BACK TO BASICS: How and Why Phosphorus Cycles through a Lake

You may have heard about nutrient cycles, otherwise known as biogeochemical cycles. But what you may not know is that these cycles also occur within a lake. One of the most important nutrient cycles in a lake is the phosphorus cycle due to its role in driving primary productivity (the ability of plants and algae to convert energy and inorganic nutrients into organic matter for the rest of the food web).

First off, biogeochemical cycles represent the movement of elements, such as nutrients, within a natural environment. These elements can be in various phases and exist within different chemical compounds. For example, the element carbon can exist in the carbon cycle as gaseous carbon dioxide (CO₂), as solid calcium carbonate (CaCO₃), and as various other compounds. These cycles can also include different compounds and phases depending on the scope, such as the movement of elements throughout terrestrial, freshwater, or oceanic ecosystems. Depending on the environment and magnitude there will be different biological and chemical reactions that each play a role within the cycle.

We will just be exploring nutrient cycles within a freshwater lake. Although this is a more focused scope, so many important reactions and processes occur in a lake within these cycles. Additionally, lakes are physically and chemically active daily and can vastly change with each change in season. This dynamic environment can impact when nutrients are available for the aquatic biota. For example, stratification within a lake can prevent the mixing of nutrients from higher to lower depths. Also, nutrients are not actively available at all stages within a nutrient cycle. Therefore, nutrient availability can have an impact on primary productivity within a lake, which can furthermore impact food web dynamics.

To understand nutrient cycles, we first need to know about the different nutrients in a lake. Nutrients are essential for all living components in a lake, however, every nutrient is of varying importance. The three most important nutrients in a lake are carbon, nitrogen, and phosphorus. These nutrients are crucial for metabolic processes and cell structure formation, thus essential components of all living things. Each of these nutrients processes and cycles within a lake are complex and unique. We are only going to focus on the phosphorus cycle.

Phosphorus (P) is an important nutrient for all living things because it is a component of DNA. It has many other essential roles especially related to internal processing of cells and growth. The nutrient phosphorus is the least abundant of carbon, nitrogen, and phosphorus, this means it is the most limiting nutrient. Therefore, the biological productivity of a lake is commonly related to the amount of available phosphorus. This theory has been famously studied at IISD-ELA since the 1970’s.

First, what are the sources of phosphorus? There are both natural and anthropogenic (or ‘originating from humans’) sources of phosphorus to a lake. The natural source of phosphorus originates from the leaching of minerals. This source will then be supplied to a lake via erosion and runoff from terrestrial landscapes. Additionally, sediment at the bottom of a lake typically acts as a large internal nutrient reserve. The phosphorus retained within the sediment is periodically released, however this release is dependant on multiple factors that can occur at the where the sediment meets the water (described in more detail below).

The input of phosphorus by humans to a lake can be a result of several different sources. If this P loading is high, it can be detrimental to lakes, resulting in harmful algal blooms. The alteration of landscapes to promote increased drainage and runoff results in faster P loading to surface waters. Also, the application of P containing fertilizers onto agricultural lands in excess can result in runoff into waterways and eventually into lakes. Other phosphorus sources include discharge from wastewater facilities, livestock manure runoff, and urban area runoff.

There are two important interfaces that play a role in the biogeochemical cycling of phosphorus. The first is the air-water interface. Interestingly phosphorus does not commonly exist in the gas phase, so this interface may be seen as irrelevant. However, movement of other elements across this interface are important for the phosphorus cycle such as oxygen.

In terms of the phosphorus cycle the sediment-water interface is incredibly important to the loading of phosphorus to a lake. The sedimentation of phosphorus occurs when organic phosphorus is released during either, excretion or decomposition, if this organic phosphorus is not taken up within the water column it will settle to the bottom of the lake. When the sediment is highly oxygenated, the phosphorus is bound to the sediment and unavailable to release—this is called complexation. However, the release of phosphorus at the sediment-water interface can be due to a few different processes. One process is the physical mixing of the water column, this typically occurs during turnover and can release the phosphorus in the sediment. Next, if the sediment is under an anoxic state (little to no dissolved oxygen), this results in oxidation-reduction reactions to occur that release phosphorus, this is called internal P loading.

There are a variety of forms of phosphorus that can exist within a lake. When phosphorus enters a lake, it is in the inorganic form as orthophosphate (PO₄^-3). The dissolved orthophosphate is the main source taken up by algae and aquatic plants. When phosphorus is within living biota such as algae, this is called particulate phosphorus. As grazers feed on the algae, and predators feed on the grazers, and so on, the phosphorus will be mobilized and used within the entire food web. Once contained within living manner in the food web, the inorganic phosphorus is converted to organic phosphorus. Which is then released in the process of either, excretion or death of animal or plant matter. The stage of decomposition within the cycle returns organic phosphorus to inorganic phosphorus. Most of the inorganic phosphorus in the water column is up taken again by algae and plants, then the cycle continues. The remainder will fall to the sediments to be retained. As previously mentioned, once in the sediment depending on the conditions, the phosphorus will either remain or be released.

Nutrient cycles are complex, and their processes vary depending on the chemical, physical, and biological processes that occur within and around a lake. The phosphorus cycle is one of the most important cycles in a lake. This cycle occurs in the lakes around you and depending on the amount of available phosphorus in the cycle, it may result in the formation of algal blooms. However, you play a role in the cycle due to the many sources of P from human activity.

So, next time you are at a lake, think about all the processes that are occurring within and be just beware that your actions can influence that system.