Impaired Leaf Litter Processing in Acidified Streams

Abstract

Anthropogenic acidification in headwater streams is known to affect microbial assemblages involved in leaf litter breakdown. Far less is known about its potential effects on microbial enzyme activities. To assess the effects of acidification on microbial activities associated with decaying leaves, a 70-day litter bag experiment was conducted in headwater streams at six sites across an acidification gradient. The results revealed that microbial leaf decomposition was strongly and negatively correlated with total Al concentrations (r = -0.99, p < 0.001) and positively correlated with Ca(2+) concentrations (r = 0.94, p = 0.005) and pH (r = 0.93, p = 0.008). Denaturing gradient gel electrophoresis analyses showed that microbial assemblages differed between non-impacted and impacted sites, whereas fungal biomass associated with decaying leaves was unaffected. The nutrient content of leaf detritus and ecoenzymatic activities of carbon (C), nitrogen (N) and phosphorus (P) acquisition revealed that N acquisition was unaltered, while P acquisition was significantly reduced across the acidification gradient. The P content of leaf litter was negatively correlated with total Al concentrations (r = -0.94, p < 0.01) and positively correlated with decomposition rates (r = 0.95, p < 0.01). This potential P limitation of microbial decomposers in impacted sites was confirmed by the particularly high turnover activity for phosphatase and imbalanced ratios between the ecoenzymatic activities of C and P acquisition. The toxic form of Al has well-known direct effects on aquatic biota under acidic conditions, but in this study, Al was found to also potentially affect microbially mediated leaf processing by interfering with the P cycle. These effects may in turn have repercussions on higher trophic levels and whole ecosystem functioning.

A global meta‐analysis of exotic versus native leaf decay in stream ecosystems

“…Boyero et al (2016) found that low quality leaves increased breakdown rates where invertebrates tend to drive decomposition rates (Boyero et al, 2011) and we observed a similar trend of lower quality leaves linked to faster decay rates at coarse mesh sizes permitting invertebrate-mediated decay. Although pH and conductivity have been shown to affect microbial decay rates and litter decay processes (Boyero et al, 2016;Clivot et al, 2013;Jenkins & Suberkropp, 1995;Mulholland, Palumbo, Elwood, & Rosemond, 1987;Webster & Benfield, 1986;Young et al, 2008), our model did not indicate that they were important for explaining large scale differences between native and exotic leaf litter decay rates. Alternatively, we also suggest it is possible that low quality leaves may become conditioned by a more palatable buffet of bacteria and fungi prior to consumption by macroinvertebrates, and thus become more attractive to invertebrate shredders than leaves of initially higher elemental quality (Gessner et al, 2010;Grac ßa, 2001;Grac ßa & Cressa, 2010;Lecerf et al, 2005).…”

“…Alternatively, we also suggest it is possible that low quality leaves may become conditioned by a more palatable buffet of bacteria and fungi prior to consumption by macroinvertebrates, and thus become more attractive to invertebrate shredders than leaves of initially higher elemental quality (Gessner et al, 2010;Grac ßa, 2001;Grac ßa & Cressa, 2010;Lecerf et al, 2005). Although pH and conductivity have been shown to affect microbial decay rates and litter decay processes (Boyero et al, 2016;Clivot et al, 2013;Jenkins & Suberkropp, 1995;Mulholland, Palumbo, Elwood, & Rosemond, 1987;Webster & Benfield, 1986;Young et al, 2008), our model did not indicate that they were important for explaining large scale differences between native and exotic leaf litter decay rates. These explanatory variables…”

Assessing impacts of unconventional natural gas extraction on microbial communities in headwater stream ecosystems in Northwestern Pennsylvania

“…Although not well studied in headwater streams, this finding is supported by previous research, which showed acidic pH was associated with lower bacterial diversity in soils (Baker et al, 1996; Hammarstrom et al, 2005). While this is the first study to assess the impacts of fracking on bacterial communities in aquatic environments, numerous other studies have demonstrated the detrimental impacts a variety of anthropogenic activities have on bacterial diversity in the environment (Wassel and Mills, 1983; Clivot et al, 2013; Sun et al, 2013). However, it should be noted that other environmental factors not measured in this study could also be contributing to decreased observed alpha diversities.…”

“…The first biotic response to environmental perturbations can be seen at the lowest trophic levels, as microbial communities can readily respond to changes in their surrounding abiotic environments. Aquatic microbial community structure changes in response to biogeochemical alterations from anthropogenic sources, including agricultural, industrial, and recreational activities (Wassel and Mills, 1983; Clivot et al, 2013; Sun et al, 2013). Despite recent increases in prevalence, the impact of unconventional natural gas extraction, referred to as hydraulic fracturing or fracking, on headwater stream ecosystems has yet to be evaluated.…”

Multi‐stressor impacts on fungal diversity and ecosystem functions in streams: natural vs. anthropogenic stress

“…In the naturally acidic sites, drainage was associated with increased metal concentrations and decreased water pH, whereas siltation impacts were not evident. A number of studies have shown that fungal assemblages respond more to changes in water chemistry than in the physical habitat (Niyogi et al 2003, Clivot et al 2013). Lack of a strong effect of drainage on water chemistry could thus explain why fungal assemblages and decomposition rates in the circumneutral streams did not respond to drainage.…”