Executive Summary – Diagnostic & Feasibility Study
John A. Downing, Jeff Kopaska et al.
Iowa
State University
The purpose of this 2-year project was
to study multiple aspects of Clear Lake,
its watershed and community to determine water-quality related problems and
their likely causes, and suggest a list of potential remedial measures. The project was funded primarily by the Iowa
Department of Natural Resources (IDNR), the City of Clear Lake, Cerro
Gordo County, Hancock
County and the Hancock and Cerro
Gordo County
offices of the Natural Resources Conservation Services (NRCS). Iowa State University (ISU) also provided
substantial cost-share. The first part
of this project, the diagnostic study, is a description of the principal
findings of the analysis of the lake, its watershed, and the social landscape.
The study was performed by a large
team of scientists and students with the help of dozens of citizen
volunteers. The work could not have been
done without the generous contribution of time and effort by hundreds of
willing citizens and the volunteer work of several ISU professors. Over the course of the project, hundreds of
thousands of meticulous measurements and analyses were made to study the lake,
the watershed and the importance of Clear
Lake to the regional social
structure.
Lake and Watershed
Characteristics
Clear
Lake is third largest of 34
natural, glacial lakes in Iowa,
and is managed for water-based recreation and fishing. It is shallow, with a maximum depth of 5.9
meters (19 ft) and an average depth of 2.9 meters (9.6 ft). Water is supplied to the lake by small
tributaries, rainfall, groundwater, and many areas of direct surface
runoff. Forty-seven percent of the water
supply flows in from a large wetland complex (Ventura Marsh). The lake’s watershed to lake-area ratio is
only 2.3:1 and the watershed is composed of 59% cropland, 10% urban areas, 9%
wetlands, 8% grasslands, 5% wooded lands, 5% roadways, 2% farmsteads, 1%
pasture and 1% State Parks.
Clear
Lake has a long history as a focal
point for recreation in Iowa, and
is currently intensively used. Use is
44% camping, picnicking and other passive uses, 28% pleasure boating, 19%
swimming, 7% fishing, 2% winter activities, and 0.2% hunting. Total use of the two State parks on the lake
(Clear Lake
State Park, McIntosh
Woods State Park)
totals more than 660,000 person-days per year and is growing
substantially. In addition to the State
Parks, the cities of Clear Lake
and Ventura maintain recreational
facilities on the lake. There are 24
public access points on the lake and 15 of these have public docks. Clear
Lake is currently managed by the
Iowa Department of Natural Resources for recreation and gamefish production.
Clear
Lake is intensively used both by
residents and by visitors from across Iowa
and the region. Clear
Lake is located in Cerro
Gordo and Hancock Counties,
which have combined populations of around 60,000. Much of the population of Cerro
Gordo County is
located quite near the lake (Mason City:
population=29,000), and this population has a higher than average income for
the State. Agriculture and related
industries are also important sources of income in this region (82%-91% of
total land area in these counties).
Economic activity associated with the lake is intense. Data on hotel/motel tax receipts indicate
that Clear Lake
has enjoyed an annual tourism impact of $30-$40 million annually for several
years. Estimates of willingness to pay
for lake water quality maintenance or improvement suggest that citizens value
the lake at between $20 and $40 million over a 5-year period, respectively. Much of the valuation of the lake is
expressed by local residents, although even visitors expressed a willingness to
personally support water quality maintenance and improvement with substantial
financial contributions. Hunting
revenues are quite small, but play an important social role.
Valuing Preservation and Improvements of Water Quality
One important indicator of social
importance of a resource such as Clear
Lake is the willingness that
citizens have to pay for maintenance or improvement of the resource. This is distinct from the amount that they
pay to use a resource (an amount that is very large indeed, and approaches the
GNP of the region associated with the resource), and indicates the unique
economic value of the resource. As part
of this study, economists performed surveys to provide information on recreational
usage of the lake, attitudes of recreators and local residents toward possible
watershed management changes, as well as estimates of visitors’ and residents’
willingness to pay for water quality improvements. Clear
Lake is very important as a recreational
resource, with visitors reporting high, persistent usage of the lake (an
average of over 6 trips per annum). Both visitors and residents report a high
willingness to pay to avoid further deterioration of the lake, about $100 for
visitors and about $550 for residents. When asked about their willingness to
pay for improvement from the current conditions, respondents indicate that they
are willing to pay only moderate amounts for a low quality improvement, but
substantial amounts for a more significant quality improvement, $215 and $600,
respectively for visitors and residents.
