PARTICLE-TRANSPORT BY BENTHIC INVERTEBRATES - ITS ROLE IN EGG BANK DYNAMICS

The ecological and evolutionary dynamics of zooplankton is in part a f unction of the numbers and ages of diapausing eggs hatching from aquat ic sediments. Successful recruitment from this 'egg bank' must depend upon the eggs being present at or near the sediment surface. Often, ho wever, zooplankton diapausing eggs are found as deep as 15 to 30 cm in the mud. Bioturbation may provide a mechanism for the regular return of buried eggs to the sediment surface. A substantial portion of the p opulation of the copepod, Diaptomus sanguineus, living in Bullhead Pon d, a small lake in Rhode Island, USA, is present as diapausing eggs. T o study the role of bioturbation in egg-bank dynamics, we introduced p olystyrene beads, the same size and specific gravity as copepod eggs, at two depths in large-diameter sediment chambers in the laboratory. T reatments included chambers with natural and reduced densities of bent hos. Consistent with other studies, our results show that the joint ac tivities of tubificid oligochaetes and chironomid larvae are responsib le for bidirectional (up and down) transport of beads in the top 2 cm of the sediment. We observed no bead movement below this depth. Thus, eggs in the top two centimeters of sediment in this lake are exposed w ith some regularity to conditions that stimulate hatching at the sedim ent-water interface. In Bullhead Pond, these eggs have a mean age of 1 2.2 years (based on Pb-210-dating). Eggs buried more deeply will only be returned to the sediment surface by relatively rare, localized dist urbances. This return of old eggs to the surface affects ecological an d evolutionary dynamics in a complex way.