Algal Blooms in Lake Iroquois, Williston, VT
Using Aerial Imagery to Evaluate Cyanobacteria Increase from 2012 to 2014
History of Cyanobacteria in Vermont and Lake Iroquois
Cyanobacteria, more commonly known as blue-green algae, is a naturally occurring algae found in fresh water across the United States. Cyanobacteria grow and reproduce rapidly during the growing season when they get enough sunlight, phosphorus, and nitrogen. While in most cases exposure to cyanobacteria is harmless or causes a minor allergic-like reaction, some forms of cyanobacteria produce dangerous chemicals called cyanotoxins. When these toxins are ingested in large quantities, organ damage and stomach problems can result in humans, pets, and other animals. Cyanobacteria are especially dangerous for children and animals due to their tendency to play near the water and inadvertently drink it. (Vermont Dept. of Health)
Vermont is not impervious to cyanobacteria and the dangers it poses. Vermont has seen an increase in cyanobacteria growth in recent years, from Lake Champlain to smaller ponds and lakes. In 2012, the Vermont Department of Health started keeping track of reports of algal blooms with an online tracker. Lake Iroquois, in Williston, VT, is one of the bodies of water that has been monitored for cyanobacteria growth. As recently as September 14th, 2017, Lake Iroquois was rated as "Generally Safe" by the VT Dept. of Health; however, there have been times when alerts about the quality of the water have been issued. Water testing is very helpful for knowing the current conditions of water, but analyzing the trends of cyanobacteria growth over time can help clean water advocates more effectively manage the lake. This project involved analyzing aerial imagery of Lake Iroquois using geographic information systems (GIS) software to determine the growth of algal blooms from 2012 to 2014.
Lake Iroquois, Williston, Vermont. Source: Middlebury Mountaineer
Blue-Green algae usually appears as a green paint or neon sheen floating in the water. Source: NOAA
Data and Methods
For my analysis, I used aerial imagery from the National Agriculture Imagery Program (NAIP) to compare algal blooms in Lake Iroquois between August 2012 and August 2014. Aerial imagery as seen below of Lake Iroquois is gathered by air- or ground-based vehicles that can detect and classify objects on Earth, usually from a distance. There are many benefits of remote sensing, including the ability to collect data from dangerous or remote areas, collect an enormous extent of data, and data can be collected more quickly and at a lower cost than traditional techniques (Dreiss, Mod1). Remote sensing has applications across disciplines, but is especially useful for detection of phenomenon that can be difficult to track otherwise. For example, in recent years, remote sensing has become widely used for wildfire detection and monitoring, where traditional on-the-ground survey techniques are impossible (Allison et al, 2016). Another new application is in the monitoring and forecasting of environmental conditions that lead to malaria outbreaks in Kenya (Sewe et al, 2017). There are countless applications of remote sensing to environmental issues that can be better understood with these analyses.
Aerial imagery of Lake Iroquois, 2012. Source: NAIP
Aerial imagery of Lake Iroquois, 2014. Source: NAIP
Remote sensing technologies use sensors that record the intensity of light reflected off the Earth's surface. Aerial imagery from NAIP contains four bands of wavelengths that make up the colors we see: red, green, blue, and near infrared. The amount of light reflected by each range of wavelengths in a given pixel of satellite imagery can tell us a lot about what is happening on the Earth. For this study, I analyzed the aerial imagery above using the Normalized Difference Vegetation Index (NDVI), which highlights greenness and healthy vegetation in satellite imagery. NDVI takes advantage of the fact that green vegetation, such as blue-green algae, absorbs most of the red band and reflects most of the near infrared. NDVI values show up as a range from -1.0 to 1.0, where higher reflectance is shown with a higher value.
Diagram explaining how NDVI works to highlight the NIR in healthy vegetation. Source: Agrobotix
NDVI is only one of many spectral indices that can be used for feature detection with remote sensing. Harris Geospatial Solutions has a comprehensive list of spectral indices used for a wide range of analyses. NDVI fits into the Vegetation Indices, but Harris also lists Geology Indices, Burn Indices, and other miscellaneous indices (Harris). Some notable indices are the Iron Oxide Ratio, Burn Area Index, Normalized Difference Built-Up Index, and Modified Normalized Difference Water Index, each one used with aerial imagery to detect natural or unnatural phenomenon that are difficult or impossible to capture with other techniques.
All of the analysis for this project was done in ArcGIS, software that allows you to work with spatial data like aerial imagery and perform complex analyses. After converting the NAIP imagery from 2012 and 2014 with the NDVI, I measured the pattern of reflectance across the lake in the most affected area of the lake off the Northern shore. This can give a valuable first look at how the reflectance has changed and where to look for algal bloom growth. The graph below shows the NDVI values across a cross section of Lake Iroquois in 2012 and 2014. Clearly, it shows that over the 2 years, there was a significant increase in vegetation in that specific cross section of the lake.
The next step in my analysis was calculating the difference in NDVI values between the 2012 and 2014 aerial imagery to get closer to the ultimate goal: measuring the amount of blue-green algae growth from 2012 to 2014. With these values, using ArcGIS, I then could use an algorithm that classifies aerial imagery into clusters based on the reflectance values. This technique gave me ten different clusters of data, one of which was the algae growth. To complete my analysis, I calculated the area of algae growth from 2012 to 2014 using these clusters.
