Litter breakdown in a stream affected by drainage from a gold mine

Abstract

The objective of the present study was to analyse the effects of drainage water from an abandoned gold mine on the breakdown of Alnus glutinosa leaves mediated by aquatic fungi and invertebrates. We analysed three sites (P = polluted, R = reference, and RP = intermediate) in the winters of 2002 and 2004. The sites had similar pH (6-7) but differed in the concentration of heavy metals (As, 0.4 mg L -1 ; Fe, 4.9 mg L -1 ; Mn and Zn, 1.2 mg L -1 in site P) and PO 4 3- (0.04 mg L -1 in site P). The breakdown of alder leaves was low, both in the absence (fine mesh bags) or presence (coarse mesh bags) of invertebrates. Sites did not differ (ANCOVA) in breakdown rates mediated by aquatic fungi (k = 0.0016 to 0.0080), but sporulation rates were significantly higher at site R in 2002 (up to 2488 spores mg AFDM -1 d -1 ). In the presence of shredding invertebrates, breakdown was significantly lower at the polluted sites in both years (k 0.0191), in spite of similar shredder biomass in all sites. The dominant shredder caddisfly Allogamus ligonifer discriminated between leaves exposed in sites R and RP in 2002 (consumption rates of 0.60 vs. 0.35 mg mg -1 AFDM day -1 ). In situ experiments revealed that shredders from site RP consumed leaves at lower rates than in site R (0.46 vs. 0.35 mg mg -1 AFDM day -1 ). Our results suggest that although pollution was not enough to suppress microbial growth and invertebrate survival, sub-lethal effects affected the ecosystem function, even when interannual variation was prominent.