Armleuchteralgen

This chapter is a short history of the knowledge about the charophytes in Germany. The first secure mention of a Characeae is in the “Pinax theatri botanici” by Caspar Bauhin (1623). Linnés “Species plantarum” (1753) contains only four species. Until around 1800 most of the presently accepted species were described. But first the work of Alexander Braun brings more clarity into the many difficult forms. In the second half of the 19th century a lot of botanists were familiar with the charophytes and overviews of some regions were published. After this period until about 1960 almost no interest was given to this plants. In the 1960is a new perception started, mostly from a phytosociological point of view. A more intensive floristic, systematic and genetic work begins in the 1980is. In 1984 there was published the first Red data book for this algae. Now the charophytes have a significant meaning in the Habitats- and Water Framework Directive of the European Union.

Charophyceae are a class of macroscopic green algae exhibiting a complex whorlshaped thallus. They are closest related to the land plants. Their systematics is subjected to ongoing tuning based on integrative approaches combining morphology, anatomy, ontogeny, molecular phylogeny, and ecology. Charophyceae are member of the division Streptophyta (Charophyta) together with six other classes including the Embryophyceae, the real land plants. The conception of genera in Charophyceae is widely resolved comprising Chara, Lamprothamnium, Lychnothamnus, Nitellopsis, Nitella, and Tolypella. However, the conception of species is still under discussion searching for a compromise between the micro-species concept, established by A. Braun and W. Migula, and the macro-species concept suggested by R.D. Wood. Evaluating the European flora of Charophyceae, about 50 micro-species and just 20 macro-species exist. The currently used “pluralistic” conception based on assessment of interbreeding, phenology and ecological niches revealed about 40 species in Europe. Here, we summarise the current status of the systematics of the family Characeae. Selected gene sequence analyses were combined with diacritic morphological characteristics. Due to the high morphological variability the traditional species circumscription is not always consistent with genetic analyses. The applicability of the conceptions differed within the genera. Whereas the results in Nitella and Tolypella support the micro-species concept, in Chara the high morphological variability is not reflected by molecular phylogenetic analyses. However, because of the high potential of micro-species for ecological inferences a strict application of the macro-species conception would result in loss of valuable information.

This chapter presents a review of the most significant steps in charophyte evolution and its fossil record, which is amongst the most complete in fossil algae. It provides enough data to document the history of the group from the Silurian until the present. Fossil charophyte remains include mainly their calcified fructifications, i.e. utricles and gyrogonites, but also their fossil thalli. Palaeozoic represents the time when charophytes reached a maximum of diversity in the Bauplan of their fructifications, with the coexistence of three charophyte orders, Sycidiales, Moellerinales and Charales. The extinction of two of these orders at the end of the Palaeozoic led to a single type of fructification, the porocharacean gyrogonite that represents the charalean ancestor of all post-Palaeozoic charophytes. However, this ancestor diverged soon in two evolutionary lines, Polyplacata and Monoplacata, based on the presence of a multicellular vs single basal plate of the gyrogonite. This fundamental difference can be assumed representing two alternative types of female gametogenesis. Charophytes with a composite basal plate, the Polyplacata, developed diverse morphologies at the base of the Mesozoic (Triassic-Jurassic), most of them are traditionally ranged within the porocharaceans, characterised by an apical opening. These polyplacate porocharaceans are considered the ancestors of extant Nitella and Tolypella (sensu stricto), recorded already in the Jurassic and Cretaceous respectively. Monoplacata diversified in several steps, the first in the Upper Jurassic and Lower Cretaceous with the appearance of the utricle-bearing clavatoraceans, and the appearance of the first characean genera with a single basal-plate, including the oldest known Sphaerochara (equivalent to Tolypella sect. Rothia). The Clavatoraceae family dominated the wetlands on the Tethyan islands during the Lower Cretaceous relegating the porocharaceans to brackish environments. At the beginning of the Late Cretaceous (Cenomanian and Turonian) there is a gap in the fossil record. After this gap the monoplacate Characeae underwent an important radiation and expanded with the appearance of many genera including modern Chara, Lamprothamnium, and Lychnothamnus. This radiation occurred in parallel with the extinction of a few last species that remained from clavatoraceans and porocharaceans near the Cretaceous-Tertiary boundary. Genus Nitellopsis is documented since the lowermost Tertiary. Characeans reached their maximum of diversification in the Middle Eocene, with numerous genera whose gyrogonites were quite different from the modern ones. These morphological types regressed during the Late Eocene and the Early Oligocene when a global climatic shift occurred. Afterwards the charophytes were progressively reduced to the morphologies of the seven modern genera. Hence the Neogene to recent flora appears as an impoverished remnant of the flourishing of charophytes in the geological past.

