Graduation Date
Fall 2019
Document Type
Thesis
Program
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
Keywords
Seaweeds, Community ecology, Environmental gradients, Estuary, Pacific Northwest
Subject Categories
Biology
Abstract
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.
Citation Style
APA
Recommended Citation
Raether, Saskia A., "Seaweed community structure along environmental gradients in an ocean-dominated estuary" (2019). Cal Poly Humboldt theses and projects. 344.
https://digitalcommons.humboldt.edu/etd/344