Rotifer vertical distribution in a strongly stratified lake: a multivariate analysis

The main source of variation of rotifer species distributions in lake Arcas -2, a small karstic lake near Cuenca (Spain), was explored by means of prin cipal components factor (PCA) and canonical correlation (CCA) analyses. PCA was performed using rotifer densities and CCA using rotifer densities plus physical and chemical parameters. Factor 1 of PCA separated summer species from winter-spring species and Factor 2 accounted for the variation in the vertical profile. Three summer species with different food habits (Polyart hra dolichoptera, Hexarthra mira and Asplanchna girodi) were grouped togeth er at the positive end of Factor 1, while Factor 2 separated the two hypoli mnetic species (Filinia hofmanni and Anuraeopsis fissa) from the rest. The relative position of rotifer species in the space determined by the CCA was roughly the same. The most significant environmental factors that became p aired with rotifer distribution in the CCA were temperature and oxygen, and parameters related to water inflow. Segregation of filter-feeding species in the spatio-temporal subenvironments is clearly shown by the multivariate analysis. The low diversity of rotifer species found in Lake Arcas-2 is attributed to the reduced dimensions of the lake and its morphology. This lake resembles a sinkhole with an abruptly sloping shoreline and poor development of the littoral zone. This morphology favors a strong oxygen stratification. Since midsummer the oxic-anoxic boundary is located in the upper metalimnion, th e vertical structure of the oxygenated water column is simplified. This low rotifer diversity contrasts with a high ciliate diversity in the anoxic wa ters.