Using a lake sediment record to infer the long-term history of cyanobacteria and the recent rise of an anatoxin producing Dolichospermum sp.

Lakes that experience recurrent toxic cyanobacterial harmful algae blooms (cyanoHABS) are often subject to cultural eutrophication, where landscape development and upland activities increase the nutrient inputs to the water column and fuel cyanoHABS. Few studies have focused on the response of a lake to nutrient inputs for which the natural geomorphic setting predisposes a nutrient-rich water column to already support abundant cyanobacteria. Here, we present a sediment core record from a lake surrounded by parkland that experiences recurrent cyanoHABs which produce dangerous levels of the neurotoxin, anatoxin-a, impacting the recreational use of the lake and park. Using photoautotrophic pigments in the sediment record, we establish cyanobacteria have long been part of the diverse and abundant phytoplankton community within the lake. Despite this long record, shotgun metagenome and other DNA analyses of the sediment record suggest that the current anatoxin-a producer Dolichospermum sp. WA102 only emerged to dominate the cyanobacterial community in the mid-1990s. A period of lakeshore farming that finished in the 1950s-1960s and possibly the stocking of rainbow trout fry (1970-2016) coincide with a progressive shift in primary production, together with a change in bacterial communities. Based on the history of the lake and contemporary ecology of Dolichospermum, we propose that the legacy of nutrient inputs and changes in nutrient cycling within the lake has encouraged the development of an ecosystem where the toxin producing Dolichospermum sp WA102 is highly competitive. Understanding the historical presence of cyanobacteria in the lake provides a context for current-day management strategies of cyanoHABs.