Macrocystis pyrifera forest development shapes the physical environment through current velocity reduction

Abstract

Marine forests of the giant kelp Macrocystis pyrifera create biogenic habitat spanning the water column, within which hydrodynamic conditions can differ strongly from those outside. Such flow alteration has implications for physical, chemical, and ecological processes across multiple spatial scales. At the forest-wide scale, M. pyrifera has been shown to decrease alongshore current velocities, but relatively little is known about how the attenuation of such currents evolves as new kelp forests emerge and mature. Here, we quantified alongshore current velocities outside and within a temperate rocky reef environment that twice underwent a transition from a barren state to one in which a thick surface canopy was present. We identified a threshold density during forest emergence at which much of the attenuation of alongshore depth-averaged velocity occurs—3 stipes m-2 with a surface canopy present. Incremental increases in damping occur as the forest matures, highlighting that relatively young, thin forests can induce substantially reduced flows. Additionally, the presence of a young forest’s subsurface canopy and its subsequent increase in height create a seasonally changing profile of varying velocities through the water column. These results indicate greater complexity in how canopy-forming kelp influence nearshore flow properties than has often been recognized. Importantly, emerging forests can alter the nearshore environment through modulation of current speeds shortly following initial recruitment, with consequences for the transport of larvae, nutrients, and sediment throughout the forest and adjacent habitats.