Nomenclature of Characeae is troubled by imprecise application of the nomenclatural laws. Application of nomenclatural laws compels the use of three hitherto rarely used names: (1) Chara subspinosa Rupr. instead of Chara rudis A. Braun, (2) Chara aculeolata Kütz. instead of Chara polyacantha A. Braun, and (3) Chara papillosa Kütz. instead of Chara intermedia A. Braun ex Lange. We refrain from changing the use of Chara aspera Willd. und Chara hispida L. with regard to a proposal to conserve Chara intermedia L. with a new type.

This chapter presents an overview about ontogenesis of charophytes. Knowledge about ontogenesis allows the reader to evaluate the importance of morphological characters in use for determination purposes. It also provides the basis for understanding the evolution of the phylogenetic lineages. In this chapter, the full generation cycle from germination of the oospore until formation of gametangia is described as far as the details are known yet. Irrespective of the great number of very detailed investigations about the formation of almost all parts of the charophyte thallus, which are presented here, there is still a main gap in knowledge. Until recently there is still an uncertainty about the exact place of meiosis, hampering the application of chromosome numbers for taxonomic investigations. On the other hand, many morphological investigations about the development and aberrations in the development of cortication and stipulode development give important hints for exact determination of field and herbar material as well as for physiological investigations. Especially the very conservative pattern of post-mitotic cell development pattern is fascinating character of charophytes, which is worth to be investigated further on.

Charophytes mainly occur in aquatic ecosystems with low nutrient concentrations where they can grow down to deep water. Under eutrophic, turbid conditions, charophytes are severely light-limited because of their small hibernacles and bottom-dwelling growth form and therefore restricted to shallow water. Due to different mechanisms of osmoregulation and turgor regulation, salinity tolerances show major differences among single species. Also mechanisms of carbon assimilation differ substantially among species. Nitella spp. mainly occur in soft water, whereas many species within the genus Chara can efficiently assimilate bicarbonate and are therefore superior competitors in calcium-rich water. While abiotic conditions such as light, salinity and carbon affect occurrence and species composition of charophytes, these plants in turn have a major impact on their abiotic and biotic environment, especially in calcium-rich lakes where they can form dense vegetation. They accumulate and immobilize nutrients, reduce resuspension and enhance sedimentation, thereby improving light availability in the water column and causing conditions that favour their own occurrence. Allelopathic effect against micro-algae has been shown in the laboratory, but the quantitative importance of this mechanism in natural ecosystems is unknown. Several investigations indicate that charophyte vegetation may have a lower refuge function for zooplankton against fish predation than other submerged macrophytes. Charophytes often harbour high macroinvertebrate densities. Their interactions with fish are complex. In charophyte-dominated lakes, waterfowl are favoured by high food availability (plants and macroinvertebrates) combined with high water clarity. Single investigations show contradictory results concerning the grazing pressure of waterfowl on charophytes. Waterfowl have an important function, however, for long-distant transport of charophyte oospores. Single species of charophytes apply far different strategies of reproduction and dispersal. Annual species with high oospore production are typical pioneer plants and mainly found in small, often temporary water bodies. Other species such as Nitellopsis obtusa have a high vegetative reproduction, but form oospores relatively rarely. Such species are restricted to larger, permanent water bodies.

Bioindication is defined as the indication of ecological quality based on composition of organisms of a habitat. The basis forms the knowledge of the relationship between organisms and habitat conditions. The history of bioindication using Charophytes is described. Charophytes have mainly been used for the indication of trophic status. The best way to use Charophytes as bioindicators is on a local scale or for defined water bodies. Communities of organisms are more suitable as bioindicators than single species. Limits of bioindication are demonstrated. Charophytes are actually used for the assessment of ecological status within the WFD and the Habitats Directive.

A compilation of available data on habitat conditions of stoneworts is presented. The focus is on data from Germany. There are different limitations and problems with data interpretation: (i) data limitations of available biological data (e.g. for some taxa like Tolypella, Lychnothamnus and Lamprothamnium there is almost no data available), (ii) data limitations of available physicalchemical data (e.g. there is many data on pH and nutrients , while the data set is limited for other parameters), (iii) extrapolation of findings (e.g. it is difficult to transfer local data to other regions) and (iv) lacking data (e.g. there is almost no information about the fitness of species in relation to habitat parameters).

The results for selected parameters are presented (pH, conductivity, salinity, total hardness, carbonate hardness, total phosphorus, total nitrogen, ammonia nitrogen).

Many Charophytes are euryoecious. Most Charophytes species grow in slightly acid and slightly alkaline water. Nitella spp. mainly occurs in slightly acid or neutral water. Chara spp., Nitellopsis and Tolypella prefer slightly alkaline or neutral water.

