Macroalgal bloom dynamics in a highly eutophic Southern California estuary

Karleen A. Boyle and Krista Kamer
University of California, Los Angeles

Friday, August 6, 1999
12:00 Noon—Pacific Forum

Anthropogenic eutrophication of estuaries in Southern California is common due to increasing urbanization of local watersheds. Episodic nutrient inputs can fuel blooms of opportunistic estuarine algae such as Enteromorpha intestinalis and Ulva expansa. Upper Newport Bay (UNB), Orange County, California, is located in an urban environment and large macroalgal blooms are common in the estuary. This study documents algal community composition and water column, sediment, and algal tissue nutrients in UNB over a 16-month period. This field monitoring was supplemented with a microcosm experiment testing nutrient limitation of macroalgal biomass in UNB.

The abiotic parameters we measured indicate a highly eutrophic system. The freshwater source for UNB was high in N (800m M) following the winter rains. Sediment N levels peaked after this period, then declined throughout the summer and fall, reaching their lowest point in early winter before the rainy season began again. We hypothesized that this decline was due to utilization of sediment nutrients by the substantial macroalgal blooms (max. of 1.11 kg wet weight m^-2) that occurred during the summer and fall. Our microcosm experiment supported this hypothesis and demonstrated that macroalgae are capable of using sediment stores of nutrients to fuel bloom events.

The algal community exhibited seasonal shifts in dominance. Large blooms of E. intestinalis began in spring and continued through early summer. U. expansa became dominant in late summer and significant amounts of Ceramium sp. were measured in the fall. In the winter the system was dominated by benthic diatom mats. Macroalgal tissue N exceeded 4 % dry weight indicating luxury uptake of N. Algal community data also indicate a highly eutrophic system, and any further nutrient inputs will only exacerbate the problem.

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