Density of benthic macroalgae in the intertidal zone varies with surf zone hydrodynamics

Abstract

ABSTRACTSurf zone hydrodynamics influence subsidies (larval settlers and phytoplankton food) to the intertidal zone; subsidies have been observed to be much higher at more dissipative shores compared to reflective shores. Benthic macroalgal populations may be favoured at more reflective surf zones due to slower water exchange with the coastal ocean, facilitating retention of spores. In addition, larval invertebrate settlers are far less abundant at more reflective surf zones, and benthic macroalgae may experience lower competition from sessile invertebrates for space. We used surf zone width as an indictor of surf zone hydrodynamics; surf zones are wider at more dissipative surf zones compared to reflective surf zones. We tested the hypothesis that as surf zone width increases, macroalgal density would decrease. We used aerial near-infrared and visible light images to calculate the normalised difference vegetation index (NDVI) at eight intertidal sites along the Oregon coast and compared NDVI with algal biomass (dry and wet weight per square metre) at each specific location; linear regressions between NDVI and dry and wet algal biomass were significant (P < 0.01); NDVI was an accurate indicator of macroalgal density. Surf zone width was measured using Google Earth images. NDVI and dry and wet algal biomass were significantly lower at wider surf zones (P < 0.01). Although the mechanism underlying this relationship is unknown, the intertidal macroalgal community clearly varied with surf zone hydrodynamics as indicated by surf zone width.