Fall 2017

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


Demographic studies allow for a better understanding of how populations change over time and establish a baseline to examine how biotic and abiotic factors influence populations. The annual alga Desmarestia herbacea, or the acid weed, accumulates sulfuric acid within cell vacuoles, likely as a chemical defense mechanism. Whether intracellular pH varies among different life history stages is poorly understood. A D. herbacea population in the Stillwater Cove, California kelp bed was assessed for two years in order to measure how internal pH varies relative to demographics, season, grazing pressure and oceanography. The timing of both spring recruitment and fall senescence varied interannually. Sporophyte recruitment occurred during the upwelling season in mid-March, two weeks earlier than previously reported, and thalli reached maximum length during the Oceanic season, then senesced during the Davidson Current season. Maximum mean thallus length varied inversely as a function of density, with smaller plants present in 2015 when densities were higher. In contrast in 2016, individuals were significantly larger, their densities were lower, and the population senescence period extended much longer into January 2017. The ontogenetic shift in intracellular pH of D. herbacea varied with life history stage and was strongly seasonal in both years and may be driven by ocean temperature. Specifically, in 2016, the pH was highest during the recruitment season (1.38 ± 0.14), followed by a decline in pH during the growth period (0.60 ± 0.01), followed by an elevation during the senescence period (0.65 ± 0.02). Benthic invertebrate grazers had a strong, significant and negative effect on the early recruitment of D. herbacea both in permanent plots and an herbivore exclusion experiment. Plots with higher herbivore grazing pressure had significantly lower recruitment. Higher densities, and smaller individuals in 2015 may have been correlated with higher temperatures in 2015 associated with ENSO events. These findings suggest that despite inter-annual variability in demographic patterns, strong, seasonal shifts in intracellular pH may reflect ontogenetic shifts in chemical defense to protect vulnerable growth phases of life history.