The effects of temperature and pH on egg release and germling development of the fucoid alga, Silvetia compressa

The rockweed, Silvetia compressa (Phaeophyceae, Fucales) is a fleshy, brown seaweed that occupies the middle intertidal zone on wave-exposed rocky shores from northern California to Baja California. Like other rockweeds in temperate latitudes worldwide, Silvetia is a canopy-forming foundation species that provides habitat and refuge from desiccation and thermal stress to a diverse community of intertidal organisms. Over the past several decades, Silvetia populations have exhibited declines driven likely by multiple anthropogenic disturbances, including alterations in environmental conditions associated with climate change, that are affecting both adults and early life history stages. Reproduction and development of Silvetia is particularly sensitive to abiotic stresses, thus, altered spawning, recruitment, and development are expected as global temperatures and carbon dioxide levels continue to rise and the ocean becomes more acidic. To examine the effects of temperature and pH on Silvetia early life history stages (egg and germling abundance, germination success, and germling development), I conducted a laboratory-controlled experiment. Collected reproductive tips of Silvetia were induced to release gametes in petri dishes (n=5 per treatment per month) with seawater conditions of four possible combinations of temperature (ambient 16℃ and a conservative future warm condition of 20℃) and pH (ambient 8.1 and a conservative future condition of 7.8) treatments. The number of eggs in petri dishes were quantified thirty minutes post-spawning and the number of germlings were counted a day later, with germination success. One-week post-spawning, the length of germlings remaining in petri dishes with maintained water conditions were measured. The effects of temperature and pH were variable among the different early life history stages of Silvetia with warming negatively impacting egg and germling counts and, potentially, germling length while future low pH conditions only reduced egg numbers under ambient temperature conditions. Despite documented peaks in reproductive output in Silvetia in winter, there were no clear temporal patterns for any measured parameter. Ocean warming may have a greater impact on Silvetia early life history stages than ocean acidification. Future impacts on early life history stages may result in continued declines of these ecosystem engineers which can have disproportionate effects on the ecosystem, including dramatic shifts in productivity, biodiversity, nutrient cycling, and ecosystem stability.

Fields L. A., 2023. The effects of temperature and pH on egg release and germling development of the fucoid alga, Silvetia compressa. MSc thesis, California State Polytechnic University, 62 p. Thesis.

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