Structural and functional responses to metal-enriched leaf material in small streams

Depositions of metals from vehicle exhaust and industrial activities impact urban watersheds worldwide. The effects on stream ecosystem function are not fully understood. I addressed this question by considering how the structure and function of stream invertebrate assemblages respond to metal-enriched basal resources. We grew alder (Alnus rubra) saplings under three mixed metal treatments corresponding to British Columbia’s interim sediment quality guidelines (ISQG) and probable effect levels (PEL) for copper, cadmium and zinc. Triplicate 5g packs of control (no metals), ISQG, and PEL leaves were deployed in six Vancouver area streams. After four weeks, we sampled leaf mass remaining and invertebrate families. Family richness and density were significantly higher in PEL leaf packs. PEL leaf breakdown was significantly faster, potentially a result of lower percent carbon and, therefore, reduced development of structural compounds. These results could have implications to carbon availability and cycling in metal-polluted watersheds.