Significance of a low oxygen layer for a Daphnia population in Lake Yunoko, Japan

Abstract

Population dynamics and vertical migration of Daphnia longispina in Lake Yunoko were studied. The Daphnia population was small in spring and early summer, probably because of high predation pressure by fish. The population grew in midsummer, when thermal stratification developed and the dissolved oxygen became very low in the deeper layer of the hypolimnion. In this season, adults of D. Zongispina concentrated in the daytime near the lake bottom, where fish were absent because of the anoxic conditions, but ascended at night to the upper layer of the hypolimnion, where food was most abundant. The low oxygen layer near the bottom kept out the predators and protected Daphnia from predation, and consequently contributed to the built-up of its population. However, the low oxygen layer was unfavorable for reproduction of Daphnia, as reflected in the low egg ratio and high percentage of males in the population. The population decreased in the fall, when thermal stratification disappeared and predation pressure seemed to increase.

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

“…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.…”

Factors Affecting Japanese Pond Smelt (<i>Hypomesus nipponensis</i>) Stock Trajectory in Lake Kasumigaura and Kitaura

“…In a study as in [11], in Lake Yunoko it was found that the population of pond smelt are almost absent in anoxic areas where the dissolved oxygen was below 3 mg/l, showing that dissolved oxygen concentration of lake water has a strong influence on the population dynamics of the pond smelt (Hypomesus olidus). Hasenbein [14] showed that negative correlation exists between turbidity of water and feeding frequency of juvenile delta smelt (Hypomesus transpacificus) from the San Francisco Bay Delta, USA.…”

“…Pond smelts have been shown to be affected by biological and environmental factors previously [11]- [14]. In a study as in [11], in Lake Yunoko it was found that the population of pond smelt are almost absent in anoxic areas where the dissolved oxygen was below 3 mg/l, showing that dissolved oxygen concentration of lake water has a strong influence on the population dynamics of the pond smelt (Hypomesus olidus).…”

How do migrating daphnids cope with fish predation risk in the epilimnion under anoxic conditions in the hypolimnion?

“…This poses a problem for zooplankters performing vertical migration as the lack of oxygen experienced by the animals during the daytime results in a reduced growth rate (Hanazato and Dodson, 1995) and a lower reproductive output (Hanazato et al, 1989). However, zooplankters are still found in low-oxygen layers (Hanazato et al, 1989;Hanazato, 1992;Duncan et al, 1993).…”

“…This poses a problem for zooplankters performing vertical migration as the lack of oxygen experienced by the animals during the daytime results in a reduced growth rate (Hanazato and Dodson, 1995) and a lower reproductive output (Hanazato et al, 1989). However, zooplankters are still found in low-oxygen layers (Hanazato et al, 1989;Hanazato, 1992;Duncan et al, 1993). A possible explanation for this phenomenon might be that these oxygen-depleted hypolimnetic layers provide not only a light refuge, because the most important planktivorous fish are visual predators, but also an oxygen refuge, as most fish are obligate aerobes and therefore unable to penetrate low-oxygen layers for longer periods (Hanazato et al, 1989;Wright and Shapiro, 1990).…”