Cardinale reply

Over the last 20 years, 59 experiments have quantified how the richness of plants and algae influence concentrations of inorganic nitrogen in soil or water. Of these, 86% have shown that the concentration of nitrogen decreases as biodiversity increases-by an average of 48%. The primary contribution of my study was to identify a biological mechanism that is likely to explain these biodiversity effects. Using stream mesocosms, I showed that the impacts of algal diversity on nitrogen dynamics are controlled by niche partitioning-a long presumed, but rarely demonstrated mechanism. Baulch, Stanley and Bernhardt have questioned whether my findings have any implications for managing water quality in-streams, as I suggested. They argue that nitrogen assimilation by algae cannot influence long-term nitrogen retention due to high turnover of algal biomass and rapid recycling of nitrogen, and they suggest that the only permanent loss of nitrogen from a stream is via denitrification.