Attitudes and Perceptions Regarding Water Quality and
Community
Because substantial improvements to
lake water quality often require social change to take place, the beliefs, attitudes
and values of residents are an important part of planned landscape change in
communities. A critical goal of this
study was to propose feasible restoration alternatives toward the improvement
of water quality in Clear Lake. Therefore, part of this study summarized
interviews with residents reflecting a broad range of experiences and
connections with the lake and community, while balancing income, gender,
education, occupation and years lived in the area. Interviews were conducted individually using
photographs as discussion points.
Analyses focused on (1) residents’ relationship to the lake, (2)
organizational and social aspects of the community, (3) perceptions of water
quality, and (4) perceived community needs as they relate to water
quality.
Clear Lake is the focal point of
the community and the region. Visual
changes are watched in detail, including water clarity, water level, fish
populations, etc.
The lake has a strong personal importance to the community
but is also important as part of the community’s link with the external world
through tourism. Although residents
expressed concern about Iowa’s water quality in general, specific views about
Clear Lake are optimistic, although there is concern for the cost of
remediation. Residents expressed
interest in additional lake-centered facilities including a bike trail around
the lake, more public docks, additional boat trailer and vehicle parking, and
small pocket parks on the lakeshore. The
community is extremely well educated about water quality and communicates very
well within itself and without. There is
a large degree of tolerance for the visual aspects of water-quality enhancing
structures (e.g., wetlands, filter strips).
Considerations for future community action indirectly related to water
quality emerged from this part of the study: community sense of place,
interpretation of local history and perceptions of public/private ownership.
Historical Changes in the Waterscape
Dr. Ken Carlander, a well-known
emeritus scientist at Iowa State University and a long-time analyst of the
Clear Lake ecosystem, began to chronicle changes in the Clear Lake shoreline in
the early 1950s by establishing a photographic record. One part of this study was to repeat his
photographs from the same vantage points to see how Clear Lake has
changed. Comparisons show striking
changes in the extent and density of emergent and submergent rooted plants in
Clear Lake. Reduced water clarity has
resulted in a reduction in the extent and biodiversity of rooted plants. These plants are important as fish and
wildlife habitat as well as the stabilization of bottom sediments and
shorelines.
Limnology and Ecology of Clear Lake
Clear Lake is a formerly oligotrophic
to mesotrophic lake that has increased in total phosphorus concentration from
around 60 ppb in the early 1970s to around 190 ppb in 2000. Total phosphorus appears to be increasing in
Clear Lake at an average rate of about 4 ppb/year. At this rate, Clear Lake would move from a
eutrophic/hyper-eutrophic lake to a hyper-eutrophic lake, attaining 340 ppb by
2040. Total phosphorus has therefore
already tripled in the last 30 years and appears to be climbing under current
watershed management scenarios.
Concurrently, water clarity has been cut to nearly a third of what it
was in the early 1970s, and probably around 10% of the clarity the lake had
near the turn of the century. Water
clarity in 1974 was about 0.9 meters (nearly 3 ft.), but is now around 0.35
meters (about 1 ft.).
Clear Lake is typical of large,
shallow, corn-belt kettle lakes. Clear
Lake receives a very elevated rate of supply of nutrients (most notably
phosphorus) from its watershed, rainfall and groundwater, resulting in a volume-weighted
average spring phosphorus concentration of 186 ppb. Much of the watershed is at the western end
of the lake, thus nutrient supply and concentrations are higher in the west end
than the east end of the lake. The very
shallow depth (maximum 5.9 m or 19 ft) means that wind mixing returns 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 gives 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-, boat- and
wind-induced mixing of sediments, are significant impediments to water quality,
since they have now turned Clear Lake into a eutrophic to hyper-eutrophic
lake. The impact of these nutrient loads
is exacerbated by a greater-than-average concentration of suspended silt that
likely arises due to wind resuspending watershed-derived silt, carp and other
benthic fish digging in sediment deposits, and power-boat wakes disturbing
sediments in shallow waters. Further
exacerbating water quality problems is a declining population and biodiversity
of rooted water plants which formerly held bottom sediments and protected
shores and shallow waters from wave erosion.
