River Algae

River-inhabiting algae are briefly presented in terms of predominant algal groups, morphological types, patterns of abundance and distribution, responses to environmental variables, ecological roles in river ecosystems, and biogeographic trends.

This chapter presents some of the more commonly encountered lotic cyanobacterial taxa. The cyanobacteria are a group of oxygenic prokaryotes present in nearly all aquatic ecosystems. While the ecological importance of this lineage is well known, much confusion exists pertaining to their systematic and taxonomic status. In order to facilitate generic-level identification, we separate the cyanobacteria into four major groupings: the Chroococcales (coccoid cells often in a mucilaginous envelop), the Oscillatoriales (filamentous forms lacking specialized cells), the Nostocales (filamentous with inducible specialized cells), and the Stigonematales (filamentous, obligatory specialized cells coupled with cell division in multiple planes). We discuss the major genera found in each lineage, the current state of the systematics, and the broad ecological roles and niches of these taxa. Dichotomous keys and images are presented to facilitate generic identifications.

The green algae represent one of the most diverse and abundant algal lineages in river systems around the world, and their evolutionary diversification led to two major lineages, the Chlorophyta and Streptophyta (with the latter including the land plants). Macroscopic and microscopic forms of green algae are common in streams, as are those living on hard substrata, epiphytically on aquatic plants or other algae, and a few free-floating forms. This chapter treats the most common genera from stream habitats, with an emphasis on the benthic forms, the macroscopic taxa, and those that are widespread in distribution. Basic descriptions of 42 genera are provided, along with illustrations and information on the habitat and phylogeny of each genus, where known.

Freshwater red algae are presently classified into five classes based on recent molecular phylogenetic and supporting morphological analyses: Bangiophyceae, Compsopogonophyceae, Florideophyceae, Porphyridiophyceae, and Stylonematophyceae. Red algae are well represented in river ecosystems and many freshwater members occur exclusively in lotic habitats. These algae are usually important constituents of stream floras, either in terms of abundance or distribution from local scale to biomes. This chapter treats all the genera from river habitats, with an emphasis on the benthic forms and macroscopic taxa. Basic descriptions of 26 genera are provided, along with illustrations and information on the habitat.

Photosynthetic stramenopiles (heterokonts) constitute at least 11 distinct lineages, including some of the most abundant algae, particularly diatoms. Diatoms are unicellular algae that possess a cell wall composed of silicon dioxide often highly ornamented, made up of two parts. Diatoms are very diverse and common in streams and rivers, with a significant role in primary production. The benthic diatoms, in particular, have developed successful strategies to establish and maintain their position in lotic environments. Fifty-nine most common diatom genera from lotic systems are described, including illustrations and a key to genera.

Freshwater brown algae can be abundant in streams, but represent just seven species in the Phaeophyceae, a class of ~2000 species, most from marine environments. Freshwater species do not form parenchyma, but are based on one of the three filamentous growth forms: (1) uniseriate, creeping filaments infrequently or frequently branched; (2) complex branched forms producing basal and vertical series of filaments forming a crust; and (3) multiseriate, frequently branched forms. Their evolutionary history and relation to marine taxa is unclear, with molecular data suggesting some closely related to existing marine taxa, while others forming a separate clade of freshwater forms. Most species are known from few locations worldwide, with all but a few reports from sites in the northern hemisphere. All species are benthic and most colonize epilithic substrata, but a few are epiphytes on macrophytes or other algae. A key to all known species is provided.

The heterokont classes Xanthophyceae and Chrysophyceae are introduced with its key characteristics and typical benthic river genera. Two relatively widespread genera are the xanthophyte siphonous Vaucheria and the filamentous Tribonema. The heterokont classs (Chrysophyceae) genus Hydrurus is also listed, which consists of a branched colony and can be widespread in cold mountain rivers with turbulent waters.

In shallow clear rivers macroalgae are a diverse component of biota and especially easily recognizable and easily accessible organisms, facilitating studies of spatial and temporal variation across environmental gradients. Their macroscopic form and appearance makes field studies of their limits and requirements in space and time possible. Some taxa occupy spatially restricted microhabitats, particularly in headwaters, while others are abundant along the length of a river. Spatial variation can occur within a site, between different stream types, or across wide regions. Temporal variation in growth or colonization by a particular species at a particular site can be driven by seasonal changes in discharge and nutrients, which may lead to regular or irregular temporal sequences. More stable temporal dynamics are observed in taxa with high adaptive capacities to resist physical disturbances or to re-establish quickly after floods. We examined the most common soft-bodied macroscopic algae (SBM) from two geographically extended datasets of temperate streams, from alpine to lowland regions in Austria and a shallower altitudinal gradient of southeastern New York State. Morphological and functional characters, combined with key environmental variables (based on median and multivariate statistics), are used to analyze general trends and causalities for species-specific spatial and temporal niches. These results provide strong arguments in favor of using a combination of on-site studies of growth form and phenology, with ecophysiological and molecular studies in the lab to improve our understanding of the factors regulating stream macroalgae occurrence in space and time in the future.

Algae in rivers are affected by light, water turbulence, and nutrient availability. These environmental factors ultimately affect algae according to their habitat, growth form, and specific physiological abilities. Water flow imposes limitations in the diffusion and availability of gases and resources, also in relation to algal size and growth form. Algae adapt physiologically to light scarcity or excess via photosynthetic mechanisms, as well as by modifying their pigment composition. The algal ability to obtain and keep resources is mediated by enzymes, and its ability to use and store materials is specific of the different algal groups. Toxicants impose a limit to algal performance and may affect photosynthesis as well as nutrient uptake, amongst other effects on algal cells.

Biogeography, or the study of the distribution of organisms in time and space, has a rich history. New molecular tools are providing another line of evidence in understanding distributions: in some instances confirming previous morphological data, while in other cases providing new insights. There have been few studies devoted to the biogeography of river algae, but much information can be gleaned from floristic surveys and systematic research. Water, wind, animals, and humans have played an important role as dispersal agents and may affect river algal biogeography. There is convincing evidence that some species of river algae are specialists and others generalists. There have been various invasions of freshwater rivers by marine algae and some of these events have raised awareness of and studies on possible transport mechanisms of microorganisms. Many recent studies of river algae have shown similar results to other microorganisms that many species are not ubiquitous, but are more geographically restricted than was previously reported. This certainly seems to be the case for freshwater Rhodophyta for which new molecular data are being amassed. Lastly, there is a need for easily accessed repositories of biogeographic data to better understand these organisms and their global distributions.

Diatoms have been widely used to detect changes in streams and rivers water quality due to their specific sensibility to a variety of ecological conditions. Their tolerances and preferences for pH, conductivity, salinity, humidity, organic matter, saprobity, trophic state, oxygen requirements, nutrients, and current velocity in freshwater streams, rivers, lakes, wetlands, and estuaries have been defined, and diatoms have also been used in paleolimnological studies. Biotic indices using diatoms based on the relative abundance of the species weighted by their autoecological values have been developed worldwide, though indices of biotic integrity based on periphyton, diatoms, non-diatom “soft” algae, including cyanobacteria, macroalgae, and macrophytes assemblages have been also developed for biological monitoring. A new approach for water quality evaluation utilizing diatoms has been increasing significantly in recent years, by applying molecular techniques using DNA sequences. Molecular identification has the potential to provide revolutionary discoveries in taxonomy that may have great benefits for bioassessment. This chapter provides an overview of the state of the art of studies related to river quality evaluation using epilithic diatom communities worldwide. Most studies highlight the use of biotic indices to summarize floristic data to assess pollution effects on aquatic communities.