A comparison of survey methods to evaluate macrophyte index of biotic integrity performance in Minnesota lakes

Aquatic macrophytes shape trophic web dynamics, provide food and refuge for macroinvertebrates and fish, and increase nutrient retention, sediment stabilization, and water clarity. Macrophytes are well-suited as indicators of ecological health because they are immobile, relatively easy to sample and identify, and respond to anthropogenic disturbance on an ecological time scale. Aquatic plant monitoring programs can provide valuable information to water resource managers, especially in conjunction with macrophyte-based indices of biotic integrity (IBI). However, there are several current sampling designs and the precision of IBI scores has not been evaluated across different surveys. We evaluated the performance of the Minnesota macrophyte-based IBI for two survey designs; a point intercept (PI) survey and a belt transect (BT) survey. PI surveys are time intensive, especially on large lakes, whereas BT are less time intensive and have been used historically in Minnesota. Our objectives were to compare the PI surveys with BT surveys on the same lakes, and to modify the BT survey (MT survey) to improve information obtained from BT surveys. BT surveys consistently overestimated IBI scores compared to the PI method (t = 6.268, df = 60, p < 0.001). Overall IBI scores calculated from MT surveys differed significantly from PI scores, but on average, MT surveys predicted scores only 3% lower than PI scores. Implementation of the Minnesota macrophyte-based IBI through the adoption of the MT survey approach would improve sampling efficiency and enable widespread documentation of the effects of landscape change, shifts in hydrologic regimes, and other anthropogenic activities on the integrity of lacustrine systems.