Impact of nutrients on photoacclimation of phytoplankton in an oligotrophic lake measured with long-term and high-frequency data: implications for chlorophyll as an estimate of phytoplankton biomass

Abstract

Chlorophyll measurements are commonly used to estimate phytoplankton biomass. However, phytoplankton readily acclimate to variations in light through a range of phenotypic responses, including major adjustments in chlorophyll pigmentation at the cellular level. The ratio of pigment chlorophyll to carbon concentration (Chl:C) is a commonly used metric in the oceanographic community to explore photoacclimation responses to varied light levels, yet is relatively rare in freshwater studies. Here we explore how nutrient variability impacted summertime Chl:C ratios of a natural phytoplankton community throughout the water column of a stratified oligotrophic lake. We utilized both long-term (18–24 years) and high-frequency (daily) data from Crater Lake, Oregon, a deep mountain lake with little anthropogenic disturbance. As expected, fluctuation in nutrients had a strong impact on phytoplankton particle density, primary productivity, light penetration, and water clarity. However, chlorophyll concentration did not register predictable changes even though the vertical location of the deep chlorophyll maximum was responsive to the overlying algal density. The impact of elevated nutrients on the Chl:C ratio was further complicated by upward shifts in chlorophyll distribution. The muted response of chlorophyll concentration to nutrients may be partially explained by variations in phytoplankton community composition or iron stress.