The recent disappearance of a persistent Planktothrix bloom: Characterization of a regime shift in the phytoplankton of Sandusky Bay (USA)

Abstract

Sandusky Bay is the drowned mouth of the Sandusky River in the southwestern portion of Lake Erie. The bay is a popular recreation location and a regional source for drinking water. Like the western basin of Lake Erie, Sandusky Bay is known for being host to summer cyanobacterial harmful algal blooms (cHABs) year after year, fueled by runoff from the predominantly agricultural watershed and internal loading of legacy nutrients (primarily phosphorus). Since at least 2003, Sandusky Bay has harbored a microcystin-producing bloom of Planktothrix agardhii, a species of filamentous cyanobacteria that thrives in low light conditions. Long-term sampling (2003-2018) of Sandusky Bay revealed regular Planktothrix-dominated blooms during the summer months, but in recent years (2019-2022), 16S rRNA gene community profiling revealed that Planktothrix has largely disappeared. From 2017-2022, microcystin decreased well below the World Health Organization (WHO) guidelines. Spring TN:TP ratios increased in years following dam removal, yet there were no statistically significant shifts in other physicochemical variables, such as water temperature and water clarity. With the exception of the high bloom of Planktothrix in 2018, there was no statistical difference in chlorophyll during all other years. Concurrent with the disappearance of Planktothrix, Cyanobium spp. have become the dominant cyanobacterial group. The appearance of other potential toxigenic genera (i.e., Aphanizomenon, Dolichospermum, Cylindrospermopsis) may motivate monitoring of new toxins of concern in Sandusky Bay. Here, we document the regime shift in the cyanobacterial community and propose evidence supporting the hypothesis that the decline in the Planktothrix bloom was linked to the removal of an upstream dam on the Sandusky River.

Keywords: Cyanobacteria; Dam removal; Microcystins; Nutrients; Planktothrix; Regime shift.

Fig. 1.

Location of sampling sites located in Sandusky Bay, Lake Erie. The inset map shows location of the Bay with respect to the whole lake. Sampling sites in bold are those used for 16S rRNA sequencing.

Fig. 2.

Water temperature measurements throughout the years 2017—2022. Error bars represent standard deviation between sites. Each point represents 7 sites within the bay.

Fig. 3.

Total nitrogen to total phosphorus ratios throughout the year. Each panel represents one year of sampling; lines represent either Inner Bay or Outer Bay sites. Note the differences in scale between years, reflecting interannual variation in springtime nutrient loads. Each point represents 3 sites for inner bay and 4 sites for outer bay.

Fig. 4.

Extracted chlorophyll-a concentrations, averaged over all sites for all dates each year. Error bars are standard deviation. The number of measurements for each year is indicated at the bottom of the bars.

Fig. 5.

Total microcystin concentrations from 2017 to 2022, measured by ELISA following EPA methods. Each point represents all sites with greater than 0 concentration on their respective DOY.

Fig. 6.

Cyanobacteria annotated at the genus level. Panel A is the percent of the total 16S reads that were cyanobacteria. Panel B is the cyanobacterial community at the genus level with taxa present at less than one percent of the total abundance was grouped together. The x-axis is the day of year (DOY), and the y-axis shows relative abundance. Each site was merged and averaged into the same DOY for analysis and each panel is a different year.

Fig. 7.

Principal coordinate analysis (PCoA) plot showing the compositional differences (Bray-Curtis dissimilarities) of cyanobacterial communities between sample years and months. Years are indicated by color, shapes indicate month of the year, and the ellipses are drawn showing 95% confidence intervals assuming multivariate normal distribution.

Fig. 8.

Aerial map images of Sandusky River at the Ballville Dam, Ballville, Ohio, retrieved from Google Maps. (A) The historical map from 2015 before the dam was removed. (B) Ballville Dam site in 2021, two years after removal of the dam. Sites were only sampled post dam removal and were chosen based on the historical location of pools.