DEVELOPMENT AND APPLICATION OF A LOTIC ECOSYSTEM TROPHIC STATUS INDEX

Abstract

We used the Matlock Periphytometer (Matlock et al., 1998) to measure in situ nutrient limitations and trophicstatus at five stream sites in the Bosque River Watershed in north-central Texas during July 1997. Periphytic chlorophyll aproduction from the Matlock Periphytometer was also used as an indicator of baseline primary productivity and ofmaximum primary productivity (MPP) in response to nutrient enrichment (nitrogen and phosphorus). The measured MPPwas interpreted to represent the rate of periphytic growth when nutrients are not limiting. We developed a lotic ecosystemtrophic status index (LETSI) using the ratio of baseline primary productivity to MPP. Nitrogen (N) and phosphorus (P)limitations were evaluated using a modified LETSI as the ratio of either N or P enriched growth to MPP. The LETSI is bydefinition a functional index, and may provide a classification tool for lotic ecosystem trophic status. Using the LETSIindices, we observed differences in nutrient limitations in the streams and detected co-limitations of nitrogen andphosphorus at two of the stream sites.