Graduation Date
Spring 2025
Document Type
Thesis
Program
Master of Science degree with a major in Biology
Committee Chair Name
Rachael Wade
Committee Chair Affiliation
Cal Poly Humboldt Faculty or Staff
Second Committee Member Name
Sean Craig
Second Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Third Committee Member Name
Mark Carr
Third Committee Member Affiliation
Community Member or Outside Professional
Fourth Committee Member Name
Brent Hughes
Fourth Committee Member Affiliation
Community Member or Outside Professional
Keywords
Northern California, Mendocino, River mouth, Bull kelp, Purple urchins, Refugia, Nereocystis luetkeana, Marine heatwaves, Atmospheric rivers, Climate change, Kelp forest, Urchin barren
Subject Categories
Biology
Abstract
Populations of Nereocystis luetkeana (bull kelp) have experienced localized extinction events in northern California since 2014 due to multiple stressor events, including the area’s most severe marine heatwave (MHW) to-date. Previous surveys in the region observed that bull kelp recovered and persisted at river mouths following the 2014-2015 MHW, suggesting that river mouths may serve as refugia – the refugial river mouth hypothesis. My study aimed to assess this hypothesis and the long-term stability of these refugia by studying how freshwater influences communities at these sites and bull kelp recruitment. Field surveys of salinity fluctuation and kelp and sea urchin densities were conducted at paired river mouth and coastal reference sites. A laboratory salinity pulse experiment examined gametophyte and sporophyte development to simulate freshwater discharge events. River plume models suggested that river mouths experienced reductions in salinity in response to high flow events (minimum of 20 PSU), such as storm events, while reference sites did not experience such fluctuations. Survey results showed significantly higher abundances of urchins at reference sites than at the river sites, with bull kelp persisting at comparable abundances at all river sites and some reference sites. Kelp gametophytes survived repeated salinity pulses as low as 8 PSU, while healthy sporophytes were only observed at or above 16 PSU. Thus, river mouths are likely not limiting bull kelp recruitment, development and survival, but are reducing herbivore pressure. These results support the refugial river mouth hypothesis and that these sites will likely continue to harbor bull kelp and should be targeted for restoration efforts.
Citation Style
APA
Recommended Citation
Jackson, Duncan, "Investigating the refugial river mouth hypothesis: assessing the effects of low salinity conditions on the microscopic life stages of Nereocystis luetkeana" (2025). Cal Poly Humboldt theses and projects. 2278.
https://digitalcommons.humboldt.edu/etd/2278