Grantee Research Project Results
Final Report: Lake Access: Making Water Quality Data Real and Relevantfor Minnesotans
EPA Grant Number: R827179Title: Lake Access: Making Water Quality Data Real and Relevantfor Minnesotans
Investigators: Barten, John
Institution: Suburban Hennepin Regional Park District, MN
EPA Project Officer: Packard, Benjamin H
Project Period: November 1, 1998 through October 31, 2000
Project Amount: $424,999
RFA: Environmental Monitoring for Public Access and Community Tracking (EMPACT) (1998) RFA Text | Recipients Lists
Research Category: Water , Sustainable and Healthy Communities , Air
Objective:
This research project was a cooperative effort by Hennepin Parks, Natural Resources Research Institute (NRRI), the University of Minnesota-Duluth, Minnesota Sea Grant, and the Minnehaha Creek Watershed District. The main objective of this research project was to provide the public with near real-time water quality data from urban lakes. Remote Underwater Sampling Stations (RUSS) units were used to monitor changes in water quality at three locations west of Minneapolis, MN. Hennepin Parks and the NRRI deployed RUSS units as an initiative for the EMPACT grant.
Summary/Accomplishments (Outputs/Outcomes):
Each RUSS unit was programmed to automatically collect water quality data (temperature, dissolved oxygen, pH, conductivity, and turbidity) at 1-meter depth intervals every 6 hours. The RUSS units then transmitted collected data to a base station computer via cellular telephone modem. All transmitted data were reviewed and uploaded to the Lake Access Web Site (http://www.lakeaccess.org/ ) as near real-time data. RUSS data also were accessible through a sister Web site, Water on the Web (http://wow.nrri.umn.edu ). The Web sites were designed to enable the general public and professional educators to view and interpret the copious amounts of data and to inform Web site visitors of the important management issues concerning area lakes.
The RUSS units initially required extensive maintenance to ensure the efficient operation for long-term monitoring. Primary complications during deployment included anchor drifting, buoy detachment, profiler malfunction, complications with the programming of the on-board computer or Remote Programming, Data Acquisition, and Retrieval (RePDAR) units, power supply interruptions, cellular modem connection problems, and fouling of the multiprobe. Collectively, these problems resulted in substantial downtime during the 2 study years, but the frequency and duration of these downtimes were greatly reduced after the initial few months of RUSS deployment. By the second year, the majority of these problems had been successfully resolved.
The RUSS units allowed the cooperating Lake Access partners and Web site visitors to see daily changes in lake conditions, and led to new insights on internal phosphorus loading dynamics in Halsteds Bay. The data collected will be invaluable in developing more informed management plans for the lakes studied in the coming years. In addition, the usership of the Web sites steadily increased during the 2 years since the project began. This increase in usership indicated the potential for raising awareness of local water quality issues by using RUSS technology in concert with the Internet.
The U.S. Environmental Protection Agency document “Delivering Timely Water Quality Information to Your Community: The Lake Access-Minneapolis Project” (EPA/625/R-00/013) provides a full description of the Lake Access Project.
RUSS Deployment. Three RUSS units were deployed on Minneapolis area lakes in 1999 and 2000 as a part of the Lake Access project. Floating RUSS buoy systems were anchored at selected sites during open-water periods and were housed in small wooden sheds placed on the ice at the same locations during the period of ice cover (see Table 1).
Deployment Period | Independence | Halsteds | West Upper |
1999 open water | 4/22-11/4 | 4/21-11/11 | 4/21-11/11 |
1999-2000 ice cover | 1/13-2/25 | 1/13-2/25 | 1/13-2/25 |
2000 open water | 3/23-11/9 | 3/22-11/12 | 3/22-11/12 |
Site Descriptions. The three RUSS units were deployed in typical urban-area Minnesota lakes, each offering different hydrology, basin morphometry, and water quality history (see Tables 2 and 3). Two RUSS units were deployed on Lake Minnetonka, in Halsteds Bay and West Upper Bay, and a third unit was deployed on Lake Independence. All three of these sites were located approximately 15 miles west of Minneapolis, MN.
