All About Phosphorus

All About Phosphorus

Phosphorus is a naturally occurring nutrient that is critical for many organisms, plants, animals, and humans. Through the phosphorus cycle, phosphorus moves through rocks, water, soil, sediments, plants, and animals. Unlike nitrogen, another common nutrient, phosphorus attaches strongly to many types of rock and soil and can stay in water bodies for a long period of time. 

However, other sources such as septic systems, fertilizer runoff from lawns, gardens, and farms, highway runoff, animal wastes, and the erosion of phosphorus rich soils can introduce more phosphorus than the naturally occurring amount. 

This excessive phosphorus builds up over decades and harms water quality, throwing aquatic ecosystems off balance by causing harmful cyanobacteria blooms and making it easier for invasive species to take over.

Here are some impacts & outcomes of when phosphorus levels become too high.

Impacts and Outcomes of Excessive Phosphorus

1.Build-up leads to overgrowth & algal blooms

Some pollutants, even other nutrients like nitrogen, spend a relatively short amount of time in a water body before flowing out of the system. Unfortunately, phosphorus sticks around for a long time. In fact, most of the phosphorus which causes algal blooms in Santuit Pond comes from the decades worth of excess phosphorus stored in the sediment at the bottom of the pond. Phosphorus from eroded soil and organic matter settles to the bottom of the pond. Over time, the sediment accumulates incredible amounts of phosphorus. Sediment weathering, low oxygen concentrations, and mixing due to boating and recreational activity release the captured phosphorus into the water body. The amount of phosphorus stored within the pond’s sediments can be so extreme and persistent that it continues to affect water quality even long after we reduce or eliminate excess phosphorus flowing into the pond from the watershed. 

Phosphorus is often the limiting nutrient for algae growth in freshwater systems like ponds. This means that plants and algae only grow until they’ve used up the available phosphorus. When phosphorus accumulates in ponds and lakes, algae can grow rapidly, leading to overgrowth. This increased algae blocks sunlight from reaching below the surface of the water, preventing other plants from growing, crowding out some important species, and destroying habitats for others. 

2. Eutrophication & hypoxia (decreased O2 levels)

Eutrophication is the process where excess nutrients in a water body lead to the overgrowth of plant life and harmful algal blooms. This overgrowth means that more oxygen is being used for respiration. As the algae and other plant life die off, the microorganisms that decompose dead algae use up all of the available dissolved oxygen in the water and produce large amounts of carbon dioxide. This creates hypoxic (low-oxygen) or anoxic (no-oxygen) zones in the water. The excess carbon dioxide also mixes with water to create carbonic acid, making the water more acidic and harming sensitive species. The low-oxygen, more acidic waters created by algal blooms cause major stress and can even lead to fish kills. 

 3. Effects on aquatic organisms

Cyanobacteria (blue-green algae) produce cyanotoxins, powerful natural poisons that have the ability to make humans, plants, pets, and animals sick. The toxin and odor-producing cyanobacteria also turn waters murky, and can easily become dominant when they have large amounts of phosphorus available for growth. Poor water quality in ponds also negatively impacts downstream water quality, affecting the ecosystem’s health and aquatic species’ viability elsewhere.

Possible Solutions

Phosphorus loading can lead to algae and aquatic plant overgrowth in a water body. Though this overgrowth can cause water quality to become hazardous for both humans and animals, there are many solutions that can reduce and prevent further water quality issues.

To truly address this problem, a clear understanding of the sources of phosphorus and their pathways into the pond is needed. Work to date has demonstrated that the major external sources of phosphorus to Santuit Pond include stormwater pollution, septic systems and agriculture. Historic inputs from these sources over decades have led to the accumulation of phosphorus rich sediment at the bottom of Santuit Pond. Under conditions of low dissolved oxygen, phosphorus is released from the bottom sediment which causes an internal source of elevated phosphorus concentrations. The internal release of phosphorus from sediment is currently the largest source of phosphorus in Santuit Pond followed by external sources attributed to polluted stormwater runoff. For more information on Santuit Pond phosphorus sources and solutions see the Santuit Pond Watershed Based Plan.

Solutions to Address Internal Phosphorus Loads

Aeration using Solarbee circulation

In 2014, the Town installed six Solarbee solar powered mixers in Santuit Pond. The purpose of the mixers was to circulate bottom water to increase the level of dissolved oxygen. At the time, it was anticipated that the introduction of more oxygen into the bottom waters of ponds would limit the amount of phosphorus released from the sediment, thereby controlling algal blooms.

However, water quality monitoring conducted in 2022 demonstrated ongoing low levels of dissolved oxygen in the bottom of the pond over the course of the day. In fact, concentrations of phosphorus remain elevated and the pond-wide incidence of harmful cyanobacteria blooms has remained since the units were installed. As a result, it was concluded that while the Solarbees may have provided some water quality improvement to Santuit Pond, this improvement has not been sufficient to address the bulk of the internal phosphorus load from the sediments.

US Army Corps of Engineers (USACE) Santuit Pond Dredge Study

A feasibility study to formulate and assess measures to address the internal phosphorus load associated with the sediment in Santuit Pond was initiated in 2019. The report concluded that sediment is distributed throughout the pond and up to 8 ft deep in some areas.  Phosphorus is released from the top 1.6 inch (4 cm) “active layer” under conditions of low dissolved oxygen.  While a final report has not been published to date, the USACE have concluded that it is cost prohibitive to dredge all the sediment from Santuit Pond to reduce the internal phosphorus load.

