05 March 2010
Lake Atitlan -News-
Why Lakes are Like Black Holes
(and what we can do about it) - Sidney Eschenbach
Eutrophication and Nutrient Balance
Eutrophication is the term used to describe one of the later stages in the natural geomorphic transformation of a lake into a swamp. To understand the process, it’s convenient to think of a lake as a black hole… a place where everything goes in but virtually nothing comes out. Due to generally positive nutrient balances, virtually every lake is simply a swamp in progress, as they receive enough silt (that doesn’t come out) and nutrients (that don’t come out) to fill and support plant and animal life of many types… and eventually turn into swamps. Therefore, the time-line of any lake can be read in its total nutrient balance.
Our problem here in Guatemala, indeed man’s general problem around the globe, is that for economic reasons we generally want to stop these natural processes of evolutionary change… while in fact our behavior speeds them up. This too is natural, because we live with a much shorter time horizon than does the planet and we have a great deal of constructed infrastructure that is dependent upon maintaining the natural status quo. However, as noted, we inevitably accelerate the very maturation processes we don’t want by inducing the principal causes of eutrophication, nutrient enrichment and siltation.
To understand what a problem this is on even the smallest scale, a review of the problems caused by cyanobacteria for aquarium owners is sobering. In the end, there is only one thing that can be done to stop the problem..
and that is to repeatedly change the water in the aquarium. However, in large bodies of water that solution is obviously not available, and it is a truly ‘inconvenient truth’ that once cyanobacteria establish themselves they have rarely been controlled and never eliminated… and the cause is invariably nutrient balance.
Specifically, what drives continued algae blooms is a natural phenomena known as ‘internal cycling’, the problem created by the ‘black hole’ physics of limnology: once something enters a relatively quiescent body of water, it never leaves. Therefore, once enough nutrients are available to support major algal growth, the natural recycling of those nutrients will be enough to sustain their continued growth even if all new nutrients are cut off. Only the reversal of the black hole phenomena, the removal of nutrients, has any hope of reversing this process. To see why and how, understanding the phenomena of ‘internal cycling’ through a brief review of the biochemistry of phosphorus is in order.
The Biochemistry of Phosphorus
Phosphorus is used as a fertilizer for the simple reason that it is required at nearly every stage of plant life. It is a vital component of DNA and RNA, and is also a principal part of ATP, the phosphorus compound formed during photosynthesis that provides the energy for all plant life. During the natural process of weathering, rocks (all phosphorus originates from the mineral ‘apatite’) gradually release the phosphorus as phosphate ions which are soluble in water, and the mineralized phosphate compounds break down. The phosphate compound PO4-3 is formed from this element as the ionized phosphorous bonds with oxygen.
Phosphates exist in three forms: orthophosphate, metaphosphate (or polyphosphate) and organically bound phosphate. Each compound contains phosphorous in a different chemical arrangement. These forms of phosphate occur in living and decaying plant and animal remains, as free ions or weakly bonded in aqueous systems, chemically fixed to sediments and soils, or as mineralized compounds in soil, rocks, and sediments. Orthophosphate forms are produced by natural processes, while the principal man-influenced sources include partially treated and untreated sewage and runoff from the fertilizers used in agriculture. Orthophosphate is readily available for use to the plant community, but is typically found in very low concentrations in unpolluted waters. Poly forms are used for treating boiler waters and in detergents. In water, they are transformed into orthophosphates, making them also available for plant uptake. Organic phosphates are typically estimated by testing for total phosphate. The organic phosphate is the phosphate that is bound or tied up in plant tissue, waste solids, or other organic material. After decomposition of the organic compound (usually through bacterial action), this phosphorus can be converted to orthophosphate and reenters the cycle of plant life as a food.
