Maintenance of a balanced, shifting boundary between the seagrass Phyllospadix and algal turf

Abstract

A dense mat of algal turf covers much of the rocky mid-intertidal substrate on beaches in southern California. Beds of Phyllospadix dominate the lower part of the intertidal zone. The study was designed to identify processes that result in this distribution pattern. to evaluate the role of direct plant/plant interactions, changes in the position of the border were measured in undisturbed vegetation and in clearings between two dominant taxa, P. torreyi S. Watson and Corallina pinnatifolia (Manza) Daws. Recovery of the substrate in clearings within the bed was monitored. Growth rates for prostrate and erect portions of each of the taxa were determined. To assess the effect of environmental factors on each of the two vegetation forms intact clumps were reciprocally transplanted, leaves were removed experimentally under several conditions, and leaf length and density, seasonally and in different habitats, were compared. Analysis of biomass data showed that longer leaves are restricted to wetter habitats. Leaves turned brown when exposed to air, indicating that shoreward extension of the Phyllospadix beds is constrained by physical factors. When the rhizome mat was removed from rocks, Carallina regenerated and within several months its turf covered the area. Rhizomes slowly regrew into turf, unimpeded by algal thalli or the layer of sand that often buries rhizomes and basal portions of thalli. Corallina and Phyllospadix share certain morphological and life history properties that are adaptive to seasonal weather patterns and the effects of sand movement on these intertidal platform beaches; each has distinct advantages on short or longer time scales within the boundary zone where one form replaces the other at a discrete border.