Temporary collapse of the Daphnia population in turbid and ultra-oligotrophic Lake Brienz

Abstract

The cyclical parthenogen Daphnia is a key species in aquatic food webs. Its abundance is influenced by environmental factors like food quantity and quality, predation, diseases, temperature and washout by discharge. In ultra-oligotrophic Lake Brienz (Switzerland), which is turbid from suspended glacial material, Daphnia density has continuously decreased since the 1990 s. In spring and summer 1999, during and after a severe flood, Daphnia density was below detection level, but the population recovered the following year. Simultaneously, a drastic two-year decline occurred in the yield of whitefish (Coregonus sp.), which mainly feed on Daphnia. Several hypotheses were tested to explain the collapse of the Daphnia population: a negative effect of the suspended particles, a covering of the diapausing eggs by sediment, and a combined washout/temperature effect. A direct negative effect of the particles and covering of diapausing eggs could be excluded. According to model calculations, the spring growth of the Daphnia population could not compensate the washout losses, as it was limited by poor food conditions due to reoligotrophication and reduced by extraordinarily low water temperatures. Moreover, ephippia abundance analysed from sediment cores was consistent with the process of eutrophication and re-oligotrophication and indicated that daphnids did not persist in the lake in the period before eutrophication (until 1955). Like most peri-alpine lakes in Europe, Lake Brienz has returned to its natural ultra-oligotrophic state and is now unable to support a large Daphnia population and fishing yield.

Repeated flood disturbance enhances rotifer dominance and diversity in a zooplankton community of a small dammed mountain pond

“…Compared to Daphnia, these smaller cladocerans have shorter generation times and hence have more ability to sustain washout losses induced by flood events. Moreover, these species seem to feed upon smaller particles and are more tolerant of large amounts of dissolved organic matter (DOC) in the water than Daphnia individuals (Rellstab et al, 2007). Due to the effect of DOC is less important for rotifers (Kirk and Gilbert, 1990;Kirk, 1991;Pollard et al, 1998), we expected DOC to be an important driver on the zooplankton community structure of the Jiřická Pond, since DOC concentrations increased during flood periods.…”

Effects of alpine hydropower operations on primary production in a downstream lake

“…Rellstab et al (2007) determined the mean intrinsic rate of growth of the Daphnia population in Lake Brienz to vary around 0.04 d -1 in spring. Model calculations by Rellstab et al (2007) indicate that reduced growth and increased flushing (due to higher throughflow) could lead to orders of magnitude lower Daphnia concentrations at the end of June. Nevertheless, no significant declines were observed in zoo-(i.e.…”

“…Gross primary production in 2002 was predicted to be~49 % higher, primarily because of the up to~60 % above average Chla mean in May and June. However, there was no complementary evidence for increased production during this period, such as uncommon phyto-and zooplankton densities (Finger, 2006;Müller et al, 2007b;Rellstab et al, 2007), low turbidity, high bio-P input, or high global radiation (Figs. 5 and 6c).…”

Lake Brienz Project: An interdisciplinary catchment-to-lake study

“…Although the intrusion depths of the particle-laden rivers changed significantly as a result of the upstream reservoirs (less dense water), the productivity did not decrease due to upstream damming. Rellstab et al (2007) demonstrate that the size of the Daphnia population has been decreasing in the last decade (Fig. 2) as a result of the declining phytoplankton density.…”

“…The particle concentration present in the lake did not have a negative effect on the fitness of Daphnia. Finally, Rellstab et al (2007) show that Daphnia had been absent from the lake before, as no resting eggs could be found in the sediment prior to 1955 -a period with similar productivity as today.…”