Why should you be concerned about internal nutrient loading in your lake?

Nutrients play a pivotal role in the health of our ponds, lakes, and reservoirs. A right nutrient balance contributes to good water quality that supports aquatic life, recreation, and water supply. But an excess of nutrients (nitrogen or phosphorus) in a lake or an imbalance can result in nuisance algae growth, or a phytoplankton community dominated by cyanobacteria, which can drastically affect the ecological health of the waterbody. If left unmanaged, these conditions may eventually lead to regulatory challenges, especially if a lake is listed as impaired by nutrients.

A key component of these comprehensive management plans is quantifying the external and internal nutrient loads that have resulted in degraded water quality. External nutrient loads are the nutrients derived from the watershed that flow into the lake, while internal nutrient loads are the nutrients derived from within the lake. Measuring the external loads is often straightforward, while understanding the internal load — the recycling of nutrients within the lake — is more challenging. Despite those challenges, lakefront residents, homeowner associations, and municipalities should all be concerned about internal nutrient loading. In this blog, I’ll focus on the internal nutrient release from lake sediments, how it can impact your lake, and how understanding it can help you proactively manage nutrient levels in your lake.

What is Internal Nutrient Loading?

Internal nutrient loading is the process of recycling nutrients between lake sediments and the overlying water column, and if left unmanaged, internal nutrient loading often promotes eutrophication (nutrient enrichment that leads to excessive algae), and degraded water quality. Everyone is familiar with the ecological benefits of recycling. But, when it comes to the recycling of nutrients in our lakes, there is little value, and often the goal is to STOP recycling! BUT that is more easily said than done.

Our lakes are often the terminal storage basins for nutrients generated from within the watershed (i.e., nutrients from outside the lake, or external loading) and when nutrients accumulate in lake sediment over time, they represent a legacy of historical conditions. Watershed sources include, but are not limited to, organic matter (e.g., leaves) delivered by stormwater, nutrients from agricultural or turf management, natural geological conditions, soil erosion, or nutrients from wastewater treatment plants. Once nutrients enter lake sediments, there are several ways for them to be released back into the water column, including physical (motorboat-induced turbulence), biological (benthic feeders such as carp that scour the sediments), or chemical (low dissolved oxygen). Often, these mechanisms work in concert with each other, which adds a layer of complexity to internal nutrient loading.

Further, anaerobic bacteria found in the sediment help break down phosphorus, a process that releases phosphate ions into the overlying waters. Phosphate ions are essentially candy to algae and cyanobacteria. The result? Excessive algae growth.

What can you do?

Whether you are a homeowner or lake association looking to improve water quality, or a municipality required to monitor for a TMDL, it is vital to understand the forms of sediment bound nutrients, the mechanisms, and site-specific lake conditions that influence internal nutrient loading in your lake. This has become a primary focus for our scientists and lake managers. As a result, we’ve developed effective management strategies to control internal nutrient loading and to restore degraded water quality conditions.

Depending on a lake’s site-specific conditions (i.e., stratified deep or shallow well-mixed) and data availability, we use a variety of methods to estimate internal nutrient loading, primarily phosphorus, because that is often the most influential nutrient that affects algae growth. These site-specific conditions are vital to understanding the range of impacts from internal nutrient loading.

Sediment Core Studies

We have found that laboratory-based sediment core studies provide flexibility to study multiple facets of internal nutrient loading at one time and can provide you with a better understanding of your lake’s dynamics.

In the field, we collect the top 10-20 centimeters of sediment for analytical chemistry testing. This top layer is where microbial decomposition of organic matter and the reduction of elements that release nutrients primarily occurs. We can then analyze the top layer of the sediment to quantify the different forms of phosphorus available in the sediment (e.g., iron-bound, manganese-bound, calcium-bound) which provides insight into the stability of the phosphorus pool that fuels internal nutrient loading.

In the laboratory, we store the sediment cores and overlying water in incubators that mimic lake temperatures, then subject the sediment cores to different experimental treatment factors such as static, oxic, or anoxic conditions to study sediment oxygen demand or the release of nutrients into the water column. Under each treatment type, and over time, we monitor dissolved oxygen, pH, nutrients, and metals concentrations in the overlying water column to determine the rate of nutrient release from the sediment.

Laboratory incubation studies can run for a few days for sediment oxygen demand experiments, to multiple weeks for nutrient release experiments. Data generated from these types of experiments provide insight into the amount of oxygen consumed by microbes in lake sediment and the rate of internal nutrient loading from the sediment under oxic or anoxic conditions.

We use this information to refine existing water quality models to better represent/validate internal nutrient loading for TMDL purposes, or to develop appropriate management strategies to help control internal nutrient loading.

Understanding internal nutrient loading is essential for maintaining the health of your lake over the long term. There are many complicating factors that influence internal nutrient release, and sediment core studies might be the solution for you.

To learn more about internal nutrient loading and the various management strategies we can use to control phosphorus in your lake, please contact me.