UBC Theses and Dissertations
Growth and persistence of the kelp Neoagarum fimbriatum in the face of intense grazer pressure Borden, Laura
Kelp forests are one of the most productive ecosystems in the world, supporting a diverse assemblage of species. Throughout their history of study, kelp forests have undergone shifts between kelp forests and urchin barrens. Urchin barrens – rocky reefs which have no remaining kelp habitat due sea urchin grazing – have important consequences for species that rely on this habitat for survival. In Howe Sound, British Columbia Neoagarum fimbriatum is the dominant habitat-forming kelp and is the essential settlement habitat for commercially important juvenile spot prawns, Pandalus platyceros. Since 2013, the abundance of Neoagarum has declined following an increase in green urchins, Strongylocentrotus droebachiensis, due to loss of top-predator sea stars. This shift in the rocky reef community has led to concerns of continued decline in Neoagarum. This study looks at two aspects of Neoagarum persistence in the face of intense grazer pressure. First, seasonal growth patterns with respect to light and temperature variation are examined. Second, the impact of high densities of green urchins on Neoagarum loss are quantified. Seasonal growth patterns showed that Neoagarum grows throughout the year with maximal growth rates during summer, reaching up to 7% d-¹. Light was determined to be the main driver of these patterns. This was apparent particularly during spring plankton blooms when periods of minimal light at the depth of the kelp beds correlated with the lowest growth rates, while highest growth correlated with the longest days and highest light intensity. These growth rates suggest that in the absence of grazer pressure, Neoagarum has the capacity to develop dense kelp beds in a matter of months. Urchin grazing experiments showed that the relationship between urchin density and the rate of kelp biomass loss scaled linearly, but only at density below 36 urchins m-². Density-dependent restriction of green urchins was apparent at densities greater than 36 urchins m-². Despite lower per capita grazing rates at high densities it appears that so long as densities of green urchins densities remain high, their ability to consume greater than just 7% kelp biomass per day will lead to further declines in Neoagarum kelp beds.
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