Parameter | Independence | Halsteds | West Upper |
Basin Surface Area | 819 ac. | 544 ac. | 879 ac. |
Watershed Area | 7,720 ac. | 6,000 ac. | NA |
Watershed Land-use | Agric. | Agric./Resid. | Resid./Agric. |
Volume | 14,742 ac. ft. | 7,100 ac. ft. | 27,249 ac. ft. |
Depth | 57 ft | 36 ft | 84 ft |
Mean Depth | 18 ft | 13 ft | 31 ft |
Lake Minnetonka: West Upper Bay and Halsteds Bay. Lake Minnetonka consists of a series of connected lake basins of glacial origin (see Figure 1). These basins, or bays, are joined by navigable channels, but the morphometry and hydrology of each bay is generally quite different from adjacent ones.
Figure 1. Map of Lake Minnetonka Showing Depth Contours and the Location of the Deployed RUSS Units (lower left)
These between-basin differences have resulted in significantly different water quality conditions. West Upper Bay and Halsteds Bay were chosen as RUSS sites to emphasize these differences and demonstrate the importance of morphometry, hydrology, nutrient loading, and stratification patterns in dictating water quality.
West Upper Bay has consistently exhibited good water quality (see Table 3). It has generally developed stable stratification by late spring or early summer, and has remained stratified until turnover in early fall (dimictic). Land use in the surrounding watershed has primarily been residential. At the time of this project's completion, there were about 90 homes along its shoreline, all of which were connected to city sewer lines.
Halsteds Bay offers a stark contrast to West Upper Bay. Halsteds is a shallow eutrophic basin that consistently exhibits the poorest water quality in Lake Minnetonka (see Table 3). The bay received raw and partially treated sewage inputs for many years until sewage discharge into the lake was stopped in 1969. Historically, Halsteds has stratified in the late spring or early summer, but has been observed to periodically mix during strong wind events or extended cool periods. This periodic mixing likely has contributed to the poor water quality of this lake, because phosphorus that is released from the sediments when the lake is stratified is made available to algae when the lake mixes.
Lake Independence. Lake Independence is a glacial pothole lake in a predominantly agricultural watershed (see Figure 2). As with many of the lakes in southern Minnesota, it has experienced a decline in water quality due to human activities in its watershed. In recent years, it has experienced frequent summer algal blooms and generally poor water quality (see Table 3). It was chosen as a RUSS site to offer a closer look at the problems that typically lead to degraded water quality in area lakes and to collect data that will help in making more informed management decisions in the future.
Figure 2. Map of Lake Independence Showing Depth Contours and RUSS Site Location
Table 3. Summary of Mean Growing Season Water Quality for Each of the Three RUSS Sites From 1995-2000. Chlorophyll-a, total phosphorus, and total nitrogen values represent surface (0-2 m) concentrations.
Site | Secchi m | Chl-a µg/L | TP µg/L | TN mg/L | TSI score* |
Halsteds Bay | 1.48 | 46 | 116 | 1.97 | 65 |
West Upper Bay | 2.32 | 6 | 27 | 0.93 | 49 |
Lake Independence | 1.45 | 31 | 73 | 1.61 | 62 |
*TSI scores were calculated using Carlson’s Trophic State Index (Carlson, 1977). |
Equipment and Methods. Due to the experimental nature of the RUSS systems, the equipment and methods used to deploy each unit were, to some extent, determined by trial and error. Several alterations to the original setup were required to correct problems that occurred with the anchoring system and buoy attachments during the first year of use. The following section details the original equipment and methods used to deploy the units, the problems that were encountered, and the changes that were made to resolve the problems.
1999 Deployment. The RUSS units were deployed according to the manufacturer’s recommendations in 1999. Anchor lines were made from small-diameter metal cables cut to lengths that were 3 to 4 times the depth at each given site, so that when deployed, they would have a 3:1 aspect ratio. Each anchor line was attached to the RUSS unit by threading the cable line through a metal eyelet on the RUSS arm and using cable clamps to secure a loop. Light aluminum anchors designed for use in areas with firm muck or sand sediments were attached to the terminal end of each line. Three cylindrical buoys, labeled “research area,” were anchored around each RUSS unit to enclose a buffer area. These buoys were anchored individually with metal cable lines and large concrete anchors. To discourage tampering, swimming pool floats were strung between the buoys to enclose the entire buffer area (see Figure 3).
