Seasonal dynamics of the macrophyte test species Myriophyllum spicatum over two years in experimental ditches for population modeling application in risk assessment
Authors: Arts Gertie H. P. , Smeden Jasper , Wolters Marieke F. , Belgers J. Dick M. , Matser Arrienne M. , Hommen Udo , Bruns Eric , Heine Simon , Solga Andreas , Taylor Seamus 
Published: November 2021

Abstract
Myriophyllum spicatum is a sediment-rooted, aquatic macrophyte growing submerged, with a wide geographical distribution and high ecological relevance in freshwater ecosystems. It is used in testing and risk assessment for pesticides in water and sediment. Population models enable effects measured under laboratory conditions to be extrapolated to effects expected in the field with time-variable environmental factors including exposure. These models are a promising tool in higher-tier risk assessments. However, there is a lack of data on the seasonal dynamics of M. spicatum, which is needed to test model predictions of typical population dynamics in the field. To generate such data, a two-year study was set up in outdoor experimental systems from May 2017 to May 2019. The growth of M. spicatum was monitored in 0.2025 m2 plant baskets installed in an experimental ditch. Parameters monitored included biomass (fresh weight [FW] and dry weight [DW]), shoot length, seasonal short-term growth rates of shoots, relevant environmental parameters, and weather data. The results showed a clear seasonal pattern of biomass and shoot length and their variability. M. spicatum reached a maximum total shoot length (TSL) of 279 m m−2 and a maximum standing crop above-ground DW of 262 g m−2. Periodical growth rates reached up to 0.072, 0.095, and 0.085 day−1 for total length, FW, and DW, respectively. Multivariate regression revealed that pH (as a surrogate for the availability of carbon species) and water temperature could explain a significant proportion of the variability in M. spicatum growth rates (p &lt; 0.05). This study has provided an ecologically relevant data set on seasonal population dynamics representative of shallow freshwater ecosystems, which can be used to test and refine population models for use in chemical risk assessment and ecosystem management. Integr Environ Assess Manag 2022;18:1375–1386. © 2021 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).
KEY POINTS
This study has generated a time-series of seasonal dynamics for the growth of M. spicatum over two years under environmental conditions found in temperate regions to be used to develop and test population models for Myriophyllum spicatum. Myriophyllum spicatum showed a clear seasonal pattern of biomass and shoot length and of their variability (increasing in summer and decreasing in winter). Multiple regression modeling revealed that water temperature and pH (the latter as surrogate for the available carbon species) explained a significant part of the variability in Myriophyllum growth rates (p-values &lt; 0.05). Over the first four experimental months in summer, exponential functions yielded a better fit for the growth of Myriophyllum spicatum than linear growth functions.