Spatial patterns of flow and their modification within and around a giant kelp forest

We present findings from two sets of measurements that quantified currents around and over the full extent of a giant kelp ( Macrocystis pyrifera) forest located at Mohawk Reef, Santa Barbara, California. Velocities were damped inside this 200- m 3 300- m forest, but not to the extent reported for larger ( kilometer- scale) kelp beds, suggesting that alongshore currents may play a greater role in exchange than has often been assumed. Secondary flow features that bear on the performance of forest organisms were observed, including a region along the forest's outer boundary where velocities exceeded incident speeds by up to 200%. An offshore current on the order of 1 cm s(-1) developed within the kelp bed, likely due to pressure gradients established across the forest coupled with topography. Wake recirculations that might have facilitated leeward retention of waterborne subsidies were not apparent. Calculations suggest that kelp beds can interact with ( and thus potentially filter) substantial portions of impinging waters; in our study, 40 - 90% of arriving waters entered the upstream end, and 20 - 70% reached the center of the forest. Seasonal changes in the size and density of the forest modified the levels of flow damping and filtration. The sum of these effects suggests potential influences on organisms throughout the forest community.