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WATERLINE - September, 2021

Dear Dr. Waterline:

I keep hearing people talk about pee in our lake. I have kept a close watch on the lake for years and have never seen anyone pee in the lake. How is so much pee getting in our lakes and what can we do to stop it?


P.O.’d in Lakeville

Dear P.O.’d,

You ask an excellent question but let me assure you that there is not an epidemic of micturition in lakes. What people are talking about is not the familiar bodily fluid, but the element phosphorus, commonly referred to by its chemical symbol “P”. Phosphorus is a vital element for life.

Phosphorus in lakes originates naturally, from the erosion of rocks in a lake’s watershed. Generally speaking, P is in short supply in most lakes relative to the needs of algae in lakes – this is what we mean when we say algae in a lake is “P-limited”. Ipso facto, if P is added to a lake, the algae grow faster. It is basically a fertilizer for algae. Therefore, lakes in areas with rocks that contain a lot of P, e.g., sedimentary rock, are usually more productive (grow more algae) than lakes in geologic areas with rocks with low P content, e.g., granite.

Rock weathering is a slow process, and so, under natural conditions, P is slowly cycled from rocks into the biosphere and then back to rock. Humans have dramatically increased the cycling of P in the environment by phosphate extraction, fertilizer application, waste generation, and P losses from cropland. The increased productivity brought by more P may increase your food supply, but it’s not so good when it finds its way into your lake.

Algae proliferate when the P going into a lake, the so-called P-loading, increases. More algae means more food for zooplankton, and more zooplankton means more food for fish, so a “productive” lake can have more and faster-growing fish. We call highly productive lakes “eutrophic”. Algae, like all plants, take in carbon dioxide to grow and release oxygen in photosynthesis during the daytime. Of course, the night follows the day, and algae stop photosynthesizing and producing oxygen in the dark; but they are still alive and continue to respire like all living things, which consumes oxygen and releases carbon dioxide. And this sets up one of the major problems with excessive P-loading – dramatic swings in oxygen concentration in the lake from day (super high) to night (super low), which is stressful for fish. In addition, all those algae cells produced by the P-loading eventually die and settle to the bottom of the lake. Bacteria then decompose the dead algae, and in doing so can consume all the oxygen at the bottom of a lake, causing anoxia and making the area inhospitable for fish.

An increase in P-loading also causes a change in the algal species present in the lake. At high P-loading rates, filamentous algae can come to dominate, which makes swimming kind of disgusting. At very high P-loading rates P can be superabundant, and other nutrients, like nitrogen, can come to limit algae growth. This can lead to the algae community becoming one where cyanobacteria dominate. Some cyanobacteria produce toxins that can poison animals. It is not uncommon to hear of dog and livestock deaths in the Pacific Northwest caused by harmful algae blooms (HABs) in lakes.

So, you may ask, what can I do about this? Well, the fundamental thing is to minimize the mobilization of P in the lake watershed. This means avoid use of P-containing fertilizers, make sure septic systems are functioning correctly, minimize soil erosion, and use best management practices for livestock and pet waste.

For further information on the P cycle, and how humans have altered it, read a recent paper by Yuan, et. al. entitled, “Human Perturbation of the Global Phosphorus Cycle: Changes and Consequences,” published in Environmental Science and Technology in 2018. Contact Dr. Waterline if you would like a copy.

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We look forward to crafting answers to your conundrums.


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