A method for examining temporal changes in cyanobacterial harmful algal bloom spatial extent using satellite remote sensing

Abstract

Cyanobacterial harmful algal blooms (CyanoHAB) are thought to be increasing globally over the past few decades, but relatively little quantitative information is available about the spatial extent of blooms. Satellite remote sensing provides a potential technology for identifying cyanoHABs in multiple water bodies and across geo-political boundaries. An assessment method was developed using MEdium Resolution Imaging Spectrometer (MERIS) imagery to quantify cyanoHAB surface area extent, transferable to different spatial areas, in Florida, Ohio, and California for the test period of 2008 to 2012. Temporal assessment was used to evaluate changes in satellite resolvable inland waterbodies for each state of interest. To further assess cyanoHAB risk within the states, the World Health Organization's (WHO) recreational guidance level thresholds were used to categorize surface area of cyanoHABs into three risk categories: low, moderate, and high-risk bloom area. Results showed that in Florida, the area of cyanoHABs increased largely due to observed increases in high-risk bloom area. California exhibited a slight decrease in cyanoHAB extent, primarily attributed to decreases in Northern California. In Ohio (excluding Lake Erie), little change in cyanoHAB surface area was observed. This study uses satellite remote sensing to quantify changes in inland cyanoHAB surface area across numerous water bodies within an entire state. The temporal assessment method developed here will be relevant into the future as it is transferable to the Ocean Land Colour Instrument (OLCI) on Sentinel-3A/3B missions.

Keywords: Cyanobacteria; Harmful algal blooms; Inland waters; MERIS; Remote sensing.

Fig. 1

Study area location map indicating all MERIS scene boundaries (A), California (B), Ohio (C), and Florida with three water management districts (D): St. Johns River (SJR), South West Florida (SWF), and South Florida (SF).

Fig. 2

Percent monthly mean cloud coverage for FL, OH, and CA assessed by satellite and extrapolated from daily MERIS imagery. Shaded regions represent summer months (June–August).

Fig. 3

Monthly temporal assessment of (A) total bloom area (km²) for FL, OH, and CA, and (B) percent bloom area for resolvable water area in FL, OH, and CA, including linear regression lines.

Fig. 4

Times series of bloom area by WHO guidance levels for recreational waters for FL (A), OH (B), and CA (C). TS of low-risk, moderate-risk, and high-risk are indicated by figure legends, including linear regression lines.

Fig. 5

Temporal assessment of Florida water management districts by surface area of bloom for each WMD (A), and bloom area by risk category for SF (B), SJR (C), and SWF (D), including linear regression lines.