Influence of Trampling Intensity versus Hydration State on Loss of Biomass from the Intertidal Rockweed,Fucus gardneri

I investigated the impact of human trampling on Fucus gardneri populations at Cattle Point on San Juan Island, Washington. I hypothesized that damage to Fucus fronds would differ depending on hydration state of the plants, number of trampling steps, and time. Over a two-day period, two levels of trampling treatment, 100- and 200-steps, were applied to 18 plots on two rocky intertidal benches. Detached biomass resulting from trampling was collected each day, weighed to the nearest 0.1 gram, re-hydrated overnight in seawater, and re-weighed in order to calculate biomass removed as a function of mean percent Fucus within each plot. Damage to plants did not differ with the hydration state of Fucus. However, loss of biomass did increase with treatment level, with 222 g of Fucus fronds removed from one 100-step plot up to 1302 g removed from a 200-step plot. Percent cover decreased differentially with treatment level, ranging from 10 percent cover in a 100-step plot up to 85 percent loss in a 200-step plot. To determine the effect of hydration on loss of Fucus biomass, future studies should include plants with a higher degree of desiccation than those in this study, more samples to increase the power of the analysis, and a longer period to detect changes due to time.