Lakes Part 2: Where Does Phosphorus Come From Within My Lake?

By Dylan Moesker

In last week’s article we discussed the trophic status of lakes in Muskoka, different sources of external phosphorus input into a lake, and how phosphorus has the ability to increase plant and algal blooms in lakes. This week’s article will follow that same path but focus on the process of internal loading (phosphorus sources from inside the lake) relating to the timing of algal blooms.

To understand the process of internal loading, we need to understand what happens to lakes in the summer. As lake water warms up, the lake begins to stratify. Cold water is more dense than warm water, causing the cooler water to settle at the bottom of the lake and warmer water on top. Oxygen is essentially trapped in both of these layers, and unable to move throughout the water column until the fall turnover.

As the unmixed bottom layer loses oxygen throughout the summer due to the decomposition of organic matter and uptake from fish, it triggers the release of phosphorus from the lake sediment into the water column – this is known as internal loading.

As phosphorus is released, it has the potential to create an additional source of nutrients for algae. The reason why we typically see algal blooms during late summer in eutrophic lakes (lakes with more than 20 µg/L of total phosphorus) is because the extra phosphorus released from sediments in a low oxygen environment adds to an already high level phosphorus lake. Essentially it fuels more growth in a highly productive lake.

On the contrary, oligotrophic lakes (lakes with under 10 µg/L of total phosphorus) are not nearly as sensitive to internal loading. This is because these lakes already have a low level of phosphorus, high dissolved oxygen, and lower amount of organic matter. Phosphorus input from internal loading is not likely to significantly increase the total phosphorus levels. In fact, due to the high dissolved oxygen and low organic matter, these lakes are less likely to even have internal loads in the first place.

Phosphorus that is released during this time will settle back into the sediment during the fall turnover as the water column is re-oxygenated.

In most lakes, internal loading is a completely natural process. As stated in last week’s article, when left undisturbed and in a natural state, a lake will find a healthy balance between nutrient input and life in the lake. The process has the potential to accelerate vegetative growth when combined with external inputs.

So what does this all mean? Well, phosphorus is a natural element necessary for aquatic plant growth, just like nitrogen is necessary for trees and plants on land. Algae develop naturally in many lakes and, unless large blooms are noticed, is not something to worry about.

Next week’s article will discuss tea-stained lakes. These lakes are actually quite different from the lakes that were discussed in this week and last week’s articles, and they are impacted in a completely different way. For now, enjoy the warm weather and make every summer moment count.

Past articles are available in this blog under the Watershed Notes Articles category or under Past Articles in the Resources section.