Low-Nitrate-Days (LND), a potential indicator of cyanobacteria blooms in a eutrophic hardwater reservoir

Abstract

When nitrate was low in a hypereutrophic, hardwater reservoir, cyanobacteria proliferated into blooms. Based on this observation an index was developed that relates an easily measurable variable, the period of Low-Nitrate-Days (LND), to the period when nuisance cyanobacteria (blue-greens) proliferate and "bloom". The bloom indicator LND (d·yr-1) was defined as the period of time during summer and early fall when nitrate concentration is below a lake-specific threshold. This concept was valuable in Fanshawe Lake, a southern Ontario reservoir of the Thames River in the Lake Eric catchment basin, where traditional bloom indicators are rare. A nitrate threshold of 1 to 2 mg·L-1 is supported by occasional observations of chlorophyll (Chl) concentration, blue-green biomass, visual inspection, and photographic documentation. Fanshawe Lake's water quality (phosphorus, Chl, and Secchi disk transparency) varied from summer to summer and LND ranged from 0 to 175 d·yr-1 with a long-term average of 62 d·yr-1 for 38 years. LND was positively and significantly correlated with average summer total phosphorus concentration (available for 8 years), but not Chl (n = 6) nor transparency (n = 11), possibly because of an invasion by the zebra mussel Dreissena. LND values agreed well with cyanobacteria biomass indicators predicted from other models. Significant relationships with 38 years of flows and the climatic index (winter North Atlantic Oscillation) reveal that during high-flow years estimated cyanobacteria blooms are infrequent, while during low-flow years bloom periods are extended and the water quality is poor. Investigations on other man-made lakes and river sections of the Thames River, and preliminary studies on natural lakes with differing trophic states show that LND may be a useful variable in all lakes and reservoirs where nutrient limitation switches from phosphorus to nitrogen during summer. Copyright © 2007, CAWQ.