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54 south-central Ontario lakes

July 26, 2006   

Excerpts from:

Hall, R.I. and Smol, J.P. 1996. Paleolimnological assessment of long-term water-quality changes in south-central Ontario lakes affected by cottage development and acidification. Can. J. Fish. Aquat. Sci. 53:1-17

Little, J.L., Hall, R.I., Quinlan, R., and Smol, J.P. 2000. Past trophic status and hypolimnetic anoxia during eutrophication and remediation of Gravenhurst Bay, Ontario: comparison of diatoms, chironomids, and historical records. Can. J. Fish. Aquat. Sci. 57:333-341.

Contents:

 Conclusions/Summary

 Exceptions:

• Declining lake water [TP] in mesotrophic lakes
  
• pH increase in alkaline lakes
  
• Initial surprise- Gravenhurst Bay
  

 Approach

 Sediment cores

• Shortcomings
  

 Study area and lake selection

 Diatom inference model

Conclusions/Summary

Postindustrial pH and [TP] changes were inferred from surface and pre-1850 sediment diatom assemblages. Results suggest that presently acidic lakes (pH <6) have acidified, and the pH of alkaline lakes (pH >7) has increased as observed in other regions receiving elevated acidic deposition. Diatom-inferred [TP] suggests that cultural eutrophication has not been widespread. In almost all lakes, present-day [TP] is not higher than before European settlement. In many mesotrophic lakes, preindustrial [TP] was higher than at present.

Several factors could account for declining lake water [TP], including:

1. lake and (or) watershed acidification processes that retain P within catchments or that increase the rate of P loss from lakes (e.g., the formation and enhanced precipitation of P-Al compounds).
   • For example, three of the lakes (Chub, Crosson, and Leech) with the largest diatom-inferred [TP] declines (9, 7, and 7 µg.L^-1, respectively) also had the greatest inferred pH declines, on the order of 0.4 to 1.0 pH units.
   • In all cases where inferred lake water pH declined significantly since before 1850, diatom-inferred [TP] never increased, suggesting at least some relationship between pH and [TP] trends within this lake set; and
   
   
2. reductions in nutrient loading from watersheds as a result of reforestation. The mature old-growth forest that existed prior to European settlement may have supplied greater P loads to lakes than the younger (50 to 100 year old), logged, and aggrading forests that surround the lakes today.

However, many other anthropogenic changes have taken place that could also reduce lake water [TP], including

1. reduction in forest fires,
2. alterations in aquatic food-web structure as a result of increases in fish stocking, fish harvesting (angling), and other lake-management activities, and
3. altered hydrology through dam construction.

pH increase in alkaline lakes:

Atleast two factors may be responsible for these inferred pH increases, including

i) watershed disturbances that increase ANC (acid-neutralizing capacity), and

ii) biogeochemical ANC-generating reactions, such as sulphate reductions by bacteria and assimilation of atmospherically deposited nitrogen compounds by plants.

Initial surprise- Gravenhurst Bay:

Study area and lake selection

The study area lies within the Great Lakes — St. Lawrence Forest Region. The landscape is predominantly forested and non-agricultural owing to the rough topography, thin soils, and cool climate. Several rural municipalities exist in the region, but seasonal recreation is the dominant human activity. The shores of most lakes are dotted with cottages, and most are serviced with septic sewage systems.

Clear-cut logging was a major industry of the past, with the last major forest clearing 60-100 years ago. Agricultural activity is almost nonexistent.

Lakes were specifically chosen to sample the broadest trophic status gradient possible, and they ranged from very oligotrophic (spring overturn [TP] =2.7 µg.L^-1) to mesotrophic (24.3 µg.L^-1). Few, if any, eutrophic lakes exist in this region. To minimize the influence of pH and pH-related variables on the distribution of diatom taxa, lakes with pH <5.5 or with [DOC] >6.5 mg.L^-1 were not included. Otherwise, the lake set included broad gradients of physical and water chemistry characteristics typical of this region.

Diatom inference model

Classical deshrinking regression equations were as follows:

for pH: y = 0.315x + 4.51, and

for [TP] (µg.L^-1): y = 0.134x + 6.56

Table: The optima and effective number of occurrences (Hill's N2) of the diatom taxa used in ordinations and weighted-average regression and calibration models

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