Refuge availability: a key to understanding the summer disappearance of Daphnia

Abstract

1. The mid-summer declines of Daphnia species in three small lakes were investigated to examine the relative roles of reduced natality and increased mortality. Reduced natality (assessed by quantifying clutch size, lipld index, and available food) could not account for the decline in daphnid abundance in any of the populations examined.2. The role of increased mortality imposed by zooplanktivorous fish was assessed by estimating the sizes of the mid-water refuge areas where daphnids could escape fish predation. The boundaries of the refuge areas were estimated from field measurements and literature values and were based on gradients of temperature, dissolved oxygen and light. Observed decreases in refuge thicknesses correlated well with the mid-summer declines of large-bodied Daphnia species in all three lakes. Intermediatesized daphnids were less affected as the refuges thinned and small-bodied species increased in abundance.3. The importance of refuge thickness in modifying zooplanktivoreinduced mortality was further tested in large enclosures where refuge thickness was experimentally modified. In the presence of zooplanktivorous fish, large-bodied Daphnia, which used the refuge, persisted when the refuge was thick but disappeared when it thinned. Daphnia galeuta mendotae, which did not occupy the refuge zone, was rapidly eliminated regardless of refuge thickness. We conclude that refuge availability plays a major role in Daphnia population dynamics.

Toward a more comprehensive theory of zooplankton diel vertical migration: Integrating ultraviolet radiation and water transparency into the biotic paradigm

“…Seasonally, as this refuge disappears, Daphnia populations decline even though food resources and reproductive rates are sustained. Perhaps even more important, except for a brief period in early spring, the depth at which visible light can serve as a potential refuge from visual predation may be so deep that oxygen levels are too low for Daphnia to survive (Wright and Shapiro 1990). While some predatory zooplankton such as M. edax and larval Chaoborus can migrate into anoxic strata (Williamson and Magnien 1982), this tolerance of anoxia has not been reported for Daphnia or other primarily herbivorous species.…”

“…Evidence for the importance of visual predation in driving DVM also comes from the fact that visual predators are size selective and that smaller, less conspicuous zooplankton often migrate less than large zooplankton (Bollens and Frost 1991b;Gonzalez and Tessier 1997;Thys and Hoffmann 2005). Similarly, there is no question about the effectiveness of DVM in reducing predation risk in more productive, less transparent lakes where fish are abundant and a refuge from visual predators exists between an upper (shallow depth) boundary of low temperature or moderate hypoxia (3 mg L 21 ) that excludes or at least minimizes the abundance of visual predators and a deep boundary of low oxygen (1 mg L 21 ) that constrains zooplankton (Wright and Shapiro 1990;Tessier and Welser 1991;Klumb et al 2004). Seasonally, as this refuge disappears, Daphnia populations decline even though food resources and reproductive rates are sustained.…”

“…In these lakes, warm-water fish species dominate and are constrained to feeding in the surface waters. The hypoxic deeper waters may actually act as an important refuge for zooplankton from fish predation (Hanazato et al 1989;Wright and Shapiro 1990;Tessier and Welser 1991) as well as visually feeding invertebrate predators, such as Bythotrephes (Manca et al 2007;Manca and DeMott 2009). In oceans, the more transparent systems are generally associated with warmer, open-ocean, low-latitude habitats, such as subtropical gyres, and therefore, in contrast to freshwater systems, are usually populated by warm-water fish species.…”

Langmuir circulations disturb the low‐oxygen refuge of phantom midge larvae

“…1, 2), thus avoiding predation by fish, the downward migrations of which are restricted by oxygen concentrations of ϳ3 mg L Ϫ1 (Wright and Shapiro 1990). Obviously, in Lake Hiidenvesi, as well as in many other lakes, chaoborids use low oxygen concentrations as a refuge against fish predation (Saunders and Lewis 1983;Hanazato 1992;Horppila et al 2000).…”

Testing zooplankton food limitation across gradients of depth and productivity in small stratified lakes

“…The coexistence of planktivorous fish and large-bodied Daphnia during summer is dependent on the presence of a deep-water refuge (Wright and Shapiro 1990;Tessier and Welser 1991). In such systems, Daphnia avoid fish predation by living in the deep-water refuge, or by migrating between surface waters and the deep refuge under cover of darkness.…”