Abstract
Microbial planktonic communities are the basis of food webs in aquatic ecosystems since they contribute substantially to primary production and nutrient recycling. Network analyses of DNA metabarcoding data sets emerged as a powerful tool to untangle the complex ecological relationships among the key players in food webs. In this study, we evaluated co-occurrence networks constructed from time-series metabarcoding data sets (12months, biweekly sampling) of protistan plankton communities in surface layers (epilimnion) and bottom waters (hypolimnion) of two temperate deep lakes, Lake Mondsee (Austria) and Lake Zurich (Switzerland). Lake Zurich plankton communities were less tightly connected, more fragmented and had a higher susceptibility to a species extinction scenario compared to Lake Mondsee communities. We interpret these results as a lower robustness of Lake Zurich protistan plankton to environmental stressors, especially stressors resulting from climate change. In all networks, the phylum Ciliophora contributed the highest number of nodes, among them several in key positions of the networks. Associations in ciliate-specific subnetworks resembled autecological species-specific traits that indicate adaptions to specific environmental conditions. We demonstrate the strength of co-occurrence network analyses to deepen our understanding of plankton community dynamics in lakes and indicate biotic relationships, which resulted in new hypotheses that may guide future research in climate-stressed ecosystems.
Highlights
Aquatic food webs are the basis for ecosystem functioning (Azam, 1998; Sommer et al, 2012)
We interpret these results as a lower robustness of Lake Zurich protistan plankton to environmental stressors, especially stressors resulting from climate change
We demonstrate the strength of co-occurrence network analyses to deepen our understanding of plankton community dynamics in lakes and indicate biotic relationships, which resulted in new hypotheses that may guide future research in climate-stressed ecosystems
Summary
Aquatic food webs are the basis for ecosystem functioning (Azam, 1998; Sommer et al, 2012). We constructed and analysed networks of protistan plankton communities in the surface (epilimnion) and deep (hypolimnion) water layers of two deep temperate lakes in the Alpine European Region, namely Lake Mondsee (Austria) and Lake Zurich (Switzerland) Both lakes were strongly influenced by anthropogenic induced eutrophication during the 20th century, but were successfully restored owing to the reduction of nutrient immissions (Dokulil & Teubner, 2005). Complete oxygen depletion in the hypolimnion is exceptional and complete water turnover (holomixis) reaching the deepest zones in Lake Mondsee is still ensured In both lakes, ciliates belong to ecologically and functionally important components of the plankton communities that are well-characterized, owing among others to their role as top predators among protists (e.g. Müller et al, 2002; Pitsch et al, 2019; Posch et al, 2015; Salbrechter & Arndt, 1994). We assume a higher vulnerability of ecosystem structures in Lake Zurich owing to its described sensitivity to climate-induced stressors (Yankova et al, 2017)
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