Leaf-litter leachate and light interactively enhance accrual of stream biofilms

We examined the individual and interactive effects of light and leaf-litter-derived leachate from red alder (Alnus rubra) on growth and elemental composition of stream biofilm. We manipulated the quantity of alder leaf litter used to produce leachate (control: no addition; low: 26.7 g d-1; high: 106.7 g d^-1) and light (shaded/ unshaded) in experimental channels to assess responses of biofilms colonizing tiles. The greatest changes in streamwater chemistry were observed in channels receiving the high leachate treatment, where streamwater ammonium increased by ∼1.4×, phosphate increased by ∼2×, and dissolved organic carbon was ∼1.5× higher than the control. Effects of light, leachate and their interaction significantly increased biofilm ash-free dry mass and chlorophyll-a. We observed weaker, non-significant effects of leachate in shaded conditions, suggesting that effects of leachate on biofilms were mostly autotrophic. Results indicate that the individual effect of leaf-litter leachates on heterotrophic components of biofilm is minor, but light interactively stimulates the effects of leaf-litter leachates on autotrophic components of biofilms in low-order streams.