Evidence for abrasion and enhanced growth of Ulva lactuca L. in the presence of colliery waste particles.

Previous studies have highlighted a reduction in occurrence and biomass of Ulva lactuca L. (Chlorophyceae) on shores where inputs of colliery waste occur. It was postulated that this was owing to an abrasive effect of colliery waste on macroalgal fronds. To test this, individual U. lactuca plants were exposed to colliery waste (three different grain size categories: < 500 microm, 500-2000 microm, and 0-2000 microm) in both shaken (turbulent) and still conditions in the laboratory. Over an 8-day period, U. lactuca plants lost weight when colliery waste was present and gained weight when no colliery waste was present. Weight loss was most pronounced in U. lactuca exposed to colliery waste of 500-2000 microm grain sizes in shaken conditions (- 18.8% +/- 4.65 SE, n = 4). However, greatest weight gain (+ 26.8% +/- 6.26 SE, n = 4) also occurred in the presence of colliery waste (0-2000 microm), but in still conditions. Weight gain was also observed after 60 days in still conditions in the presence of colliery waste (grain sizes 0-2000 microm); U. lactuca showed significant growth (+ 69% +/- 67 SE of starting weight, n = 5) compared with controls (- 51% +/- 41 SE, n = 5). These results suggest that 'large' grains of colliery waste act as a physical abrading agent on macroalgae when in turbulent conditions, and may be responsible for lowering of species richness of macroalgae where colliery waste inputs occur. However, by contrast, colliery waste in still conditions promotes the growth of U. lactuca, suggesting that, for example, rock pool flora may benefit from its presence