Very high nutrient concentrations
in Clear Lake have fueled over-abundant growth of algae, resulting in green
water and frequent algae blooms.
Phytoplankton in Clear Lake follow a seasonal pattern that is typical of
temperate, shallow, hypereutrophic lakes.
Algal biomass is generally highest in mid-summer when it forms
conspicuous “blooms” of algae coloring water an intense green color. Cyanobacteria (“bluegreen algae”) and diatoms
make up the majority of the algae.
Cyanobacteria usually dominate the algae and make up >80% of the
algae in mid- to late-summer. As is
frequently the case for eutrophic lakes, the types of Cyanobacteria present in
blooms include some of the groups that can produce toxins under certain
conditions. These groups compose an
average of 35% of the algae. Because
such toxins could become harmful to invertebrates, fish, wildlife, livestock,
and humans, reduction of nutrient levels to eliminate Cyanobacterial dominance
would be welcome.
Declines in water quality have
reduced fish and wildlife habitat substantially in Clear Lake. The number of species of aquatic plant species
found in Clear Lake has declined from 35 species in 1952 to 21 species in 1981
to 12 species in 1999. More than 80% of
the species currently present in Clear Lake have declined significantly since
1981. Aquatic plants cover about half
the area of the lake that they covered in 1981.
Rooted aquatic plants are important wildlife and fish habitat and
stabilize bottom sediments and shore zones.
Bacteria were studied intensively
in Clear Lake
and were usually found at low levels, especially in the open waters away from
shore. Concentrations of fecal
coliforms, E. coli, and fecal enterococci were highest near shore and
showed patterns that should allow remedial measures to trace bacteria sources
and eliminate or reduce these inputs.
Bacteria were found to enter the lake around much of the shore. Since much of the shore is in residential
development, significant amounts of bacteria likely result from urban
activities. In spite of this, parkland
and agricultural lands also appear to contribute substantial bacterial
inputs. Concentrations of bacteria were
found to be highest during the warmth of mid-summer, especially following
rainfall events.
Sedimentation has resulted in
substantial changes in the bottom of Clear
Lake. During the first 10,000 years of its life, we
calculate that the lake filled-in about 38% of its original basin. Almost ¼ of this volume was filled-in since
1935. Agricultural, urban and
construction activities around the basin have reduced the average depth of the
lake by one foot since 1935. Around
85,000 tons of sediment are added to the lake each year causing the lake to
lose depth at a rate of about 4 mm/year.
Assuming a constant rate of sediment addition to the lake, Clear
Lake would be completely filled-in
in 700-800 years. Normally, however,
these processes usually accelerate as lakes become shallower, so this life-time
may be over-estimated.
Internal nutrient loading via the
resuspension of benthic sediments by wind-induced waves and recreational boat
traffic is a common problem facing the managers of shallow lakes like Clear
Lake. Benthic sediment resuspension may contribute
to the suppression of fish and macrophyte communities, domination of the
phytoplankton community by potentially toxic cyanobacteria, suspension of toxic
ammonia and increased restoration time-scales.
In Clear Lake,
resuspension by wind-induced waves and recreational boat traffic may contribute
to daily, often substantial, nutrient flux with total phosphorus concentrations
increasing by over 100% and ammonia concentrations reaching levels toxic to
fish. When wind speeds exceed 10 m·s-1
(22 mph), a large proportion of the lake’s sediments may become
mobile. Sediments in Clear
Lake are most susceptible to
turbulence by wind waves and boats in the lake’s shallow western basin and
around the lake’s margins. Here, it is
likely that violations of the lake’s no wake zones may exacerbate wind-induced
resuspension and may slow the resettlement of resuspended sediments. Additionally, the frequently observed
sediment plumes passing from the western basin into the larger basins to the
east suggest that prevailing currents may transport large loads of sediments
and nutrients throughout the lake.
