How Treatments Help Your Lake Feed Its Own Problems
We’ve already covered the three root causes of lake decline, and the three targeted actions needed to reverse them. Now it’s time to turn our attention again to how we measure whether those actions are actually working and whether they’re delivering real, measurable, lasting value for money.
I’m Dave Shackleton, and I’m here to Simplify the Science of lake management so all stakeholders can understand it all better, make well informed decisions and get real value for money from their investment in their lake and their lake lifestyle.
In the 4th video in this series, we looked at how to measure oxygenation. We explained why simply taking vertical readings of dissolved oxygen isn’t enough and why you need to calculate the volume of water and sediment surface area that’s hypoxic to truly assess hypoxia to determine whether your lake is fully oxygenated.
Not many lake consultants or lake management companies do that, to reveal what is really happening down below.
Today, we move beyond hypoxia and oxygenation to focus on how to effectively control sediment nutrient recycling and measure whether your lake management program is getting rid of the accumulated sediment nutrient stockpiles that sustain it or not.
First let’s do a quick recap of the interplay and feedback loops that are set up as eutrophication takes hold of your lake.
Organic sediments cause hypoxia as they decompose. Hypoxia suffocates the animal life in the food web that would normally consume those organics and the algae they produce. Hypoxia at the bottom of the lake allows anaerobic microbes to take over and they accelerate sediment nutrient recycling. Nutrient recycling fuels more algae growth, more excess that sinks to the bottom and consumes more oxygen as it decomposes to cause more hypoxia
So you can see how the feedback loops take hold as this vicious cycle becomes more and more intensified.
Why that is important is because when these conditions exist, they create a perfect environment for toxic cyanobacteria to dominate the uptake of nutrients recycled from the sediment, displace beneficial algae and produce toxic HABs that cause the lake to be shut down and destroy your lake lifestyle.
In hypoxic conditions that sediment nutrient stockpile just keeps compounding and growing, and the more it does so, the more it sucks the lake into a downward spiral from below, and if you are not measuring that sediment accumulation, you are unaware of what’s happening deep down at the bottom of your lake until it’s too late.
I’m not a great fan of horror movies and science fiction, but one of our clients explained it using this analogy:
It’s like something out of a horror story. Deep beneath the surface, the sediment becomes a kind of Sludge Hydra, like Medusa, a beast from ancient Greek mythology, fed and sustained by decay.
Each head of the Hydra represents a visible symptom: algae blooms, dense invasive weeds, fish kills, or toxic cyanobacterial slicks. But these aren’t the root of the problem, they’re just the Medusa’s many masks that it shows when it pokes its heads up at the surface.
The true body and power of the beast is the muck below, where nutrients, waste, and organic rot accumulate, consuming oxygen, and feeding itself with its own waste. The more of this foul slimy excrement it creates, the more it can consume.
Whenever people try to fight it, with algaecides, herbicides, or phosphorus precipitants, they’re striking at the heads. The lake might briefly look better, but as the legend warns: every time you cut one head off, and two grow back.
Why? Because of the power of feedback – each treatment leaves a trail of corpses, dead algae, dead weeds, dead fish — that settle to the bottom and feed the Hydra’s bloated belly.
That is exactly what the Government Accountability Office report pointed out – algaecides end up producing more organic sediment which produces more algae – the beast feeds on the very detritus produced by these futile attempts to control it. And over time, the Medusa takes over the lake, because the visible heads are bait to fool you into feeding the monster with symptomatic chemical treatments aimed at cutting off the heads.
Over time, the lake becomes a dark kingdom ruled by the many headed monster.
Oxygen disappears. Life retreats.
The food web collapses.
Only the Hydra’s toxic offspring, cyanobacteria, can thrive.
This beast doesn’t die easily. It thrives on its own filth by consuming the waste and excrement it produces and by growing more heads every time we try to cut one off with a chemical treatment. To defeat it, we have to starve it.
That means:
Because the battle isn’t won at the surface. It’s deeper down, where decay becomes destiny, and from where the lake lifestyle is slowly subverted and strangled.
That’s why we focus our efforts on fixing what’s happening in your lake, at the bottom, not on your lake at the top.
