Lake Clarity Research Articles

Application of Landsat imagery to regional-scale assessments of lake clarity

A procedure that uses Landsat imagery to estimate Secchi disk transparency (SDT) of lakes was developed and applied to ∼500 lakes with surface areas >10 ha in the seven-county metropolitan area of Minneapolis and St. Paul, MN, USA, to assess spatial patterns and temporal trends in lake clarity. Thirteen Landsat MSS and TM images over the period 1973–1998 were used for the analysis. Satellite brightness values from lake surfaces were calibrated against available historical data on SDT ( n=∼20–40) measured nearly contemporaneously with the acquisition date of each image. Calibration regression equations for the late-summer TM images had a range of r 2 from 0.72 to 0.93. Regression analysis for three late-summer MSS images yielded r 2 values ranging from 0.60 to 0.79. Results indicate that a single late-summer image yields a reliable estimate of regional lake clarity and reasonably accurate estimates of SDT for individual lakes. An analysis of seasonal patterns on a large lake water-quality database was used to develop a model that adjusts synoptic satellite SDT estimates from different dates to a common reference, making them more comparable from year-to-year. Analysis of long-term trends shows that in spite of the large land-use changes within the region over the study period, only 49 (about 10%) of assessed lakes in the region showed significant temporal trends in SDT over the period, and more lakes had increasing SDT (34) than decreasing SDT (15).

A procedure for regional lake water clarity assessment using Landsat multispectral data

Although previous investigations have demonstrated reliable empirical relationships between satellite data and nearly contemporaneous ground observations, satellite imagery has not been incorporated into routine lake monitoring programs. This paper focuses on key issues involved in applying satellite imagery to the regional assessment of lake clarity. Ten Landsat Thematic Mapper (TM) images and four Multispectral Scanner (MSS) images of the Twin Cities Metropolitan Area (TCMA) spanning 25 years (1973–1998) were analyzed. Based on this analysis, recommendations are made for a Landsat-based procedure for water clarity assessment. Closeness of fit ( r 2 values) of regression models between satellite brightness data and measured Secchi disk transparency (SDT) decreased with increasing size of the time window between image collection and ground observation of SDT. Use of SDT data collected within ±1 day of the image date is recommended, but where SDT data are limited, windows up to ±7 days yield reasonable results, especially in late summer when water clarity is relatively constant. Average brightness data from at least nine pixels in the deep open area of a lake should be used to predict lake clarity, but the accuracy of predicted SDT did not improve much as the number of pixels in the area of interest (AOI) increased above this value. A three-coefficient regression model using the TM1/TM3 ratio and TM1 was a consistent and reliable predictor of SDT ( r 2 values of .7–.8). A similar relationship involving the MSS1/MSS2 ratio and MSS1 was a reasonable predictor of SDT for MSS data. Efforts to produce a standard prediction equation for SDT applicable to images collected on different dates were not successful, but a simple regression procedure to account for differences in atmospheric conditions among image collection dates substantially decreased the range of coefficients in the regression model.

Biostatistical evaluation of long-term lake clarity record

Biostatistical evaluation of long-term lake clarity record

Modeling Fire and Nutrient Flux in the Lake Tahoe Basin

Abstract Citizens need to understand how their ecosystem works and try to influence its behavior for the common good, but complexity, uncertainty, and cost discourage many communities. In the Lake Tahoe Basin, organizations and individuals are leveraging the community's high interest in lake clarity and fire danger to construct complex, dynamic models for adaptive management. Models show citizens how the ecosystem works so that they can make intelligent decisions about the desired future condition of their ecosystem and determine how the costs and benefits will be distributed.

Variability of Secchi disk readings in an exceptionally clear and deep caldera lake

The variability of Secchi disk readings was explored in Crater Lake, an exceptionally clear and deep subalpine caldera lake. The primary objective was to document summer monthly and annual variability in Secchi disk readings, and variation caused by observers, disk size, and surface condition of the lake. An additional objective was to determine differences between descending and ascending disk readings (the averages of the two readings representing the disk readings) relative to the depth of the Secchi disk readings, lake surface conditions, and disk size. Secchi disk data were collected from June through September, 1978-1995. Secchi disk depths typically were in the high 20- to low 30-m range. Secchi disk readings were deepest in June and shallowest in August. Average monthly disk readings exhibited considerable variation among years (9.4m in June, 13.4m in July, 13.0m in August, and 8.2m in September). On average, variation in Secchi disk readings among observers was 1 m or less. When the lake surface was calm and skies were clear or had high haze, differences between descending and ascending observations decreased slightly with increased disk depth. Waves from tour boats, drips of water from the research vessel, and wind generated ripples and chop decreased disk readings as much as 5 m relative to readings recorded when the lake surface was calm. Clarity readings using a 100-cm disk were 7.0m deeper than were 20-cm disk readings. Documentation of the extensive variation in Secchi disk clarity is needed to determine normal conditions and to judge the long-term status and trends of lake clarity. Furthermore, documenting the variation caused by slightly disturbed lake surface conditions relative to calm surface conditions and among trained observers ensures consistent interpretation of the long-term data base.

Partitioning Light Attenuation in an Acidic Lake

Although a number of researchers have reported that acidification of lakes is accompanied by an increase in transparency, there has been no systematic evaluation of the processes responsible for this transformation. In this study we partitioned the attenuation of light in acidic Dart's Lake, located in the Adirondack region of New York from May to September 1982. We observed that changes in light attenuation (Kd) and light absorption (a) were regulated largely by "gelbstoff." Substantial decreases in Kd and a occurred through the study period and were correlated with a depletion in the concentration of dissolved organic carbon (DOC). In-lake concentrations of DOC were controlled by terrigeneous loading and in-lake processes. The decrease in DOC and the attendant decreases in a and Kd were coupled to a loss of Al from the water column of the lake. We suggest that coagulation/adsorption of DOC by Al may have contributed to increases in lake clarity. Increased transparency is significant because it enhances hypolimnetic heating and decreases the thermal stability of lakes.