Grazing impacts on experimentally restored aquatic macrophytes as critical habitat for the threatened Australian...

Aquatic macrophytes, especially Vallisneria nana, provide essential spawning habitat and food resources for the threatened Australian lungfish (Neoceratodus forsteri). The availability of V. nana for lungfish has been dramatically reduced in some areas due to flood disturbances and macrophyte recovery is inhibited by reduced hydrochory due to river fragmentation by dams. Active transplanting of macrophytes may be a feasible restoration strategy to increase abundance of macrophytes, however, in the presence of macro-herbivorous aquatic animals, replanted beds may not persist due to intensive grazing. In this study, we analyzed the effects of environmental conditions on growth and expansion of transplanted V. nana in six 60 m² herbivore exclosures in the mid-Brisbane River in subtropical south-east Queensland. Following establishment, we also quantified the impact of aquatic herbivores on different patch sizes (15 m² or 45 m²) of restored V. nana to test the hypothesis that larger patch sizes may confer greater resistance to grazing using a paired control/treatment design. After initial planting, V. nana grew and spread rapidly throughout the exclosure plots, with mean % cover increasing from 5% to 71% on average and mean leaf length increasing from 20 cm to 54 cm on average over 161 days. Transplanted V. nana cover was significantly positively associated with water depth and finer substrate sizes and leaf length was significantly positively associated with water depth and water velocity but was unrelated to substrate size. Exposure to grazing significantly reduced cover of V. nana, however there was no significant effects of macrophyte patch size, grazing intensity, or environmental factors on changes in V. nana cover. Complete removal of exclosure fencing resulted in near complete loss of macrophytes within 7-22 days at all plots, indicating the macrophyte beds (15 m²-60 m²) were of insufficient size to resist grazing pressure. Our findings suggest that transplanting is a viable restoration method with exclusion of grazing, which is critical for creating resistant beds to disturbances and driving future macrophyte recovery. Further research is required to evaluate the feasibility and efficacy of alternative macrophyte restoration strategies in rivers to foster hydrochory and natural regeneration processes following flood disturbance.