We restore oxygen to the depths, ending the hypoxic hell where the Hydra thrived. We don’t attack the monster head-on. We simply rebalance the system it has corrupted and begin starving it from within.
We revive and sustain the forces that once kept it in check:
With its food supply gone, the Hydra falters.
It doesn’t go out with a bang, but with a slow collapse and surrender into irrelevance.
The monster withers. overtaken by the thriving vitality of aquatic life as the lake begins to breathe again.
If you recall, we have already described in earlier videos how we suppress nutrient recycling by oxygenating the whole water column. That has three key consequences:
· Phosphorus is less soluble in oxygenated water, so phosphorus levels plummet. Likewise, ammonia which builds up in hypoxic water at the bottom disappears so cyanobacteria no longer have an exclusive supply of abundant phosphorus and nitrogen at the bottom, that only they can utilize because of their ability to control their buoyancy and dive down to load up. That means we can rebalance the phytoplankton demographics so that beneficial algae can be restored and act as the primary food source for zooplankton, so that the foundation levels of the food web are restored.
· Once the benthic margin, where water and sediment meet, is aerobic again, benthic zooplankton can once again flourish down there, where they consume the organic detritus that settles into the sediment, so instead of accumulating, it is depleted.
· Lastly, with the water fully oxygenated and an abundance of zooplankton, the foundation levels of the food web are restored, so the full biodiversity of the food web follows and nutrient clearing capacity is restored.
We covered how to measure that in video number 4 in this series.
Creating these conditions cancels out most of the competitive advantages that cyanobacteria have, so we can begin to rebalance the phytoplankton and restore the food web – and we’ll cover how to measure that in the next video.
But how do we quantify the taming of a self-sustaining monster that lived by feeding itself more and more muck and detritus that it created for itself?
We don’t measure it in myths or metaphors, as interesting as they might be.
We measure it in inches, feet and meters and cubic yards.
As the Hydra dies, its collapse is charted by the shrinking of the sludge volume. And that’s something we can track scientifically, with data.
Some people try to poke a pole into the sediment to see how much further they need to push it before it hits hard bottom and take that as a measurement of how thick the soft mucky organic sediment layer is.
That is futile and unscientific for a number of reasons:
· It only gives a measurement in the few points where you’re poking your pole,
· You can only poke your pole in the shallow parts where you can reach the bottom – but most of the slimy organic sediment migrates to the deepest parts that you can’t reach with your pole.
· Mucky organic sediment is fluid and plastic, so it slowly flows and moves with wave action, new organic deposits and so on. So a change in one spot doesn’t mean that change has happened throughout the lake.
The only way to quantify changes in the amount of sediment in a water body is by using bathymetric sonar scanning because that takes accurate, detailed measurements, 20 times every second, that adds up to hundreds of thousands and millions of individual measurements, across the whole lake.
The first thing you need to do is establish a reference benchmark that enables you to establish the level of the water in the lake. If there is a dam wall or spillway, then that is an ideal benchmark because level of the spillway doesn’t change with the water level.
A floating dock doesn’t work because it’s level will go up and down with the water level.
So you first take a measurement of how many inches or centimeters the water is above or below that reference benchmark.
Then you need to accurately measure how many inches below the water the sonar transducer is, that is going to do the sonar scanning to collect the data.
Similarly, you state how many inches below the water level the sonar scanning transducer was set. so that Bio-Base can ensure that when the data is processed, we are comparing apples with apples.
That means when you do the bathymetric scan again in the future, Bio-Base can automatically compute and adjust for any difference in water level,
Then you scan the lake, moving slowly so that the sonar signals can be accurately collected, in a systematic manner along a preplanned track so that the whole lake is evenly covered.
Once you have the data, it is uploaded to an independent bathymetric data processing service called Bio-Base. Bio-Base has access to detailed GIS data, and the bathymetric data that you upload to it.
You also define the reference benchmark that you use and the offset – or the number of inches or centimetres that the water level was above or below that benchmark.
That means when you do the bathymetric scan again in the future, Bio-Base can automatically compute and adjust for any difference in water level, so you are always comparing apples with apples.
