On the Occurrence of Cladoceran Remains in Lake Sediments.

Exoskeletal fragments of Cladocera occur abundantly in late-glacial and postglacial lake sediments. Of the various families of the Cladocera the Bosminidae and Chydoridae are represented by the greatest diversity of remains. The morphological differentiation of the chydorids is sufficiently great that species can be identified positively in most instances from the isolated components of the exoskeleton (head shields, shells, postabdomens, postabdominal claws, ephippia), and frequently even from fragments of these.4 On the basis of his previous studies the author became convinced that probably all chydorids living in a lake at a given time became represented by morphological remains in the sediments. To test this idea, the lakes at Madison, Wisconsin, were selected for study. For more than 30 years E. A. Birge, one of the world authorities on Cladocera, studied these organisms in the Madison region. His list of species5 forms the basis of comparison for the present investigation. Lakes Mendota, Monona, Waubesa, and Kegonsa occur in that order downstream in the Yahara River valley at Madison. Wingra, a much smaller lake, had no original connection with the other lakes until a canal was dug many years ago between it and Monona. The sediment samples were collected by means of a 6-inch Ekman dredge at water depths of 25 to 40 feet in the four large lakes, and 15 feet in Wingra. Assuming a sedimentation rate of 2-3 mm a year, the surface material sampled would represent the accumulation of the past 50-70 years, which would encompass the time interval during which Birge worked on these lakes. Qualitative samples were prepared by picking out with a needle the individual skeletal remains. Quantitative slides were prepared by mounting 1/3oo cc of fresh sediment beneath a 22 mm cover slip. In both instances the sediment was first deflocculated with 10 per cent KOH on a magnetic stirrer hot plate. The most important result of the qualitative study is that, by means of a relatively small effort compared with Birge's, all 23 species of chydorids he listed for the lakes except one were recovered, and in addition six species he had not listed. Five of these latter species, which occurred in small numbers in the sediments, undoubtedly constitute only small fractions of the total living populations. Intensive collecting in the proper habitats at the proper times would be necessary to recover them, and this might be largely fortuitous. The sixth species is abundant in Waubesa and Kegonsa, but these are the two lakes in which Birge collected least intensively. The relative abundance of the various chydorids recovered from the sediments undoubtedly reflects in a general way their relative abundance in the total populations of the lakes, integrated over a period of several decades. Two conclusions of this aspect of the study are: (1) any species list of chydorids based only on living animals, even when collected intensively over a number of years, must be suspected of being incomplete, and (2) a species list of chydorids for a given locality can be assembled with least expenditure of time and greatest