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

Abstract

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 (N p ; Fig. 1). Yearly population growth is treated as the consequence of a dynamic balance between instantaneous birth (b) and death (d) rates (r ϭ b Ϫ 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 re-1

Dynamics of the invasive spiny water flea, Bythotrephes longimanus, in Lake Simcoe, Ontario, Canada

“…A combination of low prey availability and ineffective Bythotrephes predation due to poor light penetration through the water column in August and/or September could result in concurrent declines in Bythotrephes body length and population size. As seen in other lakes, however, predation by planktivorous fish at this time (Coulas et al 1998, Pothoven et al 2001, Jarnagin et al 2004, Pothoven and Vanderploeg 2004, Young and Yan 2008, Young et al 2009) may also be contributing to the midseason change in Bythotrephes body size in Lake Simcoe. The concurrent recovery of body and population size in the fall months in some years may be related to the recovery of cladoceran prey abundance and/or reduced or relaxed fish predation pressure (Pothoven et al 2001).…”

“…Bythotrephes typically recruits from diapausing (resting) eggs in May (Yurista 1992, Jarnagin et al 2004, Young 2008), yet Bythotrephes was not observed in pelagic samples in Lake Simcoe until June or July. The apparent absence of Bythotrephes in May and June may be related to low spring prey abundance because cladoceran abundance routinely did not begin increasing in the lake until June.…”

“…Many species of fish depend on crustacean zooplankton as their primary food source, particularly during the larval stages in the summer months. In years when present in summer months, highly abundant, large-bodied Bythotrephes may outcompete small, gape-limited, planktivorous fish for prey (Garton et al 1993, Barnhisel and Harvey 1995, Compton and Kerfoot 2004, Jarnagin et al 2004, which could affect fish recruitment and biomass in Lake Simcoe. In turn, top predators (e.g., lake trout, walleye) will be impacted if the abundance of their primary or preferred prey items (e.g., lake herring, rainbow smelt) are affected by changes in the abundance or biomass of lower trophic levels.…”

“…For example, variations in its morphological features and body size may relate to defense against planktivorous fishes (Straile and Halbich 2000) or energetic tradeoffs related to balancing growth and reproduction (Enz et al 2001, Branstrator 2005. The population size of Bythotrephes may be limited by bottom-up effects such as the availability of herbivorous crustaceans (Dumitru et al 2001, Strecker andArnott 2008), particularly in the spring months , as well as top-down control by fish (Coulas et al 1998, Jarnagin et al 2004, Young and Yan 2008, Young et al 2009. Temperature influences population growth rates by affecting hatching rates (Yurista 1992, Brown andBranstrator 2011), generation time (Lehman and Branstrator 1995), mode of reproduction (Straile andHalbich 2000, Manca et al 2007), or prey consumption (Yurista et al 2010).…”

“…data). In addition to these impacts on zooplankton, Bythotrephes may also have direct impacts on the fish community by competing with young-of-year fish for zooplankton prey, while its caudal spine may discourage predation by small fish (Garton et al 1993, Barnhisel and Harvey 1995, Compton and Kerfoot 2004, Jarnagin et al 2004. Bythotrephes seems to comprise a large portion of the summer diet of many Lake Simcoe fish (Rose 1998, Amtstaetter 2000, MacRae 2001, Johanson and Amtstaetter 2004, which may have consequences for fish condition and growth because Bythotrephes may provide different nutritional resources than native zooplankton prey.…”