Research Areas > Nutrients > Sediments as a Non-Point Source of Nutrients to Malibu Lagoon

Project: Sediments as a Non-point Source of Nutrients to Malibu Lagoon

Background and Objective

Like many southern California estuaries, Malibu Lagoon experiences occasional macroalgal blooms. While macroalgae are a natural component of an estuarine ecosystem, large algal blooms can decrease the overall ecological integrity of the system by reducing dissolved oxygen and leading to eutrophication. As a result of these issues, Malibu Lagoon was placed on the California’s list of impaired water bodies and several management measures were taken to address nutrient loading. There was a lack of correspondence, though, between the peak periods of nutrient input and the time when blooms were most often observed. This led to the hypothesis that wet season sediment inputs to the lagoon can store nutrients, which are later remobilized to surface waters and provide fuel for primary producer growth during dry weather periods.

The objectives of this study were to:

1) Investigate the seasonal patterns of bulk sediment and sediment pore water N and P concentrations in the lagoon.
2) Estimate wet season and long-term average annual sediment deposition rates, along with associated particulate N and P loads to the lagoon. Then determine, to the extent possible, the major anthropogenic sources associated with this deposition.
3) Estimate ambient benthic nutrient exchange over an annual cycle, as a variety of environmental conditions are observed in the lagoon.
4) Compare the importance of sediment remobilization with the magnitude of other non-point nutrient sources to lagoon.

Status

This study was conducted from 2002 to 2004.

Methods

In order to determine the significance of sediment-derived nutrients relative to other sources, SCCWRP researchers collected sediment cores and water samples throughout the year at five sites within the lagoon. At each site, water column analyses were performed for salinity, temperature, DO, total suspended sediment, chlorophyll a, particulate nutrients, particulate carbon, dissolved inorganic nitrogen, dissolved organic nitrogen, and phosphorus. Sediment cores from the same locations were assessed for bulk sediment and pore water concentrations of nutrients and carbon, as well as radionuclide tracers beryllium-7 (a natural radionuclide tracer of short-term sediment deposition) and lead -210 (used to determine the long-term average annual sedimentation rate). Percent cover of each primary producer species or functional group was likewise described at each site.

These data were analyzed to look for seasonal patterns in nutrient concentrations and estimated benthic nutrient flux rates under a variety of environmental conditions. To the extent possible, researchers tried to identify the major anthropogenic (human) sources of nutrient loading to the lagoon. Finally, they compared the importance of sediment benthic flux with other non-point nutrient sources, such as storm runoff and atmospheric deposition.

Findings

Significant findings from this study were as follows:

• Estimates of benthic flux showed a net release of dissolved inorganic and organic nutrients from sediments to surface waters. Nutrient release from sediments was a significant source to primary producers relative to other non-point inputs during the dry season.

• Wet season sediment deposition, mainly from wet weather events, increased the N and P content of Malibu Lagoon sediments as compared to pre-wet season baseline conditions. However, net loss of nutrients from sediments during the dry season depleted only about 10% of the amount deposited during the wet season.

• Excessive growth of R. maritima occurred and created hypoxic surface water conditions during the dry season. The timing of the R. maritima blooms coincided with periods of high nutrient release from lagoon sediments. Timing was also influenced by periods of lagoon closure (when the lagoon mouth was cut off from tidal influence), which affects salinity and hydrodynamic regime.

• A variety of non-point sources contributed nutrients that fueled R. maritima growth during the dry season (see table below). Nutrient release from sediments was equal to 18% of total N sources and 5% of total P sources.

Partners

This project was conducted in collaboration with the Los Angeles Regional Water Quality Control Board (LARWQCB), the Las Virgenes Municipal Water District, and Heal the Bay.