Measurement of Cyanobacterial Bloom Magnitude using Satellite Remote Sensing

Abstract

Cyanobacterial harmful algal blooms (cyanoHABs) are a serious environmental, water quality and public health issue worldwide because of their ability to form dense biomass and produce toxins. Models and algorithms have been developed to detect and quantify cyanoHABs biomass using remotely sensed data but not for quantifying bloom magnitude, information that would guide water quality management decisions. We propose a method to quantify seasonal and annual cyanoHAB magnitude in lakes and reservoirs. The magnitude is the spatiotemporal mean of weekly or biweekly maximum cyanobacteria biomass for the season or year. CyanoHAB biomass is quantified using a standard reflectance spectral shape-based algorithm that uses data from Medium Resolution Imaging Spectrometer (MERIS). We demonstrate the method to quantify annual and seasonal cyanoHAB magnitude in Florida and Ohio (USA) respectively during 2003-2011 and rank the lakes based on median magnitude over the study period. The new method can be applied to Sentinel-3 Ocean Land Color Imager (OLCI) data for assessment of cyanoHABs and the change over time, even with issues such as variable data acquisition frequency or sensor calibration uncertainties between satellites. CyanoHAB magnitude can support monitoring and management decision-making for recreational and drinking water sources.

Figure 1

Map of the study region showing the location of lakes in (A) Florida and (B) Ohio. In total, 135 lakes in Florida and 21 lakes in Ohio, were resolvable with the full resolution MERIS data and are used in this study. Land and lakes are shown in gray and blue colors respectively.

Figure 2

Schematic diagram of data processing and workflow for calculating bloom magnitude and area-normalized magnitude. Output stages in the workflow are shaded gray.

Figure 3

(A). Algal bloom magnitude in Florida lakes in 2011 before normalization and (B) after normalization by lake surface area. Area-normalized magnitude (km⁻²) of selected lakes provided as part of bar labels in parenthesis. Bar height and color are proportional to annual bloom magnitude (in A) and annual area-normalized magnitude (in B). The width of the bars is proportional to the lake surface area. Note that the color bars are log-scaled.

Figure 4

First panel: Annual area-normalized magnitude (km⁻²) in the top 50 Florida lakes. Green, orange, and red dotted lines represent equivalent WHO thresholds of 20,000, 100,000, and 1,000,000 cells mL⁻¹ limits; second panel: the inter-quartile range of area-normalized magnitude ranks in top-ranked Florida lakes over 2003–2011 ordered by their median rank over the 9 year period. Median values or ranks are highlighted in red vertical lines inside the box. Annual area-normalized magnitude rank data points are overlaid on inter-quartile boxes to highlight the variation, where the color of the scatter points indicate data year (tan: start year and deep green: end year). Third panel: green/red bar plot shows Sen’s slope (trends in rank change) during 2003–2011. Green/red color represents positive/negative trend meaning area-normalized magnitude for a lake is decreasing/increasing over time. Fourth panel: bars show Kendall’s τ (absolute values) representing consistency in rank-change trend over time. Dotted lines in Kendall’s τ plot mark the τ values at 0.2 and 0.5.

Figure 5

The number of lakes in Florida and Ohio classified as high, moderate, and low bloom categories based on recreational WHO cyanobacterial cell density limits.

Figure 6

First panel: Seasonal area-normalized magnitude (km⁻²) in Ohio lakes. Green, orange, and red dotted lines represent equivalent WHO thresholds of 20,000, 100,000, and 1,000,000 cells mL⁻¹ limits; second panel: the inter-quartile range of area-normalized magnitude ranks in top-ranked Ohio lakes over 2003–2011 ordered by their median rank over the 9 year period. Median values or ranks are highlighted in red vertical lines inside the box. Annual area-normalized magnitude rank data points are overlaid on inter-quartile boxes to highlight the variation, where the color of the scatter points indicate data year (tan: start year and deep green: end year). Third panel: green/red bar plot shows Sen’s slope (trends in rank change) during 2003–2011. Green/red color represents positive/negative trend meaning area-normalized magnitude for a lake is decreasing/increasing over time. Fourth panel: bars show Kendall’s τ (absolute values) representing consistency in rank-change trend over time. Dotted lines in Kendall’s τ plot mark the τ values at 0.2 and 0.5.

Figure 7

Bar plot showing median area-normalized magnitude (km⁻²) in Ohio lakes during the recreational season over the study period (2003–2011). Width of bars is proportional to lake surface area, height and color of the bars are proportional to the median annual area-normalized magnitude (km⁻²). Median values of the top five lakes are provided as part of bar labels inside the parenthesis.

Figure 8

First panel: Seasonal area-normalized magnitude (km⁻²) in the top 50 Florida and Ohio lakes. Green, orange, and red dotted lines represent the equivalent WHO thresholds of 20,000, 100,000, and 1,000,000 cells mL⁻¹ limits. Lakes with white and shaded bars are located in Florida and Ohio, respectively; second panel: the inter-quartile range of area-normalized magnitude ranks in top-ranked Florida lakes over 2008–2011 ordered by their median rank over the 4 year period. Median values or ranks are highlighted in red vertical lines inside the box. Annual area-normalized magnitude rank data points are overlaid on inter-quartile boxes to highlight the variation, where the color of the scatter points indicate data year (tan: start year and deep green: end year). Third panel: green/red bar plot shows trends in rank change during 2008–2011. Green/red color represents positive/negative trend meaning area-normalized magnitude for a lake is decreasing/increasing over time. Fourth panel: bars show Kendall’s τ (absolute values) representing consistency in rank-change trend over time. Dotted lines in Kendall’s τ plot mark the τ values at 0.2 and 0.5.

Figure 9

Left panel: Median seasonal bloom magnitude in 15 Florida and Ohio lakes over the study period (2008–2011) ordered by their values. Lake labels include the state name the lake is associated with and the number inside brackets represents the median area-normalized magnitude rank over the same study period. Right panel: median area-normalized magnitude for the same lakes during the same study period provided for comparison. Gray-colored bars represent lakes from Ohio.

Figure 10

Relative comparison of lake ranks calculated from the annual area-normalized magnitude and measured annual mean Chl-a concentration. Numbers associated with the lake names in the x-axis tick label represent the median lake rank as in Table 1.