Ecosystems worldwide are affected by external perturbations, like extreme climate events, invasive species, or altered nutrient dynamics. However, we still know surprisingly little about the factors that drive the temporal dynamics of ecosystems and the determinants of resistance and resilience to perturbations. Empirical data is much needed to validate the large body of theory about ecological dynamics of shallow lake ecosystems. To do so I am using some of Eawag "state of the art" equipment to create high resolution ecological time-series of experimental ecosystems (mesocosms and ponds) at Eawag Kastanienbaum and Eawag Dübendorf. 

Calibration of high-frequency multi-parameter sondes
Sondes are deployed in mesocosms (1000 L) at Eawag Kastanienbaum
Experimental ponds (15 000 L) at Eawag Dübendorf.
Variation in phytoplankton communities in the Dübendorf ponds

Temporal dynamics of aquatic ecosystems

Theory predicts that ecological dynamics of shallow lake ecosystems are determined by interactions between keystone species: submerged macrophytes compete with suspended microalgae for light and nutrients (Ibelings et al. 2007). Dominance of either group determines the state of the ecosystem: "clear" or "turbid" (Scheffer et al. 1993). However, little is known about how short term dynamics like daily patterns of ecosystem parameters and metabolism are affected by the presence of keystone species.

To investigate seasonal and diurnal dynamics of species interactions in lake ecosystems we conducted an experiment using outdoor mesocosms. We manipulated the presence and absence of macrophytes (Myriophyllum spicatum and Chara tomentosa) and monitored the evolution of the ecosystems from spring to fall with high temporal resolution. We found that densities of microalgae were lower in ecosystems with macrophytes, which also yielded higher metabolic activity (higher productivity and respiration).

  • Ibelings, B. W., R. Portielje, E. H. R. R. Lammens, R. Noordhuis, M. S. van den Berg, W. Joosse, and M. L. Meijer. 2007. Resilience of Alternative Stable States during the Recovery of Shallow Lakes from Eutrophication: Lake Veluwe as a Case Study. Ecosystems 10:4–16.
  • Scheffer, M., S. H. Hosper, M. L. Meijer, B. Moss, and E. Jeppesen. 1993. Alternative equilibria in shallow lakes. Trends in ecology & evolution 8:275–279.

 

Conceptual model for interactions and feedbacks in shallow lake ecosystems (Scheffer et al. 1993)

Response of aquatic ecosystems to perturbation

The ecological resilience of ecosystems is governed by the capacity of keystone species to resist and recover from external perturbations that threaten the ecosystem’s current state (Scheffer et al. 1993). Especially eutrophication, the inflow of nutrients, is a threat to the resilience of aquatic ecosystems worldwide (Hargeby et al 1998). However, there is little experimental evidence about how species interactions affect variation in resilience of ecosystems.

In a large scale pond experiment we are measuring the response of ecosystems with different community-compositions to perturbations with nutrients. Specifically, we are testing how interactions between macrophytes (Myriophyllum spicatum), benthic grazers (Dreissena polymorpha), and phytoplankton mediate ecosystem responses to nutrient perturbations.  

  • Blindow, I., A. Hargeby, and G. Andersson. 1998. Alternative Stable States in Shallow Lakes: What Causes a Shift? Pages 353–360 in E. Jeppesen, M. Søndergaard, M. Søndergaard, and K. Christoffersen, editors. The Structuring Role of Submerged Macrophytes in Lakes. Springer New York
  • Scheffer, M., S. H. Hosper, M. L. Meijer, B. Moss, and E. Jeppesen. 1993. Alternative equilibria in shallow lakes. Trends in ecology & evolution 8:275–279.

 

Species composition may determine the ecosystem's response to perturbations (in this case nutrient additions).


Sampling water, vegetation and benthos from our experimental ecosystems at Eawag Kastanienbaum