Nitrogen limitation of heterotrophic biofilms in boreal streams

Abstract

Summary Nutrient limitation of the biofilm is fundamental to stream ecosystem processes, as microbial activity shapes the biological availability and biogeochemical cycling of carbon and nutrients. We used nutrient‐diffusing substrata (NDS) to investigate heterotrophic nutrient limitation of microbial respiration (MR) across 20 streams draining boreal landscapes in northern Sweden. We also explored variation in microbial biomass and community structure of biofilms that developed on NDS using phospholipid fatty acid (PLFA) biomarkers. Limitation was determined as a significant response of MR and biomass production on cellulose surfaces to enrichment with nitrogen (N), phosphorus (P) or N + P, relative to controls. Microbial respiration was N‐limited, with an average 3.3‐fold increase on N‐amended NDS. Nitrogen limitation decreased, and control rates of MR increased, with greater background concentrations of inorganic N across the sites. In contrast to MR, microbial biomass was primarily N‐limited but was greatest for the N + P NDS. Accordingly, differences in respiratory versus biomass responses to nutrient addition resulted in significantly greater biomass‐specific MR on N‐amended NDS compared to all other treatments. In addition, PLFA biomarkers indicated distinct microbial communities on N and N + P NDS compared to controls and/or P NDS. Greater MR and biomass, and the development of distinct microbial communities, when supplied with inorganic N suggest that factors which alter aquatic N loading during autumn may have important implications for ecosystem processes and the biogeochemistry of boreal streams and rivers. Our findings add to a growing body of evidence that the productivity of Fennoscandian boreal landscapes is constrained by N availability.

Persistent nitrogen limitation of stream biofilm communities along climate gradients in the Arctic

“…These results contrast with studies of nutrient limitation in the North American Arctic, which have emphasized the importance of P at short (Peterson et al., ) and long (Slavik et al., ) temporal scales. However, N limitation has been observed for phytoplankton in subalpine lakes of Arctic Sweden (Bergström et al., ), as well in streams (Burrows et al., ) and lakes (Bergström, Jonsson, & Jansson, ) of boreal Sweden. The persistence of N limitation along our gradient, despite variable climatic and catchment characteristics, is not surprising given low DIN concentrations observed across sites and seasons (average 12 μg DIN/L).…”

“…Photosynthesis in streams is obviously linked to incident light (Hill, Fanta, & Roberts, ), but algal growth may also be limited by inorganic nutrients (Reisinger, Tank, & Dee, ), and constrained by physical disturbance (Biggs, ), flow velocity (Peipoch et al., ), and water temperature (Rasmussen, Baattrup‐Pedersen, Riis, & Friberg, ). Heterotrophic processes share some of these same physical and chemical drivers; in particular, nutrient supply (Burrows et al., ) and thermal regimes (Jankowski, Schindler, & Lisi, ) can strongly influence the activity of stream bacteria and fungi. However, heterotrophs are additionally influenced by the chemical structure and quantity of organic carbon available to fuel metabolism (e.g., Burrows, Laudon, Mckie, & Sponseller, ; del Giorgio & Cole, ), which derives from soils and detritus (Rasilo et al., ), and algae (Scott, Back, Taylor, & King, ).…”

“…Photosynthesis in streams is obviously linked to incident light (Hill, Fanta, & Roberts, 2009), but algal growth may also be limited by inorganic nutrients (Reisinger, Tank, & Dee, 2016), and constrained by physical disturbance (Biggs, 1995), flow velocity (Peipoch et al, 2016), and water temperature (Rasmussen, Baattrup-Pedersen, Riis, & Friberg, 2011). Heterotrophic processes share some of these same physical and chemical drivers; in particular, nutrient supply (Burrows et al, 2015) and thermal regimes (Jankowski, Schindler, & Lisi, 2014) can strongly influence the activity of stream bacteria and fungi.…”

Resource subsidies from adfluvial fishes increase stream productivity

“…Ambient NH 4 concentrations were consistently well below that predicted from excretion equations suggesting that much of the released ammonium is rapidly taken up by river microorganisms. In contrast, the productivity of nutrient poor Boreal rivers is often constrained by N availability (Burrows et al, 2015). Isotopic values for sucker eggs are indicated by the large orange symbol and associated standard deviation error bars.…”

Seasonal resource limitation of heterotrophic biofilms in boreal streams

“…Such seasonality is likely to be particularly important in boreal regions that experience extreme changes in temperature, incident light, and hydrology throughout the year. Despite this likelihood, most research in boreal streams has focused on single seasons; for example, on microbial responses to resource export during the spring flood (e.g., Ågren et al ), or on dynamics during the autumn when rates of biological activity are thought to be elevated (Burrows et al ). By contrast, very little is known about the rates and controls of microbial activity, or even of freshwater ecosystems more generally, during the long and severe winters that characterize high latitude regions (Hampton et al, ; but see Huryn et al, ).…”