Unless measures are taken to suppress the impacts of wind and boats on
the lake’s sediments, we may expect problems associated with sediment
resuspension, including increased restoration time-scales, may become more
severe as the lake’s depth continues to decline, exposing more sediments to
turbulence.
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. 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 more 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 the limnological aspects of Clear Lake, three fundamental changes
would need to take place:
- Reductions
in phosphorus loading to the lake.
- Reductions
in silt input and resuspension by fish, wind and boat action.
- Reductions
in inputs of bacteria from the watershed surrounding the lake.
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. One can thus calculate
the expected change in water quality (i.e., phosphorus concentration) by
calculating the impact of a reduction in phosphorus input. We examined the fit of more than a dozen such
models and found that current phosphorus concentration at spring circulation
could be predicted within 2% of the actual phosphorus concentration. This model is thus likely to predict the
phosphorus concentration under future remedial states. Application of these equations indicates that
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. 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
conservative 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.
Groundwater Hydrology
A potentially
important part of the nutrient budget of Clear
Lake
is groundwater inflow and outflow. An
understanding of the geology and hydrogeology of the Clear
Lake
region is thus needed to understand lake-groundwater interactions. The following objectives were investigated:
·
· determine
the thickness of Quaternary units underlying the lake and overlying the regional
bedrock aquifer;
·
· estimate
hydraulic heads in the regional aquifer and their relationship to the lake
elevation and shallow groundwater flow;
·
· determine
the nature and types geologic units affecting flow to and from the lake.
Estimation of
groundwater discharge (or seepage) to lakes is necessary to determine nutrient
load, but it is difficult task and generally involves the extrapolation of
small-scale measurements to a much larger lake area. In cases where the geology beneath the lake
is not well known and where discharge may vary, large errors are involved in
the measurement and extrapolation steps.
A geochemical
investigation of groundwater was undertaken in order to understand the presence
and absence of nutrients and contaminants in groundwater and their potential to
enter Clear
Lake. Groundwater samples from the 32 out of 33
piezometers were analyzed for total P, total N, Si, alkalinity, electrical
conductivity and pH. Additional parameters (major cations and anions, trace elements,
dissolved O2, dissolved organic carbon) were measured in order to
understand the geochemical environment in which the nutrients occur. Geochemical speciation models and soil P
measurements were used to determine potential sources of P. Selected samples were analyzed for fecal
coliform bacteria and caffeine, in order to determine potential sources of
nutrients and Cl. A radioactive isotope
of hydrogen, tritium (3H), was used to determine the relative age of
the groundwater. Nutrient and contaminant
loads from groundwater to Clear
Lake
were calculated from estimates of groundwater inflow and outflow and estimates
of the concentrations of nutrients (primarily P, N and Si) and Cl in
groundwater. Nutrient load per time was calculated by multiplying discharge (L3/T)
times concentration (M/L3).
Because of Clear Lake’s
nature as a flow-through lake, nutrients will be added to the lake in areas of
inflow and lost from the lake in areas of outflow.
Ventura
Marsh Biology, Ecology and Biomanipulation
Early in the study, we found that
Ventura Marsh (a large wetland that processes 49% of the water budget) was not
removing nutrients from the water but was a significant source of
nutrients. Experience in other shallow water
bodies in Iowa and elsewhere
indicated that this was due to impact of non-native fish (carp) on the
sediments and vegetation, creating a nutrient supply rather than a sink. We
therefore evaluated the effects of a benthivorous fish reduction. After a substantial fish removal was
obtained, water clarity increased as a result of decreased suspended sediment
and phytoplankton biomass. Water column total phosphorus declined by about 25%
from 147 ppb to 115 ppb. Prior to the
clear water phase, phytoplankton was phosphorus limited. Zooplankton grazing reduced phytoplankton
biomass during the clear-water phase.
The biomass of Daphnia and Ceriodaphnia increased
following fish removal. During this
period, grazing pressure was high and standing phytoplankton biomass remained
low. Phytoplankton appeared to be
regulated by top-down control for approximately two months before reverting
back to bottom-up control. Changes in
water quality due to wind and/or return of juvenile carp may account for the
switch back to bottom up control.
