Development and application of a lotic ecosystem trophic status index

We used the Matlock Periphytometer (Matlock et al., 1998) to measure in sit u nutrient limitations and trophic status at five stream sires in the Bosqu e River Watershed in north-central Texas during July 1997. Periphytic chlor ophyll a production from the Matlock Periphytometer was also used as an ind icator of baseline primary productivity and of maximum primary productivity (MPP) in response to nutrient enrichment (nitrogen and phosphorus). The me asured MPP was interpreted to represent the rate of periphytic growth when nutrients are plot limiting. We developed a lotic ecosystem trophic status index (LETSI) using the ratio of baseline primary productivity to MPP. Nitr ogen (N) and phosphorus (P) limitations were evaluated using a modified LET SI as the ratio of either N or P enriched growth to MPP. The LETSI is by de finition a functional index, and may provide a classification tool for loti c ecosystem trophic status. Using the LETSI indices, we observed difference s in nutrient limitations in the streams and detected co-limitations of nit rogen and phosphorus at two of the stream sites.