Temperature and nitrogen availability interact to shape N-acquisition pathways and metabolism in stream biofilms

The effects of warming and nutrient enrichment—two drivers of global change—on ecosystems have been studied in isolation for decades. We thus have a limited understanding of how they interact to influence ecosystem metabolism (gross primary production, ecosystem respiration, and net ecosystem production), which supports food webs and influences carbon (C), nitrogen (N), and phosphorus (P) cycling. To better understand stream ecosystem responses to these drivers, we asked three questions: (Q1) Do temperature and nutrients have univariate, additive, or interactive effects on ecosystem metabolism? (Q2) What is the relative effect of dissolved N versus N:P ratios on N-acquisition pathways and how are these dynamics mediated by temperature? (Q3) How do effects of temperature and nutrients on assemblage composition, biomass accumulation, and N sources combine to shape ecosystem metabolism? To answer these questions, we evaluated biofilm response to manipulations of temperature, N and P supply, and N:P ratio in three stream-side channel experiments. (Q1) In our N-limited study system, temperature and N supply had interactive effects on biofilm biomass, composition, N acquisition, and areal rates of ecosystem metabolism; all generally peaked under warm, moderate-N conditions. Biomass accumulation was more important than cellular efficiency in shaping ecosystem responses. (Q2) N uptake and N₂ fixation increased with temperature and were influenced by N supply, not P or N:P ratio. N₂ fixation was inhibited above 3.9 μM N. (Q3) Temperature and N interacted to shape biofilm metabolism by mediating biofilm biomass accumulation, autotroph taxonomic and functional composition, and N-acquisition pathways and rates. Dinitrogen fixers played a role in mediating these interactions; however, it was smaller than expected, potentially due to the relatively small contribution of N₂ fixation to total N acquisition (<30%). Taken together, our results illustrate the complex pathways through which temperature × nutrient interactions influence stream biofilms and ecosystem metabolism. We show that understanding the effects of warming and nutrient enrichment on coupled C and nutrient cycles in stream ecosystems requires consideration of N acquisition, biofilm assemblage composition, and the context-dependent influence of biomass dynamics on ecosystem fluxes.