Dam-induced disruption of hydrochory and macrophyte recovery in the Brisbane River post-flood

Extreme flood events can scour riverbeds and significantly reduce submerged aquatic macrophyte extent in river systems. In regulated systems, large dams disrupt longitudinal connectivity and may constrain post-flood recovery by limiting hydrochorous propagule transport. In 2011, the Brisbane River in south-eastern Queensland Australia, experienced a major flood that severely reduced macrophyte abundance across the whole river system. Macrophyte recovery has been slow downstream of a major dam, Wivenhoe Dam, which has not yet recovered to historical abundances. To understand how the dam affects macrophyte recovery downstream following flood events, we quantified hydrochory and the sediment seed banks upstream and downstream of the dam. Propagule drift was measured monthly over 12 months using drift nets at two upstream and two downstream sites, while sediment seed banks were sampled seasonally at three sites both upstream and downstream. The most upstream site yielded an average of 1.15 ± 1 propagules per two-hour sampling period, declining to 0.58 ± 0.23 at the second most upstream site, with no propagules captured downstream of the dam. Seed bank germination found only two species, native macrophytes Vallisneria nana and Myriophyllum verrucosum, emerging from all upstream samples and no germination from downstream sediment samples. These results indicate that Wivenhoe Dam disrupts hydrochorous dispersal and contributes to limited propagule availability downstream, delaying macrophyte recovery after flood disturbance. Overall, we suggest active restoration strategies are necessary to mitigate the impacts of floods and dams to support macrophyte regeneration post flood events.