Long-term responses of aquatic macrophyte diversity and community structure to variation in nitrogen loading

In a chain of small lakes in southeastern Michigan, USA, macrophyte community dynamics have shown a strong relationship to variations in nitrogen (nitrate and ammonia) loading that appear to correlate with variations in regional annual precipitation. We examined the community structures of two of these lakes 9 years after a drought in 1987–1988 which had temporarily reduced nutrient loading in the system, and during which the macrophytes in normally more eutrophic Shoe Lake had shifted from a Ceratophyllum-dominated community of low diversity to a more diverse community similar to that in less productive East Graham Lake downstream. With a return to the normal precipitation rates and associated nitrogen loading, the macrophytes in Shoe Lake returned to a community dominated by the non-rooted species Ceratophyllum demersum. However, overall species diversity remained relatively high in Shoe Lake after the drought event.