Macrophyte diversity and density increased after the initiation of the
clear water phase. We therefore
concluded that restoration of Ventura Marsh and carp control could be one
potentially viable remedial measure for decreasing nutrient flux into Clear Lake.
Watershed Loads,
Tributaries and Nutrient Budgets
Various tributaries to Clear
Lake were sampled to identify areas
contributing greatest nutrient loads. A
total of 37 sampling stations were established across the watershed. Inputs of various elements were calculated
multiplying concentrations by the water (hydraulic) loading rate at each
point. The study spanned one very wet
year and one very dry year. The amount
of precipitation has a large impact on the overall nutrient loading rate as
well as the distribution of the nutrient loads among the many potential
sources.
Much of the watershed lies in the agricultural region to the west of the
lake, thus much of the phosphorus entering the lake comes from agricultural
lands. The average phosphorus budget for
the lake indicates that 43% derives from the agricultural watershed, 7% from
groundwater, 6% from the City of Clear Lake, 2% from the City of Ventura, and 2% from unconsolidated county urban
lands. An average of 31% of the
phosphorus budget derives from direct rainfall on the lake, since Iowa’s rainfall phosphorus has been enriched
10-fold with airborne phosphorus over the past 30 years. The large amount of phosphorus deriving from
direct rainfall was somewhat surprising and ironically makes remediation more difficult
because of the small watershed to lake area ratio. Another surprise is the high phosphorus
concentration of groundwater.
Groundwater concentrations were quite high, suggesting that phosphorus
has moved down through the soil profile enriching the groundwater. Another surprising result was that 9% of the
lake’s overall phosphorus budget derives from internal loading from Ventura
Marsh. Ventura Marsh provides much of the
water flowing into Clear Lake and concentrations of major nutrients in
this water are very high (average 350-400 ppb of phosphorus). Because of carp activities and poor aquatic
plant development in the marsh, however, somewhat more phosphorus leaves the
marsh than enters, indicating that the marsh is a net source of sediments and
does not cleanse water as large wetlands usually do. Nutrient loading to the lake should generally
be much higher in wet years than dry ones, and the fraction of the nutrient
budget derived from rain and groundwater declines substantially under wet conditions.
Although the predominance of agricultural lands in the watershed makes
them a major overall nutrient source, nutrient losses per unit land area
indicate areas where nutrient losses are most severe. In general, phosphorus losses were somewhat
higher (per unit area) from urban lands than agricultural lands. This indicates that substantial reductions in
phosphorus input could be achieved by both urban and agricultural communities. Sediment losses from lands varied markedly,
indicating broad differences in land use management. Sediments tend to cause water quality
impairment on their own, but also carry large amounts of phosphorus with
them. Not surprisingly, agricultural
areas supply the largest amounts of nitrogen per unit area, likely owing to the
prevalent use of pure N fertilizer in Iowa agriculture. Nitrogen is not a large problem for Clear Lake, since phosphorus is generally the
production-limiting element.
Because of the importance of
agriculture in the watershed, extensive analyses of soil phosphorus and
management practices were performed.
Eighty-nine percent of the agricultural watershed was planted in corn
and soybean rotation, while 7% was planted in continuous corn and the remaining
land was under CRP, alfalfa or pasture.
Forty-eight percent of the land was managed with chisel plowing in
combination with disking and/or field cultivation. Twenty-one percent of the remaining land was
in no-till, 18% in ridge-till, 7% moldboard plow and 6% was V-ripped. Forty-four percent of the fertilizer
application is incorporated by fall plowing, disking or injecting, which is the
environmentally preferred method. On
average, P fertilizer was applied to fields 2.2 times over the last 5 years. P fertilizer was typically applied at 65 lb P2O5/acre
which is a rate that is lower than the recommendation of Iowa
State University. Eleven percent of the farmland had received
manure in the previous five years.
Manured fields in the watershed were very similar in P concentration to
those receiving high and/or frequent applications of inorganic fertilizer.
