BOLLER, M.L.*; CARRINGTON, E.; Univ. of Rhode Island; Univ. of Rhode Island: In situ measurements of hydrodynamic force on a canopy-forming intertidal macroalga

Hydrodynamic forces on intertidal macroalgae can be large due to the high water velocities that occur as waves break on the intertidal zone. This often results in the removal of an individual from the population when the hydrodynamic force exceeds the attachment strength. However, predicting the force experienced by an individual and its probability of removal is difficult because hydrodynamic forces are modified by reconfiguration (the change in shape and size of an individual due to its flexibility, resulting in decreased drag) and the canopy effect (the reduction in local flow velocity due to the macroalga living in a group). Additionally, the precise measurement of hydrodynamic forces on these organisms has been difficult due to the physical challenges of working in the wave swept intertidal. Thus, most studies of algal reconfiguration and the effects of canopies have been conducted in flow tanks, often at water velocities much lower than typical intertidal field conditions. In this study, compact digital force recorders were developed that measure 3-dimensional in situ forces on the turf forming macroalga Chondrus crispus. Recorders were deployed in the lower intertidal zone to measure water velocity and the forces experienced by individual Chondrus crispus within and outside canopies. Data was collected at a rate of 20 Hz for 1 hour and repeated over a range of wave conditions. These force measurements will be compared to previous laboratory studies, and effects of canopies will be discussed.