Interim report on the effects of Sargassum on Galveston Island's West-end Beaches: a quantitative assessment
Dellepenna, Timothy, Ph.D.
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The impact of sargassum beach cast material was measured along the beaches of the west end of Galveston Island, Texas between February 2006 and January 2007. Six monthly cross sectional profiles were obtained by using theodolite and transit systems coupled with survey grade global positioning systems that measured latitude, longitude and elevation from the toe of the dune or other limiting structure to the wet line. During the 12-month study period, the average elevation change for all study sites was -10 centimeters ranging from an independent site change of +4 centimeters to - 26 centimeters. During the study period, one meter cores were extracted along the profile lines. Four sediment cores per site were collected in February, April, September, and November 2006. The approximate coring intervals were 10 to 13 meters across-shore, starting from the toe of the dune/geo-structure and finishing at the wetline of the beach. Organic percentages ranged from 0.84% to 0.02%. The February coring cycle recorded the highest average organic percentage at 0.54%. The May coring cycle recorded the lowest at 0.25%. These monthly observations resulted in a time series of both profile and volume changes, thus enabling an analysis of beach change. There are at least six known species of sargassum, but the two most commonly found on the surface are Sargasso natans and Sargasso fluitans. The two species are very similar, and many clumps and weed lines contain both species. Monthly monitoring at the six beach sites revealed that accumulation of beach cast was markedly seasonal with largest amounts observed during the months of June and July. At this point in our study, the analysis of the cores extracted and the temporal dataset profiles does not support a direct connection between sargassum beach cast and longterm (annual) beach influences on beach geomorphology. The accretion of sand during the height of sargassum casting was short term (seasonal) as beach elevation returned to previous levels in subsequent months.