This study aimed to determine the relationships between the abundance of Nitellopsis obtusa, a controversial charophyte, locally red-listed in its native Eurasian range but invasive in North America, and the species diversity of macrophyte stands dominated by N. obtusa. Three lakes of different morphology, productivity and catchment were surveyed in the species native range. In each lake, the species composition and cover of three N. obtusa-dominated stands were determined monthly from spring to autumn and illustrated by the Shannon–Wiener diversity index. Water chemistry supplemented vegetation study. The species diversity turned out to be lake-specific and declined with the increasing share of N. obtusa, which developed mass occurrence in less mineralised and less fertile waters, leaving no space and limiting light and nutrient availability for large and branchy macrophytes. We postulate that this mechanism makes N. obtusa a superior competitor in less fertile waters and seems common to both native and invaded territories, as is the pool of macrophyte species most frequently co-occurring with N. obtusa.
The recovery or reconstruction of aquatic vegetation has recently been reported as a result of water quality improvement after anthropogenic eutrophication. The objectives of this study were: to investigate long-term trends in aquatic vegetation abundance in relation to decreasing water fertility and to present new directions in changes of the submerged aquatic vegetation structure, species richness, and biodiversity in Lake Kuźnickie (western Poland) with the perspective of the last 40 years (1978–2018). Lake Kuźnickie is an example of water quality improvement taking place without any additional reclamation measures, except a reduction in nutrient discharge into the lake from its direct catchment. Currently, the study lake represents a mesotrophic status. The Trophy State Index evidenced a decrease in the lake’s fertility compared to previous decades. The water quality improvement manifests in a significant reduction in the total phosphorus concentration. An analysis of the spatial changes in the phytolittoral evidenced a decrease in rush vegetation between 1978 and 2018 by over 2 ha. In the period 1978–2018, the aquatic vegetation structure in Lake Kuźnickie underwent significant reconstruction. Currently, charophytes play a much greater role in the lake compared to the last 40 years, contributing to the maintenance of the lake’s high water quality. Moreover, the endangered charophyte
Lychnothamnus barbatus
has recovered. Concurrently, however, the biodiversity and species richness of the submerged vegetation has decreased. At present, only four species dominate in the lake, including two charophytes
L. barbatus
,
Nitellopsis obtusa
, and two vascular plants
Ceratophyllum demersum
and
Myriophyllum spicatum
. Over 40 years, Lake Kuźnickie has changed from a eutrophic lake dominated by vascular plants to a mesotrophic lake with a codominant contribution by charophytes. The lake is characterized by good water quality, optimal for the development of aquatic vegetation, especially charophytes.
In a time of rapid environmental changes, identification of the effects of climate warming on charophytes (Characeae, Charophyta) will enable the optimization of conservation measures, especially for extremely rare species. Lychnothamnus barbatus is one of the rarest charophytes worldwide, which has decreased in the number of occupied sites over the last century. However, the recolonization of former sites has been observed in recent years (e.g. Lake Kuźnickie, Poland). The study aimed to analyse the effects of climatic changes and human pressure on the L. barbatus population.
Three 30 cm sediment cores were collected from minimum, average, and maximum depths of L. barbatus occurrence to determine the past vegetation composition. Sediment cores were dated using the radioisotopes lead‐210, caesium‐137, and carbon‐14. A spatial analysis of the lake catchment changes during the last 120 years was also conducted.
The study demonstrated L. barbatus presence in Lake Kuźnickie at the beginning of the 16th century. However, a sharp increase in the proportion of this species in the vegetation community occurred in the 19th century and during climate warming at the end of the Little Ice Age. Factors that significantly influenced the present occurrence of the L. barbatus population included improvement in water quality and the oospore bank deposited in the bottom sediments.
This study is the first palaeoreconstruction in a modern lake dominated by L. barbatus. Based on the history of L. barbatus in Lake Kuźnickie after the end of the Little Ice Age, the positive effect of climate warming on the contemporary recovery of this charophyte is postulated.
The reaction of L. barbatus to climate warming appears to differ from commonly accepted scenarios for aquatic macrophytes because its recovery in the past and at present coincided with increases in air temperature. This research indicated the appropriate management and conservation practices for lakes with L. barbatus populations.
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