Soil phosphorus was tested using 3
commonly used agronomic soil P tests and two environmental P tests. Although these methods measured different
amounts of P, most yielded similar overall results concerning identifying
high-testing areas. The survey of soil P
status and P management practices of the Clear
Lake agricultural watershed was
useful to identify areas that may be sources of large P loads to the lake and
to identify priority areas where changes in P management practices would be
desirable. Approximately one third of
the area of the watershed had soil-test P values that were twice to five times
higher than levels needed to maximize crop production. Soil test summaries since the 1960s for the two
counties surrounding the lake and our survey data from the lake watershed shows
that P management practices have markedly increased soil P tests over
time. The highest soil-test P values
were found in a very small number of fields that received either P fertilizer
or manure, which likely are the source of the major proportion of the P being
transported to the lake.
Analysis of
Clear Lake Fisheries
The fisheries of Clear Lake are an important component of recreation as
well as an important indicator of water quality. Because of their importance, fish have been
studied in Clear Lake since the early 1940s. The most striking change in the fishery has
been the near disappearance of the sunfish family (bluegill, crappie &
largemouth bass). These fish are still
present but are only occasionally caught.
The loss of these important fish is probably due to water-quality
mediated declines in aquatic vegetation which are necessary as spawning and
nursery cover. Although bullhead and
carp have been common in the lake for 50 years, they are now the dominant fish,
existing at densities of 150-300 lb/acre and 100-200 lb/acre,
respectively. They have probably filled
the void left by the bass, crappie and bluegill because of their great
tolerance of degraded water quality conditions (e.g., sediment, Cyanobacteria,
low oxygen, ammonia). Since resistant
fish like carp degrade water quality, successful improvements will need to
include management of fish populations to reduce carp dominance. This can be accomplished by (1) improving
water quality to enhance vegetation and decrease substances degrading fish
habitat, and (2) managing bottom-feeding fish (primarily carp and bullhead) to
reduce their abundance.
Point Sources
and Potential Pollutants
Part of the diagnostic study examined the point-source input of
materials to Clear Lake and the potential for toxic substances to
be concentrated in lake sediments. The
latter was done in case sediment dredging should be employed as a restoration
option. There are currently no permitted
point-source dischargers of effluents into Clear Lake. On
three occasions over the past five years, however, past exigencies have
resulted in the discharge of some pre-treatment sewage effluent into the
lake. The largest of these was the
discharge of 250,000 gallons of diluted pre-treatment sewage into the lake on June
20, 1998. Total P in this sewage was probably about 1.5
mg/L, meaning that sewage bypasses such as this, although certainly to be
avoided, only would supply about 0.02% of the lake’s phosphorus budget. This is surprising, but the sheer magnitude
of watershed inputs and rainfall are of massive proportions. Sediments contain some materials of concern,
notably cadmium, chromium, copper, lead and zinc. Potential sources of these elements are
batteries not properly disposed of, leaching from chrome-plated metals, trace
sources in agricultural fertilizers, building materials, leaded fuels, lead
shot, fishing weights, and leaching from plated steel.
Summary of Diagnostic Study
The diagnostic portion of this study shows that Clear Lake has water quality problems, due to historic
and present phosphorus and sediment loading, internal resuspension of sediment
and nutrients, and inputs of fecal-derived bacteria. These problems derive from the agricultural
and urban watersheds and from the lake bottom.
Deep lakes (i.e. >13 ft. (4 m) average depth) generally have better
water clarity, lower densities of algae, lower concentrations of suspended
particles in the water, and are more likely to lack winter fishkills or other
oxygen depletion problems. Shallow lakes
like Clear Lake (mean depth=9.6 ft (2.9 m)) have less volume for the dilution of nutrient and sediment
inputs. Accumulated sediments also
decompose and resuspend and can exacerbate oxygen and nutrient problems. Further, even these nutrient-rich sediments
derive from watershed impacts since sedimentation rates increase sharply when
eutrophication and deposition of eroded material lead to increased plankton production
and carbon-rich detritus.
Anthropogenically eutrophied lakes like Clear Lake suffer many
undesirable ecological characteristics (Table 9, Chapter 5 of the Diagnostic
Report) most of which can be remediated through better watershed management.
