Bottom-up or top-down – How is the autotrophic picoplankton mainly controlled? Results of long-term investigations from two drinking water reservoirs of different trophic state

Abstract

In two reservoirs of different trophic state, the abundance of autotrophic picoplankton was continuously assessed for 7 and 11 years, respectively. Simultaneously, other important environmental and biotic factors (nutrients, temperature, phyto-and zooplankton) were analysed. The mesotrophic Saidenbach Reservoir receives five-to sevenfold higher phosphorus imports, and exhibits twice higher phosphorus contents, and tenfold higher phytoplankton concentrations than the oligotrophic Neunzehnhain Reservoir. Despite these differences, the quantities and dynamics of autotrophic picoplankton (APP) showed remarkable similarities in both reservoirs. The APP consisted almost exclusively of Phycoerythrin-rich cyanobacteria. A notable growth of cyanobacteria never started before the onset of stratification and finished no later than the beginning of the autumnal turnover. Although the soluble reactive phosphorus (SRP) concentrations in the epilimnion were always near the limit of detection, there was no evidence for growth restriction of APP by nutrients in either of the reservoirs. However, the data reveal a strong grazing impact of all potential grazers, especially of the daphnids. This zooplankton group always showed a strict behaviour opposite of the APP during its growth periods. The grazing influence of rotifers and protozoans is less pronounced but also obvious, notable by the frequently observed inverse relationships between their quantities and those of APP. It is assumed that during the stratification period in the summer the APP dynamics is mainly top-down controlled.

Distribution of zooplankton related to environmental factors in three interconnected reservoirs: Kasseb, Mornaguia and Ghdir El Goulla (North of Tunisia)

“…These findings confirm the results of Cherbi et al (2008). Thus zooplankton is an important grazer of algae and it is well known during periods of high zooplankton abundance (Horn et al, 2008).…”

Local and regional factors influencing zooplankton communities in the connected Kasseb Reservoir, Tunisia

“…Thus, zooplankton is probably an important grazer of algae during periods of high zooplankton abundance. This is supported by the findings of a study by Horn and Horn (2008). There was no clear trophic gradient in Kasseb Reservoir; some horizontal differences in zooplankton abundance and community structure were observed.…”

Fading out of the trophic cascade at the base of the microbial food web caused by changes in the grazing community in mesocosm experiments

“…It seems that in this treatment not only ciliates and NF, but also rotifers and crustaceans, which began to increase toward the end of the experiment, played an important role in controlling the picoplankton community. Although nanoflagellates are generally considered the main APP consumers (Callieri and Stockner 2002), Horn and Horn (2008) argue that crustaceans had the strongest effect on APP, basing their long-term study on observed inverse relationships between APP and various consumers groups. In our experiment, we demonstrated an active and successful replacement of various grazers of both picoplanktonic communities: eu-APP and bacteria.…”