A study on diapause of zooplankton in a tropical river ‐ The Blue Nile

Abstract

The zooplankton of the Blue Nile overcomes the adverse conditions of the flood by entering into a state of diapause. Resting stages were found in the river bottom, flowing water and rain pools. They were blown to different localities by wind and can withstand long periods of desiccation. Some species of zooplankton go into diapause more than once a year, notably during their seasonal peaks of abundance. Release from diapause occurred in the temperature range 20-30°C and was facilitated by longer photoperiods and decreased concentrations of suspended silt. Various species of zooplankton were induced in facultative diapause when stored in complete darkness.

Effects of chemical and physical conditions on hatching success of Bythotrephes longimanus resting eggs

“…Examples of extreme thermal tolerance (near 100uC) have been reported for dormant stages of some aquatic crustaceans (Carlisle 1968), but the thermal maximum for most metazoans is between 30uC and 45uC (Willmer et al 2005). Hatching has been demonstrated for daphnid and other cladoceran resting eggs exposed to 30uC or 35uC for various amounts of time (Moghraby 1977;Schwartz and Hebert 1987). Moghraby (1977) found no hatching for eggs exposed to 36uC or 40uC.…”

“…Desiccation: Zooplankton resting eggs have been shown to tolerate drying for different lengths of time depending on taxa (Brewer 1964;Moghraby 1977;Makrushin 1978). Here, we exposed free eggs to desiccation (as gently circulating air) in year 1 for 1 h, 5 d, or 1 month (30 d); in year 2 for 1 h, 12 h, 1 d, or 2 d; and in year 3 for 30 min, 1 h, 2 h, 3 h, 4 h, 6 h, 8 h, 10 h, 12 h, or 1 d. To begin a replicate with free eggs, eggs were transferred by pipette from the egg bank into individual wells of a tissue culture plate with , 0.25 mL of residual filtered lake water per egg.…”

“…The results demonstrate that resting eggs can endure a variety of chemical extremes when kept hydrated, but are intolerant of even short-term desiccation, which is atypical of many crustacean zooplankton (Brewer 1964;Moghraby 1977;Arnott and Yan 2002). The lethal thresholds of heat and desiccation, in particular, can be applied to dispersal models and management protocols for B. longimanus.…”

“…The dormant stage is more durable and able to withstand conditions that are unfavorable or lethal to the active stage (Panov and Cá ceres 2007). For example, experiments and direct observations demonstrate that dormant stages can tolerate exposures to a variety of adverse environmental conditions, including desiccation (Brewer 1964;Moghraby 1977;Arnott and Yan 2002), freezing (Schwartz and Hebert 1987;Meijering 2003), heat (Carlisle 1968;Schwartz and Hebert 1987;Raikow et al 2007b), salinity (Bailey et al 2004;Gray et al 2005), hypoxia (Marcus et al 1994;Brown 2008), non-neutral pH (Arnott and Yan 2002;Brown 2008), and vertebrate digestion (Mellors 1975). Reports dating to Darwin (1859) that implicate wind and animals as dispersal vectors of aquatic invertebrates also imply that dormant stages can tolerate desiccation, heat, and gut passage (Cá ceres and Soluk 2002; Havel and Shurin 2004;Louette and De Meester 2005).…”

Exploring riverine zooplankton in three habitats of the Illinois River ecosystem: Where do they come from?

“…Although it has been suggested that rising water levels flush zooplankton out of off-channel habitats into the main channel, possible dilution effects have also been proposed (Saunders and Lewis 1988a,b;Thorp et al 1994). Others believe that riverine zooplankton assemblages are the result of resting eggs released from riverbottom sediments during flood events and hatch as conditions become favorable (Moghraby 1977).…”