Kym L. Butler

Survival of a submerged aquatic weed (Egeria densa) during lake drawdown within mounds of stranded vegetation

Abstract Lake drawdown, or water withdrawal, is often used to control invasive submerged macrophytes; however, regrowth of the target species often occurs rapidly. A mechanism proposed to explain such unsatisfactory results is that as water recedes and submerged aquatic weeds become exposed to the air, mounds of stranded vegetation form on the dewatered lakebed. These mounds may insulate underlying weeds, creating an environment protected from desiccation and frost. This study tests this mechanism by comparing the viability of the submerged aquatic weed Egeria densa Planch. from within mounds to the viability on the surface of these mounds during a winter drawdown of Lake Mulwala, Australia. After 22 days of exposure, no stems on the surface of weed mounds were viable, but 22% of stems on the bottom of mounds and 76% of crowns under the mounds were viable. Further, after 34 days, 12% of stems and 32% of crowns collected from the bottom of the weed mounds were still viable. We conclude that, in areas of lakes that are exposed during drawdowns, regeneration from in situ stem fragments and crowns following refilling are an important potential source of reestablishment for E. densa.

Survival of cabomba (Cabomba caroliniana) during lake drawdown within mounds of stranded vegetation

Abstract Control of submerged aquatic vegetation during drawdown is known to be hindered by mounds of stranded vegetation that protect underlying stems from desiccation. This study investigated the viability of stranded cabomba (Cabomba caroliniana) during a winter drawdown of Lake Benalla, Victoria, Australia. The viability of stems and crowns, determined by their ability to generate new shoots, was related to substrate exposure status, location in lake, mound height, and soil moisture, at 34 and 69 days after exposure (DAE). Stem viability at 34 and 69 DAE was most closely associated with exposure status, where ∼20% of stems under mounds on relatively dry substrate were viable, compared to ∼95% of stems under mounds on saturated substrate, and 92% of unexposed stems. Stem viability was similar when collected 34 and 69 DAE and was similar under naturally occurring mounds of differing thickness. In contrast, viability of exposed crowns was usually 40% or less, compared to 72% for unexposed crowns, and their viability was not influenced by substrate moisture. Instead, their viability was influenced by location in the lake and was proportional to mound thickness. Like stems, viability of crowns did not decrease over the duration of the drawdown. We concluded that for drawdown to be effective in minimizing viable stems, the substrate below them needs to dry out. Even then, viable crowns may be a source of recolonization.

Using Plant Growth Regulators to Limit Herbicide-Induced Stem Fragmentation of Aquatic Alligatorweed (Alternanthera philoxeroides)

Abstract Alligatorweed is subject to an eradication program in Victoria, Australia. In aquatic situations, the herbicides glyphosate and metsulfuron are used. Alligatorweed has been shown to break up soon after the application of these herbicides, resulting in the production of many stem fragments that are viable and capable of downstream colonization, compromising the effectiveness of the eradication program. This paper reports on an experiment to investigate the usefulness of commercially available plant growth regulators (PGRs) in reducing the number of viable propagules produced post-herbicide application. Three herbicide treatments (no herbicide, glyphosate, and metsulfuron) and four PGR treatments (no PGR, aviglycine [AVG], naphthalene acetic acid [NAA], and 2,4-D) were investigated in a factorial experiment. Chemicals were applied to alligatorweed growing in separate aquaria, the resulting stem fragments were collected and counted, and a subset was tested for viability. There was no evidence of PGRs having any effect on the total number of viable stem fragments produced. However, AVG reduced the total number of fragments produced. PGRs in combination with herbicide treatment had an antagonistic effect on the efficacy of the herbicides. PGRs increased belowground biomass of alligatorweed, as well as the number of apical growing tips present. Results indicate that although PGRs, particularly AVG, may be of benefit in reducing the number of alligatorweed propagules produced post-herbicide application, at the application rates tested here there would be no benefit from incorporating them into herbicide control programs for alligatorweed. Nomenclature: Glyphosate; metsulfuron; aviglycine; naphthalene acetic acid; 2,4-D; alligatorweed, Alternanthera philoxeroides (Mart.) Griseb. ALRPH.

Sediment Facilitates Microbial Degradation of The Herbicides Endothall Monoamine Salt and Endothall Dipotassium Salt in an Aquatic Environment

Endothall dipotassium salt and monoamine salt are herbicide formulations used for controlling submerged aquatic macrophytes and algae in aquatic ecosystems. Microbial activity is the primary degradation pathway for endothall. To better understand what influences endothall degradation, we conducted a mesocosm experiment to (1) evaluate the effects of different water and sediment sources on degradation, and (2) determine if degradation was faster in the presence of a microbial community previously exposed to endothall. Endothall residues were determined with LC-MS at intervals to 21 days after endothall application. Two endothall isomers were detected. Isomer-1 was abundant in both endothall formulations, while isomer-2 was only abundant in the monoamine endothall formulation and was more persistent. Degradation did not occur in the absence of sediment. In the presence of sediment, degradation of isomer-1 began after a lag phase of 5–11 days and was almost complete by 14 days. Onset of degradation occurred 2–4 days sooner when the microbial population was previously exposed to endothall. We provide direct evidence that the presence and characteristics of sediment are of key importance in the degradation of endothall in an aquatic environment, and that monoamine endothall has two separate isomers that have different degradation characteristics.