Bryophytes can play an important role in key ecosystem processes and represent potential candidates as bio- indicators for environmental monitoring programmes. Nitrate (NO3 −) pollution poses a growing threat to both aquatic and terrestrial ecosystems, potentially leading to imbalances in nutrient levels and altering the chemical composition of organisms, thereby impacting ecosystem function. However, the specific effects of NO3 − pollution on the elemental and isotopic composition of aquatic and semi-aquatic bryophytes remain uncertain. In this study, we examined the influence of NO3 − pollution from spring water on the elemental composition of aquatic and semi-aquatic (hygrophytic) bryophyte species and their respective water sources. Our investigation encompassed diverse land use, lithology, and climate conditions to identify suitable bryophyte species as bio- indicators of NO3 − pollution. We observed higher NO3 − concentrations in spring water from intensively farmed and urban areas compared to natural and extensively farmed areas (e.g., pastures). These higher concentrations were positively correlated with the nitrogen (N) content and $δ$15N isotope ratio in bryophytes. However, spring water NO3 − pollution did not significantly affect the overall chemical composition of the water sources, except for Cl−, Cr, and Zn. Our findings highlight Apopellia endiviifolia and Oxyrrhynchium speciosum as promising candidate species for bioindication of aquatic NO3 − pollution, due to their $δ$15N sensitivity to increasing NO3 −, i.e., they respond to variations in the ratio of $δ$15N isotopes in their environment. The identification of these species will assist land managers in effectively monitoring NO3 − pollution in freshwater systems, thereby addressing public health concerns and supporting wildlife conservation priorities.

Mediterranean spring ecosystems are unique habitats at the interface between surface water and groundwater. These ecosystems support a remarkable array of biodiversity and provide important ecological functions and ecosystem services. Spring ecosystems are influenced by abiotic, biotic, and anthropogenic factors such as the lithology of their draining aquifers, their climate, and the land use of their recharge area, all of which affect the water chemistry of the aquifer and the spring discharges. One of the most relevant characteristics of spring ecosystems is the temporal stability of environmental conditions, including physicochemical features of the spring water, across seasons and years. This stability allows a wide range of species to benefit from these ecosystems (particularly during dry periods), fostering an unusually high number of endemic species. However, global change poses important threats to these freshwater ecosystems. Changes in temperature, evapotranspiration, and precipitation patterns can alter the water balance and chemistry of spring water. Eutrophication due to agricultural practices and emergent pollutants, such as pharmaceuticals, personal care products, and pesticides, is also a growing concern for the preservation of spring biodiversity. Here, we provide a synthesis of the main characteristics and functioning of Mediterranean spring ecosystems. We then describe their ecological value and biodiversity patterns and highlight the main risks these ecosystems face. Moreover, we identify existing knowledge gaps to guide future research in order to fully uncover the hidden biodiversity within these habitats and understand the main drivers that govern them. Finally, we provide a brief summary of recommended actions that should be taken to effectively manage and preserve Mediterranean spring ecosystems for future generations. Even though studies on Mediterranean spring ecosystems are still scarce, our review shows there are sufficient data to conclude that their future viability as functional ecosystems is under severe threat.