Newport Bay/San Diego Creek Nutrient TMDL

Background Information

Excess nutrients flowing into the Newport Bay, primarily from San Diego Creek, have resulted in seasonal algae blooms that have impaired the Bay’s environment and our enjoyment of this important natural resource. The nutrients of concern are nitrogen and phosphorous, which are both essential for plant growth and necessary for healthy ecological functions in the Bay. However, high nutrient concentrations can cause excessive plant growth, including algae. This can lead to a problem called eutrophication, a condition in which excessive plant growth from nutrient enrichment impairs the capacity of a waterbody to sustain healthy ecological functions. This can lead to aesthetic problems, habitat loss, and poor biological diversity, among other adverse effects. Both nitrogen and phosphorous occur in nature, but eutrophication usually results from human activities that promote the input of these nutrients into our waterbodies. Some sources of these nutrients include agriculture, excessive garden or lawn fertilization, and pet waste.

Regulatory Description 


In 1998, the Santa Ana Regional Water Quality Control Board adopted a Total Maximum Daily Load for nutrients in the Newport Bay watershed (TMDL) to decrease the mass of nutrients flowing into the Bay, thus restoring and protecting its beneficial uses. Beneficial uses are assigned to every water body in the United States as a means of systematically assessing the quality of the nation’s surface waters, as required by the federal Clean Water Act of 1972 . If a water body’s beneficial uses are impaired, then the water body is deemed impaired and requires restoration measures. The TMDL establishes maximum nutrient loads (targets) at levels similar to those observed in the 1970s, prior to observations of eutrophic conditions. Thus, if these targets are met, signs of eutrophication in the Bay should diminish.

Photo of a sampling site in Upper Newport Bay in summer 1996, showing excessive growth of nuisance green algae
Photo of a sampling site in 1996 showing excessive green algae   

Photo of the same site as above in summer 2009, showing the absence of green algae 
Photo of the same site in 2014 showing absence of nuisance green algae

Recent Developments

The State Water Board is proposing to adopt a statewide water quality objective for biostimulatory substances along with a program of implementation as an amendment to the Water Quality Control Plan for Inland Surface Water, Enclosed Bays, and Estuaries of California.  

Data Collection and Reporting

In February 2000, a Regional Nutrient Monitoring Program (RMP) for the Newport Bay watershed was initiated by the County of Orange on behalf of the watershed cities (Costa Mesa, Irvine, Laguna Hills, Laguna Woods, Lake Forest, Newport Beach, Orange, Santa Ana, and Tustin), as mandated by the TMDL to assess compliance with the required nutrient reductions. The RMP is composed of routine and special monitoring components. The routine monitoring component consisted of weekly, bi-weekly, or monthly collection of water samples from sites throughout the watershed and monthly collection of algae samples from Upper Newport Bay.  The special monitoring component is comprised of other studies to improve our understanding of nutrient sources and dynamics in the Bay. Data (water quality and flow monitoring) collected through the RMP have been compiled into reports to the Santa Ana Regional Board.  The RMP has been revised several times since 2000, and monitoring and reporting efforts still continue for nutrients, but at a decreased frequency. 

Newport Bay Nutrient Annual Data Reports

The Newport Bay Nutrient TMDL data reports are filed in the OC Public Works Document Library  (to find the reports: in the name column, type nutrient).  


Notable accomplishments regarding implementation of the Newport Bay/San Diego Creek TMDL have included:

  • Achievement of total phosphorous and total nitrogen TMDL targets
  • Substantial reduction of algae on the Newport Bay mudflats 
  • Revision of the Regional Monitoring Program to reduce monitoring frequency, allowing resources to be spent on more pressing water quality concerns
  • The Water Quality Report Card by the State Water Board (October 2016) indicates conditions are improving for nitrogen and phosphorus in Newport Bay and San Diego Creek

Restoration and Volunteer Opportunities

There are several other initiatives underway in the watershed to help improve water quality. Plant nurseries represent the largest form of agriculture still remaining in the County and formerly contributed large inputs of nutrients into the watershed from their runoff. Today, many of these nurseries capture and reuse most or all of their irrigation water, which has substantially reduced nutrient inputs. In addition, the Irvine Ranch Water District operates natural treatment systems, including the San Joaquin Marsh. 

You can help in efforts to restore Newport Bay by: