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

Zooplankton egg banks are the accumulation of diapausing embryosof planktonic animals buried in the sediments of many aquaticecosystems. These eggs, which are analogous life history stagesto the seeds of many plants, can survive in a ready-to-hatchstate for periods ranging from a few years to greater than acentury. Their presence in ponds, lakes and near-shore marineenvironments has substantial implications for understandingtrajectories of ecological and evolutionary change. When thesediments of lakes are structured in historical sequence, diapausingeggs extracted from different sediment ages can provide a meansof studying past changes in community or population-geneticstructure. A completely different aspect of egg banks derivesfrom the fact that hatching of diapausing eggs can influence,through what can be thought of as temporal dispersal, populationand community response to environmental change. Eggs hatchingfrom diapause introduce to current environments species or genotypeslaid at times in the distant past. In addition, egg banks createextended generation overlap that can play an important rolein maintaining diversity in a fluctuating environment when differenttypes (species or genotypes) are favored at different times.These distinct aspects of egg banks (i.e., their direct impacton ecological and evolutionary processes versus their usefulnessin reconstructing historical changes), are potentially in conflictbecause for old eggs to hatch, the sediments must be at leastpartially mixed. This same mixing, however, degrades the accuracyof the historical record. Both aspects are possible, however,even within a single lake when sediment-mixing intensity isspatially heterogeneous.