Alena Wachmann

Topic

Centennial persistence and multi-decadal resilience of kelp forest ecosystems on the West Coast of Vancouver Island, Canada

Department of Geography

Date & location

• Thursday, December 11, 2025

• 2:30 P.M.

• David Turpin Building

• Room B215

Reviewers

Supervisory Committee

• Dr. Maycira Costa, Department of Geography, University of Victoria (Supervisor)

• Dr. Noémie Boulanger-Lapointe, Department of Geography, UVic (Member) 

External Examiner

• Dr. Kylee Pawluk, Ministry of Water, Land & Resource Stewardship, Government of British Columbia 

Chair of Oral Examination

• Dr. Brian Thom, Department of Anthropology, UVic

   

Abstract

Canopy-forming kelps, belonging to the order Laminaria, are essential foundation species in temperate coastal ecosystems, supporting biodiversity, fisheries, and cultural livelihoods by creating complex three-dimensional underwater forests. However, these ecosystems are increasingly vulnerable to stressors such as ocean warming, marine heatwaves, and local disturbances. Understanding how kelp forests respond to environmental variability across space and time is critical for guiding conservation and restoration amid a rapidly changing ocean. Along the West Coast of Vancouver Island, British Columbia, forests of Macrocystis pyrifera and Nereocystis luetkeana form some of Canada’s most extensive and productive nearshore ecosystems, occurring across gradients in temperature, exposure, and upwelling. However, long term, spatially consistent analyses of their dynamics have been limited, particularly in how modern trends compare to historic variability. Recent advances in remote sensing now permit consistent monitoring of floating kelp canopy extent over large stretches of the coast at decadal scales. Archived earth observation imagery from multiple satellite platforms, including Landsat, SPOT, Sentinel-2, and PlanetScope, provides opportunities to recreate the dynamics of floating kelp forests dating back to the early 1970s. This thesis uses an integrated, multi-sensor, and multi temporal approach to quantify the distribution, persistence, and resilience of WCVI floating kelp forests over the past ~165 years. Specifically, it (i) reconstructs centennial-scale persistence by harmonizing historical British Admiralty charts and Canadian Hydrographic Service fieldnotes (1858–1956) with modern Sentinel-2 imagery (2020–2023), and (ii) examines multi-decadal canopy dynamics using satellite imagery from 1972–2024 to evaluate the influence of warm and cool climate periods, including the 2014–2016 “Blob” marine heatwave. This approach identifies spatial and temporal patterns of kelp forest stability across four abiotic ecoregions, offering new insights into the environmental conditions that underpin long-term refugia and inform climate-resilient management and restoration strategies along the WCVI. Results show that colder, high-energy ecoregions 1–3 supported 97.5% of modern kelp extent and displayed high centennial persistence (87–89%). In contrast, the warm, sheltered ecoregion 4 showed lower persistence (52%), likely related to summer sea surface temperatures approaching the upper thermal limit for kelp (~18°C). Long-term time series reveal that large kelp forests in ecoregion 1, representing the coldest areas with the strongest current, remained relatively stable through warm and cool climate periods over the past 53 years. Large kelp forests in ecoregions 2 and 3, characterized by high to moderate exposure to wind and wave action, exhibited greater sensitivity to fluctuations in sea surface temperature and marine heatwave days, resulting in increased canopy variability. Conversely, smaller kelp forests in ecoregions 2, 3, and 4 declined substantially during the 2014–2016 “Blob” and showed limited recovery afterward, regardless of ecoregion designation. This may indicate a departure from historical trends, as elevated ocean temperatures since 2014 have made these forests more vulnerable to cumulative adverse effects. Overall, these findings demonstrate that WCVI kelp resilience depends on scale and climate, shaped by local oceanography, forest size, and climate influences. The framework developed here links historical persistence with modern resilience, identifying long-lasting kelp refugia vital for marine protected area design, restoration efforts, and Indigenous-led stewardship, aligning with Canada’s 30 by 30 conservation objectives.