Updated November 5th, 2002
Fish Population Responses to Experimental Acidification of a Small Ontario Lake
Author: Kennth H. Mills
Source: Early Biotic Responses to Advancing Lake Acidification (Acid Precipitation Series Vol. 6)
Date: 1984
As lakes are slowly acidified to a pH approaching or less than 5.0, fish populations decline precipitiously or disappear. However, accurate pH thresholds for reproductive failure or population decline of many fish species from natural population are poorly known and are reported generally as a wide pH range based on species presence-absense.

Physical and chemical changes

water transparency has increased, along with rates of hypolimnetic heating and thermocline deepiening

Concentrations of Mn, Na, Zn, H+, S2O4-, Al increased

Aluminum has been implicated as a major cause of fish mortality during lake acidification

S2O4-was reduced by bacteria to sulfide, followed by permanenet sedimentation as FeS. Alkalinity, genrated as byproduct of sulfate reduction , has neutralized apporximately one-third of the hydrogen ion added to the lake. Therefore, a pH refuge has persisted below throughout the acidification, but the long-term trend has been for this refuge to become progressively more acidic, although temporally lagging behind the epilimnion.

Primary production and Invertebrates

primary production has increase in Lake 223 above preacidification levels

Phytoplankton species composition has changed with Chlorophyceae and Peridineae replacing Chrysophyceae

Appearance of hypolimnetic algal peak of Chlorella

Three members of the zooplankton community Mysis relicta, Epischura lacustris, Diaptomus sicilis disappeared as pH declined to 5.4 while Daphnia catawba x schoedleri appeared

Responses of Fish Populations to Acidification As the pH of Lake 223 was lowered from 6.7 to 5.1

the fathead minnow population declined rapidly and almost disappeared when pH was 5.6. In addition, complete reproductive failure, rapid collapse of population were observed

The pearl dace population rapidly expanded to become the major minnow species when pH was 5.4. This was probably due to its greater tolerance to low pH by pearl dace than fathead minnow

White sucker (seen as relatively acid-tolerant species) showed no stress as the pH of the lake was lowered. Its individual fish growth remained consistently high.

Lake trout (relatively acid sensitive) showed decrease in population when pH was lowered from 6.7 to 5.4. However, its population did not decrease at the rate which was expected - it was much slower.

Implications of the Lake 223 acidification for Fisheries Assessment

Many changes occurred in Lake 223 before reproductive failures of lake trout and white suckers, but an ample food base for each species was maintained. It is not surprising that increase, decrease, or no change in growth have all been described for various game fish species in lakes undergoing acidification. The presence or absence of acid tolerant forage species probably dictates the responses of game fish populations to the initial phages of lake acidification. For example, if pearl dace had not been present in Lake 223, growth of lake trout might have declined rapidly after the disappearance of fathead minnows. In addition, the food web of lake 223 was very resilient to loss of some invertebrate and fish species.

Long-lived species of fish will probably remain abundant in acid lakes for many years after natural reproduction has crease further masking the effects of acidification on the fish

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Physical and chemical changes	water transparency has increased, along with rates of hypolimnetic heating and thermocline deepiening Concentrations of Mn, Na, Zn, H+, S2O4-, Al increased Aluminum has been implicated as a major cause of fish mortality during lake acidification S2O4-was reduced by bacteria to sulfide, followed by permanenet sedimentation as FeS. Alkalinity, genrated as byproduct of sulfate reduction , has neutralized apporximately one-third of the hydrogen ion added to the lake. Therefore, a pH refuge has persisted below throughout the acidification, but the long-term trend has been for this refuge to become progressively more acidic, although temporally lagging behind the epilimnion.
Primary production and Invertebrates	primary production has increase in Lake 223 above preacidification levels Phytoplankton species composition has changed with Chlorophyceae and Peridineae replacing Chrysophyceae Appearance of hypolimnetic algal peak of Chlorella Three members of the zooplankton community Mysis relicta, Epischura lacustris, Diaptomus sicilis disappeared as pH declined to 5.4 while Daphnia catawba x schoedleri appeared
Responses of Fish Populations to Acidification	As the pH of Lake 223 was lowered from 6.7 to 5.1 the fathead minnow population declined rapidly and almost disappeared when pH was 5.6. In addition, complete reproductive failure, rapid collapse of population were observed The pearl dace population rapidly expanded to become the major minnow species when pH was 5.4. This was probably due to its greater tolerance to low pH by pearl dace than fathead minnow White sucker (seen as relatively acid-tolerant species) showed no stress as the pH of the lake was lowered. Its individual fish growth remained consistently high. Lake trout (relatively acid sensitive) showed decrease in population when pH was lowered from 6.7 to 5.4. However, its population did not decrease at the rate which was expected - it was much slower.