Linking the benthic and planktonic realms: a year survey with implications for the routine monitoring of cyanobacteria

AIMS: Cyanobacteria produce and release secondary metabolites in waterways, challenging drinking water treatment plants. Benthic Cyanobacteria, a group of species living at the bottom of waterbodies, have been identified as potential Taste and Odour (T&O) compound and toxin producers. Following an increase in customer T&O complaints about water produced from the SA-L1 Reservoir, this study was conducted to better understand benthic cyanobacetria growth patterns and establish whether the source of detrimental metabolites was pelagic or benthic. METHODS AND RESULTS: A field-survey was performed from December 2014 to December 2015, during which physical samplers were deployed across a transect (1 m, 2 m, 6 m, and 14 m depth). Biofilm and water samples were analysed for microbial community composition, chlorophyll-a (Chl-a), nutrients, T&O and toxins and their associated genes. Seasons and sampling depths impacted benthic communities, Chl-a concentrations and biofilm growth. Spring and autumn were established as peak growth periods for benthic Cyanobacteria. Water geosmin concentrations significantly correlated with the abundance of benthic Cyanobacteria. The potential for cylindrospermopsin and saxitoxin production was detected in this reservoir. Mat detachment mid-spring contributed to T&O dispersion. CONCLUSIONS: Benthic Cyanobacteria are major geosmin contributors in this reservoir. The potential cylindrospermopsin-producer was demonstrated to be benthic, while the saxitoxin producer was identified as pelagic Dolichospermum circinale, which is recruited from bentic mats in spring. Utilities should consider regular monitoring of benthic mats, which provides the necessary evidence to better anticipate benthic and pelagic events, to in turn provide safe and palatable drinking water.