Influence of kelp forests on flow around headlands

Kelp forests affect coastal circulation but their influence on upwelling around headlands is poorly understood. Tidal-cycle surveys off two headlands with contrasting kelp coverage illustrated the influence of kelp forests on headland upwelling. Underway acoustic Doppler current and backscatter profiles were collected simultaneously to surface water temperature. Surveys occurred along three off-headland transects in July 25–29, 2018, off Isla Natividad, located midway on the western coast of the Baja California peninsula. Flows and water temperature distributions off the headland with no kelp coverage were consistent with headland upwelling. In contrast, the kelp around the headland with dense coverage: 1) attenuated the ambient flow; 2) favored an increase in effective radius of flow curvature; 3) promoted flow ducting, which consists of enhancing flow through channels unobstructed by kelp; and 4) suppressed headland upwelling. Kelp suppressed upwelling by channeling the flow away from the headland, keeping nearshore waters warmer than offshore. Plain language abstract: This study documents a way in which biology can affect physics in coastal ocean environments. In particular, the study describes how a kelp forest suppresses the upward pumping of cool subsurface waters that is typically found around headlands. Such suppression of subsurface waters injection occurs via a process that we refer to as ‘flow ducting.’ In flow ducting, coastal flows are channelized through kelp gaps, concentrated in bands <30 m wide, and kept away from the morphological influences of a headland. This ducting is analogous to the tortuous flow through porous media.