Aquatic Macrophytes Alter Metabolism and Nutrient Cycling in Lowland Streams

Macrophytes influence the physical, chemical, and biological characteristics of lowland streams, so may be critically important in stream management. We investigated the role of macrophytes in regulating metabolism and nutrient cycling in three lowland, agricultural streams. We measured stream metabolism over the growing season and following experimental macrophyte removal, and used short-term nutrient additions of phosphate (P) and ammonium to assess macrophyte influences on nutrient uptake. Primary production was closely correlated with macrophyte cover across all streams and dates, and decreased greatly with macrophyte removal, whereas ecosystem respiration was not correlated with macrophyte cover and was not altered by macrophyte removal. Phosphate uptake velocity was negatively related to primary production, suggesting that macrophyte activity actually slowed P uptake. Ammonium uptake was not correlated with macrophyte cover or metabolism metrics. Stream nitrate concentrations typically exceeded concentrations of incoming groundwater, suggesting little net nitrate retention in these macrophyte-dominated streams. Phosphorous demand by macrophytes was 10-fold lower than observed uptake rates, indicating that macrophyte P demand was much lower than that of other stream biota. Nitrogen demand by macrophytes was nearly equal to ammonium uptake and was not sufficient to affect the high nitrate flux. These results indicate that macrophytes drive ecosystem metabolism but have limited influence on water column nutrient concentrations because macrophyte demand is much lower than the supply available from the water column. Thus macrophytes in our streams had a large impact on stream trophic state, but offered little potential to influence nutrient removal via management.