter |
Units |
Mean |
Standard Error |
n |
Total
Phosphorus |
mg/L as P |
180 |
6 |
251 |
Total
Nitrogen |
mg/L
as N |
2.19 |
0.06 |
251 |
Nitrate-Nitrogen |
mg/L
as N |
0.26 |
0.01 |
182 |
Ammonia-Nitrogen |
mg/L
as N |
0.19 |
0.03 |
182 |
Chlorophyll
a |
µg/L |
45 |
12 |
36 |
Secchi
depth |
m |
0.43 |
0.02 |
37 |
Alkalinity |
mg/L
as CaCO3 |
142 |
3 |
149 |
Dissolved
Oxygen |
mg/L |
8.31 |
0.19 |
232 |
Specific
Conductance |
mmhos/cm |
336 |
3 |
244 |
Total
Suspended Solids |
mg/L |
53 |
2 |
223 |
pH |
neg.
log H+ conc. |
8.32 |
0.03 |
244 |
TABLE 6. Summary
table of summer measurements made on Clear Lake during study period
(1998-2000), during the 1990 and 2000 state lake surveys and on all Iowa lakes
during the 1990 and 2000 state lake surveys.
Parameter |
Units |
Clear Lake 1998-2000 |
Clear Lake 1990 |
Iowa lakes average 1990 |
Clear Lake 2000 |
Iowa lakes average 2000 |
Total
Phosphorus |
mg/L as P |
188 |
155 |
163 |
129 |
187 |
Total
Nitrogen |
mg/L
as N |
2.39 |
4.1 |
3.4 |
1.97 |
2.15 |
Chlorophyll
a |
mg/L |
42 |
53.4 |
48.1 |
31 |
28 |
Secchi
depth |
m |
0.41 |
0.4 |
1.1 |
0.4 |
1.0 |
TABLE 7. Fraction of total biomass composed of different
algae taxa in Clear Lake, Iowa during 1999 and 2000. Total biomass data were calculated directly from volumetric
approximations of algae counts. The
data represent the average for all stations and all dates sampled.
Taxon |
Overall
Average |
%
Bacillariophyceae |
44 |
%
Cyanobacteria |
38 |
%
Chlorophyceae |
12 |
%
Chrysophyceae |
4 |
%
Cryptophyceae |
1 |
%
Dinophyceae |
<1 |
Total
Biomass (mg/L) |
47.0 |
Table
8. Hydraulic budget for Clear Lake.
Month |
Precip. |
Pan Evap. |
Change in Storage |
Precip. |
Runoff |
Evap. |
Groundwater |
Outflow |
|
(cm) |
(cm) |
(m3) |
(m3) |
(m3) |
(m3) |
(m3) |
(m3) |
Aug-98 |
14.0 |
16.2 |
1985720 |
2060273 |
2460529 |
-1758944 |
-48050 |
-728088 |
Sep-98 |
4.6 |
17.9 |
-547569 |
674339 |
805345 |
-1943661 |
-46500 |
-37092 |
Oct-98 |
8.8 |
8.2 |
1895531 |
1289068 |
1539499 |
-884986 |
-48050 |
0 |
Nov-98 |
2.8 |
0.0 |
860611 |
413545 |
493886 |
0 |
-46500 |
-319 |
Dec-98 |
1.0 |
0.0 |
410277 |
152751 |
305576 |
0 |
-48050 |
0 |
Jan-99 |
3.6 |
0.0 |
1516969 |
521588 |
1043431 |
0 |
-48050 |
0 |
Feb-99 |
4.1 |
0.0 |
1756372 |
599826 |
1199946 |
0 |
-43400 |
0 |
Mar-99 |
3.2 |
0.0 |
1166813 |
473155 |
946541 |
0 |
-48050 |
-204833 |
Apr-99 |
20.8 |
10.9 |
2769206 |
3043839 |
3591777 |
-1179981 |
-46500 |
-2639928 |
May-99 |
