Phenology and Water Quality Impacts of an Invasive Water Chestnut (Trapa bispinsoa Roxb. Var iinumai Nakano) in Northern Virginia, USA and Evaluation of Early Detection/Rapid Response (EDRR) Practices in Its Control



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Species of the genus Trapa, specifically Trapa natans, have plagued the northeastern US, including the tidal Potomac for over 100 years. Much has been learned about the ecology and management of T. natans, during this period. In 2014, a new species of Trapa identified as T. bispinosa Roxb. var iinumai Nakano) was discovered in the tidal Potomac River and in subsequent years it has spread to nearby waterbodies. This species is different from T. natans as it has pink flowers instead of white flowers of T. natans; two horned fruit instead of four horned fruit; and reddish underside of leaves instead of green leaves of T. natans. T. bispinosa (hereafter referred as Trapa) has been identified at over 65 different locations in VA, since 1995. The purpose of this study is to describe the phenology of growth, flowering, and fruit production of T. bispinosa in a group of Fairfax County ponds and to assess its impact on WQ parameters such as temperature, dissolved oxygen (DO), pH, specific conductance (SPC) and turbidity (NTU). The final goal is to evaluate laws, regulations, incentives, knowledge gaps, and to assess the usefulness of Early Detection and Rapid Response (EDRR) as a policy option in its control. A total of 8 Trapa ponds were sampled over the two-year study (2019 and 2020). Some had a history of herbicide treatment and others had no known treatment history. A total of 4 non-Trapa ponds were also sampled in 2020 and WQ was compared with Trapa ponds. In untreated ponds with dense and healthy Trapa, the beds rapidly developed in spring and remained robust until fall. Rosette diameters were bigger in July and August. Trapa flowers, fruit and reproductive phenology were also higher in late summer and early fall. Trapa growth was highly impacted by herbicide applications in treated ponds resulting in more erratic plant development, with less flowers and fruits. Even though Trapa appeared later in treated ponds compared to untreated, the maximum flower and fruit phenology was during the similar months. Herbicide did not completely block the flowering and fruiting capacity of Trapa. All phenological parameters were negatively correlated with DO (mg/L). WQ parameters also varied according to the density of Trapa and were more consistent in untreated ponds compared to treated ponds except pH. DO ((mg/L) and %) was significantly depleted in untreated Trapa ponds compared to treated Trapa ponds and non-Trapa ponds. This indicated that the dense carpet of Trapa was obstructing reaeration and photosynthesis in the underlying layers. pH was low in fall and alkalinity was low in spring in Trapa ponds compared to non-Trapa ponds. In most of the studies, DO (mg/L) was mostly below 5 but above 2 in summer in the most intensely studied untreated pond, VGA-VA where Trapa was healthy, dense and abundant. DO % was above 25 in summer but higher than 40 in spring and rest of the fall months. The short-term hydroponic experiment assessing the effect of low densities of Trapa found that Trapa could sustain higher DO levels in uncrowded conditions due to photosynthesis by underwater leaves. Trapa management is a multijurisdictional issue in VA. Counties, researchers, and volunteers have initiated some management options for its control. EDRR could help manage the distribution and spread of Trapa and efforts are being made to apply EDRR to this situation. This study by providing some basic ecological and phenological information on Trapa growth in both treated and untreated ponds will inform the EDRR process.