Patterns and Controls of Nitrogen in Tallgrass Prairie Streams

A descriptive and experimental approach was used to study patterns of nitrogen concentrations in tallgrass prairie streams in Kansas. Nitrate concentrations varied depending on the year, season, time of day, stream flow conditions, source of water, and location where the sample was taken. Mean nitrate concentrations during the dormant season decreased from 1982 to 1984, corresponding to above—normal precipitation during this same period. Nitrate concentrations decreased from third—order (grass/shrub riparian vegetation) to fourth—order channels (gallery forests). Nitrate, organic N, and total N were higher in a third—order intermittent stream than in a third—order perennial stream. Water from seeps and tributaries had higher nitrate concentrations than stream water; however, streams and tributaries had higher organic N than seeps. Maximum nitrate concentrations during storm flows were directly related to the frequency of storm events. The activity of terrestrial vegetation influenced nitrate concentrations within streams, seeps, and tributaries; nitrate concentrations were always higher during periods of vegetation dormancy. Aquatic biota influenced both spatial patterns (downstream decrease in nitrate concentrations and increase in organic N) and diel patterns of nitrate concentration (night > day). Uptake rates of nitrate by biota were greater during the day than during the night as determined by experimental addition of nitrate. Nitrogen (N) and/or phosphorus (P) limitations of algal growth were tested experimentally using nutrient—diffusing substrata (clay saucers filled with enriched agar) and enrichment of artificial channels. Addition of N and P in combination increased algal biomass (chlorophyll a and ash—free dry mass) over controls or single addition of N or P. Thus, algal biomass was N and P limited; however, individual taxa responded to specific N and/or P concentrations or atomic N:P ratio.