礫の転がしによる大型糸状緑藻カワシオグサの剥離実験

Abstract

The large filamentous green alga Cladophora glomerata was experimentally scraped off cobbles by rolling the cobbles by hand along a natural riverbed. We estimated standing crop (loss on ignition) of the algae by following methods. The shape of a tuft of the alga was likened to a spindle, and the volume of the tuft was estimated by finding the volume of the spindle using the thickness and the length. In advance we found a regression equation predicting loss on ignition (LOI) of the tuft from the volume (V): LOI (mg) = 0.023758V (mm^3). Therefore, by measuring the thickness and the length of every tuft of the algae on a cobble we estimated the standing crop (loss on ignition) of the algae on the cobble. In the experiment about a half of the standing crop was scraped by every 15m rolling, regardless of initial amount of the standing crop before rolling. It suggests that large standing crop raised probability that algae hit the riverbed, and consequently larger standing crop might be scraped. Densely attached tufts of the algae were slightly less scraped both in the number of tufts and in the standing crop, whereas sparsely attached tufts were slightly more scraped. It suggests that dense tufts tend to be only shorted whereas sparse tufts are cut at the roots. Roundness of cobbles had little effect on scraping the algae. But, after rolling the cobbles long over 20 m, the algae tend to be slightly more scraped on the well rounded cobbles. We did not find any effect of the weight of cobbles on scraping.