A Multi-Value Macrophyte Based Assessment for Lakes in the Twin Cities Metro, Minnesota

Abstract

Macrophytes are an important component of lake ecosystems and approaches to assess their community are needed. Previously published aquatic macrophyte based indices often fail to address invasive species, single-species dominance, and mean rake density. Indices also have a single number output, which may be too general to accurately portray all aspects of the macrophyte community. My study developed a comprehensive multi-value aquatic macrophyte-based assessment for lakes in the Twin Cities Metro area of Minnesota, integrating metrics of invasive species, mean rake density, and species dominance. A database of 438 point-intercept surveys across 131 lakes in the seven-county metro area was used to evaluate existing indices and individual metrics. Among available indices of macrophyte community health I focused on two, Beck’s Index of Biotic Integrity (B-IBI) and the Aquatic Macrophyte Community Index (AMCI). Both were similar and highly correlated, but B-IBI was developed for Minnesota and does not confound invasives species with community health. I then created two additional indices, one for Invasive Species and another for Single-Species Dominance. The Invasive Species Index includes three metrics: frequency of occurrence of invasive macrophytes, mean rake density of curlyleaf pondweed, and mean rake density of Eurasian watermilfoil. The Single Species Dominance Index uses two metrics: the frequency of occurrence of the most abundant species relative to the littoral area vegetated and mean rake density of the most abundant species. The two new indices were used in conjunction with B-IBI, which accurately summarized general macrophyte community health, to create our multi-value assessment of individual lakes. Case studies involving lakes subjected to alum and herbicidal treatments demonstrate the indices' responsiveness to management actions. For instance, invasive species scores and dominance scores decreased in lakes treated with herbicides, reflecting effective invasive species control. Conversely, alum treatments primarily enhanced macrophyte community health without significantly impacting invasive species or dominance scores. My approach allows for the retrospective analysis of management practices, offering insights into the effectiveness of different interventions as well as prospective assessments for management outcomes. Overall, the multi-value assessment framework provides a useful tool for lake managers and regulators, enabling more informed decisions about aquatic macrophyte management and health.