Date of Award

Summer 8-22-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Marine Biology

First Committee Advisor

Douglas Rasher

Second Committee Member

Kristina Cammen

Third Committee Member

Damian Brady

Additional Committee Members

Emma Elliott Smith

Mark Carr

Abstract

Macroalgae serve as foundation species in coastal seas around the world, providing both habitat and energy resources that facilitate productive and diverse ecosystems. In many temperate coastal oceans, kelps (Laminariales) create underwater forests, which support fish assemblages and food webs; thus, the decline of kelps due to climate change can reshape the structure, dynamics, and functioning of ecosystems. In the 21st century, the loss of kelp forests is often followed by the rise of small, red algae, which are creating novel “turf reef” ecosystems around the world. In the Gulf of Maine, a kelp-to-turf state shift has already drastically altered the physical structure of coastal rocky reef habitats, providing an opportunity to assess the impacts of this new ecosystem configuration on local fauna and food web structure. However, we lack knowledge on the basic ecological linkages between kelp and fishes in this region to use as a baseline for assessing the impacts of climate and habitat change.

My dissertation seeks to fill these knowledge gaps by investigating the ecology of rocky reef fishes, with a particular focus on their connections to kelp forests and turf reef habitats that characterize the modern nearshore ecosystem. I surveyed reefs fishes and measured environmental covariates across the coast, within habitats ranging from those dominated by kelps (i.e., kelp forests) to diminutive red algae (i.e., turf reefs), to gain insights into the natural variation and drivers of fish assemblages (Chapter II). Since structured habitat provided by macroalgae can be most important for fish during their vulnerable juvenile stages, I further assessed patterns of fish recruitment and juvenile habitat use (Chapter III). Causal models assessing the drivers of fish assemblages suggested that kelp forest loss likely had little impact on modern species assemblages or productivity, and fish recruits on rocky reefs did not particularly associate with kelp habitats. Therefore, we can infer that the species composition of the small piscine predator assemblage that dominates modern rocky reefs has not been significantly altered by the state shift (Chapter II and III).

Food webs are structured both by their species composition and the interactions between those species. Foundation species like kelp can regulate those interactions by expanding ecological niche spaces and directly contributing energy to the interaction pathways that connect trophic levels. To assess these food web structures on Gulf of Maine rocky reefs, I analyzed the degree to which the predominant fishes rely on similar trophic resources, and on energy produced by kelp and other macroalgae (Chapter IV). This research suggested that kelp forests increase predatory fishes’ overall ecological niche breadth and niche partitioning between species, and contribute energy, which is heavily used in local food webs. Patterns of niche space occupancy and energy use changed where kelp forests were lost (Chapter IV).

Overall, my research shows that the Gulf of Maine’s coastal reefs contain unique, low-diversity fish assemblages, whose composition has been defined by past anthropogenic impacts, but today appear relatively resilient to the ongoing climate change-induced kelp-to-turf state shift. My dissertation reveals much about the role of fishes and kelp in the structure and functioning of Gulf of Maine nearshore food webs, and sets a benchmark to understand ongoing change. In addition, it contributes to knowledge on the role of North Atlantic kelp forests within global kelp forest ecology, and sheds light on the food web implications of emerging turf-dominated reefs around the world.

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