How the biochemistry drives eutrophication
As stated above, due to the low amounts of total P normally found in nature (usually around 0.01 to .5% in soil) and given normal aerobic conditions, the natural cycles that slowly fill and enrich lakes is measured in geologic time. If, however, for any reason the levels of nutrients are increased and the nutrient balance becomes excessively positive, plankton and algae of many kinds begin to grow more rapidly than normal. This ‘excessive’ growth also creates ‘excessive’ die off of the plants and algae, as sunlight is blocked at lower levels. In that dead organic material bacteria then go to work consuming the dead plant matter. However, in this work they consume the oxygen, release more nitrogen and phosphorus, and thereby start the process known as "nutrient recycling” or “internal cycling" of existing nutrients. Some of the phosphate may be precipitated as iron phosphate and stored in the sediment where it can then be released if anoxic conditions develop. This is virtually identical to the process that enriches forest floors, as organic material is recycled after plant death, freeing the phosphorus to be taken up again by other plants present in the forest. Unfortunately, unlike the positive nature of nutrient recycling that takes place on land, in aquatic environments the internal cycling of the dead cyanobacteria can seriously and continuously deteriorate the water body.
That is because conditions worsen as more phosphates and nitrates are added to the water, all of the oxygen gets used up by bacterial action as they consume and decompose all of the waste from the dead plants… and anaerobic conditions ensue. Under these conditions, not only is all non-plant (zooplankton and fish) aquatic life threatened or eliminated in growing ‘dead zones’, but different bacteria continue to carry on decomposition reactions with drastically different products. The carbon is now converted to methane gas instead of carbon dioxide, and sulfur is converted to hydrogen sulfide gas. Some of the phosphorus released from the organic molecules is precipitated as iron phosphate. However, unlike the aerobic conditions, under anaerobic conditions the iron phosphate that precipitates in the sediments can be released from the sediments… only to make the phosphate bioavailable once again! This is the last step in the ‘lake to swamp’ growth and decay cycle. The pond, stream, or lake then gradually fills with partially decomposed plant materials and becomes, over geologic time frames, a swamp. This is the natural process. The problem is that, through mans actions, this otherwise natural process gets significantly accelerated, and the human economic infrastructures that depended upon a particular state of nature can no longer exist as one state of nature changes and another emerges.
Put simply, due to the laws of growth, decay and gravity, the ‘biochemistry of a black hole’, even if no new nutrients are introduced into the water body, eutrophication cannot and will not be halted simply because of internal phosphorus cycling: the plant food never leaves. For that reality to change, the underlying biochemical balances must be reversed… and that can only be through the removal of the nutrients from the black hole.
How to Escape the Black Hole
Fundamentally, there are only two ways to reverse the process and reduce nutrients; chemical intervention or physical removal. Chemical removal involves treating the water with some compound, usually copper sulfide, alum or lanthanum, or any other products that precipitate or sequester the ionized orthophosphates. This involves very high costs, further environmental risks, and, at best, problematic results. For example, one of the best treatments is made by an Australian company that produces a product called “Phoslock”, a compound that removes PO4 at a ratio of 100:1 by weight. That is, for every 100 kilos of Phoslock applied, approximately 1 kg of PO4 is removed. Phoslock costs approximately $2,500 per ton, which means that, including transportation and application costs, it would cost approximately $3,000 for every kilo of PO4 removed.
A second method of phosphorus removal is also available, and that involves the physical removal of the plant as a natural reservoir and concentrator of phosphorus. In the specific species that threatens Lake Atitlan, total phosphorus is slightly less than 1% by dry weight. Thus, like the Phoslock, with the removal of every 100 kg. of dry algae along comes 1 kilo of phosphorus…. with the added benefit that it doesn’t cost $3,000 per kilo to remove.
There is also a third, theoretical and very cheap method to strike at the motor of internal recycling, and that is by ‘corralling’ the blooms and holding them inside of floating barriers in the middle of the lake until they die. At that point, the theory is that they would slowly fall to the very bottom of the lake, some 1,000 feet down, taking them out of the epilemnic waters used by the Lyngbya and removing the phosphorus contained within their structures from the bacteria driven cycle that would release them again for reuse as PO4. At a very minimum, this would at least achieve a few goals: first, it would be ‘doable’, as it is both technically and economically within the abilities of the nation. Second, even if the Lyngbya species that we have in the lake is found to use the hypolimnion at some point of its life-cycle, the ultimate re-release of the PO4 back into the deep lake waters would represent an amount so insignificant that it would cease to be a ‘tipping point’ factor driving the Lyngbya life cycle.
