Management of submerged aquatic weeds by pond liming

Abstract

The aquatic weed infestations are proliferating at an alarming rate resulting in various adverse effects such as the water quality degradation, water flow disruption, reduced water storage capacity, and the establishment of habitats for vectors. Though physical or mechanical measures can be adopted for their control, it is not feasible due to practical difficulty or cost and drudgery involved. There is no registered herbicide for effectively managing aquatic macrophytes. Hence, a study entitled “Management of submerged aquatic weeds by pond liming” was conducted at the Department of Agronomy, College of Agriculture, Vellanikkara, Kerala Agricultural University, during 2022-23. The objective was to assess the efficiency of different liming materials against three common submerged aquatic weeds (Cabomba furcata, Lymnophila heterophylla, and Hydrilla verticillata), and its effects on water and sediment quality. The impact of different liming doses on phytotoxicity, chlorophyll degradation, water, and sediment quality over time was analysed through ANOVA, and the significant differences between treatments were studied using TUKEY's test. Correlation and regression studies on percentage control, phytotoxicity, chlorophyll degradation, water, and sediment quality were conducted to elucidate the trends over time. The experiment consisted of three tank studies and one field study. The tank studies were conducted to standardize the doses of various liming materials. The treatments in the first tank study were three different liming materials (calcite, quicklime, and dolomite) at varying doses of 50, 100, 200, 400, and 600 mg L-1. As these doses were found ineffective, in the second tank study, treatments included higher concentrations (0.8, 1.0, 1.2, and 1.4 g L-1) of calcite, quicklime, and dolomite. Only CaO was found effective in the second tank study. Hence varied doses of CaO @ 2, 4, 6, 8, and 10 g L-1 were included in the third tank study and higher doses were found more effective. In field study, varying doses of quicklime @ 2, 4, and 6 g L-1 was tested in lined irrigation channels of Kole lands infested with limnophila during May 2023.The total quantity of CaO to be applied was arrived at based on quantity of water in the waterbody. From the first and second tank studies, it was revealed that, calcite and dolomite were not effective in controlling the weeds whereas quicklime at 1.0 g L 1 and above was effective. However, there was a noticeable influence on pH and EC of water when calcite and dolomite was used. Conversely, the higher dose of quicklime resulted in considerable effect on both water and sediment quality. From the third experiment, the efficacy of quicklime on weeds was evident across all liming doses (2, 4, 6, 8, and 10 g L-1). Notable impacts on chlorophyll content and phytotoxicity were observed from 7 days after application. More than 50 per cent reduction in chlorophyll content was noted at higher doses after 14 days of liming. By 21 days, a substantial decrease from 0.98 mg g-1 to 0.07 mg g-1 was observed in cabomba and from 1.65 mg g-1 to 0.07 mg g-1 in hydrilla, achieving 85 per cent control. Similarly, in limnophila, chlorophyll content reduced from 0.83 mg g-1 to 0.15 mg g-1 with a 75 per cent control. In the field experiment, when applied in lined channels, quicklime applied @ 6 g L-1 of water in the channel resulted in complete weed control (100 %), while 4 g L-1 resulted in 50 per cent control only. The water quality parameters like pH, EC, carbonates, bicarbonates, alkalinity, total hardness, calcium, and Chemical Oxygen Demand (COD) were positively correlated with increasing doses of quicklime. However, all parameters except COD were in the acceptable range by 30 days after liming. pH values were in the range of 9.27-9.71 by 30 days at higher doses. Alkalinity, total hardness, carbonates, bicarbonates, and calcium increased by 15 days after application and thereby, a slight reduction was observed. Bicarbonates ranged from 2.83 meq L-1 to 9.0 meq L-1. Alkalinity was within the permissible limit (600 mg L-1) and ranged from 53.33 mg L-1 to 266.66 mg L-1 in treated tanks. In all the treatments COD was above the acceptable limit of 1 mg L-1. By 30 days after liming, nutrient content in both water and sediment parameters showed a negative correlation with the increasing dose of quicklime. In field study pH was within a narrow range of 6.47 to 6.87 by 21 days. The pH, EC, organic carbon, and calcium content of sediment were positively correlated with doses of quicklime. Whereas the available N, P, K, magnesium, iron, and manganese were negatively correlated. Organic carbon content linearly increased with increasing quicklime concentration in all the treatments and was in the medium range (0.50 % to 0.75 %). The current study revealed that the quicklime can effectively control submerged aquatic weeds with no regrowth. In tank study, chlorophyll degradation progressed with time and complete chlorophyll degradation was observed by 21 days after liming in higher doses starting from 6 g L-1 and by 30 days complete control was observed even at lower doses. However, in the field study, the lower doses of 2 and 4 g L-1 were less effective and the higher dose of 6 g L-1 resulted in complete chlorophyll degradation and death of weeds by 21 days. When quicklime was applied, there was an immediate hike in water quality parameters and over time, followed by a gradual decline and reached the acceptable range by 21 days after liming. Similarly, no adverse effects on aquatic fauna such as fishes and snails were observed. Hence, the study indicates the possibility of CaO application as an eco friendly and effective means for managing submerged freshwater aquatic weeds. However, the size and nature of the waterbody, weed density and volume of water contained are the three important factors which decides the practical feasibility of the technology. The present technology has to be extended to ponds and crop lands for a comprehensive investigation and validation of its effectiveness in managing submerged aquatic weeds and algae.