Lake Water Quality: Introduction

Satellite imagery cannot be used to measure every aspect of a lake’s physical, chemical and biological characteristics, but it can efficiently measure the following optically related properties, which are key indicators of a lake’s water quality and ecological status. Clicking each of the water quality properties listed in the right column leads to additional information about the nature and relevance of each measure.

Reflectance spectra for waters dominated by various optical properties.

Above: Reflectance spectra for waters dominated by various optical properties. The spectrum for water is from a highly oligotrophic, very clear mine pit lake in northern Minnesota, and its spectrum is close to that for pure water. The chlorophyll and CDOM-chlorophyll spectra have minima in reflectance at ~670 nm, where chlorophyll has an absorbance peak and have secondary minima around 620 nm due to phycocyanin (thus indicating the presence of cyanobacteria in the waters). The three spectra labeled with CDOM all have low reflectance in the blue region (< ~500 nm) because of high CDOM absorbance in this region. The very low (red) spectrum is for a water that is optically dominated by very high CDOM, which absorbs nearly all incoming light, leaving very little to be reflected, even at longer wavelengths.

The key characteristic of these variables is that they affect the optical properties of water bodies, each in different ways. Specifically, they absorb and/or scatter visible light (and infrared radiation) at different wavelengths and thus affect the amount and spectral character of light reflected back to satellite sensors. The reflected light measured by sensors can be used to measure different water quality variables.

As the figure on the right shows, the spectral-radiometric properties of light reflected from inland water bodies depends on the relative abundance of three important “optical” constituents: plant pig­ments (chlorophyll and phycocyanin), sus­pended matter, and colored dissolved organic matter (CDOM). Methods used to retrieve data on the abundance of these constituents in inland waters typically use satellite sensor bands that enable measurement of these constituents with minimal interference from other optical properties. For example, retrieval equations for chlorophyll in inland waters typically use the ratio of the scattering peak for phytoplankton cells at ~ 705 nm and the reflectance trough at ~ 670 nm, which is where the peak in chlorophyll absorbance occurs. These longer wavelengths are less affected by CDOM absorbance, as shown by the difference between the yellow and green lines for spectra in the figure. In contrast, marine remote sensing scientists measure chlorophyll using spectral bands related to chlorophyll absorbance peaks in the 450-550 nm region, but CDOM levels are low in the oceans and do not interfere.

The Upper Great Lakes region has tens of thousands of inland lakes, networks of rivers and streams, and the three largest Great Lakes. The region’s plentiful aquatic resources support recreational activities (including swimming, boating and fishing), as well as transportation, agriculture, industry, and also serve as both drinking water supplies and recipients of treated waste water. The heavy reliance of infrastructure and economy on the region’s water resources translates into high vulnerability to potential changes in water quality.

Human activities impact surface water quality in many ways. Lake eutrophication (nutrient over-enrichment resulting in excessive production of algae and cyanobacteria) is a pervasive problem in the Upper Great Lakes region. Its primary causes are phosphorus and nitrogen inputs from municipal wastewater, urban and agricultural runoff, and near-shore land use. Suspended sediment loadings related to land management practices are a cause of poor water clarity in southern Minnesota; many organic and heavy metal contaminants also are associated with suspended sediment loadings. Mercury pollution is a widespread problem, especially in northern Minnesota, and mercury levels in fish are associated with CDOM levels. Understanding the links between surface water quality and environmental drivers, such as land use and climate change, invasive species, and changes in management practices, is a major focus for our research.

Protecting and monitoring lake water quality is a major activity for many federal, state, tribal, and local agencies and citizen groups. Such efforts provide a vital service in understanding and managing our surface waters, but because of cost and human resource limitations, only a small fraction of lakes and rivers can be sampled annually; fewer still can be assessed at frequencies high enough to detect rapid change. Satellite remote sensing can be a cost-effective way to gather spatially comprehensive water quality information for lakes (e.g., > 12,000 in Minnesota).

Long-term water quality information on a broad regional scale is essential for effective environmental planning and management. Although one cannot go back in time and collect additional water quality data by conventional field methods, Landsat satellite images have been collected regularly since the early 1970s, allowing the possibility of extracting historical water quality data from archived images, as the page on water clarity demonstrates.

The extraction of historic and current water quality data from satellite images, coupled with existing data collection, facilitates the development of comprehensive regional databases (e.g., LAGOS) that can be used to evaluate regional differences and trends over time, as well as to evaluate the effects of land use practices on lake conditions. Results of such analyses will help local and state agencies make informed policy decisions and improve the management of our surface water resources.