Goose Poop in Lakes

(or Guanotrophication)

Canada Goose - Ryan Hyde on Unsplash

The recovery of the Canada goose population in North America is a remarkable success story. Where 50 years ago there were only six hundred thousand or so Canada geese, there are now millions. Not only have we humans protected geese, we have also inadvertently created some fine goose habitat. Geese have taken full advantage of the human tendency to put grassy lawns near bodies of water. Many geese no longer migrate, choosing instead to hang out in a single area year-round. This increased goose “residency” has created problems for some human lake users, not the least of which is the sheer volume of feces these majestic birds create.

The Canada goose is a big bird, typically weighing up to 13 pounds. As you might expect, the Canada goose can eat a lot of food. These geese digest food inefficiently, so high input means high “output”. Of the 82 grams of poop (dry weight) a Canada goose “generates” each day, 1.23 grams is phosphorus.

Phosphorus is naturally occurring in our lakes, and that’s good. It provides a necessary nutrient for the algae that make up the base of the aquatic food web. Without phosphorus we would have lakes with no aquatic life to enjoy. Unfortunately, many of our lakes have too much phosphorus, and too much phosphorus means too much algae. Excess algae can create many problems like unsightly blooms, big oxygen swings that stress or kill fish, bad tasting drinking water, and water that is toxic.

A valid question is “does a burgeoning goose population have a negative effect on water quality?” To answer the question, we need to know where did the nutrients come from, how much poop enters the lake, and what happens to the poop in the lake? While there are real concerns with bacteria (such as E. coli) in goose poop leading to beach closings, for this article we’re going to focus on phosphorus.

Where did the nutrients come from?

If the goose eats plants from the lake and then poops in the lake it is simply returning the nutrients to the lake (this is called “nutrient cycling”) and, while there is a transformation inside the goose (from food to poop), there is no net increase of phosphorus. If a goose eats from a field outside the watershed and then poops in the lake, it is importing nutrients to the lake. Conversely, if a goose eats plants in the lake, then flies away to poop, it is exporting nutrients.

How much poop gets in the lake?

There are papers full of calculations estimating the quantity of poop a goose generates, the probability a poop will end up in the water versus on land, and how long food spends in a goose. Some unfortunate graduate students had to count and weigh goose droppings and count time between feeding and pooping. Ultimately, the more time a goose spends in the water, the more likely its poop is going to end up in the lake. Because of soil binding and plant uptake, poops that fall in terrestrial feeding areas contribute fewer nutrients to the lake than poops that fall directly into the lake.

What happens to poop in the lake?

Goose droppings in the water will sink to the bottom of the lake and much of the phosphorus will not be immediately available for the algae to use. One short-term study where goose poop was added to water tanks showed little nutrient increase in the water column. Over time, those nutrients are likely to migrate upward as sediments are reintroduced to the water column by foraging carp and geese (called bioturbation), microbial decomposition, and lake mixing.

Canada geese accompany Travis Robinett as he monitors cyanotoxin at Spring Valley Lake in Kansas City

Let’s take a closer look using some numbers. For this exercise, let’s imagine 2 lakes. One is 150-acre Lamar City Lake in Barton County, the other is Barry Harbor, a small, 5.4-acre, suburban neighborhood lake in the Kansas City area. Lake size and number of geese matter for these calculations. Lamar City Lake is about 30 times larger than the Barry Harbor in both surface area and volume. 300 geese on Lamar City Lake will have as much impact as 10 geese will on Barry Harbor Lake. If we arbitrarily assume that ¼ of a goose’s poop will end up in the lake and that the nutrients did not originate in the lake we can make the following "back of the envelope" estimations:

• A typical goose will poop nearly 1 pound of phosphorus per year, so ¼ pound of phosphorus enters either lake from each goose.

• If we hold at 3 Barry Harbor geese and 90 Lamar Lake geese, phosphorus in each lake could theoretically increase by about 7 µg/L each year. This estimation does not account for nutrient exports like water going over the spillway or harvested fish.

• The amount of in-lake phosphorus from goose poop is small compared to the amount of phosphorus entering the lake via runoff. Using the same goose numbers above (3 geese for Barry Harbor and 90 for Lamar), goose poop would contribute 2.9% of Barry Harbor’s phosphorus and 0.9% of Lamar’s phosphorus each year (based on landcover estimates).

In addition to the math, there are some ecological considerations as well. For example, as geese graze down the vegetation in a lake, the lake may become more turbid (cloudy) as soil particles are suspended in the water. We often see this when grass carp are added to a lake. However, some work shows that as goose poop is deposited on the bottom of the lake it encourages additional plant growth by providing nutrients directly to the substrate where plant roots are. One study shows that the diversity of the algal community is diminished in the presence of abundant goose poop but notes that, at least in the short term, this does not lead to increased cyanobacteria (bluegreen algae) and thus isn’t likely to contribute to high toxin levels in the water. Long-term data may tell a different story.

So, to answer the question “does a burgeoning goose population have a negative effect on water quality?”, the answer is maybe. Certainly, Canada geese introduce phosphorus to our lakes. However, that may be the cost of doing business with nature. If we are really serious about improving water quality, it makes sense to look to other sources of pollution. Runoff from urban and agricultural land can introduce much more new phosphorus to a lake than a typical flock of migrating geese or a few resident geese. If the population of resident geese gets out of hand, you’ll have to call in the professionals. Organizations like GeesePeace can help with resident goose management.

Canada Goose - Anchor Lee on Unsplash