Changes among chydorid assemblages in Lake La Cruz sediments, from a core of 178 cm length, have been studied using community structure indices and multivariate methods (cluster analysis, principal components analysis). The results revealed the existence of two main sources of variation in these assemblages. One (axis I of PCA1) is associated with the trophic level of the lake, which is hypothetised to be greater in the bottom and upper part of the core. Both zones are characterised by an increase in the relative frequency of Chydorus sphaericus, accompanied by a marked reduction in the relative frequency of the most abundant species throughout the history of the lake, Acroperus neglectus. The other source of variation (axis II of PCA1) may be interpreted as the alternation of periods of dryer and wetter weather. Our analyses show the separation of two assemblages which alternatively prevail in the sedimentary sequence, one constituted by Pleuroxus laevis, Alona guttata, Graptoleberis testudinaria and, sometimes, Alona rectangula, and another dominated by Chydorus sphaericus, Alona quadrangularis, Alona affinis and sometimes Leydigia species. According to the ecological preferences of these species, the dominance of the first chydorid assemblage can be related to episodes of higher temperature and with very low and fluctuating water level, or with a high water level but a very reduced extent of shallow water in the lake. Prevalence of the second group corresponds to episodes with a colder environment and permanent waters, the level of which could be variable but in such a way that a benthic sublittoral zone may develop. The structure of the community is strongly affected by changes in lake level. High indices of fluctuation in community structure and small mean body size of the chydorid assemblage marked the period with more extreme dry conditions, characterised by Alona rectangula maxima. The peaks of C. sphaericus coincided with peaks of the index of fluctuation D-0 and prevalence of smaller species of the chydorid assemblage.
