A test of food web hypotheses by exploring time series of fish, zooplankton and phytoplankton in an oligo-mesotrophic lake

Abstract

The strength of trophic cascades in the pelagic area of lakes at low productivity has been discussed intensively, but predictions of trophic coupling differ strongly. Many studies suggest that trophic cascades are weak in oligotrophic lakes, but some models discussed that trophic interactions might be strong at low nutrient concentration. Here, we used time series over 9 (phytoplankton and zooplankton) or 6 (fish) years from the oligo-mesotrophic Lake Stechlin (Germany) to explore correlative relationships between biomasses of these trophic levels. The fourth trophic level of piscivorous fish was almost absent in the pelagic area. The biomass of planktivorous coregonid fishes was not at all correlated to total zooplankton biomass, which was dominated by calanoid copepods. However, there was a strong negative correlation of adult coregonid biomass to the proportion of Daphnia in zooplankton, and a strong positive correlation of adult fish to proportion of calanoid copepods. Zooplankton and phytoplankton biomasses were not correlated except for a significantly negative correlation between Daphnia biomass and biomass of Cyanobacteria. Overall, our results suggest that planktivorous fish may modify the zooplankton structure, but not the zooplankton biomass, in lakes of low productivity. However, this top-down effect by fish does not cascade further down to the phytoplankton biomass or community structure, confirming earlier hypotheses that trophic cascades are weak in oligotrophic lakes.

Crustacean zooplankton communities in 13 lakes of Yunnan-Guizhou plateau: Relationship between crustacean zooplankton biomass or size structure and trophic indicators after invasion by exotic fish

“…The invasion of exotic planktivorous fish had a negative effect on water quality. In previous studies, lakes stocked with amounts of copepods, and low nutrient are usually associated with planktivorous fish (e.g., Benndorf et al, 2000;Mehner et al, 2008). Other study had also provided evidence supporting the above conclusion that the increase of copepod biomass or size structure, caused by the predation of planktivorous fish, had a negative effect on phytoplankton control and water quality (Blumenshine and Hambright, 2003).…”

“…In these lakes, cladoceran biomass and size structure were only positively related to the biomass and size structure of copepods and showed no significant correlation with water quality indicators. Earlier studies suggest that herbivorous calanoid copepods can exert little influence on phytoplankton biomasses due to their low overall biomasses, slow growth and low metabolic rates (Mehner et al, 2008). An increase of cyclopoid copepods would result in great mortality of herbivorous zooplankton and have a negative influence on the control of phytoplankton biomass (Blumenshine and Hambright, 2003).…”

“…However, little is known about the relationship between the crustacean zooplankton community and water quality after invasion by exotic fish (Zhang et al, 2005). Mehner et al (2008) found a strong correlation between planktivorous fish and the proportion of copepods in the crustacean zooplankton community. Liu (2001) also found the high grazing pressure of N. taihuensis led to an obvious reduction in the abundance of small bodied calanoids, with an increase in the abundance of a large bodied calanoid Neodiaptomus schmackeri (Z.R.…”

Molecular and microscopic diversity of planktonic eukaryotes in the oligotrophic Lake Stechlin (Germany)

“…Since 1994, a regular sampling program has been carried out with, at average, biweekly sampling, that led to (1) detailed study of species composition and succession pattern of phytoplankton (Krienitz & Scheffler, 1994, Padisák et al, 1998, (2) discovery of a freshwater deep chlorophyll maximum formed by the picocyanobacterium Cyanobium (Gervais et al, 1997;Padisák et al, 1997), (3) description of population dynamics of phycogeographically restricted unicellular diatoms (Cyclotella tripartita and Stephanocostis chantaicus -Scheffler & Padisák, 1997, Scheffler & Padisák, 2000, (4) description of a new species of Eustigmatophyta (Pseudotetraedriella kamillae; Hegewald et al, 2007), and (5) by now provided a data bank that is suitable for analyses of long-term trends of phytoplankton changes (Padisák et al, 2010a, b). As the grazer food chain in the lake is rather weak (Mehner et al, 2008), microbial loop involving a number of mixo-and heterotrophic protists, is of prior importance especially in the winter isothermal period and during the clear water phase. However, due to the limitations of the inverted microscope method, identification of these flagellates is confused and incomplete.…”

Seasonal dynamics of plankton communities coupled with environmental factors in a semi arid area: Sidi Sad reservoir (Center of Tunisia)

“…Zooplankton is an important link in freshwater aquatic ecosystems, grazing on and controlling the phytoplankton (Mitra et al, 2007;Mehner et al, 2008;Li et al, 2009) and bacterioplankton (Wikner et al, 1990). Primary producers in aquatic ecosystems are affected by resources ("bottom-up" effects) and consumers ("topdown" effects) (McQueen et al, 1986(McQueen et al, , 1989.…”