Experimental test of abiotic and biotic factors driving restoration success of Vallisneria americana in the Lower Bay of Green Bay

Abstract The eutrophication of aquatic habitats is a primary driver of ecosystem degradation, often culminating in a switch from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid water state. While numerous studies have documented the ecological implications of this switch, subsequent reductions in nutrient and sediment loading do not consistently result in predictable reversals to macrophyte dominance. Re-introduction of rooted aquatic macrophytes at appropriate scales and species combinations may disrupt these negative feedbacks, although our current limited understanding of these complex mechanisms hinders the development of effective, targeted restoration strategies. We evaluated the potential for restoration of Vallisneria americana (wild celery) in the Lower Bay of Green Bay by altering restoration size and co-planting with the emergent species Schoenoplectus acutus (hardstem bulrush). Wild celery survival among all sites exceeded 90% in 2015 and 110% in 2016. However, in contrast to our expectations the effect of restoration size and interspecific facilitation on survival was either marginal or non-significant. Instead, various environmental effects focused largely on the interaction of water depth, substrate characteristics, and the abundance of unrestored floating macrophytes (i.e. Lemna , Ceratophyllum , and Utricularia spp.) drove variability in restoration success, suggesting that future work focus on identifying restoration methods that can withstand a highly dynamic environment. Our results provide insight into the factors continuing to limit the re-establishment of aquatic macrophytes in degraded systems, suggesting a more limited role of water quality and greater role of interspecific competition and propagule limitation or seedling establishment than previously recognized.