Species-environmental relationships of aquatic macrophytes in Northern Ireland

Abstract A data-set from 574 lakes in Northern Ireland was used to analyse the individual relationships of the occurrence of aquatic macrophytes to the lake environment. The 32 most frequent species were used in the analyses. The 21 environmental parameters recorded for each lake were divided into five groups; (1) geographical features; (2) lake physical variables; (3) lake-water chemistry summary variables; (4) lake-water major ions; and (5) lake-water nutrients. The species–environmental relationships were analysed using generalised additive models (GAM) and canonical correspondence analysis (CCA). The results were interpreted in terms of expected range, tolerance, and optimum of each species. Predictive models were made for each of the most frequent aquatic macrophytes along individual environmental gradients. The most influential variables are related to local-scale chemical and nutrient composition, both of which are highly correlated with altitude because hardwater, nutrient-rich lakes are restricted to the lowlands. Major ions and nutrients constrain the species into two groups associated with either low or high ionic concentration. The high ionic concentration group of species may be further divided into those associated with calcareous lakes or lakes with high sodium and chloride. Local-scale variation in lake chemistry and the strong environmental responses of the aquatic macrophytes suggest a strong local influence on species composition of different lakes, leading to the interpretation that the occurrence of a species in a lake is predominantly controlled by the catchment use, particularly farming and fertiliser use.