Research Summary: Interannual Variability of Cyanobacterial Blooms in Lake Erie

Abstract

After a 20-year absence, severe cyanobacterial blooms have returned to Lake Erie in the last decade, in spite of negligible change in the annual load of total phosphorus (TP). Medium-spectral Resolution Imaging Spectrometer (MERIS) imagery was used to quantify intensity of the cyanobacterial bloom for each year from 2002 to 2011. The blooms peaked in August or later, yet correlate to discharge (Q) and TP loads only for March through June. The influence of the spring TP load appears to have started in the late 1990 s, after Dreissenid mussels colonized the lake, as hindcasts prior to 1998 are inconsistent with the observed blooms. The total spring Q or TP load appears sufficient to predict bloom magnitude, permitting a seasonal forecast prior to the start of the bloom.

Introduction

Lake Erie suffered from intense blooms of cyanobacteria in the 1970 s. Following phosphorus abatement strategies these blooms disappeared in the 1980 s [1]–[3]. The blooms reappeared in the 1990 s, with blooms dominated by Microcystis aeruginosa common in the last decade[4]–[5]. During this time the annual total phosphorus (TP) load has not changed, but annual soluble reactive phosphorus (SRP) loads have increased [6]. In addition, the 1990 s saw ecological disruptions caused by invasive mussels of the genus Dreissena, which have been hypothesized to promote cyanobacterial blooms [7]–[9]. The Maumee River (Figure 1), the single largest watershed draining into the Laurentian Great Lakes, has also been hypothesized to supply the needed nutrients to fuel the Microcystis spp. blooms [5]. Over 80% of the land within the watershed is used for agriculture [10], and it discharges into the shallowest portion of Lake Erie.

Map of western Lake Erie. (Credit: Stumpf, et al.)

Microcystis produce noxious and toxic compounds that cause a variety of detrimental impacts[11]. These impacts include animal mortalities and human health risks from the toxin microcystin, as well as taste and odor problems in finished drinking water, if not specifically treated [11]–[12]. Therefore a seasonal prediction of the blooms would aid managers in planning mitigation strategies. Warmer temperatures may exacerbate blooms, increasing the severity of these impacts [13].

Satellite imagery can provide data on the areal extent of cyanobacterial blooms [14]–[16]. Of the several instruments, the Medium-spectral resolution imaging spectrometer (MERIS) permits quantification of blooms even in water with suspended sediments [17]–[18], including Lake Erie[15], [19]. MERIS data is available since 2002, allowing a comparison of the bloom intensity with Maumee River loads for each of the last ten years. With several bands in the red and the “red edge” portion of the near-infrared, MERIS data allow spectral shape algorithms that can target severe blooms [19]–[21]. Spectral shape methods use a computational equivalent to the second derivative [15]; these include fluorescent line height (FLH) [20]; maximum chlorophyll index (MCI) [21], and the cyanobacteria index (CI) [19]. The MCI was demonstrated to be effective in coastal ocean blooms with data that has not been atmospherically corrected [21]. This power of spectral shape algorithms means that far more data can be retrieved under thin cloud and glint conditions than with standard algorithms based on water-leaving radiance. The CI, which is the negative of the FLH [20], has been quite effective at identifying cyanobacterial blooms in Lake Erie [15], [19], and appears to be less sensitive to high sediment loads than the MCI.

Full study, including methods, results and discussion, published Aug. 1, 2012 under open-access license in PLOS ONE.