Long-term effects of a 1940s fertilization experiment on diatoms from Cache Lake (Algonquin Provincial Park, Ontario, Canada)

Abstract

Cache Lake, an ultra-oligotrophic Precambrian Shield lake located in Algonquin Provincial Park (Ontario, Canada), was manipulated in the summers of 1946 and 1947 when approximately 9.4 tons (cumulatively) of 12-24-12 (nitrogen-phosphorus-potassium) fertilizer was added to promote plankton growth and the possible enhancement of sh populations. Little is known about the long-term impacts of this lake manipulation, nor the effects of previous disturbances such as logging in the late-1800s, increased tourism following the extension of the Grand Trunk Railway to Cache Lake in 1896, limited cottage development starting ca. 1905, and subsequent construction of Highway 60 in the 1930s. We used a diatom-based paleolimnological approach to track limnological changes during the past ~ 200 years. Prior to the 1940s, assemblage changes were minimal, although small increases in, for example, Aulacoseira ambigua, likely tracked modest nutrient inputs from historical watershed disturbances. The most striking change was an increase in planktonic Fragilaria tenera in tandem with the near extirpation of the previously abundant Discostella stelligera from the sediment record that was concurrent with the 1946-1947 lake fertilization experiment. Although other lake sediment archives in the region record increases in D. stelligera with limnological changes linked to recent climate warming, this taxon never recovered to pre-disturbance abundances in Cache Lake following nutrient enrichment, but continued to be replaced by other planktonic taxa, also linked to recent warming (e.g. F. tenera, Asterionella formosa). The 1940s experimental fertilization has left a long-term legacy on Cache Lake diatoms, despite its return to ultra-oligotrophic conditions.