AGE AND SURVIVORSHIP OF DIAPAUSING EGGS IN A SEDIMENT EGG BANK

We determined the densities of diapausing eggs of the copepod Diaptomu s sanguineus in sediments from two small freshwater lakes in Rhode Isl and. Sediment cores, sliced at 1-cm intervals, showed that egg densiti es 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 on e lake. Between 10 and 50% of these eggs hatched in short-term laborat ory experiments, and actual egg viability is probably higher. Pb-210-d ating revealed relatively constant sedimentation rates in both lakes, and we use this information to estimate egg ages. In one lake, the mea n 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 ag e is 48.9 yr (median age = 35.9 yr) and the maximum age of eggs we hat ched was 112 yr. We calculated egg mortality rates by regressing In(eg g density) on the age of the sediment from which the eggs were taken t o obtain estimates of 1.1 and 1.5% mortality/yr for the two lakes. Dia pausing 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 evoluti onary dynamics.