Graduation Date
Spring 2025
Document Type
Thesis
Program
Master of Science degree with a major in Biology
Committee Chair Name
Dr. Sean F. Craig
Committee Chair Affiliation
Cal Poly Humboldt Faculty or Staff
Second Committee Member Name
Dr. Mark H. Carr
Second Committee Member Affiliation
Community Member or Outside Professional
Third Committee Member Name
Dr. Jeffrey W. White
Third Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Keywords
Northern California, Rocky reef community, Scuba, Community ecology
Subject Categories
Biology
Abstract
In 2014, an unprecedented ecosystem shift impacted the rocky reef communities of northern coastal California following a rapid succession of severe disturbance events. Rocky reef communities shifted from kelp forests to large scale urchin barrens due to the demise of the sunflower sea star (Pycnopodia helianthoides), an unprecedented marine heatwave event (MHW) and increased purple sea urchin densities. This “perfect storm” caused a decrease in benthic species richness and led to a change in community structure due to intense sea urchin grazing. Here I examine the richness of the benthic community across four survey sites over an 8-year time span (from 2014 to 2021) using annual SCUBA surveys to investigate community structure and the specific assemblages of taxa at three depths in northern California.
Purple sea urchins (Strongylocentrotus purpuratus) were the most abundant overall species censused and had the greatest range of observed densities among all 68 study taxa (ranging from 0-205 individuals/m2). Richness was highest in 2014 before the MHW. Random Forest models were employed to identify a suite of taxa in order to predict community richness. Results predict that high benthic taxon richness occurs when purple sea urchin density is between 1.5 and 8 individuals/m2. The Random Forest models also highlighted taxa such as the blood star (Henricia leviuscula), giant acorn barnacle (Balanus nubilus) and the stalked tunicate (Styela montereyensis), as potential indicators of higher richness in the rocky reefs.
Across the four sites surveyed, community composition changed over time, with 2014 being significantly different than all other years (2015-2021). However, post- “perfect storm” communities (2017-2021) did not differ significantly among years within this 5-year period. Analyses show clearly defined communities at different depths. Reefs at 4-meter depths were dominated by forest-forming brown algae as well as herbivores such as gumboot chiton (Cryptochiton stelleri), purple sea urchin (Strongylocentrotus purpuratus), and red abalone (Haliotis rufescens). Deep rocky reefs at 18-meter depths were more closely correlated with a community composed of red sea urchin (Mesocentrotus franciscanus), rock scallop (Crassadoma gigantea), giant acorn barnacle (Balanus nubilus) and leafy hornmouth (Ceratostoma foliatum).
This study shows that rocky reef communities within northern California are still experiencing change. Those taxa identified in this study that are associated with the more diverse rocky reef communities could help inform where recovery of kelp forests could happen more quickly following grazer suppression. This study builds upon our understanding of ecosystem phase shifts and community dynamics within rocky reef communities along the poorly studied far northern California coast.
Citation Style
author-date style
Recommended Citation
Moitoza, Franklin D., "Benthic community structure changes in Northern California following a marine heatwave" (2025). Cal Poly Humboldt theses and projects. 2298.
https://digitalcommons.humboldt.edu/etd/2298