19.2 |
18.7 |
350207 |
2820301 |
3327999 |
-2029126 |
-48050 |
-3720916 |
Jun-99 |
13.1 |
18.2 |
-898881 |
1914973 |
2259698 |
-1976744 |
-46500 |
-3050308 |
Jul-99 |
28.3 |
22.0 |
1249376 |
4154076 |
1629469 |
-2393046 |
-48050 |
-2093073 |
Aug-99 |
5.5 |
17.8 |
-1701496 |
808461 |
224886 |
-1927119 |
-48050 |
-759675 |
Sep-99 |
5.5 |
14.8 |
-799738 |
804736 |
52094 |
-1610068 |
-46500 |
0 |
Oct-99 |
2.8 |
16.7 |
-1423028 |
409819 |
26529 |
-1811326 |
-48050 |
0 |
Nov-99 |
1.9 |
0.0 |
254977 |
283148 |
18329 |
0 |
-46500 |
0 |
Dec-99 |
1.9 |
0.0 |
778713 |
271971 |
554792 |
0 |
-48050 |
0 |
Jan-00 |
2.7 |
0.0 |
1175107 |
402368 |
820789 |
0 |
-48050 |
0 |
Feb-00 |
4.1 |
0.0 |
1801111 |
607277 |
1238783 |
0 |
-44950 |
0 |
Mar-00 |
3.1 |
0.0 |
725231 |
450801 |
322480 |
0 |
-48050 |
0 |
Apr-00 |
4.3 |
17.7 |
-1021026 |
633357 |
310965 |
-1918848 |
-46500 |
0 |
May-00 |
12.1 |
23.9 |
7303 |
1777125 |
872532 |
-2594304 |
-48050 |
0 |
Jun-00 |
13.2 |
21.2 |
95663 |
1929876 |
517110 |
-2304823 |
-46500 |
0 |
Jul-00 |
14.3 |
42.2 |
-2069334 |
2093803 |
561035 |
-4582076 |
-48050 |
-94046 |
|
|
|
|
|
|
|
|
|
Total |
194.9 |
266 |
10338116 |
57180951 |
25124021 |
-28915052 |
-1133050 |
-13328278 |
Table 9. Tabular representation of the
usual changes in aquatic ecosystems corresponding with alterations in the
phosphorus concentrations of freshwater lakes.
Parameter |
Oligotrophic |
Mesotrophic |
Eutrophic |
Hyper-eutrophic |
Total
P (ppb) |
0-20 |
20-70 |
70-200 |
>200 |
Clarity |
Excellent |
Good |
Poor |
Very Poor |
Oxygen |
Abundant |
Adequate |
Hypoxic |
Anoxic |
Toxic
Algae |
Absent |
Absent |
Frequent |
Constant |
Bacteria |
Rare |
Rare |
Abundant |
Very Abundant |
Silt
/ Filling |
Very Slow |
Slow |
Rapid |
Very Rapid |
Toxin
Mobility |
Bound |
Bound |
Mobile |
Very Mobile |
NH3 Toxicity |
Improbable |
Infrequent |
Frequent |
Constant |
Biodiversity
& Stability |
High |
Good |
Poor |
Very Poor |
Fish
Habitat |
Good |
Excellent |
Poor |
Very Poor |
Wildlife
Habitat |
Good |
Excellent |
Poor |
Very Poor |
Fish
Production |
Low |
Moderate |
High |
Moderate |
Fish
Community |
High Quality |
Good Quality |
Poor Quality |
Rough Fish |
FIGURE 1. Trend in total phosphorus and Secchi disk transparency since the early 1970’s. Data are from EPA and State of Iowa surveys. The dashed line is a linear regression analysis (r2=0.70) showing a slope of around 4 ppb/year.
FIGURE 2. Open water
sampling points in Clear Lake, Iowa.
