AGE AND SURVIVORSHIP OF DIAPAUSING EGGS IN A SEDIMENT EGG BANK

We determined the densities of diapausing eggs of the copepod Diaptomus sanguineus in sediments from two small freshwater lakes in Rhode Island. Sediment cores, sliced at 1-cm intervals, showed that egg densities ranged between 4 x 10(4) and 8 X 10(4) eggs/m(2) near the sediment surface and declined to very low values at depths of 10-15 cm in both lakes, although eggs were found as deep as 30 cm in the sediment of one lake. Between 10 and 50% of these eggs hatched in short-term laboratory experiments, and actual egg viability is probably higher. Pb-210-dating revealed relatively constant sedimentation rates in both lakes, and we use this information to estimate egg ages. In one lake, the mean diapausing-egg age is 70.4 yr (median age = 45.9 yr) and the maximum age of eggs we hatched was 332 yr. In the other lake, the mean egg age is 48.9 yr (median age = 35.9 yr) and the maximum age of eggs we hatched was 112 yr. We calculated egg mortality rates by regressing In(egg density) on the age of the sediment from which the eggs were taken to obtain estimates of 1.1 and 1.5% mortality/yr for the two lakes. Diapausing eggs of zooplankton represent a long-lived life history stage of an otherwise short-lived organism. They provide generation overlap that can have substantial significance for both ecological and evolutionary dynamics.