Seasonal dynamics of the standard test species Lemna sp. in outdoor microcosms
Authors: Arts Gertie H. P. , van Smeden Jasper , Wolters Marieke F. , Belgers J. Dick M. , Matser Arrienne M. , Hommen Udo , Bruns Eric , Heine Simon , Solga Andreas , Taylor Seamus 
Published: March 2024

Abstract
Lemna L. sp. is a free-floating aquatic macrophyte that plays a key role as a standard test species in aquatic risk assessment for herbicides and other contaminants. Population modeling can be used to extrapolate from laboratory to field conditions. However, there are insufficient data on longer-term seasonal dynamics of this species to evaluate such models. Therefore, several long-term growth experiments were conducted in outdoor microcosms (surface area 0.174 m2). Monitoring parameters included biomass, frond numbers, water parameters, and weather data. Three different datasets were generated: frond numbers and biomass from weekly to monthly destructively sampled microcosms; a year-round dataset of frond numbers from five continuously monitored microcosms; and seasonal growth rates without the effect of density dependence over 1–2 weeks in freshly inoculated microcosms. Lemna sp. reached a maximum of approximately 500 000 fronds m−2 and 190 g dry weight m−2. During the first winter, the microcosms were covered by ice for approximately four weeks, and Lemna sp. populations collapsed. The second winter was warmer, without any ice cover, and Lemna sp. populations maintained high abundance throughout the winter. Dry weight per frond was not constant throughout the year but was highest in autumn and winter. Growth rates without density dependence under outdoor environmental conditions reached 0.29 day−1 for frond number, 0.43 day−1 for fresh weight, and 0.39 day−1 for dry weight. In linear regressions, these growth rates were best explained by water temperature. For the populations continuously monitored throughout a year, the nitrogen-to-phosphorus ratio best explained the growth rate of frond numbers. This study yielded a relevant dataset for testing and refining Lemna population models used in chemical risk assessment as well as for managing ecosystems and combating the effects of eutrophication. Integr Environ Assess Manag 2024;20:1625–1638. © 2024 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).
Key Points
Growth rates of Lemna sp. in the field can reach values close to those required in standard laboratory tests designed for high exponential growth but that are affected by light, temperature, nutrient availability, and density dependence. Water temperature was found to be a major driver in the short-term experiments without density dependence, and the nitrogen-to-phosphorus ratio was the factor that best explained the growth rate of Lemna sp. in continuously monitored microcosms. Dry-weight-to-frond ratios of Lemna sp. were highest in autumn and winter, probably the result of the storage of reserves; these ratios are not constant, as assumed in the Lemna sp. model, but vary over the seasons. This study generated ecologically relevant, long-term data series for testing and refining and/or as input parameters for Lemna sp. population models to be applied in the higher tiers of the risk assessment framework for plant protection products.