Typical for species of brackish water are high conductivity levels. Some of these species are restricted to oligo-euhalin water (Chara baltica, C. canescens, C. horrida, Lamprothamium papulosum, Tolypella nidifica) while species like Chara aspera, C. globularis and C. contraria occur both in freshwater and brackish water.

There are some species which tolerate moderate pollution (C. aspera, C. baltica, C. contraria, C. globularis, C. horrida, C. vulgaris, Nitella flexilis, Nitella mucronata, N. opaca and Nitellopsis obtusa).

In Germany Charophytes are used as bioindicators within the assessment of the Water Frame Work Directive and the Habitats Directive. Recommendations for the adjustment of indicator species data sets based on evaluated physico-chemical data sets are given.

This chapter deals with the stoneworts communities in Germany (class: Charetea F. Fukarek 1961 ex Krausch 1964).

The basis of the stoneworts communities in Germany are the phytosociological reviews by Krausch (1964) and Krause (1969). In total 28 characeen associations were listed in German freshwater systems in the order Nitelletalia flexilis Krause 1969 and in the order Charetalia hispidae Sauer 1937 ex Krausch 1964. In brackish waters of the Baltic Sea and in salt inland waters four characeen associations were found in the alliance Charion canescentis Krausch 1964 (“Halo-Charion” sensu Krausch). Also communities with stoneworts and other algae (Vaucheria dichotoma, Cladophora glomerata) or water plants (Potamogeton, Ruppia, Stratiotes and Zostera spp.) were described.

This chapter describes the threats of charophytes and their habitats and specifies appropriate actions for their conservation and protection. Main reasons for endangering are habitat destruction, water pollution, increase of utilization (e.g. water management, river regulation, disturbances by fishery, recreation activities), drainage, lack of management, rarity, climate change, introduction of foreign plants and inadequate legal protection. Following the latest Red Data Books 81 % of the charophyte-species and 88 % of the charophyte-plant communities in Germany are threatened. An overview about the degrees of endangering of charophytes in other European countries is presented additionally. The legal foundations for protection of charophytes and their habitats in Germany are analysed with special emphasis to European directives (European Habitats Directive, European Water Framework Directive) and international conventions (e.g. Convention on Biological Diversity, Global Strategy for Plant Conservation). Different measures for conservation and protection of charophytes are discussed. The most important aim is the protection of extant populations against damage and their prevention from nutrient loading. Because many charophytes are poor competitors most of their sites (especially small water bodies and ditches) need a special management as periodical remove of vegetation/mud and creation of vegetation-free areas. Other measures discussed are e.g. an improvement or re-establishment of natural dynamics, development of seminatural fish stocks, creation of new waterbodies, implementation of buffer strips, improvement of legal protection and establishment of nature reserves. Alternative concepts (e.g. Important Stonewort Areas, Plant Micro-Reserves) must be verified, too. The continuous decline of charophytes requires urgent measures to stop the loss of species and habitats. Therefore the implementation of a valid national action plan for protection and conservation of charophytes and their habitats in Germany focussing on the most endangered species is proposed. A subdivision into 5 action plans for 15 threatened charophytes is suggested. In addition local action plans must be realized for 6 species with present occurrence in less than five sites. The proposed actions aim especially at an improvement of knowledge of the present and former occurrence of the target species, their ecological demands, the identification of the present national hot spots for conservation of charophytes in Germany, the promotion of public relations and the realization of management measures including transplantation of oospores.

For geological research in Germany, fossil charophytes generally play a rather exotic, but sometimes significant role. This is especially true for the biostratigraphy (dating and correlation of rock successions) within non-marine sediments holding carbonates and marls (for Germany: mainly upper Jurassic to basal Cretaceous and Oligocene to Miocene). More generally, charophytes are often used for a simple first estimation of the depositional environments of sedimentary rocks. Here, charophyte remains (usually quite easily recognizable) simply serve as indicators for non-marine environments (fresh or brackish waters). For most geologists and carbonate specialists, the latter is certainly by far the most important benefit of the fossil remains of this group of algae.

On the contrary, the few findings of charophytes in Carboniferous, Permian and Triassic and Middle Jurassic rocks of Germany are rather insignificant and – in an international context – of minor importance.

During the following period of time (middle to to upper part of the Lower Cretaceous, the entire Upper Cretaceous and the first section of “Tertiary” (Paleocene/most of the Eocene), charophytes were very rare and played a minor role again.

This changes again very significantly for the Oligocene and Miocene series. Some of the charophyte deposits in Germany played an important role for the development of the European charophyte biozonation for the Paleogene and Neogene, mainly deposits in the Oligocene of the Mainz Basin (European Rhabdochara major, Chara microcera and C. notata zones).