Therefore, if policies were effected that would limit the introduction of new phosphorus into the lake while at the same time reducing through physical removal or chemical precipitation the total amount of orthophosphate (remember, that’s the dilute phosphate) available to the cyanobacteria, it should be theoretically possible to eliminate the engine of internal phosphorus recycling that keeps the levels of phosphorus high enough to sustain the bacteria.
What is the scale of the problem?
In the first table below is much of the data needed in order to calculate what the nutrient balance is and was, and serves as the basis for calculations regarding the scale of the remedial actions needed in order to return to a cyanobacterial status quo ante:
With the above data one can calculate nutrient balances, past and present, and get an idea of what must be done in order to eliminate or significantly reduce the cyanobacterial infestation at the lake. The following table shows the results of those calculations and shows the marked difference between what the situation probably was prior to 1960, and what it is today. It must be noted that neither of these two tables take into account the major contribution municipal septic outfalls make to the lakes nutrient balances. That will be addressed next.
What this table (above) shows is that the current nutrient balance, even excluding municipal septic inflows is highly unfavorable, given that the lake increases its accumulated total of PO4 by nearly 50,000 kg of PO4 every year. What this means in milligrams per liter can be seen in the following box. Here (below), annual and total post-Stan dilute totals are shown, both with and without the additional septic loads introduced by the municipalities around the lake:
(Note: Because PO4 is dissolved, it should only be counted as adding to the epilimnion. If there is no or little mixing, it would have no independent quality that would carry it into the hypolimnion.)
The total increase of nearly .03 mg/lt of additional PO4 in the lake since Hurricane Stan can arguably be considered the smoking gun of eutrophication, and as there was no significant amount of cyanobacteria in the lake prior to 2005, it could also be used as the goal for total removal, the amount needed to be taken out of the lake in order to halt the eutrophication process and reverse the black hole bio-chemistry.
What that translates into when the additional loads produced by municipal septic systems are added in:
This is by any standard a huge amount of phosphorus, all of it added in the past four and one-half years. In order to extract it by any of the methods described above, would mean the combined application of (something like) Phoslock and the removal of bio-mass totaling approximately 35,000 tons between them.
That is the scale of the problem.
Obviously, even at a ratio of 3:1 (Phoslock:removal), we’re looking at costs ranging as high as Q800,000,000.00… which explains why victory is so rare in the eutrophication wars. No one said that exiting a black hole would be easy.
Given the data, the following actions need to be taken:
- First, to stop the situation from worsening:
- The immediate halting of all municipal effluents from entering the Lake.
- The immediate prohibition of the use of all phosphate based soaps in the lake basin.
- The immediate implementation of mandatory policies promoting soil protection and fertilizer reduction throughout the watershed.
- Second, to begin to reverse the process:
- Implement a detailed study of the limnology of the lake, and the arrival at a definite answer to the questions surrounding the lake hydrology.
- Specifically, the details of its relative thermic stability.
- A detailed study of the Lyngbya life-cycle.
- Specifically, its use of and ability to use the hypolimnion if needed.
- The preparation of plans to physically remove the next and all subsequent algal blooms
- Based upon the results of the above studies, design the cheapest and best system to remove the bio-mass from the lake.
- Coordinate with the local authorities to implement the above.
While the task is certainly daunting, it is not by any means impossible.
However, with every month that passes another 6,500 kg of PO4 are added to the lake… another 6,500 kg. of PO4 that then must be removed in order to stop and reverse the process. Given that the continued wellbeing of nearly 200,000 people depends upon the continued health of the lake and the lake basin, doing anything less than the above amounts to criminal negligence.
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