Trajectories of long-term ecological change in shallow lakes: allochthonous drivers and autochthonous stability

Background/Question/Methods

Shallow lakes can be subject to two ecological extremes; a clear-water, macrophyte-dominated or turbid, algal-dominated state. The drivers of ecological instability that promote the success of these end-member states can overlap and interact making management of these ecosystems frustrating. Direct links between landscape processes and ecological state remain elusive. The main focus of the majority of research has been on contemporary, short-term autochthonous processes. We pose the question: Are the long-term ecological trajectories an effective means of describing the underlying mechanisms of ecological stability in shallow lakes? To address this question we couple contemporary and paleo-limnological methods from a number of shallow lakes in the Prairie Pothole Region (PPR) of North America. We selected lakes which currently exist in either a turbid or clear state. The PPR is rich in shallow lakes and wetlands and the landscape has been almost exclusively converted to agriculture, which is often associated with a dramatic change in allochthonous inputs. 

Results/Conclusions

Our research has highlighted that significant differences exist between states in short-term autochthonous processes (e.g. turnover rate of DOM), however these processes do not translate into strong differences in the net ecosystem production between states. Over decadal timescales we find that the variability of OC burial does not correspond to ecological state. We find a number of allochthonous inputs and drivers influence each lake over variable timescales, but the overall long-term ecological stability of the lake is controlled largely by trophic interactions. In this talk we present long-term case studies that highlight allochthonous drivers - such as phosphorus inputs, hydrologic alteration, and clastic detrital inputs from the watershed – and the in-lake response. We show that the ecological stability of a clear-water state can be controlled by food web stability (i.e. lack of planktivorous and benthivorous fish), the physical setting of the lake, and nutrient inputs. Our work highlights the importance of both allochthonous inputs and in-lake processes in promoting long-term stable, clear-water shallow lakes.