1. Project Title: Mitigation of Low pH in Salmonid Streams
2. Contact
Gilles L. Lacroix
Department of Fisheries and Oceans, Biological Station, St. Andrews, NB E0G 2X0
Tel.: (506) 529-8854; FAX: (506) 529-5862
3. Agencies Involved
Department of Fisheries and Oceans
Nova Scotia Department of Fisheries
4. Restoration Goal
Increase stream and interstitial water pH to protect fish at times of acidic episodes.
5. Project Type
Addition of limestone gravel to the bed of a section of a small acidified brook to form bars for water contact and percolation to increase pH and mitigate low pH effects on fish.
6. Background and Rationale
Fifteen Mile Brook, Nova Scotia, a tributary of the Medway River, is regularly used by Atlantic salmon and brook trout for spawning and provides nursery habitat for these species. However, as a result of annual acidic episodes that occur in autumn lasting through the winter, juvenile salmon rarely survived to be yearlings. Brook trout did survive, but were stressed physiologically by the low pH.
In September 1987, the lower section of Fifteen Mile Brook was limed by adding crushed limestone to the bed of the brook in several areas. Limestone was again applied on a smaller scale in November 1991 to replenish some areas.
The intent was first to provide a better environment for egg, alevin and fry survival, and then to limit the lethal effects on juveniles during acidic episodes. In contrast to headwater lake liming, the application of limestone to stream beds is relatively inexpensive if road access is available and the limestone should not have to be replenished on an annual basis. The method was selected for its suitability for use in running waters in small streams with no headwater lakes that could be used for liming. However, the methodology had only been tested on several occasions and with limited success, and no long-term biological assessment had previously been done.
7. Regulatory Considerations
An Initial Environmental Assessment form, an internal review procedure, was completed prior to project initation. For other participants, habitat modification permits would be required.
8. Criteria
The stream selected for liming must be small with low water discharge; usually <1 m3/s and, preferably, <10 m3/s at times of peak flows. The amount and size of limestone required is a function of stream size and flow. However, effectiveness is ultimately related to water contact time with the limestone and water temperature. The bars selected for adding crushed limestone should be shallow and provide a large surface area for water contact and percolation through the gravel. The bars must also be located in channel sections where conditions and features lead to bar stability to prevent loss of the limestone downstream to pools where it loses contact with much of the flowing water, severely reducing its effectiveness.
The physical parameters of the stream sites are of key importance to the success of this type of limestone mitigation technique. The stability of the limestone bars is dependent on stream hydrology and the physical characteristics of the channel and stream bed. Straight, shallow and evenly-based stream sections are the most stable sites for successful gravel application. Winding, deep, or unevenly-based sections prove to be unstable for treatment.
Limestone gravel is best applied during low flow periods in late summer. Sites should be assessed beforehand to assure stability and should be chosen according to the following guidelines: (a) The site should have a relatively level substrate of cobble, gravel, and boulders; (b) Sites should have a run or riffle morphology; (c) A site may have a pool located upstream, but no pool immediately downstream; and (d) The site should not have sharp turns or bends in the stream bed, especially if the site is less than 15 m wide.
9. Project Design
The lower 800 m of Fifteen Mile Brook to the mouth of the brook were used as an experimental area. The brook enters the Medway River which is located on the southwest coast of Nova Scotia. A control section of 250 m, farthest upstream, was left unlimed and undisturbed. A 300 m section of the brook, downstream of the control section was limed by adding crushed limestone to 8 bars across the brook at 25-75 m intervals. This was done to avoid creating a barrier on one location at low flow, to modify existing spawning habitat, and to establish a longitudinal pH gradient. A section of 250 m, downstream of the last limestone bar and extending to the mouth of the brook, was left undisturbed.
The limestone bars were positioned at the downstream end of pools and at the head of riffles or in runs to simulate what is considered to be preferred spawning habitat characteristics for salmonids. The bars were shaped to create minimal disturbance to channel characteristics, water depth, and stream flow routing. The depth of crushed limestone added was also limited to ensure that the bars did not impede migratory fish movement.
10. Implementation
Approximately 200 tonnes of crushed, graded dolomitic limestone were added to the bed of Fifteen Mile Brook in 1987 and an additional 100 tonnes were added in 1991. The 4 cm grade "Fluxstone" limestone was delivered to several sites along the brook by the Moser Limestone Co. Ltd. of Upper Musquodoboit, Nova Scotia. The limestone was initially distributed over eight separate areas using light machinery to form bars 30-50 cm deep across the brook. The second limestone application supplemented 4 of these bars. The limestone was moved with a small front-end loader (approximately 2 m3 bucket) to the desired location for application using several entry points along the stream. The machinery was driven along exposed sections of stream bed at times of extremely low water flows. After dumping, the limestone was spread and leveled by dragging the bucket in an upstream direction. Some further spreading and shaping of the bars was done manually using rakes to achieve the desired stream bed morphology and flow characteristics preferred by salmonids for spawning.