Figure 3. Diagram of the Original RUSS Unit Setup Used in 1999, Showing Relative Anchor Line Positions and Buoy Placement
Several equipment problems were encountered shortly after the RUSS units were deployed in 1999: (1) the anchors did not hold the RUSS in position during windy periods; (2) the line of swimming pool floats periodically detached from the buoys and entangled in the anchor lines or RUSS profiler; (3) the metal cable anchor lines cut through the aluminum attachment points on the RUSS and several of the lines broke or became detached from the anchors during the year; and (4) bird wastes accumulated on the solar panels, which led to power failures.
To resolve these problems, the light aluminum anchors were replaced with heavier concrete anchors, the lines of swimming pool floats were removed, and coils of wire were placed onto the solar panels to deter bird roosting. The broken and detached anchor lines were replaced or reattached, but no changes were made to the overall design of the anchor lines in 1999.
2000 Deployment. In response to the problems experienced in 1999, several changes were made to the setup of each RUSS unit. These included: (1) changes to the anchor lines to prevent drifting and to simplify deployment and maintenance; (2) a simplified buoy system; and (3) an improved bird-deterrent system.
Anchor Lines and Buoy System Improvements. The new anchor lines were set up as in Figure 4 below. The marker buoys were directly attached to the anchor lines and were held in position by adding an additional small weight to the line between the buoy and anchor. Small swimming pool-type floats were positioned and secured along the sections of anchor line between the RUSS unit and the buoys to prevent the anchor line from submerging and entangling the profiler when collecting surface measurements.
Figure 4. Diagram of RUSS Anchoring and Buoy System (not to scale). Three such lines were used to anchor each RUSS unit.
To ensure that the anchors would hold, the anchor line length (scope) was increased from that as used in 1999 such that the aspect ratio (distance/depth) of the deployed line was greater than 5:1. The anchoring lines were made with 3/8-inch floating nylon rope. Loops were tied along the line to create points for attachment. The position of each attachment loop was determined based on required buoy distance, site depth, and sediment composition (see Table 4).
Table 4. Attachment Loop Positions on Anchor Lines, Given as Linear Distance From the RUSS
Attachment Loop | Linear Distance |
Terminal Loop (surface end) | 0 (attaches to the RUSS) |
Buoy Attachment | 10 m |
Mid-line Weight Attachment | 10 m + (0.75 x depth) |
Terminal Loop (anchor end) | 5-7 x depth * |
* Longer lines were used at sites with large fetches or hard sediments. |
These loops were doubled up (tied on a bight), and reinforced with sections of vinyl tubing. Loose ends were wrapped with duct tape to reduce the wear on the loops and minimize the chance of line failure.
Each piece of equipment was attached to the appropriate loop on the anchor line using a large metal carabiner with a threaded sleeve closure. Each marker-buoy attachment also was equipped with a swivel device to prevent twisting of the anchor line.
Anchors and Weights. Anchors were made by filling 5-gallon pails half full with concrete mix and inserting large threaded eye bolts (bent slightly to prevent slipping) before the concrete solidified. After curing, the anchor was removed from the plastic pail. Anchors of this size and weight proved to be very effective at holding the RUSS units stationary, and yet were relatively easy to deploy, reposition, and remove.
Small weights (8-12 lbs) were made by filling 1-ft long sections of 4-inch diameter PVC pipe with cement mix, and inserting large threaded eye bolts (bent slightly to prevent slipping) before the cement mix solidified. These weights were attached to the loop between the buoy and the terminal anchor on each line. When deployed, these weights were suspended at about one half of the total water depth. This ensured that the line outside of the marker buoys was held deep enough to avoid being cut by passing motorboats and resulted in constant tension on the lines, thus maintaining proper RUSS and buoy position. To ensure proper line tension, the small weights were attached to the loops only after all three anchor lines were in place.
