Research Areas > Nutrients > Sediments as an Internal Source of Nutrients to Upper Newport Bay California

Project: Sediments as an Internal Source of Nutrients to Upper Newport Bay

Background and Objectives

Newport Bay is one of the largest estuarine embayments in southern California. The upper portion is a State Reserve, providing foraging areas and breeding grounds for a number of threatened and endangered species. Over time, land use changes in the watershed have led to increased nutrient loads, which fuel macroalgal blooms in Upper Newport Bay (UNB) and have created hypoxia and other associated impacts. As a result, the Santa Ana Regional Water Quality Control Board placed the Bay on the federal 303(d) list of impaired water bodies. This listing resulted in the development and adoption of a Total Maximum Daily Load (TMDL) for nitrogen (N) and phosphorus (P) for the Bay in 1998. Current water quality objectives are based on surface water loads and do not account for internal sources of nutrients (such as sediments). However, sediment-derived nutrients are biologically available to primary producers and may cause blooms to persist even when nutrient loading from the watershed is reduced.

This project had four major objectives:

1) Determine the load of N and P associated with wet-season input of sediments into UNB
2) Measure the exchange of N and P between the surface waters and the sediment (a.k.a. benthic nutrient flux)
3) Identify the major processes controlling benthic nutrient flux
4) Evaluate the importance of benthic nutrient flux relative to other nonpoint inputs to UNB

Status

This project was conducted from 2003 to 2006.

Methods

The project consisted of four major components:

• Sampling of sediments, surface waters, and macroalgae in order to:

- Describe major spatio-temporal patterns in nutrient pools found in these three components
- Using radioisotope tracers, estimate seasonal and annual sedimentation rates and associated nutrient loads
- Estimate the flux of nutrients from sediments (i.e., diffusive flux) based on concentration gradients between sediment pore waters and surface waters

• Direct measurement of nutrient fluxes in situ within the Bay, using benthic flux chambers

• Investigation of the interactions among macroalgae, surface water nutrient concentrations, and dissolved oxygen under controlled laboratory conditions

• Integration of results to estimate the annual flux of nutrients from sediments into the water in UNB, and compare this to other non-point nutrient sources

Findings

Significant findings were:

• Nutrient, metal, total inorganic carbon and oxygen concentrations and fluxes indicated that UNB is an anthropogenically-impacted, eutrophic estuary.

• Deposition of particulate N and P in sediments provided a biologically-available source of nutrients to surface waters and thereby primary producers such as macroalgae. Specifically, a net release of ammonia, phosphate and dissolved organic N and P from the sediments occurred throughout the year.

• Benthic release of nutrients had the most significant biological impact during the dry summer season, when other factors such as light availability and increased temperature enhanced the growth of macroalgal blooms.

• Actual measured nutrient fluxes were higher than either controlled measurements of in situ benthic nutrient flux or calculated estimates of diffusive flux. Advective transport (e.g., due to bioirrigation or tidal pumping) was identified as one of the major factors controlling benthic nutrient flux. Other factors such as oxygen availability and the presence of certain algal species could also modify benthic nutrient fluxes and have a major impact on nutrient cycling.

Partners

This project was conducted in collaboration with the Santa Ana Regional Water Quality Control Board, University of Southern California, and California Institute of Technology.