Ecophysiological adaptations of northeast Pacific kelp communities to ultraviolet light stress

Kelp are differentially stratified along a gradient of UV exposure (via water depth). Ultraviolet light's role in seaweed zonation has not been fully explored. Differential UV tolerance may be a determinant of kelp zonation. Barkley Sound waters reduced UV exposure by 100% at a depth of 5--10 m, suggesting UV influences were shallower than the maximal depth of observed kelp growth (15 m). Nearshore and offshore waters possess similar UV transparencies and proximity to kelp beds did not affect seawater UV transparency. Intertidal organisms at 1.5 m(LLW) experience 5--7.5X more UV than at 0 m(LLW). Peak intertidal UV influence occurs during April--May, and August--September in the subtidal. Meiospore size differences exist within and among kelp species. Size correlated with depth distribution of adult plants, with the largest meiospores originating from shallow kelp. Under laboratory UV, meiospores from high-UV environments had greater survivorship than those from low-UV environments. Differential tolerance to UV, possibly determined by meiospore size, may limit the upper settlement position of kelp species and individuals. Kelp phlorotainnins may, as an exudate, affect coastal absorbancy characteristics. A dilution series of seawater, incubated with kelp, reduced UV-B in an inverse relationship. Phlorotannin levels within contact liquids were inversely related to UV-B exposures. Increasing kelp exudate/phlorotannin levels, which filtered light treatments, increased meiospore survivorship during UV-B stress. Short-term exposure to UV-A and UV-B increased phlorotannin levels within kelp tissues as compared to PAR exposed or unexposed (dark) plants. UV-A increased contact liquid absorbance (300 nm), attributable to phlorotannins, while UV-B decreased absorbance (at 300 nm). Results suggest kelp phlorotannins are produced and released into seawater during periods of UV-A stress but degraded by UV-B and they can reduce biological damage. UV-absorbing compounds (UVAC) within kelp tissues were maximal following seasonal periods of high UV exposure. UVAC was highest within shallow, high UV exposed plants. Meristematic and soral tissues possessed higher levels of UVAC than surrounding tissues types. UVACs were higher in blade tissues than structural tissues. Eight Barkley Sound Laminariales and one Fucales possessed a distinct absorbance peak at 333--334 nm. UVACs may act as cellular UV chemical defenses through selective absorbance of UV. This thesis demonstrates intra and interspecific differences in kelp meiospore call size/UV tolerance and production of biochemical UV sunscreens. These UV adaptations likely contribute towards the structure and distribution of near-shore kelp communities.