Bird-Deterrent System. The new bird-deterrent system was designed to be more effective and easier to handle during maintenance activities (see Figure 5). The new deterrents consisted of sections of plastic coated wire fence (2 x 4-inch grid) placed over the three arms of each RUSS unit. Every other crosspiece in the 2 x 4-inch grid was cut and bent upward to form a "bed of nails." Each panel was attached to the distal end of each RUSS arm such that they could be hinged off for easy access to the solar panels and electronic connections. The system was most effective when each deterrent panel was suspended at least 4 inches above the deck of the RUSS so the sides of each panel were bent down to provide rigid supports that rested on the deck. These deterrent panels proved to be very effective at preventing bird roosting on the RUSS units.
Figure 5. Picture of a RUSS Unit Equipped With the New Bird-Deterrent System
Collectively, these changes to the anchoring system, solar panels, and buoy configuration proved to be very effective in resolving the problems encountered in 1999 and the amount of downtime was greatly reduced.
RUSS Operation. Each RUSS unit consisted of the following components: (1) three-armed floating raft (triple hull); (2) an on-board computer (RePDAR unit); (3) profiler device; (4) multisensor probe; (5) solar panels and a marine battery; and (6) wireless communication (cellular modem).
Figure 6. Schematic of RUSS Unit Components
The RePDAR unit initiated automatic collection of water quality data at 1-meter depth intervals every 6 hours. A multisensor probe water quality sonde (YSI 6820) was attached to the profiling device (leveler) that moved the sonde vertically within the water column. The sonde measured temperature, dissolved oxygen, pH, conductivity, and turbidity. The RUSS units logged data collected by the sondes and then periodically transmitted all data to a base station computer via cellular telephone modem. The transmitted data then were reviewed and uploaded to the Lake Access Web Site (http://www.lakeaccess.org/ ) as near real-time data for public access.
Operational Problems and Solutions. Initially, the RUSS units required extensive maintenance to ensure efficient operation for long-term monitoring. In addition to the problems discussed in the previous section (RUSS Deployment), other issues such as profiler malfunction, complications with the programming of the RePDAR units, power supply interruptions, cellular modem connection problems, and fouling of the multiprobe resulted in downtime. As with the deployment problems, by the second year, the majority of these operational issues had been successfully resolved or reduced.
The profiler unit periodically experienced some malfunctions related to buoyancy. If unable to stay positively buoyant, the profiler and sonde settled into the soft sediments and required manual retrieval for repairs. At other times, the profiler was unable to submerge, either due to entanglement with buoy lines or improper weighting and purging of the ballast tank. Under either of these conditions, the profiler was unable to move vertically through the water column and data were not collected. These problems were resolved through alterations to the buoy line configuration and by employing a regimented schedule of ballast tank purging.
Some problems were encountered with RePDAR operation. Periodically, one of the RePDAR units would "lock up" and would need to be manually reset. The source of this malfunction was not determined, but was believed to be the result of programming error, power supply malfunction, or some environmental condition such as moisture or heat. These issues were resolved on an individual basis and required the assistance of Apprise Technologies staff.
The power supply to the RUSS units was periodically disrupted, thus preventing the scheduled data collection. This usually occurred as the result of bird droppings blocking light to the solar panels or from long cloudy periods coinciding with frequent profiler operation (high-power demand). These issues were resolved through the installation of the bird-deterrent system detailed in the previous section and through careful monitoring of battery voltage. On a few occasions, it was necessary to install a fresh battery.
The signal strength of the modem occasionally was insufficient to maintain a reliable connection for data transmission and remote programming. Using higher quality cellular antennae at the sites eventually reduced the frequency of signal loss, but some connection problems still occurred.