FIGURE 3. Trends in water
temperature in Clear Lake, Iowa during 1998-2000. Temperature data are averages of measures taken at each sampling
point in the lake. The dots indicate dates and depths of sampling. Vertical
lines indicate that the lake is mixed from top to bottom, while the more
horizontal lines indicate periods of relative stratification.
FIGURE 4. Trends in
water clarity in Clear Lake, Iowa from 1998-2000. Larger numbers indicate greater water clarity.
FIGURE 5. Trends in
oxygen concentrations in Clear Lake, Iowa during 1998-2000. Data are averages of measures taken at each
sampling point in the lake. The dots indicate dates and depths of sampling.
Vertical lines indicate that the lake is mixed from top to bottom, while the
more horizontal lines indicate periods of relative stratification.
FIGURE 6. Trends in
alkalinity concentrations in Clear Lake, Iowa during 1998-2000. Data are averages of measures taken at each
sampling point in the lake. The dots indicate dates and depths of sampling.
Vertical lines indicate that the lake is mixed from top to bottom, while the
more horizontal lines indicate periods of relative stratification.
FIGURE 8. Trends in
total dissolved solids (TDS) in Clear Lake, Iowa during 1998-2000. Data are averages of measures taken at each
sampling point in the lake. The dots indicate dates and depths of sampling.
Vertical lines indicate that the lake is mixed from top to bottom, while the
more horizontal lines indicate periods of relative stratification.
FIGURE 9. Trends in
silicon in Clear Lake, Iowa during 1998-2000.
Data are averages of measures taken at each sampling point in the lake.
The dots indicate dates and depths of sampling. Vertical lines indicate that
the lake is mixed from top to bottom, while the more horizontal lines indicate
periods of relative stratification.
FIGURE 10. Trends in
whole-lake, volume weighted concentrations of silica in Clear Lake, Iowa during
1998-2000.
FIGURE 11. Comparison of
total suspended solids concentrations in Clear Lake with those found in the
2000 Iowa Lake Water Quality Survey (Downing & Ramstack 2001).
FIGURE 12. Trends in
total suspended solids (TSS) in Clear Lake, Iowa during 1998-2000. Data are averages of measures taken at each
sampling point in the lake. The dots indicate dates and depths of sampling.
Vertical lines indicate that the lake is mixed from top to bottom, while the
more horizontal lines indicate periods of relative stratification.
FIGURE 13. Volume
weighted trends in suspended solids in Clear Lake, Iowa during 1998-2000.
FIGURE 14. Trends in
inorganic suspended solids (ISS) in Clear Lake, Iowa during 1998-2000. Data are averages of measures taken at each
sampling point in the lake. The dots indicate dates and depths of sampling.
Vertical lines indicate that the lake is mixed from top to bottom, while the
more horizontal lines indicate periods of relative stratification.
FIGURE 15. Trends in
volatile suspended solids (VSS) in Clear Lake, Iowa during 1998-2000. Volatile suspended solids represent the
organic fraction of the suspended sediment and therefore contain algae,
detritus and soil particles. Data are
averages of measures taken at each sampling point in the lake.
FIGURE 16. Comparison of
total phosphorus concentrations in Clear Lake with those found in the 2000 Iowa
Lake Water Quality Survey (Downing & Ramstack 2001).
FIGURE 17. Comparison of
total phosphorus and total nitrogen concentrations in Clear Lake with those
found in the 1990 (Bachmann et al 1992) and 2000 Iowa Lake Water Quality
Surveys (Downing & Ramstack 2001), as well as world lake data (Downing and
McCauley 1993).
FIGURE 18. Trends in
total nitrogen in Clear Lake, Iowa during 1998-2000. Data are averages of measures taken at each sampling point in the
lake. The dots indicate dates and depths of sampling. Vertical lines indicate
that the lake is mixed from top to bottom, while the more horizontal lines
indicate periods of relative stratification.
FIGURE 19. Trends in
nitrate in Clear Lake, Iowa during 1998-2000.
