Factors Regulating Nearshore Periphyton Blooms in Lake Tahoe

Periphyton blooms may be increasing in oligotrophic lakes globally in response to increases in water temperature and nutrient loads, often associated with climate change. In the Sierra Nevada of California, climate change is altering the duration of snow cover, the frequency and severity of drought, and storm intensity, all which have the potential to affect periphyton growth rates. In Lake Tahoe, there are anecdotal reports that the frequency and severity of nearshore periphyton blooms have increased in recent years. Such blooms decrease both water quality and the aesthetic value of nearshore areas. While recent studies have documented changes in periphyton biomass, it is difficult for in situ studies to disentangle the individual and interactive effects of temperature and nutrients on periphyton growth rates. Consequently, we used laboratory experiments to examine the role of temperature, nutrients, and their interactive effects on periphyton metabolic rates in Lake Tahoe. By measuring changes in dissolved oxygen in sealed incubation chambers containing rocks collected from the nearshore, we quantified rates of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP). We used a 2 x 4 factorial design, manipulating nutrient concentrations (ambient and augmented) and water temperature (ambient, +3, +6, and +9 °C above ambient) to simulate warming during different seasons, and nutrient loading associated with increased runoff. The effects of warming on GPP and ER were found to vary seasonally, with temperature showing stimulatory effects on GPP and ER during some seasons and no significant effect during others. We also found interactive effects between temperature and nutrients to vary seasonally, suggesting that the seasonal timing of nutrient pulses can influence periphyton metabolic response to warming. By providing a mechanistic understanding of periphyton metabolic responses, these results will allow us to predict future algal growth rates under climate change scenarios, and help develop management strategies for Lake Tahoe and other clear water lakes.