| Summary: | The aim of this study was to assess whether geothermal effluent from the Krafla power plant, which runs into the stream Hlíðardalslækur, affects the stream algal biomass and community structure. The site is located in northeast Iceland and about 10 % of the geothermal effluent from the power plant is discharged into the stream Hlíðardalslækur. In order to address the aim of the study, basaltic tiles were installed in the stream in July 2015 at four different stations: one upstream from the discharge (KR1-2), one shortly downstream of the discharge (KR1-4), one further downstream from the discharge (KR1-6) and one in the tributary (KR2-2) between KR1-4 and KR1-6, which was the control stream. Samples from these tiles were collected in October 2015, January and June 2016. The chlorophyll a concentration was measured using spectrophotometry and the algae were identified in microscope with 200-400 times magnification. The algae were classified to genus level and the cells with chloroplasm were counted. The cell density of each genus (cells per cm2 of tile), the fraction of total number of cells for each genus and total cell density were calculated. The temporal variation in chlorophyll a concentration was more prominent than the spatial variation, with the lowest concentrations in January for all stations and highest in October for station KR1-4. The highest diversity in the algal community was at the reference station (KR2-2) and the lowest just downstream the discharge site (KR1-4). Diatoms dominated the algal community at KR2-2, while they were practically absent at KR1-4. The cell density of cyanobacteria was much higher at KR1-4 (below power station) than at the upstream KR1-2. Considering the unaffected station KR2-2 as control, the effluent has a large impact on the algal community at the discharge site. The biomass (chlorophyll a) increased, the diversity decreased and diatoms were practically absent. The results indicated that the algal ecosystem recovers with increasing distance from the discharge site, as ...
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