Temporal dispersal: ecological and evolutionary aspects of zooplankton egg banks and the role of sediment mixing.

Zooplankton egg banks are the accumulation of diapausing embryos of planktonic animals buried in the sediments of many aquatic ecosystems. These eggs, which are analogous life history stages to the seeds of many plants, can survive in a ready-to-hatch state for periods ranging from a few years to greater than a century. Their presence in ponds, lakes and near-shore marine environments has substantial implications for understanding trajectories of ecological and evolutionary change. When the sediments of lakes are structured in historical sequence, diapausing eggs extracted from different sediment ages can provide a means of studying past changes in community or population-genetic structure. A completely different aspect of egg banks derives from the fact that hatching of diapausing eggs can influence, through what can be thought of as temporal dispersal, population and community response to environmental change. Eggs hatching from diapause introduce to current environments species or genotypes laid at times in the distant past. In addition, egg banks create extended generation overlap that can play an important role in maintaining diversity in a fluctuating environment when different types (species or genotypes) are favored at different times. These distinct aspects of egg banks (i.e., their direct impact on ecological and evolutionary processes versus their usefulness in reconstructing historical changes), are potentially in conflict because for old eggs to hatch, the sediments must be at least partially mixed. This same mixing, however, degrades the accuracy of the historical record. Both aspects are possible, however, even within a single lake when sediment-mixing intensity is spatially heterogeneous.