UC Berkeley

Harmful blooms of green macroalgae, known as green tides, have been increasing in frequency and intensity world-wide over the last decade. Composed mainly of the macroalgae, Ulva, these blooms occur in areas of low wave energy and high nutrient input from anthropogenic sources; they often result in massive die-offs in the impacted ecosystem. My dissertation addressed three key questions concerning the potential for green tides to occur in central San Francisco Bay: 1) what species of Ulva inhabit the bay and which of them have been identified as bloom-forming taxa in other locations? 2) does an overwintering, benthic bank of dormant propagules contribute to the rapid growth of spring Ulva populations? 3) does waterborne spore availability limit the recruitment of Ulva populations? Within central San Francisco Bay, I identified six species of Ulva using genetic barcoding, only four of which were previously reported within the bay. Several of these species are known to produce economically costly green tides in other regions of the world. While previous studies on the control of algal blooms focused on post-recruitment processes such as herbivory and space competition, I investigated two pre-recruitment processes: banks of benthic microscopic forms and supply of waterborne propagules. A multifactorial lab experiment demonstrated that banks of dormant benthic microscopic Ulva forms (i.e. gametes, zoospores or thallus fragments) survive winter incubation periods (12 weeks of darkness undisturbed at 11°C), but at a depressed growth rate. Successful recruitment from these banks is strongly influenced by seasonal increases in water temperature. A regular regime of water column sampling and laboratory culturing demonstrated that waterborne propagule supply varied with location within the central bay, and that the abundance of propagules in the water column varied strongly with season. Although water samples from all sites yielded recruits, sites north of the bay mouth yielded more Ulva recruits than southern sites during the spring recruitment period while sites south of the bay mouth yielded more recruits in fall. Together, this information indicates that propagule supply, hydrographic patterns that mediate dispersal, and environmental and physiological constraints on juvenile development impose important limitations on dynamics of green tide algae in the central San Francisco Bay. These interacting factors must be included in management as future global climate change is expected to stimulate more rapid development of spring green algal blooms.