Biotic and Abiotic Influences on Bull Kelp (Nereocystis luetkeana) Abundance and Distribution in the Salish Se

Bull kelp, Nereocystis luetkeana, is an annual species of subtidal marine macroalgae that serves as major biogenic habitat in the Salish Sea of Washington State. Kelp forests are ecologically important because they provide structure for a variety of other organisms and contribute substantial primary production to food webs within and below the photic zone. Like all kelp, N. luetkeana has a complex life history with alternating macroscopic sporophyte and microscopic gametophyte life stages. The mature sporophyte reaches up to 30 meters in length and is the most recognizable life stage, but the surface of a bull kelp bed only hints at the complexity below. Juvenile bull kelp need light and space and must compete with perennial understory kelp as soon as they begin to grow. Additionally, survivorship is low for those recruits that do begin to grow; the narrow stipes and single blades of bull kelp recruits are likely far more vulnerable to damage by herbivores than mature bull kelp with thicker stipes and many blades. Conversely, adult bull kelp have ample access to light, but are far more susceptible to the effects of current flow and the accompanying drag than juveniles. Because juvenile bull kelp sporophytes (stipe length < 40 cm) face very different biotic and abiotic pressures than adult bull kelp that have reached the surface, it is crucial to improve our understanding of this canopy-forming kelp across multiple life stages. The four major goals of this dissertation are: 1) track the appearance and survivorship of N. luetkeana recruits across seasons 2) examine how propagule availability and removal of competitors influence N. luetkeana recruitment at two sites within and across seasons; 3) quantify how morphology and material properties of N. luetkeana change across ontogeny, investigating morphological scaling of both juvenile and mature bull kelp and testing how juvenile stipes fail; and 4) determine feeding patterns of the Northern Kelp Crabs ( Pugettia producta) in laboratory experiments and assess how protecting juvenile N. luetkeana from large consumers in the field influences growth.