The amount of limestone initially added to form each bar ranged from 7.5 to 15 m3 (16.7 - 33.5 tonnes). The amount added to four of the bars on the second application was about the same as that initially spread on each of these bars. Throughout the procedure, care was taken to avoid pools and to minimize disturbances instream to habitat adjacent to the limed areas and to stream banks.
11. Degree of Environmental Intervention
This liming methodology involves the application of gravel of foreign origin, dolomitic limestone, to the stream bed. This resulted in changes in water chemistry parameters such as pH, alkalinity, calcium, and magnesium. It also resulted in changes in morphometry, microhabitat water velocities, and bed material characteristics in treated sections. Application was done with small machinery and manually, and involved minimal disturbance to habitat in and along the stream when done at times of low water flow in late summer. No spawning fish were present in the stream at that time, and young-of-the-year had already moved to deeper habitat than the shallow areas used for limestone application. They would, therefore, not have been directly affected by physical disturbance associated with application of the limestone or modification of microhabitat in those areas.
For several days after initial application of the limestone, the water turned milky because of rinsing of limestone dust from the crushed material. Water turbidity and pH increased markedly. This condition was repeated at the time of the first major increase in stream flow when areas of exposed limestone were submerged for the first time and remaining fines were washed out of the limestone. However, no significant adverse effects on biota were detected as a result of this flushing. Some in situ bioassays with juvenile Atlantic salmon and brook trout were conducted during one such flushing episode. The application of limestone gravel to the stream bed only needs to be repeated infrequently; it is a relatively long-lasting substance which dissolves according to temperature and flow. This is in contrast to methods using a limestone slurry addition to the stream or the application of powdered lime to headwater lakes which require frequent intervention. Therefore, the approach used minimizes the impact and degree of environmental intervention directly associated with liming for mitigation of low pH.
12. Cost
1987 Limestone (200 tonnes) $1 100
Trucking $2 926
Application $1 000
Total $5 026
1991 Limestone (100 tonnes)
and trucking $2 000
Application $2 500
Total $4 500
NOTE: Costs of chemical and biological assessments are not included.
13. Biological Assessment
An assessment of the impact of limestone application on water chemistry and on fish populations in the brook was conducted over 8 years. This assessment involved comparisons of limed and unlimed sections of the brook. Water chemistry was monitored on a weekly to monthly basis in both sections, and periodically at sites along the brook. Stream discharge was monitored weekly and water temperature was recorded continuously. Interstitial conditions in the stream bed also were monitored in both sections over several winters.
The density of all fish species present in both sections of the brook was estimated twice annually using the sequential-removal method, and their biomass was determined separately by age-class. Counts of Atlantic salmon spawning redds were made annually in both sections of the brook. The emergence success of Atlantic salmon and brook trout alevins was assessed on several occasions. Microhabitat use by brook trout and juvenile Atlantic salmon was also examined over two years in response to acidic episodes and the longitudinal pH gradient related to liming, and in relation to changes in available microhabitat as a result of limestone application. In situ bioassays to assess the survival of juvenile Atlantic salmon and brook trout were also conducted over three years in limed and unlimed sections of the brook during acidic episodes in autumn and spring. These bioassays were complemented with detailed measurements of the physiological responses of the fish to chemical conditions in each section of the brook.
14. Measures of Success
The following could serve as indicators or measures of some degree of success of the application of limestone to the stream bed to mitigate low pH in a section of stream:
increase in stream and interstitial water pH and associated chemical parameters;
intensive use of limestone bars for redd digging and spawning by salmon;
increased survival of salmon and brook trout eggs, alevins, and fry;
survival of age-0 Atlantic salmon past the acidic episodes in autumn;
decreased physiological stress for brook trout and juvenile salmon;
increased density and biomass of all age classes of juvenile salmon and brook trout;
stability of microhabitat use by juvenile salmon and brook trout along the limed section;
recolonization by other fish species sensitive to low pH and their survival.
Success Rating: 4
Some degree of chemical and biological success was evident but with gradual return towards pre treatment conditions over an 8-year period.
15. Key References
Lacroix, G. L. 1992. Mitigation of low stream pH and its effects on salmonids. Environ. Pollut. 78: 157-164.
Lacroix, G. L. 1995. Stability of microhabitat use by brook trout and juvenile Atlantic salmon after stream acidification. Trans. Amer. Fish. Soc. (in press)
Mayhew, H. 1989. The effects of limestone gravel application to two acidic Nova Scotian streams. N.S. Dep. Fish., Econ. Reg. Dev., Agreement Rep. No. 22, Pictou, N.S.
Correct citation for this contribution:
Lacroix, G.L. 1995. Mitigation of low pH in salmonid streams, p. 59-63. In J.R.M. Kelso and J.H. Hartig [editors]. Methods of modifying habitat to benefit the Great Lakes ecosystem. CISTI (Can. Inst. Sci. Tech. Inf.) Occas. Pap. No. 1.