Date of Award

Summer 8-31-2025

Level of Access Assigned by Author

Open-Access Thesis

Language

English

Degree Name

Doctor of Philosophy (PhD)

Department

Marine Biology

First Committee Advisor

Douglas Rasher

Second Committee Member

Damian Brady

Third Committee Member

Kristina Cammen

Additional Committee Members

Peter Countway

Aaron Hartmann

Abstract

Kelp forests are ecologically important marine ecosystems that support rich biodiversity, fisheries, and coastal productivity. However, kelp forests are rapidly declining worldwide due to climate change and other anthropogenic stressors, leading to significant biodiversity loss and ecosystem degradation. In a rapidly warming Gulf of Maine, for example, kelp forests are collapsing and being replaced by low-lying turf algae, fundamentally transforming rocky reef habitats. In this dissertation, I investigate why kelp forests in the Gulf of Maine are transitioning to turf algae-dominated reefs, the ecological consequences of this ecosystem state shift, and what feedback mechanisms reinforce this shift.

Through three years of field surveys at 32 sites spanning 200 linear km of the Maine coastline, I document that a state shift from kelp-to-turf dominance has continued to progress across Maine’s rocky reef ecosystem. Through modeling efforts, rising ocean temperatures emerged as a key factor, where warming has pushed conditions beyond kelps’ thermal limits, especially in the southern Gulf of Maine, giving an advantage to heat-tolerant turf algae. These results implicate climate change and associated environmental stressors as the dominant forces precipitating the spread of turf algae through this region.

As kelp forests give way to turf algae, the ecological consequences are profound. Through trait-based analysis I show that turf-dominated algal communities have different functional characteristics compared to kelp forests. Turf algae are generally small, fast-growing, and form dense mats, lacking the height and structural complexity of kelp. This shift causes a loss of three-dimensional habitat with implications for the system’s capacity to support marine life. Functional trait analyses also suggest diminished carbon storage and nutrient uptake – kelp fronds store carbon and absorb nutrients efficiently, but turf algae store far less biomass. In essence, the transformation from kelp forests to turf reefs “flattens” the ecosystem both physically and functionally, potentially undermining key services and processes within these rocky reefs.

Through metagenomic analysis of reef biofilms, I further revealed that when kelp is replaced by turf algae, the composition of the reef microbiome shifts. Kelp-dominated reefs harbored diverse microbial communities including bacteria that help cycle nutrients (for example, aiding in nitrogen turnover and organic matter breakdown). In contrast, turf-dominated reefs showed a different microbial profile, with indications of fewer microbes that facilitate nitrogen recycling. This suggests that the ecosystem’s nutrient cycling capacity is altered on turf-dominated reefs: essential nutrients like nitrogen may become less readily regenerated in forms usable for kelp growth, potentially creating a nutrient environment less favorable to kelp.

Lastly, my research revealed that the turf-dominated state is not merely a passive consequence of kelp loss but is actively reinforced by feedback mechanisms – locking the system into a turf-dominated state. I discovered a chemical mechanism in which turf algae inhibit the recruitment of kelp. Through comprehensive chemical profiling (non-targeted metabolomics) of water and algal samples, coupled with lab bioassays, I found that turf algae release bioactive chemical compounds into the surrounding water, suppressing kelp recruitment. This provides direct evidence that allelopathy (chemically-mediated competition) is occurring on temperate reefs. Hence, turf algae are not only outcompeting kelp for space and light; they are also chemically inhibiting kelp reproduction. This research therefore provides continued evidence that Maine’s southern reefs are locked in a stable, alternative configuration dominated by turf algae, entrenched by feedback mechanisms that make a return to kelp forests unlikely.

In conclusion, this dissertation provides a deeper, mechanistic understanding of kelp forest collapse in the Gulf of Maine and its aftermath. It chronicles a climate-driven ecological transformation – from vibrant kelp forests to sprawling turf algae – and unpacks the multitude of factors that cause and cement this state shift. This research not only diagnoses the problem (warming oceans causing turf proliferation) but also reveals why the problem perpetuates (feedback loops involving chemicals). These insights are significant globally: many temperate reefs around the world are experiencing similar changes, and the Gulf of Maine now stands as a case study of how such ecosystem state shifts play out and what challenges they pose. By clearly identifying the processes involved, this work contributes to ecological theory on state shifts and could inform applied efforts to conserve and restore kelp ecosystems. The overarching lesson is that safeguarding kelp forests under climate change will require holistic approaches – addressing both the external stressors and internal feedbacks that reinforce the ecosystem state shift. Only by understanding and managing both can we hope to preserve the rich underwater forests that are so vital for marine life and coastal communities.

Download

Files over 10MB may be slow to open. For best results, right-click and select "save as..."

Share