The Relative Effects of Willow Invasion, Willow Control and Hydrology on Wetland Zooplankton Assemblages

Abstract

Few studies have examined zooplankton assemblages associated with grey willow (Salix cinerea) invasions in wetlands. Our aim was to quantitatively examine zooplankton composition among S. cinerea stands within the South Taupō Wetland, New Zealand, to determine whether these assemblages are affected by willow growth and willow control treatment using the herbicide metsulfuron (C14H15N5O6S). Alternatively, we examined whether wetland hydrology had an over-riding influence. Sampling was performed on three occasions (late-summer, mid-winter, and early-summer). Using Multidimensional Scaling and ANOSIM, we found no significant differences in zooplankton composition or environmental variables among native vegetation, live and dead S. cinerea sites, except for a difference in willow canopy density between late summer and winter. However, zooplankton composition differed on either side of a sand bar, suggesting areas separated by this barrier functioned independently. Overall, we found zooplankton communities to be regulated more by wetland hydrology than by willow presence. A limited willow effect was possibly due to the wetland being at an early stage of invasion, representing stand-alone individuals, with a continuous canopy not yet having formed. Alternatively, willows have lesser effects on invertebrates in wetlands than in streams. Ground control treatment of S. cinerea using metsulfuron had no apparent impact.