Coupling energy and elements in a warming world : how temperature shapes biofilm ecosystem structure and function

Abstract

Freshwater ecosystems are key contributors to global fluxes of energy and materials. Within freshwater ecosystems, benthic biofilms (i.e. thin streambed mats of algae, bacteria and detrital matter) act as biogeochemical hotspots, contributing to these important fluxes. Understanding how temperature shapes the structure and function of biofilm communities, and thus the coupling of energy and material fluxes, is important to our ability to predict the effects of climate change. We cultivated stream benthic biofilm communities in experimental streamside channels under a range of warming scenarios (7.5-23.6°C) that maintained natural diel and seasonal temperature variation. We quantified autotrophic community structure, biomass, ecosystem metabolism, stoichiometry, and nutrient uptake. Biological N 2-fixation was quantified as part of a concurrent study (Welter et al. in review). We found that temperature had strong effects on many metrics of ecosystem structure and function. Biofilm communities were dominated by cyanobacteria at all temperatures, which comprised >91% of total biovolume. Temperature had strong positive effects on biofilm biomass (2.8 to 24-fold variation) and net ecosystem productivity (44 to 317-fold variation). Temperate had minimal effects on biofilm stoichiometry; carbon:nitrogen (C:N) was constant across all temperatures, and carbon:phosphorus (C:P) declined slightly with temperature (a product of high C:P at the coldest temperature). Although ammonium uptake increased with temperature (2.8 to 6.8-fold variation), the magnitude of this response was not sufficient to meet total predicted N demand. We found that this shortfall was met by N 2-fixation, particularly at warmer temperatures. In contrast, increases in dissolved SRP uptake across the thermal gradient were sufficient to meet the predicted demand. This study is one of few to isolate the effects of temperature on benthic biofilms, improving our understanding of how climate change may impact freshwater ecosystems.