Algal spore settlement and germling removal as a function of flow speed

Abstract

The settlement success of zoospores from the green alga Ulva intestinalis (syn. Entero- morpha intestinalis) was studied in relation to the small-scale hydrodynamic forces acting close to the substratum. In moderate flow speeds (free-stream velocity 20 cm s -1 ) U. intestinalis spore settlement was strongly reduced, with only 5% of spores settled compared with settlement in still water. The decrease in settlement occurred when calculated flow speeds at the height of settling spores exceeded the swimming speed of spores (approximately 0.2 mm s -1 ). This indicates that the relatively low swimming speed of the spores is important for successful settlement in the viscous sublayer. A possible longer-term effect of flow during settlement was also tested by measuring the removal of 2 wk old germlings exposed to a flow speed of 10 m s -1 , which is approximately the maximum speed encountered on a rocky shore. The percentage removal was lower from the less densely populated panels, i.e. panels exposed to 20 cm s -1 during settlement. After removal, biomass was nearly equal on all panels regardless of the initial density of settling spores. The footprint of reduced settlement in 20 cm s -1 flow speed did not persist, in terms of biomass, after the removal treatment. Even though rough surfaces and stochastic variations in hydrodynamic forces are likely to affect spores and germlings in nature, the results from this study can give an estimate of flow effects for tested conditions.