Data are averages of measures taken at each sampling point in the lake.
The dots indicate dates and depths of sampling. Vertical lines indicate that
the lake is mixed from top to bottom, while the more horizontal lines indicate
periods of relative stratification.
FIGURE 20. Trends in
ammonia nitrogen in Clear Lake, Iowa during 1998-2000. Data are averages of measures taken at each
sampling point in the lake. The dots indicate dates and depths of sampling.
Vertical lines indicate that the lake is mixed from top to bottom, while the
more horizontal lines indicate periods of relative stratification.
FIGURE 21. Trends in
volume weighted concentrations of various forms of nitrogen in Clear Lake, Iowa
during 1998-2000.
FIGURE 22. Trends in
total phosphorus concentration in Clear Lake, Iowa during 1998-2000. Data are averages of measures taken at each
sampling point in the lake. The dots indicate dates and depths of sampling.
Vertical lines indicate that the lake is mixed from top to bottom, while the
more horizontal lines indicate periods of relative stratification.
FIGURE 23. Trend in
volume weighted concentrations of total phosphorus in Clear Lake, Iowa during
1998-2000.
FIGURE 24. Trends in
the volume weighted ratio of total nitrogen to total phosphorus concentration
in Clear Lake, Iowa during 1998-2000.
FIGURE 25. Trends in the
volume weighted chlorophyll a concentrations in Clear Lake, Iowa during
1998-2000.
FIGURE 26. Trends in
percentage taxonomic composition of phytoplankton in Clear Lake, Iowa during
1998-2000 (by biomass).
FIGURE 27. Photograph of
Bacillariophycaea (Melosira sp.) from Clear Lake.
FIGURE 28. Photograph of
Bacillariophycaea (Asterionella sp.) from Clear Lake.
FIGURE 29. Photograph of
Cyanobacteria (Spirulina sp.) from Clear Lake.
FIGURE 30. Photograph of
Cyanobacteria (Oscillatoria sp.) from Clear Lake.
FIGURE 31. Photographs
of Cyanobacteria (Anabaena sp.) from Clear Lake.
FIGURE 32. World trend in Cyanobacteria abundance as
related to the total phosphorus concentrations in lakes. The percentage
Cyanobacteria composition of phytoplankton in Clear Lake, Iowa during 1998-2000
(by biomass) is shown as a star.
FIGURE 33. Secchi disk measurements and macrophyte abundance in Clear Lake from 1896 to 2000.
FIGURE 34.
Clear Lake bathymetric map, 2000.
FIGURE 35. Clear Lake bathymetric map, 1935.
FIGURE 36. Clear Lake bathymetric
map, original post-glaciation estimate.
FIGURE 37. Clear Lake sediment
deposition map, 1935-2000.
FIGURE 38. Clear Lake sediment
deposition map, original lake basin estimate-2000.
FIGURE 39. Little lake section of
Clear Lake sediment deposition map, original lake basin estimate-2000.
Figure 40. Changes in ambient total phosphorus concentration in Clear Lake predicted from the Canfield/Bachmann model for various levels of reduction in phosphorus input from the watershed. The upper (red) line is an approximately upper 70th percentile of predictions, while the lower (green) line is an approximately lower 70th percentile of predictions. The black line indicates the approximate expected phosphorus concentration in an average year, given a certain level of decrease in phosphorus input.
Figure 41. Changes in ambient water clarity in Clear Lake predicted from the Canfield/Bachmann model for various levels of reduction in phosphorus input from the watershed. The upper (red) line is an approximately upper 70th percentile of predictions, while the lower (green) line is an approximately lower 70th percentile of predictions. The black line indicates the approximate expected water clarity in an average year, given a certain level of decrease in phosphorus input. Water clarity is predicted for Clear Lake based on relationships between total phosphorus and water clarity observed in other Iowa Lakes (Bachmann et al. 1994).