Responses of an emergent macrophyte, Zizania latifolia, to water-level changes in lakes with contrasting hydrological management

Twenty-four lakes associated with the Yangtze floodplain and Huaihe basin, China, with different degrees of disconnection from the river systems, exhibited managed hydrologies ranging from minimally fluctuating reservoir-like lakes, through intermittently fluctuating lakes to those with large, quasi-natural fluctuations in level. We hypothesized that annual water-level fluctuations limit growth and survival of the emergent macrophyte Zizania latifolia. We investigated adaptations to submergence and sought to define the tolerances of Z. latifolia to the amplitude and timing of water-level fluctuations in these types of lake, at different stages in its phenology and life cycle. Shoots from rhizome buds emerged in early spring and reached maximum extension with high water levels in summer. Z. latifolia did not occur in lakes with the highest amplitude (> 5 m) of fluctuation. Height growth in lakes with low amplitude (reservoir-like) was smaller than in lakes with greater amplitude (intermittent to quasi-natural fluctuations), giving the appearance of ‘short’ and ‘tall’ phenotypes. Across all lakes, however, maximum height was linearly related to water depth in June and to annual amplitude of water level, indicating a continuous phenotypic response. Peak biomass was weakly affected by these environmental drivers. Field experiments showed that seedlings tolerated water depths of c. twice their height (0.6 m), and submergence rates similar to their maximum extension growth rate (2 cm.d-1). Sprouting of rhizome buds was unaffected by submergence to a depth of 0.4 m, but then declined with depth. This study reveals the effects of large-scale hydrological engineering on an emergent macrophyte of economic and conservation importance and informs the management of its populations under seasonally fluctuating water-level regimes.