Zooplankton life cycles: Direct documentation of pelagic births and deaths relative to diapausing egg production

Zooplankton demographics operate over multiple time scales corresponding to pelagic parthenogenetic generations, an overwintering stage, and an ‘‘egg bank.’’ Comparisons of pelagic birth and death rates with seasonal diapausing egg production provide insights into local species persistence. The spiny cladoceran (Bythotrephes) possesses morphological adaptations (spines and thick-walled diapausing eggs) that lessen, but do not eliminate, predation effects by fish. Because caudal spines are retained throughout life and are indigestible in fish guts, spine accumulation in sediment traps directly documents patterns of seasonal mortality. Here, we contrast deposition of spines and diapausing eggs in a recently colonized inland lake (Lake Michigamme) with deposition in a larger source lake (Lake Michigan). Direct mortality estimates at the inland lake correspond well with traditional indirect estimates from egg ratio techniques. Although spines are effective against abundant small fish in the inland lake, individuals are susceptible to ingestion by late-season young of year and larger fish. Laboratory experiments confirm that fish are responsible for spine breakage in the water column, accounting for an estimated 62–71% of seasonal mortality. Effects of late-season predation are lessened because diapausing eggs survive gut passage, yet yearly production of diapausing eggs is low in Lake Michigamme. A more balanced pelagic birth(death) to diapausing egg relationship exists in Lake Michigan, probably because egg-carrying adults find better refuge in central, deeper waters. Since spines catch on seines and diapausing eggs survive gut passage, use of bait fish and live wells promotes Bythotrephes dispersal. Zooplankton studies in lakes traditionally concentrate on seasonal pelagic dynamics of local populations (Np; Fig. 1). Yearly population growth is treated as the consequence of a dynamic balance between instantaneous birth (b) and death (d) rates (r 5 b 2 d). Netting of plankton allows seasonal estimates of brood sizes and population density. Temperature-dependent regressions for developmental duration convert egg ratio measures into instantaneous birth rates (b), whereas changes in pelagic population size allow independent estimates of the realized population growth rate (r). The difference between the instantaneous birth rate and the re1 Present address: U.S. EPA Landscape Ecology Branch, 12201 Sunrise Valley Drive, Reston, Virginia 20192-0002. 2 Corresponding author ([email protected]). 3 Present address: Marine Science Program, Univ. of California, Santa Barbara, California 93106.