The influence of nitrate pollution on elemental and isotopic composition of aquatic and semi-aquatic bryophytes

Bryophytes can play an important role in key ecosystem processes and represent potential candidates as bioindicators 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 bioindicators 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 δ¹⁵N 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 δ¹⁵N sensitivity to increasing NO₃⁻, i.e., they respond to variations in the ratio of δ¹⁵N 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.