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

Abstract

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.

Are large macroalgal blooms necessarily bad? nutrient impacts on seagrass in upwelling‐influenced estuaries

“…In many of these systems, marine-derived nutrients seasonally dominate estuaries, as a result of nearshore oceanographic processes rather than terrestrial nutrient loading. Here, blooms of ulvoid macroalgae can be as large as those associated with seagrass declines elsewhere, but few negative effects on the dominant seagrass species, Zostera marina L. have been detected (Thom 1990, Kentula and DeWitt 2003, Brown et al 2007, Jorgensen et al 2010, Hessing-Lewis et al 2011, Hessing-Lewis and Hacker 2013, but see Nelson and Lee 2001, Olyarnik and Stachowicz 2012. Further, in these systems, the effects of high nutrient conditions (driven by upwelling events) on eelgrass growth are unclear.…”

Predicting consequences of climate change for ecosystem functioning: variation across trophic levels, species and individuals

“…Protected by a~2 km sand spit, Bodega Harbor includes both shallow mudflats (at 0 to 0.7 m above mean lower low water [MLLW]) and deeper eelgrass (Zostera marina) beds (at 3 to 0 m below MLLW). Inside the harbour, what is most likely Ulva lactuca covers the mudflats between the spring and fall, providing above-sediment habitat structure and food where it is otherwise lacking, and also intermixes with eelgrass in deeper, colder water (Olyarnik & Stachowicz, 2012;Best & Stachowicz, 2014). Immediately outside of the harbour, Ulva of the same and/or additional species also blooms in spring and summer in the rocky intertidal-zone of the outer coast, where it is the most widespread ephemeral, both forming monotypic patches and intermixing with perennial macroalgae.…”

Phenotypic and phylogenetic evidence for the role of food and habitat in the assembly of communities of marine amphipods

“…1 for names; Appendix A: Table A1 for additional details). These species are epifaunal, inhabiting extensive mudflats covered with beds of macroalgae (Ulva spp., hereafter Ulva), beds of Zostera marina (hereafter eelgrass) which also contain variable amounts of Ulva (Olyarnik and Stachowicz 2012), and the encrusting invertebrates and macroalgae (again, Ulva) growing on floating docks and pilings. Because eelgrass beds are deeper (range from 3 to 0 m below mean lower low water [MLLW]), than Ulva beds on mudflats (0 to 0.7 m above MLLW), Ulva beds reach higher water temperatures than eelgrass beds at low tide.…”