ABSTRACT

The premise of this review is that climate effects on lakes can be quantified most effectively by the integration of process-oriented limnological studies with paleolimnological research, particularly when both disciplines operate within a common conceptual framework.  To this end, the energy (E) – mass (m) flux framework (Em flux) is developed and applied to selected retrospective studies to demonstrate that climate variability regulates lake structure and function over diverse temporal and spatial scales through four main pathways: rapid direct transfer of E to the lake surface by irradiance, heat, and wind; slow indirect effects of E via changes in terrestrial development and subsequent m subsidies to lakes; direct influx of m as precipitation, particles, and solutes from the atmosphere, and; indirect influx of water, suspended particles, and dissolved substances from the catchment.  Sedimentary analyses are used to illustrate the unique effects of each pathway on lakes, but suggest that interactions among mechanisms are complex and depend on the landscape position of lakes, catchment characteristics, the range of temporal variation of individual pathways, ontogenetic changes in lake basins, and the selective effects of humans on m transfers.  In particular, preliminary synthesis suggests that m influx can overwhelm the direct effects of E transfer to lakes, especially when anthropogenic activities alter m subsidies from catchments.

ABSTRACT

Paleolimnological approaches were used to assess the ecological and environmental implications of diatom assemblage compositional changes recorded over the last ca. 200 years from four sites in the Lake of the Woods (LoW), Ontario, Canada. Comparisons between a reference site (Whitefish Bay) and three disturbed sites (Bigstone Bay, Paleolimnological Project site #1 (PP-1), and Forrest Island) provide insights into the effects that multiple stressors (dam construction, total phosphorus (TP) changes, and recent warming) have had on the ecology of this highly complex freshwater system. Overall patterns of diatom compositional changes from high-resolution 210Pb-dated sediment cores revealed a strong temporal coherence ca. 1910 and then again over the last few decades among all sites. Hydromanagement activities at the turn of the 19th century and recent warming over the last few decades played key roles in the LoW diatom changes. Diatom compositional changes at all sites were significantly related to trends in nearby air temperature records and to changes in lake ice phenology over the past ca. 40 years from Whitefish Bay. Turn-of-the-century hydromanagement activities do not appear to have had long-term effects on diatom-inferred total phosphorus (DI-TP), particularly at the disturbed sites. Clear decreases in DI-TP over the last few decades were evident at all sites, but at the reference site were well below pre-disturbance levels, which we link in part to recent warming. Substantial changes in climate will amplify the effects of multiple stressors on lakewater properties that have major implications on algal communities.

ABSTRACT

Comparisons between diatom assemblages preserved in modern and pre-industrial sediments were made to provide a lake-wide paleolimnological assessment of environmental change in 17 sites located throughout the Ontario portion of the Lake of the Woods (LoW). Diatom changes were consistent across all sites, although the magnitude of these changes varied along a trophic gradient (i.e., main channel sites versus isolated bays). The most notable taxonomic shift was towards a higher relative abundance of small, centric Cyclotella taxa and other planktonic, pennate diatoms (e.g., Asterionella formosa, Fragilaria crotonensis) in the modern sediments, with corresponding lower relative abundances of heavily silicified Aulacoseira taxa and small benthic Fragilaria taxa. To aid in determining whether changes in nutrients can explain the diatom trends, weighted averaging partial-least-squares (WA-PLS) techniques were used to develop a diatom-based inference model to reconstruct changes in total phosphorus (TP) concentrations. Diatom-inferrred TP (DI-TP) reconstructions revealed that 88% of the sites showed either no change or a slight, but not significant decline in DI-TP since pre-industrial times. Diatom-based inferences suggest that TP concentrations at many sites in the Ontario portion of the LoW were moderately elevated in nutrients prior to any major human disturbances (i.e., pre-1850). Results suggest that substantial increases in temperature over the last few decades, and the associated changes to ice cover and water column properties, have likely exacerbated the effects of existing stressors on the system and were key factors influencing a lake-wide restructuring of the diatom communities over the past ca. 150 years.

ABSTRACT

Chironomid and Chaoborus (midge) remains preserved in a dated sediment core from Poplar Bay, Lake of the Woods (LOW), Ontario, Canada, were used to assess the effects of multiple stressors (e.g., recent warming and shoreline development) on water quality over the past ~200 years. As monitoring data for LOW do not extend beyond recent decades, paleolimnological methods are used to reconstruct long-term limnological trends and to establish pre-disturbance conditions. The effects of recent warming and shoreline development on Poplar Bay water quality are examined using an index of hypolimnetic oxygen (O2) status based on the ratio of Chaoborus to chironomid remains (chaob:chir) and a midge-inferred volume-weighted hypolimnetic oxygen (VWHO) model. Our paleolimnological data indicate that hypolimnetic [O2] in Poplar Bay have been historically hypoxic (1-4 mg O2 L-1) but have declined further (generally <2 mg O2 L-1) over the last few decades. Significant relationships between air temperature and midge data indicate that substantial warming starting in the late-1970s has triggered a marked response in the midge assemblages that pre-dates the onset of cottage development (mid-1990s). These findings complement a diatom-based study on the same sediment core, likewise suggesting that recent warming has played a prominent role in structuring limnetic communities. However, it is likely that the full, compounded effects of recent warming and shoreline development have not yet been realized. Our study highlights the complexity of multiple stressor systems, such as Poplar Bay, and emphasizes the benefits of using multiple, independent lines of paleoenvironmental evidence in gaining a more complete understanding of historical water quality.