Ecohydrology engineering provides a valuable framework for addressing emerging environmental challenges by integrating ecological and environmental engineering principles. In this study, we discuss the potential of parsimonious, physically-based ecohydrological models through the lens of three case studies: sustainable irrigation, urban heat island mitigation via green roofs, and mangrove restoration for climate change mitigation. First, we investigate sustainable irrigation strategies, illustrating the trade-offs between water conservation and soil salinization. This highlights the delicate balance required to optimize crop yield while mitigating soil degradation. Second, we explore the role of green roofs in urban heat island mitigation, showing how vegetation and water dynamics on rooftops can enhance latent heat flux, thereby potentially reducing urban temperatures and improving building energy efficiency. Lastly, we assess the climate mitigation potential of mangrove restoration, accounting for the impacts of salinization and sea-level rise. We demonstrate that carbon sequestration in mangrove ecosystems may be strongly limited by productivity reduction due to salinity and reduced area availability under sea-level rise. These examples highlight the value of simple ecohydrological models in providing critical insights into sustainable environmental management. Ecohydrological engineering, therefore, offers promising avenues for developing innovative solutions that leverage the intricate connections between water and biota to address emerging challenges.

Phytoplankton stoichiometry modulates the interaction between carbon, nitrogen and phosphorus cycles, yet most biogeochemical models represent phytoplankton C:N:P as constants. This simplification has been linked to Earth System Model (ESM) biases and potential misrepresentation of biogeochemical responses to climate change. Here we integrate key elements of the Adaptive Trait Optimization Model (ATOM) for phytoplankton stoichiometry with the Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) ocean biogeochemical model. Within a series of global ocean-ice-ecosystem retrospective simulations, ATOM-COBALT reproduced observations of particulate organic matter N:P, and compared to static N:P, exhibited reduced phytoplankton P-limitation, enhanced N-fixation, and increased low-latitude export, leading to improved consistency with observations. Two mechanisms together drove these patterns: the growth hypothesis and frugal P-utilization during scarcity. The addition of translation compensation- differential temperature dependencies of photosynthetic relative to biosynthetic processes- led to relatively modest strengthening of N:P variations and biogeochemical responses relative to growth-plus-frugality. Comparison of the multi-mechanism model herein against frugality-only models suggest that both can capture observed N:P patterns and produce qualitatively similar biogeochemical effects. There are, however, quantitative response differences and different responses across N:P mechanisms are expected under climate change- with the growth rate mechanism adding a distinct biogeochemical footprint in highly-productive low-latitude regions. These results suggest that variable phytoplankton N:P makes some biogeochemical processes resilient to environmental changes, and support using dynamic N:P formulations with the ocean biogeochemical component of next generation of ESMs.