| Summary: | Iceland has an abundance of fissures that are parallel to the Mid-Atlantic ridge where bedrock cracks as a result of continental rifting. In some areas fissures penetrate the aquifer and expose the groundwater within the bedrock. The main objective of the study was to measure the biological diversity in groundwater fissures in Iceland. A large part of the substrate in fissures is rock wall and they often reach great depth. As a result, the sampling sites were at variable depth on both rock wall as well as fissure’s bottom. The present study was constrained to macrozoobenthos, while presence or absence of fish was also noted. Invertebrates were found to be living within and around a biofilm that was mostly made of Cyanobacteria and diatoms. Invertebrate assemblages were studied in three fissures: Silfra and Flosagjá in SW Iceland and Stekkjagjá in NE Iceland. All samples and measurements were acquired by scuba diving. Invertebrate assemblages in fissures varied significantly between geographical regions in regards to taxa richness, density and diversity. All crustacean groups were in significantly greater densities in NE Iceland while chirononmids were found in similar densities in both geographical regions. A closer inspection revealed differences in chironomid assemblages as Arctopelopia sp. was only found in NE Iceland while Diamesa zernyi gr. was found in significantly greater densities in SW Iceland. Assemblages in fissures in the same region were similar except for differences in densities of cladocerans. Chydorus sp. was found in high densities and is the most common cladoceran in Flosagjá while it was almost absent from Silfra and Alona werestschagini, the second most common cladoceran in Flosagjá, was entirely absent from Silfra. Within fissures most taxa were fairly evenly distributed regardless of physical habitat (e.g. depth, rock wall vs. rock bottom). Biofilm mats cover the entire substrate in fissures. These mats are made from Cyanobacteria and benthic diatoms, which are successful under low light ...
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