The causes and consequences of macroalgal blooms on an eelgrass (Zostera marina) community in Bodega Harbor, CA

Due to their sessile nature and high light requirements, seagrasses are particularly vulnerable to macroalgal blooms. However, most studies of the effect of macroalgal blooms on seagrasses have focused on the short-term, acute effects of blooms, and have been conducted in locations that have already begun the shift to macroalgal dominance due to eutrophication. In other places, seasonal blooms occur naturally, but it is less clear what is driving them and whether they have similar detrimental effects. The following research explores the long-term variation in macroalgal blooms (Ulva sp.) in Bodega Harbor, California, USA, and the effects on eelgrass ( Zostera marina) and its associated community. Chapter One presents a three-year field experiment in which we manipulated the density of Ulva. The Ulva blooms in each of the four summers varied in magnitude and duration, ranging from less that 0.5 to over 4 kg m-2. It was only during the largest bloom in 2006 that there was a significant effect of Ulva, resulting in declines in eelgrass shoot density greater than 50% that persisted for 4-6 months. Ulva did not affect rates of shoot growth, reproductive shoot density, epiphyte load, or sediment organic content. In Chapter Two, we present the effects of Ulva on the epifaunal invertebrates in the same experimental treatments. Ulva had strong effects on invertebrate abundance and species composition in 2006 only. The main effect of Ulva was an increase in the density of gammarid amphipods. Results from a mesoscom experiment suggest Ulva provides an important predation refuge for this group. In Chapter Three, I discuss the top-down effects of herbivores and the bottom-up effects of nutrients on Ulva biomass in this system. Although there was evidence of herbivore control of Ulva in a mesocosm experiment, there was only a weak effect of herbivores on Ulva biomass in a field experiment that crossed herbivore and nutrient treatments. Surprisingly, nutrients had no effect on change in Ulva biomass in either trial of the field experiment. These results suggest management of Ulva may be difficult, as neither nutrients nor loss of grazers seem to play an important role.