Multi-year study of the effects of Ulva sp. blooms on eelgrass Zostera marina

Macroalgal blooms have contributed to declines in foundation species such as corals and seagrasses across the globe. Most studies of macroalgal bloom effects on seagrasses focus on the short-term effects, and have been conducted in locations that already begun the shift to macroalgal dominance, usually due to eutrophication. Our goal was to determine the degree to which the timing and magnitude of ephemeral, green macroalgal blooms (Ulva sp.) vary in Bodega Bay, California, USA, where there is little evidence for eutrophication, and how such blooms affect eelgrass Zostera marina. Over 38 mo, we conducted (1) an unmanipulated control treatment, and 3 manipulative treatments: (2) Ulva removal, (3) ambient Ulva, and (4) doubleambient Ulva ('2×'). We observed 4 blooms of varying magnitude and duration, ranging from <0.5 to >4 kg m−2. It was only during the largest bloom in 2006 (peak density 8 times that of the smallest observed bloom) that we saw a significant effect on eelgrass, resulting in declines in shoot density of >50% that persisted for 4 to 6 mo. During this time, the 2× treatment reduced shoot biomass by up to 90%, an effect that persisted for up to 9 mo. Ulva did not affect rates of individual shoot growth, reproductive shoot density, epiphyte load, or sediment organic content at any time during the experiment, suggesting the effect occurs through shoot mortality and reduced shoot production. Interannual variation in the magnitude timing and duration of algal blooms is large and can dramatically alter the effects of blooms on eelgrass. These factors must be considered when interpreting the results of experimental algal additions.