We used the Matlock Periphytometer (Matlock et al., 1998) to measure in situ nutrient limitations and trophic
status at five stream sites in the Bosque River Watershed in north-central Texas during July 1997. Periphytic chlorophyll a
production from the Matlock Periphytometer was also used as an indicator of baseline primary productivity and of
maximum primary productivity (MPP) in response to nutrient enrichment (nitrogen and phosphorus). The measured MPP
was interpreted to represent the rate of periphytic growth when nutrients are not limiting. We developed a lotic ecosystem
trophic 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 by
definition a functional index, and may provide a classification tool for lotic ecosystem trophic status. Using the LETSI
indices, we observed differences in nutrient limitations in the streams and detected co-limitations of nitrogen and
phosphorus at two of the stream sites.