The epikarstic waters of a restricted sector of the Ojo Guareña cave (north Iberian Peninsula) were investigated to characterize
the physico-chemical variation in an annual cycle, to improve the scarce knowledge of the aquatic cave fauna on the Iberian
Peninsula, to look for distribution patterns of species per habitat along the annual cycle, and to search for the environmental
basis (either physico- or hydro-geochemical) that could explain species distribution in the epikarst. The habitats studied included
all puddles and gours present in the sector selected making a total of 51 chemical and 42 biological samples for the entire
cycle. The waters show no appreciable contaminationand exhibit small chemical variations throughout the year which are
patently affected by external weather conditions. The 53 taxa found belong to ten higher taxonomic groups (Oligochaeta,
Turbellaria, Mollusca, Copepoda, Ostracoda, Isopoda, Bathynellacea, Tardigrada, Acarina and Cnidaria), and consist mainly of
crustacea with a total of 27 species. Fourteen species were stygobionts (belonging to ten genera), nine of which are new to science
and ten of which are endemic. Cave pools that appeared to be more stable in terms of water volume and mineralization, had
a lower pCO2 and were carbonate oversaturated, harboured the greatest number of taxa. It is these pools that can maintain strictly
cave dwelling species. Pools with lower levels of mineralization and greater water volume fluctuations had a lower diversity
of fauna and in general lacked stygobiotic species.