Research Areas > Nutrients > Evaluation of the Impact of Terrestrial Runoff on Coastal Ocean Biological Response

Project: Evaluation of the Impact of Terrestrial Runoff on Coastal Ocean Biological Response

Background and Objectives

Episodic winter storm events in southern California can create overland runoff laden with pollutants, which eventually discharges through rivers and storm sewers to the ocean. Stormwater plumes, particularly from urban watersheds, are sometimes followed by blooms of planktonic algae. It is difficult to discern the relative role of stormwater in the development of these periodic phytoplankton blooms, though, since there are no tools that predict the effect of climatic and anthropogenic events on the variability in algal growth within the coastal zone.

However, interactions among physical, chemical, and biological processes occurring in the coastal ocean can be assessed through a combination of observational remote sensing tools and predictive modeling tools. These tools can then be used to relate land-based nutrient fluxes (e.g., via stormwater) to biological response in the coastal ocean. With such tools available, environmental managers can assess the effects of stormwater discharges from southern California coastal watersheds relative to changes in climate and implementation of stormwater management measures. The models can also be used to evaluate how the presence or absence of a spatially extensive estuary along the coast affects runoff plume properties and pollutant loadings to the ocean.

Using an integrated analysis approach that utilizes multi-sensor satellite data and computational modeling, the objectives of this project were to:

1) Quantify nutrient loadings (relative to watershed properties and climate variability) during stormwater runoff events from several southern California watersheds
2) Quantify the coastal ocean biological response (i.e., planktonic blooms) to stormwater pollutant (especially nutrient) loadings, considered in the context of event-scale, seasonal, and interannual processes

Status

This project was initiated in 2006 and completed in 2009.

Methods

Analyses of the relationship between watershed land use properties and nutrient runoff/loading were developed for stormwater plumes from the Santa Clara River, Ballona Creek, Los Angeles River, San Gabriel River, and Santa Ana River watersheds.

The spatial and temporal distributions of plume parameters, primarily temperature, salinity, suspended solids (plus associated sorbed constituents), fecal indicator bacteria (FIB), and dissolved substances (e.g., nutrients, dissolved organic matter) were evaluated using satellite, in situ observations, and modeling.

Variability in winter and spring phytoplankton biomass (e.g., algal blooms) in the Santa Monica and San Pedro Bay/Basin regions were quantified using remotely sensed chlorophyll levels, as well as in situ (onsite) measures for assessing chlorophyll concentrations deeper in the water column. In situ measurements also served as a point of comparison for satellite-derived chlorophyll levels. Variability in the chlorophyll time-series was analyzed relative to: 1) nutrient flux, 2) light attenuation (both in situ and remotely derived), and 3) physical forcing mechanisms over a range of temporal and spatial scales.

Findings

Exceedances of bacterial standards were observed in most of the systems following storms. However, the areas of impact were generally spatially limited, and contaminant concentrations typically declined to below California Ocean Plan (COP) standards within 2 to 3 days. Maximum nitrate concentrations (~40 μM) occurred in the San Pedro Shelf region near the mouth of the Los Angeles River.

Based on the results of general linear models, individual sources of stormwater differ in nutrient concentrations and fecal indicator bacteria (FIB) concentration and composition. While nutrients appeared to decrease in plume waters due to simple mixing and dilution, FIB concentrations in plumes depended on other factors. The relationships between contaminants (nutrients and FIB) and plume indicators (salinity and total suspended solids (TSS)) were not strong, indicating the presence of other potentially important sources and/or sinks for both nutrients and FIB.

The COP standards were often exceeded in waters containing greater than 10% stormwater (salinity range < 28-30). Relationships between colored dissolved organic matter (CDOM) and salinity, and between TSS and beam attenuation, indicated that readily measurable, optically active variables can be used as proxies to provide at least a qualitative, if not quantitative, evaluation of the distribution of dissolved and particulate components of stormwater plumes. In this context, both CDOM absorption and the beam attenuation coefficient can be derived from satellite ocean color measurements of inherent optical properties, suggesting that remote sensing of ocean color should be useful in mapping the spatial areas and duration of impacts from these contaminants.

Partners

This project was conducted in collaboration with the NASA Jet Propulsion Laboratory, University of California Los Angeles, University of Southern California, and the U.S. Geological Survey.