Results
My analysis clearly shows that algal blooms have significantly increased from 2012 to 2014. The map at the right shows the areas where there has been algal bloom growth. Based on my analysis, there was 23,151 square meters of new algal blooms in 2014 as compared to 2012. This new growth is shown in
electric green on the map.
Most of the algal blooms are located along shorelines and at inflows around the lake, both in the Northern section and at the Eastern shore. Algal blooms flourish when they are "overfed," which occurs when nutrients like carbon, phosphorus, and nitrogen build up in farmlands and lawns and flow into bodies of water at higher than normal rates (US Dept of Commerce). Therefore, it makes sense that the highest density of algal bloom growth over the two-year span would be near the shorelines and inflows around the lake. Researchers at the University of Vermont are studying the source of excess amounts of phosphorus in Lake Iroquois to try to eventually come up with a mitigation plan for the lake. However, this research can also be applied to larger bodies of water and watersheds like the Lake Champlain basin, which has also experienced increased algal blooms in recent years (Milideo).
Limitations and Sources of Error
This analysis clearly shows that between August 2012 and August 2014, there was significant blue-green algae growth in Lake Iroquois. However, due to the limitations of this approach, the exact value of growth may not be correct. There are several potential sources of error:
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After using a tool in ArcGIS to gather the data into ten distinct clusters, I chose the values that represented the growth in blue-green algae by simply comparing it visually with the aerial imagery. Only one of the clusters was chosen as representing blue-green algae, but it is possible that this subjective analysis missed some of the growth in the lake.
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It is possible that some of the growth that my analysis discovered was not all blue-green algae. While the NDVI approach singles out vegetation, there could be other forms of vegetation growth in this pond. This is probably not a large source of error though, because many organizations have recognized that there is significant blue-green algae growth in the lake, including the Vermont Department of Environmental Conservation.
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This analysis only used two images for the analysis, which means that it cannot be generalized to other time periods. Between August 2012 and August 2014, my analysis may have captured the growth, but without a more in-depth analysis, we cannot know whether this is an outlier or the norm.
Management Implications
Based on my analysis, Lake Iroquois is experiencing significant growth in blue-green algae due to an overflow of nutrients like phosphorus into the water. Organizations like the Lake Iroquois Association have recognized this and are working to better monitor water conditions in the lake. Steps that have already been taken by the Vermont Department of Health, including starting the Cyanobacteria Tracker and offering a wealth of free information online about the dangers and management of blue-green algae, are necessary to educate the public and keep officials aware of the issue. The Lake Iroquois Association also has started a number of initiatives focused on monitoring water quality, prevention, and infrastructure improvements to manage the problems. On August 17, 2017, the Lake Iroquois Association completed the Lake Iroquois Stream Remediation and Erosion Control Project, which focused on reducing erosion and the decreasing the amount of sediment carried into the lake. You can read more details about the project here. Based on my analysis, projects like these are the most effective way to reduce more algal bloom growth in the future.
Lake Iroquois, Williston, Vermont. Source: Flickr
Further management actions may include:
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Education for local residents and patrons of the lake about the potential dangers of blue-green algae, potentially in the form of increased signage or a press release
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More infrastructure improvement projects to implement riparian buffers and other natural forms of erosion/overflow control
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Investment in monitoring of water quality in feeder streams and possibly aerial imagery to understand trends over time
In conclusion, as the world continues to fight against climate change and figures out how to deal with more and more extreme weather events, it is more important than ever that organizations at the local and state level focus on preserving natural resources like Lake Iroquois for future generations. The work of education, prevention, and management that is already being done must be continued so that we can better understand the causes and work to mitigate the consequences of blue-green algae growth.
Sources
Allison, Robert S. et al. “Airborne Optical and Thermal Remote Sensing for Wildfire Detection and Monitoring.” Ed. Ingolf Willms. Sensors (Basel, Switzerland) 16.8 (2016): 1310. PMC. Web. 16 Sept. 2017.
“Cyanobacteria (Blue-Green Algae).” Vermont Department of Health, 28 June 2017, www.healthvermont.gov/health-environment/recreational-water/cyanobacteria-blue-green-algae
Dreiss, Lindsay. "Mod1: Remote Sensing and Detection." Web. 17 Sept. 2017.
Milideo, Lauren. “UVM Researchers Studying Lake Iroquois.” Shelburne News, 26 July 2017, www.shelburnenews.com/2017/07/27/uvm-researchers-studying-lake-iroquois/.
Sewe, M. O., et al. "Using Remote Sensing Environmental Data to Forecast Malaria Incidence at a Rural District Hospital in Western Kenya." Scientific Reports, vol. 7, no. 1, 2017, SCOPUS, www.scopus.com, doi:10.1038/s41598-017-02560-z.
“Spectral Indices.” Harris Geospatial Docs Center, Harris Geospatial Solutions, www.harrisgeospatial.com/docs/SpectralIndices.html.
US Department of Commerce, National Oceanic and Atmospheric Administration. “Why Do Harmful Algal Blooms Occur?” NOAA's National Ocean Service, 1 Aug. 2014, oceanservice.noaa.gov/facts/why_habs.html.