The probes on the sonde experienced rapid fouling and required frequent maintenance. It was anticipated that the sonde would only need to be calibrated monthly, but quality assurance data indicated that the water quality sonde required more frequent calibration to ensure data accuracy. Additional evaluation indicated that the decay in accurate dissolved oxygen measurement was attributable to the depth at which the sonde was suspended within the water column. When the sonde was not collecting data, each leveler device was parked (suspended) at a depth of 5 meters. During the summer, the lakes typically stratified and became hypoxic at 5 meters. Hydrogen sulfide concentrations within the hypoxic region (hypolimnion) of the lakes eventually reached levels sufficient to react with the potassium chloride solution in the dissolved oxygen probe and resulted in significant error for dissolved oxygen measurement. Consequently, the profiler parking depth was changed to ensure that the probes were out of the hypolimnion. The actual parking depth was dependent on lake stratification patterns in each given basin. This resulted in more stable sonde performance and reduced the amount of time spent on probe maintenance and calibration. The turbidity probes on the water quality sondes also were frequently fouled by a biofilm believed to consist of periphyton, bacteria, and aquatic insect eggs. Consequently, the optical surfaces had to be cleaned frequently to ensure the collection of accurate turbidity measurements.
Results. The RUSS units allowed the cooperating Lake Access partners and the Web site visitors to see daily changes in lake conditions and led to new insights on internal phosphorus loading dynamics in Halsteds Bay. The data collected will be invaluable in developing more informed management plans for the lakes studied in the coming years. From an educational standpoint, the continuous and real-time nature of the data collected offered a unique opportunity for students, educators, and the general public to witness the dynamic nature of northern lakes. The following sections detail the most notable observations and insights.
Lake Access Outreach. The objective of Lake Access was to increase public understanding of factors affecting water quality for informed decisionmaking about land use in a metropolitan region in Minnesota. Members of our Lake Access outreach team developed and implemented various methods to assure that our timely and accurate water quality information would be understandable, relevant, and accessible to the general public and decisionmakers. We solicited a variety of sources to gather input on how to best present our information and promote the opportunity for people to access real-time data by conducting a survey and convening a focus group. These results assisted us in reaching our target audiences through the marketing of our Lake Access Web Site at meetings, festivals, and through announcements in the print media.
The 20-question survey of 450 randomly selected Hennepin County residents in 23 select cities in our target area was conducted from November 1999-January 2000. The majority of survey respondents were male (64 percent) and between the ages of 25 and 65 (81 percent). We also found that 61 percent of our survey respondents use lakes more than 5 days per year, while 95 percent were somewhat to very concerned about the quality of lakes and shoreland areas in the target area. This knowledge about our audience guided the development of information on our Lake Access Web Site to be both engaging and relevant to this group of citizens.
We also found that although the top factor survey participants perceived to impact water quality of the lake they use most was lawn fertilizers and chemicals, 85 percent of those that care for a lawn have never had their soil tested, and 26 percent fertilize their lawns 3-4 times per year. These results have demonstrated a gap between the knowledge and behavior of survey respondents performing lawn care practices. We used these responses to support the new directional focus our group has taken in our newly funded Lake Access II proposal.
The survey served a third purpose as we prioritized the methods needed to make our information accessible to our target audiences. Survey results indicated that 52 percent of respondents would use the Internet to learn more about the lakes in their region, while 36 percent would use an interactive kiosk. The Internet was the most popular choice (31 percent) when asked for the most convenient way for them to access in-depth news and information about their lakes, while the major city newspaper (24 percent) would be the best way for them to notice a brief announcement about their lakes. This outcome reinforced our work in presenting information in these two formats, while reminding us that future efforts should expand on these methods if we want to reach a broader audience.
Our Lake Access team also convened a focus group on February 29, 2000, comprised of 14 educators and naturalists from Hennepin Parks and other organizations in the Minneapolis/St. Paul metropolitan area. This group guided our Web site development by helping us identify the content and methods that would most effectively engage our audience. Their feedback was used to reevaluate our goals and objectives as well as reorganize the Web site to include a more appealing and user-friendly interface. These educators also described topics that would most interest our target audience and common misinformation issues that we could address on our Web site. The educators were very enthusiastic about promoting Lake Access in their educational programming with families and lakes users.
We promoted Lake Access at meetings and festivals by distributing specially designed brochures and magnets that advertise the Web site. Further promotion of the Web site was achieved through printed materials that introduced the new site at special events. These activities likely played a major role in the observed increase in user traffic on the Lake Access Web Site from May 2000–April 2001 (see Figure 7).
