The impact of environmental factors on the distribution pattern of aquatic macrophytes in a middle-sized Mediterranean stream

Abstract

We studied the aquatic macrophyte assemblage structure along a Mediterranean lotic ecosystem from the upper reaches to the most downstream areas. We examined how environmental characteristics affect the composition and abundance of macrophyte communities spatially and temporally. Field survey data were collected from the Acheron river catchment, between 2005 and 2007. The identification of plant assemblages and the hierarchical structure of each group of species were analyzed using hierarchical classification (Bray–Curtis) and Indicator Species Analysis (ISA). The relationships between macrophyte community composition and water quality were examined using Redundancy Analysis (RDA) while the temporal variability was assessed using Canonical Variate Analysis (CVA). A total of 81 aquatic macrophyte taxa were recorded and Bray–Curtis classification identified 6 different vegetation groups. ISA results produced bryophyte and pteridophyte indicator species for the fast flowing sites near river springs while lower parts were dominated by Phragmites australis, Potamogeton pectinatus and macroalgae species. In the mid-river sections 4 different vegetation groups were identified: (i) Scirpus holoschoenus–Polygonum salicifolium–Plantago major occupied fast flowing waters and riffle habitat, (ii) Arundo donax–Scirpus lacustris–Fontinalis antipyretica located at urban sites, (iii) Brachythecium rutabulum–Pellia endiviifolia–Equisetum telmateia located at shaded sites with low nutrient concentration; and (iv) hydrophytes and helophytes which were found to be dominant in Acherons river tributaries and were associated with slower flowing water, low water clarity and high nitrate and ammonia nitrogen concentrations.Life form analysis indicated that mosses and liverworts (bryophytes) dominate the upper part of the river basin while emergent plants (helophytes) were abundant both in upper and middle reaches. Monte Carlo permutation testing revealed that NO3–N, NH4–N, pH, conductivity, water temperature, mean water velocity are strongly correlated with aquatic macrophyte distribution. Canonical Variates Analysis (CVA) and Canonical Correspondence Analysis (CCA's) showed a strong seasonal signal with nitrogen nutrients (nitrite, nitrate, and ammonium), dissolved oxygen, pH and water temperature all significant discriminating variables. Nitrogen appeared to be the most important nutrient since it was found to be significant as nitrate, nitrite and ammonium forms in all seasons.