Dwilette G. McFarland

Interrelationships between the growth of Hydrilla verticillata (L.f.) Royle and sediment nutrient availability

Abstract On sediment planted with Hydrilla verticillata (L.f.) Royle compared with control (unplanted) sediment we measured >90% and >30% reductions in concentrations of exchangeable N and extractable P following two consecutive 6-week periods of growth. Diminished N availability, owing to prior plant uptake, resulted in a 30% reduction in Hydrilla growth. Concomitant increases (>30%) in the concentration of exchangeable K suggest that this element, obtained via foliar uptake from overlying water, may be exchanged by roots of Hydrilla for ammonium in sediment. We postulate that under N-limiting conditions, K translocation in this species from shoots to roots potentially increases the availability of sediment N. An important implication of these findings is that under some circumstances the growth of Hydrilla may be self-limiting.

Investigations of the Availability and Survival of Submersed Aquatic Vegetation Propagules in the Tidal Potomac River

The establishment of submersed aquatic vegetation (SAV) at unvegetated sites in the freshwater tidal Potomac River was limited primarily by factors other than propagule availability. For two years, traps were used to quantify the amount of plant material reaching three unvegetated sites over the growing season. The calculated flux values provided a gross estimate of the flux of propagules that could potentially survive if other site factors were suitable. The mean flux ofHydrilla verticillata and all other species (≥0.01 gdw m−2 d−1) appeared sufficient to favor the establishment of vegetation, particularly considering the high viability (70–100%) of whole plants and fragments under controlled conditions. However, median water clarity values (i.e., for light attenuation, Secchi depth, total suspended solids, and chlorophylla) were below SAV restoration goals at all unvegetated sites. Additionally, sediments from unvegetated sites showed a potential for nitrogen limitation of the growth ofH. verticillata. Our findings support the hypothesis that in the tidal Potomac River, water clarity and nutrient (especially nitrogen) levels in sediment are key to plant community establishment.