Research Areas > Nutrients > Relationship between Dissolved Oxygen and Macroalgal Distribution in Upper Newport Bay

Project: Relationship between Dissolved Oxygen and Macroalgal Distribution in Upper Newport Bay

Background and Objectives

Upper Newport Bay (UNB) is the second largest estuarine embayment in southern California and serves a variety of beneficial uses. It is both an important recreational area and a state ecological reserve that provides refuge, foraging areas and breeding grounds for a number of terrestrial and aquatic species. However, beneficial uses of the Bay are threatened by numerous sources of pollutant loading. To address issues of nutrient overenrichment, the Santa Ana Regional Water Quality Control Board established a Total Maximum Daily Load (TMDL) for nitrogen and phosphorus in 1998.

Management decisions for UNB require an understanding of the factors that control its trophic condition. However, there are several technical challenges to making such assessments and measuring compliance with the TMDL. First, it is difficult to synoptically evaluate the abundance of macroalgae throughout the bay. Second, the spatial and temporal extent of seasonal hypoxia in the bay is unknown. Third, the relationship between macroalgal abundance and DO, and hence its utility as an indicator of eutrophication, is unknown.

This project's objectives were to answer the following questions:

• Was macroalgal distribution accurately assessed by false color infrared (CIR) aerial photography as an alternative or complement to ground-based methods?

• What was the extent of macroalgal cover over the summer-autumn season and how did estimates of macroalgal cover based on ground measures vs. aerial photography compare?

• What were the spatial and temporal patterns of hypoxia/anoxia in UNB?

• Were frequency and intensity of hypoxia related to macroalgal abundance? What other environmental factors (physical and chemical parameters) appeared to influence hypoxia and observed variations of macroalgal abundance?

Status

This project was conducted from 2005 to 2006.

Methods

The project approach consisted of two components. First, water column parameters DO, temperature, salinity, depth, and pH were continuously monitored in both surface and bottom waters at three sites in UNB, using automated data sondes. Second, macroalgal distribution was surveyed with high-resolution CIR aerial photography during daytime low tides on three occasions: July 26, September 17, and October 31, 2005. To provide a comparative data set, macroalgal abundances were also measured on the ground by SCCWRP and Moss Landing Marine Laboratories (MLML) personnel during each overflight. For the land-based method, the percent cover of macroalgae was measured with a quadrat at approximately 30 random locations throughout UNB.

Findings

Significant findings from this project were:

• Color infrared photography is a good tool for evaluating macroalgal abundance on exposed, intertidal mudflats. Two classes of macroalgae were distinguishable from these photographs and correlated well with ground-based measurement. Data extrapolated from the ground surveys likely overestimates macroalgal abundance, while estimates from aerial imagery are likely conservative. Aerial photo-interpretation was not able to provide any information on the thickness of macroalgal mats and, therefore, not appropriate for estimating biomass.

• During the year sampled, overall algal extent was high. There was typically a longitudinal gradient in macroalgal abundance with more algae near the head of the estuary and less in downstream areas.

• Events of hypoxia primarily occurred following algal blooms in late summer and early fall, and under particular water quality conditions (i.e., vertical stratification of the water column where bottom water and surface water do not mix).

• Macroalgal abundance was not quantitatively related to the frequency of hypoxia, and explained only about half of the variability in DO values observed in surface and bottom waters.

• Wind-driven mixing and tidal mixing in UNB may limit the occurrence of hypoxia, even during macroalgal bloom events.

Partners

This project was conducted in collaboration with the Orange County Resources and Development Management Department, Irvine Ranch Water District, and Moss Landing Marine Laboratories.