Structure and function of food webs in acid mine drainage streams

Acid mine drainage (AMD) is a significant environmental issue worldwide, which often causes severe contamination and marked species losses in receiving streams. However, little is known about how this stress alters food webs and ecosystem function. I conducted a literature review, which revealed that AMD-impacted streams generally had depauperate benthic communities dominated by a few tolerant species and impaired ecosystem processes. Next, using survey and experimental-based approaches, I investigated food web structure and energy flow in these highly stressed streams, which typically have low pH (< 3), high concentrations of dissolved metals (Al, Fe), and substrata coated with metal hydroxide precipitates, on the South Island, New Zealand. Inputs of AMD caused substantial loss of consumers and reduced the overall number of links between species generating small and simplified food webs, with few invertebrates and no fish. Comparative analysis of food webs from a survey of 20 streams with either anthropogenic or natural sources of acidity and metals, indicated that anthropogenic sources had a stronger negative effect on food web properties (size, food chain length, number of links); an effect driven primarily by differences in consumer diversity and diet. However, the presence of fewer trophic levels and reduced trophic diversity (detected using isotopic metrics), were common structural attributes in AMD-impacted webs along a pH gradient, regardless of impact level. Furthermore, complementary dietary analyses of consumer gut contents and stable isotope signatures (δ13C and 15N) confirmed that primary consumers fed generally on basal resources and that there were few predatory interactions, which reflected low densities of small-bodied chironomids. This suggests that food quantity was unlikely to limit primary consumers but that reduced prey availability may be an additional stressor for predators. In these radically re-structured food webs, trophic bottlenecks were generated at the primary consumer level and energy flow to higher consumers was disrupted. However, streams still retained some limited function, including slow leaf litter breakdown, which provided detrital resources and supported the small food webs. Overall, my findings have furthered our understanding of these highly stressed stream ecosystems by providing new insights into interactions among species and trophic levels that structure food webs and enable function.

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