Date of Award

Fall 2023

Project Type

Dissertation

Program or Major

Biological Sciences

Degree Name

Doctor of Philosophy

First Advisor

Jennifer A Dijkstra

Second Advisor

Steven Dudgeon

Third Advisor

Brezo Martínez

Abstract

Many coastal ecosystems are under threat from the combined pressures of climate change and introduced species. The Gulf of Maine is one such region under siege, being one of the fastest warming bodies of water in the world and being host to a variety of introduced species. In this dissertation, I investigate the invasion potential of the invasive red alga Dasysiphonia japonica in the Gulf of Maine and beyond through a combination of field, laboratory, and modeling studies. Despite being a recent introduction, Dasysiphonia has rapidly spread along the New England coastline. In areas where it has become established, Dasysiphonia often forms dense monocultures, transforming the seascape and reducing local algal diversity. In field studies, settlement plates were deployed for either short-term or long-term periods over the course of two years to examine patterns of short-term seasonal recruitment and longer-term succession. It was found that algal diversity in this system is high, but depends on the regular opening of new bare space to maintain diversity of rarer species. This makes sense in the view of the classic intermediate disturbance hypothesis. On long-term settlement plates, succession showed a gradual decline in species richness over time, moving towards a red turf-dominated state. Dasysiphonia was found year-round on both short and long-term plates, demonstrating that it is active even when other species are seasonally limited. In two laboratory experiments, the ability of Dasysiphonia to produce asexual propagules and the growth rates of those propagules were investigated under summer and winter conditions. It was found that Dasysiphonia can produce a large number of viable propagules, even under winter conditions. Those released fragments were able to grow to mature plants in eight weeks under summer conditions. Growth was slowed under winter conditions, but survival of propagules showed they were capable of overwintering through the New England winter conditions. Species distribution models were constructed to determine the potential suitable habitat currently vulnerable to invasion by Dasysiphonia, as well as how that area may shift under future projected climate conditions. It was found that Dasysiphonia has the potential to continue spreading along this coastline as far north as the Bay of Fundy and as far south as the Chesapeake Bay. Additional vulnerable regions include the temperate coasts of South America, Africa, and Australia. Under projected climate conditions, this suitable area will shift slightly poleward, but the total amount of vulnerable area will not be significantly impacted. In combination, these three studies paint a picture of Dasysiphonia as being a very effective invader in the Gulf of Maine and beyond. Its ability to reproduce asexually year-round through the production of propagules gives it a distinct advantage over native species with seasonally limited recruitment windows, including the native kelp Saccharina latissima. In the Gulf of Maine we should expect to see Dasysiphonia continue to dominate the subtidal ecosystem, as it rapidly takes advantage of newly opened bare space during seasons when other species are inactive. Currently, the Gulf of Maine consists of a complex mosaic of kelp and red algal turf habitats in an unstable equilibrium. With warming waters negatively impacting kelp, we may see a shift in the subtidal ecosystem from a kelp-dominated to a red turf-dominated system.

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