Seasonal dynamics of the macrophyte test species Myriophyllum spicatum over two years in experimental ditches for population modelling application in risk assessment

Summary

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 2-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 and dry weight), 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. Myriophyllum spicatum reached a maximum total shoot length of 125.3 m.m-2 and a maximum standing crop aboveground dry weight of 262 g.m-2. Periodical growth rates reached up to 0.072.d-1, 0.095.d-1 and 0.085.d-1 for total length, fresh weight and dry weight, respectively. Multivariate regression revealed that pH (as a surrogate for the availability of carbon species) and water temperature could explain a significant part of the variability in Myriophyllum spicatum growth rates (p-values < 0.05). This study has delivered an ecologically relevant dataset 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.