Abstract: Aquatic plants, or macrophytes, play many prominent roles in river ecosystems, from supporting local biodiversity and intercepting nutrients, sediment, and pollutants, to shading out other primary producers, growing invasively, and interfering with human water use. Because of these varied environmental impacts, it is often useful to model macrophyte growth computationally. However, riverine macrophytes have received relatively little modeling attention compared with plants in many other aquatic habitats such as lakes and estuaries, and models developed in other habitats may not adequately describe riverine macrophyte growth due to the unique geophysical conditions in rivers. To address this, we performed a systematic literature review of riverine macrophyte growth models, and developed a new model as a case study. Our objectives were to compile existing approaches and highlight productive future directions for model development, create a conceptual framework to guide formulations of macrophyte growth, and use our case study as an example, producing a generalizable, broadly applicable modeling tool in the process.
We identified 12 published models or model families simulating riverine macrophyte growth over time. We found that almost all of them included light availability, nutrient and temperature limitation of photosynthesis, and mortality; however, processes such as dispersal, herbivory, scour, burial, and desiccation were scarce. In our case study, simulating the growth of Podostemum ceratophyllum in the Middle Oconee River in Georgia, we show that the conceptualization of growth, herbivory, and scour can have strong impacts on model outputs. This highlights both the importance of detailed natural history data on study species, and the power of models to help address questions in macrophyte biology. We propose a framework to guide subsequent model formulation of macrophyte growth including four components: the specific nature of growth, macrophyte requirements for growth, external factors affecting growth and survival, and feedback effects of macrophytes on their environment. Finally, we encourage future modelers to incorporate principles of accessibility and modularity to facilitate sharing, refining, and adapting models across systems and research questions.
