Disproportion among Cladocera (Crustacea) skeletal components in lake sediment taphocoenoses and significance with respect to two methods of sub-fossil enumeration

Analysis of Cladocera (Crustacea: Branchiopoda) subfossil remains in lake sediments features prominently in paleolimnological studies. It is well known, however, that species composition in a taphocoenosis (assemblage of subfossil remains in sediments) does not represent perfectly that of the original living community (biocoenosis) from which it came. We analyzed the representation of different Cladocera skeletal components in sediments of 27 Russian water bodies to compare two methods of enumerating relative abundances of cladoceran remains: (1) recording the number of most abundant fragments of each taxon to represent the number of individuals, and (2) recording each fragment of a taxon as belonging to an individual specimen. Overall, for all cladoceran taxa and all water bodies sampled, proportions of different skeletal components differed from what would be expected based on those in live individuals. Carapaces were the most abundant component in 23 of 27 water bodies. Head shields were common, but predominated in only four samples, whereas postabdomens were rare, accounting for < 10% of the whole sample set. Several hypotheses are proposed to explain the different proportions of cladoceran skeletal remains in sediment samples. Both methods for counting remains yielded similar relative abundance estimates for different taxa, which can be regarded as evidence that both methods are adequate, with some exceptions. For example, Alona affinis was represented by a relatively high portion of postabdomens, a component that for other taxa was usually poorly represented. In general, recording of the total number of skeletal components, as compared to using the most abundant fragment method (MAF), slightly overestimates the significance of some taxa in the community. Disproportionate preservation of different skeletal components among cladoceran taxa does not compromise the results of cladoceran-based paleoecological analyses using either counting method, but should be considered when interpreting results.