Beach kelp wrack surveys following construction of Wheeler North Reef in San Clemente, California

摘要

The accumulation of drift kelp (kelp wrack) along the shoreline is documented at five 500-foot transects spanning a 3.7-mile study area on the beach at San Clemente, California, 1999-2005 and 2008-2013. Large amounts of kelp wrack were found to accumulate on the beach immediately after some, but not all, large wave storm/swell events. Evidence was found that a combination of swell size and directionality shift had as much to do with kelp breaking free from its substrate as mere wave height (Hs). Further, there were larger volumes of kelp wrack during southern California's dry summer season than during its rainy winter season, even though it is usually during the winter that most of the large storm events occur. This may have been due to the fact that kelps ability to stay attached to the bottom weakens as a function of increasing temperatures over the summer season, as well as to large southern swell events that occasionally arise from early spring through fall. Overall, higher average kelp wrack amounts were observed during the spring, summer, and fall months than during the winter months of January-March. On an annual basis, the largest amount of kelp wrack appeared in the last two years of both the initial six-year study, from 1999 to 2005, and the later five-year study from 2008 to 2013. This is most likely because individual giant kelp plants (Macrocystis pyrifera) in southern California have a life expectancy of 4-6 years. Two of the five 500-ft transects, San Clemente Pier and San Mateo Point, consistently showed the greatest amount of kelp wrack per survey. This is likely due to the trapping abilities of the pier pilings of San Clemente Pier and the focusing of wave energy by San Mateo Point.