Figure 7. Graph of Web Site Activity at www.lakeaccess.org . The vertical axis is given in terms of number of "hits" or Web Site visits per week.
Limnological Observations and Insights. The RUSS units facilitated the collection of large amounts of lake data that provided a high-resolution view of many interesting limnological processes. Although most of these processes have been discussed in limnological texts, the Lake Access project offered a great resource for educators, students, and citizens to see these occurrences in area lakes as they occur.
Development of Lake Stratification. Natural lakes located within temperate climates stratify as water temperature increases after ice out. The water's physical properties and the lake’s morphology often determine the onset of stratification. Water is most dense at 4°C and becomes less dense at higher and lower temperatures. In the spring, surface water absorbing heat from the increased air temperatures becomes less dense than the deeper, cooler water. Eventually, the large differences in density at higher temperatures result in distinct layers of warmer and cooler water. During the onset of stratification, these layers can become mixed due to wind action. Deeper lakes may not mix thoroughly from top to bottom because too much energy is required to mix the upper layers with the lower layers. Consequently, deeper lakes will tend to stratify earlier than shallower lakes because of a larger density differential due to temperature; the lower layers are less vulnerable to wind mixing with the upper layers. The deepest basin monitored with RUSS technology was West Upper, Lake Minnetonka. According to the 1999 and 2000 RUSS data, stratification developed at West Upper Bay in early May (see Figures 8 and 9). Lake Independence, a moderately deep basin, also stratified in early May. However, Halsteds Bay in Lake Minnetonka was relatively shallow and thus more vulnerable to whole-lake wind mixing. Consequently, Halsteds Bay did not stratify until early June.
As the summer progressed, the stratification in each lake became more distinct. The upper layer that consists of continuously mixed warmer water (epilimnion) slowly warmed. The water near the bottom of each lake (hypolimnion) remained relatively cool and began to show signs of oxygen depletion. As the temperature difference between these layers grew wider, the stratification grew more stable due to the difference in density. We found that there were no significant changes in stratification with West Upper or Lake Independence during the summer. However, Halsteds Bay experienced two unexpected incidences in which the whole lake was mixed (temperature became uniform throughout the water column). This whole-lake mixing likely resulted from prolonged periods of strong wind. The disruption of weak stratification by wind is fairly normal in large shallow lakes in Minnesota and has been well documented. The RUSS unit provided a high-resolution view of these events because detailed profile data were collected four times daily.
Development of Anoxic Conditions. With the onset of stratification, the hypolimnion within temperate productive lakes often becomes anoxic due to the decomposition of organic matter and changes in redox potential. Typically, anoxic conditions begin to become apparent as stratification develops. The rate at which oxygen is depleted depends on lake productivity and sediment composition. The RUSS data showed that anoxia began to develop in Halsteds Bay and Lake Independence immediately after initial stratification (see Figures 8 and 9). However, the development of anoxic conditions within West Upper Bay occurred substantially later than the onset of stratification. In 1999, dissolved oxygen levels in West Upper Bay decreased at approximately 8 m below the surface, increased by 2 mg/L in the area between 9 and 13 m, and then decreased again near the lake bottom (18 m). This dissolved oxygen spike within the hypolimnion persisted until late July 1999. This pattern was not seen in either of the two other lakes.
Figure 8. Continuous Records of 1999 Temperature in West Upper Bay, Halsteds Bay, and Lake Independence
Figure 9. Continuous Records of 1999 Dissolved Oxygen in West Upper Bay, Halsteds Bay, and Lake Independence
Internal Loading. The development of anoxic conditions in lakes results in the release of phosphorus and nitrogen from the bottom sediments. In relatively deep basins, these nutrients often accumulate in the hypolimnion due to stratification and are not readily available for algal production. However, nutrients released from sediments can become available to algae in relatively shallow basins due to wind-mixing events that periodically degrade stratification (internal loading). The two major mixing events that were observed in Halsteds Bay were further substantiated by the development of uniform dissolved oxygen levels throughout the water column.
