Conflicting management goals: Manatees and invasive competitors inhibit restoration of a native macrophyte

Vallisneria americana is a native macrophyte in freshwater and oligohaline ecosystems, often forming meadows that significantly affect ecosystem carbon and nutrient cycling and provide structural habitat. Vallisneria has declined in abundance in several Florida lakes, rivers, and estuaries. We conducted an 11-month field experiment in Kings Bay, Florida, USA, to determine factors that might affect successful transplantation of Vallisneria and to guide potential future restoration efforts by water resource management agencies. To determine the effects of herbivores and other primary producers on Vallisneria production, we conducted a 2 x 2 factorial experiment at three sites in which we allowed or denied: (1) relatively large herbivores (>3 cm) and (2) other primary producers (e.g., Myriophyllum spicatum, Hydrilla verticillata, Lyngbya sp.) access to 1.5 x 1.5-m transplanted plots of Vallisneria. Within one month, Vallisneria disappeared from 80% of herbivore-access plots due primarily to consumption by manatees. Vallisneria density was reduced a variable amount (0-50%) in response to competitors, due to site-specific variation in natural abundance of other primary producers (at two sites, we observed extensive colonization by other primary producers and a strong treatment effect). Most of this competitive effect was attributable to Eurasian watermilfoil (Myriophyllum spicatum); after pooling Vallisneria density data across sites, we observed a negative exponential relationship between shoot density of Vallisneria and stem density of Myriophyllum for every sampling date. We observed variable recruitment of Vallisneria transplants into larger size classes among sites, but not between treatments, and large, established plants grew at similar rates whether in monospecific or mixed stands, with evidence for reduced mass-specific growth in dense stands. Hence, a probable mechanism by which Myriophyllum reduces Vallisneria occurs via space limitation, and reduction in Vallisneria densities and/or recruitment and growth of new shoots. We conclude that two different management goals (restoration of Vallisneria and protection of manatees) are in conflict and must be simultaneously considered to devise effective and sustainable ecosystem management scenarios. Due to the highly managed nature of freshwater systems worldwide, the limitations of extrapolating experimental results and the importance of site-specific pilot experiments should also be recognized.