Influence of streambed substrate type and watershed properties on seston algal abundance

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] In this age of growing water quality issues, it is crucial to identify factors which impact stream water chemistry. In the Gulf of Mexico a growing area of low oxygen in the water column has resulted in the absence of shrimp and fish within the effected region. The dead zone has been linked to increased nitrogen concentrations in the Mississippi River as a result of fertilizer application in the Midwest. With an understanding of factors controlling nutrients such as nitrogen and phosphorus, and algae (both suspended in the stream and attached to the channel bed), predictive models can be constructed and used as indicators of stream health. Some potential sources of influence on suspended algae are land use (crop, pasture or forest) within the watershed, total watershed area, and stability of the stream bed material. Application of fertilizer to cropped land, and in some cases pasture as well, is susceptible to removal via runoff from rain and delivery to the local stream. Large watershed areas increase the volume of stream flow and residence times, allowing algae more time to grow in suspension. Stability of stream bed material refers to whether the substrate is disturbed by runoff producing rain events. Stable substrates such as bedrock, boulders, and gravel tend to grow lush coats of attached algae in nutrient rich streams. Unstable substrates like sand and mud are disturbed on a regular basis and seldom display luxuriant growths of attached algae. One hypothesis is that suspended algal cells in stream water were sloughed from attached mats, which would result in lower quantities of suspended algae in streams with unstable substrates. This study combined the results from research on 105 Missouri streams in order to develop an understanding of the extent to which land use, watershed area and substrate stability influence nutrients and suspended algae in streams. Streams from the Plains (northern Missouri), the Border region (along the Missouri and Mississippi Rivers) and the Ozark Highlands (southern Missouri) were included. The Plains region has more crop land than the other regions and streams have sand or mud beds. The Highlands are mostly forested and the streams have stable substrates. Streams in the Border region also generally have stable substrates, with more crop land than the Highlands. Both percent of the watershed area in crop and total watershed area were important factors in describing nitrogen, phosphorus and suspended algae concentrations. For watersheds with less than 10% crop, phosphorus and algae in streams increased strongly with %crop. For streams with more than 10% crop, watershed area was the most important factor. In all cases nitrogen was strongly predicted by %crop alone. The link between a stream and land use in its watershed is clearly demonstrated by this strong response of nutrients and algae to percent crop. Stream bed substrate, however, had no influence on suspended algal concentrations, even when only small streams with short residence times were considered. This finding suggests attached algae may not be the primary source of suspended algae, particularly in streams with unstable substrates, and that growth in the flowing water is a determining factor. Further research into the taxa of suspended algae present in these stream types is the next step in this line of research. The EPA had directed states to establish criteria to be used to determine whether nutrient levels in streams are acceptable or must be remediated. The results of this study and others like it provide the scientific basis for regulator's decisions.--From public.pdf