Water samples collected from the epilimnion and hypolimnion indicated that phosphorus accumulated in the hypolimnion during periods of anoxia and was mixed into the epilimnion during mixing events. These mixing events within Halsteds Bay are suspected to have resulted in significant internal loading that likely contributed to the high levels of algae observed during the summer.
This insight into the importance of internal loading in Halsteds Bay led to further investigation in 2000 to quantify the amount of phosphorus being released. High-resolution bathymetry maps were generated by using global-positioning technology in tandem with a depth-sonar device. These paired data were used to map the three-dimensional surface of the lake bottom and ultimately to determine changes in the volumes of the epilimnion and hypolimnion based on thermocline position in the water column. At this time, the internal loading of phosphorus has not been calculated due to the complexity of modeling the dynamic nature of the stratification in Halsteds Bay, but the RUSS data make it possible to track daily changes in stratification and may ultimately prove to be very useful for modeling internal loading in dynamic systems such as Halsteds Bay.
Algal Blooms. The occurrence of algae blooms is typical throughout the summer in urban-area lakes. The RUSS units were capable of monitoring changes in algal population density indirectly through turbidity measurements. Turbidity is an indicator of the amount of particulate material suspended within the water column. In addition to turbidity data, secchi depth measurements and chlorophyll-a samples periodically were collected at the RUSS sites. Peaks in turbidity measurements taken from the epilimnion corresponded with the occurrence of algal blooms and high-chlorophyll-a concentrations. Secchi depth measurements suggested that Halsteds Bay experienced more severe and more frequent algal blooms than West Upper Bay and Lake Independence. Excessive algal blooms can cause drastic diel changes in dissolved oxygen due to the photosynthetic/respiratory cycles of algal. In Halsteds Bay, dissolved oxygen levels tended to increase during daytime hours due to plant photosynthesis and decrease dramatically during the nighttime hours due to plant respiration.
Changes in pH and Conductivity. We observed changes in pH and conductivity levels relative to depth within Lake Minnetonka and Lake Independence. Algal photosynthetic activity can effect pH and conductivity. Typically, carbon dioxide dissolves in water and dissociates into bicarbonate ions (HCO3) and carbonic acid (H2CO3). During stratification, photosynthetic activity within the upper layers removes dissolved carbon dioxide from the water. Consequently, the pH increases and conductivity decreases. In contrast, the cold and dark deeper layers are not conducive to photosynthetic activity. As a result, carbon dioxide from bacterial respiration accumulates, causing pH to decrease and conductivity to increase.
Occurrence of Lake "Turnover." In late summer and early autumn, the water temperature in northern lakes begins to decrease, thus reducing the density differences between the epilimnion and the hypolimnion, weakening the lakes’ stratification. Eventually, wind mixes the water column, causing the thermocline to deepen until the entire water column is mixed. When surface and bottom waters approach the same temperature and density, autumn winds can mix the entire lake very rapidly, causing a lake “turn-over” event. Shallower lakes tend to turn over more readily than deeper basins. After continuous mixing of the water column, temperature and dissolved oxygen levels become uniform throughout the water column.
Lake Independence and West Upper Bay generally "turned over" in late September and mid October respectively (see Figures 8 and 9). Both of these basins are relatively deep and experience stable summer stratification. West Upper Bay turns over slightly later than Lake Independence due to its greater depth and volume. As discussed earlier, Halsteds Bay experienced periodic whole-lake mixing events throughout the year. These mixing events were followed by subsequent restratification. Halsteds Bay experienced its final turnover in early September.
Supplemental Keywords:
water, watersheds, ecological effects, suspended particulates, ecosystem, indicators, innovative technology, conservation, environmental, environmental chemistry, biology, ecology, limnology, monitoring, analytical, central, Midwest, Minnesota, MN, EPA Region 5., Scientific Discipline, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Environmental Chemistry, State, Monitoring/Modeling, Ecological Risk Assessment, aquatic ecosystem, EMPACT, environmental monitoring, lake access, Minnesota, MN, community-based approach, public information, remote underwater sampling, web site development, public access, water quality, public outreach, real-time monitoring, land managementRelevant Websites:
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Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.