While the importance of grasslands in terrestrial silicon (Si) cycling and fluxes to rivers is established, the influence of large grazers has not been considered. Here, we show that hippopotamuses are key actors in the savannah biogeochemical Si cycle. Through a detailed analysis of Si concentrations and stable isotope compositions in multiple ecosystem compartments of a savannah-river continuum, we constrain the processes influencing the Si flux. Hippos transport 0.4 metric tons of Si day⁻¹ by foraging grass on land and directly egesting in the water. As such, they bypass complex retention processes in secondary soil Si pools. By balancing internal processes of dissolution and precipitation in the river sediment, we calculate that hippos affect up to 76% of the total Si flux. This can have a large impact on downstream lake ecosystems, where Si availability directly affects primary production in the diatom-dominated phytoplankton communities.

The response of aquatic plants to abiotic factors is a crucial study topic, because the diversity of aquatic vegetation is strongly related to specific adaptations to a variety of environments. This biodiversity ensures resilience of aquatic communities to new and changing ecological conditions. In running water, hydrodynamic disturbance is one of the key factors in this context. While plant adaptations to resource stress (nutrients, light…) are well documented, adaptations to mechanical stress, particularly flow, are largely unknown. The submerged species Egeria densa was used in an experiment to detect whether the presence or absence of hydrodynamic stress causes plant thigmomorphogenetic responses (i) in terms of plant biogenic silica (BSi), cellulose and lignin concentrations, and (ii) in terms of plant strength. Plant silica concentrations, as well as lignin concentrations were significantly higher in presence of hydrodynamic stress. These physiological changes are accompanied by some significant changes in stem biomechanical traits: stem resistance to tensile forces (breaking force and breaking strength) and stiffness were higher for plants exposed to hydrodynamic stress. We conclude that the response of this aquatic plant species to mechanical stress is likely the explaining factor for a higher capacity to tolerate stress through the production of mechanically hardened shoots.

Climate change is causing the alteration of atmospheric dynamics, leading to extreme precipitation events and floods. On the other hand, landscape modification and increased imperviousness due to urbanization exacerbate the impacts of flooding. In order to become more permeable, cities are increasingly embracing aquatic Nature-based Solutions which, using natural processes, allow for the mitigation of water-related hazards. One of these solutions is floodable parks, where pluvial runoff is conveyed for its temporal storage into, firstly, permanent retention ponds and, eventually, the partial or totality of their surface. Floodable parks are still a novel aquatic Nature-based Solution and have not yet been investigated. In this paper, a systematic review on current floodable parks was performed in order to study (1) the conditions needed for their implementation, (2) their design, and (3) the connection between design and ecosystem services. A subsequent systematic review was performed to understand (4) the processes occurring within the park. With the obtained information, a conceptual model of floodable parks was developed. The results indicate that both the vegetation surrounding the permanent pond of the floodable park and the biodiversity within the pond enhance the performance of this solution and allow potential water reuse. The implementation of floodable parks will therefore facilitate the transformation of urban areas to create sustainable, climate-resilient, and circular cities.