Experimental Warming and Short‐Term Nutrient Effects on Nearshore Periphyton Metabolism in a Large, Oligotrophic Lake

Periphyton blooms may be increasing in oligotrophic lakes due to warming water temperatures and increased nutrient loads associated with climate change. Such blooms decrease both water quality and the aesthetic value of nearshore areas, but identifying the mechanisms driving periphyton blooms in situ is complex. We conducted laboratory experiments using periphyton‐covered rocks collected from the nearshore of oligotrophic Lake Tahoe, CA, to examine (a) baseline seasonal variability in periphyton biomass and metabolism, (b) effects of warming, nutrients, and their interaction on periphyton metabolism, and (c) seasonal variability in these effects on periphyton metabolism. We quantified rates of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) under 2 nutrient treatments (ambient and enriched) and 4 warming treatments (ambient, +3°C, +6°C, and +9°C above ambient). Overall, warming stimulated GPP, NEP, and ER (Q 10 temperature coefficients of 1.6, 1.4, and 2 respectively), with stronger effects during colder months. Warming also stimulated metabolic rates in the absence of nutrient additions. Short‐term nutrient effects were more variable across seasons and alternated between depressing or stimulating metabolic rates (ranged from a 9%–11% decrease in rates to a 13%–27% increase across seasons). The relative importance of warming and nutrient effects were seasonally dependent as nutrients stimulated metabolic rates more than warming in October, and warming more so than nutrients in February and November. These results indicate that climate‐driven alterations to temperature and nutrient regimes will have important and seasonally explicit consequences for the ecosystem energetics and periphyton community structure of oligotrophic lakes.