Rapid Ecological Change in Two Contrasting Lake Ecosystems: Evidence of Threshold Responses, Altered Species Dynamics, and Perturbed Patterns of Variability

Abstract:

Studying threshold responses to environmental change is often made difficult due to the paucity of monitoring data prior to and during change. Progress has been made via theoretical models of regime shifts or experimental manipulation but natural, real world, examples of threshold change are limited and in many cases inconclusive. Lake sediments provide the potential to examine abrupt ecological change by directly observing how species, communities, and biogeochemical proxies responded to environmental perturbation or recorded ecosystem change. These records are not problem-free; age uncertainties, uneven and variable temporal resolution, and time-consuming taxonomic work all act to limit the scope and scale of the data or complicate its analysis.

Here I use two annually laminated records

1. Kassjön, a seasonally anoxic mesotrophic lake in N Sweden, and
2. Baldeggersee, a nutrient rich, hardwater lake on the central Swiss Plateau

to investigate lake ecosystem responses to abrupt environmental change using ideal paleoecological time series.

Rapid cooling 2.2kyr ago in northern Sweden significantly perturbed the diatom community of Kassjön. Using wavelet analysis, this amelioration in climate also fundamentally altered patterns of variance in diatom abundances, suppressing cyclicity in species composition that required several hundred years to reestablish. Multivariate wavelet analysis of the record showed marked switching between synchronous and asynchronous species dynamics in response to rapid climatic cooling and subsequent warming.

Baldeggersee has experienced a long history of eutrophication and the diatom record has been used as a classic illustration of a regime shift in response to nutrient loading. Time series analysis of the record identified some evidence of a threshold-like response in the diatoms. A stochastic volatility model identified increasing variance in composition prior to the threshold, as predicted from theory, and a switch from compensatory to synchronous species dynamics, concomitant with eutrophication, was observed.

These results document in high resolution how two aquatic systems reacted to abrupt change and demonstrate that under ideal conditions sediments can preserve valuable evidence of rapid ecological change.