Similarly, you state how many inches below the water level the sonar scanning transducer was set. so that Bio-Base can ensure that when the data is processed, we are comparing apples with apples.
That means when you do the bathymetric scan again in the future, Bio-Base can automatically compute and adjust for any difference in water level.
So what does the bathymetric data tell you?
Firstly, it produces a contour map or depth profile of the lake. Over time, that allows you to see how the bottom profile of the lake changes.
For instance, you can see how at this end of the reservoir, it was really shallow – 2 or 3 feet in 2017. Five years later you can see that where we only had a 2 or 3 foot contour we now have 5, 6 and 7 foot contours.
So we can clearly see that the reservoir is deepening which means that the volume of sediment has reduced.
We can draw a transect down the length of the lake and flip this view by 90 degrees to look at it in profile and to see how the depth has changed.
The sonar scans also quantify the density of aquatic vegetation, in this case invasive milfoil and hydrilla that was growing in the organic muck that had accumulated in these shallow areas.
The color gradients show red where these invasive weeds are densest and occupying virtually the whole water column, and transitions to orange, yellow, green and then blue where there is no aquatic vegetation in the water column.
This blue line highlights the 10 foot contour. As you can see, in 2017, these invasive weeds had taken over everywhere that was 10 feet deep or less.
Over a period of 5 years our bio-dredging program digested away that organic sediment that the invasive weeds were rooting in.
This is how the depth profile changed as that organic sediment was digested enzymatically and cleared via the restored food web, and here is the aquatic vegetation heat map that confirms that the amount of aquatic vegetation had been restored to a healthy level.
The bathymetric data also enables us to calculate the total volume of water in a lake.
In 2017 there was 155 acre feet of 250,000 cubic yards of water.
In 2022 there was 240 acre feet or 388,000 cubic yards of water – an increase in water volume of or 138,000 cubic yards. What that means is that the amount of sediment was reduced by 138,000 cubic yards. That means the storage capacity of the reservoir increased by over 50%
Conservative estimates are that it costs at least $75 per cubic yard to physically dredge away sediment from a lake – so that would have cost about 10 million dollars.
By bio-dredging, not only was the cost over five years a few hundred thousand dollars, there was no disruption to the shoreline, no messy slimy muck being hauled out, no heavy trucks damaging the roads as they hauled that muck away for disposal.
To haul away 138,000 cubic yards in 15 cubic yard truck would take 9,000 truck loads. Can you imagine the cost of that – not just in the fuel and operating cost of the trucks, but the cost of rebuilding the damage to the roads once the job was completed!
The main reason that so many consultants and lake management companies don’t do this sort of accurate scientific measurement is because it will expose how the use of algaecides and herbicides phosphorus precipitants actually increases the amount of organic sediment and reduces the water volume in the lake.
We saw that here at Lake Rogerene, where the weeds were so dense that it was decided to nuke them with herbicides in March 2020 to clear the way for a bio-dredging program to take place.
We did a bathymetric scan of the lake before the herbicides were applied, and the average depth of the lake was 6.3 feet and the total water volume was just over 70,000 cubic meters.
2 months after the herbicide had been applied, and the weeds had died down and decomposed, we repeated the scan. It showed that the average depth of the lake had reduced by 6 inches to 5.8 feet – so that is an average of 6 inches of additional decomposed nutrient-rich organic biomass that had been added to the sediment. That was the equivalent of nearly 5 and a half thousand cubic meters of sediment that had been added.
Fortunately, by the end of 2022, nearly 10,000 cubic meters of organic sediment had been bio-dredged away and the average depth of the lake had increased to 7.1 feet.
That is how you measure and quantify the effect that algaecides and herbicides are having on your sediment nutrient stockpiles and on how effective bio-dredging is in getting rid of it.
If your annual lake report isn’t giving that data, its time you demanded it, because it you are blind to what is really happening at the bottom of your lake. You have no idea how far organic sediment accumulation has progressed or to what extent it has created a self-sustaining supply of recycled nutrients feeding that monster, that is going to help cyanobacteria to dominate and produce toxic HABs that will become a public health threat, causing your lake to be shut down, your property value to plummet and your lake lifestyle to be lost.
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