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|Title:||Investigations of the production, transport, and survival of monoecious Hydrilla propagules in the tidal Potomac River|
|Authors:||Aquatic Plant Control Research Program (U.S.)|
McFarland, Dwilette G.
Barko, John W.
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: The availability of viable propagules of Hydrilla verticillata (L.f.) Caspary in the tidal Potomac River was examined in a series of studies conducted in 1994. The studies were performed over time frames selected to coincide with the seasonal production of different propagule types. Five sites were chosen based on observations made during the previous (1993) growing season, indicating that two of the sites were vegetated - the Greenway Flats (GF) site supporting Myriophyllum spicatum L. and Hydrilla , and the Dempsey Dumpster (DD) site, Vallisneria americana Michx. - while the three remaining sites, DM-4R (DM), Pohick Bay (PB), and Hunting Creek (HC), were unvegetated. All five sites were sampled in March for overwintered tubers and turions. Nearly all of the propagules collected (99 percent, 𝑛 = 131) were found at the GF site. Maximum propagule densities (66 tubers and 157 turions m⁻²) at this location occurred between 1.0- and 1.5-m sampling depths. Under laboratory conditions, germination was 78 percent for tubers (𝑛 = 49) and 60 percent for turions (𝑛 = 81) and was closely related to propagule fresh weight. Measurements of propagule transport from May through September confirmed that submersed macrophyte propagules (predominately Hydrilla fragments) were reaching the three unvegetated sites. Mean rates of transport for Hydrilla were 1.3, 5.6, and 37 fragments m⁻² day⁻¹, with total fresh weights of 0.58, 1.21, and 13.9 g m⁻² day⁻¹ at sites HC, PB , and DM, respectively. A strong seasonal trend was noted in the transport of Hydrilla fragments, with peak numbers and fresh weights occurring during senescence in late September. In Hydrilla fragments tested in the laboratory from the three unvegetated sites, the overall viability for the season was approximately 70 percent (𝑛 = 641). Viability peaked at 90 percent (𝑛 = 98) in early June, generally coinciding with the period of rapid increases in length and biomass of Hydrilla plants in the field. Over the remainder of the summer, viability percentages gradually declined, reaching a minimum of 55 percent (𝑛 = 203) by early fall. Laboratory studies of Hydrilla fragments (apical stems) left floating in the river for 0, 1, 2, and 4 weeks showed that all fragments were viable after each treatment. Increases in biomass after planting were greatest in fragments that had floated for 2 weeks, probably due to their having the highest carbohydrate levels and roots for immediate uptake of sediment nutrients. Fragments that had floated for 4 weeks were in relatively poor condition due to breakage and other damage and consequently produced the least biomass after planting. Viability studies revealed that live fragments of Hydrilla were being deposited at the unvegetated sites; however, these propagules were largely unsuccessful in establishing new colonies. Possible limiting factors are considered based on collective experimental results, including potential effects of nitrogen and phosphorus in sediments limiting propagule growth.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Technical Report|
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