Macroalgal Distribution and Impacts on Intertidal Flats, With Emphasis on the Exotic Species Agarophyton Vermiculophyllum

Agarophyton vermiculophyllum (formally known as Gracilaria vermiculophylla) is an invasive, bloom-forming, marine macroalga originating from Japan. A. vermiculophyllum has invaded estuaries along both eastern and western coasts of North America, the Atlantic coast of Europe, and the northwest African coastline. A. vermiculophyllum can be found on intertidal mudflats, which are soft-sediment environments, usually without rooted vegetation. In many estuaries A. vermiculophyllum is now the dominant, or only macroalga on mudflats. This species affects mudflat ecosystems by increasing the abundance, and in some cases diversity of epifauna. Whether these community differences lead to ecosystem-scale effects in food webs, or biomass, has not been investigated. Additionally, the generality of epifaunal community changes across abiotic gradients is not well understood. The goal of this dissertation was to understand the drivers of A. vermiculophyllum spatial distribution and its effect on mudflat trophic structure across abiotic gradients. A > 900 km² dynamic coastal seascape across which mudflats varied in elevation, topographic complexity, wave exposure, and water residence time was used for a study system. To understand the role of geomorphology for mudflat producers and consumers I developed a novel index variable describing topographic variability on mudflats. I investigated the effect of A. vermiculophyllum on primary producers, macroinvertebrates, and shorebirds across a gradient of topographic variability. Through behavioral observations, large-scale manipulations, surveys and models, I found community-level changes caused by A. vermiculophyllum were decoupled from ecosystem-scale biomass changes. Shorebird communities partitioned mudflats based on the distribution of A. vermiculophyllum, with species preferring or avoiding macroalgal habitat based on their foraging mode specialization. However, the biomass of benthic microalgae, macroinvertebrates, and shorebirds, as well as the individual body sizes of the macroinvertebrate and shorebird communities, did not change when mudflats were colonized with A. vermiculophyllum. Mudflat topography explained A. vermiculophyllum spatial distribution, and the abundance and body size distribution of macroinvertebrates and shorebirds. Once accounting for mudflat topography and sediment characteristics, effects of A. vermiculophyllum for mudflat consumer biomass were not evident. This may have been due to compensatory changes in community body size distribution of invertebrates and birds driven by topography and sediment characteristics. Highly invasive species do not always have ecosystem-level effects, and while monitoring and more experimentation are certainly required, it appears for now that Agarophyton vermiculophyllum may be one of these cases.