Print PageHydrilla verticillata, a rooted submersed aquatic plant native to Southeast Asia, has spread throughout the southern US, continuing northward and westward. Hydrilla, one of the world’s most troublesome weeds, has been particularly successful in invading large, multipurpose, reservoirs – often occurring in large monospecific expanses. Man-made systems are susceptible to invasion because they generally lack competing native plants and often contain fertile sediments that have accumulated over many years. While hydrilla is quite efficient at nutrient uptake, it is not so well adapted for nutrient retention and much of its accumulated store is lost during winter senescence. This profligate mode of growth should ultimately lead to hydrilla’s replacement by more conservative species.
Hydrilla often invades, rapidly disperses throughout the littoral zone, grows to great excess, and then declines over a period of time. One possible explanation for this pattern is that accumulated nutrients, which fuel excessive early growth, become depleted and subsequent growth is curtailed. Understanding the nutritional ecology of hydrilla should enable better management of infestations and restoration of more desirable (nutrient retentive) native aquatic plants.
Because nitrogen has been shown to be the most likely limiting nutrient, we studied the influence of sediment (and water) nitrogen supply on growth and nutrient uptake of hydrilla and determined “critical” (limiting) shoot nitrogen cencentration as a diagnostic indicator of plant nutritional status.
Results/Conclusions
In greenhouse tank experiments, hydrilla growth was unresponsive to additions of nitrogen to either sediment or water, indicating that even ambient (low) levels of nitrogen were adequate to support problem-level growth. Shoot nitrogen concentrations did decrease to a critical level under the lowest levels of supply, but this did not affect biomass production, indicating that other factor(s) limited growth.
In an additional experiment, we show that availability of inorganic carbon limited hydrilla’s ability to convert accumulated nitrogen into shoot biomass. Given adequate inorganic carbon, hydrilla readily exploits either sediment- or water-supplied nitrogen to develop a canopy. Finally, in a pond experiment simulating a new infestation in which inorganic carbon should not have been limiting, growth of hydrilla was, yet again, largely unaffected by sediment nitrogen.
While nitrogen supply should ultimately limit biomass development in hydrilla, efficient uptake mechanisms, a stoloniferous growth form, and a relatively low tissue nitrogen requirement enable this species to develop problem levels of biomass even under conditions of low nitrogen availability.