Nutrient inactivation with alum

Alum (aluminum sulfate) is a compound commonly used by public water suppliers to clarify drinking water during the treatment process. When added to water, alum forms a solid, heavy cluster called a “floc”, which traps phosphorus and settles it onto the bottom of a water body. As these phosphorus-rich flocs settle and line the bottom sediment, they create a barrier that also makes it harder for phosphorus in the sediments to be re-released into the water. This treatment called “nutrient inactivation” binds phosphorus in sediment, making it unavailable to fuel algae growth. 

Nutrient inactivation has been used in dozens of water bodies over the last few decades, including a number on Cape Cod:

  1. Cliff Pond – Brewster / Orleans 
  2. Long Pond – Harwich 
  3. Hamblin Pond – Barnstable 
  4. Ashumet Pond – Mashpee / Falmouth 
  5. Great Pond – Eastham 
  6. Herring Pond – Eastham 
  7. Mystic Pond – Barnstable 
  8. Lovells Pond – Barnstable 
  9. Stillwater Pond – Chatham 
  10. Lovers Pond – Chatham
  11. Parker Pond, Barnstable/Osterville

The ponds treated with nutrient inactivation have a marked improvement in water quality.

View the FAQ Document: Nutrient Inactivation Treatment to Address Internal Loads of Phosphorus in Sediment at Santuit Pond Mashpee, MA

Boat restrictions to limit resuspension of phosphorus laden sediment

Motorboats can kick up phosphorus-rich sediment, mixing it back into the water and making more phosphorus readily available for algae production. Reducing boating speeds can help keep phosphorus locked in the sediment and out of the water column.

To learn more about all the options considered to address the internal phosphorus loads in Santuit Pond, please see the Santuit Pond Focused Feasibility Study.

Solutions to Address External Loads of Phosphorus

What can homeowners can do to address polluted stormwater?

Stormwater runoff from housing developments and roadways surrounding the pond is the second largest source of phosphorus. Stormwater runoff can be generated right in your backyard. When water runs off impervious surfaces (areas that prevent rainfall from seeping into the ground) such as the roof of your home or your driveway, it creates stormwater runoff. Without management, stormwater runoff from your property can impact on the entire watershed, including lakes and ponds in close proximity to waterways like Santuit Pond. Fortunately, homeowners can do something about addressing the potential impacts of stormwater runoff.

Choices about landscaping and lawn care practices can impact water quality. Runoff from lawns can directly enter nearby waterbodies through surface runoff and through the stormwater system. When leaf litter, pet waste, fertilizer, or other potential pollutant sources common to residential lots make their way to Santuit Pond, the excess phosphorus they contain leads to water quality issues.

For more information about what homeowners can do to reduce phosphorus please see the following resources:

What is the Town of Mashpee doing to address polluted stormwater?

The Town of Mashpee has taken steps to identify opportunities to reduce phosphorus contained in stormwater runoff from roadways around the pond, including retrofitting existing stormwater drainage systems using green infrastructure techniques to capture and remove phosphorus from stormwater. Currently, twenty-one retrofit locations have been identified and prioritized for implementation, with the highest priority opportunities being implemented along Timberlane Drive and the Town Landing. 

Septic system maintenance, upgrades, and centralized treatment

Septic systems can become a source of phosphorus pollution if not properly maintained. Phosphorus found in wastewater primarily comes from toilet waste, as well as bath, laundry, and kitchen wastewater. For the most part, septic systems work by removing phosphorus through absorption and precipitation methods within soil treatments and the drain field.

Making sure your septic system is inspected, maintained, and pumped on a regular basis can reduce phosphorus pollution and the risk of infrastructure damage and overload. It is also important to regularly maintain your drain field and have a professional check your soil to make sure water from your septic system is draining properly. If the drain field is not filtering properly, overloaded with too much liquid, or clogged with solids, it will flood the field with sewage, increasing phosphorus levels in nearby water bodies. Septic system sources are also being addressed through the Town of Mashpee Comprehensive Wastewater Management Plan (CWMP). The Town of Mashpee has a commitment to eliminate all Title V Septic Systems.


Historic agricultural use around Santuit Pond – including several cranberry farms – has contributed to the accumulation of phosphorus in the Pond’s bottom sediment. The water from these bogs historically discharged into Santuit Pond, increasing phosphorus levels and further contributing to rapid algae and plant growth. In 2002, the Town of Mashpee purchased the historic cranberry bogs on the east and south shores of Santuit Pond in a 287-acre acquisition called the “Santuit Pond Preserve”. All remaining bogs are inactive (e.g., Baker, Brackett, and Chop Chaque). Studies are underway to determine how to restore these former cranberry bogs to wetlands. 

Santuit Pond Watershed Based Plan

The ongoing public health threat caused by persistent algal blooms is driving the need to evaluate and implement all potential measures to control phosphorus and manage the ongoing public health threat created by harmful blooms. Improvements to water quality will involve a number of solutions implemented over time. A Santuit Pond watershed based plan has been prepared in accordance with the guidelines under Section 319 of the Clean Water Act. The goal of the Watershed Plan is to build off of the activities to date, help guide implementation measures moving forward, and support efforts to identify implementation-funding opportunities for both the Town and the Mashpee Wampanoag Tribe. 

View the Watershed Based Plan