**3.5 Algae in the role of indicators in the assessment of stream condition**

Algae are considered to be primary producers in aquatic ecosystems powering both food webs and biogeochemical cycling [126], even rare metals [305]. Algae are present in almost every aquatic environment including fresh, brackish, marine, and hypersaline water [306, 307]. Algae communities in rivers are usually diverse and inhomogeneous [308, 309], and their types are as in **Table 2**.

The floristic composition of algae in the benthos could be employed in water quality, stream condition, and eutrophication monitoring [24, 90, 108, 311–315]. A number of studies show a preference for diatoms since diatom-based methods in bio-monitoring approaches demonstrating a tendency for higher success rate to be the most successful [29, 316]. In practice, other algal groups present bigger difficulties in sampling and quantitative estimation in comparison with diatoms. Moreover, common river algae, in particular the green algae [78], show a demonstrable lack of identification keys although partial country-wide lists exist, for example, [317] where 321 out of 500 genera are identified. However, these other groups may provide information that diatom-based measures cannot provide easily; for example, eutrophication can be monitored by cyanobacterial and green algae biomass and diversity could be used to monitor eutrophication [108, 313, 318–320].

Diatoms also play important roles in biotechnology, engineering, biology, and material science [321] but their main role in the general riverine environmental condition [29, 322, 323], water quality ecological assessment of aquatic systems [107, 152, 324–328], eutrophication [78, 314, 329], pollution [330, 331], bioassessment [107, 116, 332–334], and urbanization [335–338].
