The dictionary tells us that turbidity is the quality of being cloudy, opaque, or thick with suspended matter. Something we don’t want to see in our lakes. Scientists have studied the causes and effects of turbidity in lake water. Here are some of their findings:
- Wave action and turbulence from motor boats in shallow water produce sediment suspension and release nutrients and pollutants into the water column. (Alexander and Wigart, 2013 ; Bastien et al., 2009; Gélinas et al., 2005; Wang et al., 2009; Zoumis et al., 2001.)
- Boat waves contribute to shoreline erosion and poor water clarity… nearshore turbidity is elevated in many waterways during periods of high boating activity … shorelines that are exposed to high boating pressure… experience high erosion that cannot be attributed to wind wave energy. (Biklovic et al., 2019)
- The mass of suspended material from boat disturbance was relatively large, and the total phosphorus increased. Lenzi et al. (2013)
These findings led Sebastien Raymond and his colleagues at the University of Laval in Quebec to conduct a study in 2015 “to assess the impact of wakeboat motorized boats, in order to broaden knowledge about the impact of navigation on lakes, including sediment resuspension.” They aimed to define the impact of the wake boat propellor stream, and to measure the re-suspension of sediments in the water.
They ran wake boats on two lakes in Quebec, at various speeds. They placed an Acoustic Doppler Current Profiler and other instruments at various depths underwater to measure the velocity of the water, its temperature, turbidity, and level of dissolved oxygen. They ran the wake boats in various modes and measured the results. The researchers found that in ballasted wake surfing mode, traveling at 12 miles per hour, the impact of the wake extended down at least 15 feet. And its turbulent effects lasted from 72 to 80 seconds.
According to the authors of the study, “Bottom sediments are often reservoirs of phosphorus in lakes. If the sediments are re-suspended by the passage of a boat, this can contribute significantly to the transfer of phosphorus into the water column… All of these conditions promote the release of phosphorus under oxic conditions and thus promote the phenomenon of eutrophication or accelerated aging of the lake.”
“Under the conditions studied, the wake surf boat has the potential to impact the water column and remobilize bottom sediments up to 5 meters (16 feet) for more than a minute.”
The authors recommended to the folks in charge of the lakes in Quebec as follows:
“Boating on Quebec’s lakes is constantly increasing. Wake Surfing practices and the power of boat engines continue to grow. These practices have a significant impact on the water column and would increase water turbidity, total phosphorus and orthophosphate concentration, dissolved oxygen near the bottom and thus the potential for oxydo-reduction and would reduce the sediment consolidation. Total phosphorus release and especially orthophosphate may be a factor in premature aging of lakes called eutrophication. This increase in phosphorus in the water column can also promote the development of cyanobacteria (Blue-Green Algae), which is becoming a major problem in many Quebec lakes.”
“Thus, for a responsible and sustainable navigation it is necessary to prevent the impact of boats on shoreline erosion, on the suspension of sediments, and thus the availability of phosphorus in the water column. It is therefore necessary to advocate a practice of Wake Surf and Wake Board (with 350HP boats) in areas 600 meters (1968 feet) wide and at least 5 meters (16 feet) deep. If one of these conditions is not met, then these navigation practices must be limited or framed, as they impact the environment.”
And this was just one wake boat operating. Imagine the effects of four or five running past simultaneously.
The turbidity caused by wake boats in shallow water can:
- make our lakes less valuable for other uses,
- encourage eutrophication,
- promote the growth of potentially toxic blue-green algae blooms.
The rule proposed by Responsible Wakes for Vermont Lakes aims to mitigate the turbidity effects of wake boats by keeping them in the deep center of only our largest lakes, where they do less damage. Keep them 1000 feet from shore in water more than 20 feet deep.
The DEC’s proposal of 500 feet from shore as not enough. It will only move our clear, clean Vermont lakes in the direction of becoming cloudy, opaque, and thick with suspended matter. We have seen enough of cloudy air in the past days; we do not wish the same for turbid lakes if wake boats are not adequately managed.
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Put another way…
They say that increased turbidity
Can hasten a lakes’s morbidity.
All over the state and the nation,
This trend is called eutrophication.
Sediments rise up to nourish
Plants and critters that flourish.
(A good thing if you are an alder;
A bad thing if you are a paddler.)
Wake boats with angled propellors
Push phosph’rous and other such fellows
Up from the bottom to feed
Algae and critters and weeds.
These grow in abundance and thrive:
It’s natural, these plants are alive.
Flora and fauna do cloudy appear
On the lake that was once crystal clear.
References:
Alexander, M.T., Wigart, R.C., 2013. Effect of motorized watercraft on summer nearshore turbidity at Lake Tahoe, California–Nevada. Lake and Reservoir Management 29, 247–256.
Defining boat wake impacts on shoreline stability toward management and policy solutions, Donna Marie Bilkovic et al., 2019, Ocean & Coastal Management, Volume 182, 1 December 2019, 104945
Lenzi, M., Finoia, M.G., Gennaro, P., Mercatali, I., Persia, E., Solari, J., Porrello, S., 2013. Assessment of resuspended matter and redistribution of macronutrient elements produced by boat disturbance in a eutrophic lagoon. Journal of Environmental Management 123, 8–13.
Bastien, D., Demers,A., Named P., L., Rancourt, E., 2009. Environmental experts. Final mandate report. Environmental impacts of motorized boats and water sports on Lake Massawippi. 123pp.
Gélinas, R., Bouchard Valentine, M., Roy., M-S., 2005. Impacts of motorized boats on the release of phosphorus from sediments: literature review and analysis for Lake St. Augustine. Ville de Québec – Environment Department.46pp.
Raymond, Bastien, 2015, Impact de la navigation en milieu lacustre – étude sur la remise en suspension des sédiments : Cas du Lac Masson et du Lac des Sables, https://www.lacdessablesapels.com/point-de-vue-scientifique/impact-de-la-navigation-en-milieu-lacustre-etude-sur-la-remise-en-suspension-des-sediments-cas-du-lac-masson-et-du-lac-des-sables/
Wang, S., Jin, X., Zhao, H., Wu, F., 2009. Phosphorus release characteristics of different trophic lake sediments under simulative disturbing conditions. Journal of Hazardous Materials 161, 1551–1559.
Zoumis, T., Schmidt, A., Grigorova, L., Calmano, W., 2001. Contaminants in sediments: remobilisation and demobilisation. Science of the Total Environment 266, 195–202.