Graduation Date

Fall 2019

Document Type



Master of Science degree with a major in Biology

Committee Chair Name

Frank Shaughnessy

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Paul Bourdeau

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Joe Tyzburczy

Third Committee Member Affiliation

HSU Faculty or Staff

Fourth Committee Member Name

Erik Jules

Fourth Committee Member Affiliation

HSU Faculty or Staff

Subject Categories



Macrophyte community structure in estuaries that have been modified by the addition of hard substrata are poorly documented, especially in relation to the physical and environmental gradients that shape them. Therefore, this study describes the summer macrophyte flora that occurs on hard and soft substrata throughout Humboldt Bay, CA and tests how the macrophyte communities correlate to measures of horizontal and vertical environmental gradients. The percent cover of macroalgae, vascular plants, cyanobacteria, lichens, diatom film, sessile invertebrates, and substratum types were quantified during the summers of 2017 and 2018 at eleven intertidal locations in the bay and outer coast. Horizontal environmental gradients of salinity, water temperature, and wave exposure were quantified by hydrodynamic and wind-wave models. Vertical environmental gradients of air and water temperature per zone were measured using temperature loggers, while gradients of cumulative radiation, dew point, relative humidity, and wind speed were calculated from local weather station data during emergence periods. Macrophyte community structure and diversity measurements were analyzed using NMDS ordinations with environmental correlations, PERMANOVA, and indicator species analyses. Overall, in the bay there were 97 macroalgae, 22 salt marsh vascular plants, and one seagrass species. Horizontal and vertical environmental gradients were strongly correlated with changes in community structure. Horizontally, hard-bottom communities near the mouth of the estuary experienced more stable water temperatures and salinity and higher wave exposure than soft-bottom sites furthest from the mouth, which had greater fluctuations in temperature and salinity and reduced wave exposure. Hard-bottom sites closest to the mouth of the bay shared many algal species seen on the outer coast and had higher species richness and diversity than inland soft-bottom sites, which hosted fewer but abundant chlorophytes and eelgrass, in addition to diverse vascular plant communities. Vertically, elevation, desiccation pressure, and substratum type were the strongest predictors of community structure for both hard and soft-bottom sites. At hard-bottom sites, richness, evenness, and diversity increased from high to low intertidal while at soft-bottom sites, these measurements increased from low to high intertidal. As such, at sites with artificial riprap, a few annual chlorophytes and rhodophytes in the high intertidal transitioned to dense, morphologically diverse, and perennial red and brown algae at lower elevations. At soft bottom sites, diverse vascular salt marsh plants in the high intertidal gave way to some chlorophytes and eelgrass beds in the low intertidal. Compared to other natural and modified estuaries on the NE Pacific and Atlantic Oceans, species diversity in Humboldt Bay decreases further into the estuary; however, species composition differs greatly from Northern Atlantic estuarine rocky intertidal habitats. Meanwhile, vertical community structure of marine macrophytes follows similar zonation patterns to other estuaries. Taken together, these results suggest that although modified oceanic estuaries harbor great diversity and unique macroalgal communities, they have come at the expense of natural soft bottom habitat that is important for many plants and animals and, with the threat of climate change, is vulnerable to sea level rise.

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