Date

Summer 2025

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

In California, recent large-scale declines in kelp forest cover, the formation of sea urchin barrens, and the subsequent biodiversity loss have heightened the need to further investigate mechanisms of kelp forest recovery and sea urchin barren dynamics. To better understand the kelp recruitment potential of these deforested areas it is important to determine the effect sea urchins have on the microscopic portion of the kelp lifecycle and if a size refuge exists. Laboratory studies were used to examine the effect of the purple sea urchin Strongylocentrotus purpuratus on the density, percent cover and reproductive output for three microscopic stages of Giant Kelp, Macrocystis pyrifera. Three separate experiments were initiated, each beginning at a different microscopic stage: newly settled zoospore (2 days old), prefertile gametophyte (13 days old), and embryonic sporophyte ( 4 weeks old). Each experiment ran for three weeks. In all experiments, sea urchins decreased the densities of microscopic stages within 1 week. In all experiments sea urchins also reduced gametophyte percent cover. Purple sea urchins significantly reduced embryonic sporophyte density and hindered sporophyte recruitment in all experiments. The impact of purple sea urchin on per capita oogonia and sporophytes, as well as the density of gametophytes with oogonia and sporophytes, was assessed in the experiments starting at the youngest stages (zoospore and prefertile gametophytes). Sea urchins had no effect on the number of oogonia or sporophytes produced per gametophyte. In the prefertile gametophyte experiment, sea reduced the density of gametophytes with oogonia and sporophytes. This study found no evidence for a size refuge, i.e., no microscopic stage was too small for sea urchins to affect. These findings suggest that microscopic stages, existing prior to urchin barren formation, may not be a reliable source for recruitment. Additionally, this study suggests that urchins also have the potential to suppress recruitment of sporophytes from new zoospore input. We propose that the impact of urchins on the microscopic stages of M. pyrifera is a significant factor in explaining the persistence of purple urchin barrens for prolonged periods, particularly those isolated from established kelp forests without continuous zoospore input and settlement.

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