Impacts of stream acidification on litter breakdown: implications for assessing ecosystem functioning

Abstract

Summary1. Scientific understanding of acidification in aquatic ecosystems relies on effective assessment, which at present is mostly limited to chemical and sometimes structural biological variables. Effects on ecosystem functioning are, in contrast, largely neglected. Litter breakdown is a potentially useful, highly integrative and crucial process that could enhance such assessment programmes. 2. Breakdown rates of beech Fagus sylvatica leaves were determined in 25 woodland headwater streams along an acidification gradient in the Vosges Mountains, France. Additional data relating to micro-organisms (microbial respiration, fungal biomass and degree of conditioning measured as leaf palatability) and macroinvertebrates (shredder diversity, abundance and biomass) associated with decomposing leaves were collected to elucidate the mechanisms underlying leaf breakdown. 3. Breakdown rates varied more than 20-fold between the most acidified and circumneutral sites ( k = 0·0002-0·0055 day). Stream water alkalinity and total Al concentration together accounted for 88% of the variation in litter breakdown rates among streams. Microbial factors associated with decaying leaves, particularly microbial respiration, declined with increasing stream acidity and were significantly related to Ca 2+ and total Al concentrations. 4. Total abundance, biomass and richness of leaf-shredding invertebrates associated with decomposing leaves were not related to stream acidity. However, the abundance and biomass of the amphipod Gammarus fossarum , an acid-sensitive and particularly efficient leaf-shredder, showed a strong positive relationship with leaf breakdown rate. Gammarus abundance and microbial respiration together accounted for 85% of the variation in litter breakdown rates among streams. 5. Synthesis and applications . These results indicate that leaf-litter breakdown responds strongly to stream acidification, with both microbial decomposers and invertebrate detritivores markedly affected. Measuring leaf breakdown rate may be developed into a simple, powerful and low-cost tool for assessing a critical component of ecosystem functioning. We advocate further investigation of this approach for the routine biomonitoring of freshwaters affected by, or recovering from, other anthropogenic stresses.

Influence of episodic acidification on leaf breakdown in neighboring streams of the western Adirondacks, USA

“…Shredder density did not differ between the circumneutral stream (S6) and two episodically acidic streams (S5 and S10) in summer 2005 even though one acid-tolerant shredder, Neumoridae, was greater in S5 than in S6. The lack of response of shredders to acidification is not consistent with the findings of other studies (Allard and Moreau 1986;Mulholland et al 1987;Griffith and Perry 1993;Dangles and Gue´rold 1998, Dangles et al 2004, Pretty et al 2005. However, our study streams lacked a key shredder, Peltoperla sp., which is an important component of shredder assemblages in other streams of the eastern United States (e.g., Mulholland et al 1987;Griffith and Perry 1993).…”

“…We found no differences in beech breakdown among streams in summer, and beech breakdown was greatest at S5 in autumn, which contradicted the findings of Dangles et al for Fagus spp. (Dangles andGue´rold 1998, 2001;Dangles and Chauvet 2003;Dangles et al 2004). They measured the breakdown of European beech (Fagus sylvatica) rather than American beech (F. grandifolia) as was used in our study, and the duration of their studies was much longer (4150 days) than our study (42-49 days).…”

“…Therefore, leaf litter processing (i.e., leaf breakdown) integrates the actions of biological communities over time and can be used as an indicator of stream ecosystem health (Gessner et al 1999;Gessner and Chauvet 2002;Dangles et al 2004;Young et al 2008). Leaf breakdown has been shown to respond to several anthropogenic disturbances, including land-use change (e.g., Sponseller and Benfield 2001;Hagen et al 2006;Paul et al 2006), excess nutrients (e.g., Meyer and Johnson 1983;Suberkropp and Chauvet 1995;Rosemond et al 2002;Gulis and Suberkropp 2003), organic pollution (Pascoal et al 2001(Pascoal et al , 2003, and toxic chemicals (e.g., Niyogi et al 2001;Chaffin et al 2005).…”

“…This decrease in breakdown has been linked to reduced activity of microbes (Chamier 1986;Griffith and Perry 1994;Griffith et al 1995), macroinvertebrate shredders (Griffith and Perry 1993;Gue´rold 1998, 2001;Pretty et al 2005), or both (Allard and Moreau 1986;Mulholland et al 1987;Dangles et al 2004). Dangles et al (2004) tested whether breakdown of leaf litter could be used to functionally assess the effects of stream acidification. They measured breakdown of Fagus sylvatica in several streams of the Vosges Mountains in northeastern France that differed in catchment geology.…”

Kinetics of the aerobic decomposition of Talauma ovata and Saccharum officinarum

“…The acidity presents negative effects on the decomposition (WEBSTER; BENFIELD, 1986), reducing the microbial metabolism, the richness and abundance of invertebrates and making the leaf breakdown substantially slower, as demonstrated by numerous studies in acid natural systems (e.g. DANGLES; GUÉROLD, 1998;DANGLES et al, 2004;SUBERKROPP, 1995). On the other hand, Suberkropp (2001) analyzed leaf breakdown in a circumneutral stream and in a basic one (pH 6.7 and 8.0, respectively) and observed higher rates of decomposition and fungal production in the basic stream.…”