Leaf Litter - The phosphorus contribution of trees

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I was contacted by a gentleman whose neighborhood has a lake in need of attention. It was a pretty typical conversation, with complaints of decreasing clarity and water getting greener with each passing year. I was preparing to give my usual reply about looking to the watershed and scrutinizing the landscape for possible nonpoint source pollution, when he told me that this lake is a retired quarry. It doesn’t have much of a watershed.

Apparently, limestone was mined from this site for railway construction. Mining was abandoned when the pit filled with water, forming the lake. Largely fed by groundwater, the lake has no inflowing or outflowing stream. Since the lake doesn’t flush, any nutrients entering it will tend to cycle indefinitely.
So how are nutrients entering the lake?

This lake doesn’t drain any land, and is surrounded by woods. It is possible that a broken sewer line is contributing nutrients to the groundwater feeding the lake. However, detecting such a problem was a tall order, so I kept looking. One other possible source of nutrients is leaf litter.

A typical Ozark forest drops about 2.1 tons of leaf litter per acre each year (Stambaugh et al 2006). Based on a few estimates of the phosphorus content of leaf litter, that works out to be between 5.8 and 7 pounds of phosphorus per acre of forest annually. While this is a small, 1 acre lake, it has a long, skinny shape. As a result, it has more shoreline and more trees at its edge than a round lake with the same surface area.

According to one paper (France 1995), leaves in forested landscapes travel, on average, less than two feet after they fall. Drawing a two foot wide corridor around the lake gives an area of about a tenth of an acre. If we assumed (arbitrarily) that 30% of those leaves blew into the lake, we could be looking at around 0.15 pounds of phosphorus entering the lake each year. If this occurred each of the lakes 60 years, it would yield a possible total of 9.12 pounds (or 4135 grams) of phosphorus. Assuming the lake to have an average depth of 10 feet, we get a total possible concentration of 335 micrograms of phosphorus per liter from leaf litter alone. That is a lot of phosphorus.

Measured phosphorus concentrations were an order of magnitude less, at only 44 ug/L.

In reality, the amount of leaf litter entering the lake is probably considerably less. This is a neighborhood park, not an Ozark forest after all. And much of the phosphorus that does enter the lake will settle to the bottom and remain there in particle form. Some phosphorus will be incorporated into animal tissue. There is a community of shredding invertebrates in aquatic environments that break leaves and other organic material down into smaller pieces. These “benthic” organisms feed larger animals like fish and birds, which can leave the lake, taking nutrients with them.

The potential input of phosphorus from leaf litter is pretty high. This potential is exacerbated by the quarry lake’s lack of flushing. This doesn’t mean that lake owners should chop down trees near their lake, however. The benefits of trees far outweigh the “negatives.” In addition to providing shade and beauty, trees retain phosphorus that otherwise might enter the lake via runoff water.

The lake’s nutrient concentration has likely been stable over the last several decades. The surrounding homes are roughly 50 years old, so there haven’t been any new activities in the area. In the last 20 years, a trail has been added that allows its users to see the lake. Perhaps shade trees have been cut down, allowing more sunlight to reach the lake.

The increased algal growth may be the result of top down effects. Perhaps someone has been harvesting bass from the lake, and the fish that eat algae-grazing organisms are overpopulating. Also, this is a small lake, and small lakes tend to be very dynamic, with conditions changing quickly and often. Whatever has happened, monitoring the lake is a good place to start.

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