Long‐term dynamics of macrophyte dominance and growth‐form types in two north‐west German lowland streams

Abstract

Summary Continuous multiyear data sets can help to detect long‐term trends and allow predictions with respect to environmental and biological variables. While the use of taxon‐based approaches for biomonitoring of watercourses is well established, little information exists concerning long‐term variability of dominance and growth form composition of aquatic macrophyte communities. We analysed patterns in dominance and growth forms of macrophytes over a period of 21 years in relation to changes in water chemistry, disturbance regime and extreme weather conditions in two reference streams in the north‐west German lowlands. Rank–abundance curves of annual observations indicated five core species. The pattern of abundance resembled a broken‐stick rather than a log‐normal distribution. The core species, which reached both the highest absolute and average cover values, were Sparganium emersum, Elodea canadensis, Callitriche hamulata, Potamogeton natans and Ranunculus peltatus. An analysis of change in dominance showed that in 67.5% of all observed cases, the same species became dominant in the subsequent year. Sparganium emersum (80.6%) and E. canadensis (68.8%) were the most successful species in retaining their relative dominance. Calculation of transition probabilities showed a strong attractor function of states dominated by Sparganium and Elodea. However, dominance patterns broke down over time and after 20 years, only one of the sites still showed the same dominance type. On average, dominance was preserved for around 4 years. Reaches were mostly dominated by six growth forms, namely vallisnerids, potamids, peplids, nymphaeids, myriophyllids and graminids. In each reach, two, three or five growth forms shared the highest cover percentages. An analysis of traits of successfully established and dominant species identified three strategies: a facultative wintergreen growth habit in combination with effective means of vegetative reproduction; strong maintenance traits (in particular rhizomes); short‐life span in combination with seed and turion production. Strong oscillations of growth form types were observed. Six growth form clusters were defined, up to four of which may occur in one reach. Most frequent clusters were one characterised by strong vallisnerid dominance and another characterised by dominant potamids with peplids and myriophyllids. Both clusters showed the highest frequencies, durations and transition probabilities from other clusters. On average, growth form clusters persisted for 4.5 years. An analysis of the influence of environmental factors on growth form dynamics showed that chemical and physical parameters distinguish the sample sites but cannot explain the observed changes. We conclude that dynamics of species dominance as well as the occurrence of growth form assemblages are not controlled by physicochemical environmental factors such as nutrient composition at the scale of observation in this type of river. Instead, stochastic interactions between species characteristics and single disturbance events or weather extremes must be assumed. Implications for nature conservation, biomonitoring and reconstruction of reference sites are discussed. Continuous multiyear sampling is essential to understand the temporal behaviour of the system.