Wave surge increases rates of growth and nutrient uptake in the green seaweed Ulva pertusa maintained at low bulk flow velocities

Effects of wave surge on growth and nutrient uptake in the green seaweed Ulva pertusa were measured in small tanks fed natural seawater via a dump bucket that could be either locked in place to deliver water continuously, or set to tip at particular volumes. Rates of bulk water flow through the tanks were low (<13 mm s(-1)), as may be typical of highly sheltered marine and estuarine environments. At the maximum bulk flow rates tested, wave surge increased growth in wet mass 1.6-fold. Wave surge increased the rate of ammonium uptake 1.5-fold over the full range of bulk flow rates tested (0.2 to 2.9 mm s(-1)). In winter, when light probably limited U. pertusa in our outdoor cultures, growth in wet mass was relatively unaffected by wave surge and completely unaffected by the experimental addition of nutrients (N and P). However, in summer, when ambient nutrients probably limited growth, wave surge increased growth in wet mass at low (0.21 mm s(-1)) and high (4.06 mm s(-1)) bulk flow rates, and nutrients increased growth rates at low but not high bulk flow rates. Together, these results indicate that the effect of wave surge on U. pertusa at low bulk flow rates (ca. <2 mm s(-1)) is to enhance the seaweed's access to nutrients. However, at higher bulk flow rates wave surge has a different effect, which we speculate is due either to provision of a micronutrient or removal of an inhibitory substance from the concentration boundary layer. Our work demonstrates