Sediment from watershed runoff has
had a major impact on this lake over its lifetime. Sediment flux has reduced the volume of Clear Lake to 38% of its original post-glacial volume,
and nearly 25% of that sediment was deposited since 1935. The rate of sediment deposition in the lake
may have been reduced in the last few decades due to improved erosion
management. Sediment deposition still
occurs, however, so Clear Lake is becoming shallower and smaller with the
passing years.
Runoff from the watershed
contributes bacteria, nutrients, and turbidity to the water and leads to algal
blooms, reduced transparency, and great concentrations of suspended
solids. In the long term, sediments
accumulate in the lake basin and cause water quality problems that are common
to shallow lakes. Eventually, lake
basins can fill to the point that they are no longer useful for
recreation. Nutrients in the excess
quantities found in Clear Lake impair many aspects of water quality.
Feasibility of Lake Restoration
Clear
Lake is an excellent lake of
outstanding potential for recreation and is of very great economic
importance. Improvement of water quality
could benefit from the activities of many at all levels, including citizens and
municipal, county, state and federal government agencies. The
following restoration alternative suggestions are designed to reverse the
eutrophication and sedimentation processes by improving the nutrient retention
of the watershed and by deepening parts of the lake. Preventative measures in the watershed are
necessary to slow the input of new nutrients and sediments into the lake, so
that the restored lake can have an enhanced lifetime and improved water
quality.
Principle restoration measures
suggested are:
- reduction
in phosphorus inputs to the lake,
- reduction
in bacteria inputs to the lake,
- improved
management of bottom sediments, siltation, erosion, and fish populations
to reduce turbidity and nutrients due to sediment.
We project that
phosphorus loading to the lake can be reduced by 50-60% by implementing
practices designed to address these issues.
This will lead to a substantial increase in water clarity and improved
biological function.
Improving watershed management and increasing the depth of certain parts
of the lake by dredging are two important aspects of this lake restoration
plan. These management approaches
compliment each other; dredging helps to restore the basin by increasing water
depths and watershed restoration helps improve water quality and decelerate the
rate at which the lake will degrade in the future. We suggest that both approaches would be most
effective if adopted together; for it will do little good to remove the
sediments from the lake if soil erosion and nutrient loads rapidly return
sediment and phosphorus to the lake, while watershed restoration would only
restore the water quality of the supply to a lake of short life and dubious
aquatic potential.
Watershed management activities
identified that would benefit Clear Lake include: land conservation by planting
permanent vegetation, pond and wetland installation, Ventura Marsh renovations,
water control structure renovations, dredging, fish barrier construction, and
post-restoration lake monitoring. The total
estimated cost for these activities is $15,555,300. Respondents to the Clear
Lake survey indicated a willingness
to pay of $19.5 million to avoid the deterioration of Clear
Lake. Alternatively, respondents are willing to pay
about $40 million for quality improvements at the lake. These numbers represent the value to visitors
and residents of water quality improvements.
In considering whether investments to clean up the lake are worth the
costs, these value estimates provide the appropriate baseline for
comparison. These large values associated
with water quality improvements at the lake are consistent with the lack of
good substitutes and the potential quality of this unique resource.
If
no restoration options are employed, the lake will become more and more
eutrophic, and thus become a less attractive recreation resource. Additionally, in time ,the lake will fill to
the point that its value as a recreational lake declines dramatically. Long before that time, however, severe water
quality problems will be encountered.
The combination of watershed improvements and lake dredging will enhance
the recreational value of the lake and greatly prolong its useful life. Watershed restoration and lake deepening will
act to reduce nutrient inputs and dilute their effect in the lake, however, the
lake is still likely to have some water quality problems due to algae blooms
and low transparency. This is due to the
limits on restoration imposed by phosphorus-rich precipitation and groundwater.
How long will it take? It should be noted that the time-course of
response to nutrient abatement is likely to be quite long. Researchers have analyzed a number of cases in which external
nutrient loads were reduced substantially.
They found that short-term (<5 years) improvements were only noted in
about half the lakes and that most lakes take more than 5 years for changes to
begin to be detected. In shallow lakes,
the problem is exacerbated by internal loading, so changes may be very slow. Lakes may improve over a decade or two before
the equilibrium level is approached. The
time-course of restoration should therefore be expected to be 5-30 years
depending upon the speed and degree to which restoration activities are
undertaken.