Impacts of temperature variation on duckweed population growth and distribution in a changing climate

Abstract

Understanding the impacts of climate change on aquatic plants involves examining how temperature fluctuation patterns influence their temperature-dependent vital rates and distribution. Duckweeds, small aquatic plants with both economic significance and ecological concern, can pose challenges due to overgrowth and the spread of invasive species. The impact of climate-induced temperature changes on aquatic plants remains poorly understood, as many studies use constant conditions that do not account for natural variability in temperature. Research focused on increased average temperatures has shown general ectotherm responses tied to geographic location, such as enhanced growth in temperate regions. However, when temperature fluctuations are considered, responses differ from those under constant conditions due to nonlinear and asymmetrical thermal performance. Increased autocorrelation, with prolonged sequences of unusually high or low temperatures, can affect population growth rates, while nighttime warming alters diel temperature variation and potentially influences time-sensitive processes like photosynthesis and respiration. This thesis investigates the thermal performance and distribution of duckweed species under varying temperature regimes associated with climate change, incorporating both controlled experiments and predictive modeling. The second chapter uses a Maxent species distribution model to predict the potential range expansion of Landoltia punctata (dotted duckweed), an invasive, herbicide-resistant species. Habitat suitability is modeled under current and future climate scenarios, using satellite-derived water temperature data and constraining model features to match the shape of thermal performance curves obtained from laboratory experiments. Results indicate high suitability for this species in Western Europe and Southern Canada, with the Great Lakes region becoming increasingly suitable in the future due to climate warming. These projections underscore the importance of climate-informed management strategies to mitigate the ecological impact of invasive species. The third chapter investigates how diel temperature variability and climate change affect the reproduction of Lemna minor (common duckweed) during spring and summer. Experimental results highlight the importance of temperature variance as opposed to the timing of warming. While increased mean spring temperatures enhance duckweed performance, reduced temperature variance during high summer temperatures in regions such as Canada helps mitigate the negative impacts of otherwise excessively hot fluctuating conditions. These findings emphasize the varying effects of climate change on duckweed's thermal performance across different seasons. The fourth chapter examines the effects of temperature autocorrelation on both common and dotted duckweed reproduction and survival. Experiments show that strongly autocorrelated sequences result in mortality due to heat stress when hot temperature sequences begin with elevated heat. In contrast, autocorrelation has limited impacts under cooler average conditions, likely due to slower physiological responses. These findings align with broader predictions of increased extinction risks for ectotherms under persistent and extreme temperature patterns caused by climate change. This work is a step towards a more realistic understanding of aquatic plant responses to climate change by considering thermal performance responses, diverse temperature fluctuation patterns, and water temperatures. Our results can be used in population dynamics models to make more realistic predictions of climate change responses. The experimental and modeling findings in this thesis advance our understanding of aquatic plant responses to climate change and support the development of informed strategies to manage their ecological impacts and sustainable production in a warming world.