Elevated temperature may intensify the positive effects of nutrients on microbial decomposition in streams

Abstract

Summary Climate change scenarios predict an increase in global temperature and alterations in precipitation regimes, which may change nutrient concentrations in waterbodies. In forested streams, decomposition of allochthonous organic matter is a key ecosystem process that is affected by the quality of plant litter entering the streams and several environmental factors, including nutrient concentrations, whose interactive effects are difficult to predict. We examined the concomitant effects of increased temperature, concentration of inorganic nutrients in stream water and litter quality on leaf decomposition and activity of microbial decomposers. Leaves of alder (Alnus glutinosa) and oak (Quercus robur), representative of high and low initial N content, respectively, were immersed in a stream (NW Portugal) to allow microbial colonisation and then were exposed in microcosms to increasing concentrations of N‐NO3 (0.09–5 mg L−1; six levels) and P‐PO4 (0.003–0.3 mg L−1; three levels) alone or in all possible combinations. One set of microcosms was kept at 12 °C, a temperature typically found in Iberian streams in autumn, and the other set at 18 °C to simulate a warming scenario. Nitrogen immobilisation was higher in alder than in oak leaves, and increased with temperature and N concentration in stream water for both leaf species. Leaf decomposition, fungal biomass accrual and reproduction were not affected by P concentration, but overall microbial activity increased asymptotically (Michaelis–Menten kinetics) with N concentration. Increased temperature led to an increase in maximum activity of fungal decomposers and to a decrease in N concentration needed to achieve it, especially in alder leaves. Under the predicted warming scenario, leaf decomposition may become faster in streams with lower nutrient levels, especially those receiving high‐quality leaf litter.

Aquatic hyphomycetes, benthic macroinvertebrates and leaf litter decomposition in streams naturally differing in riparian vegetation

“…Our results suggest that aquatic hyphomycete reproductive output at the stream level may depend more on the availability of organic resources than on dissolved inorganic nutrients. This may be due to the high needs for carbon during conidial production and to nutrient demands by fungi being met at relatively low concentrations of dissolved inorganic nutrients (Rosemond et al 2002;Ferreira et al 2006b;Gulis et al 2006;Fernandes et al 2014). …”

Spring stimulates leaf decomposition in moderately eutrophic streams

“…Under controlled conditions, water temperature affects leaf decomposition rates, by enhancing microbial activity on leaves (Ferreira and Chauvet 2011;Fernandes et al 2014) and stimulates leaf consumption by invertebrate shredders (González and Graça 2003;Friberg et al 2009). In fact, correlative studies have found a positive relationship between water temperature and decomposition rates of leaves incubated along latitudinal (Boyero et al 2011) or geothermal gradients (Friberg et al 2009).…”

Ecosystem structure and function in two branches of an eastern Minnesota, USA, trout stream

“…One might expect a move towards a fish community more dominated by warmwater species (Buisson et al 2013) and a decrease in density in macroinvertebrate communities (Li et al 2012) with a shift towards smaller and shorter-lived species (Lawrence et al 2010). In addition, an increase in GPP (Yvon-Durocher et al 2010), periphyton growth and leaf decomposition rates (Fernandes et al 2014, Mora-Gomez et al 2015) might be expected. Our results gave mixed results concerning these predictions.…”