Threat of Algal Blooms in Lake Iroquois
Similar to other Vermont waters, Lake Iroquois has seen an increase in the bloom of cyanobacteria (blue-green algae) over recent years. While this algae naturally occurs in water, it releases natural toxins and poisins (cyanotoxins), which in large amounts, can cause serious health problems with skin exposure or if swallowed. These algal blooms are due to excessive nutrients entering the lake because of events such as stormwater runoff and are expected to worsen with climate change.
In order to assess the growth of cyanobacteria blooms in Lake Iroquois, data between 2012 and 2014 was compared. There was an increase of ~22 square miles of algal cover over the two years and as evident in the map below, the northern end of the lake in Williston, VT should be a priority to monitor.
How was this done?
Remote sensing is the science of collecting information about Earth from a distance, usually with sensors mounted on a satellite or aircraft, and is commonly used with a geographic information system (GIS) with a variety of applications from to tracking hurricanes to monitoring land use.
There are three main bands of solar radiation that reaches the Earth's surface: ultraviolet, visible (red, green, blue) and infrared. At the Earth's surface, this radiation is absorbed and reflected by various plants, buildings, animals etc and the sensors measure the percentage of reflectance of red, green, blue and near-infrared (NIR) bands. Different materials have different spectral signatures, meaning unique percentages of reflectance in relation to wavelength. Since each material has a unique spectral signature, algorithms can be run on GIS to identify these specific features--in this case algal blooms in the lake.
In this case, GIS was used to analyze digital aerial imagery from the USDA Farm Service Agency National Agriculture Imagery Program (NAIP).
The red, green, blue and NIR bands in the NAIP imagery were used to derive the normalized difference vegetation index (NDVI) which identified the algae in the lake. The NDVI was created by dividing the difference of the NIR and red bands by the sum of them.
NDVI= (NIR-RED) / (NIR+RED)
This is done as plants reflect lots of infrared light as explained on the right.
NDVI values are represented as a ratio ranging in value from -1 to 1, where ratios closer to -1 indicate water and ratios closer to 1 indicate vegetation. Limitations of the data are easily seen here where A sample of NDVI values was taken within within the northern part of Lake Iroquois to determine whether the seemingly large algal bloom was significant.
While the graph shows that there seems to be a correlation between where the presumed algae blooms are and higher NDVI values, investigation on the ground is still important to affirm the data-- especially since the ratio for vegetation is usually higher than 0. The spatial resolution (size of the pixels) of the image limits the degree of accuracy one can monitor the algal blooms in the lake as well. In addition, the NAIP 2014 Imagery being the most recent, there is also a gap in data between then and now in 2017. The analysis done was only looking at a short span of two years and more data would be needed in order to better predict patterns of growth.