Plant Diversity and Community Composition in Temperate Northeast Atlantic Salt Marshes Are Linked to Nutrient Concentrations

Question: Studies in the northwest Atlantic have shown that nutrient enrichment can severely impact salt marshes, undermining their broad range of high-value ecosystem services. However, biogeographical differences in plant communities may preclude extrapolation of these findings to other regions. The few investigations of such impacts elsewhere, including in Europe, have been strongly localised, restricting generalisations, and have not addressed community-level properties. Location: South and east coasts of Ireland (northeast Atlantic). Methods: We conducted a field survey of vascular plants and soil properties in fifteen salt marshes representing broad environmental gradients but comparable biogeographical settings, to test if salt marsh plant (a) diversity and (b) community composition are related to soil nutrients. Results: Species richness and Shannon diversity were both negatively related to soil NOx-, and Shannon diversity had a hump-shaped relationship to labile P. Multivariate community composition was highly significantly related to NOx-, to a lesser degree to P, and marginally significantly to NH4+. This was most clearly evident in the positive association of the ground cover of an evergreen dwarf shrub, Atriplex portulacoides, and a weaker negative association of a forb, Plantago maritima, with NOx-, indicating competition between these key and functionally contrasting mid-salt-marsh species. The relationship between community composition and P was most evident through the positive association of P with the invasive cordgrass, Spartina anglica. Conclusions: Physiognomic and functional differences between the herb-rich Pl. maritima community and monospecific A. portulacoides stands likely have implications for ecosystem services, as would a continued expansion of S. anglica. Further studies to ascertain causality could determine whether such transitions can be driven by nutrient enrichment.